Dissertations / Theses on the topic 'Seismic swarms'

Une séquence sismique est un ensemble de tremblements de terre qui se produisent avec une grande proximité spatiale et temporelle. Un essaim de séisme est un type de séquence sismique caractérisé par des tremblements de terre dont la localisation change au cours du temps et qui n’est pas dominé par un seul grand tremblement de terre.Dans cette thèse, j'étudie l'évolution spatio-temporelle d'essaims sismiques naturels et induits dans le but d'identifier les processus physiques qui les produisent et de caractériser les propriétés des systèmes de failles activés. Plus spécifiquement, je me concentre sur trois aspects clés de l'activité sismique : l'évolution temporelle du taux de sismicité, la localisation des séismes et la distribution des magnitudes. L'étude se concentre sur deux essaims sismiques. Le premier, d'origine naturelle, s'est produit dans la vallée de la Maurienne (Alpes françaises) entre 2017 et 2019, tandis que le second a été induit par des opérations de fracturation hydraulique à Preston New Road, au Royaume-Uni, en 2019. Pour assurer une analyse à haute résolution de ces séquences, j'ai d'abord créé des catalogues améliorés qui intègrent de nombreux événements nouvellement détectés ainsi que des magnitudes et des localisations plus précises.La migration des séismes pendant l'essaim de Maurienne suggère que cette séquence a pu être activée par une combinaison de multiples diffusions de fluides à haute pression et d'interactions entre séismes. De plus, la proportion de petits et grands événements (paramètre b de la loi de Gutenberg-Richter) varie dans l'espace, et ce changement peut être lié à la dimension des failles actives.En plus de l'étude de l'activité sismique, j'ai appliqué l'interférométrie du bruit sismique ambiant pour évaluer si les changements de contraintes pendant l'essaim ont créé des variations détectables de la vitesse des ondes sismiques. Les changements de vitesse semblent être principalement influencés par un processus saisonnier probablement lié aux variations de pression interstitielle dues aux précipitations. Cependant, pendant la période principale d'activité sismique, ces changements ont pu être amplifiées par les secousses du sol résultant de l'occurrence prolongée de séismes.La séquence de Preston New Road est caractérisée par un taux de sismicité et une distribution des magnitudes qui évoluent progressivement au fur et à mesure que les fluides sont injectés de manière répétée et que le volume de la zone sismogénique grandit. Ceci suggère que l'activité sismique au cours d'une phase d'injection dépend de l'historique des phases précédentes.Les séquences de Maurienne et de Preston New Road ne sont que deux exemples du phénomène plus large des essaims sismiques et induits. Néanmoins, ces deux séquences illustrent le fait que les essaims induits et naturels peuvent présenter des propriétés similaires dans leur évolution spatio-temporelle, comme la migration des séismes et la dépendance de la valeur de b à l'échelle du système de failles. Cela souligne la possibilité d'appliquer les connaissances acquises lors de l'étude d'un type d'essaim pour améliorer notre compréhension de l'autre
A seismic sequence is a cluster of earthquakes that occur in close spatial and temporal proximity. One type of seismic sequence is a seismic swarm, which is typically characterized by earthquakes whose location changes over time and by the absence of a single, dominant, large earthquake.In this thesis, I investigate the spatio-temporal evolution of natural and induced seismic swarms with the aim of identifying the physical processes that drive them and characterizing the properties of the activated fault systems. More specifically, I focus on three key aspects of the seismic activity: temporal evolution of the seismicity rate, earthquake location and frequency-magnitude distribution. The study focuses on two seismic swarms. The first one, of natural origin, occurred in the Maurienne valley (French Alps) between 2017 and 2019, while the second one was induced by hydraulic fracturing operations at Preston New Road, UK, in 2019. To ensure a high-resolution analysis of these sequences, I first created improved catalogs, which incorporate newly detected events and more accurate magnitudes and hypocenter locations.The migration of earthquakes during the Maurienne swarm suggests that this sequence may have been triggered by a combination of multiple pulses of high-pressure fluids and earthquake-to-earthquake interactions. Additionally, the proportion of small and large events (i.e., the b-value of the Gutenberg-Richter law) varies in space, and this change may be linked to the size of the active fault systems.In addition to the study of the seismic activity, I applied ambient noise interferometry to assess if stress changes during the Maurienne swarm produced detectable variations in seismic wave velocity. The velocity changes appear to be primarily influenced by a seasonal process possibly related to pore pressure variations due to rainfall. However, during the main period of seismic activity, such changes may also be accentuated by the continuous ground shaking resulting from the prolonged occurrence of earthquakes.The Preston New Road sequence is characterized by a seismicity rate and a frequency-magnitude distribution that gradually evolve as fluids are repeatedly injected and the seismogenic volume expands in size. This suggests that the seismic activity during an injection stage depends on the injection history of past stages.The Maurienne and Preston New Road sequences are just two examples of the broader phenomenon of seismic and induced swarms. Nonetheless, these two sequences illustrate that induced and natural swarms can exhibit similar patterns in their spatio-temporal evolution, such as earthquake migration and the dependence of the b-value on the scale of the fault system. This emphasizes the potential of applying the knowledge gained from studying one type of swarm to improve our understanding of the other

