Crustal Earthquakes Research Articles

Development of synthetic ground motion at a specific site in Yogyakarta town, Indonesia utilizing the PSHA Method

This paper presents the development of synthetic ground motion at specific sites in Yogyakarta town. In the 2019 Indonesian Seismic Code [1] provides an alternative method in the analysis of building structures by applying the dynamic time history analysis. At least 11-pairs of earthquake recordings must be used in the analysis. Synthetic ground motion utilizing the Method of Probability Seismic Hazard Analysis (PSHA) was used in this study. A selected site in Yogyakarta town was chosen as a pilot study considering that there were many fatalities and building damage caused by the 2006 Yogyakarta earthquake. The Uniform Hazard Spectra (UHS) based on the shallow crustal earthquake source is higher than the Megathrust. The risk targeted spectrum demand MCEr has been considered, which on average 12.3% greater than the UHS. The synthetic ground motions (SGM) are accordingly based on the shallow crustal earthquakes. The dominant magnitude and distance are MD = 6.5 and RD = 14.5 km. They show that the contribution of the Opak River fault to the hazard in Yogyakarta town is very dominant because the distance is very close. Based on the obtained MD and RD, spectral matching, and testing significant duration D595, the 12-synthetic ground motions were successfully developed.

ФРАКТАЛЬНАЯ ГЕОМЕТРИЯ РЕЧНОЙ СЕТИ И НЕОТЕКТОНИКА ЮЖНОГО СИХОТЭ-АЛИНЯ

A new complex parameter is proposed to characterize the self-similarity of a river network, that is, the parameter of river networks self-similarity (PRNS), which is a combination of the exponent in the distribution of channels along the lengths, fractal dimension of channels, and fractal dimension of points of channel order change, which is more correct when compared with neotectonic movements. A comprehensive analysis of the self-similarity of the river network model for the southern Sikhote-Alin derived from the Digital Elevation Model (DEM) based on the Shuttle Radar Topography Mission (SRTM) image was performed. Comparison of the results of morphostructural and fractal analyses showed a good correlation of these two methods. PRNS relative maxima coincide with areas where ascending neotectonic movements are of the largest amplitude (increment of relief), while minima coincide with areas either of the least increment of relief or where erosion rates are the greatest. At the same time, most epicenters of crustal earthquakes are confined to the boundary zones between PRNS relative maxima and minima, that is, uplifts are flanked by elongated seismic areas, which is related to the block nature of neotectonic vertical movements.

Unusually deep earthquakes in the central Sierra Nevada (California, USA): Foundering ultramafic lithosphere?

AbstractUsing a network of temporarily deployed broadband seismometers, we characterize an unusual region of crustal earthquakes in the west-central Sierra Nevada, California (USA). We locate 131 earthquakes, which occurred from 3.1 to 47.1 km deep during June 2005 to May 2006. We detect more events, at greater depths, than are present in the Northern California Seismic Network catalog during this period. Most of the events occur at depths of 20–35 km and cluster into two distinct groups. In addition, some of the events appear to be repeating due to the similarity of their waveforms and locations. We calculate focal mechanisms for 52 of these events, and about half exhibit reverse faulting, which represents a state of horizontal compressional stress that is distinct from the regional stress field. From first arrivals, we calculate a one-dimensional model of crustal P-wavespeeds, which resolves a gradational increase from 5.8 km/s near the surface to 6.7 km/s at 35 km depth. The events overlie a significant variation in the character of the Moho, and two long-period events occur near the seismically imaged Moho at nearly 40 km depth. We suggest that these earthquakes could be the seismogenic response of the crust to active foundering of mafic-ultramafic lithosphere and resultant asthenospheric upwelling beneath the central Sierra Nevada.

