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1
van Ginkel, Janneke, Elmer Ruigrok et Rien Herber. « Using horizontal-to-vertical spectral ratios to construct shear-wave velocity profiles ». Solid Earth 11, no 6 (9 novembre 2020) : 2015–30. http://dx.doi.org/10.5194/se-11-2015-2020.
Texte intégralRésumé :
Abstract. For seismic hazard assessment and earthquake hypocentre localization, detailed shear-wave velocity profiles are an important input parameter. Here, we present a method to construct a shear-wave velocity profiles for a deep unconsolidated sedimentary layer by using strong teleseismic phases and the ambient noise field. Gas extraction in the Groningen field, in the northern part of the Netherlands, is causing low-magnitude, induced seismic events. This region forms an excellent case study due to the presence of a permanent borehole network and detailed subsurface knowledge. Instead of conventional horizontal-to-vertical spectral ratios (H∕V ratios) from amplitude spectra, we calculate power spectral densities and use those as input for H∕V calculations. The strong teleseisms provide resonance recordings at low frequencies, where the seismic noise field is too weak to be recorded well with the employed geophones and accelerometers. The H∕V ratios of the ambient noise field are compared with several forward modelling approaches to quality check the teleseism-based shear-wave velocity profiles. Using the well-constrained depth of the sedimentary basin, we invert the H∕V ratios for velocity profiles. A close relationship is observed between the H∕V spectral ratios from the ambient noise field, shear-wave resonance frequencies and Rayleigh-wave ellipticity. By processing only five teleseismic events, we are able to derive shear-wave velocities for the deeper sedimentary sequence with a 7 % bias in comparison with the existing detailed velocity model for the Cenozoic sediments overlying the Groningen gas field. Furthermore, a relation between resonance frequency and unconsolidated sediment thickness is derived, to be used in other areas in the Netherlands, where detailed depth maps are not available.
2
Evans, John R., et Andrew M. Pitt. « Reliable automatic detection of long-period volcanic earthquakes at Long Valley caldera, California ». Bulletin of the Seismological Society of America 85, no 5 (1 octobre 1995) : 1518–22. http://dx.doi.org/10.1785/bssa0850051518.
Texte intégralRésumé :
Abstract Capturing the rare long-period (LP) volcanic earthquakes occurring in and near Long Valley caldera, California, is important to ongoing volcanic-hazards monitoring. It is difficult, however, because LP events are weak, emergent, and almost devoid of energy above a few hertz. Automatic systems designed for tectonic earthquakes routinely fail to capture LP events. We applied a PC-based teleseism-specific event-detection computer program to capturing these events. Retuning the software for LP events involved only changing parameters originally designed for change in this algorithm. Our retuned algorithm has captured every known LP event at Long Valley from October 1992 through the end of 1994. We monitored up to 16 stations known to produce good records of LP events, saving those events that triggered enough of these stations (typically 10 of 16) within a specified time window. The principal difficulty has been the algorithm's sensitivity to regional earthquakes, which have waveforms similar to LP events. During our test, the 1992 Landers, 1994 Northridge, and 1994 Double Springs Flat (Nevada) earthquakes each have swamped the detector, requiring careful, active management of PC disk resources. The efficacy of this retuned algorithm and the poor performance of tectonic-earthquake detectors during some volcanic emergencies make this algorithm an attractive candidate for volcano monitoring.
3
Olsen, Kim B., James C. Pechmann et Gerard T. Schuster. « Simulation of 3D elastic wave propagation in the Salt Lake Basin ». Bulletin of the Seismological Society of America 85, no 6 (1 décembre 1995) : 1688–710. http://dx.doi.org/10.1785/bssa0850061688.
Texte intégralRésumé :
Abstract We have used a 3D finite-difference method to model 0.2 to 1.2 Hz elastodynamic site amplification in the Salt Lake Valley, Utah. The valley is underlain by a sedimentary basin, which in our model has dimensions of 48 by 25 by 1.3 km. Simulations are carried out for a P wave propagating vertically from below and for P waves propagating horizontally to the north, south, east, and west in a two-layer model consisting of semi-consolidated sediments surrounded by bedrock. Results show that in general, sites with the largest particle velocities, cumulative kinetic energies, duration times of motion, and spectral magnitudes overlie the deepest parts of the basin. The maximum values of these parameters are generally found above steeply dipping parts of the basin walls. The largest vector particle velocities are associated with P or SV waves that come from within 10° of the source azimuth. Low-energy S and surface waves follow the strongest arrivals. The largest peak particle velocities, cumulative kinetic energies, signal durations, and spectral magnitudes in the simulations are, respectively, 2.9, 15.9, 40.0, and 3.5 times greater than the values at a rock site measured on the component parallel to the propagation direction of the incident P wave. Scattering and/or mode conversions at the basin boundaries contribute significantly to the signal duration times. As a check on the validity of our simulations, we compared our 3D synthetic seismograms for the vertically incident plane P wave to seismograms of nearly vertically incident teleseismic P waves recorded at an alluvium site in the valley and at a nearby rock site. The 3D synthetics for the alluvium site overestimate the relatively small amplification of the initial P wave and underestimate the large amplification of the coda. Using 2D simulations, we find that most of the discrepancies between the 3D synthetic and observed records can be explained by an apparently incorrect total sediment thickness, omission from the model of the near-surface low-velocity unconsolidated sediments and of attenuation, and the inexact modeling of the incidence angle of the teleseism. The records from a 2D simulation in which these deficiencies are remedied (with Q = 65), and which also includes topography and a near-surface velocity gradient in the bedrock, provide a better match to the teleseismic data than the records from the simple two-layer 3D simulation. Our results suggest that for steeply incident P waves, the impedance decrease and resonance effects associated with the deeper basin structure control the amplification of the initial P-wave arrival, whereas reverberations in the near-surface unconsolidated sediments generate the large-amplitude coda. These reverberations are caused mainly by P-to-S converted waves, and their strength is therefore highly sensitive to the incidence angle of the source.
4
Vasco, D. W. « Deriving source-time functions using principal component analysis ». Bulletin of the Seismological Society of America 79, no 3 (1 juin 1989) : 711–30. http://dx.doi.org/10.1785/bssa0790030711.
Texte intégralRésumé :
Abstract Factors such as source complexity, microseismic noise, and lateral heterogeneity all introduce nonuniqueness into the source-time function. The technique of principal component analysis is used to factor the moment tensor into a set of orthogonal source-time functions. This is accomplished through the singular value decomposition of the time-varying moment tensor. The adequacy of assuming a single source-time function may then be examined through the singular values of the decomposition. The F test can also be used to assess the significance of the various principal component basis functions. The set of significant basis functions can be used to test models of the source-time functions, including multiple sources. Application of this technique to the Harzer nuclear explosion indicated that a single source-time function was found to adequately explain the moment tensor. It consists of a single pulse appearing on the diagonal elements of the moment-rate tensor. The decomposition of the moment tensor for a deep teleseism in the Bonin Islands revealed three basis functions associated with relatively large singular values. The F test indicated that only two of the principal components were significant. The principal component associated with the largest singular value consists of a large pulse followed 16-sec later by a diminished pulse. The second principal component, a long-period oscillation, appears to be a manifestation of the poor resolution of the moment-rate tensor at low frequencies.
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Lewis, Brian T. R., et LeRoy M. Dorman. « Recording teleseisms on the seafloor ; an example from the Juan de Fuca plate ». Bulletin of the Seismological Society of America 88, no 1 (1 février 1998) : 107–16. http://dx.doi.org/10.1785/bssa0880010107.
