Journal articles on the topic 'Aftershock studies'
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Horner, R. B., R. J. Wetmiller, M. Lamontagne, and M. Plouffe. "A fault model for the Nahanni earthquakes from aftershock studies." Bulletin of the Seismological Society of America 80, no. 6A (December 1, 1990): 1553–70. http://dx.doi.org/10.1785/bssa08006a1553.
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Abstract Relative locations of 323 large aftershocks (M 3.0 or greater) in the period from 5 October 1985 to 25 March 1988 show that the Ms 6.6 event on 5 October 1985 initiated at 62.208°N, 124.217°W, about 2.5 km northeast of the Ms 6.9 main shock on 23 December 1985. The overall aftershock distribution suggests the October rupture was primarily a west-dipping, low-angle thrust. In subsequent aftershock activity, the main rupture plane was marked by a distinct quiescent area of about 200 km2 that persisted until the 23 December event. Most of the stress drop and slip occurred in this area. Following the 23 December rupture, a similar sized quiescent zone was also observed; however, it was only evident during the first 24 hr of the aftershock sequence, and the area was about 50 per cent too small to yield the overall stress drop. The additional area appeared to come from secondary rupture zones that developed coincident with the main shock rupture. Precise locations of 182 small (M 3.0 or less) aftershocks recorded during a third field survey from 12 to 21 September 1986 indicated at least one and probably three high-angle faults. Composite mechanism solutions showed thrust faulting except in a region directly south of the main shock rupture areas where there is a bend in one of the secondary fault zones and a concentration of aftershock activity. Mechanism solutions calculated for five of the largest aftershocks in the same region also indicated a similar variability. Development of secondary fault zones explained the increased complexity of the December event and may also provide an explanation for the vertical peak acceleration exceeding 2 g that was recorded about 10 sec after the December rupture initiated.
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Sabu, Berlin, and S. Deepa Balakrishnan. "A Review on the Selection of Real and Artificial Seismic Sequences for Analysis." Proceedings of the 12th Structural Engineering Convention, SEC 2022: Themes 1-2 1, no. 1 (December 19, 2022): 909–13. http://dx.doi.org/10.38208/acp.v1.601.
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A huge mainshock activates several aftershocks, divulging the public to serious risk and impeding building repair, rehabilitation, and restoration works. The performance of a structure during multiple earthquakes depends on structural properties and the characteristics of ground motion. Hence, the selection of seismic sequence plays a vital role in the analysis. Presumption of spatial and temporal characteristics of aftershocks is needed for the identification of these repeated motions. This paper further looks into statistical variability connected with aftershock sequences. A critical review of different real and artificial seismic sequences taken for the analysis is carried out. A few studies reveal that a strong motion database for procuring actual mainshock aftershock sequences could underrate the aftershock effects as the database is inadequate and imperfect. It is also observed that artificial sequences can take the place of real sequences especially when an ample data set of real mainshock aftershock sequences are not accessible.
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Yang, Fan, Dewen Liu, Min Lei, Yanping Zheng, Tiange Zhao, and Liang Gao. "Seismic fragility analysis of the inter-story isolated structure for the influence of main-aftershock sequences." Advances in Mechanical Engineering 15, no. 1 (January 2023): 168781322211457. http://dx.doi.org/10.1177/16878132221145791.
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The inter-story isolated structure is an effective and feasible structure seismic technology and system, but most studies on inter-story isolated structures only consider the mainshock. A strong mainshock is usually accompanied by multiple aftershocks, the structure will be damaged under the action of the mainshock. Because of the short time interval between the main shock and the aftershocks, the structure is often not repaired in time, so it will be further damaged under the action of the aftershock. Therefore, it is meaningful to study the fragility of inter-story isolated structures under the action of main-aftershock sequences. In this study, the incremental dynamic analysis method was used, and the inter-story isolated structure of a frame shear wall was established. The vulnerability curves of each substructure under the action of a single mainshock and main-aftershock sequence were compared. A series structure system was used to calculate the overall vulnerability of the inter-story isolated structure. The vulnerability curves of different isolation layer setting positions and isolation bearing stiffness under the action of a single mainshock and main-aftershock were compared, and the collapse margin ratio (CMR) of the structure given. The results show that aftershocks increase the exceedance probability of each substructure, and with an increase in the limit state, the influence of aftershocks is more obvious. An appropriate isolation layer design reduces the influence of aftershocks and the exceedance probability of the entire structure.
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Ogata, Yosihiko, and Takahiro Omi. "Statistical Monitoring and Early Forecasting of the Earthquake Sequence: Case Studies after the 2019 M 6.4 Searles Valley Earthquake, California." Bulletin of the Seismological Society of America 110, no. 4 (May 26, 2020): 1781–98. http://dx.doi.org/10.1785/0120200023.
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ABSTRACT This study considers the possible implementation of the operational short-term forecasting, and analysis of earthquake occurrences using a real-time hypocenter catalog of ongoing seismic activity, by reviewing case studies of the aftershocks of the Mw 6.4 Searles Valley earthquake that occurred before the Mw 7.1 Ridgecrest earthquake. First, the short-term prediction of spatiotemporal activity is required in real time along with the background seismic activity over a wide region to obtain practical probabilities of large earthquakes; snapshots from the continuous forecasts during the Searles Valley and Ridgecrest earthquake sequence are included to monitor the growth and migration of seismic activity over time. We found that the area in and around the rupture zone in southern California had a very high background rate. Second, we need to evaluate whether a first strong earthquake may be the foreshock for a further large earthquake; the rupture region in southern California had one of the highest such probabilities. Third, short-term probability forecast of early aftershocks are much desired despite the difficulties with data acquisition. The aftershock sequence of the Mw 6.4 Searles Valley event was found to significantly increase the probability of a larger earthquake, as seen in the foreshock sequence of the 2016 MJMA 7.4 Kumamoto, Japan, earthquake. Finally, detrending the temporal activity of all the aftershocks by stretching and shrinking the ordinary time scale according to the rate given by the Omori–Utsu formula or the epidemic-type aftershock sequence model, we observe the spatiotemporal occurrences in which seismicity patterns may be abnormal, such as relative quiescence, relative activation, or migrating activity. Such anomalies should be recorded and listed for the future evaluation of the probability of a possible precursor for a large aftershock or a new rupture nearby. An example of such anomalies in the aftershocks before the Mw 7.1 Ridgecrest earthquake is considered.
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Savage, Martha Kane, and Robert P. Meyer. "Aftershocks of an M = 4.2 earthquake in Hawaii and comparison with long-term studies of the same volume." Bulletin of the Seismological Society of America 75, no. 3 (June 1, 1985): 759–77. http://dx.doi.org/10.1785/bssa0750030759.
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Abstract Study of the aftershocks recorded in a 3-hr period after a 4.2 magnitude event on the East Rift Zone of Kilauea volcano, Hawaii, on 12 April 1982 shows that the aftershocks occurred on different planes than the main shock, probably as a result of stress redistribution; the aftershock locations are probably controlled by preexisting structures. This study also suggests that these relatively small aftershocks occurred in the same seismicity patterns as larger events recorded in the same volume over a period of 10 yr. Slips on most of the aftershocks and the main shock are in the same direction, perpendicular to the East Rift Zone, as has been found in studies of other, larger earthquakes. However, fault-plane solutions varied more, as did the tensional axes, and several of the smaller events showed movement in the opposite direction from the main shock and the rest of the aftershocks, suggesting some rebound was occurring near the edges of the aftershock zone. Because ten times as much energy was released in the aftershocks in a narrow linear region as elsewhere, and since the main shock epicenter was oceanward of all the aftershocks, we suggest that rupture began at the main shock hypocenter and propagated landward, implying an almost “one-dimensional” fault. For the aftershocks, the relationship between moment and magnitude was: log M0 = (1.18 ± 0.17) ML + (17.3 ± 0.17). Differences in amplification lead to site differences of up to 0.8 units in local magnitude and 1.5 orders of magnitude in energy release. These correlated somewhat with station time corrections in that the stations with the longest delay times also had greatest amplification.
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Astiopoulos, A. C., E. Papadimitriou, V. Karakostas, D. Gospodinov, and G. Drakatos. "SEISMICITY CHANGES DETECTION DURING THE SEISMIC SEQUENCES EVOLUTION AS EVIDENCE OF STRESS CHANGES." Bulletin of the Geological Society of Greece 43, no. 4 (January 25, 2017): 1994. http://dx.doi.org/10.12681/bgsg.11390.