The most recent intense earthquake swarm in the Vogtland lasted from 6 October 2008 until January 2009. Greatest magnitudes exceeded M3.5 several times in October making it the greatest swarm since 1985/86. In contrast to the swarms in 1985 and 2000, seismic moment release was concentrated near swarm onset. Focal area and temporal evolution are similar to the swarm in 2000. Work hypothysis: uprising upper-mantle fluids trigger swarm earthquakes at low stress level. To monitor the seismicity, the University of Potsdam operated a small aperture seismic array at 10 km epicentral distance between 18 October 2008 and 18 March 2009. Consisting of 12 seismic stations and 3 additional microphones, the array is capable of detecting earthquakes from larger to very low magnitudes (M<-1) as well as associated air waves. We use array techniques to determine properties of the incoming wavefield: noise, direct P and S waves, and converted phases.

On observe depuis 2005 une sismicité intense à 10 km d'Oulan Bator ce qui a permis d'identifier une faille active, Emeelt, sur le terrain. Après le calcule d'un modèle de vitesse 3D, j'ai appliqué la tomographie double différence pour obtenir une localisation précise des séismes. Ils marquent au moins trois branches parallèles orientées N147° comme la faille vue en surface. L'activité sur la faille principale d'Emeelt (MEF) s’étend sur 15 km, les branches Ouest et Est, moins actives, sur 10 km. La profondeur de l'activité s'étend entre 4 et 15 km. L'activité sismique semble concentrée à l'intersection avec des failles Mésozoïques et les contrastes Vs/Vs suggèrent la présence de fluides. Les 10 essaims identifiés montrent une activité croissante et une migration spatiale avec le temps. Le calcul de 2 scénarios possibles, un M ~ 6.4 et un M ~ 7, indique un important impact sur la ville d'Oulan Bator, avec une intensité minimum de VIII et localement IX pour M=6.4 et X pour M=7
We observe since 2005 a high seismic activity at 10 km from Ulaanbaatar that allowed us to identify a new active fault, Emeelt, in the field. After computing a 3D velocity model, I applied Double-Difference tomography to obtain a precise localization of earthquakes. They trace at least three parallel branches oriented N147° like the fault seen at surface. The seismic activity on the Main Emeelt Fault (MEF) is along at least 15 km, on the West and East branches, less active, along 10 km. The depth of the seismicity extends between 4 and 15 km. The activity seems concentrated at the intersection with Mesozoic faults and Vp/Vs contrast suggests the presence of fluids. The 10 swarms identified show an increasing activity and a spatial migration with time. The calculation of 2 possible scenarios, one M ~ 6.4 and one M ~ 7, shows an important impact on Ulaanbaatar, with a minimum intensity of VIII and IX for M=6.4 and X for M=7

