Global Centroid Moment Tensor Research Articles

Tectonic activity in Gulf of Guinea and Sub-Sahara West Africa: A validation of Freeth (1977) using focal mechanism solutions

Fault plane solutions for a group of 104; 4.0 ≤ Mw ≤ 7.1 earthquakes between January 1979 and December 2016, extracted from the Global Centroid Moment Tensor Project catalog. Were used to investigate the regional tectonic stress regime of the Gulf of Guinea region. The idea is to validate the theory of membrane tectonics put forward by Freeth (1977)[1] in which the tectonic of the Gulf of Guinea and the sub-Sahara West Africa region were described based on Freeth (1977)[1]. The tectonic of the Gulf of Guinea and the sub-Sahara West Africa region are based on the movement of the African plate, we emphasized the use of rigorous statistical tests to decide on the quality and variability of the earthquake focal mechanisms (FMSs) utilized for the stress tensor inversion analysis. To constrain our analysis, we have applied both the Algorithm of Michael and Gauss technique in our stress tensor inversion analysis of FMS obtained from the region, and the results are found to be coherent and in good agreement with each other. Both Michael (1984)[2] and Zalohar and Vrabec (2007)[3] techniques show that the regional tectonic stress regime of the Gulf of Guinea and the sub-Sahara West Africa is extensional, which is in good agreement with the work of Freeth (1977)[1]. However, our investigation concluded that the orientation of the extensional stress regime is the same as the orientation of the movement of the African plate, which is towards the Euro-Asia plate.

Stress Variations in Southern Tonga Slab Derived From Deep‐Focus Earthquakes

AbstractTonga is a convergent plate boundary between the Pacific and Australian plates and is the fastest and the most seismically active deep subduction system in the world. We focused on southern Tonga (south of latitude 22°S) and the mantle transition zone (depths of 410–670 km), where seismic activity forms two subparallel bands of events in the east and west. We performed stress analysis by inverting focal mechanisms of earthquakes available in the Global Centroid Moment Tensor catalog and revealed two distinct stress regimes in the slab. While the stress orientation in the eastern slab segment conforms to the down‐dip compressional stress along the entire slab, the stress orientation in the western slab segment is different, having the maximum compression in the vertical direction. This suggests that the western segment can represent a stagnant slab with flattening and bending, as proposed by modeling studies. Its connection with the younger actively subducting slab is supported by the horizontal westward shift at 520 km depth. The stress analysis also indicates substantially different fault orientations in both segments. In the actively dipping slab, the majority of activated faults are predominantly sub‐horizontal. However, they are significantly inclined from vertical in the stagnant slab segment. A higher scatter in fault orientations in the stagnant slab suggests deformation, fragmentation and rheological complexity resulting from bending and flattening.

Apparent Non-Double-Couple Components as Artifacts of Moment Tensor Inversion

Compilations of earthquake moment tensors from global and regional catalogs find pervasive non-double-couple (NDC) componentswith a mean deviation from a double-couple (DC) source of around 20%. Their distributions vary only slightly with magnitude, faulting mechanism, or geologic environments. This consistency suggests thatfor most earthquakes, especially smaller ones whose rupture processes are expected to be simpler, the NDC components are largely artifacts of the moment tensor inversion procedure. This possibility is also supported by the fact that NDC components for individual earthquakes with Mw<6.5 are only weakly correlated betweencatalogs. We explore this possibility by generating synthetic seismograms for the double-couple components of earthquakes around theworld using one Earth model and inverting them with a different Earth model. To match the waveforms with a different Earth model, the inversion changes the mechanisms to include a substantial NDC component while largely preserving the fault geometry (DC component). The resulting NDC components have a size and distribution similar to those reported for the earthquakes in the Global Centroid Moment Tensor (GCMT) catalog. The fact that numerical experiments replicate general features of the pervasive NDC components reported in moment tensor catalogs implies that these components are largely artifacts of the inversions not adequately accounting for the effects of laterally varying Earth structure.

Which Global Moment Tensor Catalog Provides the Most Precise Non-Double-Couple Components?

