Search

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]

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]

Based on subdaily kinematic GNSS solutions, the fault slip properties during the very early postseismic phase after the 2021 M 8.2 Chignik earthquake are investigated in this paper. The very early postseismic deformations captured by near-field GNSS sites can be well depicted by the power model. The comparison of afterslip determined by daily and subdaily GNSS solutions suggests that neglecting very early afterslip can result in the underestimation of postseismic slip. Compared with coseismic slip, the cumulative afterslip of the first 24 h is mainly focused in the southeast of the hypocenter, and the shallow updip afterslip appears after this earthquake. The spatio-temporal evolution of the afterslip reveals that the patch of afterslip is immediately generated after the earthquake, and then the postseismic slip gradually grows along the afterslip patch. The magnitude of the afterslip patch varies remarkably within the 24 h following the earthquake, especially in the first several hours. Meanwhile, the spatio-temporal patterns of aftershocks and afterslip exhibit strong similarity during the first 24 h, suggesting that very early afterslip may be a possible driving factor of aftershocks. Moreover, most of the afterslip patches and aftershocks occurring immediately after this earthquake are situated in the area covered by positive Coulomb Stress Change (CSC), which implies that the immediate afterslip and aftershock activities can be influenced by the coseismic CSC. The following afterslip process further releases coseismic CSC and then influences the spatio-temporal variations of aftershock activities. Thus, the afterslip may be a possible triggering mechanism of very early aftershocks for this earthquake, alongside the effects of the CSC generated by coseismic rupture. [ABSTRACT FROM AUTHOR]

This manuscript investigates the bias introduced by scaling aftershock ground motions when evaluating the performance of structures subjected to earthquake sequences. The study focuses on different hysteretic behaviors exhibited by structures and selects eight intensity measures as scale indicators. A benchmark database comprising 274 recorded mainshock–aftershock ground motions is utilized for analysis. The findings reveal that scaling aftershock records using intensity measures such as SI (seismic intensity), PGV (peak ground velocity), IC (Arias intensity), and Sa (spectral acceleration) relative to mainshock records effectively controls the mean bias within 30% throughout the entire period range, given a maximum scale factor of 10.0. However, it is observed that the additional damage in systems exhibiting un-degrading hysteretic behavior is more significantly affected by aftershock ground motion scaling compared to systems with degrading hysteretic behavior. Furthermore, scaling aftershock ground motions upwards using relative Sa tends to overestimate the additional damage incurred by structures. These results emphasize the importance of considering the specific hysteretic behavior of structures when applying aftershock ground motion scaling, as well as selecting appropriate intensity measures for accurate evaluation of structural performance. [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 Aftershocks are a fundamental characteristic of seismicity, and their generation mechanism is mainly characterized by two physical models, stress transfer from large earthquakes and afterslip-induced stress loading. However, the contribution of each mechanism to aftershock generation remains unclear. Here we investigate the spatiotemporal variations in aftershock activity following the 2011 Tohoku-Oki earthquake by applying the Hierarchical Space–Time Epidemic-Type Aftershock Sequence (HIST-ETAS) model to the decade of recorded seismicity since the mainshock. Using the estimated HIST-ETAS model, we categorize the aftershocks into background earthquakes (which are caused by aseismic phenomena) and triggered earthquakes (which are caused by earthquake-to-earthquake interactions). Most of the earthquakes that occurred updip of the large coseismic slip zone along the Japan Trench are triggered earthquakes, consistent with the lack of afterslip in this area. Conversely, background earthquakes are the predominant earthquake type in the long-term downdip of the large coseismic slip zone, and they positively correlate with the afterslip evolution. Our results suggest the importance of combining these two end-member aftershock generation models to explain aftershock activity and thus provide new insights into the relationship between afterslip and spatiotemporal aftershock distribution. Our classifications may also contribute to the monitoring of afterslip in a given region that hosts a large earthquake, particularly where geodetic observation networks are too sparse to evaluate afterslip evolution.

Abstract The 2022 Mw6.2 Pasaman earthquake took place in central-west Sumatra in association with activity in the Sumatran Fault system. This study clarifies the spatial and temporal distribution of the Pasaman earthquake sequence and forecasts the earthquake sequence’s impact on the seismicity in the vicinity and in the Sumatran Fault system. We first examined the seismicity before the mainshock and observed temporal low b-value anomalies, shedding light on the earthquake’s precursor by monitoring b-values prior to the event. Based on the aftershocks in the first 18 days, we modeled the temporal distribution of the aftershocks according to the modified Omori’s law, which suggested this sequence could last 49–473 days. By further considering Båth’s law and the Gutenberg–Richter law, we estimated the temporal distribution of the maximum magnitudes in the aftershock sequence. To understand the spatial pattern of the aftershocks, we calculated the coseismic Coulomb stress change imparted by the Pasaman mainshock. Considering uncertainties of the Coulomb stress calculations from rupture geometry, mainshock parameters, friction coefficients, and strike angles of the receiver plane, the patterns of the Coulomb stress changes are similar that the stress increases extended northwest and southeast, consistent with aftershock distribution. We further evaluated rupture probability for each segment of the Sumatran Fault. Considering the stress perturbation imparted by the Pasaman earthquake, we expected a seismicity rate increase of ca. 40% at the Sumpur and Sianok segments in the short term. To quantify long-term rupture probability, the recurrence interval and the time elapsed since the previous earthquake were incorporated based on the time-dependent Brownian passage-time model. The earthquake probability at the Sumani segment in the coming 50 years was determined to be 72%. The results of this study have significant implications for subsequent probabilistic seismic hazard assessments, not only for Sumatra but also for certain metropolitan areas in Malaysia and Singapore.

Abstract: Earthquake triggering by seismic waves has been recognized as a phenomenon for nearly 30 years. However, our ability to study dynamic triggering has been limited by our ability to capture the triggering stresses accurately and record the resultant earthquakes. Here we use full waveforms from a dense seismic network and a modern, high‐resolution seismic catalog to measure triggering in Southern California from 2008 to 2017 based on interevent time ratios. We find that the fractional seismicity rate change, which we term triggering intensity or triggerability, as a function of peak strain change for the period of ∼20 s due to distant earthquakes is monotonically increasing and compatible with earlier measurements made with a disjoint data set from 1984 to 2008. A triggering strain of 1 microstrain is equivalent to the local productivity generated by an M1.8 earthquakes. This result implies that a prediction of seismicity rate changes can be made based on recorded ground shaking using the same formalism as currently used for aftershock prediction. For a teleseismic event, this small level of triggering occurs throughout the region and thus aggregates to a regional effect. We find that the triggering rate decays after the triggerer follows an Omori‐Utsu law, but at a much slower rate than a typical aftershock sequence. The slow decay rate suggests that an ancillary process such as creep or fluid flow must be part of dynamic triggering. The prevalence of triggering in areas of creep or fluid involvement reinforces this inference. A triggering cascade of secondary earthquakes is insufficient to explain the data.

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.

