Centroid Moment Tensor Solutions Research Articles

Seismic moment tensor solutions and geophysical settings in the Philippine Sea basin mapped by GMT

The study is focused on mapping geophysical settings on the margins of the Philippine Sea Plate (PSP). Active volcanism and seismic situation along the margins of the Philippine Sea Basin (PSB) shows earthquake events demonstrating high seismicity of the region caused by tectonic plate subduction. The aim was to perform cartographic mapping of the earthquake events using Generic Mapping Tools (GMT) modules, to analyze seismic situation through visualization of the geophysical datasets. The data was processed by GMT using multi-source data: GEBCO, CMT and ISC seismic data. Technically, a set of GMT modules was used ('psmeca', 'psvelo', 'grdtrack', 'grdimage') for data modelling and mapping. An application of GMT functionality is presented by selected code snippets with selected code snippets for 'psmeca' and 'psvelo', 'img2grd', 'img2grd' modules. Current paper presents a report on the cartographic techniques applied for geophysical mapping. Centroid moment tensor solutions are visualized for shallow depth earthquakes of Mw <10 along the PSB margins from 1976 to 2010. Earthquakes along the tectonic plate subduction zone are shown by GMT. Data from Global Centroid-Moment-Tensor (CMT) Project. Presented and explained GMT code snippets contribute to the development of methods in geological mapping using GMT scripting toolset, for visualizing focal mechanisms.

Relation between rheological properties and the stress state in subducting slabs

The distribution of different stress states in the subducting slab indicated by centroid moment tensor solutions for intra-slab earthquakes can help constrain the rheological properties of the slabs. A comparison of slabs in the western and eastern Pacific realms shows contrasting patterns in the stress states down to depths of ~ 350 km. The majority of slabs in the western Pacific show a pair of down-dip compression (DC) and down-dip tension (DT) domains in the upper and lower parts of the slab reflecting the effects of the slab unbending during progressive subduction. In contrast, slabs in the eastern Pacific show predominantly in-plane DT stress irrespective of slab geometry. Two-dimensional numerical simulations assuming constant slab thickness and viscosity indicate that the development of slabs with in-plane DT stress at depths of 100–300 km requires the slabs to be thin and have a low viscosity (1023 Pa s). Weak slabs bend easily and tend to fold when they encounter increased resistance to downward movement at the 660-km boundary. The associated DC stresses are not transmitted up the slab so negative buoyancy of the slab and DT stress dominates at intermediate depths for this type of slab. Most experimentally derived rheological parameters predict a high viscosity (> 1024 Pa s) for such slabs. However, two-dimensional numerical simulations using temperature- and pressure-dependent viscosity show that a relatively low activation energy (~ 110 kJ/mol) for diffusion creep is a possible explanation for the observed distribution of stresses in the slabs. Such low activation energies are compatible with recent experimental work on diffusion creep of polyphase mantle materials in which a low effective activation energy for creep results from a slow grain growth due to pinning effect of the secondary phase. The simulations provide a mechanical explanation for the observed dominantly DT stress state at 100–300 km depths for young slabs and paired DT and DC stress states at the same depth range for old slabs.Graphical

Rupture Pattern of the 2015 Alor Earthquake Sequence, Indonesia

Abstract A sequence of earthquakes occurred on Alor Island, Nusa Tenggara Timur, Indonesia, beginning in November 2015 with the mainshock (Mw 6.2) on 4 November 2015. We calculate the centroid moment tensor (CMT) solutions for nine of the earthquakes with Mw≥3.9, which occurred between November 2015 and March 2016 using records from a temporary array of 30 broadband instruments in eastern Indonesia and Timor Leste (YS network). Our CMT results reveal an interesting pattern of ruptures in this order: (a) three foreshocks of Mw 4–5.3 all with strike-slip mechanisms that occurred with a centroid depth of ∼13 km in the three days prior to the mainshock, (b) the mainshock on 4 November 2015, with Mw 6.2 that occurred with a deeper centroid (∼25 km) and a strike-slip mechanism similar to the foreshocks, (c) followed by five aftershocks with Mw>3.9 at depth ∼3–15km. We further determine the fault plane and rupture direction of the mainshock and the largest foreshock (Mw 5.3) by relocating the hypocenter and examining its geometrical location with respect to the centroid. We find that the fault plane strikes 97°±9° from north and that the fault ruptures westward. We propose that the rupture of this sequence of events initiated at depth ∼10 km, propagating westward and triggering the mainshock to rupture at a deeper depth (within lower crust) on a similar faulting system. The aftershocks migrate back to shallower depths and occur mainly at depth <10 km.