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The statistical properties of the aftershock occurrence are among the main issues in investigating the earthquake generation process. Seismicity rate changes during a seismic sequence, which are detected by the application of statistical models, are proved to be precursors of strong events occurring during the seismic excitation. Application of these models provides a tool in assessing the imminent seismic hazard, oftentimes by the estimation of the expected occurrence rate and comparison of the predicted rate with the observed one. The aim of this study is to examine the temporal distribution and especially the occurrence rate variations of aftershocks for two seismic sequences that took place, the first one near Skyros island in 2001 and the second one near Lefkada island in 2003, in order to detect and determine rate changes in connection with the evolution of the seismic activity. Analysis is performed through space–time stochastic models which are developed, based upon both aftershocks clustering studies and specific assumptions. The models applied are the Modified Omori Formula (MOF), the Epidemic Type Aftershock Sequence (ETAS) and the Restricted Epidemic Type Aftershock Sequence (RETAS). The modelling of seismicity rate changes, during the evolution of the particular seismic sequences, is then attempted in association with and as evidence of static stress changes
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Wang, Chisheng, Junzhuo Ke, Jincheng Jiang, Min Lu, Wenqun Xiu, Peng Liu, and Qingquan Li. "Visual analytics of aftershock point cloud data in complex fault systems." Solid Earth 10, no. 4 (August 27, 2019): 1397–407. http://dx.doi.org/10.5194/se-10-1397-2019.
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Abstract. Aftershock point cloud data provide direct evidence for the characteristics of underground faults. However, there has been a dearth of studies using state-of-the-art visual analytics methods to explore the data. In this paper, we present a novel interactive visual analysis approach for visualizing the aftershock point cloud. Our method employs a variety of interactive operations, rapid visual computing functions, flexible display modes, and various filtering approaches to present and explore the desired information for the fault geometry and aftershock dynamics. The case study conducted for the 2016 Central Italy earthquake sequence shows that the proposed approach can facilitate the discovery of the geometry of the four main fault segments and three secondary fault segments. It can also clearly reveal the spatiotemporal evolution of the aftershocks, helping to find the fluid-driven mechanism of this sequence. An open-source prototype system based on the approach is also developed and is freely available.
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Jiang, C. S., and Z. L. Wu. "PI forecast with or without de-clustering: an experiment for the Sichuan-Yunnan region." Natural Hazards and Earth System Sciences 11, no. 3 (March 7, 2011): 697–706. http://dx.doi.org/10.5194/nhess-11-697-2011.
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Abstract. Pattern Informatics (PI) algorithm uses earthquake catalogues for estimating the increase of the probability of strong earthquakes. The main measure in the algorithm is the number of earthquakes above a threshold magnitude. Since aftershocks occupy a significant proportion of the total number of earthquakes, whether de-clustering affects the performance of the forecast is one of the concerns in the application of this algorithm. This problem is of special interest after a great earthquake, when aftershocks become predominant in regional seismic activity. To investigate this problem, the PI forecasts are systematically analyzed for the Sichuan-Yunnan region of southwest China. In this region there have occurred some earthquakes larger than MS 7.0, including the 2008 Wenchuan earthquake. In the analysis, the epidemic-type aftershock sequences (ETAS) model was used for de-clustering. The PI algorithm was revised to consider de-clustering, by replacing the number of earthquakes by the sum of the ETAS-assessed probability for an event to be a "background event" or a "clustering event". Case studies indicate that when an intense aftershock sequence is included in the "sliding time window", the hotspot picture may vary, and the variation lasts for about one year. PI forecasts seem to be affected by the aftershock sequence included in the "anomaly identifying window", and the PI forecast using "background events" seems to have a better performance.
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Mahat, Pushpa, Piyush Pradhan, Rabindra Adhikari, Andre Furtado, Dipendra Gautam, and Rajesh Rupakhety. "Seismic Sequence Vulnerability of Low-Rise Special Moment-Resisting Frame Buildings with Brick Infills." Applied Sciences 12, no. 16 (August 17, 2022): 8231. http://dx.doi.org/10.3390/app12168231.
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When buildings are exposed to earthquake sequence, damage aggravation is expected to occur. Although several studies report seismic vulnerability of reinforced concrete (RC) buildings under the mainshock–aftershock sequence, indicating damage aggravation due to aftershock, none, to the best of our knowledge, quantifies seismic vulnerability of buildings under foreshock–mainshock–aftershock sequences. Since foreshock–mainshock–aftershock sequences are also expected in many active seismic regions, we aim to quantify the level of vulnerability under seismic sequences considering the seismically highly active Himalayan region as the case study location. Fragility functions are derived considering foreshock, foreshock–mainshock sequence, and foreshock–mainshock–aftershock sequence for a low-rise special moment-resisting frame (SMRF) building that represents a typical low-rise owner-built construction system in Nepal, one of the most active seismic regions in the world. The results highlight that the foreshock significantly increases seismic vulnerability of the structures with respect to the often-considered case of a mainshock–aftershock sequence.
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Gerstenberger, M. C., L. M. Jones, and S. Wiemer. "Short-term Aftershock Probabilities: Case Studies in California." Seismological Research Letters 78, no. 1 (January 1, 2007): 66–77. http://dx.doi.org/10.1785/gssrl.78.1.66.
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Iervolino, Iunio. "Generalized Earthquake Counting Processes for Sequence‐Based Hazard." Bulletin of the Seismological Society of America 109, no. 4 (June 11, 2019): 1435–50. http://dx.doi.org/10.1785/0120180271.
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Abstract Sequence‐based probabilistic seismic hazard analysis (SPSHA) allows us to account for the effect of aftershocks in the assessment of seismic structural‐design actions (Iervolino et al., 2014, 2018). In fact, it generalizes classical probabilistic seismic hazard analysis (PSHA; Cornell, 1968), combining it with aftershock‐PSHA (Yeo and Cornell, 2009). SPSHA associates in time aftershocks to mainshocks and, therefore, retains a desirable property of classical PSHA; that is, events (earthquakes in PSHA and mainshock–aftershock sequences in SPSHA) occur according to homogeneous Poisson processes (HPPs). Nevertheless, the number of earthquakes in SPSHA is not Poisson‐distributed. This is addressed herein, in which the probability distribution is formulated and discussed for the following random variables: (1) the count of all earthquakes pertaining to sequences originating in any time interval; (2) the count of all earthquakes occurring in any time interval; (3) the count of all earthquakes that cause exceedance of an arbitrary ground‐motion intensity threshold at the site of interest, generated by sequences originating in any time interval. An application referring to central Italy is also developed to help the discussion. The three main findings are that: (1) the formulated SPSHA counting processes further generalize PSHA; that is, they degenerate in the corresponding mainshock HPPs, if aftershocks are neglected; (2) to associate the aftershocks to the corresponding mainshocks in time is fit for hazard assessment purposes; and (3) the variance‐to‐mean ratio of the counting distributions is significantly larger than one; consequently, the occurrence processes cannot be approximated by Poisson processes. These results, which complete the SPSHA framework, can be a reference for model calibration exercises when SPSHA is computed via simulation and in those cases in which the probability of an exact number of exceedances is of interest, rather than that of observing at least one exceedance (e.g., for seismic damage accumulation studies).
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Jia, Ke. "Modeling the Spatiotemporal Seismicity Patterns of the Longmen Shan Fault Zone Based on the Coulomb Rate and State Model." Seismological Research Letters 92, no. 1 (September 23, 2020): 275–86. http://dx.doi.org/10.1785/0220200088.
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Abstract In the past two decades, three major earthquakes have occurred near the Longmen Shan fault zone, Sichuan, China (the 2008 Mw 7.9 Wenchuan, 2013 Mw 6.6 Lushan, and 2017 Mw 6.5 Jiuzhaigou earthquakes), in response to the continuous collision of the Indian and Eurasian plates, and have produced numerous aftershocks. Recent studies have demonstrated that physics-based aftershock forecasting holds the potential capability to meet the demands of earthquake forecasting. I have successfully modeled the spatiotemporal seismicity of the Longmen Shan fault zone by applying the coulomb rate and state model by including high-quality data products (e.g., source models and receive faults) and optimized rate-and-state parameters in the calculation. I also investigate the roles of secondary triggering of aftershocks and the friction coefficient in seismicity modeling. The findings suggest that the friction coefficient plays an important role in modeling the observed seismicity, and that the secondary triggering of aftershocks in the Longmen Shan fault zone moderately affects the predicted seismicity.
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Jones, Laura E., and Susan E. Hough. "Analysis of broadband records from the 28 June 1992 Big Bear earthquake: Evidence of a multiple-event source." Bulletin of the Seismological Society of America 85, no. 3 (June 1, 1995): 688–704. http://dx.doi.org/10.1785/bssa0850030688.