Texte intégralRésumé :
Abstract In 1991, during an experiment to compare low-frequency seismic noise on a basaltic and a sediment covered seafloor (NOBS), we recorded teleseisms on the Juan de Fuca ridge, the Gorda ridge, and the adjacent Cascadia Basin with the SNAG ocean-bottom seismometers (OBS). These data provide an indication of the type of data that may be obtained from future experiments to record teleseisms and may be helpful in designing these experiments and analyzing the results. We found that although seafloor noise is dominated by microseisms in the band 0.1 to 0.3 Hz, there is a well-developed minimum in noise from about 0.03 to 0.1 Hz (the noise notch). In this noise notch, teleseisms can be most easily detected. In the Cascadia area, the overall noise levels are such that only teleseismic events with magnitude greater than 6.5 were usefully recorded. A magnitude 6.6 event in the New Britain area (Δ = 89°) produced usable P- and surface-wave data only in this noise notch. In the band 0.03 to 0.1 Hz, the character of compressional waves is very sensitive to water depth and the type of sensor. We show that pressure sensors are especially sensitive to reverberation in the ocean and that motion sensors (seismometers) are less sensitive to ocean reverberations and will record teleseismic phases with less distortion than pressure sensors. The Cascadia data indicate enhanced P amplitudes at sites on the ridge axes that could be due to focusing caused by a low-velocity lens. These data suggest that amplitude information may be as, or even more, useful than P delay times for determining upper mantle structure.
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Li, Yingping, William Prothero, Clifford Thurber et Rhett Butler. « Observations of ambient noise and signal coherency on the Island of Hawaii for teleseismic studies ». Bulletin of the Seismological Society of America 84, no 4 (1 août 1994) : 1229–42. http://dx.doi.org/10.1785/bssa0840041229.
Texte intégralRésumé :
Abstract Ambient seismic noise conditions at several sites on the Island of Hawaii have been studied by analyzing noise power spectral density (PSD) and signal-to-noise ratio (SNR) of teleseisms. Broadband digital waveform data used in this study were collected during Project ALOHA. Direct comparison of noise PSD and SNR of teleseisms simultaneously recorded at stations on Hawaii and at station KIP on Oahu indicates that the stations on Hawaii are quieter than station KIP on Oahu in the frequency band of 0.1 to 1 Hz, suggesting that the Island of Hawaii can provide better noise conditions than station KIP. Our interpretation of this observation is that the area and volume of Hawaii are much larger than those of Oahu and distances to the coast for the stations on Hawaii are larger than that for station KIP on Oahu. Stronger attenuation and relatively low coherency for teleseismic signals on Hawaii in frequencies of 0.4 to 1.2 Hz are attributed to the presence of partial melt in the upper mantle. Teleseismic signal coherency is very low at frequencies above 2 Hz for a small array with a radius about 2 km. The relatively high coherency of the tangential component is strongly indicative of severe scattering effects caused by the heterogeneity in structure of the active volcanic area.
7
Short, Kevin M. « Detection of Teleseismic Events in Seismic Sensor Data Using Nonlinear Dynamic Forecasting ». International Journal of Bifurcation and Chaos 07, no 08 (août 1997) : 1833–45. http://dx.doi.org/10.1142/s0218127497001400.
Texte intégralRésumé :
In this paper we consider the use of nonlinear dynamic (NLD) forecasting as a signal processing tool for seismic applications. The specific problem considered here arises in monitoring nuclear tests and nuclear treaty compliance, where the presence of ubiquitous background noise obscures the seismic signals associated with the tests. The problem is that the signal from a distant teleseismic event can be attenuated so that it is lost in the background noise, and since the noise overlaps the frequency band occupied by the teleseisms, frequency-based techniques provide only marginal improvements in detection capabilities. For the work in this paper, we studied a test set of actual seismic sensor data prepared by the Air Force Technical Applications Center (AFTAC). The data set was composed of background seismic noise which contained or had added to it a number of hidden teleseismic signals. This data was analyzed to determine if techniques of NLD forecasting could be used to detect the hidden signals. For this test case, it was possible to predict the behavior of the seismic background sufficiently well that when the predicted background behavior was removed, the hidden signals became evident. However, some of the weaker signals were very close to the residual noise level, so the ability to detect these events is compromised.
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Ambarsari, Riana, Madlazim . et Utama Alan Deta. « PENCITRAAN RUPTURE GEMPABUMI DI LOMBOK TIMUR MW 6,9 PADA 19 AGUSTUS 2018 DENGAN METODE BACK PROJECTION ». Inovasi Fisika Indonesia 10, no 3 (10 décembre 2021) : 24–32. http://dx.doi.org/10.26740/ifi.v10n3.p24-32.
Texte intégralRésumé :
Abstrak
Gempabumi yang terjadi di Lombok pada 19 Agustus 2018 berkekuatan Mw 6,9 merupakan gempa mainshock kedua sebagai akibat dari rangkaian gempabumi Lombok pada Juli – Agustus 2018. Penelitian ini bertujuan untuk meneliti karakteristik rupture gempabumi di Lombok Timur Mw 6,9 yang terjadi pada tanggal 19 Agustus 2018 dari hasil pencitraan arah, kecepatan, durasi, dan panjang rupture gempabumi. Metode yang digunakan dalam penelitian ini dengan teknik pemrosesan array teleseismik menggunakan metode Multiple Signal Back-Projection (MUSICBP) dalam software MATLAB. Data yang digunakan adalah data sekunder dari data seismogram teleseismik dengan 54 stasiun-stasiun seismik array AU (Australia) berformat .SAC dari repositori IRIS Wilber 3. Data tersebut diolah dengan menggunakan program MUSICBP serta dilakukan filter bandpass dengan rentang frekuensi 0,05 – 0,25 Hz melalui proses cross correlation. Hasil keluaran yang diperoleh berupa plot durasi, arah, dan panjang rupture gempa dari MUSICBP serta nilai kemiringan grafik untuk menentukan kecepatan dari rupture gempa dari hasil regresi linear. Hasil dari penelitian ini menunjukkan bahwa pencitraan rupture menggunakan metode MUSICBP adalah sesuai yang divalidasi dengan lokasi gempa susulan-gempa susulan (aftershock) yang terjadi di zona segmentasi rupture tersebut. Berdasarkan hasil dan pembahasan, pada penelitian ini dapat disimpulkan bahwa karakteristik rupture yang terjadi pada gempabumi di Lombok Timur Mw 6,9 tanggal 19 Agustus 2018 adalah arah rupture merambat ke arah Timur secara unilateral sepanjang ~25 km berdurasi 25 s dengan kecepatan rupture 1,74 km/s yang termasuk dalam rentang kecepatan rendah. Hasil dari pencitraan rupture menunjukkan gempabumi yang terjadi di Lombok Timur pada 19 Agustus 2018 tersebut disebabkan oleh aktivitas Flores back arc thrust di wilayah Pulau Lombok bagian Utara dengan arah rupture merambat menuju ke arah Timur (eastward) dengan mekanisme tipe sesar naik (thrust fault).
Kata Kunci: Rupture Gempa, Back Projection, Gempa Teleseismik, Sesar Flores Back-Arc.
Abstract
The earthquake that occurred in Lombok on August 19, 2018, with a magnitude of Mw 6.9 was the second mainshock earthquake as a result of the Lombok earthquake series in July – August 2018. This study aims to examine the characteristics of the earthquake rupture in East Lombok Mw 6.9 which occurred on 19 August 2018 from the results of imaging the direction, speed, duration, and length of earthquake rupture. The method used in this research is a teleseismic array processing technique using the Multiple Signal Back-Projection (MUSICBP) method in MATLAB software. The data used is secondary data from teleseismic seismogram data with 54 AU (Australia) seismic array stations in .SAC format from the IRIS Wilber 3 repository. The data was processed using the MUSICBP program and a bandpass filter was performed with a frequency range of 0.05 – 0.25 Hz through the cross-correlation process. The outputs obtained are plots of duration, direction, and length of earthquake rupture from MUSICBP and the slope value of the graph to determine the velocity of earthquake rupture from linear regression results. This study indicates that the rupture imaging using the MUSICBP method is appropriate which is validated by the location of the aftershocks that occur in the rupture segmentation zone. Based on the results and discussion, in this study, it can be concluded that the characteristics of the rupture that occurred in the earthquake in East Lombok Mw 6.9 on 19 August 2018 were the direction of the rupture spreading towards the East unilaterally along ~25 km with a duration of 25 s with a rupture speed of 1.74 km/s which is included in the low-speed range. The results of the rupture imaging show that the earthquake that occurred in East Lombok on August 19, 2018, was caused by the activity of Flores back arc thrust in the northern part of Lombok Island with the direction of the rupture spreading towards the East (eastward) with a thrust fault type mechanism.