Describing coseismic groundwater level rise using tank model in volcanic aquifers, Kumamoto, southern Japan

The change of groundwater levels after the 2016 Mw 7.0 Kumamoto crustal earthquake was evaluated using a simple conceptual hydrological model in an attempt to show the presence, intensity, and probable mechanism of water level rise observed in Kumamoto where a comprehensive observation-well network exists. A tank model was applied to verify 16 wells in the study field. In the model groundwater levels were first calibrated for the periods in ca. 2 years before the main shock using several hydrological parameters including precipitation, evapotranspiration, water recharge and discharge, and artificial recharge by irrigation. Water levels were then simulated by extrapolating this law of water fluctuating patterns for ca. 2.5 years after the main shock of the earthquake, without considering hydrogeological changes due to the earthquake. A difference in groundwater levels between observation and simulation results yields a degree of coseismic water level rises for each well. The coseismic abnormal water level increase was calculated to be ~11 m in 4–5 month after the main shock and was most significantly on the western slope of the Aso caldera rim mountains. The spatial distribution of the coseismic water increases clarified that the most dominate increasing anomalies prevail at mountain feet surrounding the plains, suggesting the occurrence of coseismic mountain water release resulting in the rise of water levels in downslope aquifers. Identified coseismic water level increases still continue up to 2.5 years after the earthquake, probably because changes in hydrogeological properties in mountain aquifers, i.e., permeability, are still sustained. Our forecasting water recovering trends require ca. 3.5–5 year after the earthquake for complete recovery to the original conditions. We demonstrated that our approaches are capable of describing coseismic water level changes and could potentially be applied to other fields.

Evidence of ridge propagation in the eastern Gulf of Mexico from integrated analysis of potential fields and seismic data

Integrated analysis of gravity, magnetic, and seismic data reveals two phases of spreading in the eastern Gulf of Mexico (GOM) including two distinct spreading centers, suggesting a major ridge reorganization during the opening of the eastern part of the GOM. Ridge propagation between the two spreading episodes explains the following observations: 1) the drastic asymmetry in the oceanic domain of northeastern GOM, 2) the presence of two distinct crustal zones with dramatically different thickness and physical properties, and 3) the observed seismicity within the oceanic domain that is not aligned with any known tectonic structure. The initial Late Jurassic spreading center (~160 Ma) resulted in thin (~5 km) and uniform oceanic crust with a fast compressional velocity (7 km/s). Based on our analysis, the estimated full spreading rate of this older spreading event is less than 1 cm/yr. The spreading regime changed in Early Cretaceous around 150 Ma, resulting in a propagation (i.e., jump) of the spreading center. The new spreading episode was characterized by a change in spreading direction and increased magma supply as it produced thicker (up to 9 km) oceanic crust with a typical two-layered structure. Despite the increase in magmatic material, the full rate of this younger spreading event estimated from our analysis is only slightly faster (1.1 cm/yr assuming that spreading ceased at 137 Ma). The later conclusion is consistent with the morphology of the spreading centers mapped by seismic data. Our analysis shows that recent deep crustal earthquakes in the middle of the Gulf of Mexico are aligned with the boundary between the two identified distinct oceanic zones, referred here as a pseudofault.

Search for Traces of Strong Ancient Earthquakes in the Western Caucasus: Archeoseismological Studies in Ancient Gorgippia

The ancient city of Gorgippia is surrounded by well-known (North Black Sea, Kerch, and South Azov) seismogenic zones, all of which are submarine, and only some of their branching faults reach the surface in the coastal zone. On the surface, in the vicinity of the ancient city, there are also seismically active faults and flexure rupture zones, which can generate strong earthquakes. Our investigations in the ancient city of Gorgippia have revealed a number of seismogenic deformations and destructions in its walls: systematic tilts and shifts of the upper parts of the walls, as well as their arc-shaped bends in plan view; breaks of the walls; rotations of walls and other building elements around the vertical axes; and squeezing of the mouths and shafts of water wells. Some of the mentioned deformations could be formed during the strong historical earthquakes in the 3rd and 1st centuries BC, as well as those in the 1st and 3rd centuries AD, and all these seismic events were accompanied by local destructions and fires. Summarizing all materials on historical earthquakes of the Taman Peninsula, the regional seismic potential can be determined by the possible occurrence of a crustal earthquake with M ≥ 7.0 and average recurrence interval of once in few hundred years. Seismic shaking in the source zones of such natural events can be I0 = IX or even more.