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Abstract The 28 June 1992 Big Bear earthquake occurred at 15:05:21 GMT and is considered to be an aftershock of the earlier Mw = 7.3 Landers earthquake. From overall aftershock locations and long-period focal studies, rupture is generally assumed to have propagated northeast. No surface rupture was found, however, and the mainshock locations determined from both strong motion and TERRAscope data are mutually consistent and do not lie on the assumed fault plane. Further, directivity analysis of records from the TERRAscope array suggests significant short- and long-period energy propagating northwest along the presumed antithetic fault plane. This observation is supported by significant early aftershocks distributed along both the presumed rupture plane and the antithetic plane to the northwest. An empirical Green's function (eGf) approach using both the Mw = 5.2, 28 June 1992 14:43 GMT foreshock and the Mw = 5.0 17 August 1992 aftershock produces consistent results and suggests that the Big Bear event comprised at least two substantial subevents. From the eGf results, we infer that the second and possibly a third subevent occurred on the presumed (northeast striking) mainshock rupture surface, but that significant moment release occurred on the antithetic northwest striking surface. We present results from line-source fault modeling of broadband displacement recordings of the Big Bear mainshock, which indicate that a two-fault event is necessary to produce the observed waveforms. The limitations imposed by the mainshock location and directivity analysis require that the initial rupture be towards the northwest, striking 320°. This was followed approximately 4 sec later by bilateral rupture along a northeast-southwest fault that strikes 50° east of north.
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Wooddell, Kathryn E., and Norman A. Abrahamson. "Classification of Main Shocks and Aftershocks in the NGA-West2 Database." Earthquake Spectra 30, no. 3 (August 2014): 1257–67. http://dx.doi.org/10.1193/071913eqs208m.
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Previous studies have found a systematic difference between short-period ground motions from aftershocks and main shocks, but have not used a consistent methodology for classifying earthquakes in strong motion data sets. A method for unambiguously classifying earthquakes in strong motion data sets is developed. The classification is based on the Gardner and Knopoff time window, but with a distance window based on a new distance metric, CRJB, defined as the shortest horizontal distance between the centroid of the surface projection of the potential aftershock rupture plane and the surface projection of the main shock rupture plane. Class 2 earthquakes are earthquakes that have a CRJB distance less than a selected limit and within a time window appropriate for aftershocks. All other earthquakes are classified as Class 1. For maximum CRJB of 0 km and 40 km, 11% and 36% of the earthquakes in the NGA-West2 database are Class 2 events, respectively.
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Gulia, Laura, Stefan Wiemer, and Gianfranco Vannucci. "Pseudoprospective Evaluation of the Foreshock Traffic-Light System in Ridgecrest and Implications for Aftershock Hazard Assessment." Seismological Research Letters 91, no. 5 (July 8, 2020): 2828–42. http://dx.doi.org/10.1785/0220190307.
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Abstract The Mw 7.1 Ridgecrest earthquake sequence in California in July 2019 offered an opportunity to evaluate in near-real time the temporal and spatial variations in the average earthquake size distribution (the b-value) and the performance of the newly introduced foreshock traffic-light system. In normally decaying aftershock sequences, in the past studies, the b-value of the aftershocks was found, on average, to be 10%–30% higher than the background b-value. A drop of 10% or more in “aftershock” b-values was postulated to indicate that the region is still highly stressed and that a subsequent larger event is likely. In this Ridgecrest case study, after analyzing the magnitude of completeness of the sequences, we find that the quality of the monitoring network is excellent, which allows us to determine reliable b-values over a large range of magnitudes within hours of the two mainshocks. We then find that in the hours after the first Mw 6.4 Ridgecrest event, the b-value drops by 23% on average, compared to the background value, triggering a red foreshock traffic light. Spatially mapping the changes in b values, we identify an area to the north of the rupture plane as the most likely location of a subsequent event. After the second, magnitude 7.1 mainshock, which did occur in that location as anticipated, the b-value increased by 26% over the background value, triggering a green traffic light. Finally, comparing the 2019 sequence with the Mw 5.8 sequence in 1995, in which no mainshock followed, we find a b-value increase of 29% after the mainshock. Our results suggest that the real-time monitoring of b-values is feasible in California and may add important information for aftershock hazard assessment.
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Simanjuntak, Andrean V. H., and Olymphia Olymphia. "Perbandingan Energi Gempa Bumi Utama dan Susulan (Studi Kasus : Gempa Subduksi Pulau Sumatera dan Jawa)." Jurnal Fisika FLUX 14, no. 1 (July 21, 2017): 19. http://dx.doi.org/10.20527/flux.v14i1.3776.
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Indonesia is located on the third meeting of the active tectonic world plates which are Eurasian Indian - Australia and the Pacific Plate. This condition makes Indonesia as a tectonically active area with seismicity level or pattern of high seismicity. Occurrence of devastating earthquake followed by aftershocks of earthquakes, often increase the level of social unrest. The purpose of this study is calculate the energy of devastating earthquakes and followed by aftershocks, so it can be compared to the amount of energy released by both of them. By obtaining these comparisons, the study is based on scientific studies can be used as a reference in providing information on the possible impact of an occurrence of the earthquake and its aftershocks. Empirical formula of Guttenberg- Richter was used to calculate the energy value, historical data with aftershock earthquakes was obtained from ISC (International Seismological Center) for five major earthquakes are Bengkulu, Pangandaran, Simeulue, West Sumatra, and Tasikmalaya earthquake. Earthquake aftershocks taken within three months after a major earthquake. From analysis and energy calculations of earthquake aftershocks of a major earthquake with a magnitude of five large, energy-earthquake aftershocks ranging from 0.1% to 33%, with a random pattern. By comparing the energy aftershocks of earthquakes, the results are generally 10%, it is estimated that the earthquake with strike-slip mechanism having earthquake aftershocks with a total energy is less than 10%. While earthquakes with earthquake aftershocks have thrust mechanism with a total energy of more than 10%.
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Chen, L., J. G. Chen, and Q. H. Xu. "Correlations between solid tides and worldwide earthquakes <i>M</i><sub>S</sub> ≥ 7.0 since 1900." Natural Hazards and Earth System Sciences 12, no. 3 (March 6, 2012): 587–90. http://dx.doi.org/10.5194/nhess-12-587-2012.
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Abstract. Most studies on the correlations between earthquakes and solid tides mainly concluded the syzygies (i.e. new or full moons) of each lunar cycle have more earthquakes than other days in the month. We show a correlation between the aftershock sequence of the ML = 6.3 Christchurch, New Zealand, earthquake and the diurnal solid tide. Ms ≥ 7 earthquakes worldwide since 1900 are more likely to occur during the 0°, 90°, 180° or 270° phases (i.e. earthquake-prone phases) of the semidiurnal solid earth tidal curve (M2). Thus, the semidiurnal solid tides triggers earthquakes. However, the long-term triggering effect of the lunar periodicity is uncertain. This proposal is helpful in defining possible origin times of aftershocks several days after a mainshock and can be used for warning of subsequent larger shocks.
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Konca, A. Ozgun, Sezim Ezgi Guvercin, Seda Ozarpaci, Alpay Ozdemir, Gareth J. Funning, Ugur Dogan, Semih Ergintav, Michael Floyd, Hayrullah Karabulut, and Robert Reilinger. "Slip distribution of the 2017 Mw6.6 Bodrum–Kos earthquake: resolving the ambiguity of fault geometry." Geophysical Journal International 219, no. 2 (July 25, 2019): 911–23. http://dx.doi.org/10.1093/gji/ggz332.
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SUMMARY The 2017 July 20, Mw6.6 Bodrum–Kos earthquake occurred in the Gulf of Gökova in the SE Aegean, a region characterized by N–S extension in the backarc of the easternmost Hellenic Trench. The dip direction of the fault that ruptured during the earthquake has been a matter of controversy where both north- and south-dipping fault planes were used to model the coseismic slip in previous studies. Here, we use seismic (seismicity, main shock modelling, aftershock relocations and aftershock mechanisms using regional body and surface waves), geodetic (GPS, InSAR) and structural observations to estimate the location, and the dip direction of the fault that ruptured during the 2017 earthquake, and the relationship of this event to regional tectonics. We consider both dip directions and systematically search for the best-fitting locations for the north- and south-dipping fault planes. Comparing the best-fitting planes for both dip directions in terms of their misfit to the geodetic data, proximity to the hypocenter location and Coulomb stress changes at the aftershock locations, we conclude that the 2017 earthquake ruptured a north-dipping fault. We find that the earthquake occurred on a 20–25 km long, ∼E–W striking, 40° north-dipping, pure normal fault with slip primarily confined between 3 and 15 km depth, and the largest slip exceeding 2 m between depths of 4 and 10 km. The coseismic fault, not mapped previously, projects to the surface within the western Gulf, and partly serves both to widen the Gulf and separate Kos Island from the Bodrum Peninsula of SW Anatolia. The coseismic fault may be an extension of a mapped, north-dipping normal fault along the south side of the Gulf of Gökova. While all of the larger aftershocks are consistent with N–S extension, their spatially dispersed pattern attests to the high degree of crustal fracturing within the basin, due to rapid trenchward extension and anticlockwise rotation within the southeastern Aegean.