Aftershocks are a fundamental characteristic of seismicity, and their generation mechanism is mainly characterized by two physical models, stress transfer from large earthquakes and afterslip-induced stress loading. However, the contribution of each mechanism to aftershock generation remains unclear. Here we investigate the spatiotemporal variations in aftershock activity following the 2011 Tohoku-Oki earthquake by applying the Hierarchical Space–Time Epidemic-Type Aftershock Sequence (HIST-ETAS) model to the decade of recorded seismicity since the mainshock. Using the estimated HIST-ETAS model, we categorize the aftershocks into background earthquakes (which are caused by aseismic phenomena) and triggered earthquakes (which are caused by earthquake-to-earthquake interactions). Most of the earthquakes that occurred updip of the large coseismic slip zone along the Japan Trench are triggered earthquakes, consistent with the lack of afterslip in this area. Conversely, background earthquakes are the predominant earthquake type in the long-term downdip of the large coseismic slip zone, and they positively correlate with the afterslip evolution. Our results suggest the importance of combining these two end-member aftershock generation models to explain aftershock activity and thus provide new insights into the relationship between afterslip and spatiotemporal aftershock distribution. Our classifications may also contribute to the monitoring of afterslip in a given region that hosts a large earthquake, particularly where geodetic observation networks are too sparse to evaluate afterslip evolution. [ABSTRACT FROM AUTHOR]

The earthquake catalog notes that there have been earthquakes with Mw > 7 that hit the Flores area, three of which occurred in the Flores Sea in 1992, 2015, and 2021. Revealed that the seismic activity of Eastern Indonesia is thought to be influenced by the isolated thrust fault segment of the island of Flores and the island of Wetar. The study of the rising fault segment on Flores Island and Wetar Island helps in further understanding the fault behavior, earthquake pattern, and seismic risk in the Flores Sea region. In earthquakes with giant magneto, an aftershock can occur due to the interaction of ground movements. This research analyzes and compares the data from the evaluation of the classification algorithm and the regression algorithm. The initial stages of this research include requesting IRIS DMC Web Service data. The data is then subjected to a cleaning process to obtain the expected feature extraction. The next stage is to perform the clustering process. This stage is carried out to label dependent data by adding new features as data clusters. The following procedure divides the validation value, which consists of training and test data. The estimation results show that the classification algorithm's evaluation value is better than that of the regression algorithm. The evaluation value of several algorithms indicates this, with an accuracy rate between 80% and 100%. [ABSTRACT FROM AUTHOR]

The 2022 Mw6.2 Pasaman earthquake took place in central-west Sumatra in association with activity in the Sumatran Fault system. This study clarifies the spatial and temporal distribution of the Pasaman earthquake sequence and forecasts the earthquake sequence’s impact on the seismicity in the vicinity and in the Sumatran Fault system. We first examined the seismicity before the mainshock and observed temporal low b-value anomalies, shedding light on the earthquake’s precursor by monitoring b-values prior to the event. Based on the aftershocks in the first 18 days, we modeled the temporal distribution of the aftershocks according to the modified Omori’s law, which suggested this sequence could last 49–473 days. By further considering Båth’s law and the Gutenberg–Richter law, we estimated the temporal distribution of the maximum magnitudes in the aftershock sequence. To understand the spatial pattern of the aftershocks, we calculated the coseismic Coulomb stress change imparted by the Pasaman mainshock. Considering uncertainties of the Coulomb stress calculations from rupture geometry, mainshock parameters, friction coefficients, and strike angles of the receiver plane, the patterns of the Coulomb stress changes are similar that the stress increases extended northwest and southeast, consistent with aftershock distribution. We further evaluated rupture probability for each segment of the Sumatran Fault. Considering the stress perturbation imparted by the Pasaman earthquake, we expected a seismicity rate increase of ca. 40% at the Sumpur and Sianok segments in the short term. To quantify long-term rupture probability, the recurrence interval and the time elapsed since the previous earthquake were incorporated based on the time-dependent Brownian passage-time model. The earthquake probability at the Sumani segment in the coming 50 years was determined to be 72%. The results of this study have significant implications for subsequent probabilistic seismic hazard assessments, not only for Sumatra but also for certain metropolitan areas in Malaysia and Singapore. [ABSTRACT FROM AUTHOR]

Liquefaction caused by long-term cyclic loads in loose saturated soil can lead to ground subsidence and superstructure failures. To address this issue, this study aimed to emulate the liquefaction phenomenon based on a shaking table test while especially focusing on the soil behavior mechanism due to the reliquefaction effect. Liquefaction and reliquefaction behaviors were analyzed by ground conditions where an embankment was located on the coastal ground. Silica sand was used for the experiment for various thickness and liquefiable conditions, and the embankment model was constructed above the model ground. For seismic waves, sine wave excitation was applied, and a total of five excitations (cases) were conducted. When the upper ground layer consisted of a non-liquefiable layer, liquefaction did not occur due to the first excitations but occurred by the third excitation. The results indicated that as the earthquake was applied, the water level in the liquefiable layer increased to the height of the non-liquefiable layer and liquefaction could occur. It was identified that even if liquefaction did not occur for the main earthquake, liquefaction could occur due to aftershocks caused by a rise in the groundwater level due to a series of earthquakes. In a general seismic design code, liquefaction assessment is performed only for soil layers below the groundwater level; however, when successive earthquakes occur, unexpected liquefaction damage could occur. Therefore, to mitigate the earthquake risk of liquefaction for coastal embankments, it is necessary to evaluate the liquefaction by aftershocks even when the groundwater level of the ground layer under an embankment is low. [ABSTRACT FROM AUTHOR]

Evaluation of the residual seismic capacity (RSC) of post-earthquake damaged buildings is instrumental to the formation of reasonable recovery strategies. At present, incremental dynamic analysis (IDA) that considers the mainshock and aftershock is the method most frequently used to evaluate the RSC of damaged structures. However, the mainshock-induced structural damage determined using the IDA method may be inconsistent with the damage observed in actual engineering. This inconsistency could potentially lead to an unreasonable evaluation result. To overcome this drawback, it is necessary to evaluate the RSC of damaged structures according to their observed damage instead of that obtained by the IDA. In this paper, a method of evaluating the RSC of damaged reinforced concrete (RC) columns is proposed. First, the damage degree and distribution of the damaged columns were evaluated via visual inspection after mainshocks. Then, a numerical model was developed to predict the residual behavior of damaged columns subjected to aftershocks. After that, the RSC of damaged columns was estimated based on fragility analysis. The degradation of the collapse capacity of damaged columns was quantified by the collapse fragility index (CFI), and a parameter analysis was conducted to study the effect of structural parameters on the CFI of damaged columns. Lastly, an empirical model for predicting the CFI was proposed, facilitating the application of this study in actual post-earthquake assessments. The parameter analysis indicates that the axial load ratio of the columns and the degree of damage degree accumulated during mainshocks have a significant effect on the CFI. Additionally, the proposed empirical model can effectively predict the degradation of the collapse capacity of RC columns in existing test data, with an accuracy of 0.82. [ABSTRACT FROM AUTHOR]