Abstract The availability of digital seismic waveform data enabled compilation of seismic moment tensor catalogs that provide information about earthquake source processes beyond what could be derived from earlier methods that assume double-couple sources representing slip on planar faults. This additional versatility involves additional complexity. Moment tensors are determined by inversions minimizing the misfit between observed and synthetic waveforms, and depend on the specifics of the data inverted, the inversion algorithm, and the Earth structure assumed. Hence, substantial uncertainties arise in moment tensors and quantities derived from them, which can be assessed by comparing moment tensors from multiple global and regional catalogs using different data and inversion procedures. While the double-couple (DC) components of moment tensors are generally determined with greater certainty, non-double-couple (NDC) components for the same earthquake sometimes differ significantly between catalogs. This observation raises questions about the reliability of their determination and hence their geological significance. Using the correlation between NDC components in different catalogs, we quantify the reliability of NDC components in moment tensor catalogs through the determination of the effects of unmodeled and inaccurately modeled effects contained in them. We determine that the NDC components in the Global Centroid Moment Tensor catalog are, on average, more precise than in other catalogs, and thus studies on NDC components should be based on this catalog. Furthermore, their uncertainties are largely unrelated to uncertainties in the DC components. Therefore, the reliability of fault angles derived from a moment tensor is largely independent from the reliability of its NDC components.

SEISMICITY of the ARCTIC in 2018–2019

The article provides an overview and analysis of seismicity within the boundaries of the Arctic region for 2018–2019, a description of seismic station networks, and processing methods. The consolidated catalog of earthquakes in the Arctic region was compiled on the basis of catalogs of several organizations and seismological centers. In total, 1177 earthquakes for 2018–2019 are included in the consolidated catalog. Most of the earthquakes that occurred in 2018–2019, including all the strongest earthquakes, were located within the mid-ocean ridges of Mon, Knipovich. and Gakkel. In the offshore territories, most of the earthquakes were confined to the Svalbard archipelago, in particular, to the seismically active zone in the Sturfjord strait. Within the shelf areas, seismicity is also characteristic of the "continent-ocean" transition zone of the Barents-Kara region, Bely (Kvitøya) Island and Novaya Zemlya archipelago. For 28 earthquakes, the focal mechanism parameters are presented according to the Global CMT catalog.

Seismotectonic Stress Regime and Airborne-Gravity Lineament Investigation of the Caribbean region and its Kinematic implications

The tectonic stress regime of the Caribbean region has been studied by stress tensor inversion analysis for the area bounded by Latitudes 17.32o to 23.79o N and Longitudes 81.47o to -60.56o E. A total of two hundred and twenty (220) fault plane solutions (FPS), made up of the seismic (earthquake) events that have occurred in the region of the study were found on the data catalog of the project website of the global centroid moment tensor, were employed in this study. To characterize the stress regimes of the region of study, the classification of the region based on the global moment tensor project is as follows: Cuba-zone, Haiti-zone, Dominican-republic, Monapassage, Puerto Rico, Virgin-Island and the Leeward Island. The gravity geophysical method revealed the presence of multiple or swarms of fractured lines within the same geographical range of study which might have developed over the years as a result of the interaction and coupling effect of the plate boundaries that have close proximity to the region of study. The seismicity pattern indicates that none of the provinces or countries located within the Caribbean region is devoid of large-magnitude earthquake occurrences capable of wreaking severe havoc on the immediate environment. Obviously, our stress tensor inversion result indicates two types of stress regimes namely compression and the strike-slip with different orientations. Cuba-zone, Haiti-zone, Dominican Republic, Monapassage, and Leeward Island are governed mainly by compression stress regimes. In contrast, the strike-slip stress regimes mainly govern the Puerto Rico and the Virgin Island.