Keywords: Earthquake Rupture, Back Projection, Teleseismic Earthquake, Flores Back-Arc Thrust.
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Sari, Sendy Oktaviana, et Madlazim . « ANALISIS DINAMIKA RUPTURE GEMPABUMI PADA 5 AGUSTUS 2018 DI LOMBOK UTARA Mw 6,9 MENGGUNAKAN METODE MUSICBP ». Inovasi Fisika Indonesia 10, no 3 (10 décembre 2021) : 33–38. http://dx.doi.org/10.26740/ifi.v10n3.p33-38.
Texte intégralRésumé :
Abstrak
Gempa bumi Mw = 6,9 yang terjadi di Lombok Utara pada 5 Agustus 2018 disebabkan karena adanya aktivitas seismik dari sesar Flores. Penelitian ini bertujuan untuk mengestimasi rupture gempabumi yang terjadi di daerah Lombok Utara, Nusa Tenggara Barat. Parameter rupture yang diestimasi berupa nilai durasi, panjang, kecepatan, dan arah rupture. Metode yang digunakan ialah Multiple Signal Back-Projection (MUSICBP) dengan menggunakan filter band pass pada rentang 0,25 Hz – 1 Hz. Data yang digunakan didapatkan dari website IRIS Wilber 3 berupa format SAC dengan jarak epicentral berupa teleseismik menggunakan stasiun Array Australia sebanyak 36 stasiun yang sudah diproses dengan metode cross correlation sehingga diperoleh sinyal yang koheren dan sefase. Hasil dari penelitian ini yaitu pada gempabumi Lombok Utara memiliki arah rupture unilateral menuju ke barat. Hal tersebut sesuai dengan orientasi sesar Flores, sehingga dapat disimpulkan bahwa gempabumi ini diakibatkan karena aktivitas seismik sesar Flores. Aktivitas sesar Flores mengakibatkan gempabumi ini memiliki jenis sesar naik (reverse fault) dengan durasi rupture lebih dari 50 sekon sehingga didapatkan nilai kecepatan rambat rupture yaitu 3,1 km/s sepanjang 72 km. Hasil-hasil dinamika rupture yang didapatkan dalam penelitian ini menunjukkan bahwa lokasi gempa-gempa susulan berada di jalur zona segmentasi rupture gempabumi ini sehingga diharapkan dapat mengestimasi lokasi-lokasi gempabumi susulan secara cepat dan akurat.
Kata Kunci: parameter rupture, back projection, teleseismik, sesar Flores.
Abstract
The Mw = 6.9 earthquake that occurred in North Lombok on August 5, 2018, was caused by seismic activity from the Flores fault. This study aims to estimate the earthquake rupture that occurred in North Lombok, West Nusa Tenggara. The estimated rupture parameters are the duration, length, speed, and rupture direction. The method used is Multiple Signal Back-Projection (MUSICBP) using a bandpass filter in the range of 0.25 Hz – 1 Hz. The data used is obtained from the IRIS Wilber 3 website in SAC format with an epicentral distance in the form of teleseismic using the Array Australia station as many as 36 stations that have been processed by the cross-correlation method so that a coherent and in-phase signal is obtained. The results of this study are that the North Lombok earthquake has a unilateral rupture direction towards the west. This is following the orientation of the Flores fault, so it can be concluded that this earthquake was caused by the seismic activity of the Flores fault. The Flores fault activity caused this earthquake to have a reverse fault type with a rupture duration of more than 50 seconds so that the rupture propagation velocity value was 3,1 km/s along 72 km. The result of the rupture dynamics obtained in this study indicate that the location of the aftershocks is in the segmentation zone of this earhquake rupture so that it is expected to be able to estimate the location of the aftershocks quickly and accurately.
Keywords : Parameter rupture, back projection, teleseismic, Flores fault.
10
Fan, Chengliang, Gary L. Pavlis, Arthur B. Weglein et Bogdan G. Nita. « Removing free-surface multiples from teleseismic transmission and constructed reflection responses using reciprocity and the inverse scattering series ». GEOPHYSICS 71, no 4 (juillet 2006) : SI71—SI78. http://dx.doi.org/10.1190/1.2217369.
Texte intégralRésumé :
We develop a new way to remove free-surface multiples from teleseismic P- transmission and constructed reflection responses. We consider two types of teleseismic waves with the presence of the free surface: One is the recorded waves under the real transmission geometry; the other is the constructed waves under a virtual reflection geometry. The theory presented is limited to 1D plane wave acoustic media, but this approximation is reasonable for the teleseismic P-wave problem resulting from the steep emergence angle of the wavefield. Using one-way wavefield reciprocity, we show how the teleseismic reflection responses can be reconstructed from the teleseismic transmission responses. We use the inverse scattering series to remove free-surface multiples from the original transmission data and from the reconstructed reflection response. We derive an alternative algorithm for reconstructing the reflection response from the transmission data that is obtained by taking the difference between the teleseismic transmission waves before and after free-surface multiple removal. Numerical tests with 1D acoustic layered earth models demonstrate the validity of the theory we develop. Noise test shows that the algorithm can work with S/N ratio as low as 5 compared to actual data with S/N ratio from 30 to 50. Testing with elastic synthetic data indicates that the acoustic algorithm is still effective for small incidence angles of typical teleseismic wavefields.
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Kennett, B. L. N., et M. S. Sambridge. « Earthquake location — genetic algorithms for teleseisms ». Physics of the Earth and Planetary Interiors 75, no 1-3 (décembre 1992) : 103–10. http://dx.doi.org/10.1016/0031-9201(92)90121-b.
Texte intégral
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Liu, Shaolin, Dinghui Yang, Xingpeng Dong, Qiancheng Liu et Yongchang Zheng. « Element-by-element parallel spectral-element methods for 3-D teleseismic wave modeling ». Solid Earth 8, no 5 (28 septembre 2017) : 969–86. http://dx.doi.org/10.5194/se-8-969-2017.
Texte intégralRésumé :
Abstract. The development of an efficient algorithm for teleseismic wave field modeling is valuable for calculating the gradients of the misfit function (termed misfit gradients) or Fréchet derivatives when the teleseismic waveform is used for adjoint tomography. Here, we introduce an element-by-element parallel spectral-element method (EBE-SEM) for the efficient modeling of teleseismic wave field propagation in a reduced geology model. Under the plane-wave assumption, the frequency–wavenumber (FK) technique is implemented to compute the boundary wave field used to construct the boundary condition of the teleseismic wave incidence. To reduce the memory required for the storage of the boundary wave field for the incidence boundary condition, a strategy is introduced to efficiently store the boundary wave field on the model boundary. The perfectly matched layers absorbing boundary condition (PML ABC) is formulated using the EBE-SEM to absorb the scattered wave field from the model interior. The misfit gradient can easily be constructed in each time step during the calculation of the adjoint wave field. Three synthetic examples demonstrate the validity of the EBE-SEM for use in teleseismic wave field modeling and the misfit gradient calculation.
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Frederiksen, A. W. « Transfer functions between teleseismic data components ». Geophysical Journal International 221, no 2 (17 février 2020) : 1248–63. http://dx.doi.org/10.1093/gji/ggaa085.
Texte intégralRésumé :
SUMMARY Different data components of teleseismic waveforms are related by transfer functions that depend only on receiver-side structure. This is the common basis of a number of teleseismic techniques, including receiver functions and shear wave splitting analysis. Common trace misfits used in these analysis techniques are shown to be equivalent to band-limited comparisons of real and synthetic transfer functions. The data deconvolution used in receiver function analysis leads to reduced structural resolution compared to direct trace-based misfits such as cross-convolution, with direct transfer function modelling of a data trace having the particular advantage of a physically meaningful misfit. Having established that the intertrace transfer function contains all available structural information, the sensitivity of transfer functions to structure is examined for a series of teleseismic scenarios. Transfer functions for the teleseismic P coda show a strong sensitivity to shallow low-velocity structures such as sedimentary basins; the Sp precursors used in S receiver functions are less affected. Examination of transfer functions for shear wave splitting shows that response complexities occur at frequencies too high to be observable in teleseismic studies, and that the dominant control on the response is the splitting intensity.