On 21 May 2021, the MS 6.4 earthquake struck Yangbi County, Dali City, Yunnan Province. Minor seismic activities were common both before and after the earthquake; the foreshock sequence activity characteristics of this earthquake were studied to gain a better understanding and more perspective. First, between May 18 and 5 July 2021, we collected data of the seismic events from the Yangbi MS 6.4 earthquake sequence, determined the minimum magnitude of completeness of the sequence based on the magnitude-frequency relationship, filtered out the more complete earthquake sequences, and performed double-difference earthquake relocation using the HypoDD method. Then, we improved the nearest-neighbor distance algorithm and used it in conjunction with the Gaussian mixed model fitting method to conduct a comprehensive multiple factor analysis of the Yangbi MS 6.4 earthquake sequence. The main findings of our preliminary analysis are as follows: 1) We distinguished the foreshocks, mainshocks, and aftershocks of the Yangbi earthquake sequence as well as their higher-order aftershocks, using the method described in this paper, and we can obtain clear intergenerational relationships between them. We identified eight “foreshocks” with statistically greater physical significance than the others and found that majority of the shocks occurred shortly after the mainshock were direct aftershocks, with secondary or higher-order aftershocks gradually increasing in the later stage of the sequence. 2) Combining the double-difference earthquake relocation and the event distribution on the fault plane, we found that the b-value of the Yangbi foreshock sequence was clearly on the lower end of the spectrum, with an obvious nonlinear amplification process, and can be divided into three foreshock sub-sequences: sub-sequence I with the lowest b-value and a concentrated spatial distribution, which is an F-value foreshock sequence; subsequence II with an overall shallower source depth and an obvious rebound in b-value, which is an explosive aftershock sequence; subsequence III with a lower b-value and a rapid rupture spread, which is a typical U-F-ρ foreshock. 3) We constructed a topological tree of the Yangbi foreshock sequence in Yunnan Province using the nearest-neighbor distance algorithm, combining the correlation between earthquake sequence type and fault rupture intensity. We analyzed the distribution and topology of the three sub-sequences (combined) of the Yangbi MS 6.4 foreshock sequence and found the following: the topology of subsequence I was more linear than the sequence II, each was the parent event of the subsequent event, and its rupture mode was similar to the fluid intrusion rupture in a specific channel; sequence II had a relatively simple topology, exhibiting a spray topology, and independently formed a main-aftershock mini-sequence, probably triggered by its shallow brittle rupture, which caused the fluid intrusion rupture to spread faster and eventually triggered the Yangbi MS6.4 mainshock.

Based on subdaily kinematic GNSS solutions, the fault slip properties during the very early postseismic phase after the 2021 M 8.2 Chignik earthquake are investigated in this paper. The very early postseismic deformations captured by near-field GNSS sites can be well depicted by the power model. The comparison of afterslip determined by daily and subdaily GNSS solutions suggests that neglecting very early afterslip can result in the underestimation of postseismic slip. Compared with coseismic slip, the cumulative afterslip of the first 24 h is mainly focused in the southeast of the hypocenter, and the shallow updip afterslip appears after this earthquake. The spatio-temporal evolution of the afterslip reveals that the patch of afterslip is immediately generated after the earthquake, and then the postseismic slip gradually grows along the afterslip patch. The magnitude of the afterslip patch varies remarkably within the 24 h following the earthquake, especially in the first several hours. Meanwhile, the spatio-temporal patterns of aftershocks and afterslip exhibit strong similarity during the first 24 h, suggesting that very early afterslip may be a possible driving factor of aftershocks. Moreover, most of the afterslip patches and aftershocks occurring immediately after this earthquake are situated in the area covered by positive Coulomb Stress Change (CSC), which implies that the immediate afterslip and aftershock activities can be influenced by the coseismic CSC. The following afterslip process further releases coseismic CSC and then influences the spatio-temporal variations of aftershock activities. Thus, the afterslip may be a possible triggering mechanism of very early aftershocks for this earthquake, alongside the effects of the CSC generated by coseismic rupture.

This manuscript investigates the bias introduced by scaling aftershock ground motions when evaluating the performance of structures subjected to earthquake sequences. The study focuses on different hysteretic behaviors exhibited by structures and selects eight intensity measures as scale indicators. A benchmark database comprising 274 recorded mainshock–aftershock ground motions is utilized for analysis. The findings reveal that scaling aftershock records using intensity measures such as SI (seismic intensity), PGV (peak ground velocity), IC (Arias intensity), and Sa (spectral acceleration) relative to mainshock records effectively controls the mean bias within 30% throughout the entire period range, given a maximum scale factor of 10.0. However, it is observed that the additional damage in systems exhibiting un-degrading hysteretic behavior is more significantly affected by aftershock ground motion scaling compared to systems with degrading hysteretic behavior. Furthermore, scaling aftershock ground motions upwards using relative Sa tends to overestimate the additional damage incurred by structures. These results emphasize the importance of considering the specific hysteretic behavior of structures when applying aftershock ground motion scaling, as well as selecting appropriate intensity measures for accurate evaluation of structural performance.

Abstract Geodetic evidence for slow slip recurrence changed by stress perturbations was rare, especially from afterslip following a nearby large earthquake. The first observed slow slip events in the southernmost Ryukyu subduction had occurred in 2005, 2009, and 2015 following the nearby 2002 March 31 Mw 7.1 Hualien offshore earthquake. To investigate whether the Mw 7.1 earthquake had influenced the occurrence times of the slow slip, we calculated the coseismic slip distribution and afterslip distribution using the surface displacements from onshore Global Navigation Satellite System observations. The stress perturbation on the slow-slip regions caused by the coseismic slip was quantified using the Coulomb failure criteria. We also examined the aftershock distribution and the evolution with time to clarify the stress perturbations from the afterslip offshore. Our results show that the primary afterslip distribution may have overlapped the 2009 and 2015 slow-slip patch at the downdip of the earthquake. The coseismic stress perturbation may have influenced the SSEs area directly by a Coulomb stress increase of probably 0.10 MPa. However, the 2005 SSE patch in the updip depths had only a little coseismic slip and afterslip with the Coulomb stress increase of approximately 0.06 MPa. We find that most of the aftershocks had occurred in the 2009 and 2015 slow-slip region that evolved into a typical aftershock sequence at least 2.5 years after the earthquake. The surface geodetic displacements reveal that the afterslip may have lasted longer than 4.5 years after the earthquake. The evidence for the afterslip proves that the 2009 and 2015 slow-slip patch was influenced by the afterslip directly for years. We suggest that the ongoing afterslip may have modulated the coseismic stress perturbation. It may explain the delay of the 2009 slow slip occurrence compared with the recurrence interval between the 2009 and 2015 slow slip events. Graphical Abstract

Four RC moment frames (RCMFs) with 4–20 stories are designed, modeled, and subjected to IDA procedures by considering various levels of prior damage caused by mainshock (MS). The collapse of the structures is then identified by gradual scale-up of the aftershock until dynamic instability is observed. The percentage of the energies dissipated by various parts of the structures and via different modes is next studied at three states during the collapse-inducing excitation. These states include the final residual state, the maximum drift state, and the maximum kinetic energy state. While an almost constant contribution of around 30% is provided by the inherent damping of the nodal masses, the share of beams' hysteretic energy varies between 10% in the low-rise shear-dominated frame and 30% in the high-rise flexure-dominated RCMFs. The contribution of the columns' hysteresis also changes in a reverse manner so that the sum of ratios provided by the beams and columns hysteresis is kept constant and equal to around 70%. The prior damage caused by the MS also leads to changes in the contribution of various elements and stories due to the deterioration MS induces to the strength of the members. [ABSTRACT FROM AUTHOR]

Featured Application: Resource and environmental effects of the Tibetan Plateau. Precursory earth tidal triggering is believed to influence earthquake timing preferentially when a region is critically stressed. However, whether and how the recurrence of aftershocks after a giant earthquake is affected by tidal triggering remains perplexing. To provide insight into this study, we utilized the Schuster test to explore the tidally induced stress variation correlated with the 2011 Mw 9.0 Tohoku earthquake aftershock sequence by determining the tidal phase angle at the occurrence time of events and the periodic characteristics of the aftershocks. Our results show that the aftershocks were triggered by short-period tides, including semidiurnal and diurnal tides. The rupture associated with the mainshock likely resulted in a critical stress state in the focal region, which is conducive to tidal triggering. We subdivided the aftershock catalog into several subsets, using a depth of 30 km and a magnitude of 5 as discriminators. The analysis of these subsets reveals that weaker and deeper earthquakes are best correlated with Earth tides, which will be helpful to investigate the mechanisms of tidal correlation. [ABSTRACT FROM AUTHOR]

