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On October 28, 2022, a moment magnitude (Mw) 3.8 earthquake occurred in Goesan, South Korea, typically characterized as a stable continental region. Herein, we analyze 42 earthquakes, including the Mw 3.8 earthquake, the largest foreshock (Mw 3.3), which preceded the mainshock by 17 s, and the largest aftershock (Mw 2.9). The primary aim of this study is to identify interactions among the seismic events. To this end, we utilized the permanent seismic networks with the closest station at 8.3 km from the epicenter, and the temporary network deployed eight hours after the mainshock's occurrence. Relocation results delineate that the mainshock occurred at the southeastern tip of the hypocenter distribution of three foreshocks, trending west-northwest–east-southeast. The aftershocks form an overall spatially diffused seismic pattern that propagates toward both ends of the inferred lineament in the downdip direction. The rupture directivity of the mainshock, along with waveform similarity across the mainshock and foreshocks, confirms the inferred geometry, corresponding well with the focal mechanisms of the mainshock and the largest foreshock. We demonstrate that the change in Coulomb failure stress (ΔCFS) by the largest foreshock was positive where the mainshock occurred and that the mainshock generated ΔCFS capable of triggering the propagation of the aftershocks. [ABSTRACT FROM AUTHOR]

Foreshocks and aftershocks occurred before and after the ML6.8 (Mw7.0) earthquake in eastern Taiwan on 18 September 2022. We explore the epicentral distribution and temporal variations for the mainshock, foreshocks, and aftershocks. Most of the events were located in the area around the Longitudinal Valley. Most foreshocks occurred around the mainshock, while the aftershocks happened outwards from the foreshock area. The temporal variations in seismic-wave energy show that the largest foreshock and the mainshock were responsible for releasing most of the energy during the earthquake sequence. In addition, the b values of the Gutenberg-Richter frequency-magnitude law were 0.62 for foreshocks, 0.87 for aftershocks, and 0.71 for the whole seismic activity by using the least squares method and 0.52 for foreshocks, 0.84 for aftershocks, and 0.65 for the whole seismic activity by using the maximum likelihood method. The b values increase from foreshocks to aftershocks, suggesting the possibility that the fluid pressure of faults during foreshocks is higher than that of the faults during aftershocks due to the outward migration of water. The p-value of the Omori-Utsu law for the aftershock sequence was estimated to be 0.92 for all aftershocks in the study, 1.39 for the aftershocks occurred in the first 6 days, and 1.30 for the aftershocks occurred in the first 12 days. The foreshock sequence could not be described by the inverse Omori law. [ABSTRACT FROM AUTHOR]

The February 6, 2023, Turkey earthquake with a moment magnitude (Mw=7.8) will be recognized as one of the most powerful earthquakes to strike a large metropolitan area in recent memory. This quake occurred in southern Turkey near the northern border of Syria along the southern western branch of the East Anatolian Fault (EAF). This major event was followed by numerous significant aftershocks, with 14,107 earthquakes occurring as of March 5, 2023. Preliminary data analysis in this study of aftershocks a month after the main earthquake (Mw=7.8) implies that the majority of the aftershock sequence was focused near the epicenter of the main shock displaced numerous fault segments within the EAF zone and can have indirect effects on neighboring fault systems. As a result, notable earthquake activity was observed along the northern section of the Dead Sea Transform (DST) fault system in Syria, Lebanon, and Palestine. According to statistical seismological analysis, 81% of aftershocks with magnitudes less than 3 occurred after the main shock, while 38 aftershocks with magnitudes 5 or greater occurred within the first 6 days, which includes 7.5 and 6.8 magnitude shocks. The depth distribution of the large main shocks and the aftershocks was located at shallower crustal depth. The aftershock sequence is mostly distributed in the first 15 km of the earth's crust, with significant occurrences occurring between 5 and 19 km deep. The expected aftershock scenario of such a large earthquake is to continue for several months or longer, possibly years. The interaction of the EAF and the DST fault is strongly recommended as an important research issue since it may well provide insights into the general tectonic activity and assist in better predicting future earthquakes in the region. [ABSTRACT FROM AUTHOR]

