Modeling of earthquake swarms: occurrence patterns, evolution mechanism and possible triggering

The study of earthquake swarms is one of the most modern topics in the field of Seismology due to their unique triggering and evolution mechanisms. Earthquake swarms are a type of earthquake sequence, where a dominant in magnitude earthquake is absent. The triggering mechanism of earthquake swarms could be either due to tectonic loading or to the existence of fluids or anthropogenic activity in an area, such as fluid intrusion in wells for commercial use. The main goal of this study is to investigate the spatiotemporal properties of earthquake swarms in Greece aiming to associate their occurrence with the triggering of strong earthquakes and contribute to the seismic hazard assessment. The main study area is the Gulf of Corinth, and especially its western part, where frequent seismic excitations occur. This study also uses data from a local network that operated in the area of Florina (NW Greece) for six months, in order to interpret the triggering mechanism of earthquake swarms. First, all available seismological data are gathered in order to compile earthquake catalogs for each area. These catalogs are checked regarding the magnitude scales that were used and are homogenized into one. The fundamental part of this thesis is the relocation of the earthquakes using the double difference method and waveform cross correlation techniques. The high accuracy in earthquake location allows the study of the spatiotemporal properties of earthquake swarms. It was then possible to identify seismic excitations using declustering algorithms. The identified seismic excitations are associated with certain fault segments and their characteristics are defined based on the three dimensional distribution of the earthquake focal parameters. An important tool in order to verify the geometry of the identified fault segments is the computation of fault plane solutions, which also contribute to the understanding of the stress field in the study area. Regarding the temporal properties, seismic excitations are distinguished into earthquake swarm and mainshock – aftershock sequences, taking into account the seismic moment release history in each excitation. Then, the temporal properties of the identified earthquake swarms are investigated in detail by applying stochastic models and examining the interevent time distribution. Regarding the triggering mechanism and the evolution of earthquake swarms, an attempt to associate their occurrence with the existence of fluids is made. The migration of the epicenters, which is commonly observed in earthquake swarms, is investigated in this study. Coulomb stress changes could also play an important role to earthquake triggering and the evolution of an earthquake swarm. Repeating earthquakes (i.e. earthquakes with identical seismograms) are observed during the evolution of earthquake swarms, especially when their occurrence is due to fluid intrusion.Η μελέτη των σμηνοσειρών αποτελεί ένα από τα πλέον σύγχρονα πεδία έρευνας στον τομέα της Σεισμολογίας, λόγω των ιδιαίτερων χωρο-χρονικών τους χαρακτηριστικών και του τρόπου δημιουργίας και εξέλιξής τους. Οι σμηνοσειρές συνιστούν μία κατηγορία σεισμικών εξάρσεων στην οποία απουσιάζει «ο κύριος σεισμός», δηλαδή αυτός που υπερέχει σε μέγεθος σε σχέση με τους υπόλοιπους σεισμούς της έξαρσης. Τα αίτια που προκαλούν τις σμηνοσειρές μπορεί να οφείλονται στην τεκτονική φόρτιση της περιοχής αλλά επίσης και στην ύπαρξη ρευστών ή σε ανθρωπογενείς παράγοντες, όπως στη διάνοιξη γεωτρήσεων και εισπίεση ρευστών που στοχεύει στην οικονομική εκμετάλλευση. Σκοπό της παρούσας διατριβής αποτελεί η διερεύνηση των ιδιοτήτων των σμηνοσειρών στον ελληνικό χώρο με βάση τη μεταβολή των χωρο-χρονικών τους ιδιοτήτων στοχεύοντας στη συσχέτισή τους με τις διαδικασίες γένεσης των ισχυρών σεισμών και τελικό σκοπό τη συμβολή στην εκτίμηση της σεισμικής επικινδυνότητας. Ο κύριος όγκος της έρευνας αφορά την περιοχή του Κορινθιακού κόλπου, με έμφαση στο δυτικό τμήμα του, ο οποίος αποτελεί μία από τις πιο ενεργά σεισμικές περιοχές του ελληνικού χώρου. Επιπλέον, χρησιμοποιήθηκαν δεδομένα από τοπικό πείραμα στην περιοχή της Φλώρινας (ΒΔ Ελλάδα) με σκοπό την ερμηνεία του μηχανισμού δημιουργίας των σμηνοσειρών στην περιοχή. Αρχικά, συλλέγονται τα απαραίτητα σεισμολογικά δεδομένα με σκοπό τη σύνταξη καταλόγων σεισμών για κάθε περιοχή, οι οποίοι ελέγχονται ως προς τις κλίμακες μεγεθών που έχουν χρησιμοποιηθεί και ανάγονται σε μία κλίμακα. Το σημαντικότερο τμήμα της διατριβής αποτελεί ο προσδιορισμός των εστιακών συντεταγμένων με μεγάλη ακρίβεια, χρησιμοποιώντας τις μεθόδους των διπλών διαφορών και της διασυσχέτισης κυματομορφών. Η ακρίβεια στις εστιακές συντεταγμένες κάνει εφικτή τη μελέτη των χωρο-χρονικών ιδιοτήτων των σμηνοσειρών. Έτσι, χρησιμοποιώντας κατάλληλους αλγορίθμους αποσυσταδοποίησης καταλόγων σεισμών, αναγνωρίζονται οι σεισμικές εξάρσεις. Οι σεισμικές εξάρσεις συσχετίζονται με τμήματα ρηγμάτων και προσδιορίζονται οι γεωμετρικές τους ιδιότητες με βάση τη χωρική κατανομή των σεισμών στις τρεις διαστάσεις. Σημαντικό εργαλείο για την επαλήθευση της γεωμετρίας αυτής αποτελούν οι μηχανισμοί γένεσης οι οποίοι συμβάλλουν επιπλέον στην ερμηνεία του πεδίου τάσεων στην περιοχή μελέτης. Ως προς τις χρονικές ιδιότητες οι σεισμικές εξάρσεις διακρίθηκαν σε σμηνοσειρές και εξάρσεις του τύπου μετασεισμικών ακολουθιών με κυριότερο κριτήριο την έκλυση της σεισμικής ροπής σε μία έξαρση. Στη συνέχεια, εξετάστηκαν ενδελεχώς οι χρονικές ιδιότητες κυρίως των σμηνοσειρών εφαρμόζοντας τόσο στοχαστικά μοντέλα, όσο και μελετώντας την κατανομή των χρόνων μεταξύ διαδοχικών σεισμών. Αναφορικά με τον μηχανισμό δημιουργίας και εξέλιξης των σμηνοσειρών, γίνεται συσχέτιση της ύπαρξης ρευστών στις περιοχές μελέτης με την εκδήλωση σμηνοσειρών. Το κυριότερο χαρακτηριστικό που οδηγεί σε αυτή την ερμηνεία είναι η μετανάστευση των εστιών, η οποία παρατηρείται κατά την εκδήλωση των σμηνοσεισμών. Επίσης, σημαντικό ρόλο διαδραματίζουν οι μεταβολές των τάσεων Coulomb στη χωρο-χρονική εξέλιξη των σμηνοσειρών. Κατά τη διάρκεια των σμηνοσειρών, ειδικότερα σε περιπτώσεις όπου η εκδήλωση τους οφείλεται σε ύπαρξη ρευστών, παρατηρούνται επαναληπτικοί σεισμοί, δηλαδή σεισμοί οι οποίοι έχουν όμοια σεισμογράμματα