Some characteristics of the earthquake catalog and seismic process are considered. The catalog is obtained according to the KNET network (KN-code in FSDN, International Federation of Digital Seismograph Networks, that is operated by Research Station of RAS, RS RAS) and contains more than 10000 earthquake parameters from 1994 to 2020. The catalog and seismicity characteristics were determined the whole catalog as well as the catalog that is limited by the coordinates of the Bishkek geodynamic polygon (BGP). Statistical analysis of arrival times of direct P- and S-waves registered at KNET network stations was carried out. The maximum P- and S-waves were registered at AAK station, the minimum – at ULHL station. The territory of the BGP is covered by great P-traces density. We analyzed earthquake localization errors, i.e. the difference between the observed and calculated arrival times of P-and S-wave fronts (RMS), horizontal error (ERH, epicenter) and vertical error (ERZ, depth). Occurred on the BGP territory earthquakes have minimum values of the considered errors. The representative sample is defined: for the whole catalog it includes K≥7.2 earthquakes and for the limited by BGP coordinates catalog – K≥6.7. Statistical characteristics of the representative part of the catalog in terms of time and depth are determined. The temporal distribution of earthquakes by energy classes is constructed and the absence of positive or negative trends in the number of events is noted. Spatial distribution of earthquakes by depth – 0–5, 5–10, 10–15 and more than 15 km is constructed. The smallest errors of earthquake depth determination have the events that occurred on the BGP territory. During the study 46 moderate earthquakes with K≥12 occurred. The most part of these events happened in the North Tien Shan seismgenic zone. 22 events were determined with K≥10 followed by aftershock sequences and give some aftershock characteristics. The most events with aftershocks occurred in the eastern part of the Kyrgyz ridge. The distribution of earthquake numbers and STD intensity is constructed. Zones of seismic activity and maximum intense Earth’s crust deformation are identified. The Gutenberg – Richter law and provisions of nonextensive statistical physics were used to describe the energy distribution function of earthquakes.

Liquefaction caused by long-term cyclic loads in loose saturated soil can lead to ground subsidence and superstructure failures. To address this issue, this study aimed to emulate the liquefaction phenomenon based on a shaking table test while especially focusing on the soil behavior mechanism due to the reliquefaction effect. Liquefaction and reliquefaction behaviors were analyzed by ground conditions where an embankment was located on the coastal ground. Silica sand was used for the experiment for various thickness and liquefiable conditions, and the embankment model was constructed above the model ground. For seismic waves, sine wave excitation was applied, and a total of five excitations (cases) were conducted. When the upper ground layer consisted of a non-liquefiable layer, liquefaction did not occur due to the first excitations but occurred by the third excitation. The results indicated that as the earthquake was applied, the water level in the liquefiable layer increased to the height of the non-liquefiable layer and liquefaction could occur. It was identified that even if liquefaction did not occur for the main earthquake, liquefaction could occur due to aftershocks caused by a rise in the groundwater level due to a series of earthquakes. In a general seismic design code, liquefaction assessment is performed only for soil layers below the groundwater level; however, when successive earthquakes occur, unexpected liquefaction damage could occur. Therefore, to mitigate the earthquake risk of liquefaction for coastal embankments, it is necessary to evaluate the liquefaction by aftershocks even when the groundwater level of the ground layer under an embankment is low.

Geodetic evidence for slow slip recurrence changed by stress perturbations was rare, especially from afterslip following a nearby large earthquake. The first observed slow slip events in the southernmost Ryukyu subduction had occurred in 2005, 2009, and 2015 following the nearby 2002 March 31 Mw 7.1 Hualien offshore earthquake. To investigate whether the Mw 7.1 earthquake had influenced the occurrence times of the slow slip, we calculated the coseismic slip distribution and afterslip distribution using the surface displacements from onshore Global Navigation Satellite System observations. The stress perturbation on the slow-slip regions caused by the coseismic slip was quantified using the Coulomb failure criteria. We also examined the aftershock distribution and the evolution with time to clarify the stress perturbations from the afterslip offshore. Our results show that the primary afterslip distribution may have overlapped the 2009 and 2015 slow-slip patch at the downdip of the earthquake. The coseismic stress perturbation may have influenced the SSEs area directly by a Coulomb stress increase of probably 0.10 MPa. However, the 2005 SSE patch in the updip depths had only a little coseismic slip and afterslip with the Coulomb stress increase of approximately 0.06 MPa. We find that most of the aftershocks had occurred in the 2009 and 2015 slow-slip region that evolved into a typical aftershock sequence at least 2.5 years after the earthquake. The surface geodetic displacements reveal that the afterslip may have lasted longer than 4.5 years after the earthquake. The evidence for the afterslip proves that the 2009 and 2015 slow-slip patch was influenced by the afterslip directly for years. We suggest that the ongoing afterslip may have modulated the coseismic stress perturbation. It may explain the delay of the 2009 slow slip occurrence compared with the recurrence interval between the 2009 and 2015 slow slip events. [ABSTRACT FROM AUTHOR]

Abstract This study comprehensively investigates the source mechanisms associated with the mainshock and aftershocks of the Mw = 6.3 Yogyakarta earthquake which occurred on May 27, 2006. The process involved using moment tensor inversion to determine the fault plane parameters and joint inversion which were further applied to understand the spatial and temporal slip distributions during the earthquake. Moreover, coseismal slip distribution was overlaid with the relocated aftershock distribution to determine the stress field variations around the tectonic area. Meanwhile, the moment tensor inversion made use of near-field data and its Green’s function was calculated using the extended reflectivity method while the joint inversion used near-field and teleseismic body wave data which were computed using the Kikuchi and Kanamori methods. These data were filtered through a trial-and-error method using a bandpass filter with frequency pairs and velocity models from several previous studies. Furthermore, the Akaike Bayesian Information Criterion (ABIC) method was applied to obtain more stable inversion results and different fault types were discovered. Strike–slip and dip-normal were recorded for the mainshock and similar types were recorded for the 8th aftershock while the 9th and 16th June were strike slips. However, the fault slip distribution from the joint inversion showed two asperities. The maximum slip was 0.78 m with the first asperity observed at 10 km south/north of the mainshock hypocenter. The source parameters discovered include total seismic moment M0 = 0.4311E + 19 (Nm) or Mw = 6.4 with a depth of 12 km and a duration of 28 s. The slip distribution overlaid with the aftershock distribution showed the tendency of the aftershock to occur around the asperities zone while a normal oblique focus mechanism was found using the joint inversion.

Abstract We quantitatively examined the influence of pore fluid pressure and coseismic stress changes on the seismicity rate changes that followed the 2016 Kumamoto earthquake, on the basis of two approaches. One is a numerical calculation of the classic stress metric of ∆CFS, and the other is an inversion analysis of pore fluid pressure fields with earthquake focal mechanism data. The former calculation demonstrated that seismicity rate changes were consistent with the expectation from ∆CFS in 65% of the target region, whereas they were not in the remaining 35% of the region. The latter analysis indicates that seismicity rates increased in the regions where pore fluid pressure before the Kumamoto earthquake sequence was remarkably enhanced above hydrostatic, regardless of values of ΔCFS. This suggests that the increase in pore fluid pressure is one of the important physical mechanisms triggering aftershock generation. We obtained evidence that pore fluid pressure increased around the southern part of the main rupture zone after the mainshock, examining temporal changes in types of focal mechanism data. The average increases in pore fluid pressure were estimated to be 17, 20, and 17 MPa at depths of 5, 10, and 15 km, respectively. These large increases in pore fluid pressure cannot be explained under the undrained condition. The spatial derivative of the pore fluid pressure field in the depth direction implies that fluid supply from greater depths may have controlled increases in seismicity rates that followed the large earthquake.