14
Morozov, Igor B., Elena A. Morozova, Scott B. Smithson et Leonid N. Solodilov. « On the nature of the teleseismic Pn phase observed on the ultralong-range profile “Quartz,” Russia ». Bulletin of the Seismological Society of America 88, no 1 (1 février 1998) : 62–73. http://dx.doi.org/10.1785/bssa0880010062.
Texte intégralRésumé :
Abstract The most prominent secondary phase observed in the records from the ultralong profile “Quartz” crossing northern Eurasia is the high-frequency long-range (teleseismic) Pn. This phase, propagating with the group velocity of about 8.1 to 8.2 km/sec to over 3000 km distances, dominates the records within the frequency range above about 5 Hz. Within this teleseismic Pn, we distinguish onsets of several branches having higher apparent velocities between 8.5 and 8.6 km/sec. Using travel-time modeling techniques, we associate the teleseismic Pn with whispering gallery modes traveling within the top 160 km of the mantle. The long incoherent coda of this phase results from scattering and from reverberations of seismic waves within the crust. The contrast in frequency contents between the teleseismic Pn and deeper refracted and reflected phases is explained by the increase of inelastic attenuation within the prominent low-velocity zone (LVZ) below the depth of about 150 km.
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Pirli, M., et J. Schweitzer. « Regional and teleseismic event detection capability of the small-aperture Tripoli seismic array, Greece ». Bulletin of the Geological Society of Greece 40, no 3 (5 juin 2018) : 1246. http://dx.doi.org/10.12681/bgsg.16877.
Texte intégralRésumé :
The Tripoli Seismic Array (TRISAR) is a small-aperture array designed to monitor and locate the seismicity in the area of Greece. In this study, its detection capabilities are discussed for regional and teleseismic events. A reference event list is compiled, consisting of events ofmb>5.0for regional and teleseismic distances (A>6°), according to the ISC On-line Bulletin. TRISAR automatically detected approximately 25% of these events over the entire investigated distance range. Although TRISAR slowness vector residuals are rather large, as expected for an array of such small aperture, the benefits resulting from the use of such a system for reporting regional and teleseismic activity is obvious.
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Moshou, A., P. Papadimitriou et K. Makropoulos. « Teleseismic body wave inversion ». Bulletin of the Geological Society of Greece 40, no 3 (5 juin 2018) : 1177. http://dx.doi.org/10.12681/bgsg.16854.
Texte intégralRésumé :
Body wave inversion methodology is developed to determine the earthquake source parameters in teleseismic distances. The generalized inversion technique, based on the singular value decomposition method, is applied to determine the deviatone moment tensor which is decomposed in two parts. The first one is related to the pure Double Couple (DC) and the second one to the compensated linear vector dipoles (CLVD). The best solution of the overdetermined problem is obtained by minimizing the misfit between observed and synthetic seismograms. The proposed methodology is applied for the four strongest earthquakes that occurred recently in Greece (2001-2006)
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Nishida, K., et R. Takagi. « Teleseismic S wave microseisms ». Science 353, no 6302 (25 août 2016) : 919–21. http://dx.doi.org/10.1126/science.aaf7573.
Texte intégral
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Shragge, J., B. Artman et C. Wilson. « Teleseismic shot-profile migration ». GEOPHYSICS 71, no 4 (juillet 2006) : SI221—SI229. http://dx.doi.org/10.1190/1.2208263.
Texte intégralRésumé :
The shot-profile migration approach of wave-equation migration generates subsurface images using the interferometric principle of crosscorrelating two passive wavefields. These wavefields are typically a source wavefield containing energy from an excited source and a receiver wavefield comprised of scattered-source wavefield energy by the discontinuous earth structure. Shot-profile migration can be recast as a novel way of imaging the earth’s lithosphere using teleseismic wavefield data, where the source wavefield is the directly arriving wavefront and the receiver wavefield is the following wavefield coda. We demonstrate that the shot-profile technique can be tailored to suit teleseismic acquisition geometry and wavefields. Assuming an acoustic framework and 2.5D experimental geometry, we develop procedures that enable kinematic and structural imaging (migration) using both transmission and free-surface reflected passive wavefields. Experiments with synthetic data demonstrate the method’s applicability and illustrate the negative imaging consequence of using inaccurate migration velocity profiles. We apply shot-profile migration to a suite of teleseismic events acquired during the IRIS-PASSCAL CASC-1993 experiment in central Oregon. The imaging results are interpreted to show the Juan de Fuca plate subducting beneath the North American plate. We attribute the observed dissimilarities between these results and other Juan de Fuca subduction- zone images to the combination of different imaging goals and the use of more accurate migration velocity profiles.
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Dijkstra, H. Paul, Emin Ergen, Louis Holtzhausen, Ian Beasley, Juan Manuel Alonso, Liesel Geertsema, Celeste Geertsema et al. « Remote assessment in sport and exercise medicine (SEM) : a narrative review and teleSEM solutions for and beyond the COVID-19 pandemic ». British Journal of Sports Medicine 54, no 19 (30 juin 2020) : 1162–67. http://dx.doi.org/10.1136/bjsports-2020-102650.
Texte intégralRésumé :
BackgroundThe COVID-19 pandemic forces sport and exercise medicine (SEM) physicians to think differently about the clinical care of patients. Many rapidly implement eHealth and telemedicine solutions specific to SEM without guidance on how best to provide these services.AimThe aim of this paper is to present some guiding principles on how to plan for and perform an SEM consultation remotely (teleSEM) based on a narrative review of the literature. A secondary aim is to develop a generic teleSEM injury template.ResultseHealth and telemedicine are essential solutions to effective remote patient care, also in SEM. This paper provides guidance for wise planning and delivery of teleSEM. It is crucial for SEM physicians, technology providers and organisations to codesign teleSEM services, ideally involving athletes, coaches and other clinicians involved in the clinical care of athletes, and to gradually implement these services with appropriate support and education.ConclusionteleSEM provides solutions for remote athlete clinical care during and after the COVID-19 pandemic. We define two new terms—eSEM and teleSEM and discuss guiding principles on how to plan for and perform SEM consultations remotely (teleSEM). We provide an example of a generic teleSEM injury assessment guide.
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Baer, M., et U. Kradolfer. « An automatic phase picker for local and teleseismic events ». Bulletin of the Seismological Society of America 77, no 4 (1 août 1987) : 1437–45. http://dx.doi.org/10.1785/bssa0770041437.
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Abstract An automatic detection algorithm has been developed which is capable of time P-phase arrivals of both local and teleseismic earthquakes, but rejects noise bursts and transient events. For each signal trace, the envelope function is calculated and passed through a nonlinear amplifier. The resulting signal is then subjected to a statistical analysis to yield arrival time, first motion, and a measure of reliability to be placed on the P-arrival pick. An incorporated dynamic threshold lets the algorithm become very sensitive; thus, even weak signals are timed precisely. During an extended performance evaluation on a data set comprising 789 P phases of local events and 1857 P phases of teleseismic events picked by an analyst, the automatic picker selected 66 per cent of the local phases and 90 per cent of the teleseismic phases. The accuracy of the automatic picks was “ideal” (i.e., could not be improved by the analyst) for 60 per cent of the local events and 63 per cent of the teleseismic events.
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Zhang, Jia, et Charles A. Langston. « Separating the scattered wavefield from teleseismic P using curvelets on the long beach array data set ». Geophysical Journal International 220, no 2 (24 octobre 2019) : 1112–27. http://dx.doi.org/10.1093/gji/ggz487.
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SUMMARY A dense seismic array, composed of over 5000 stations with an average spacing close to 120 m was deployed in Long Beach, CA, by NodalSeismic and Signal Hill Petroleum as part of a survey associated with the Long Beach oilfield. Among many interesting wave propagation effects that have been reported by others, we observe that the coda of teleseismic P waves display waves caused by obvious local scattering from the Signal Hill popup structure between strands of the Newport-Inglewood fault. The density of the seismic array allows space-based methods, such as the Curvelet transform, to be investigated to separate the teleseismic and local scattered wavefields. We decompose a synthetic wavefield composed of a teleseismic plane wave and local scattered spherical wave in the curvelet domain to test the plausibility of our curvelet analysis and then apply the technique to the Long Beach array data set. Background noise is removed by a soft thresholding method and a declustering technique is applied to separate the teleseismic and local scattered wavefield in the curvelet domain. Decomposed results illustrate that the signal-to-noise ratio of the teleseismic P wave can be significantly improved by curvelet analysis. The scattered wavefield is composed of locally propagating Rayleigh waves from the pop-up structure and from the Newport Inglewood fault itself. Observing the wavefield both in space and time clearly improves understanding of wave propagation complexities due to structural heterogeneity.