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Langer, C. J., M. G. Bonilla, and G. A. Bollinger. "Aftershocks and surface faulting associated with the intraplate Guinea, West Africa, earthquake of 22 December 1983." Bulletin of the Seismological Society of America 77, no. 5 (October 1, 1987): 1579–601. http://dx.doi.org/10.1785/bssa0770051579.
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Abstract This study reports on the results of geological and seismological field studies conducted following the rare occurrence of a moderate-sized West African earthquake (mb = 6.4) with associated ground breakage. The epicentral area of the northwestern Guinea earthquake of 22 December 1983 is a coastal margin, intraplate locale with a very low level of historical seismicity. The principal results include the observation that seismic faulting occurred on a preexisting fault system and that there is good agreement among the surface faulting, the spatial distribution of the aftershock hypocenters, and the composite focal mechanism solutions. We are not able, however, to shed any light on the reason(s) for the unexpected occurrence of this intraplate earthquake. Thus, the significance of this study is its contribution to the observational datum for such earthquakes and for the seismicity of West Africa. The main shock was associated with at least 9 km of surface fault-rupture. Trending east-southeast to east-west, measured fault displacements up to ∼13 cm were predominantly right-lateral strike slip and were accompanied by an additional component (5 to 7 cm) of vertical movement, southwest side down. The surface faulting occurred on a preexisting fault whose field characteristics suggest a low slip rate with very infrequent earthquakes. There were extensive rockfalls and minor liquefaction effects at distances less than 10 km from the surface faulting and main shock epicenter. Main shock focal mechanism solutions derived from teleseismic data by other workers show a strong component of normal faulting motion that was not observed in the ground ruptures. A 15-day period of aftershock monitoring, commencing 22 days after the main shock, was conducted. Eleven portable, analog short-period vertical seismographs were deployed in a network with an aperture of 25 km and an average station spacing of 7 km. Ninety-five aftershocks were located from the more than 200 recorded events with duration magnitudes of about 1.5 or greater. Analysis of a selected subset (91) of those events define a tabular aftershock volume (26 km long by 14 km wide by 4 km thick) trending east-southeast and dipping steeply (∼60°) to the south-southwest. Composite focal mechanisms for groups of events, distributed throughout the aftershock volume, exhibit right-lateral, strike-slip motion on subvertical planes that strike almost due east. Although the general agreement between the field geologic and seismologic results is good, our preferred interpretation is for three en-echelon faults striking almost due east-west.
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Cochran, Elizabeth S., Emily Wolin, Daniel E. McNamara, Alan Yong, David Wilson, Marcos Alvarez, Nicholas van der Elst, Adria McClain, and Jamison Steidl. "The U.S. Geological Survey’s Rapid Seismic Array Deployment for the 2019 Ridgecrest Earthquake Sequence." Seismological Research Letters 91, no. 4 (January 15, 2020): 1952–60. http://dx.doi.org/10.1785/0220190296.
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Abstract Rapid seismic deployments following large earthquakes capture ephemeral near-field recordings of aftershocks and ambient noise that can provide valuable data for seismological studies. The U.S. Geological Survey installed 19 temporary seismic stations following the 4 July 2019 Mw 6.4 and 6 July 2019 (UTC) Mw 7.1 earthquakes near the city of Ridgecrest, California. The stations record the aftershock sequence beginning two days after the mainshock and are expected to remain in the field through approximately January 2020. The deployment augments the permanent seismic network in the area to improve azimuthal coverage and provide additional near-field observations. This article summarizes the motivation and goals of the deployment; details of station installation, instrumentation, and configurations; and initial data quality and observations from the network. We expect these data to be useful for a range of studies including detailing near-field variability in strong ground motions, determining stress drops and rupture directivity of small events, imaging the fault zone, documenting the evolution of crustal properties within and outside of the fault zone, and others.
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Ommi, S., and H. Zafarani. "Probabilistic aftershock hazard analysis, two case studies in West and Northwest Iran." Journal of Seismology 22, no. 1 (September 24, 2017): 137–52. http://dx.doi.org/10.1007/s10950-017-9696-7.
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Zhang, Xu, Li-Sheng Xu, Lei Yi, and Wanpeng Feng. "Confirmation and Characterization of the Rupture Model of the 2017 Ms 7.0 Jiuzhaigou, China, Earthquake." Seismological Research Letters 92, no. 5 (April 14, 2021): 2927–42. http://dx.doi.org/10.1785/0220200466.
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Abstract On 8 August 2017, an Ms 7.0 earthquake struck the Jiuzhaigou town, Sichuan Province, China, rupturing an unmapped fault, which is adjacent to the Maqu seismic gap in the Min Shan uplift zone in the easternmost part of the Bayan Har block. Having summarized the previous studies on the source of this earthquake, we confirmed the rupture model by jointly inverting the teleseismic P-wave and SH-wave data, Interferometric Synthetic Aperture Radar line-of-sight displacement data, and the near-field seismic and strong-motion data, a most complete dataset until now. The confirmation showed that a scalar seismic moment of 6.6×1018 N·m was released (corresponding to a moment magnitude of Mw 6.5), and 95% of the release occurred in the first 10 s. The slip area was composed of two asperities, with a horizontal extension of ∼20 km and a depth range of ∼2–15 km. A bilateral extending occurred at shallow depths, but the rupturing upward from deep depth dominated in the early time. The rupture process was found generally featuring the slip-pulse mode, which was related to the weak prestress condition. The aftershocks almost took place in gaps of the mainshock slip because of the coulomb stress change. Combining the aftershock relocations, aftershock focal mechanism solutions, and our confirmed rupture model, we suggest that the seismogenic fault was a northward extension of the mapped Huya fault. The occurrence of this earthquake made the Maqu seismic gap at a higher level of seismic risk, in addition to the moderate to high strain accumulation on the easternmost tip of the Kunlun fault system and the weak lower crust below.
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Emanov, A. F., A. A. Emanov, and A. V. Fateev. "Stable Structures of the 2003 Chuya Earthquake Aftershocks." Russian Geology and Geophysics 63, no. 1 (January 1, 2022): 72–84. http://dx.doi.org/10.2113/rgg20204234.
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Abstract —The 2003 Chuya earthquake aftershocks are studied using the data obtained during experiments with dense networks of stations. Density maps of the foci of more than 50,000 aftershocks are compared with the day surface faults and the block structure and tectonics of the focal area. The large shearing strain caused by the Chuya earthquake is accompanied by a spatially intermittent aftershock structure stretching along it. The density maps of long-lasted aftershocks differ in structure from the maps of seismic activity in the initial aftershock area. The study has revealed a relationship between the block structure of the epicentral area and the structure of the aftershock process. The nodes of the intersection of faults with the aftershock area are characterized by reduced aftershock activity. The aftershock process is only partly confined to the block-separating faults. In many cases, the aftershock process is shifted relative to these faults or wanders from them.
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Mizrahi, Leila, Shyam Nandan, and Stefan Wiemer. "The Effect of Declustering on the Size Distribution of Mainshocks." Seismological Research Letters 92, no. 4 (February 17, 2021): 2333–42. http://dx.doi.org/10.1785/0220200231.
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Abstract Declustering aims to divide earthquake catalogs into independent events (mainshocks), and dependent (clustered) events, and is an integral component of many seismicity studies, including seismic hazard assessment. We assess the effect of declustering on the frequency–magnitude distribution of mainshocks. In particular, we examine the dependence of the b-value of declustered catalogs on the choice of declustering approach and algorithm-specific parameters. Using the catalog of earthquakes in California since 1980, we show that the b-value decreases by up to 30% due to declustering with respect to the undeclustered catalog. The extent of the reduction is highly dependent on the declustering method and parameters applied. We then reproduce a similar effect by declustering synthetic earthquake catalogs with known b-value, which have been generated using an epidemic-type aftershock sequence model. Our analysis suggests that the observed decrease in b-value must, at least partially, arise from the application of the declustering algorithm on the catalog, rather than from differences in the nature of mainshocks versus fore- or aftershocks. We conclude that declustering should be considered as a potential source of bias in seismicity and hazard studies.
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François-Holden, Caroline, Stephen Bannister, John Beavan, Jim Cousins, Bryan Field, Rob McCaffrey, Graeme McVerry, et al. "The Mw 6.6 Gisborne earthquake of 2007." Bulletin of the New Zealand Society for Earthquake Engineering 41, no. 4 (December 31, 2008): 266–77. http://dx.doi.org/10.5459/bnzsee.41.4.266-277.