Early Source Characterization of Large Earthquakes Using W Phase and Prompt Elastogravity Signals

Abstract In the minutes following a large earthquake, robust characterization of the seismic rupture can be obtained from full wavefield records at local distances or from early signals recorded by regional broadband seismometers. We focus here on the latter configuration, and evaluate the individual and joint performances of the early low-frequency elastic phases (W phase) and the recently discovered prompt elastogravity signals (PEGS). The 2011 Mw 9.1 Tohoku–Oki earthquake is a natural target for this evaluation, because the high quality of global and regional networks enabled to gather the best PEGS data set so far. We first confirm that the well-established W-phase method, using records from global seismological networks, is able to provide a reliable centroid moment tensor solution 22 min after the earthquake origin time. Using regional stations, an accurate W-phase solution can be obtained more rapidly, down to 10 min after origin time. On the other hand, a PEGS-based source inversion can provide even earlier, starting 3 min after origin time, a lower bound of the seismic moment (Mw 8.6) and constraints on the focal mechanism type. However, relying solely on PEGS introduces uncertainties caused by the hindering seismic noise and trade-offs between source parameters that limit the accuracy of source determination. We show that incorporating even a few early W phase signals to the PEGS data set reduces these uncertainties. Using more complete W phase and PEGS data sets available 5 min after origin time enables to converge towards a result close to the Global Centroid Moment Tensor solution.

Joint inversion of centroid moment tensor for large earthquakes by combining teleseismicP-wave andW-phase records

SUMMARYCentroid moment tensor inversion involves the determinations of the moment tensor solution, which is usually done with low-frequency signals to meet the requirement of the point-source approximation, and the moment centroid, which requires high-frequency signals to improve the resolution. Traditional centroid moment tensor inversion techniques, such as the W-phase inversion, mainly use low-frequency data to estimate the magnitude and fault parameters, which limits the resolution of the moment centroid. In this study, we combine the P wave and W phase to constrain both the moment tensor and moment centroid. The rupture directivity is considered in the P-wave inversion, and the rupture velocity is resolved by inverting the P wave solely. The moment centroid is estimated by the rupture velocity from the P-wave inversion and the grid search in the W-phase inversion. The final moment tensor solution is determined based on the moment centroid by jointly inverting both P-wave and W-phase data. The resulting centroid moment tensor solution can fit a broad frequency band (0.001–0.1 Hz) of waveforms. Through synthetic inversion tests and applications to several large earthquakes, we demonstrate that the magnitudes and fault parameters from our joint inversion are more stable than the P-wave inversion, and the moment centroids, especially the centroid depths, seem more reasonable than those from the W-phase inversion.

The 2022 Delingha, China, Earthquake Sequence and Implication for Seismic Hazard near the Western End of the Qilian–Haiyuan Fault

Abstract Although the Qilian–Haiyuan fault is known to be responsible for major earthquakes up to M 8, the potential of damaging earthquakes near its western end is not well understood. Since January 2022, three moderate earthquakes (M 5.8, M 6.0, and M 5.4) occurred around the Halahu region of Delingha, China, near the western end of the Qilian–Haiyuan fault. These earthquakes are unusual M 5+ events in this low-seismicity region, and both the U.S. Geological Survey and Global Centroid Moment Tensor solutions suggest that the focal mechanisms of the three mainshocks are distinct from the activity characteristics of the nearby mapped faults. Thus, determining the precise source parameters and identifying the causative fault of this earthquake sequence are important to analyze its seismogenic settings and seismic hazard in this region. In this article, we determined the point-source parameters of the three moderate events via regional waveform modeling and found that these earthquakes are strike-slip events with the nodal planes striking nearly north–south and east–west directions. We then resolved its seismogenic faults by analyzing the aftershock distribution and the rupture directivity. The results show that both the M 5.8 and M 6.0 events ruptured along the north–south nodal plane and expanded toward the south for ∼4 km and ∼3 km, respectively, and the ruptured fault of the M 5.4 is difficult to distinguish. Furthermore, we constrained the relative location using the Interferometric Synthetic Aperture Radar observation, and the result is consistent with that obtained from seismic waveform data. We proposed that this earthquake sequence ruptured along an unmapped dextral fault, which forms a conjugate fault system with the sinistral strike-slip Qilian–Haiyuan fault. Its deformation mechanism may be controlled by long-term, protracted, nearly north–south-trending, right-lateral simple shear in the Qilian Shan fold-thrust belt. The accumulated static Coulomb stress changes resulted in the northwest of this ruptured fault being closer to failure in the future.