The relationship between crust-lithosphere structures and seismicity on the southeastern edge of the Tibetan Plateau

The crust-lithosphere structure, especially the discontinuities such as Moho and lithosphere-asthenosphere boundary (LAB), is important in the investigation of geodynamic process implications and tectonic evolution of the lithosphere and regional seismic activity. We stacked the S receiver functions from 51 permanent broad-band stations to investigate the crust-lithosphere structures beneath the southeastern edge of the Tibetan Plateau and further discussed the mechanism of lower-crust earthquakes, the regional tectonics deformation characteristics, and the relationship between the two discontinuities and seismicity in this region. The results show that the Moho depth increases from 40 to 52km beneath the Sichuan Basin to 56–74km on the both western side of the Longmenshan (LMS) fault and Anninhe-Zemuhe fault. The LAB depth ranges from 130 to 170km beneath the Sichuan Basin, and a shallow belt ranges from 100km to 130km around the Sichuan Basin. On the Moho depth contour map, 374 moderate earthquakes that accounted for 72.2%, corresponding to the depth range of 44–52km and 60–68km on the both sides of a narrow strip (longitude 102°~104°). On the LAB depth contour map, 321 earthquakes accounted for 62.15% in the depth range of 130–150km. The variation of LAB corresponds to the increasing crustal thickness, intensive lower crustal earthquakes (hypocenters about 60km and magnitude 4.0≤Ms.≤4.9) and surface abrupt elevations. It implied that Normal strong earthquakes generate the lower crustal earthquakes or aftershocks that resulted in the shear zones where fluid-induced metamorphic transformations, the eclogitization of dry granulite, and further affect the crustal thickness and deformation to control the spatial distribution of deep faults and seismicity.

Приамурье и Приморье

The review of seismicity of Amur and Primorye in 2013 according to the data of permanent seismic sta-tions’s network of the Sakhalin branch of the GS RAS is presented. Maps of the representative registration of earthquakes (Mmin), of epicenters of earthquakes and explosions are presented. Distributions of shallow earthquakes by T.G. Rautian energy classes КR, and of deep-focused by magnitude MPVA are shown; total amount of seismic energy released in six seismoactive areas is given. The results of observations of after-shock activity of Skovorodino earthquake on October 14, 2011 (M=6.2) are presented. Seismicity recorded in 2013 is compared with the results of long-term observations. In general, the seismic regime of the Pri-amurye and Primorye region in 2013 is identical to that in 2012. The Turano-Bureinsky district (No. 4) con-tinues to be the dominant region in terms of released seismic energy of crustal earthquakes, which is similar to the previous year. The more stable operation of seismic stations in 2013 had a positive effect on the quality of seismic events in the catalog. There is a noticeable shortage of small events in the catalog (with КР≤6.5) despite this. This is mostly relevant to Stanovoy District No. 1 and Yankan-Tukuringra-Dzhagdinsky No. 2 regions. There was a significant increase in the total amount of released seismic energy of deep-focus earthquakes in the Sikhote-Alin area No. 5 in 2013. The peculiarity of 2013 was a slight in-crease in the number of recorded technogenic events, the largest of which are located on the territory of northern China. In most parts of the Amur and Primorye region the level of seismic activity in 2013 re-mained moderate.

ОЩУТИМЫЕ в МОЛДОВЕ в 2013 г. ЗЕМЛЕТРЯСЕНИЯ: 11 августа с КР=11.8, 6 октябряс КР=14.1, 15 октября с КР=12.3 (Румыния–Молдова)