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Gisborne city experienced recorded peak ground accelerations exceeding 0.25g for the third time since 1966 in the magnitude Mw 6.6 earthquake at 075516 UT (8:55 pm local time) on 20 December 2007. The earthquake was at a hypocentral distance of 64 km from Gisborne at a depth of 40 km, well within the mantle of the subducted slab of the Pacific plate as it dips beneath the North Island of New Zealand. At this location the plate interface is about 10-15 km deep. The main event was followed by sparse aftershocks consistent with a rupture of the subducted plate, with the largest aftershock of magnitude 4.6 occurring on December 22nd. The GeoNet website received 3,257 felt reports, with a strongest intensity of MM8 (heavily damaging) assigned to the main shock.
The 122 strong motion records of this event show a clear regional directional variation in the wave propagation, as well as a distinct 2 Hz peak widely observed throughout the country. At a local scale, three sites in the Gisborne region recorded accelerations around 0.2g. Recordings in Gisborne city also revealed a predominant displacement direction, parallel to the main street where most of the damage occurred.
Source studies from moment tensor solution, aftershock relocations, GPS and strong motion data showed that the earthquake occurred within the subducted plate on a 45 degree eastward dipping fault plane. The mainshock rupture area is about 10 km2 reaching a maximum slip of 2.6 m. The computed high stress drop value of 17 MPa is as expected for an intraslab event and consistent with observations of very energetic seismic waves as well as heavy structural damage.
GPS data recorded by continuous GPS instruments have also shown that slow slip occurred for about three weeks after the main shock. The slow slip was triggered on the subduction interface, rather than on the same fault plane as the aftershocks. This is the first clear-cut case worldwide of triggered slow slip, although three non-triggered slow-slip events have occurred in the same region since 2002.
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Shen, Zheng-Kang, Bob X. Ge, David D. Jackson, David Potter, Michael Cline, and Li-yu Sung. "Northridge earthquake rupture models based on the global positioning system measurements." Bulletin of the Seismological Society of America 86, no. 1B (February 1, 1996): S37—S48. http://dx.doi.org/10.1785/bssa08601b0s37.
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Abstract We use global positioning system (GPS) data to study the rupture mechanism of the 1994 Northridge earthquake in southern California. We include data from 62 observation sites, of which two (Palos Verdes and Jet Propulsion Lab) are permanent GPS geodetic array (PGGA) sites. We use a grid-search scheme to study the range of single- and dual-plane, uniform and varied slip models consistent with the data. We find that in order to fit the geodetic data with a fault model whose primary fault patch is confined to a plane through the aftershocks, a secondary fault plane is required above the primary fault plane. The moment release of the secondary fault can be as large as 1.9 × 1018 N-m, 14% of the moment release of the primary fault. This result implies significant deformation in the shallow crust associated with the mainshock. Our preferred model has a 14 × 14 array of dislocation patches on a plane through the main aftershock cluster and a 5 × 6 array of patches in the hanging wall west of the epicenter. We estimate the displacements on the patches by linear inversion with a first-order smoothness constraint. The estimated displacements on the main fault for this model are confined to a simple region between depths of 5 and 18 km, in the interior of the modeled fault surface. The mainshock lies at the bottom of the aftershock zone, near which about 1-m slip is shown on our modeled fault surface. The maximum slip on the fault surface is about 2.2 m, located at 34.28° N, 118.55° W, and 12.4 km at depth. The seismic moment release estimate of 1.34 ± 0.15 × 1019 N-m on the main fault at the 95% confidence is consistent with the estimate from strong-motion studies.
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Jain, Richa, B. K. Rastogi, and V. P. Dimri. "Fractal Dimension of the 1999 Chamoli Earthquake from Aftershock Studies in Garhwal Himalaya." Pure and Applied Geophysics 160, no. 12 (December 1, 2003): 2329–41. http://dx.doi.org/10.1007/s00024-003-2405-1.
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Jeon, Jong-Su, Reginald DesRoches, Laura N. Lowes, and Ioannis Brilakis. "Framework of aftershock fragility assessment-case studies: older California reinforced concrete building frames." Earthquake Engineering & Structural Dynamics 44, no. 15 (July 14, 2015): 2617–36. http://dx.doi.org/10.1002/eqe.2599.
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Dyagilev, Ruslan, Filipp Verkholantsev, Yuliya Varlashova, Denis Shulakov, Irina Gabsatarova, and Aleksey Epifanskiy. "Katav-Ivanovsk earthquake on 04.09.2018, mb=5.4 (Urals)." Russian Journal of Seismology 2, no. 2 (June 23, 2020): 7–20. http://dx.doi.org/10.35540/2686-7907.2020.2.01.
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The article summarizes the instrumental and macroseismic data obtained in the area of Katav-Ivanovsk earthquake, which occurred on September 4, 2018, in Chelyabinsk region, Russia. The earthquake was the strongest instrumentally recorded earthquake in the Urals (mb=5.4) and at the same time, it had the most seismic intensity among other earthquakes in Russia in 2018 (I0=6 points). The uniqueness of this event was given by the fact that after it for the first time for the Urals the aftershock process was recorded, the active stage of which lasted more than 1 year. Like the mainshock, some aftershocks had a significant macroseismic effect. The work contains the results of studies that allowed to determine the exact coordinates of the epicenter in conditions of lack of near stations using the relative location technique. New processing approaches also made it possible to estimate the depth of the focus through a function of phase spectrum matching. Finally, a considerable amount of macroseismic data formed the basis of the macroseismic field map.
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Scrivner, Craig W., and Donald V. Helmberger. "Finite-difference modeling of two aftershocks of the 1994 Northridge, California, earthquake." Bulletin of the Seismological Society of America 89, no. 6 (December 1, 1999): 1505–18. http://dx.doi.org/10.1785/bssa0890061505.
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Abstract Recent evidence for nonlinear behavior of strong motions generated by the Northridge, California, earthquake relies explicitly on the analysis of aftershock data. Thus, correcting these observations for propagational and source effects becomes a crucial step in these studies. Here, we demonstrate that the dipping structure near the edges of the San Fernando Valley can strongly alter the local site responses, especially for shallow events. We model two aftershocks with similar epicenters, but with shallow (4 km) and deep (16 km) hypocentral depths. Waveforms from 12 portable instruments deployed across the basin and recording these events were processed and studied in the 0.3-4-Hz bandpass. For the shallow event, distinctive features are a broad direct S phase and large-amplitude surface waves at the basin stations, and high-frequency S phases at stations in the Santa Monica Mountains. The deep event is less strongly affected by the basin but displays strong evidence of the source radiation pattern. An array analysis indicates 2D propagation for most of the strong arrivals. Some of these features are explained by a simple 2D basin model containing a strong shallow contrast, a factor of 2 jump in seismic velocity at a depth of 1 km within the basin, and a moderate deeper gradient.
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Zhai, Changhai, Duofa Ji, Weiping Wen, Weidong Lei, Lili Xie, and Maosheng Gong. "The Inelastic Input Energy Spectra for Main Shock–Aftershock Sequences." Earthquake Spectra 32, no. 4 (November 2016): 2149–66. http://dx.doi.org/10.1193/121315eqs182m.
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This study investigates the input energy spectra for inelastic single-degree-of-freedom (SDOF) systems under main shock–aftershock sequences. The input energy spectra quantitatively reveal the effects of aftershocks on input energy, which verifies the necessity of incorporating aftershocks in energy-based seismic design. The investigation selects the sequences including one aftershock or two aftershocks respectively, according to the proposed criteria for selecting earthquake records. Then, the input energy for sequences is normalized by mass, m, and expressed in terms of the equivalent velocity, V E, seq. Next, the variation of V E, seq is studied in consideration of the hysteretic models, ductility values, periods of vibration, site conditions, relative intensities of aftershocks and number of aftershocks. The results indicate that the effects of aftershocks on input energy are significant in almost the whole period region. Finally, a simplified expression of input energy is proposed for incorporating aftershocks in energy-based seismic design.
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Trifunac, M. D., and M. I. Todorovska. "Can aftershock studies predict site amplification factors? Northridge, CA, earthquake of 17 January 1994." Soil Dynamics and Earthquake Engineering 19, no. 4 (June 2000): 233–51. http://dx.doi.org/10.1016/s0267-7261(00)00011-7.
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Mori, Jim. "Estimates of velocity structure and source depth using multiple P waves from aftershocks of the 1987 Elmore Ranch and Superstition Hills, California, earthquakes." Bulletin of the Seismological Society of America 81, no. 2 (April 1, 1991): 508–23. http://dx.doi.org/10.1785/bssa0810020508.