Immense liquefaction damage was observed in the 2011 off the Pacific coast of Tohoku Earthquake. It was reported that, in Chiba Prefecture, Japan, the main shock oozed muddy water from the sandy ground and the aftershock which occurred 29 min after the main shock intensified the water spouting; thus, the aftershock expanded the liquefaction damage in the sandy ground. For comprehending such a phenomenon, using a soil–water–air coupled elastoplastic finite deformation analysis code, a rise in groundwater level induced by main shock is demonstrated, which may increase the potential of liquefaction damage during the aftershock. The authors wish to emphasize that these results cannot be obtained without soil–water–air coupled elastoplastic finite deformation analysis. This is because the rise in groundwater level is caused by the negative dilatancy behavior (plastic volume compression) of the saturated soil layer which supplies water to the upper unsaturated soil layer, and it is necessary to precisely calculate the settlement of ground and the amount of water drainage/absorption to investigate the groundwater level rise. This study provides insight into the mechanism of ground liquefaction during a series of earthquakes. [ABSTRACT FROM AUTHOR]

This article proposes a framework to support postearthquake building safety and reoccupancy decisions by quantifying the change in building collapse risk following a mainshock earthquake event. This risk may be exacerbated by both an increase in seismic hazard due to aftershock activity and a reduction in building collapse resistance due to structural damage. To address these factors, the framework is based on a hazard that includes (1) both the steady-state and the aftershock occurrence rates, that is, the elevated hazard that accounts for the dependence on the mainshock magnitude and the aftershock rate that decays over time, and (2) revised collapse fragility functions that account for structural damage sustained during the mainshock. The framework is capable of addressing region-specific questions such as (1) What are the mainshock magnitudes for which aftershocks pose a life-safety concern? (2) How long does it take for the elevated risk due to aftershocks to dissipate? and (3) What gaps in current knowledge deserve further attention from the earthquake engineering and seismology communities? The framework addresses these questions for a 20-story building in San Francisco, assuming three different, hypothetical mainshock events of magnitudes 7,7.5, and 8 M W on the San Andreas fault. This is followed by a parametric study that considers a range of buildings and provides a graphical representation of the elevated risk to inform building evaluation (tagging) decisions, based on the intact building's collapse capacity, the amount of structural damage, and the length of time after the mainshock. [ABSTRACT FROM AUTHOR]

Damaging aftershock sequences often exhibit considerable spatio‐temporal complexity. The stress changes associated with coseismic slip and aseismic afterslip are commonly proposed to drive aftershock sequences, but few systematic studies exist and do not always support strong, universal driving relationships. To investigate the roles that these two sources of stress changes may play in driving aftershocks, we assess the spatio‐temporal relationships between coseismic slip, afterslip, and on‐fault (within 5 km) aftershock density following seven Mw6.0–7.6 continental‐settings earthquakes, using available high‐quality slip models and regional seismic data. From previous empirical work and frictional considerations, near the mainshock we expect coseismic slip and afterslip to be anti‐correlated, and aftershocks to occur where coseismic slip is low/zero, near high slip gradients, and/or to migrate with afterslip. However, we find that spatial relationships between afterslip and coseismic slip, and between afterslip and aftershock density differ between earthquakes. Aftershock density correlates with coseismic slip following five of the earthquakes, and with total cumulative slip (coseismic slip + afterslip) following six: indicating that on‐fault aftershock distributions may be approximated by total slip (at current resolutions). Additionally, we find that the gradients of coseismic slip and afterslip (proxies for new stress concentrations) do not clearly correlate with aftershock distributions and that the choice of spatial domain over which relationships are tested can affect results significantly. A possible explanation of these results is that fault zones contain considerable fine‐scale structural and frictional heterogeneity. Nonetheless, the empirical evidence for frequently assumed relationships between coseismic slip, afterslip and on‐fault aftershocks is mixed. Plain Language Summary: The spatial distributions of aftershocks following major earthquakes can be complex and varied. We investigate how the locations of aftershocks on the main fault relate to the distributions of slip from the mainshock itself and from a gentle relaxation process that occurs on the fault in the following weeks, months, and years, called afterslip: we expect aftershocks to broadly occur where afterslip is and where mainshock slip is low/zero. We investigate seven interesting mainshocks that occur within comparable continental settings and show that spatial relationships between coseismic slip, afterslip, and aftershocks vary between these case studies. Aftershocks generally occur where slip of some kind (either mainshock slip or afterslip) has been modeled, which may be useful for estimating the broad distribution of on‐fault aftershocks in the future. As mainshock slip, afterslip, and aftershocks often appear to be occurring in exactly the same regions (contradicting common assumptions), we propose that fault zones contain finer scale detail than can currently be resolved. Key Points: We explore how coseismic slip and afterslip distributions may influence on‐fault aftershock density following seven key continental earthquakesTotal modeled slip often correlates well with on‐fault aftershock density thus could be used as an indicator of aftershock evolutionCoseismic slip, afterslip, and aftershocks commonly co‐locate, contrary to expectations, possibly due to unresolved slip heterogeneity [ABSTRACT FROM AUTHOR]