The article presents instrumental and macroseismic data on earthquakes of the Carpathian region in 2013, felt on the territory of the Republic of Moldova. Three earthquakes, with an intermediate focal depth, occurred in the Vrancea zone: August 11 with h=93 km, КР=11.8; October 6 with h=140 km, КР=14.1; Octo-ber 15 with h=144 km, КР=12.3. Also in 2013, a swarm of small earthquakes was recorded, near Galati (Ro-mania) and Giurgiulesti (Moldova). The swarm arose September 23, its shocks were recorded for two months. More than 370 seismic events happened, of which three had КР  11 (September 29, 30, and Octo-ber 4 with КР=11.2, 10.9, 10.6). The intensity of shaking reached 5. For earthquakes, September 29, 30 and October 4 regional solutions for the focal mechanism from the Mostryukov and Petrov catalog (GO "Borok" IPE RAS) were received. The event of August 11, 2013, with КР=11.8, occurred at a depth of 93 km on the northeastern margin of the focal zone of Vrancea, felt at the epicenter with intensity I=4. In Chisinau, only some people felt weak shocks (I=2). The two largest earthquakes of 2013 occurred on October 6 at 01h 37m (КР=14.2, mb=5.3, I0=6) and on October 15 at 19h 33m (КР=12.3, mb=4.8), in the central part of the focal zone Vrancea. Between these events, three more earthquakes were noted with ML=3–3.5. However, the most extensive macroseismic manifestations arose from the main shock – October 6, which caused shaking in the territory of four states – in Romania, in the north of Bulgaria, in the south-west of Ukraine, and in Moldova. The reason for such a significant spread of tremors lies in a large focal depth of 140 km. But for the same reason, the intensity of the macroseismic impact did not exceed I=5 in Romania and I=4–5 in Moldova and Ukraine. As a result, a map of the intensity distribution of the October 6 earthquake is given, built on the basis of data processing in 104 locations. Various solutions for the focal mechanisms of intermediate earthquakes in 2013 are presented. The consistent manifestation of a swarm of crustal earthquakes and a number of earthquakes of intermediate depth confirms the unity of tectonic processes in the Carpathian region. The accumulated seismic stresses were preliminarily realized in the form of crustal shocks, then powerful discharge occurred in the Vrancea region, which interrupted a series of events associated with Sfintul-Gheorghe and Peceneaga–Camena faults.

Changes to the microbial communities within groundwater in response to a large crustal earthquake in Kumamoto, southern Japan

Post-seismic microbial changes in aquifer systems have rarely been documented, and the processes to explain these changes have yet to be explicated. Microbial communities within groundwater in the Kumamoto area, Japan were characterized in 2013 and 2014 prior to the occurrence of a large earthquake (7.0 Richter scale) in 2016, which provided a rare opportunity to compare samples obtained following the seismic activity, with information from the previous, and subsequent years 2017 and 2018. Microbial changes were monitored for 4 representative wells in a regional groundwater flow system utilizing a previously proposed post-seismic hydrogeological change model. Site-specific microbial communities changed and became homogeneous for all of the wells after the earthquake. The original microorganisms such as methylotrophs had decreased; however, exogenous microorganisms, such as Propionibacterium acnes, increased at all sites in the 2–3 months following the earthquake, these changes were consistent with the coseismic surface water drawdown model. However, the prevalence of P. acnes clones decreased in the 2–3 years following the earthquake. Notably, none of the microbial communities recovered to their original condition prior to the earthquake. This suggested that a new water supply from mountain aquifers could have caused these second stage post-seismic microbial changes. Our study demonstrated that large crustal earthquakes could dramatically alter microbial communities within aquifer systems caused by post-seismic hydrogeological changes. However, future monitoring is required to reveal further changes during the microbiological recovery process.

Post-seismic hydrochemical changes in regional groundwater flow systems in response to the 2016 Mw 7.0 Kumamoto earthquake

An understanding of the detailed mechanisms and processes of hydrochemical changes in regional groundwater flow systems in response to large earthquakes has been hindered because of a lack of spatiotemporal datasets to describe these changes. To clarify these issues we documented hydrochemical changes observed during the 2016 Mw 7.0 Kumamoto crustal earthquake using a large amount of analytical records for an area of ca. 945 km2 from 139 wells for 82 measured parameters. Spatiotemporal distribution patterns of these changes were characterized based on a factor notation calculated using a 10-year data trend. Our results suggest that alteration of hydrogeological properties, i.e., seismic rupturing, coseismic mountain water release, and release of soil pore waters, caused changes in hydrochemical signatures at the regional scale, most prominently reflected in elevated dissolved Si, diluted Cl-, and increased CO2 and organic compounds contents. Other hydrochemical anomalies detected at the local scale were induced by deep fluid upwelling and liquefaction. This study identified the combined mechanisms and processes that generate the overall water chemistry changes in active groundwater flow systems in Kumamoto. Our results provide new insights into coseismic hydrochemical changes that could be applied to other aquifer systems and, importantly, the use of groundwater resources during and after disasters.