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Abstract Event record sections, which are constructed by plotting seismograms from many closely spaced earthquakes recorded on a few stations, show multiple free-surface reflections (PP, PPP, PPPP) of the P wave in the Imperial Valley, California. The relative timing of these arrivals is used to estimate the strength of the P-wave velocity gradient within the upper 5 km of the sediment layer. Consistent with previous studies, a velocity model with a value of 1.8 km/sec at the surface increasing linearly to 5.8 km/sec at a depth of 5.5 km fits the data well. The relative amplitudes of the P and PP arrivals are used to estimate the source depth for the aftershock distributions of the Elmore Ranch and Superstition Hills main shocks. Although the depth determination has large uncertainties, both the Elmore Ranch and Superstition Hills aftershock sequences appear to have similar depth distribution in the range of 4 to 10 km.
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Lu, Ngo Thi, Phung Thi Thu Hang, Nguyen Huu Tuyen, Ha Thi Giang, and Nguyen Thanh Hai. "The Characteristics of Aftershock Activities of Dien Bien Earthquake on 19 February 2001 and Their Relation to the Local Geomorphological, Tectonic Features." Ekológia (Bratislava) 38, no. 2 (June 1, 2019): 189–200. http://dx.doi.org/10.2478/eko-2019-0015.
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AbstractThis article examines in detail the characteristics of Dien Bien earthquake on 19 February 2001 and its aftershocks. On the basis of the temporal development of aftershocks and the spatial distribution of tectonic faults, five aftershock series have been determined. The analysis of spatial distribution and temporal evolution of these five aftershock series has clarified the development in the source zone of Dien Bien earthquake, which is closely related to the active and recent activities of tectonic faults in the area, especially Lai Chau Dien Bien fault. The comparison between characteristics of aftershock activities of Dien Bien earthquake and geomorphological features as well as tectonic activities in the area has indicated that the magnitude of these aftershocks and their temporal evolution (early or late) depend not only on the closer or further distance compared to the mainshock and the active faults that cause them but also on terrain elevation, slope index, lineament density and their positions relative to other tectonic faults in the studied area.
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Tsuchiyama, Ayako, Taka’aki Taira, Junichi Nakajima, and Roland Bürgmann. "Emergence of Low-Frequency Aftershocks of the 2019 Ridgecrest Earthquake Sequence." Bulletin of the Seismological Society of America 112, no. 2 (January 4, 2022): 750–62. http://dx.doi.org/10.1785/0120210206.
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ABSTRACT Low-frequency earthquakes (LFEs) generally have relatively stronger spectral components in the lower frequency range compared with what is expected for regular earthquakes based on their magnitude. LFEs generally occur in volcanic systems or deep (>∼15 km) in plate boundary fault zones; however, LFEs have also been observed in nonvolcanic, upper crustal settings. Because there are few studies that explore the spatiotemporal behaviors of LFEs in the shallow crust, it remains unclear whether the shallow-crustal LFEs reflect local attenuation in their immediate vicinity or differences in their source mechanism. Therefore, it is important to identify shallow-crustal LFEs and to characterize their spatiotemporal activity, which may also improve our understanding of LFEs. In this study, we focus on detecting shallow-crustal LFEs and explore the possible generation mechanisms. We analyze 29,646 aftershocks in the 2019 Ridgecrest, California, earthquake sequence, by measuring the frequency index (FI) to identify candidate low-frequency aftershocks (LFAs), while accounting for the magnitude dependency of the FI. Using small earthquakes (ML 1–3) recorded in the borehole stations to minimize the attenuation effects in near-surface layers, we identify 68 clear LFAs in total. Based on their distribution and comparisons with other seismic parameters measured by Trugman (2020), the LFAs possess distinct features from regular events in the same depths range, including low corner frequencies and low stress drops. Events in the close vicinity of LFAs exhibit lower average FI values than regular aftershocks, particularly if the hypocentral distance between an LFA and its neighbors is less than 1 km. Our results suggest that LFAs are related to local heterogeneity or a highly fractured fault zone correlated with an abundance of cross faults induced by the aftershock sequence at shallow depths. Zones of high pore-fluid pressure in intensely fractured fault zones could cause the bandlimited nature of LFAs and LFEs in general.
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Jamalreyhani, Mohammadreza, Mehdi Rezapour, Simone Cesca, Torsten Dahm, Sebastian Heimann, Henriette Sudhaus, and Marius Paul Isken. "Insight into the 2017–2019 Lurestan arc seismic sequence (Zagros, Iran); complex earthquake interaction in the basement and sediments." Geophysical Journal International 230, no. 1 (February 11, 2022): 114–30. http://dx.doi.org/10.1093/gji/ggac057.
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SUMMARY Despite its high-seismogenic potential, the details of the seismogenic processes of Zagros Simply Folded Belt (SFB) remains debated. Three large earthquakes (Mw 7.3, 5.9 and 6.3) struck in the Lurestan arc of the Zagros SFB in 2017 and 2018. The sequence was recorded by seismic stations at regional, and teleseismic distances. Coseismic surface displacements, measured by Sentinel-1A/B satellites, provide additional data and a unique opportunity to study these earthquakes in detail. Here, we complement previous studies of the coseismic slip distribution of the 12 November 2017 Mw 7.3 Ezgeleh earthquake by a detailed analysis of its aftershocks, and we analysed the rupture process of the two interrelated earthquakes (25 August 2018 Mw 5.9 Tazehabad and the 25 November 2018 Mw 6.3 Sarpol-e Zahab earthquakes). We model the surface displacements obtained from Interferometric Synthetic Aperture Radar (InSAR) measurements and seismic records. We conduct non-linear probabilistic optimizations based on joint InSAR and seismic data to obtain finite-fault rupture of these earthquakes. The Lurestan arc earthquakes were followed by a sustained aftershock activity, with 133 aftershocks exceeding Mn 4.0 until 30 December 2019. We rely on the permanent seismic networks of Iran and Iraq to relocate ∼700 Mn 3 + events and estimate moment tensor solutions for 85 aftershocks down to Mw 4.0. The 2017 Ezgeleh earthquake has been considered to activate a low-angle (∼17°) dextral-thrust fault at the depth of 10–20 km. However, most of its aftershocks have shallow centroid depths (8–12 km). The joint interpretation of finite source models, moment tensor and hypocentral location indicate that the 2018 Tazehabad and Sarpol-e Zahab earthquakes ruptured different strike-slip structures, providing evidence for the activation of the sinistral and dextral strike-slip faults, respectively. The deformation in the Lurestan arc is seismically accommodated by a complex fault system involving both thrust and strike-slip faults. Knowledge about the deformation characteristics is important for the understanding of crustal shortening, faulting and hazard and risk assessment in this region.
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Xu, Jie. "Fractal Analysis to Study the Structural Distribution of Wenchuan Earthquake in China." Advanced Materials Research 243-249 (May 2011): 4097–100. http://dx.doi.org/10.4028/www.scientific.net/amr.243-249.4097.
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A comprehensive study of fractal property applied in earthquakes is analyzed based on the aftershock of 2008 Wenchuan earthquake. Different fractal parameters are analyzed to study the magnitude, epicenter and hypocenter structural distributions in time or space. Theb-valueis found to be 0.86 closed to which is usually 1.0 observed worldwide. This indicates there is a relative abundance of small magnitude events than large ones in the studied range. The spatial correlation is calculated using correlation integral technique, indicating that epicenters are approaching a two-dimensional region and the aftershocks are uniformly distributed along the trend of the aftershock zone. The rate of the fall of aftershock activity with time reflects the decrease of stress is modestly slow. Temporal correlation is 0.59 for aftershocks of M >4.0, indicating a non continuous aftershock activity. Geometrical probability dimension reflecting epicenter clustering degrees of the region was also analyzed. Also the volume fractal dimension of the aftershock region has been calculated using the box-counting technique to study the hypocenter distributions. From the assessment of slip on different faults it is inferred that 67.9% displacement is accommodated on the primary fault and the remainder on secondary faults.
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Langston, Charles A., and Mariaclare Franco-Spera. "Modeling of the Koyna, India, aftershock of 12 December 1967." Bulletin of the Seismological Society of America 75, no. 3 (June 1, 1985): 651–60. http://dx.doi.org/10.1785/bssa0750030651.