Evaluation of the residual seismic capacity (RSC) of post-earthquake damaged buildings is instrumental to the formation of reasonable recovery strategies. At present, incremental dynamic analysis (IDA) that considers the mainshock and aftershock is the method most frequently used to evaluate the RSC of damaged structures. However, the mainshock-induced structural damage determined using the IDA method may be inconsistent with the damage observed in actual engineering. This inconsistency could potentially lead to an unreasonable evaluation result. To overcome this drawback, it is necessary to evaluate the RSC of damaged structures according to their observed damage instead of that obtained by the IDA. In this paper, a method of evaluating the RSC of damaged reinforced concrete (RC) columns is proposed. First, the damage degree and distribution of the damaged columns were evaluated via visual inspection after mainshocks. Then, a numerical model was developed to predict the residual behavior of damaged columns subjected to aftershocks. After that, the RSC of damaged columns was estimated based on fragility analysis. The degradation of the collapse capacity of damaged columns was quantified by the collapse fragility index (CFI), and a parameter analysis was conducted to study the effect of structural parameters on the CFI of damaged columns. Lastly, an empirical model for predicting the CFI was proposed, facilitating the application of this study in actual post-earthquake assessments. The parameter analysis indicates that the axial load ratio of the columns and the degree of damage degree accumulated during mainshocks have a significant effect on the CFI. Additionally, the proposed empirical model can effectively predict the degradation of the collapse capacity of RC columns in existing test data, with an accuracy of 0.82.

The paper presents the results of calculations, as well as further analysis of the kinematical parameter Vp/Vs in the source area of the strong earthquake (Mw = 7.3), which occurred on December 20, 2018 at the junction of the Kurile-Kamchatka and Aleutian trenches. Calculations of the Vp/Vs field based on the results of processing the parameters of moderate and weak earthquakes (Mw ≤ 6.5) in the earthquake source area and its surroundings, registered by the system of seismic observations of the Kamchatka branch of Federal Research Center « Geophysical Survey of RAS» in the period from 2009 to 15.06.2019. The results of the analysis of the Vp/Vs field and its comparison with the mechanisms of strong events in the vicinity of the source area of the studied earthquake lead us to the conclusion that the kinematic parameter Vp/Vs characterizes the elastic and strength properties of the geophysical environment, and also reflects the nature and changes in the stress field. The paper discusses also the probable effect of deformation regime on the type and form of distribution of the Vp/Vs parameter. This may be of particular importance for the development of methods of earthquake prediction and diagnosis of the environment.

Abstract The relative locations between mainshocks and their aftershocks have long been studied to characterize the mainshock–aftershock relationships, yet these comparisons may be subjected to biases inherited from various sources, such as the analysis method, data, and model parameters. Here, we perform both a relocation analysis of interplate events to obtain accurate relative centroid locations and a slip inversion analysis of the mainshock slip relative to the relocated events, with some of the relocated events used as empirical Green’s functions, to retrieve the spatiotemporal slip features of the mainshock relative to all of the relocated events. We perform these analyses on the large ( $$6.0 \le M_{W} \le 7.3$$ 6.0 ≤ M W ≤ 7.3 ) interplate earthquakes that occurred near four giant ( $$M_{W} \ge 8.5$$ M W ≥ 8.5 ) megathrust earthquakes: the 2007 $$M_{W} \;8.5$$ M W 8.5 Bengkulu (Indonesia), 2005 $$M_{W} \;8.6$$ M W 8.6 Nias (Indonesia), 2010 $$M_{W} \;8.8$$ M W 8.8 Maule (Chile), and 2011 $$M_{W} \;9.1$$ M W 9.1 Tohoku-Oki (Japan) earthquakes. Most of the spatiotemporal slip features of the mainshocks are consistently recovered using different empirical Green’s functions. We qualitatively and quantitatively demonstrate that the large interplate aftershocks within 5 years of the four analyzed mainshocks are largely located on the periphery or outside of the large-slip regions of these four giant megathrust earthquakes.

The paper presents the first results of integrated research of the strong seismic event (Uglovoye Podnyatiye earthquake (UPE) with a magnitude of Mw=7.3) that occurred on December 20, 2018 in the junction zone between the Kuril-Kamchatka and Aleutian island arcs. The authors has provided macroseismic information about the UPE, calculated peak ground accelerations and the seismic moment tensor, and described briefly the forshock and aftershock processes, as well as summarized data on earthquake precursors. UPE was reported to be felt at distances of up to 500 km. The maximum shaking was recorded as I = 5–6 at an epicentral distance of Δ~90 km on Bering Island, where the maximum instrumental acceleration of ~60 cm/s2 was recorded. It is shown that foreshocks remote in time occurred 2 years prior to the UPE. The distinctive grouping of the aftershocks in three spatially isolated clusters characterized by different parameters of the seismicity time course is related to the long and intense aftershock process of the UPE. The strongest aftershock had magnitude ML=6.6, another 25 events corresponded to ML≥5.0. UPE was preceded by precursor anomalies in the parameters of geophysical fields and the seismic process. The authors estimated the size of the UPE rupture, the depth of the equivalent source, the magnitude of the focal slip, and the duration of the rupture. The seismic moment tensor UPE has a large non-dipole component.

Abstract To clarify the relationship between earthquake occurrence and fluid, we analyzed data from a dense aftershock-observation network with 69 high-gain short-period seismographs installed immediately after the mainshock occurrence (October 21) in the aftershock area of the 2016 Central Tottori Prefecture earthquake. We determined the hypocenters and focal mechanisms of the aftershocks very precisely in the period from October 22 to December 15. We then investigated the temporal changes in the spatial distributions of hypocenters and T-axis azimuths of focal mechanisms. The distributions of aftershock hypocenters and T-axis azimuths are basically temporally stable, except those in limited portions in the shallow layer near the western edge of the aftershock area, where rapid decrease of aftershocks with T-axis azimuths of WSW to west was observed. If fluid rises from the lower crust due to fault rupture, the locations of aftershocks and focal mechanisms may change over time, especially in the deepest part of the aftershock region. However, the temporal change in these parameters was not apparent at depth. These observations suggest that the aftershock activity of the Central Tottori Prefecture earthquake was controlled mainly by stress concentration rather than strength reduction due to high fluid pressure.

The April 25, 2015 Mw 7.9 Gorkha earthquake in Nepal was characterized by a peak slip of several meters and persisting aftershocks. We report here that, in addition, a dense seismic swarm initiated abruptly in August 2017 at the western edge of the afterslip region, below the high Himalchuli-Manaslu range culminating at 8156 m, a region seismically inactive during the past 35 years. Over 6500 events were recorded by the Nepal National Seismological Network with local magnitude ranging between 1.8 and 3.7 until November 2017. This swarm was reactivated between April and July 2018, with about 10 times less events than in 2017, and in 2019 with only sporadic events. The relocation of swarm earthquakes using proximal temporary stations ascertains a shallow depth of hypocenters between the surface and 20 km depth in the High Himalayan Crystalline slab. This swarm reveals an intriguing localized interplay between orogenic collapse and stress adjustments, involving possibly CO2-rich fluid migration, more likely post-seismic slip and seasonal enhancements.