Exploring the link between large earthquakes and magma transport at the onset of the Mayotte volcano-seismic crisis

The archipelago of Comoros was generally considered a moderately seismic region. However, since May 2018, unusual seismicity has been observed off the east coast of Mayotte Island. Following this increase of seismic activity, oceanographic campaigns led to the discovery of a new submarine volcano, indicating that the observed seismicity had a volcanic origin. In this study, we estimate Centroid Moment Tensor (CMT) solutions of M w ≥5 earthquakes of this sequence using 3D Green’s functions and analyze their non-double-couple (non-DC) components. Consistently with previous reports, our results indicate that seismicity migrated upward in May–June 2018 with an increasing number of non-DC events. We show that non-DC components observed in our solutions and previously published catalogs cannot correspond to dike opening or closing as the observed rupture durations suggest unrealistically large magma flow rates. Given that waveforms can be relatively well explained with pure-shear sources, we postulate that non-DC components are most likely artifacts due to unmodeled shallow structural heterogeneities. Most M w ≥5 earthquakes have a strike-slip mechanism consistent with the rupture of pre-existing faults loaded by the regional stress regime and triggered by the increment of stress produced by the upward magma transfer.

Rapid Magnitude Determinations for Tsunami Early Warning: A sample case, October 30, 2020, Samos Island Earthquake

It is vital to calculate the magnitude of an earthquake to give a response and disseminate the early warning to evacuate the people as soon as possible. Generally, the magnitude determination process takes a couple of minutes and has some difficulties because the propagation effect depends on the epicentral distances. The moment magnitude is calculated for large earthquakes, in general, using body waves that it takes at least ten minutes to calculate. This time is too much for giving an alarm and evacuating the people who live near the coastal areas. This study is concentrated improve and implementing the P-wave moment magnitude calculations to use in early warning issues by using a sample case that occurred on October 30, 2020, on Samos Island that is near to Aegean coast of Turkey. It was destructive effects on İzmir. In total, 115 people have lost their lives and more than a thousand people are wounded. The magnitude of this event was announced as 6.6 (Mw) by the Disaster and Emergency Presidency (AFAD), Earthquake Institute. A small-scale tsunami wave arrived in the Sığacık-Seferihisar region. The P-wave moment magnitude is calculated as 7.0 in this study, and it is similar to the Global Centroid Moment Tensor Solution (Mw-GCMT) result. This study shows the technique can be used for early warning purposes at regional distances.

Rupture Characteristics Analysis of the 2020 Mw 7.4 Oaxaca, Mexico Earthquake Using Teleseismic, High-Rate GPS, and InSAR Data