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Abstract Short-period teleseismic and near-regional long-period waveforms from a large aftershock (mb = 5.3, origin time 06:18:36.8 UTC) of the 10 December 1967 Koyna main event were modeled to determine source depth and focal mechanism. Identification of pP and sP on the short-period waveforms yielded a well-constrained source depth of 3.5 to 4.0 km. The focal mechanism was determined using a systematic trial and error (grid-testing) technique in which seven P first motions, two SH first motions, two teleseismic pP/P ratios, one teleseismic sP/P ratio, and one regional SH/P ratio were included. This heterogeneous, but sparse, data set yielded a constrained normal fault mechanism at 100° ± 20° strike, 40° ± 10° dip, and 240° ± 20° rake. Rupture directivity is inferred from consistently high surface reflection/direct wave amplitude ratios. A circular fault of 1 km radius with a rupture initiation point at the lower edge and an assumed rupture velocity of 0.9 Vs explains the anomalously high amplitude ratios and yields a seismic moment of 3.8 × 1023 dyne-cm. The shallow source depth is similar to that found for the main event (4.5 km) and the 13 September foreshocks (5 km) in previous modeling studies and is consistent with the premise that the Koyna earthquakes were triggered by the impoundment of the Koyna reservoir. However, the normal fault mechanism differs substantially from the previous events which were left-lateral strike-slip faults trending NNE. The differing aftershock mechanism is probably due to the reactivation of old faults in the Koyna area caused by stress readjustment after the Koyna main event.
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Feng, Guangcai, Zhiwei Li, Xinjian Shan, Lei Zhang, Guohong Zhang, and Jianjun Zhu. "Geodetic model of the 2015 April 25 Mw 7.8 Gorkha Nepal Earthquake and Mw 7.3 aftershock estimated from InSAR and GPS data." Geophysical Journal International 203, no. 2 (September 17, 2015): 896–900. http://dx.doi.org/10.1093/gji/ggv335.
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Abstract We map the complete surface deformation of 2015 Mw 7.8 Gorkha Nepal earthquake and its Mw 7.3 aftershock with two parallel ALOS2 descending ScanSAR paths’ and two ascending Stripmap paths’ images. The coseismic fault-slip model from a combined inversion of InSAR and GPS data reveals that this event is a reverse fault motion, with a slight right-lateral strike-slip component. The maximum thrust-slip and right-lateral strike-slip values are 5.7 and 1.2 m, respectively, located at a depth of 7–15 km, southeast to the epicentre. The total seismic moment 7.55 × 1020 Nm, corresponding to a moment magnitude Mw 7.89, is similar to the seismological estimates. Fault slips of both the main shock and the largest aftershock are absent from the upper thrust shallower than 7 km, indicating that there is a locking lower edge of Himalayan Main Frontal Thrust and future seismic disaster is not unexpected in this area. We also find that the energy released in this earthquake is much less than the accumulated moment deficit over the past seven centuries estimated in previous studies, so the region surrounding Kathmandu is still under the threaten of seismic hazards.
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Jafari, Mohammad Kazem, Mohammad Reza Ghayamghamian, Mohammad Davoodi, Mohsen Kamalian, and Abdollah Sohrabi-Bidar. "Site Effects of the 2003 Bam, Iran, Earthquake." Earthquake Spectra 21, no. 1_suppl (December 2005): 125–36. http://dx.doi.org/10.1193/1.2098266.
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The site amplification characteristics of the 2003 Bam, Iran, earthquake were investigated based on geological studies as well as geophysical, microtremor and aftershock measurements conducted by IIEES in the study area. A site effect microzonation map was prepared classifying the ground conditions of the city into five distinct categories, based on their stiffness, thickness, and frequency characteristics. The highest percentage of damage was concentrated in sites with stiff shallow and medium depth soils, which possessed considerable amplification potentials in high frequency ranges.
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Ventura-Valentín, Wilnelly, and Michael R. Brudzinski. "Characterization of Swarm and Mainshock–Aftershock Behavior in Puerto Rico." Seismological Research Letters 93, no. 2A (February 9, 2022): 641–52. http://dx.doi.org/10.1785/0220210329.
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Abstract The recent Indios, Puerto Rico earthquake sequence has drawn attention, as the increased seismicity rate in this area was unprecedented. The sequence began on 28 December 2019, caused a 6.4 magnitude earthquake on 7 January 2020, and remained active over a year later. This sequence fits the nominal definition of an earthquake swarm in that it had an abrupt onset, a sustained high rate of seismicity without a clear triggering mainshock or evidence for Omori decay, and a lack of adherence to Bath’s law. However, the sequence also had several prominent mainshock–aftershock (MS–AS) sequences embedded within it. We applied three-station waveform cross correlation to the early part of this sequence using the Puerto Rico Seismic Network (PRSN) catalog as templates, which confirmed the mixture of swarm and MS–AS patterns. In an effort to place this intriguing sequence in the context of the previous seismicity in Puerto Rico, we investigated the existence of swarms and MS–AS sequences recorded by the PRSN since 1987 by identifying sequences with increased seismicity rate when compared to the background rate. About 59 sequences were manually verified and characterized into swarms or MS–AS. We found that 58% of the sequences follow traditional swarm patterns and 14% adhere to traditional MS–AS behavior, whereas 29% of the sequences have a mixture of both swarm and MS–AS behaviors. These findings suggest that it is not unusual for the Indios sequence to have a mixture of both the characteristics. In addition, the detection of many swarms distributed over a broad area of the subduction interface indicates stress heterogeneity and low-coupling consistent with prior studies indicating that the potential for a magnitude ∼8 megathrust earthquake along the Puerto Rico trench is unlikely.
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Cassidy, John F. "Rupture directivity and slip distribution for the Ms 6.8 earthquake of 6 April 1992, Offshore British Columbia: An application of the empirical Green's function method using surface waves." Bulletin of the Seismological Society of America 85, no. 3 (June 1, 1995): 736–46. http://dx.doi.org/10.1785/bssa0850030736.
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Abstract On 6 April 1992 at 13:55 UT, an earthquake of Ms = 6.8 occurred along the Revere-Dellwood-Wilson (RDW) fault, in the triple-junction region at the northern end of the Cascadia subduction zone. This was the first significant event in this region recorded by modern digital broadband seismic networks, thus providing the first opportunity to examine the rupture process of a major earthquake along this young oceanic transform fault. In this article, an empirical Green's Function technique is applied to regional and teleseismic surface waves to estimate the rupture directivity, the extent of rupture, and the slip distribution along the Revere-Dellwood-Wilson fault associated with this earthquake. The 20-sec low-pass-filtered relative source time functions (RSTF's) are single pulses with an azimuthal variation in the pulse width. This suggests that the rupture propagated to the NW (315° ± 20°), along the Revere-Dellwood-Wilson fault (striking 326°). Higher frequency RSTF's reveal two discrete subevents. The clear azimuthal variation in the time separation of these subevents requires that relative to the first subevent, the second is located 13 to 20 km in the direction 345° ± 20°. Using the RSTF at HRV (perpendicular to the rupture direction), a total rupture length of 35 km is estimated, with the bulk of the slip concentrated in a 20-km-long segment of the RDW fault to the northwest of the epicenter. Two peaks are observed in the estimated slip distribution, with maximum values of 1.8 and 1.1 m, respectively. The rupture model derived from this analysis is similar to that obtained from the analysis of body waves and is consistent with the results of aftershock studies. The latter indicate a paucity of aftershock activity (and low moment release) in the 20-km-long segment of the RDW fault to the NW of the epicenter. A distinct peak in aftershock activity 30 to 40 km to the NW of the epicenter likely represents the termination of rupture. The good agreement between the results of this study, the rupture model estimated from body-wave analysis, and the aftershock distribution bode well for the application of the empirical Green's function method using surface waves. It suggests that this method could be applied to large, historic earthquakes in this region, for which regional and teleseismic surface waves are often the most reliable data set.
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Meng, Qingjun, Sidao Ni, Aizhi Guo, and Yong Zhou. "Ground Surface Deformation Caused by the Mw 5.8 Early Strong Aftershock following the 13 November 2016 Mw 7.8 Kaikōura Mainshock." Seismological Research Letters 89, no. 6 (September 5, 2018): 2214–26. http://dx.doi.org/10.1785/0220180019.
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ABSTRACT The Mw 7.8 Kaikōura earthquake on 13 November 2016 is one of the most complex events ever recorded, with surface rupture found on more than a dozen faults. Within about 10 minutes after the mainshock, an Mw 5.8 event occurred and caused an 8 cm static displacement at high‐rate Global Positioning System (GPS) station KAIK, which was not accounted for in previous mainshock studies. In this article, we focus on the Mw 5.8 aftershock including (1) relocating the hypocenter using the hypo2000 method, (2) conducting a grid search for its point‐source mechanism and centroid location using seismic waveforms at four nearby stations, (3) inverting finite‐fault models of this event based on grid‐searched fault mechanism, and (4) calculating the surface ground deformation and estimating the deformation in the line of sight (LoS) directions of the ascending and descending Advanced Land Observation Satellite‐2 (ALOS‐2). Although we are not able to resolve the ruptured fault of the Mw 5.8 aftershock because of limited observation data, we estimate that this event can generate 10–20 cm ground surface displacement and affect the ground displacement observed on the Interferometric Synthetic Aperture Radar (InSAR) data near the Kaikōura Peninsular.