Microearthquake (< M3.0) swarms occur frequently in volcanic environments, but do not always culminate in an eruption. Such non-eruptive swarms may be caused by stresses induced by magma intrusion, hydrothermal fluid circulation, or regional tectonic processes, such as slow-slip earthquakes. Strandline Lake, located 30 km northeast of Mount Spurr volcano in south-central Alaska, experienced an intense earthquake swarm between August 1996 and August 1998. The Alaska Volcano Observatory (AVO) catalog indicates that a total of 2,999 earthquakes were detected during the swarm period, with a maximum magnitude of Mw 3.1 and a depth range of 0-30 km below sea level (with the majority of catalog hypocenters located between 5-10 km BSL). The cumulative seismic moment of the swarm was 2.03e15 N-m, equivalent to a cumulative magnitude of Mw 4.2. Because of the swarm's distance from the nearest Holocene volcanic vent, seismic monitoring was poor and gas and GPS data do not exist for the swarm period. However, combined waveforms from a dense seismic network on Mount Spurr and from several regional seismic stations allow reanalysis of the swarm earthquakes. I first developed a new 1-D velocity model for the Strandline Lake region by re-picking and inverting precise arrival times for 27 large Strandline Lake earthquakes. The new velocity model reduced the average RMS for these earthquakes from 0.16 to 0.11s, and the average horizontal and vertical location errors from 3.3 to 2.5 km and 4.7 to 3.0 km, respectively. Depths of the 27 earthquakes ranged from 10.5 to 22.1 km with an average depth of 16.6 km. A moderately high b-value of 1.33 was determined for the swarm period, possibly indicative of magmatic activity. However, a similarly high b-value of 1.25 was calculated for the background period. 28 well-constrained fault plane solutions for both swarm and background earthquakes indicate a diverse mixture of strike-slip, dip-slip, and reverse faulting beneath Strandline Lake. Finally, five Interferometric Synthetic Aperture Radar (InSAR) images spanning the swarm period unambiguously show no evidence of surface deformation. While a shallow volcanic intrusion appears to be an unlikely cause of the Strandline Lake swarm based on the new well-constrained earthquake depths and the absence of strong surface deformation, the depth range of 10.5 to 22.1 km BSL for relocated earthquakes and the high degree of FPS heterogeneity for this swarm are similar to an earthquake swarm beneath Lake Tahoe, California in 2003 caused by a deep intrusion near the base of the crust (Smith et al, 2004). This similarity suggests that a deep crustal magmatic intrusion could have occurred beneath the Strandline Lake area in 1996-1998 and may have been responsible for the resulting microearthquake activity.

Earthquake swarms occur frequently in West Bohemia, Central Europe. Their occurrence is correlated with and propably triggered by fluids that escape on the earth's surface near the epicentres. These fluids raise up periodically from a seemingbly deep-seated source in the upper mantle. Moment tensors for swarm events in 1997 indicate tensile faulting. However, they were determined under assumption of seismic isotropy although anisotropy can be observed. Anisotropy may obscure moment tensors and their interpretation. In 2000, more than 10,000 swarm earthquakes occurred near Novy Kostel, West Bohemia. Event triggering by fluid injection is likely. Activity lasted from 28/08 until 31/12/00 (9 phases) with maximum ML=3.2. High quality P-wave seismograms were used to retrieve the source mechanisms for 112 events between 28/08/00 and 30/10/00 using > 20 stations. We determine the source geometry using a new algorithm and different velocity models including anisotropy. From inversions of P waves we observe ML<3.2, strike-slip events on steep N-S oriented faults with additional normal or reverse components. Tensile components seem to be evident for more than 60% of the processed swarm events in West Bohemia during the phases 1-7. Being most significant at great depths and at phases 1-4 during the swarm they are time and location dependent. Although tensile components are reduced when anisotropy is assumed they persist and seem to be important. They can be explained by pore-pressure changes due to the injection of fluids that raise up. Our findings agree with other observations e.g. correlation of fluid transport and seismicity, variations in b-value, forcing rate, and in pore pressure diffusion. Tests of our results show their significance.