The June 23 2020 Oaxaca Mw 7.4 interplate thrust earthquake struck the state of Oaxaca in Mexico, generating strong shaking and a long-lived tsunami. This earthquake is well recorded by the teleseismic, high-rate Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data, which provides an opportunity to understand the rupture characteristics of the Mexican subduction zone. Here, an integrated inversion strategy involving centroid moment tensor inversion and kinematic finite-fault inversion is used to study the rupture history of the 2020 Oaxaca earthquake. The fault geometry and source duration time derived from the centroid moment tensor solution are used as prior information in linear kinematic finite-fault joint inversion. The rupture initial point and relative weight of each dataset are determined to estimate a well-constrained rupture model. The finite-fault model shows the rupture expanded bilaterally around the hypocenter, the peak slip is 3.5 m, the main slip was located at a depth of 15–30 km, the whole rupture lasted about 20 s, and a 95% moment rate was released at 15 s. The half-duration of the finite-fault inversion is consistent with the centroid moment tensor inversion results (half-duration 9 s), which shows the good resolution of the temporal information. The total scalar moment was 1.5 × 1020 Nm, equivalent to a moment magnitude of Mw 7.4. The integrated inversion strategy used in this study is useful since the prior information can be derived and used to constrain the rupture process. Both the centroid moment tensor and finite-fault inversion mainly rely on identical temporal information provided by teleseismic P waveforms. The 2020 Oaxaca earthquake was mainly the interaction between Cocos and the North American plate, and the slow slip events may be the key factor affecting the seismogenic zone width in the Oaxaca region.

Global centroid moment tensor solutions in a heterogeneous earth: the CMT3D catalogue

SUMMARYFor over 40 yr, the global centroid-moment tensor (GCMT) project has determined location and source parameters for globally recorded earthquakes larger than magnitude 5.0. The GCMT database remains a trusted staple for the geophysical community. Its point-source moment-tensor solutions are the result of inversions that model long-period observed seismic waveforms via normal-mode summation for a 1-D reference earth model, augmented by path corrections to capture 3-D variations in surface wave phase speeds, and to account for crustal structure. While this methodology remains essentially unchanged for the ongoing GCMT catalogue, source inversions based on waveform modelling in low-resolution 3-D earth models have revealed small but persistent biases in the standard modelling approach. Keeping pace with the increased capacity and demands of global tomography requires a revised catalogue of centroid-moment tensors (CMT), automatically and reproducibly computed using Green's functions from a state-of-the-art 3-D earth model. In this paper, we modify the current procedure for the full-waveform inversion of seismic traces for the six moment-tensor parameters, centroid latitude, longitude, depth and centroid time of global earthquakes. We take the GCMT solutions as a point of departure but update them to account for the effects of a heterogeneous earth, using the global 3-D wave speed model GLAD-M25. We generate synthetic seismograms from Green's functions computed by the spectral-element method in the 3-D model, select observed seismic data and remove their instrument response, process synthetic and observed data, select segments of observed and synthetic data based on similarity, and invert for new model parameters of the earthquake’s centroid location, time and moment tensor. The events in our new, preliminary database containing 9382 global event solutions, called CMT3D for ‘3-D centroid-moment tensors’, are on average 4 km shallower, about 1 s earlier, about 5 per cent larger in scalar moment, and more double-couple in nature than in the GCMT catalogue. We discuss in detail the geographical and statistical distributions of the updated solutions, and place them in the context of earlier work. We plan to disseminate our CMT3D solutions via the online ShakeMovie platform.

Full-waveform centroid moment tensor inversion of passive seismic data acquired at the reservoir scale

SUMMARY Passive seismic inversion at the reservoir scale offers the advantages of low cost, negligible environmental impact and the ability to probe a target area with low-frequency energy not afforded by even the most modern active-source seismic technology. In order to build starting models suitable for full-waveform wave speed tomography, characterization of earthquake sources is an indispensable first step. We present a workflow for the centroid moment tensor (CMT) inversion of seismic events identified in a passive seismic data set acquired by a large and dense array of three-component broad-band seismic sensors in a mountainous setting in the Himalayan foothills. The data set comprised 256 instruments operating for 2×4 months over an area of 8000 km2. An initial 3-D wave speed model was determined for the region via the analysis of first-arriving traveltime picks. Of the 2607 identified seismic events that were well recorded at frequencies between 0.2–50 Hz, 86 with magnitudes 1.3 ≤ M ≤ 3.0 initially had their CMT focal mechanisms determined by a waveform fitting procedure built on a Green’s function approach in a 1-D layered average wave speed model, for stations within an offset of 10 km, in the frequency range 0.2–1.4 Hz. Here, we obtain updated CMT mechanisms for the 86 events in that catalogue via multicomponent full-waveform inversion in the 3-D wave speed model. Our workflow includes automated data- and model-driven data selection using a combination of different metrics derived from signal-to-noise considerations and waveform-fitting criteria, and relies upon spectral-element simulations of elastic wave propagation in the 3-D wave speed model, honouring topography. Starting from the initial CMT solutions, we seek improvement to the data fit within the frequency band 0.5–2.5 Hz by minimizing the waveform difference between observed and synthetic data, while accommodating wave speed-model errors by allowing for small time-shifts. We balance uneven data coverage and tune their contributions via data-space weighting functions. We quantify the improvements to the data fit in terms of different metrics. We summarize the changes to the CMT solutions, and present and analyse the resulting catalogue for the region, including their breakdown into double-couple and non-double couple components, and their relation to mapped faults.