Towards fast focal mechanism inversion of shallow crustal earthquakes in the Chinese mainland

We have developed an automatic regional focal mechanism inversion system based on the Earthquake Rapid Report (ERR) system and the real-time three-component seismic waveform stream of 1 000 broadband seismic stations provided by the China Earthquake Networks Center (CENC). The system can rapidly provide a double couple solution and centroid depth within 5–15 ​min after receiving earthquake information from the ERR system. The data processing is triggered by earthquake information obtained from the ERR system. The system is capable of determining the focal mechanism of all shallow-depth earthquakes in the Chinese mainland with a magnitude of 5.5–6.5. It utilizes waveform data recorded by seismic stations located within 500 ​km from the epicenter, enabling the reporting of a focal mechanism solution within 5–15 ​min of an earthquake occurrence. Additionally, the system can assign a corresponding grade (A B C) to the focal mechanism solution. We processed a total of 301 earthquakes that occurred from 2021 to June 2022, and after the quality control, 166 of them were selected. These selected solutions were manually checked, and 160 of them were compiled in a focal mechanism catalog. This catalog can be conveniently downloaded online via the Internet. The automatic focal mechanism solution of earthquakes in eastern China exhibits a good agreement with that provided by the Global Centroid Moment Tensor (GCMT), when available. The average Kagan angle between this catalog and GCMT is 22°, and the average difference in MW is 0.17. Furthermore, compared with GCMT, the minimum magnitude of our catalog has been reduced from approximately 5.0 to 4.0. The correlation between the centroid depth and crustal thickness in the Chinese mainland confirms the distribution of the centroid depth.

CANVAS: An Adjoint Waveform Tomography Model of California and Nevada

AbstractWe present the California‐Nevada Adjoint Simulations (CANVAS) model, an adjoint waveform tomography model of the crust and uppermost mantle of the states of California and Nevada. We used WUS256 (Rodgers et al., 2022, https://doi.org/10.1029/2022jb024549) as the starting model and iteratively decreased the minimum period of CANVAS from 30 to 12 s. CANVAS was iterated in two distinct stages: the first stage with source mechanisms from the Global Centroid Moment Tensor (GCMT) catalog and the second stage with inverted moment tensors (MT) using the CANV_WUS model (Doody et al., 2023, https://doi.org/10.1029/2023jb026463). We show that updating the MTs with 3D Green's functions improved waveform fits and azimuthal coverage of windowed data used to calculate the gradients. As for the model itself, we improved waveform fits over WUS256, particularly in the dispersed surface waves. CANVAS resolved tectonic features seen in other models and accurately defined the depth to basement of major basins, including the Central Valley and the Ventura Basin. We propose CANVAS as a starting model for crustal tomography models on smaller scales.

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.

Ransiki earthquakes, northeastern Bird’s Head Peninsula, northwestern New Guinea, Indonesia: Deformation partitioning in oblique plate convergence

The plate boundary between the Pacific-Caroline and Australian plates in northwestern New Guinea is associated with a geographic concentration of earthquakes developed in the Ransiki region of the northeastern Bird’s Head Peninsula (West Papua, northwestern New Guinea) at the intersection of the Ransiki and Yapen faults. We examine these earthquakes based on regional geomorphological and lithostratigraphical frameworks, field observations of surface ruptures and liquefaction phenomena, and focal mechanisms of historical earthquakes. The Ransiki earthquakes are a set of 29 earthquakes from the Global Centroid Moment Tensor catalogue in the period 1977–2019 (magnitudes of Mw4.9 to Mw7.5). In the east, focal mechanisms show sinistral movement along the east-west trending Yapen Fault including the Mw6.7 earthquake on 21 April 2012. The largest earthquake was on 10 October 2002 (Mw7.5) and along with other earthquakes mainly in the southwest were associated with dextral movement indicated by focal mechanism solutions on the northwest trending Ransiki Fault south of its intersection with the Yapen Fault. The southern part of the Ransiki Fault therefore indicates local north-northeast compression that is also evident in the newly recognised Wainoei Fault south of Yapen Island. The two largest earthquakes (10 October 2002, 21 April 2012) show ground effects associated with liquefaction, indicated by surface offsets, open fissures, and sand blows, that all occurred in saturated sediments of the Ransiki delta. Earthquakes in the Ransiki region show that west-southwest oblique plate convergence between the Australian and Pacific-Caroline plates is partitioned into east-west sinistral strike-slip motion along the Yapen Fault and north-northeast compression associated with the Ransiki Fault.