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Pratt, Thomas L., et Lisa S. Schleicher. « Characterizing Ground-Motion Amplification by Extensive Flat-Lying Sediments : The Seismic Response of the Eastern U.S. Atlantic Coastal Plain Strata ». Bulletin of the Seismological Society of America 111, no 4 (29 juin 2021) : 1795–823. http://dx.doi.org/10.1785/0120200328.
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ABSTRACT We examine the effects that Atlantic Coastal Plain (ACP) strata have on ground motions in the eastern and southeastern United States. The ACP strata consist of widespread, nearly flat-lying sediments, the upper portions of which are unconsolidated or semiconsolidated. The ACP sediments are deposited primarily on crystalline basement rocks, creating large velocity and density contrasts with the underlying rocks. At 211 sites on ACP strata to thicknesses of 4000 m, we compute spectral ratios relative to the average of four bedrock sites west or northwest of the strata. Sites consist of stations of Earthscope’s USArray Transportable Array (TA), and temporary deployments in the Southeast Suture of the Atlantic Margin Experiment (SESAME), Eastern North American Margin (ENAM) experiment, and the DCShake deployment in Washington, D.C. For the TA and SESAME stations, we use signals from 13 teleseisms and three regional earthquakes as input, combining the north and east components of motion after taking the Fourier transforms. We also include similarly processed site responses from the ENAM and DCShake arrays that were computed in earlier studies. Results show prominent, fundamental resonance peaks at frequencies determined by reverberations in the entire sediment column, and that often define the largest amplifications for each frequency. As frequencies increase, these resonance peaks migrate to thinner ACP strata and increase in amplitude. The peaks are well defined at frequencies below about 1 Hz, but become narrower and less defined regionally at higher frequencies. We develop simple equations to characterize amplification versus ACP thickness, which we approximate by cosine and Gaussian curves with amplifications of 1 on bedrock and rising to the resonance peak, and then decreasing to an average amplification at thicknesses greater than twice the resonance peak. Comparisons with other site corrections for the central and eastern United States based on sediment thickness show similarities on thin ACP strata but divergence on thicker sediments. The results also demonstrate the effectiveness of using teleseismic arrivals to characterize the site responses of sedimentary sequences.
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Bostock, M. G., D. W. Eaton et D. B. Snyder. « Teleseismic studies of the Canadian landmass : Lithoprobe and its legacyThis article is one of a series of papers published in this Special Issue on the theme Lithoprobe — parameters, processes, and the evolution of a continent.ESS Contribution 20090305. » Canadian Journal of Earth Sciences 47, no 4 (avril 2010) : 445–61. http://dx.doi.org/10.1139/e09-040.
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Although teleseismic research was only modestly represented within the Lithoprobe program, the analysis of deeper lithospheric structure beneath Canada using teleseismic methods has intensified in the past decade. This development is due in large part to a legacy of improved understanding of shallower lithospheric structures afforded by Lithoprobe. Most recent teleseismic experiments have been conducted in regions lying within Lithoprobe transects and coverage is particularly good in the Slave Province, southern and eastern Ontario, and southwestern British Columbia. A number of key results have arisen out of this collective body of work. Studies on the Slave Province and environs have placed strong constraints on the origin of the high-velocity continental root that underlies most of the Canadian Shield. Fine-scale, anisotropic stratigraphy in this region has been definitively tied to underplated lithosphere, indicating that shallow subduction has played a fundamental role in craton stabilization. Modification of continental lithosphere by the underlying convecting mantle has been extensively documented in the southeastern Canadian Shield and Slave Province, yielding insights into the forces driving plate motion and those that induce intraplate volcanism. Teleseismic investigations in British Columbia point to the importance of water in controlling the structure and dynamics of subduction zones, but have rekindled controversy concerning the location and characterization of the downgoing oceanic plate.
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Sun, Weijia, et B. L. N. Kennett. « Receiver structure from teleseisms : Autocorrelation and cross correlation ». Geophysical Research Letters 43, no 12 (28 juin 2016) : 6234–42. http://dx.doi.org/10.1002/2016gl069564.
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Gusev, Alexander A., et Danila Chebrov. « On Scaling of Earthquake Rise‐Time Estimates ». Bulletin of the Seismological Society of America 109, no 6 (1 octobre 2019) : 2741–45. http://dx.doi.org/10.1785/0120180214.
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Abstract The scaling behavior of rise times Tr determined within earthquake source inversions that used strong‐motion data is determined using estimates as accumulated in the SRCMOD database. The Tr versus M0 trend derived from this data set is close to logTr=1/3logM0+ const; this agrees with the assumption of self‐similarity of earthquake ruptures. No biasing effect of station distance on Tr was found. The result was compared to recent scaling estimates based on mass teleseismic inversions. Absolute levels of teleseismic and local inversions match well; the slope of the trend of teleseismic estimates is somewhat more gradual. The absolute levels of Tr versus M0 trends recovered from finite source inversions may need reduction when used to predict parameters of near‐source ground motion. The observed scaling behavior of Tr is incompatible with the assumption that Tr defines the second corner frequency of the source spectrum.
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Neely, James S., Seth Stein et Bruce D. Spencer. « Large Uncertainties in Earthquake Stress-Drop Estimates and Their Tectonic Consequences ». Seismological Research Letters 91, no 4 (20 mai 2020) : 2320–29. http://dx.doi.org/10.1785/0220200004.
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Abstract Earthquake stress drop, the stress change on a fault due to an earthquake, is important for seismic hazard analysis because it controls the level of high-frequency ground motions that damage structures. Numerous studies report that stress drops vary by tectonic environment, providing insight into a region’s seismic hazard. Here, we show that teleseismic stress-drop estimates have large uncertainties that make it challenging to distinguish differences between the stress drops of different earthquakes. We compared stress drops for ∼900 earthquakes derived from two independent studies using teleseismic data and found practically zero correlation. Estimates for the same earthquake can differ by orders of magnitude. Therefore, reported stress-drop differences between earthquakes may not reflect true differences. As a result of these larger uncertainties, some tectonic environment stress-drop patterns that appear in one study do not appear in the other analysis of the same earthquakes. These large uncertainties in teleseismic estimates might lead to erroneous inferences about earthquake hazards. In many applications, it may be more appropriate to assume that earthquakes in different regions have approximately the same average stress drop.
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Rafea, Huda Farhan, Emad A. Al-Heety et Wathiq Abdulnaby. « Crustal Velocity Structure of Western Iraq from Inversion of Receiver Functions at Anbar Seismic Station ». Iraqi Geological Journal 55, no 2B (31 août 2022) : 25–34. http://dx.doi.org/10.46717/igj.55.2b.3ms-2022-08-19.
Texte intégralRésumé :
The inversion of the teleseismic P-wave receiver functions was employed to determine the structure of the crust under Anbar broadband seismic station. Some of the computer programs in seismology were used to analyze the receiver functions of nine teleseismic earthquakes. Results of inversion show that the crustal structure (Moho depth) beneath the ANB1 station is 44 km. The sedimentary cover thickness (basement depth) at the ANB1 station is 12km. In general, the obtained sedimentary cover and crustal thicknesses are consistent with the results of some previous studies in the Mesopotamian plain. As well as the inversion results show that the crustal velocity model beneath the ANB1 station has four distinct discontinuities; the first at 6km with Vs of 2.83km/s, the second one at 12km with Vs of 3.63, the third one at 36km with Vs of 4.21km/s, and the fourth one at 44km with Vs of 4.37km/s. The obtained velocity model may be employed to locate and relocate the local and teleseismic earthquakes and in moment tensor solutions.