Surface Rupture on a Secondary Fault Associated with the 8 August 2020 Mw 5.1 Sparta North Carolina Earthquake

Abstract On 8 August 2020, northwest North Carolina experienced an Mw 5.1 earthquake that caused damage to buildings and roads in the city of Sparta. A regional centroid moment tensor solution shows that the earthquake was the result of slip on a reverse fault with a minor strike-slip component. Interferometric Synthetic Aperture Radar (InSAR) data, from the Japan Aerospace Exploration Agency’s Advanced Land Observing Satellite #2 (ALOS2) satellite, reveal a deformation field that is more complex than expected from a single reverse fault earthquake. The data also reveal an apparent fault rupture at the Earth’s surface that caused damage to local roads. Modeling of the InSAR deformation field indicates the fault rupture is associated with a very shallow normal faulting event with an equivalent Mw of about 5.1 that overprinted the reverse fault deformation field and possibly occurred aseismically.

Tectonic Geomorphic of Sulawesi Island and Its Implication for Future Large Earthquake

This paper analyzes how resource of past and prospective great earthquake on the Central Sulawesi Arm, adhere on topography analysis from several space-based source. To answer the question, we analysis the tectonic geomorphic, stream pattern, exhumed fault, geological mapping and seismicity data. Detailed tectonic geomorphic studies in Sulawesi still lacking due to tectonic and fault obscures. For instance, Palu Koro Fault (PKF) was unpredictable, because the historical seismic records inevitably remain poorly documented and unrecognized fault strand, which was buried beneath abundant Quaternary alluvium subsequently obscured the fault trace. In other hand, the faults have been active during Quaternary must take into account because potentially dangerous, also the inactive faults during instrumental period must be re-evaluated in order to have awareness for large future large earthquake. Surprisingly, recent seismic activity of PKF generate super shear rupture a Mw 7.5 earthquake on 28th September 2018 with average slip 41 mm/year, which over the past two decade quiet from any seismic activity. The seismic potential for large fault is essential, since it has been silent during the instrumental period. Therefore, our motivation in this study to produce detail tectonic geomorphic map of the region in local scale, which is currently not available to prepare better knowledge and awareness for the large future earthquake. We have use Shuttle Radar Topography Mission (SRTM) with resolution ~30m, which run by ArcGIS software to observed tectonic geomorphic evidence of fault system and supplement with structural, geological and bathymetric data’s as ware available to us. We relate this analysis with seismicity data from Centroid Moment Tensor Solution (CMT) to recognize the seismic source. Our results show the tectonic geomorphic of Central Sulawesi Arm due to nature extension of NNW-SSE left-lateral slip curving to WNW-ESE of Palu-Koro Fault (PKF), then transcript to N-S circular normal fault of Poso Fault (PF). The PF indicate replica of PKF curving, where has not been mapped previously. We have mapped 60 major onshore fault systems, 10 faults showed evidence maximal to rapid rate tectonic activity along instrumental periods. Based on our CMT analysis, Sulawesi Island is greatly dominated by oblique fault.