Les zones de subduction sont les endroits les plus sismiquement actifs du monde et sont également le siège de méga-tremblement de Terre comme celui qui frappa le Chili en 1960 d’une magnitude estimée à 9.5. Les dernières grandes ruptures ayant frappées le Chili ont permis de mettre en évidence des intéractions sismiques-asismiques complexes. Afin d’étudier ces interactions nous nous sommes penchés sur deux crises sismiques : l’essaim sismique d’Avril 2017 ; la phase préparatoire du séisme d’Iquique (M w 8.1) du 1er Avril 2014. L’essaim sismique qui pris place près de la ville de Valparaiso dans une région connue pour avoir eu l’expérience de très grands séismes par le passé en 1730 et en 1906 notamment. Dans le but d’étudier la dynamique de cet essaim nous avons construit un catalogue riche de plus de 2000 séisme composant la séquence. Une activité sismique intense commença deux jours avec le séisme principal de la séquence M w 6.9 et fut accompagné par un glissement progressif le long de l’interface que nous avons observé à la fois dans les données GPS mais également à l’aide de repeaters. Enfin, un séisme de magnitude 8,2, survenu le 1er avril 2014 près de la ville de Iquique, a rompu un tiers de la lacune sismique au nord du Chili. Cet séisme a été précédé d’une séquence d’essaims sismiques précurseurs qui semblent avoir été également accompagnés par des glissements stables de l’interface de subduction. A l’aide d’un catalogue de séisme plus complet, nous avons pu analyser finement la préparation du séisme d’Iquique. Nous mettons en évidence, à travers une approche statistique, l’apparition d’une quiescence sismique dans la région du choc principale. Nous lions cette quiescence à un glissement asismique profond potentiellement relié à la circulation de fluide sous pression le long de l’interface de subduction
The subduction zones are the most seismically active places in the world and are also the seat of megathrust-earthquakes such as the one that stroke Chile in 1960 with a magnitude estimated at 9.5. The last major ruptures in Chile have revealed complex seismic-aseismic interactions. In order to study these interactions, we investigated on at two seismic crises: the seismic swarm of April 2017; the preparatory phase of the Iquique earthquake (M w 8.1) from April 1 st , 2014. The seismic swarm took place near the city of Valparaiso in an area known to have experienced mega-earthquakes in the past: in 1730 and in 1906. In order to study the dynamics of this swarm we have built a rich catalog of more than 2000 earthquakes composing the sequence. An intense seismic activity began on April 22 nd , two days before the main earthquake of the sequence M w 6.9 and was accompanied by a gradual slip along the interface that we observed both in the GPS data and using repeating-earthquakes. Our analysis suggest that the swarm was driven by aseismic slip. The second study concerns an earthquake of mag-nitude 8.2 which occurred on April 1 st 2014 near the city of Iquique and broke one-third of the seismic gap in northern Chile. This earthquake was preceded by a sequence of precursor seismic swarms that appeared to have been accompanied by stable slip in the subduction interface as well. By building a more complete earthquake catalog, we were able to thoroughly analyze the preparation of the Iquique earthquake. We show, following a statistical approach, the occurrence of a large-scale seismic quiescence in the region of the mainshock. We link this quiescence to a downdip transient-slip potentially related to fluid channeling along the subduction interface

La partie ouest du rift de Corinthe, en Grèce, s'ouvre à une vitesse d'environ 15 mm par an générant un taux de déformation parmi les plus élevés au monde, quelques séismes destructeurs de magnitude M>6 par décennie, et une forte activité microsismique irrégulière spatialement et temporellement. Afin de mieux comprendre les mécanismes liés à cette déformation crustale et de préciser les structures majeures actives, ce travail de recherche exploite la base de données sismologiques du Corinth Rift Laboratory de 2000 à 2015 en analysant finement les microséismes et leur évolution spatio-temporelle. La relocalisation globale des sources sismiques ainsi que leur classification en multiplets ont permis de préciser la géométrie des failles et d'identifier des comportements mécaniques différents. La zone ouest, au milieu du golfe, est affectée par des variations de pressions de fluides dans une couche géologique, entraînant des migrations des essaims de microséismes à des vitesses d'environ 50 m par jour. Les multiplets profonds de la partie centrale, près de la côte nord, sont persistants et semblent déclenchés par des épisodes de glissements lents asismiques sur un détachement immature pouvant atteindre la croûte ductile. Le faible pourcentage de déclenchement dynamique par les ondes sismiques suggère que l'état global du système de failles n'est pas au seuil critique de rupture. La magnitude des séismes est corrélée à l'impulsivité initiale de la rupture. Ces résultats précisent la dynamique de déformation du rift, les interactions sismique-asismiques, et permettront d'améliorer les modèles d'aléas sismiques de la région
The western part of the Corinth Rift in Greece is opening at about 15 mm per year, generating one of the highest deformation rates in the world, some destructive earthquakes of magnitude M>6 per decade, and high microseismic activity irregular in space and time. In order to better understand the mechanisms related to this crustal deformation and to specify the major active structures, this research work makes use of the seismological database of the Corinth Rift Laboratory from 2000 to 2015 by finely analyzing microearthquakes and their spatio-temporal evolution. The global relocation of the seismic sources and their classification into multiplets enable to refine the geometry of the faults and to identify different mechanical behaviors. The western zone, in the middle of the gulf, is affected by fluctuations of fluid pore pressures in a geological layer, resulting in microseismic swarm migrations at a velocity of about 50 m per day. The deep multiplets of the central part, near the northern coast, are persistent and appear to be triggered by episodes of slow aseismic slip along an immature detachment, which can reach the ductile crust. The low percentage of dynamic triggering by passing seismic waves suggests that the overall state of the fault system is not at the critical breaking point. The magnitude of earthquakes is correlated with the initial impulsiveness of the rupture. These results specify the dynamics of the rift deformation, the seismic-aseismic interactions, and will make possible the improvement of the seismic hazard models of the region