Multisegment ruptures and Vp / Vs variations during the 2020–2021 seismic crisis in western Corinth Gulf, Greece

International audienceSUMMARY On 2020 December 23, a seismic crisis initiated in the western Corinth Gulf offshore Marathias, lasted several months, and generated thousands of small magnitude earthquakes. The Gulf of Corinth is well known for earthquake swarm occurrence and short-lived burst-like earthquake sequences, mostly triggered by crustal fluids. Here, we perform a detailed seismic analysis aiming to identifying earthquake clusters within the seismic crisis and define their spatial and temporal characteristics. Thanks to the dense seismic station coverage in the area, operated by the Hellenic Unified Seismological Network and Corinth Rift Laboratory, we relocate shallow seismicity and compile a high-resolution earthquake catalogue containing ∼1400 earthquakes spanning the first two months of the seismic crisis. We identify 19 earthquake clusters by applying spatio-temporal criteria and define the geometry (strike and dip) using principal component analysis for 11 of them. Our results are consistent with moment tensor solutions computed for the largest earthquake in each cluster. A striking feature of the seismic activity is the west-towards-east migration with a notable increase in Vp/Vs values for each cluster and a slight increase of the dip angle for the identified fault segments. Furthermore, we find that each cluster contains several burst-like, short interevent time, repeating earthquakes, which could be related to aseismic slip or fluid migration. Overall, we show that the 2020–2021 seismic crisis consists of earthquake clusters that bifurcate between swarm-like and main shock–aftershock-like sequences and ruptured both north- and south-dipping high-angle fault segments. The 2020–2021 seismic activity is located between 5 to 8 km, shallower than the low angle north-dipping (∼10°) seismic zone which hosts long-lived repeating sequences at ∼9–10 km depth. This study supports a hypothesis that the low-angle north-dipping seismicity defines the brittle-ductile transition in the western Corinth Gulf, with seismic bursts occurring at shallower depths in the crust