New insight into the 24 January 2020, Mw 6.8 Elazığ earthquake (Turkey): An evidence for rupture-parallel pull-apart basin activation along the East Anatolian Fault Zone constrained by Geodetic and Seismological data

In this study, we investigated the main features of the causative fault of the 24 January 2020, Mw 6.8 Elazığ earthquake (Turkey) using seismological and geodetic data sets to provide new insight into the East Anatolian Fault Zone (EAFZ). We first constrained the co-seismic surface deformation and the rupture geometry of the causative fault segment using Interferometric Synthetic Aperture Radar (InSAR) interferograms (Sentinel-1A/B satellites) and teleseismic waveform inversion, respectively. Also, we determined the centroid moment tensor (CMT) solutions of focal mechanisms of the 27 aftershocks using the regional waveform inversion method. Finally, we evaluated the co-seismic slip distribution and the CMT solutions of the causative fault as well as of adjacent segments using the 27 focal solutions of the aftershocks, superimposed on the surface deformation pattern. The CMT solution of the 24 January 2020Elazığ earthquake reveals a pure strike-slip focal mechanism, consistent with the structural pattern and left-lateral motion of the EAFZ. The rupture process of the Elazığ event indicated that the rupture is started at 12 km around the hypocenter, and then propagated bilaterally along the NE-SW but mainly toward the southwest. The rupture slip has initially propagated toward the southwest (first 10 s) and northeast (4 s), and again toward the southwest (9 s). Maximum displacement is calculated as 1.3 m about 20 km southwest of the hypocenter at 6 km depth (centroid depth). The rupture stopped to down-dip around 20 km depth toward the southwest. The distribution of the slip vectors indicates that the rupture continued mostly through a normal oblique movement. Most of the moment release was released SW of the hypocenter and the rupture reached up to around 50 km. The focal mechanisms of analyzed 27 aftershocks show strike-slip, but mostly normal and normal oblique-slip faulting with an orientation of the tensional axes (NNE-SSW), indicating a normal oblique-slip, “transtensional” stress regime, parallel-subparallel to the strike of the EAFZ, consistent with SW-rupture directivity and co- seismic deformation pattern. Finally, based on the co-seismic surface deformation compatible with the distributional pattern of normal focal solutions, normal and normal oblique-slip focals of the aftershocks evidence the rupture-parallel pull-apart basin activation as a segment boundary of the left-lateral strike-slip movement of the EAFZ.

The <i>M</i><sub>w</sub> 7.5 Tadine (Maré, Loyalty Islands) earthquake and related tsunami of 5 December 2018: seismotectonic context and numerical modeling

Abstract. On 5 December 2018, a magnitude Mw 7.5 earthquake occurred southeast of Maré, an island of the Loyalty Islands archipelago, New Caledonia. This earthquake is located at the junction between the plunging Loyalty Ridge and the southern part of the Vanuatu Arc, in a tectonically complex and very active area regularly subjected to strong seismic crises and earthquakes higher than magnitude 7 and up to 8. Widely felt in New Caledonia, it was immediately followed by a tsunami warning, confirmed shortly after by a first wave arrival at the Loyalty Islands tide gauges (Maré and Lifou), and then along the east coast of Grande Terre of New Caledonia and in several islands of the Vanuatu Archipelago. Two solutions of the seafloor initial deformation are considered for tsunami generation modeling, one using a non-uniform finite-source model from USGS and the other being a uniform slip model built from the Global Centroid Moment Tensor (GCMT) solution, with the geological knowledge of the region and empirical laws establishing relationships between the moment magnitude and the fault plane geometry. Both tsunami generation and propagation are simulated using the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM), an open-source modeling code solving the shallow-water equations on an unstructured grid allowing refinement in many critical areas. The results of numerical simulations are compared to tide gauge records, field observations and testimonials from 2018. Careful inspection of wave amplitude and wave energy maps for the two simulated scenarios shows clearly that the heterogeneous deformation model is inappropriate, while it raises the importance of the fault plane geometry and azimuth for tsunami amplitude and directivity. The arrival times, wave amplitude and polarities obtained with the uniform slip model are globally coherent, especially in far-field locations (Hienghène, Poindimié and Port Vila). Due to interactions between the tsunami waves and the numerous bathymetric structures like the Loyalty and Norfolk ridges in the neighborhood of the source, the tsunami propagating toward the south of Grande Terre and the Isle of Pines is captured by these structures acting like waveguides, allowing it to propagate to the north-northwest, especially in the Loyalty Islands and along the east coast of Grande Terre. A similar observation results from the propagation in the Vanuatu islands, from Aneityum to Efate.