Active stress field and fault kinematics of the Greater Caucasus

This work contributes to depict the current seismicity, fault kinematics, and state of stress in the Greater Caucasus (territories of Georgia, Azerbaijan and Russia). We merged and homogenized data from different earthquake catalogues, relocated ~1000 seismic events, created a database of 366 selected focal mechanism solutions, 239 of which are new, and performed a formal stress inversion. Preferential alignments of crustal earthquake foci indicate that most seismic areas are located along the southern margin of the belt and in the north-eastern sector. This is consistent with the presence of dominant active WNW-ESE faults, parallel to the mountain range. In the entire Greater Caucasus, a dominant NNE-SSW-oriented greatest principal stress (σ1) controls the over-all occurrence of earthquakes of minor and major magnitude. Main earthquakes are characterized by a vertical least principal stress (σ3), corresponding to reverse kinematics. Reverse slip is more common along the southwestern and north-eastern foothills of the Greater Caucasus, although in these areas there are also scattered strike-slip events. This suggests the presence of local stress fields with horizontal σ1 and σ3. In the central-southern part of the mountain belt, in correspondence of the local collision between the Lesser and the Greater Caucasus, σ1 rotates to NNW-SSE. The strike-slip events, instead, dominate along the southern flank of the central-eastern mountain range; this is interpreted as the effect of the collision that promotes eastward escape of the tectonic blocks located to the east.

Deep Earthquakes in the Caucasus: Recalculation Results

The question of the existence of deep earthquake sources in the Caucasus is extremely important both for solving geodynamics problems, assessing seismic hazard, and seismic zoning of the region. It was previously believed that earthquakes with depths not exceeding 150 km can occur in the Caucasus. However, in recent years, this has become argumentative, reflected in a number of publications. This article shows that in the Caucasus, in addition to crustal earthquakes, rather deep mantle earthquakes also occur. This, in addition to direct determinations, is confirmed by data published in Caucasian earthquake catalogs and bulletins. The presence of deep earthquake sources in the Caucasus significantly changes our views on the regional structure and geotectonics. Obviously, crustal earthquakes dominate in the region’s seismicity, but it is clear that mantle earthquakes also make a significant contribution. The fact that they are fewer than crustal earthquakes in the total number may indicate that the crust is harder and more brittle than the upper mantle. Keywords: earthquakes, hypocenters, catalogs, bulletins

Testing and Developing the Macroseismic Intensity Attenuation Relationships for the Vrancea (Romania) Crustal Earthquakes

A correct seismic hazard assessment and intensity-based shake maps of a seismic zone depend on the determination of parameters that emphasize the distribution and the macroseismic intensity attenuation for that seismogenic zone. Due to the reduced number of strong ground motion records, macroseismic intensity information available for previous earthquakes which have occurred in studied area were the only source of information utilized for developing attenuation relationship for that zone. The aim of this paper is to test the existing equations published for various crustal seismic sources and develop a new macroseismic intensity attenuation law for the Vrancea crustal seismic source. The Vrancea crustal seismogenic zone had experienced moderate earthquakes in the past with magnitudes which did not exceed 5.9 [1] and macroseismic maralps had been constructed by various authors for these earthquakes representing intensity pattern and decrease of this parameter with distance on different azimuths. Some of the selected attenuation laws were tested for different values of epicentral intensity and with reference to eight directions. The input data consist of macroseismic intensities collected for 4 small and moderate earthquakes (Mw ≥ 3.9) that had occurred during the last hundred years with epicentral/maximum intensity in the range IV to VI MSK degrees. The seismic events used to verify the attenuation laws were selected based on the magnitude and number of macroseismic observations collected for each event. Using this data, attenuation relationships of macroseismic intensity with distance will be tested and modified. The deduced attenuation equation might be used for rapid intensity assessment of the potential future earthquakes occurred in this seismic zone.