Variation of b-Value of Normal, Reverse and Strike-Slip Faulting Earthquakes with Focal Depth

A crucial factor in evaluating seismic activity, seismotectonics, and seismic hazard assessment is the distribution of earthquake sizes (b-value). The goal of this research is to explore the relation between depth and the b-constant values of normal, inverse, and strike-slip faulting earthquakes. The earthquake catalog used in this work was extracted from the Global Centroid Moment Tensor Catalog (GCMTC) and covers the period from 2008 to 2017. The focal depth of the selected tremors ranges from zero to 700 km. The magnitude of completeness for normal and reverse faulting events is 5.2 and 5.1 for strike-slip faulting events. The value of the b- constant was estimated using the maximum likelihood technique. The findings of regression analysis showed that there is an insignificant negative poor correlation relation between the b- value of the normal and strike-slip faulting events and depth, while that relation is an insignificant positive poor correlation for reverse faulting. The depth of the turning point for the b-constant value of normal and reverse faulting earthquakes is near the mantle seismic discontinuities 410 and 520 km. The depth of inflection points of b-constant value strike-slip faulting earthquakes is near to lithosphere-asthenosphere boundary (LAB) at depth of 250km and the mantle seismic discontinuity of 520km.

How many strong earthquakes will there be tomorrow?

In this note, we study the distribution of earthquake numbers in both worldwide and regional catalogs: in the Global Centroid Moment Tensor catalog, from 1980 to 2019 for magnitudes Mw 5. 5+ and 6.5+ in the first case, and in the Italian instrumental catalog from 1960 to 2021 for magnitudes Mw 4.0+ and 5.5+ in the second case. A subset of the global catalog is also used to study the Japanese region. We will focus our attention on short-term time windows of 1, 7, and 30 days, which have been poorly explored in previous studies. We model the earthquake numbers using two discrete probability distributions, i.e., Poisson and Negative Binomial. Using the classical chi-squared statistical test, we found that the Poisson distribution, widely used in seismological studies, is always rejected when tested against observations, while the Negative Binomial distribution cannot be disproved for magnitudes Mw 6.5+ in all time windows of the global catalog. However, if we consider the Japanese or the Italian regions, it cannot be proven that the Negative Binomial distribution performs better than the Poisson distribution using the chi-squared test. When instead we compared the performances of the two distributions using the Akaike Information Criterion, we found that the Negative Binomial distribution always performs better than the Poisson one. The results of this study suggest that the Negative Binomial distribution, largely ignored in seismological studies, should replace the Poisson distribution in modeling the number of earthquakes.

Determining the Success of Strong Motion Records in Calculating P-Wave Moment Magnitude

Earthquake early warning systems have high importance and improving continuously using different analysing techniques. The system depends on calculating a magnitude in a short time with as high accuracy as possible. Large earthquakes that have tsunami potential occurred and will be occurred especially in the Mediterranean and Aegean Seas Turkey has settlements along these coastlines. In practice, weak-motion (velocity) records are generally used to determine the magnitude, but these seismic records have a disadvantage: a clipping problem around the epicentre. Up to now, there is not much work on the study of the strong-motion (acceleration) records to calculate the magnitude of an earthquake in a short time with high accuracy. This study focused on calculating the P-wave moment magnitude using P-wave body waves recorded at strong motion seismic stations caused by possible tsunami source region earthquakes. For that reason, we used three earthquakes that have a magnitude larger than 6.0 Mw. The difference between the Global Centroid Moment Tensor (GCMT) moment magnitude is about 0.2 m.u. which is acceptable considering an early warning purpose.

The coulomb stress changes for correlation analysis of tectonic earthquake activity and volcanic of the 2001–2010 Merapi eruption based on focal mechanism data

The incident of increased seismic activity of the Merapi volcano was initiated by several tectonic earthquake phenomena. Analysis of the mechanism of tectonic earthquakes can be studied based on the combination of static stress on the rock. This paper describes the distribution of the Coulomb stress changes due to the interaction that occurs in tectonic earthquakes with volcanic activity. The data in the calculation of the Coulomb stress distribution are secondary in the form of tectonic seismicity data downloaded from the ISC (International Seismological Center), USGS (United States Geological Survey), and Global CMT (Global Centroid Moment Tensor) catalogs between 2001 and 2010. Calculation results and data analysis is the occurrence of an increase in Coulomb stress response in the Merapi Volcano area with a value range of 0.2–10 ​kPa at a depth of 2–4 ​km below the peak of Merapi Volcano. The Coulomb stress changes distribution is thought to indicate the accumulation of stress due to tectonic earthquakes that downward at the top of the volcano, thereby affecting the stability of the magma chamber and causing volcanic activity at the Merapi volcano.