Revisiting moment tensors in Switzerland: Unraveling source characteristics in Central Alps and their foreland

Studies on moment tensors (MT) and focal mechanisms are of great importance for assessing regional and local seismotectonic processes, especially when a high-quality, dense network is in operation. However, common MT inversion methods are largely restricted to magnitudes > 3.5. In order to lower the completeness of MT catalogs, improved Green’s functions and/or hybrid inversion techniques are needed. In this study, we revisit small-to-moderate earthquakes, which occurred in Switzerland and surrounding regions by means of various MT inversion methods and assess the potential to improve completeness of MT catalogs in Central Alps region. To accomplish this, we implement state-of the art methods for MT inversion using either full waveform data or combinations of first-motion polarities with amplitudes and amplitude ratios. Methods based on full waveform inversion considered in this study are ISOLA (Sokos & Zahradnik 2013) and Grond (Heimann et al. 2018), as well as techniques based on amplitudes and/or polarities (HybridMT (Kwiatek et al. 2016), MTfit (Pugh & White 2018)), which can solve MTs for smaller magnitude earthquakes. Hence, the combination of multiple techniques allows to compute full or deviatoric MTs for a broader range of magnitudes and enrich the existing catalogs. We first apply these methods to recent earthquake sequences occurred in the Central Alps between 2019 and 2021. During that period, several earthquake sequences, like the one associated with the 2021 M4.1 Arolla earthquake, occurred and show complexity on the waveforms, due to their shallow focal depths. In addition, several of the standard MT solutions calculated by the Swiss Seismological Service (SED) for these earthquakes indicate complex moment tensors with unusually high percentage of the CLVD component. To check whether such CLVD component is real and not an artifact caused, for instance, by unmodeled heterogeneities, we invert for full and deviatoric MTs using multiple 1D velocity models and algorithms. Additionally, we perform MT inversions for several earthquakes either within selected earthquake sequences or regional background seismicity. The resulting MT solutions are compared to existing high-quality focal mechanisms computed using first motion polarities as well as to high-precision double difference locations. Uncertainties of MT solutions are estimated using bootstrap-based methods. This work contributes towards an enriched high-quality focal mechanisms database for Switzerland, which could be used to revisit the regional to local stress field at unprecedented resolution and provides new insights into the complexities of active fault systems in the Central Alps region

Analysis of the 2021 Milford, Utah earthquake swarm: Enhanced earthquake catalog and migration patterns

An earthquake swarm occurred in Spring 2021 in south-central Utah near the town of Milford. The University of Utah Seismograph Stations located 125 earthquakes between March 19 and May 10 with magnitudes ranging from 0.5 to 3.2. We implement a matched-filter technique in order to identify additional earthquakes that went undetected during the routine network location. The 125 network-located earthquakes are used as templates and are cross-correlated with continuous data for the dates Feb 17—June 10. This time period corresponds to approximately 1 month before the earthquake swarm began through 1 month after it ended. For the matched-filter analysis, we rely heavily on station FOR1, which is located within 5 km of most template events. Four other stations within 20–30 km of most template events provide a supplement to the closest station. The matched-filter implementation results in the detection of over 600 earthquakes in addition to the original 125 catalog events. This is one of the largest swarms ever recorded in Utah, and no previous large swarms have been recorded in this location. We use HypoDD to obtain relative double-difference locations of the catalog events. Both routine locations and HypoDD relocations of the catalog events suggest a fault dipping west beneath the mountain range, opposite of typical Basin and Range normal faults that dip beneath alluvial valleys. Moment tensors for the largest five events show normal faulting consistent with the west-dipping fault seen in the seismicity. Hydrothermal features in the area, including a geothermal power plant, suggest that fluids may be a contributing factor to the earthquake swarm triggering. We examine the role of fluids by exploring bounds on diffusion parameters and investigating spatial migration characteristics of the swarm seismicity. We conclude that this swarm is the result of heterogenous stress conditions in a prefractured region.ISSN:2296-646