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Monteiller, Vadim, Stephen Beller, Bastien Plazolles et Sébastien Chevrot. « On the validity of the planar wave approximation to compute synthetic seismograms of teleseismic body waves in a 3-D regional model ». Geophysical Journal International 224, no 3 (12 décembre 2020) : 2060–76. http://dx.doi.org/10.1093/gji/ggaa570.
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SUMMARY Injection methods are a very efficient means to compute synthetic seismograms of short-period teleseismic body waves in 3-D regional models. The principle is to inject an incident teleseismic wavefield inside a regional 3-D Cartesian spectral-element grid. We have developed an opern-source package that allows us to inject either an incident plane wave, computed with a frequency–wavenumber method, or the complete wavefield, computed in a spherically symmetric reference earth model with AxiSEM. The computations inside the regional spectral-element grid are performed with SPECFEM3D_Cartesian. We compare the efficiency and reliability of the two injection methods for teleseismic P waves, considering a wide range of epicentral distance and hypocentral depths. Our simulations demonstrate that in practice the effects of wave front and Earth curvature are negligible for moderate size regional domains (several hundreds of kilometres) and for periods larger than 2 s. The main differences observed in synthetic seismograms are related to secondary phases that have a different slowness from the one of the reference P phase.
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Kanao, Masaki, Akira Yamada et Mikiya Yamashita. « Characteristic Seismic Waves Associated with Cryosphere Dynamics in Eastern Dronning Maud Land, East Antarctica ». International Journal of Geophysics 2012 (2012) : 1–19. http://dx.doi.org/10.1155/2012/389297.
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Several kinds of natural source signals are recorded by seismic exploration stations on the continental ice sheet in Eastern Dronning Maud Land, East Antarctica, during 2002 austral summer. They include not only tectonic earthquakes, but also ice-related phenomena possibly involving recent global climate change. The recorded signals are classified into (1) teleseismic events, (2) local ice quakes, and (3) unidentified events (X-phases). The teleseismic waves show the high signal-to-noise ratio in spite of the small magnitude of the event; this indicates that it is highly feasible to study not only the local shallow structure but also the deep structure of the earth by using teleseismic events. Frequency spectra of the all waveforms represent discordances along the observation seismic profile. The abrupt change of topography in the valley along the seismic profile might cause both the anomalous frequency content and travel times. Finally, an origin of the X-phases is speculated as the intraplate earthquakes or possibly large ice-quakes (glacial earthquakes) around Antarctica, involving global warming appeared in polar region.
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Amukti, Rian, et Wiwit Suryanto. « Analisa Receiver Function Teleseismic untuk Mendeteksi Moho pada Stasiun Bkb Data Meramex ». INDONESIAN JOURNAL OF APPLIED PHYSICS 3, no 02 (21 mai 2016) : 195. http://dx.doi.org/10.13057/ijap.v3i02.1272.
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<span>It has been done a research to determine internal earth using receiver function teleseismic analysis <span>method. This method have been done by using MERAMEX (MErapi Amphibious Experiment) data from <span>broadband seismometer BKB. Event of teleseismic is chosen from Honshu Japan with radius 30<span>o <span>and <span>magnitude 7.2. This research begun by analysing radial and vertical characteristic of teleseismic event<br /><span>and using bandpass filter with range 0.003 Hz – 0.5 Hz. Then Iteractive Deconvolution is used to get <span>velocity model. The result of this model shows crustal model that has 4 Km thick upper crust, a 26 Km <span>thick lower crust and 10 Km thick Moho transition zone, with velocity increasing gradually.</span></span></span></span></span></span><br /></span></span></span>
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Bharadwaj, Pawan, Chunfang Meng, Aimé Fournier, Laurent Demanet et Mike Fehler. « Redshift of earthquakes via focused blind deconvolution of teleseisms ». Geophysical Journal International 223, no 3 (2 septembre 2020) : 1864–78. http://dx.doi.org/10.1093/gji/ggaa419.
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SUMMARY We present a robust factorization of the teleseismic waveforms resulting from an earthquake source into signals that originate from the source and signals that characterize the path effects. The extracted source signals represent the earthquake spectrum, and its variation with azimuth. Unlike most prior work on source extraction, our method is data-driven, and it does not depend on any path-related assumptions, for example, the empirical Green’s function. Instead, our formulation involves focused blind deconvolution (FBD), which associates the source characteristics with the similarity among a multitude of recorded signals. We also introduce a new spectral attribute, to be called redshift, which is based on the Fraunhofer approximation. Redshift describes source-spectrum variation, where a decrease in high-frequency content occurs at the receiver in the direction opposite to unilateral rupture propagation. Using the redshift, we identified unilateral ruptures during two recent strike-slip earthquakes. The FBD analysis of an earthquake, which originated in the eastern California shear zone, is consistent with observations from local seismological or geodetic instrumentation.
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Withers, Mitchell, Richard Aster et Christopher Young. « An automated local and regional seismic event detection and location system using waveform correlation ». Bulletin of the Seismological Society of America 89, no 3 (1 juin 1999) : 657–69. http://dx.doi.org/10.1785/bssa0890030657.
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Abstract We report on the development of an automated Local Waveform Correlation Event Detection System (LWCEDS) and its application to the New Mexico Tech Seismic Network. LWCEDS is an adaptation of a global system, WCEDS, a matched filtering algorithm for global Comprehensive Test Ban Treaty (CTBT) monitoring applications developed at Sandia National Laboratories and New Mexico Tech. Although the current CTBT monitoring system is based on teleseismic phase detection, effort is being placed on research to highlight specific areas of the globe for which local and regional seismic networks could be employed. An automated waveform correlation regional location system could also serve as a rapid alert and automated location system by providing magnitude and hypocenter information within a few minutes of the occurrence of a hazardous earthquake. In the LWCEDS algorithm, processed waveforms are correlated with theoretical travel-time envelopes, and a grid search is performed to identify the space-time solutions that yield the highest correlations. High correlation indicates that an event has occurred and that a good approximation to the correct origin time and hypocenter has been determined; explicit phase identification is not required. To avoid the large computational expense of calculating a complete correlation for each grid point, we use a laterally homogeneous velocity model and reformulate the problem into a single matrix multiplication and matrix assessment for each time step. LWCEDS has been successfully tested on a suite of local and regional seismic events selected to span the range of expected event quality. Preliminary results from our sparse network show typical epicentral errors of less than 3 km for local events and, with notable exceptions, to within 10 to 20 km for regional events. Similar results were obtained during an on-line experiment conducted to generate daily bulletins during the time period from 3 December 1996 through 7 January 1997. Data from 156 triggers were processed, including 33 teleseisms, 102 regional events (includes explosions), and 21 local earthquakes. Results from this expanded test set are encouraging but reveal the need for a method to mask or flag various electronic and telemetry spikes.
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Boué, P., P. Poli, M. Campillo, H. Pedersen, X. Briand et P. Roux. « Teleseismic correlations of ambient seismic noise for deep global imaging of the Earth ». Geophysical Journal International 194, no 2 (15 mai 2013) : 844–48. http://dx.doi.org/10.1093/gji/ggt160.
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Abstract We present here a global analysis showing that wave paths probing the deepest part of the Earth can be obtained from ambient noise records. Correlations of seismic noise recorded at sensors located various distances apart provide new virtual seismograms for paths that are not present in earthquake data. The main arrivals already known for earthquake data are also present in teleseismic correlations sections, including waves that have propagated through the Earth's core. We present examples of applications of such teleseismic correlations to lithospheric imaging, study of the core mantle boundary or of the anisotropy of the inner core.
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Bostock, M. G., et S. Rondenay. « Migration of scattered teleseismic body waves ». Geophysical Journal International 137, no 3 (1 juin 1999) : 732–46. http://dx.doi.org/10.1046/j.1365-246x.1999.00813.x.
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Earle, P. S. « Polarization of the Earth's teleseismic wavefield ». Geophysical Journal International 139, no 1 (1 octobre 1999) : 1–8. http://dx.doi.org/10.1046/j.1365-246x.1999.00908.x.
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Korenaga, Jun. « Teleseismic migration with dual bootstrap stack ». Geophysical Journal International 196, no 3 (17 décembre 2013) : 1706–23. http://dx.doi.org/10.1093/gji/ggt475.