Foreshocks and aftershocks occurred before and after the ML6.8 (Mw7.0) earthquake in eastern Taiwan on 18 September 2022. We explore the epicentral distribution and temporal variations for the mainshock, foreshocks, and aftershocks. Most of the events were located in the area around the Longitudinal Valley. Most foreshocks occurred around the mainshock, while the aftershocks happened outwards from the foreshock area. The temporal variations in seismic-wave energy show that the largest foreshock and the mainshock were responsible for releasing most of the energy during the earthquake sequence. In addition, the b values of the Gutenberg-Richter frequency-magnitude law were 0.62 for foreshocks, 0.87 for aftershocks, and 0.71 for the whole seismic activity by using the least squares method and 0.52 for foreshocks, 0.84 for aftershocks, and 0.65 for the whole seismic activity by using the maximum likelihood method. The b values increase from foreshocks to aftershocks, suggesting the possibility that the fluid pressure of faults during foreshocks is higher than that of the faults during aftershocks due to the outward migration of water. The p-value of the Omori-Utsu law for the aftershock sequence was estimated to be 0.92 for all aftershocks in the study, 1.39 for the aftershocks occurred in the first 6 days, and 1.30 for the aftershocks occurred in the first 12 days. The foreshock sequence could not be described by the inverse Omori law.

Immense liquefaction damage was observed in the 2011 off the Pacific coast of Tohoku Earthquake. It was reported that, in Chiba Prefecture, Japan, the main shock oozed muddy water from the sandy ground and the aftershock which occurred 29 min after the main shock intensified the water spouting; thus, the aftershock expanded the liquefaction damage in the sandy ground. For comprehending such a phenomenon, using a soil–water–air coupled elastoplastic finite deformation analysis code, a rise in groundwater level induced by main shock is demonstrated, which may increase the potential of liquefaction damage during the aftershock. The authors wish to emphasize that these results cannot be obtained without soil–water–air coupled elastoplastic finite deformation analysis. This is because the rise in groundwater level is caused by the negative dilatancy behavior (plastic volume compression) of the saturated soil layer which supplies water to the upper unsaturated soil layer, and it is necessary to precisely calculate the settlement of ground and the amount of water drainage/absorption to investigate the groundwater level rise. This study provides insight into the mechanism of ground liquefaction during a series of earthquakes.

This paper aims to evaluate the vertical-to-horizontal, V/H, spectral acceleration ratios from the earthquake ground motions recorded during the February 6, 2023 Turkey-Syria seismic events, which include doublet events with moment magnitudes of M w 7.7 and M w 7.6 that struck the city of Kahramanmaraş and a considerable aftershock with M w 6.7 that mainly struck the city of Nurdağı. Statistics of the observed V/H spectral ratios computed from records that have peak ground acceleration larger than 0.1 g showed that most records of the three earthquakes yielded spectral V/H ratios higher than 2/3, which is a typical value recommended by several seismic codes. Additionally, comparison of the observed V/H spectra with respect to the Ground Motion Predictive Models, GMPMs, for V/H ratios introduced by Bozorgnia and Campbell (2016), BC model, and Bommer et al. (2011), BAK model, revealed that both GMPMs mostly underestimate the V/H ratios at long spectral periods. Residual analysis showed that the capability of the BAK model for large source-to-site distances is more than that for small distances for which the ratios may be underestimated specially at long periods, while the estimations of the BC model do not significantly depend on the distance. Additionally, comparison of the observed V/H ratios with those computed based on the current 2018 edition of the Turkish seismic code indicated that the three considered earthquakes mostly yielded V/H ratios considerably higher than the code-based values in the long period range that could have higher damaging potential than those ratios in short period range. • Evaluation of the spectral vertical-to-horizontal, V/H, during the 2023 Turkey-Syria seismic events. • V/H spectra based on GMPMs and the Turkish seismic code. • Evaluation of the predictive equations using residuals analysis. [ABSTRACT FROM AUTHOR]