Effect of permanent stress on the time-dependent earthquake probability: implication for seismic hazard assessment in the African continent

SUMMARY Earthquake hazard assessment is the first step towards implementing prevention, preparedness and response or faster recovery actions to reduce the risk of seismic disasters. In this paper, we present a comprehensive study on present-day seismicity in terms of the estimated waiting time and conditional probability in Africa by 2022–2072 using four competing distribution models (Brownian passage time (BPT), gamma, lognormal and Weibull). We also investigate how much Coulomb stress change $\ ( {\Delta {\rm CFF}} )$ induced by previous earthquakes that occurred in neighbouring active sources should revise the probability of occurrence at the location of the next events. We analyse large earthquakes with moment magnitude, ${M}_{\rm w} \ge 6.0$, collating data from the Global Centroid Moment Tensor and from several published literature that list fault plane solutions of large African earthquakes since 1900. We assume that the data set is stationary and consists of independent events. First, the model's parameters are estimated and the results of the statistical analysis of the interevent times show clear evidence of quasi-periodic recurrence behaviour for large earthquakes ${M}_{\rm w} \ge $ 6.0 in different seismotectonic regions in Africa. Next, a comparison among the distribution models is performed with the aim of selecting the most suitable one. The results in terms of the maximum likelihood criterion and its extension (Akaike Information Criterion) indicate that, in general, the BPT and Weibull models had similar fits to each other, and performed slightly better than gamma and lognormal models. Then, we use Gaussian random distributions to treat parameter uncertainties (e.g. aperiodicity, maximum expected magnitude, slip rate and mean recurrence time) of the distribution models associated with each seismotectonic region. From repeated Monte Carlo draws, we assess uncertainties of the 50-yr conditional probability values for the next earthquake obtained from two distribution models (BPT and BPT + $\Delta {\rm CFF}$) related to the 50th percentile. The results of the BPT distribution indicate very high chances of future earthquakes in the study region where the conditional probability of a large earthquake reaches 99.5, 95.6, 83.1 and 82.2 per cent for the western branch of the East African Rift System (EARS), northwest Africa, the Afar region, and the eastern branch of EARS, respectively. Taking into account the effect of stress change from interacting sources (BPT + $\Delta {\rm CFF}$), these probabilities are slightly modified to 99.8, 98.4, 89.9 and 87.3 per cent for the western branch of EARS, northwest Africa, the Afar region and the eastern branch of EARS, respectively. These marginal increases suggest that the estimated effect of the earthquake interaction introduced by the coseismic slip of previous earthquakes on neighbouring active sources is minor if compared with the uncertainties affecting the renewal models used for the basic time-dependent conditional probability assessment.

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.

Regression relationships for conversion of body wave and surface wave magnitudes toward Das magnitude scale, Mwg

A reliable and standardized estimation of earthquake size is a fundamental requirement for all tectonophysical and engineering applications. Several investigations raised questions about the determinations of smaller and intermediate earthquakes using Mw scale. Recent investigations (Das et al. in Bull Seismol Soc Am 108(4):1995–2007, 2018b) show that the moment magnitude scale Mw is not applicable for lower and intermediate ranges throughout the world and does not efficiently represent the seismic source potential due to its dependence on surface wave magnitudes; therefore, an observed seismic moment (M0)-based magnitude scale, Mwg, which smoothly connects seismic source processes and highly correlates with seismic-radiated energy (Es) compared to the Mw scale is suggested. With the goal of constructing a homogeneous data set of Mwg to be used for earthquake-related studies, relationships for body wave (mb) and surface wave magnitudes (Ms) toward Mwg have been developed using regression methodologies such as generalized orthogonal regression (GOR) (GOR1: GOR relation is expressed in terms of the observed independent variable; and GOR2: GOR relation is used inappropriately in terms of theoretical true point of GOR line) and standard least-square regression (SLR). In order to establish regression relationships, global data have been considered during 1976–2014 for mb magnitudes of 524,790 events from the International Seismological Centre (ISC) and 326,201 events from the National Earthquake Information Center (NEIC), Ms magnitudes of 111,443 events from ISC along with 41,810 Mwg events data from the Global Centroid Moment Tensor (GCMT). Scaling relationships have been obtained between mb and Mwg for magnitude range 4.5 ≤ mb ≤ 6.2 for ISC and NEIC events using GOR1, GOR2 and SLR methodologies. Furthermore, scaling relationships between Ms and Mwg have been obtained for magnitude ranges 3.0 ≤ Ms ≤ 6.1 and 6.2 ≤ Ms ≤ 8.4 using GOR1, GOR2 and SLR procedures. Our analysis found that GOR1 provides improved estimates of dependent variable compared to GOR2 and SLR on the basis of statistical parameters (mainly uncertainty on slope and intercept, RMSE and Rxy) as reported in Das et al. (2018b). The derived global scaling relationships would be helpful for various seismological applications such as seismicity, seismic hazard and Risk assessment studies.