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Vinnik, L. P., et R. Kind. « Ellipticity of Teleseismic S-Particle Motion ». Geophysical Journal International 113, no 1 (avril 1993) : 165–74. http://dx.doi.org/10.1111/j.1365-246x.1993.tb02537.x.
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Frederiksen, A. W., et J. Revenaugh. « Lithospheric imaging via teleseismic scattering tomography ». Geophysical Journal International 159, no 3 (décembre 2004) : 978–90. http://dx.doi.org/10.1111/j.1365-246x.2004.02414.x.
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Bostock, M. G. « Kirchhoff-approximate inversion of teleseismic wavefields ». Geophysical Journal International 149, no 3 (26 mai 2002) : 787–95. http://dx.doi.org/10.1046/j.1365-246x.2002.01687.x.
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Huang, Ching-Jer, Hsin-Yu Chen, Chung-Ray Chu, Ching-Ren Lin, Li-Chen Yen, Hsiao-Yuen Yin, Chau-Chang Wang et Ban-Yuan Kuo. « Low-Frequency Ground Vibrations Generated by Debris Flows Detected by a Lab-Fabricated Seismometer ». Sensors 22, no 23 (29 novembre 2022) : 9310. http://dx.doi.org/10.3390/s22239310.
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A lab-fabricated ocean bottom seismometer was modified and deployed terrestrially to detect low-frequency (<10 Hz) ground vibrations produced by debris flows. A frequency–response test of the new seismometer revealed that it can detect seismic signals at frequencies of 0.3–120 Hz. Its seismic ground motion detection ability was investigated by comparing its measurements of seismic signals produced by rockfalls with those of a geophone. Two new seismometers were deployed at the Aiyuzi Stream, Nantou County, Taiwan, in September 2012. Seismic signals produced by two local earthquakes, two teleseisms, and three debris flows detected by the seismometer in 2013 and 2014 were discussed. The seismic signal frequencies of the local earthquakes and teleseisms (both approximately 1800 km apart) were 0.3–30 and <1 Hz, respectively. Moreover, seismometer measurements revealed that seismic signals generated by debris flows can have minimum frequencies as low as 2 Hz. Time-matched CCD camera images revealed that debris flow surge fronts with larger rocks have lower minimum frequencies. Finally, because the seismometer can detect low-frequency seismic waves with low spatial decay rates, it was able to detect one debris flow approximately 3 min and 40 s before it arrived.
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Mercier, Jean-Philippe, Michael G. Bostock et Adam M. Baig. « Improved Green’s functions for passive-source structural studies ». GEOPHYSICS 71, no 4 (juillet 2006) : SI95—SI102. http://dx.doi.org/10.1190/1.2213951.
Texte intégralRésumé :
Over the past two decades, teleseismic receiver functions have proved to be a useful tool to investigate crustal structure. Because they represent a first-order approximation to the [Formula: see text]-wave component of the teleseismic-[Formula: see text] Green’s function, receiver functions provide valuable information on physical properties related to shear modulus. However, the implicit use of the [Formula: see text]-component seismogram as a proxy for the source precludes the recovery of information on discontinuous structure involving contrasts in compressional modulus. By deconvolving improved estimates of complex source time functions generated by earthquakes, one may move beyond the conventional receiver function paradigm to a more accurate approximation of the earth’s Green’s function. Using a new deconvolution method, we present estimates of the [Formula: see text]-component of the teleseismic-[Formula: see text] Green’s functions at several stations of the Canadian National Seismic Network (CNSN) that clearly show the receiver-side pure [Formula: see text]-wave crustal multiple. The identification and characterization of these signals in studies of the lithosphere will afford better constraints on subsurface lithology and represent a narrowing of the gap between active- and passive-source seismic imaging.
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Tsuboi, Seiji, Paul M. Whitmore et Thomas J. Sokolowski. « Application of Mwp to deep and teleseismic earthquakes ». Bulletin of the Seismological Society of America 89, no 5 (1 octobre 1999) : 1345–51. http://dx.doi.org/10.1785/bssa0890051345.
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Abstract The broadband moment magnitude Mwp (Tsuboi, et al., 1995) allows for the effective determination of earthquake magnitude by using broadband P waveforms. It was developed to determine moment magnitude of shallow earthquakes around the Japanese Islands for early tsunami warnings. Tsuboi et al. (1995) demonstrated that Mwp shows good agreement with the Mw from Harvard centroid moment tensor (CMT) solutions. In the present study, we show that Mwp is also applicable to deep earthquakes and earthquakes recorded at teleseismic distances. The Mwp proves to be useful for quick, accurate size estimates of earthquakes recorded at both regional and teleseismic distances occurring at any depth.
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Hwang, Lorraine J., et Hiroo Kanamori. « Teleseismic and strong-motion source spectra from two earthquakes in eastern Taiwan ». Bulletin of the Seismological Society of America 79, no 4 (1 août 1989) : 935–44. http://dx.doi.org/10.1785/bssa0790040935.
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Abstract The 20 May and 14 November 1986 Hualien earthquakes occurred in a seismically active region of Taiwan. Locally determined focal mechanisms and aftershock patterns from the Taiwan Telemetered Seismographic Network indicate that both earthquakes occurred on steeply dipping reverse faults that trend NNE. This agrees with teleseismic first-motion data for the May event but not for the November event. This discrepancy is due to a moderate foreshock before the November event. Surface-wave analysis gives a solution for the November event of: dip 57°, rake 100°, and strike 43°, which is similar to the locally reported focal mechanism. The seismic moment of the November event is M0 = 1.7 × 1027 dynecm and the magnitudes determined from WWSSN data are m^b = 6.4, Ms = 7.3. Teleseismic source spectra show that the two events also have similar spectral signatures above 0.15 Hz. Reference acceleration spectra are computed from the average teleseismic source spectra and compared to the averaged acceleration spectra computed from strong-motion stations for both events. Correlations between the spectral amplitudes of the strong-motion spectra obtained from the main portion of the SMART 1 array and the teleseismically estimated reference spectra are poor above 0.2 Hz. Data from the hard-rock site situated outside of the basin indicates that amplification of the ground motion between 0.17-1.7 Hz is due to the alluvial valley where the SMART 1 array is located. The amplitude of the observed spectrum is five times the reference spectrum at the hard-rock site. This is consistent with similar observations from the 1985 Michoacan and 1983 Akita-Oki earthquakes. The analysis of these and more teleseismic and strong-motion records will lead to a better understanding of the relationship between their spectra.
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Bryan, Carol J. « A possible triggering mechanism for large Hawaiian earthquakes derived from analysis of the 26 June 1989 Kilauea south flank sequence ». Bulletin of the Seismological Society of America 82, no 6 (1 décembre 1992) : 2368–90. http://dx.doi.org/10.1785/bssa0820062368.
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Abstract Examination of short-period seismic data from the ML = 6.1 Kilauea south flank earthquake and aftershock sequence indicates that the rupture process in large Hawaiian earthquakes is more complex than previously modeled. In contrast to the low-angle thrust solution determined for the mainshock from long-period teleseismic body waves by other workers, I find an intermediate- to high-angle reverse solution; I find, however, that focal mechanisms for coastal aftershocks of ML > 3.0 are similar to the teleseismic mechanism for the mainshock. A difference in focal mechanisms determined from short-period local-network seismic data and from long-period teleseismic data has been noted for other recent large Hawaiian earthquakes. Both the mapping of surface cracks and the focal mechanism derived from short-period seismic data for the ML = 6.6 1983 Kaoiki earthquake show strike-slip motion, whereas the centroid moment tensor solution shows low-angle thrusting. The focal mechanism calculated from short-period seismic data for the ML = 7.2 1975 Kalapana mainshock shows low-angle thrusting according to some workers, but intermediate- to high-angle reverse faulting according to others, whereas focal mechanisms calculated from long-period seismic data show low-angle thrusting. This result suggests that rupture initiation in large Hawaiian earthquakes, as represented by the short-period focal mechanisms, differs significantly from the overall rupture process, as represented by the teleseismic mechanisms. I propose that small earthquakes trigger the large-scale energy release at the bases of the volcanic edifices, the type of energy release often observed in large Hawaiian earthquakes. These triggering events may occur along rupture surfaces that differ from those along which the long-period moment release occurs and thus may represent release of a local stress concentration superposed upon the regional stress field.