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Michelini, A., and B. A. Bolt. "Application of the principal parameters method to the 1983 Coalinga, California, aftershock sequence." Bulletin of the Seismological Society of America 76, no. 2 (April 1, 1986): 409–20. http://dx.doi.org/10.1785/bssa0760020409.
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abstract A new technique, called the method of the principal parameters (Ebblin and Michelini, in press), to infer the orientations of active fault planes of an aftershock sequence has been applied to the sequence that followed the Coalinga, California, earthquake of 2 May 1983 (ML = 6.7). The method is based on observed clustering in time and space. Clustering is a characteristic feature of aftershock sequences, and it suggests interdependence of the events. It follows that the spatial locations of time-successive foci may provide additional information about the geometries of the rupturing fault system. The method involves sliding of a temporal window of a fixed number of foci along the sequence and estimating the eigenvalues and eigenvectors of a spatial matrix for each window-set. This matrix can be interpreted as an ellipsoid which is fitted through the foci. During the earthquake sequence, the different trends of the seismicity pattern can be isolated by selecting and averaging the greatly flattened ellipsoids. In the case of the Coalinga sequence, the trends are generally consistent with one of the focal planes obtained from the fault-plane solutions given in earlier published studies. The method appears to offer a simple way to infer average active fault geometries in complex areas from hypocentral locations only.
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Khatami, Mostafa, Mohsen Gerami, Ali Kheyroddin, and Navid Siahpolo. "The Effect of the Mainshock–Aftershock on the Estimation of the Separation Gap of Regular and Irregular Adjacent Structures with the Soft Story." Journal of Earthquake and Tsunami 14, no. 02 (December 6, 2019): 2050008. http://dx.doi.org/10.1142/s1793431120500086.
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One way for decreasing the effect of pounding is to set the separation gap between two adjacent buildings. On the one hand, earthquakes in earthquake-prone zones often occur as a chain of successive earth movements in the form of foreshock, mainshock and aftershock. On the other hand, the existence of soft story in the lowest story of the structure is the most common type of irregularity in lateral stiffness. This paper investigates the effect of seismic sequences to estimate the separation gap at the highest collision level of two adjacent structures. For this purpose, 335 adjacent combinations of regular and irregular steel moment-resisting frames are evaluated which have a soft story on the first story. Separation gap demand is calculated using dynamic analysis of nonlinear time history under a set of seismic sequences which are a combination of the mainshock and aftershock. Results of the total of analysis done show the seismic sequence effects are significant and should be considered in the process of determining the normal separation gap (here after, NSG). Finally, based on the done studies, an empirical relationship is presented to estimate the seismic sequence effects on separation gap of two regular and irregular adjacent structures.
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Ziv, A. "On the Role of Multiple Interactions in Remote Aftershock Triggering: The Landers and the Hector Mine Case Studies." Bulletin of the Seismological Society of America 96, no. 1 (February 1, 2006): 80–89. http://dx.doi.org/10.1785/0120050029.
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Mulabisana, T., M. Meghraoui, V. Midzi, M. Saleh, O. Ntibinyane, T. Kwadiba, B. Manzunzu, O. Seiphemo, T. Pule, and I. Saunders. "Seismotectonic analysis of the 2017 moiyabana earthquake (MW 6.5; Botswana), insights from field investigations, aftershock and InSAR studies." Journal of African Earth Sciences 182 (October 2021): 104297. http://dx.doi.org/10.1016/j.jafrearsci.2021.104297.
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Emanov, Aleksandr, Aleksey Emanov, Aleksandr Fateev, Victor Soloviev, Elena Shevkunova, Egor Gladyshev, Ilya Antonov, et al. "Seismological studies in the Altai-Sayan mountain region." Russian Journal of Seismology 3, no. 2 (June 29, 2021): 20–51. http://dx.doi.org/10.35540/2686-7907.2021.2.02.
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The paper provides a brief overview of seismological studies in the Altai-Sayan mountain region. The de-velopment of a network of seismological stations and experiments with temporary stations in the epicen-tral zones of large earthquakes is described. It is shown that the background seismicity of the region is or-dered over time into structures with a hierarchy in the rate of occurrence. Large earthquakes in some cases occur in places that do not match with the areas of increased background seismicity. Major earthquakes in Eastern Tuva (Busingol, Belin-Biy-Khem, etc.) occur as shifts and rotations of blocks near rift depressions. Large earthquakes of the Western Sayan Ridge and the Academician Obruchev Ridge (Tuvan First and Second earthquakes, Sayan earthquake) are associated with faults transverse to these structures and are the result of the uneven extension of blocks of the Tuva hollow and the Tuva highlands to the north. Stud-ies in the Altai Mountains found that after a long period (about 10 years) of the aftershock process of the Chui earthquake dominating the seismicity, a period of seismic activation of adjacent (60-80 km) and dis-tant (within a radius of approximately 260-280 km) structures occurred. The center of seismic activity shifted from the epicenter of the 2003 Chui earthquake to the epicenter of the 2019 Aigulak earthquake. Experimental work with powerful vibrators has determined the capabilities of a network of seismological stations in vibroseismic monitoring of the Earth's crust.
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Szafranski, Dawid, and Benchun Duan. "Exploring Physical Links between Fluid Injection and Nearby Earthquakes: The 2012 Mw 4.8 Timpson, Texas, Case Study." Bulletin of the Seismological Society of America 110, no. 5 (August 11, 2020): 2350–65. http://dx.doi.org/10.1785/0120200090.
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ABSTRACT In this work, we integrate a fluid-flow model of 3D deformable porous media with a dynamic rupture model of earthquakes in 3D heterogeneous geologic medium. The method allows us to go beyond fault failure potential analyses and to examine how big an earthquake can be if part of a fault reaches failure due to fluid injection. We apply the method to the 17 May 2012 Mw 4.8 Timpson, Texas, earthquake as a case study. The simulated perturbations of pore pressure and stress from wastewater injection at the time of the mainshock are high enough (several MPa) to trigger an earthquake. Dynamic rupture modeling could reproduce the major observations from the Mw 4.8 event, including its size, focal mechanism, and aftershock sequence, and thus building a more convincing physical link between fluid injection and the Mw 4.8 earthquake. Furthermore, parameter space studies of dynamic rupture modeling allow us to place some constraints on fault frictional properties and background stresses. For the Timpson case, we find that a dynamic friction coefficient of ∼0.3, a value of ∼0.1 m for the critical slip distance in the slip-weakening friction law, and uniform effective normal stress are associated with the Timpson earthquake fault. By reproducing main features of the aftershock sequence of the mainshock, we also demonstrate that the method has potential to become a predictive tool for fluid injection design in the future.
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Gibbons, Steven J., Natalia A. Ruppert, Ezgi Karasözen, Kasey Aderhold, and Ian Dickson. "Resolving Northern Alaska Earthquake Sequences Using the Transportable Array and Probabilistic Location Methods." Seismological Research Letters 91, no. 6 (July 22, 2020): 3028–38. http://dx.doi.org/10.1785/0220200142.
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Abstract Between 2014 and 2017, almost 200 new seismic stations were installed in Alaska and northwestern Canada as part of the EarthScope USArray Transportable Array. These stations currently provide an unprecedented capability for the detection and location of seismic events in regions with otherwise relatively sparse station coverage. Two interesting earthquake sequences in 2018 and 2019 in the northeastern Brooks Range were exceptionally well recorded because of this deployment. First is the aftershock sequence of the Mw 6.4 and Mw 6.0 Kaktovik earthquakes of 12 August 2018, the largest earthquakes recorded to date in the region. The second is the Niviak swarm, southwest of the Kaktovik sequence. Since July 2018, >4000 earthquakes between magnitudes 1 and 4.3 have been recorded across a region exceeding 5000 km2. We explore how the Bayesloc probabilistic multiple seismic-event location algorithm can better resolve features of these two sequences, exploiting the large numbers of readings that the improved station coverage provides from events down to magnitudes below 2. The Bayesloc calculations consistently move events in the Kaktovik sequence a few kilometers to the northeast, providing an almost linear east-southeast-striking southern limit to the aftershock zone. Analysis of the Bayesloc joint probability distribution of corrections to travel-time predictions indicate that anomalously fast wave propagation to the southwest is likely the most significant contribution to the seismic-event mislocation. The joint relocations are more consistent with Interferometric Synthetic Aperture Radar–inferred coseismic displacement than the network location estimates. The Bayesloc relocation of the Niviak events confirms that the earthquakes are distributed between many distinct clusters of seismicity that have clearer spatial separation following the relocation. The probabilistic relocations motivate both double-difference studies to better resolve clustered seismicity at the smallest spatial scales and systematic multiple event relocation studies to calculate structure and travel-time corrections over larger scales.