碩士
國立交通大學
多媒體工程研究所
95
Particle Swarm Optimization (PSO) is adopted to detect parameter pattern (e.g. circle, ellipse, hyperbola and asymptote.) Each particle is represented as parameters of patterns, then swarm of particles search the optimal solution in parameter space. We define mathematical formulas to represent various kinds of parameter patterns, and define the distance from points to patterns. Experiments on simulated image get good detection. The method is also applied to detect the parameters of direct wave (line) and reflected wave pattern (hyperbola) in simulated and real one-shot seismogram, the results can improve seismic interpretation and further seismic data processing.

In my doctoral thesis I have investigated earthquake swarms from two com- pletely different tectonic areas, West Bohemia/Vogtland and Southwest Iceland, with the aim of gaining a deeper insight into the nature of earthquake swarms in diverse tectonic environments. I analysed swarm-like activities from West Bo- hemia and Southwest Iceland from the perspective of statistical characteristics (magnitude-frequency distribution, interevent time distribution), seismic moment release, and space-time distribution of events. I found that the ratio of small to large events and the event rates are similar for all the activities in both areas, while the rate of the seismic moment release is significantly higher for the South- west Icelandic swarms. Seismic moment released step by step is characterised for the West Bohemia swarms, whereas seismic moment released in one dominant short-term phase is typical of Southwest Icelandic earthquake swarms. All the West Bohemian swarms took place in a bounded focal zone Nový Kostel that is fairly complex, consisting of several fault segments. The Southwest Icelandic swarms are distributed at much larger area along the Mid Atlantic Ridge up to its branching in the Hengill triple junction, the individual swarms clearly reflect a tectonic structure of respective focal areas. I have...

Title: Methods of processing macroseismic data and their application to the 2008 earthquake swarm in western Bohemia Author: Bc. Pavla Procházková Department: Department of Geophysics Supervisor: doc. RNDr. Oldřich Novotný, CSc. Consultant: RNDr. Pavla Hrubcová, PhD. Supervisor's e-mail address: on@karel.troja.mff.cuni.cz Abstract: Abstract: This work evaluates macroseismic observations of the earthquake swarm which occurred in West Bohemia in 2008. The first chapters are devoted to the explanation of basic terms, the geological description of the region and the history of earthquakes in the region. The historical development of macroseismic scales and a detailed description of the EMS-98, used for the evaluation of the 2008 earthquake swarm data, are contained in the following chapters. The main part of the present work deals with the manual macroseismic evaluation of the swarm followed by a description of the program for the automatic evaluation of data. The program results were found to agree with the manual evaluation. In the next part there is a detailed description of a program for drawing isoseismal lines. In the appendix the reader can find an overview of macroseismic scales, sample questionnaires and suggestions to improve the Czech questionnaire. Keywords: macroseismical intensity, EMS-98...