The Significance of Tomographic Edge Zones for Large Earthquakes in Taiwan

AbstractSeismogenesis of large earthquakes in Taiwan are often controlled by regional tectonics. Despite decades of efforts, the detailed relationship between seismogenesis of disastrous earthquakes and the geophysical properties of the earth materials remains unclear. In this study, we conduct high‐resolution tomography inversion of seismic velocities (Vp and Vs) and the Poisson's ratio (σ), and the results are used to infer the three‐dimensional (3‐D) distributions of crack density (ε) and saturation rate (ζ). We find that more than 70% of large (moment or local magnitudes ≥6, M6+) earthquakes occurred along tomographic edge zones (TEZ) within the crust where the spatial gradient exceeds 0.1, including all three disastrous events in the past two decades: the 1999 M7.6 Chi‐Chi, 2016 M6.5 Meinung, and the 2018 M6.4 Hualien earthquakes. We attribute the preference of large crustal earthquakes along TEZ as the consequence of fluid migration from the more saturated side to the opposite, leading to the build‐up of high pore pressure along TEZ. Locations of historical M6+ earthquakes are consistent with the inference of excessive fluids from either the extensive dehydration of hydrous minerals within the subducting Philippine Sea plate or the sedimentary compaction of alluvium and clastic deposits in the foreland basin west of the orogenic belt. Positive correlation between M6+ earthquakes and TEZ is not unique to Taiwan. Several famous earthquakes in regions with different tectonic settings appear to be associated with local TEZ. Our study could provide insights to long‐term mitigation of regional seismic hazards in Taiwan and probably elsewhere.

Attenuation of Macroseismic Intensity for Crustal Romanian Earthquakes: Calibrating the Bakun-Wentworth’s Method

The purpose of the present study is to elaborate a new model of the macroseismic intensity attenuation using all observed intensities data for the crustal earthquakes in Romania and then, based on this relationship, to calibrate the Bakun-Wentworth method, one of the modern methods most used to estimate the parameters of the historical earthquakes (localization, moment magnitude-Mw). The study is based on 14 calibrating earthquakes (4.5 < Mw < 5.7, 811 IDPs, II < Ii < VIIIO EMS) and 7 validation earthquakes (3.5 < Mw < 5.4, 215 IDPs, II <Ii < VIIIO EMS) produced mainly in western part of Romania and for which the reported macroseismic intensities were revised in a homogeneous manner and their locations and moment magnitudes were instrumentally determined. The method was tested using earthquakes produced in neighbouring regions, expanding its application space for earthquakes produced, for example, in the extra-Carpathian areas and Pannonian Depression. The comparative analysis with the results obtained with other methods confirms the high performance and the scientific rigour of the new attenuation relationship and of the Bakun-Wentworth method and their effective applicability for testing and validation of the results obtained in the process of the Romanian earthquakes catalogue revision.

Seismotectonics of SE Aegean inferred from precise relative locations of shallow crustal earthquakes

We relocated the seismicity in SE Aegean that was recorded by both permanent and temporary seismic networks during 2004–2018 in order to investigate its correlation with the active faults in this area. P and S-phase travel times of best quality events were used to estimate a minimum 1D velocity model with station delays by utilizing VELEST. This velocity model and station delays were then used to obtain absolute locations of 3055 events, utilizing the probabilistic nonlinear algorithm NLLOC. The double-difference algorithm was used along with catalog and cross-correlation differential times to relocate all the events, resulting in 2200 precise relative locations with horizontal and vertical uncertainties of less than 1.0 km. The precise locations delineated faults along the Gulf of Gokova, SW of Nisyros, and Karpathos area. Based on the comparison of the resulting seismicity distribution with the regional stress field, it can be concluded that seismicity only occurred along faults with ENE-WSW strike in the area north of Tilos and N-S strike in the area south of Tilos. Seismogenic layer thickness estimated from the hypocentral depth distribution was found to vary between 12.1 and 15.4 km. Expected moment magnitudes of faults in this area were calculated by using their geometrical properties and the seismogenic layer thickness, yielding magnitudes in the range of 5.9–6.9. The fact that most of the seismically active faults in SE Aegean lie offshore increases the probability that a major earthquake will be followed by a tsunami and calls for the close monitoring of seismicity in this area.