Precursory earth tidal triggering is believed to influence earthquake timing preferentially when a region is critically stressed. However, whether and how the recurrence of aftershocks after a giant earthquake is affected by tidal triggering remains perplexing. To provide insight into this study, we utilized the Schuster test to explore the tidally induced stress variation correlated with the 2011 Mw 9.0 Tohoku earthquake aftershock sequence by determining the tidal phase angle at the occurrence time of events and the periodic characteristics of the aftershocks. Our results show that the aftershocks were triggered by short-period tides, including semidiurnal and diurnal tides. The rupture associated with the mainshock likely resulted in a critical stress state in the focal region, which is conducive to tidal triggering. We subdivided the aftershock catalog into several subsets, using a depth of 30 km and a magnitude of 5 as discriminators. The analysis of these subsets reveals that weaker and deeper earthquakes are best correlated with Earth tides, which will be helpful to investigate the mechanisms of tidal correlation.

Abstract Earthquake triggering by seismic waves has been recognized as a phenomenon for nearly 30 years. However, our ability to study dynamic triggering has been limited by our ability to capture the triggering stresses accurately and record the resultant earthquakes. Here we use full waveforms from a dense seismic network and a modern, high‐resolution seismic catalog to measure triggering in Southern California from 2008 to 2017 based on interevent time ratios. We find that the fractional seismicity rate change, which we term triggering intensity or triggerability, as a function of peak strain change for the period of ∼20 s due to distant earthquakes is monotonically increasing and compatible with earlier measurements made with a disjoint data set from 1984 to 2008. A triggering strain of 1 microstrain is equivalent to the local productivity generated by an M1.8 earthquakes. This result implies that a prediction of seismicity rate changes can be made based on recorded ground shaking using the same formalism as currently used for aftershock prediction. For a teleseismic event, this small level of triggering occurs throughout the region and thus aggregates to a regional effect. We find that the triggering rate decays after the triggerer follows an Omori‐Utsu law, but at a much slower rate than a typical aftershock sequence. The slow decay rate suggests that an ancillary process such as creep or fluid flow must be part of dynamic triggering. The prevalence of triggering in areas of creep or fluid involvement reinforces this inference. A triggering cascade of secondary earthquakes is insufficient to explain the data.

The effect of aftershocks on the fragility of single-story masonry structures is investigated using probabilistic seismic demand analysis Finite element models of an unreinforced masonry (URM) structure and a confined masonry (CM) structure are established and their seismic response characteristics when subjected to mainshock, aftershock, and the mainshock-aftershock sequence are then comparatively investigated. The effects of aftershocks and the use of confining members on the seismic response are studied. Probabilistic seismic demand models of the structures are built, and fragility curves under various conditions are derived to investigate the effect of aftershocks on structural fragility. The maximum roof displacement and maximum inter-story drift ratio are lower in the confined masonry model than in the unreinforced masonry model; additionally, the probability of exceedance (PE) values of each damage limit state reduced, and those of the mainshock-damaged models subjected to aftershock significantly increase compared to those directly subjected to a same-intensity aftershock. The probability of severe damage or collapse compared with the mainshock-damaged CM model is greater than when each is subjected to a same intensity aftershock. The use of confining members benefits aftershock resistance and reduces the failure probability of the mainshock-damaged structure. The PE values significantly increase with the aftershock scaling factor δ. Therefore, the effect of aftershocks should be considered in the seismic design and analysis of masonry structures.

Abstract We reconstructed the spatiotemporal evolution of seismicity associated with the 2018 M w 5.6 northern Osaka earthquake, Japan, to discuss the source fault model of the mainshock rupture, the possible link between this rupture and known active faults, and subsequent crustal deformation. We first relocated the hypocenters listed in the earthquake catalog determined by the Japan Meteorological Agency using a double-difference relocation algorithm. We then searched for the earthquake waveforms that closely resembled those of the relocated hypocenters by applying a matched filtering technique to the continuous waveform data. The relocated hypocenters revealed two distinct planar alignments with different fault geometries. A combination of the relocated hypocenters and focal mechanisms suggests that the mainshock rupture initiated on a NNW–SSE-striking thrust fault, dipping ~ 45° to the east, with the rupture propagating to an adjacent sub-vertical ENE–WSW-striking strike-slip fault ~ 0.3 s after the initial mainshock rupture, resulting in the simultaneous propagation of dynamic rupture along the two faults. The strike-slip fault is oblique to the strike of the Arima-Takatsuki Fault, indicating that blind strike-slip faulting occurred. While the east-dipping thrust fault is located deeper than the modeled extent of the Uemachi Fault, a simple extrapolation of the near-surface geometry of the Uemachi fault partially overlaps the mainshock rupture area. Although it is unclear as to whether a blind thrust fault or a deep portion of the Uemachi Fault ruptured during this mainshock–aftershock sequence, a mainshock rupture would have transferred a static stress change of > 0.1 MPa to a portion of the east-dipping thrust fault system. Intensive aftershocks have persisted along the northern and southern edges of the source area, including moderate-magnitude events, whereas the seismicity in the central part of the source area has shown a rapid decay over time. Delayed triggered aftershocks were clearly identified along the northern extension of the rupture area. Because the background seismicity is predominant in this northern area, we interpret that aseismic deformation, such as cataclastic flow lubricated by crustal fluids, triggered this off-fault seismicity.

The paper discusses the results of the detection of seismic and infrasonic waves generated by a major earthquake and its aftershock (the moment magnitude MW=4.9 and MW=4.2 respectively), which occurred in northern Mongolia under Lake Hovsgool on December 5, 2014. The joint analysis of waveforms of seismic and infrasonic oscillations has shown that the signal recorded by the infrasound station of the Geophysical Observatory of the Institute of Solar-Terrestrial Physics SB RAS (ISTP SB RAS) is formed from sources of three generation types: local, secondary, and epicentral. This analysis enables us to propose a hypothesis of generation of epicentral infrasonic signal by flexural waves in an elastic ice membrane on the surface of Lake Hovsgool, which appear during the passage of seismic wave packets. This hypothesis explains the similarity between seismic and epicentral infrasonic signals, negative initial phase of epicentral infrasonic waves, and detection of a weak signal after a small-magnitude aftershock.

Numerous landslide dams have been induced in recent years as a result of frequent earthquakes and extreme climate hazards. Landslide dams present serious threats to lives and properties downstream due to potentially breaching floods from the impounded lakes. To investigate the factors influencing the stability of landslide dams, a large database has been established based on an in-depth investigation of 1,737 landslide dam cases. The effects of triggers, dam materials, and geomorphic characteristics of landslide dams on dam stability are comprehensively analyzed. Various evaluation indexes of landslide dam stability are assessed based on this database, and stability evaluation can be further improved by considering the dam materials. Stability analyses of aftershocks, surges, and artificial engineering measures on landslide dams are summarized. Overtopping and seepage failures are the most common failure modes of landslide dams. The failure processes and mechanisms of landslide dams caused by overtopping and seepage are reviewed from the perspective of model experiments and numerical analyses. Finally, the research gaps are highlighted, and pathways to achieve a more complete understanding of landslide dam stability are suggested. This comprehensive review of the recent advances in stability and failure mechanisms of landslide dams can serve as a key reference for stability prediction and emergency risk mitigation.