Earthquake swarms and complex seismic sequences in tectonic and volcanic areas

ISSN:2296-646

SCDetect: Near real-time computationally efficient waveform cross-correlation based earthquake detection during intense earthquake sequences

Aftershock sequences or earthquake swarms generate a high number of seismic events that are not detected by standard regional network routine processes. Undetected earthquakes are mostly due to low signal to noise ratio, overlapping earthquakes, and a network configuration that targets earthquake detection with a homogeneous magnitude of completeness. Furthermore, the analyst’s workload is increasing dramatically during an intense earthquake sequence, which results in prompt manual review of the largest events, only. We present a computationally efficient and highly customizable tool (SCDetect) to detect earthquakes in near real-time by applying waveform cross-correlation in the time domain based on a set of template events. SCDetect is a free and open-source SeisComP extension module fully integrated into the SeisComP environment. It may be used to process both archived waveform data, when operated in playback mode, as well as real-time data. In either of the use cases, waveform data is accessed through SeisComP’s standard RecordStream interface. Multiple template event based detectors may be configured. The individual detector configuration is fully stream based which allows for generic multi-stream event detection. Event parameter products for newly detected events (i.e. origins, picks, amplitudes, station magnitudes) may be sent to SeisComP's messaging system for further processing. In addition to earthquake detection, we implement amplitude calculation by measuring amplitudes on the horizontal components. SCDetect offers multiple magnitude estimation methods based on the amplitudes of the template earthquakes and the new detections (i.e regression, amplitude ratios). Magnitude estimation is configurable using SeisComP’s bindings configuration. We applied SCDetect to recent earthquake sequences in Switzerland between 2019 and 2021. The dense seismic network operated by the Swiss Seismological Service offers a unique opportunity to evaluate the performance of the proposed module. Our first results show that these extended earthquake catalogs contain at least ten times more earthquakes than the national earthquake catalogue

Persistent shallow microseismicity near a glacier in southwestern Switzerland (Arolla VS) revealed by enhanced earthquake catalogs

On October 05, 2021 an Mw4.0 earthquake struck 6 km south of the village of Arolla, near the tongue of the Arolla Glacier. Almost one year prior to this earthquake, an M3.5 event occurred on November 08, 2020 in the same location. Both earthquakes were followed by a few aftershocks that were detected and located by the Swiss Seismological Service (SED). The unusually shallow depth of 1-2 km of these earthquakes, indications for a mostly thrust-type mechanisms within a region characterized by a predominantly extensional stress regime, and unusual high CLVD (50-70%) components of SED’s routine moment tensor solutions raised questions regarding the triggering mechanism. To understand and explain the possible existence of shallow thrust earthquakes in the area, we perform a thorough seismotectonic analysis that is based on enhancing the existing earthquake catalog of the SED and complementary moment-tensor solutions computed by multiple algorithms. The original SED earthquake catalog contains 83 earthquakes that occur between January 01, 2020 and December 31, 2021 and locate ~5 km around the two mainshocks. Using a deep learning based algorithm (EQTransformer), we detect additionally 253 events, thus the new catalog contains 4 times more earthquakes than the original SED bulletin. Absolute locations for the additional earthquakes are obtained using the probabilistic NonLinLoc method in combination with a recently updated Vp and Vs crustal 3D velocity model. In addition, we compute local magnitudes (MLhc) using SED’s standard procedure, in order to compile a homogeneous catalog consistent with the SED bulletin. The enhanced catalog events are used as templates for a match filtering scheme, which increased the number of detections by at least one magnitude order. Last, we relocate the final catalog using the double difference method towards obtaining a high resolution enhanced earthquake catalog. Spatially, the main cluster shows an intense seismic activity, stretched in N-S direction that matches the strike of the fault planes derived from moment tensor inversion. An additional cluster, that is not present in the SED bulletin locations, is identified next to the area were the aftershock activity of the two main events locates. Furthermore, the enhanced catalog shows a smother temporal evolution with more background events than previously recorded. Overall, we explore the possibility of fluid driven microseismicity that might be related to the nearby glacier. With our study we emphasize the importance of enhanced earthquake catalogs using both machine learning pickers and template matching algorithms. These approaches lead to unravel prior unmapped structures and improve our understanding of the seismotectonic regime in the study area