SETAS: A Spherical Version of the Space–Time ETAS Model

AbstractTo analyze and forecast seismicity globally or in high-latitude regions, and particularly in relation to remote seismicity interactions on a worldwide scale, we reformulate the space–time epidemic-type aftershock sequence (ETAS) model from a planar to a spherical version and implement it in a FORTRAN software package, spherical ETAS (SETAS). This article explains the details of the related algorithms and the usage of the package, which can fit the SETAS model to seismicity data from regions specified by three types of boundaries: no, quadrilateral, and spherical polygon boundaries. The software is tested with the Global Centroid Moment Tensor (Global CMT) catalog and compared to the original planar ETAS model by fitting both the versions to local seismicity in Japan and Alaska. The results show that the spherical model has a high computational efficiency, even with a large range of global data, and is consistent with the well-established planar version at the local scale.

A Cautionary Tale: examples of the mis-location of small earthquakes beneath the Tibetan plateau by routine approaches

SUMMARYEarthquake moment tensors and centroid locations in the catalogue of the Global CMT (gCMT) project, formerly the Harvard CMT project, have become an essential resource for studying active global tectonics, used by many solid-Earth researchers. The catalogue’s quality, long duration (1976–present), ease of access and global coverage of earthquakes larger than about Mw 5.5 have transformed our ability to study regional patterns of earthquake locations and focal mechanisms. It also allows researchers to easily identify earthquakes with anomalous mechanisms and depths that stand out from the global or regional patterns, some of which require us to look more closely at accepted interpretations of geodynamics, tectonics or rheology. But, as in all catalogues that are, to some extent and necessarily, produced in a semi-routine fashion, the catalogue may contain anomalies that are in fact errors. Thus, before re-assessing geodynamic, tectonic or rheological understanding on the basis of anomalous earthquake locations or mechanisms in the gCMT catalogue, it is first prudent to check those anomalies are real. The purpose of this paper is to illustrate that necessity in the eastern Himalayas and SE Tibet, where two earthquakes that would otherwise require a radical revision of current geodynamic understanding are shown, in fact, to have gCMT depths (and, in one case, also focal mechanism) that are incorrect.

Analysis of Coulomb Stress Changes in Aceh Earthquake on Sibayak Volcano

Based on geological conditions, Indonesia is located between the Indo-Australian plate, the Eurasian plate, and the Pacific plate, making Indonesia prone to tectonic earthquakes. However, since the big earthquake that occurred in Aceh on December 26, 2004, the geological conditions in Sumatra have undergone significant changes marked by stressful situations in the western to southern regions of Sumatra, especially on Sibayak Volcano. This study used data from BMKG and Global CMT, which included magnitude (Mw), depth, earthquake coordinates (longitude and latitude), type of earthquake, strike, dip, and rake. From the analysis using Coulomb 3.3, the highest coulomb stress value of Sibayak Volcano was obtained in 2015 with an average change in coulomb stress of 0.235 bar, shear 0.1909 bar, normal 0.1106 bar. However, the lowest coulomb stress value occurs in 2021 with a moderate shift in coulomb stress of 0.0593 bar, shear 0.0251, normal 0.0849 bar.