Local earthquake data was used to determine a three-dimensional (3D) seismic attenuation structure around the aftershock source region of the 2018 Lombok earthquake in Indonesia. The aftershocks were recorded by 13 seismic stations from August 4 to September 9, 2018. The selected data consist of 6,281 P-wave t∗ values from 914 events, which had good t∗ quality in at least four stations. Our results show that the two aftershock clusters northwest and northeast of Lombok Island have different attenuation characteristics. A low P-wave quality factor (low-Qp), low P-wave velocity (Vp), and high ratio of P-wave velocity and S-wave velocity (Vp/Vs), which coincide with a shallower earthquake (20 km) northeast of Lombok Island, might be associated with an area that lacks fluid content. The difference in fluid content between the northwest and northeast regions might be the cause of the early generation of aftershocks in the northwest area. The significant earthquake that happened on August 5, 2018, took place in a region with moderate Qp, close to the contrast of high and low-Qp and high Vp, which suggests that the earthquake started in a strong material before triggering the shallower aftershocks occurring in an area affected by fluid content. We also identified an old intrusive body on the northeast flank of the Rinjani volcano, which was characterized by a high-Qp, high-velocity, and a high Bouguer anomaly.

This study comprehensively investigates the source mechanisms associated with the mainshock and aftershocks of the Mw = 6.3 Yogyakarta earthquake which occurred on May 27, 2006. The process involved using moment tensor inversion to determine the fault plane parameters and joint inversion which were further applied to understand the spatial and temporal slip distributions during the earthquake. Moreover, coseismal slip distribution was overlaid with the relocated aftershock distribution to determine the stress field variations around the tectonic area. Meanwhile, the moment tensor inversion made use of near-field data and its Green's function was calculated using the extended reflectivity method while the joint inversion used near-field and teleseismic body wave data which were computed using the Kikuchi and Kanamori methods. These data were filtered through a trial-and-error method using a bandpass filter with frequency pairs and velocity models from several previous studies. Furthermore, the Akaike Bayesian Information Criterion (ABIC) method was applied to obtain more stable inversion results and different fault types were discovered. Strike–slip and dip-normal were recorded for the mainshock and similar types were recorded for the 8th aftershock while the 9th and 16th June were strike slips. However, the fault slip distribution from the joint inversion showed two asperities. The maximum slip was 0.78 m with the first asperity observed at 10 km south/north of the mainshock hypocenter. The source parameters discovered include total seismic moment M0 = 0.4311E + 19 (Nm) or Mw = 6.4 with a depth of 12 km and a duration of 28 s. The slip distribution overlaid with the aftershock distribution showed the tendency of the aftershock to occur around the asperities zone while a normal oblique focus mechanism was found using the joint inversion. [ABSTRACT FROM AUTHOR]

Previous earthquakes have shown that for structures that experience a significant ground motion with only minor damage, there is a likelihood of severe damage or complete collapse during the aftershocks. Due to the importance of such seismic sequences in structural engineering investigations, in this study, the vulnerability of RC frames under subsequent earthquakes is evaluated using computational intelligence (CI) in three phases. In the first phase, an optimized fuzzy system (Fuzzy-TLBO) is presented to estimate the used capacity of the story by considering the damage state of beams, columns, joints, and also the drift. The second phase includes two soft computing models to determine the used capacity of the frame under mainshock using indicators of the soil, record, and the structure, simultaneously. The final phase utilizes two soft computing methods with the same goal of the second phase in which the initial vulnerability of the frame concluded from the mainshock is also introduced to the estimators. These three approaches, with high accuracy in their outputs, have several benefits and applications, especially in seismic studies. They can calculate the overall output of an RC frame under loads without needing a nonlinear time-history analysis. Furthermore, they consider the effects of beams, columns, joints, and also the drift in the final output. Therefore, the proposed models in this research are a very comprehensive approach compared to the available methodologies, which only consider drift as the engineering demand parameter. [ABSTRACT FROM AUTHOR]

Aftershock records have a considerable effect on the results of collapse assessments conducted on buildings. Thus, they should be selected cautiously. As the number of recorded aftershocks is not sufficient, mainshock records are often utilized instead. In order to increase the correlation between the aftershock time history and the seismic response of a structure, this research intends to investigate several Intensity Measures (IMs). For this study, three RC frames were considered. Forty-four far-field records from FEMAP-695 were selected as main and aftershock. Each building analysis was conducted under 44 mainshock–aftershock chains. According to the results, use of the summation of the first mode spectral acceleration value of aftershocks as the second part of a vector IM can lead to the sufficiency of the IM. [ABSTRACT FROM AUTHOR]

We quantitatively examined the influence of pore fluid pressure and coseismic stress changes on the seismicity rate changes that followed the 2016 Kumamoto earthquake, on the basis of two approaches. One is a numerical calculation of the classic stress metric of ∆CFS, and the other is an inversion analysis of pore fluid pressure fields with earthquake focal mechanism data. The former calculation demonstrated that seismicity rate changes were consistent with the expectation from ∆CFS in 65% of the target region, whereas they were not in the remaining 35% of the region. The latter analysis indicates that seismicity rates increased in the regions where pore fluid pressure before the Kumamoto earthquake sequence was remarkably enhanced above hydrostatic, regardless of values of ΔCFS. This suggests that the increase in pore fluid pressure is one of the important physical mechanisms triggering aftershock generation. We obtained evidence that pore fluid pressure increased around the southern part of the main rupture zone after the mainshock, examining temporal changes in types of focal mechanism data. The average increases in pore fluid pressure were estimated to be 17, 20, and 17 MPa at depths of 5, 10, and 15 km, respectively. These large increases in pore fluid pressure cannot be explained under the undrained condition. The spatial derivative of the pore fluid pressure field in the depth direction implies that fluid supply from greater depths may have controlled increases in seismicity rates that followed the large earthquake. [ABSTRACT FROM AUTHOR]

In this paper, we present a model for studying aftershock sequences that integrates Coulomb static stress change analysis, seismicity equations based on rate-state friction nucleation of earthquakes, slip of geometrically complex faults, and fractal-like, spatially heterogeneous models of crustal stress. In addition to modeling instantaneous aftershock seismicity rate patterns with initial clustering on the Coulomb stress increase areas and an approximately 1/t diffusion back to the pre-mainshock background seismicity, the simulations capture previously unmodeled effects. These include production of a significant number of aftershocks in the traditional Coulomb stress shadow zones and temporal changes in aftershock focal mechanism statistics. The occurrence of aftershock stress shadow zones arises from two sources. The first source is spatially heterogeneous initial crustal stress, and the second is slip on geometrically rough faults, which produces localized positive Coulomb stress changes within the traditional stress shadow zones. Temporal changes in simulated aftershock focal mechanisms result in inferred stress rotations that greatly exceed the true stress rotations due to the main shock, even for a moderately strong crust (mean stress 50 MPa) when stress is spatially heterogeneous. This arises from biased sampling of the crustal stress by the synthetic aftershocks due to the non-linear dependence of seismicity rates on stress changes. The model indicates that one cannot use focal mechanism inversion rotations to conclusively demonstrate low crustal strength (≤10 MPa); therefore, studies of crustal strength following a stress perturbation may significantly underestimate the mean crustal stress state for regions with spatially heterogeneous stress.