Recent felt earthquakes (Mw 5.0–5.9) in Mizoram of north‐east India region: Seismotectonics and precursor appraisal

Eight medium magnitude (Mw 5.0–5.9) severely felt earthquakes, mostly at a shallower depth (<35 km), occurred in a short period of 7 months only, April–October, 2020, in Mizoram State, north‐east region (NER) of India, and it created much panic in the state. The increasing population growth as well as rapid urbanization in Aizawl, the capital city of Mizoram, may be under severe threat for an impending large earthquake in the region. The NER, India, is jawed between two arcs, the Himalayan Arc collision zone to the north and the Indo–Burma (Myanmar) Arc subduction zone to the east, and witnessed two great earthquakes (Mw ≥ 8.0, 1897 and 1950) and a total of 25 large earthquakes (Mw ≥ 7.0) in the past since 1869. The 2020 sudden burst of seismic activity in Mizoram, along with a strong (Mw 6.7) earthquake in Manipur in 2016 and a medium magnitude (Mw 5.7) earthquake in 2017 in Tripura, adjacent to the Mizoram State, are critically examined to understand the ongoing seismotectonics and the precursor implications, if any. The 2016 Manipur earthquake is interpreted to be an intra‐plate earthquake triggered by the transverse Kopili Fault by strike‐slip motion. The 2017 Tripura earthquake is also interpreted to be typically an intra‐plate strike‐slip event in the eastern margin of the Bengal Basin. The seismic activity in Mizoram, on the other hand, is found to have occurred in the accretionary Indo–Burma Wedge, between the Churachandpur Mao Fault and Eastern Boundary Thrust. The centroid moment tensor solutions of the events suggest the dextral‐slip motion of the wedge along the Indo–Burma Arc. A preliminary analysis of the Mizoram seismicity recognizes a precursor anomaly, a temporal variation of much higher seismic activity, for an impending large earthquake in the study area.

Bayesian Inference of Centroid Moment Tensors of the 2019 Ambon (Mw 6.5) Aftershock Earthquake Sequence, Indonesia: A Preliminary Result

The Ambon Mw 6.5 earthquake on September 26th, 2019, had contributed to give severe damages and significantly increased seismicity around Ambon Island and surrounding areas. Mainshock was followed by aftershocks with spatial distribution added to the impact of destructions in this region. We investigated aftershocks sequences to reveal the effect of mainshock toward the change in the in-situ stress field, including the possibility of the existing faults reactivation and the generation of aftershocks. We inferred centroid moment tensor (CMT) for significant aftershock events with Mw more than 4.0 using waveform data recorded from October 18th to December 15th, 2019. The aftershock focal mechanism was determined using the Bayesian full-waveform inversion code ISOLA-Obspy. This approach provides the uncertainty of the CMT model parameters. From ten CMT solution we had inferred in three seismic clusters, we found that majority of events have a strike-slip mechanism. Four events located on the south of the N-S trendings have a dextral strike-slip fault type, reflected the rupture of the mainshocks fault plane. Three events in the cluster of Ambon Island are dextral strike-slip, confirming the presence of the fault reactivation. Meanwhile, three CMT solutions in the north show the dextral strike-slip faulting and may belong to the mainshock main fault, connected with the cluster in the south.

Probabilistic Near‐Field Tsunami Source and Tsunami Run‐up Distribution Inferred From Tsunami Run‐up Records in Northern Chile

AbstractUnderstanding a tsunami source and its impact is vital to assess a tsunami hazard. Thanks to the efforts of the tsunami survey teams, high‐quality tsunami run‐up data exist for contemporary events. Still, it has not been widely used to infer a tsunami source and its impact mainly due to the computational burden of the tsunami forward model. In this study, we propose a TRRF‐INV (Tsunami Run‐up Response Function‐based INVersion) model that can provide probabilistic estimates of a near‐field tsunami source and tsunami run‐up distribution from a small number of run‐up records. We tested the TRRF‐INV model with synthetic tsunami scenarios in northern Chile and applied it to the 2014 Iquique, Chile, tsunami event as a case study. The results demonstrated that the TRRF‐INV model can provide a reasonable tsunami source estimate to first order and estimate tsunami run‐up distribution well. Moreover, the case‐study results agree well with the United States Geological Survey report and the global Centroid Moment Tensor solution. We also analyzed the performance of the TRRF‐INV model depending on the number and the uncertainty of run‐up records. We believe that the TRRF‐INV model has the potential for supporting accurate hazard assessment by (1) providing new insights from tsunami run‐up records into the tsunami source and its impact, (2) using the TRRF‐INV model as a tool to support existing tsunami inversion models, and (3) estimating a tsunami source and its impact for ancient events where no data other than estimated run‐up from sediment deposit data exist.