Ground‐Motion Prediction Equations for Western Saudi Arabia

AbstractGround‐motion prediction equations (GMPEs) for western Saudi Arabia are developed by employing a mixed‐effects regression model to modify the Boore et al. (2014) Next Generation Attenuation‐West2 (NGA‐West2) project GMPEs. NGA‐West2 addressed several key issues concerning GMPEs for shallow crustal earthquakes in active tectonic regions. However, the NGA‐West2 results do not include many earthquakes in extensional regimes such as those occurring in Saudi Arabia. This deficiency is corrected by calculating a magnitude scaling of the new Saudi Arabia GMPEs compared to those of Boore et al. (2014). Furthermore, there is a clear difference in distance scaling for the Arabian GMPEs in comparison with the NGA‐West2 GMPEs. This difference is especially significant at large distances and is mainly due to lower anelastic attenuation in the crystalline crust of western Saudi Arabia. Our empirical data demonstrate the GMPEs presented here are in good agreement with observed earthquake ground motions in western Saudi Arabia.

Rupture Directivity Analysis of the 2018 Hokkaido Eastern Iburi Earthquake and Its Seismotectonic Implication

ABSTRACTThe Mw 6.6 Hokkaido Eastern Iburi earthquake striking southern Hokkaido Island on 5 September 2018 was a disastrous and peculiar event. In contrast to the usually shallow crustal earthquakes, this event occurred at a hypocentral depth about 37 km, close to the Moho discontinuity. To infer the rupture feature of the 2018 Hokkaido earthquake, we determine focal mechanism and centroid depth of the event with inversion of teleseismic waveforms. The result reveals that the centroid (at depth about 26 km) of this thrust earthquake is shallower than the hypocenter, which suggests the upward rupture propagation and dominant rupture in the lower crust. We also investigate the causative fault and rupture directivity based on waveform modeling. The steeply dipping fault (70°) with strike in the north–south direction is preferred to be the causative fault. The total dimension of rupture is estimated to be about 30 km, based on the aftershock distribution and rupture directivity. We propose that a seismogenic model with low temperature and complex stress field in the lower crust above the subduction‐zone interface may explain this event.

Active Crustal Deformation in the Trans‐Mexican Volcanic Belt as Evidenced by Historical Earthquakes During the Last 450 Years

AbstractThe Trans‐Mexican Volcanic Belt (TMVB) is oblique to the subduction zone. Background seismicity is very low. Nevertheless, two large instrumental earthquakes were recorded: the 1912 Acambay (Mw6.9) and the 1920 Jalapa (Mw6.4) earthquakes. Macroseismic data were systematically searched for crustal earthquakes on the TMVB contained in historical accounts. The macroseismic reports were inverted for the best fitting magnitude and epicenter. The western end of the TMVB shows the highest level of seismic activity. An earthquake is reported in 1568 (Mw7.2 ± 0.2) and a second large event took place in 1875 (Mw7.0 ± 0.2). Two other earthquakes are reported in the western TMVB in 1611 and 1749 (Mw6.4). The 1858 earthquake (Mw7.6 ± 0.3), in the central TMVB occurred on faults located to the west of the 1912 Acambay event. The eastern TMVB is less active. An earthquake in 1575 (Mw5.7) occurred east of the city of Puebla. The 1887 (Mw6.2 ± 0.2) and the 1920 Jalapa earthquakes are the larger events in this region. The seismicity observed from the instrumental and historical seismicity catalogs show that active tectonic deformation occurs throughout the TMVB and that it is not confined to specific regions where instrumental earthquakes have been located and active faults mapped. The number of large earthquakes in the TMVB, suggest unique large‐scale deformation of this complex volcanic belt. The presence of large earthquakes in this tectonic province, where 40% of the population of Mexico live, underlines the importance of reevaluating hazard in this densely populated region.