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Shimizu, Kousuke, Yuji Yagi, Ryo Okuwaki et Yukitoshi Fukahata. « Development of an inversion method to extract information on fault geometry from teleseismic data ». Geophysical Journal International 220, no 2 (1 novembre 2019) : 1055–65. http://dx.doi.org/10.1093/gji/ggz496.
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SUMMARY Teleseismic waveforms contain information on fault slip evolution during an earthquake, as well as on the fault geometry. A linear finite-fault inversion method is a tool for solving the slip-rate function distribution under an assumption of fault geometry as a single or multiple-fault-plane model. An inappropriate assumption of fault geometry would tend to distort the solution due to Green’s function modelling errors. We developed a new inversion method to extract information on fault geometry along with the slip-rate function from observed teleseismic waveforms. In this method, as in most previous studies, we assumed a flat fault plane, but we allowed arbitrary directions of slip not necessarily parallel to the assumed fault plane. More precisely, the method represents fault slip on the assumed fault by the superposition of five basis components of potency-density tensor, which can express arbitrary fault slip that occurs underground. We tested the developed method by applying it to real teleseismic P waveforms of the MW 7.7 2013 Balochistan, Pakistan, earthquake, which is thought to have occurred along a curved fault system. The obtained spatiotemporal distribution of potency-density tensors showed that the focal mechanism at each source knot was dominated by a strike-slip component with successive strike angle rotation from 205° to 240° as the rupture propagated unilaterally towards the south-west from the epicentre. This result is consistent with Earth’s surface deformation observed in optical satellite images. The success of the developed method is attributable to the fact that teleseismic body waves are not very sensitive to the spatial location of fault slip, whereas they are very sensitive to the direction of fault slip. The method may be a powerful tool to extract information on fault geometry along with the slip-rate function without requiring detailed assumptions about fault geometry.
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Steck, Lee K., et William A. Prothero. « Observations of direct P-wave slowness and azimuth anomalies for teleseisms recorded in Long Valley caldera, California ». Bulletin of the Seismological Society of America 83, no 5 (1 octobre 1993) : 1391–419. http://dx.doi.org/10.1785/bssa0830051391.
Texte intégralRésumé :
Abstract Using particle motion, frequency-wavenumber analysis, and planewave fitting, we observe large anomalies in the bearing and apparent velocity of 0.5 to 1.0 Hz teleseismic P-waves incident at stations near the resurgent dome of Long Valley caldera, California. Our 3-component regional and teleseismic data come from three 2-mo deployments: (1) a 10 station, 2.9-km aperture array, (2) a 5 station, 1.9-km aperture array, and (3) a 3 station network, spanning about 5 km. We find stacked particle motion to be the most informative polarization method when the signal-to-noise ratio (SNR) is high and wavefronts are nonplanar. However, the insensitivity of the broadband frequency-wavenumber method (BBFK) to noise makes it more useful as SNR declines. Numerical simulations and data analyses using the BBFK method show that for a 10 station, 3-km aperture array, precision of roughly 8° in bearing and 0.008 sec/km in apparent slowness can be obtained with average teleseismic data in regions having complex structure. Bearing anomalies at Long Valley caldera reach up to 124°, whereas apparent slowness differ from expected values by up to a factor of 2. The anomalies are most likely caused by strong lateral velocity variations in the subcaldera crust between depths of 6 to 35 km.
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Vinnik, Lev P., et Barbara A. Romanowicz. « Origin of precursors to teleseismic S waves ». Bulletin of the Seismological Society of America 81, no 4 (1 août 1991) : 1216–30. http://dx.doi.org/10.1785/bssa0810041216.
Texte intégralRésumé :
Abstract In the literature, there are descriptions of precursors to teleseismic S waves that are polarized as teleseimic P. They are commonly regarded as phases converted from SV to P underneath the seismographic stations. We present observations of precursors at the broadband digital GEOSCOPE stations in the distance range from 45° to 95° and at periods around 10 sec. These precursors are polarized as the teleseismic P but cannot be interpreted in terms of conversion underneath the station. We propose that they are formed by conversion and scattering from S to P at the free surface and scattering of the resulting P in the lithosphere of the region between the source and the receiver. The apparent velocity of S in the region of scattering is usually around 7 km/sec, which implies a long wavepath of the converted P in the continental crust. Apparently, S-to-P and P-to-P scattering play an important role in forming teleseismic wave fields even in the relatively long-period band around 10 sec. A possibility of observing true S-P converted phases originating in the lithosphere underneath the station is certainly not ruled out. However, our analysis is a warning that scattered waves sometimes can be erroneously taken for converted phases.
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Jiang, Mei, Hui Qian et Yingiun Ma. « Teleseismic anistotropy and its features in the upper mantle beneath the Tibet Plateau and neighboring areas. Metallogenic implications ». Global Tectonics and Metallogeny 8, no 1-4 (1 janvier 2003) : 87–88. http://dx.doi.org/10.1127/gtm/8/2003/87.
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Clark, R. A., et R. G. Pearce. « Identification of multiple underground explosions using the relative amplitude method ». Bulletin of the Seismological Society of America 78, no 2 (1 avril 1988) : 885–97. http://dx.doi.org/10.1785/bssa0780020885.
Texte intégralRésumé :
Abstract The relative amplitude method is applied to the few available good quality teleseismic P-wave seismograms from five presumed double nuclear explosions and one known multiple chemical explosion, under the “naive” assumption that the observed multiple arrivals correspond to P, pP, and sP from a single earthquake—an interpretation which is indeed consistent with the body-wave arrival time data in most cases. The purpose is to investigate the ability of relative amplitudes to identify correctly such multiple events for which established discrimination criteria may give earthquake-like or ambiguous results. For five of the examples, observed relative amplitudes from only four azimuthally well-distributed array seismograms are sufficient to exclude the single-earthquake interpretation. Deliberate attempts to simulate earthquake teleseismic P wave-forms using multiple explosions are restricted to simulation studies, and one of these is analyzed here using the same approach. We conclude that relative amplitudes can act as a valuable aid to source discrimination in cases where complexity gives rise to fallibility of conventional discriminants, even where only a small number of well-distributed teleseismic short-period array seismograms are available, their signal-to-noise ratios being maximized by suitable array design and careful choice of array site. The network need not be dense, since closely spaced observations of the focal sphere generally embody a large measure of redundancy.
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Dewanto, Bondan Galih, Ramadhan Priadi, Leni Sophia Heliani, Al Shida Natul, Muhammad Yanis, Indranova Suhendro et Admiral Musa Julius. « The 2022 Mw 6.1 Pasaman Barat, Indonesia Earthquake, Confirmed the Existence of the Talamau Segment Fault Based on Teleseismic and Satellite Gravity Data ». Quaternary 5, no 4 (2 novembre 2022) : 45. http://dx.doi.org/10.3390/quat5040045.
Texte intégralRésumé :
A Mw 6.1 earthquake on 25 February 2022, at around 8:39 a.m. local time, struck Pasaman Barat Regency, West Sumatra, Indonesia, and was felt in Singapore and Malaysia. The hypocenter of this earthquake was 12 km deep and preceded by an Mw 4.9 foreshock a few minutes earlier. The earthquakes originated on a blind fault and triggered a landslide at Mount Talamau. Herein, the slip distribution and asperities along the plane fault during the earthquake were examined by teleseismic inversion and the fault location was identified by Global Gravity Model plus (GGMPlus) satellite gravity data. The slip distribution was calculated from the source parameters (strike: 136°; dip: 70°; rake: 174°) by inversion techniques based on teleseismic data. Based on the slip distribution, the earthquake was generated by stress from the Sianok fault that spread to the north and reached the uncertain fault segment in the Talamau area. In addition, the results of the First Horizontal Derivative and Second Vertical Derivative from the GGMplus data revealed a straight Simple Bouguer Anomaly pattern, confirming the existence of the uncertain Talamau fault as part of the Great Sumatra Fault. This work shows the potential application of the combination of teleseismic and gravity observation for delineating the fault structure that caused the 2022 Mw 6.1 Pasaman earthquake, which can also be applied to other locations of similar geological backgrounds.