Abstract In order to investigate the seismic velocity structure in the region of concentrated severe damage during the 2016 Kumamoto earthquake sequence, we conducted continuous seismic observations in the central area of Mashiki Town, Kumamoto Prefecture. During 4 days of observations at eight temporary sites, 2 months after the mainshock, recordings from 30 aftershocks (1.7 ≤ M j ≤ 4.3, 1.9 km ≤ depth ≤ 13.5 km) were obtained. The aftershock data showed that site amplifications at approximately 1 Hz are dominant in a zone where almost no buildings were damaged along the Akitsu riverside, whereas site amplifications at higher than 3 Hz are observed in the heavily damaged zones. Our data also showed that the peak acceleration and velocity amplitudes, as well as seismic intensities for the small events in the less damaged zone, are clearly larger than those in the damaged zones, implying that the damage distribution is inconsistent with site response based on linear site amplifications. The estimated phase velocities of Rayleigh waves using the aftershock and microtremor data show dispersive characteristics in the lower frequency range (0.26 ≤ f ≤ 1.27 Hz), but the values are substantially smaller than those derived from the P–S logging model at the nearest KiK-net strong-motion observation station KMMH16. The derived microtremor horizontal-to-vertical spectral ratios and earthquake radial-to-vertical (R/V) spectral ratios show common distinct peaks at around 0.4 Hz, which are possibly related to the response of deep sedimentary layers beneath the area. The refined velocity structure model that better accounts for both the phase velocity and common dominant peak indicates that the values of S wave velocity (Vs) above the bedrock layer (Vs = 2700 m/s) are smaller than those inferred from the logging model and the depth to the bedrock layer could be much deeper (about 600 m) in comparison with the logging model (234 m). The derived R/V spectral ratio at station KMMH16 also shows a distinct peak at 0.4 Hz, suggesting that there is no large difference of deep sedimentary structure between the observation area and station KMMH16.

The relative locations between mainshocks and their aftershocks have long been studied to characterize the mainshock–aftershock relationships, yet these comparisons may be subjected to biases inherited from various sources, such as the analysis method, data, and model parameters. Here, we perform both a relocation analysis of interplate events to obtain accurate relative centroid locations and a slip inversion analysis of the mainshock slip relative to the relocated events, with some of the relocated events used as empirical Green's functions, to retrieve the spatiotemporal slip features of the mainshock relative to all of the relocated events. We perform these analyses on the large ( 6.0 ≤ M W ≤ 7.3 ) interplate earthquakes that occurred near four giant ( M W ≥ 8.5 ) megathrust earthquakes: the 2007 M W 8.5 Bengkulu (Indonesia), 2005 M W 8.6 Nias (Indonesia), 2010 M W 8.8 Maule (Chile), and 2011 M W 9.1 Tohoku-Oki (Japan) earthquakes. Most of the spatiotemporal slip features of the mainshocks are consistently recovered using different empirical Green's functions. We qualitatively and quantitatively demonstrate that the large interplate aftershocks within 5 years of the four analyzed mainshocks are largely located on the periphery or outside of the large-slip regions of these four giant megathrust earthquakes. [ABSTRACT FROM AUTHOR]

Featured Application: The Acoustic Emission (AE) technique can be used to perform structural monitoring of historical buildings including tall masonry towers. In addition, the AE data detected on the structure, during the earthquake activity, can be used to discriminate foreshock and aftershock intervals. Historical churches, tall ancient masonry buildings, and bell towers are structures subjected to high risks due to their age, elevation, and small base-area-to-height ratio. In this paper, the results of an innovative monitoring technique for structural integrity assessment applied to a historical bell tower are reported. The emblematic case study of the monitoring of the Turin Cathedral bell tower (northwest Italy) is herein presented. First of all, the damage evolution in a portion of the structure localized in the lower levels of the tall masonry building is described by the evaluation of the cumulative number of acoustic emissions (AEs) and by different parameters able to predict the time dependence of the damage development, in addition to the 3D localization of the AE sources. The b-value analysis shows a decreasing trend down to values compatible with the growth of localized micro and macro-cracks in the portion of the structure close to the base of the tower. These results seem to be in good agreement with the static and dynamic analysis performed numerically by an accurate FEM (finite element model). Similar results were also obtained during the application of the AE monitoring to the wooden frame sustaining the bells in the tower cell. Finally, a statistical analysis based on the average values of the b-value are carried out at the scale of the monument and at the seismic regional scale. In particular, according to recent studies, a comparison between the b-value obtained by AE signal analysis and the regional activity is proposed in order to correlate the AE detected on the structure to the seismic activity, discriminating foreshock, and aftershock intervals in the analyzed time series. [ABSTRACT FROM AUTHOR]

This study examines the possibility of tidal triggering in the aftershock sequence of the MS 8.0 Wenchuan earthquake on 12 May 2008. Two methods, the Schuster spectrum and KORRECT (correlation with a rectangular pulse) methods, were used to explore the periodicities of the aftershock sequence. Firstly, we detected the latent periodicities through the Schuster spectrum method; then, we used the KORRECT method to verify and confirm these periodicities. Moreover, the aftershock catalogue was divided into two subsets using 16 km focal depth to discuss the impact of focal depth on tidal triggering. Results show that the aftershocks in the first few days following the mainshock were mainly triggered by semidiurnal and diurnal tides. In the Longmen Shan Thrust Belt, shallow earthquakes were triggered by diurnal tides and deep earthquakes were triggered by semidiurnal tides. [ABSTRACT FROM AUTHOR]

To clarify the relationship between earthquake occurrence and fluid, we analyzed data from a dense aftershock-observation network with 69 high-gain short-period seismographs installed immediately after the mainshock occurrence (October 21) in the aftershock area of the 2016 Central Tottori Prefecture earthquake. We determined the hypocenters and focal mechanisms of the aftershocks very precisely in the period from October 22 to December 15. We then investigated the temporal changes in the spatial distributions of hypocenters and T-axis azimuths of focal mechanisms. The distributions of aftershock hypocenters and T-axis azimuths are basically temporally stable, except those in limited portions in the shallow layer near the western edge of the aftershock area, where rapid decrease of aftershocks with T-axis azimuths of WSW to west was observed. If fluid rises from the lower crust due to fault rupture, the locations of aftershocks and focal mechanisms may change over time, especially in the deepest part of the aftershock region. However, the temporal change in these parameters was not apparent at depth. These observations suggest that the aftershock activity of the Central Tottori Prefecture earthquake was controlled mainly by stress concentration rather than strength reduction due to high fluid pressure. [ABSTRACT FROM AUTHOR]

This study focused on the state of stress in the aftershock areas of the strongest earthquakes registered in Tien Shanon 11 August 1974 (MS=7.3), 24 March 1978 (MS=7.1), 01 November 1978 (MS=6.8), 19 August 1992 (MS=7.3), and 05 October 2008 (MS=6.9). These earthquakes occurred in similar geodynamic conditions in the zone of the nearly N–S oriented compression caused by the India-Eurasia collision. The parameters of the state of stress were determined by Yu.L. Rebetsky method of cataclastic analysis (MCA) of displacements on faults, based on the focal mechanisms of aftershocks. In the first stage of MCA, we determined the orientations of the principal stress axes, the Lode-Nadai coefficients, and the geodynamic types of the state of stress in the study area. The stress fields reconstructed from the aftershocks of different energy levels show that the principal deviatoric stress axes pattern becomes less orderly as the magnitude of seismic events decreases. A peculiar feature of the aftershock process is the misalignment of the axes of maximum compression in the mechanisms of the weakest aftershocks and the compression axis in the focal mechanism of the main seismic event. Most of the aftershocks were recorded west of the location of the main earthquake.