Complementing regional moment magnitudes to GCMT: a perspective from the rebuilt International Seismological Centre Bulletin

Abstract. Seismologists and geoscientists often need earthquake catalogues for various types of research. This input usually contains basic earthquake parameters such as location (longitude, latitude, depth, and origin time), as well as magnitude information. For the latter, the moment magnitude Mw has become the most sought after magnitude scale in the seismological community to characterize the size of an earthquake. In this contribution we provide an informative account of the Mw content for the newly rebuilt Bulletin of the International Seismological Centre (ISC, http://www.isc.ac.uk, last access: May 2021), which is regarded as the most comprehensive record of the Earth's seismicity. From this data, we extracted a list of hypocentres with Mw from a multitude of agencies reporting data to the ISC. We first summarize the main temporal and spatial features of the Mw provided by global (i.e. providing results for moderate to great earthquakes worldwide) and regional agencies (i.e. also providing results for small earthquakes in a specific area). Following this, we discuss their comparisons, by considering not only Mw but also the surface wave magnitude MS and short-period body wave magnitude mb. By using the Global Centroid Moment Tensor solutions as an authoritative global agency, we identify regional agencies that best complement it and show examples of frequency–magnitude distributions in different areas obtained both from the Global Centroid Moment Tensor alone and complemented by Mw from regional agencies. The work done by the regional agencies in terms of Mw is fundamental to improve our understanding of the seismicity of an area, and we call for the implementation of procedures to compute Mw in a systematic way in areas currently not well covered in this respect, such as vast parts of continental Asia and Africa. In addition, more studies are needed to clarify the causes of the apparent overestimation of global Mw estimations compared to regional Mw. Such difference is also observed in the comparisons of Mw with MS and mb. The results presented here are obtained from the dataset (Di Giacomo and Harris, 2020, https://doi.org/10.31905/J2W2M64S) stored at the ISC Dataset Repository (http://www.isc.ac.uk/dataset_repository/, last access: May 2021).

Seismotectonics and stress perspective of the Kumaon Himalaya: A geophysical evidence of a Lesser Himalayan duplex

The Kumaon Himalaya lies in the central seismic gap and therefore bears the potential to host a great Himalayan earthquake. The seismicity and seismotectonics of the Kumaon Himalaya and the adjacent region have been investigated based on local earthquake data recorded at 18 seismological stations. The seismically active zone of the eastern segment of the Kumaon Himalaya deviates from the usual pattern of seismicity in the Himalayan Seismic Belt of the NW Himalaya. Shallow-focus earthquakes in this region largely concentrate in the Chiplakot Crystalline Belt (CCB) immediately south of the Vaikrita Thrust. The Focal Mechanism Solutions of 41 earthquakes computed through waveform inversion technique along with the solutions of 12 earthquakes obtained from the Global Centroid Moment Tensor solution catalog are used to investigate the kinematics of the region. The study reveals a complex faulting pattern in the Inner Lesser Himalaya. The stress inversion results show a widely distributed stress pattern and low frictional coefficient which are attributed as one of the major causes of clustered seismicity observed in the CCB. Careful examination of the fault orientations indicate the presence of a hinterland dipping Lesser Himalayan Duplex over the ramp structure on the Main Himalayan Thrust. The high compressive stress and deformation rate in the CCB are partially accommodated by this duplex structure. The large concentration of shallow-focus earthquakes in the CCB is the result of the presence of fluid-rich zone as well as strain localization and large stress build-up due to locking in the ramp structure on the Main Himalayan Thrust beneath the CCB.