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One hundred years ago, Fusakichi Omori died. Our paper is dedicated to his memory. Omori made an outstanding contribution to the physics of earthquakes. In 1894 he formulated the law of aftershock evolution. Omori's Law states that after the main shock of an earthquake, the frequency of aftershocks decreases hyperbolically with time. In this paper, we briefly describe one of the directions of modern aftershock research. We present Omori's law in the form of a differential equation describing the evolution of aftershocks. The evolution equation allows us to solve the inverse problem of the earthquake source "cooling down" after the main shock. The solution of the inverse problem allowed us to reveal the existence of the so-called Omori epoch. It is established that at the end of the Omori epoch something similar to a bifurcation of the source state occurs. A logistic equation supposedly describing the bifurcation phenomenon is proposed. [ABSTRACT FROM AUTHOR]

The liquefaction that occurred in the city of Palu on September 28, 2018, was caused by a series of significant earthquakes that took place in a relatively short time around 25 minutes after the main earthquake of magnitude 7.5. This event was followed by aftershocks of magnitudes 6.4, 6.2, and 6.1. The magnitude 6.2 aftershock occurred at 10.16 UTC, while the magnitude 6.1 aftershock occurred at 10.25 UTC. These were both located very close to the liquefaction locations in Balaroa, Petobo, and Jono Oge. We investigated the mainshock and the three aftershocks using the NCEER method based on Vs30 measurements and data from the drill liquefaction locations at Balaroa, Petobo, and Jono Oge. We found that the liquefaction was not only caused by the main earthquake but also by the subsequent aftershocks that occurred within 25 minutes after the mainshock. [ABSTRACT FROM AUTHOR]

The existence of aftershocks in an earthquake sequence can impact the analysis of the mainshock. In this study, we present a method for deleting an aftershock sequence based on the spatial relationship between earthquakes and faults. This method improves the performance of space window selection in the classical K-K method by eliminating aftershocks with an ideal fault buffer zone. The determination of fault buffer zones is based on a trial-and-error analysis of 69,714 earthquake records from the China Seismic Network Center (CENC) collected between 1980 and 2020. We selected 20 typical big earthquakes ( M L 7.0–8.0 or ~ M s 6.6–8.0; for earthquakes above magnitude M s 7 or M L 7.2 , M L is approximately equal to M s ) as the mainshocks to establish the fault buffer zones. We also propose an empirical formula to determine the distance of the fault buffer zone by counting the aftershock deletion effect at different buffer distances. Compared with the classical K-K method, our method considers the correlation between the spatial distribution of aftershocks and faults, eliminates earthquake groups that are not related to the mainshock, greatly reduces the spatial range of aftershocks, improves the performance of deleting aftershocks of different magnitudes, and provides a new rule and reference for aftershock deletion. [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]

We collected a data set of mainshocks and their respective largest foreshocks of 38 earthquake sequences in Taiwan. The plot of local magnitude, ML, of a mainshock (denoted by MLm) versus ML of its largest foreshock (denoted by MLf) shows an increase in MLm with MLf. This indicates that for Taiwan’s earthquakes the bigger the largest foreshock is, the larger the mainshock is. The plot of the epicentral distance, Δ (in km), from the largest foreshock to the mainshock versus ML of the mainshock exhibits a weak increase in Δ with MLm as Δ1 day.

Through the study of the real seismic behavior, we find that large mainshocks are often followed by aftershocks. Mainshocks occur instantaneously, while aftershocks feature relatively slow relaxation. Based on the Olami–Feder–Christensen with relaxation (OFCR) model, the long-range interactions and new energy distribution rule are added into our model. At the same time, we find that the statistical distributions of seismic magnitude, and time intervals of mainshock and aftershock conform to the power law behaviors. By comparing the curves of aftershock and mainshock behaviors, we find that they have similar distribution characteristics, which indicates that the study of aftershock behavior is of practical significance for the study of large earthquakes. Meanwhile long-range interactions OFCR model can provide a good supplement for the large earthquake data. [ABSTRACT FROM AUTHOR]

Reinforced concrete intake towers are considered as main hydraulic structures constructed in dams' reservoirs, connected to the dam or as a separate structure. If an intake tower is damaged, the powerhouse or downstream of the dam may experience serious problems. In this research, the influence of mainshock-aftershock seismic sequences is studied on real intake tower. The intake tower of Encino dam is modeled by using the finite element method. The dynamic time history analyses are conducted with the real as-recorded earthquakes and the nonlinear behavior of concrete and steel bars is considered. The tower is modeled with reservoir and foundation, and their interactions are considered. The earthquake records are applied to the tower in three cases: (i) one horizontal component, (ii) two horizontal components, and (iii) two horizontal components with vertical component. The main dynamic responses of the tower including relative deflections and nonlinear material responses are presented. The obtained results are discussed, and the influence of the main shock-aftershock sequences are thoroughly examined. [ABSTRACT FROM AUTHOR]

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]

Several recent studies have investigated the risk posed to structures by earthquake sequences, proposing state-dependent fragility/vulnerability models for assets in damaged conditions. However, a critical component for such efforts, i.e., ground-motion record selection, has received relatively minor consideration. Specifically, utilization of "consistent" mainshock (MS)–aftershock (AS) ground motions is desirable in practical applications. Such consistency in selecting MS–AS sequences requires proper consideration of the correlations between and within the intensity measures of MS and AS ground motions. Most of the studies in this domain utilize spectral accelerations as the considered ground-motion intensity measures and rely on empirical linear correlation models between the intensity measures of MS and AS ground motions for developing, for instance, record selection approaches. This study proposes a generalized ground-motion model (GGMM) to estimate consistent 30 × 1 vectors of intensity measures for mainshocks (denoted as IMMS) and aftershocks (denoted as IMAS) using a framework of successive long-short-term-memory (LSTM) recurrent neural network (RNN). The vectors of IMMS and IMAS consist of geometric means of significant duration ( D 5 - 95 , g e o m ), Arias intensity ( I a , g e o m ), cumulative absolute velocity ( C A V geom ), peak ground velocity ( P G V geom ), peak ground acceleration ( P G A geom ) and RotD50 spectral acceleration ( S a T ) at 25 periods for both MS and AS ground motions. The proposed RNN-based GGMM is trained on a carefully selected set of ~ 700 crustal and subduction recorded MS–AS sequences. The inputs to the framework include a 5 × 1 vector of source and site parameters for MS and AS recordings. The residuals of the trained LSTM-based RNN are further used to develop empirical covariance structures for IMMS and IMAS. The proposed framework is finally illustrated to select MS–AS ground motions based on IMMS and IMAS using a multi-criteria objective function. The selected MS–AS ground motion sequences are then used to perform non-linear time-history analyses of a case-study two-spanned symmetric bridge structure. The obtained engineering demand parameters are evaluated and critically discussed. [ABSTRACT FROM AUTHOR]

The 28 April 2021 earthquake Mw 6.1 (epicentre 26.69° N, 92.36° E; centroid depth ~ 40 km GCMT report, www.globalcmt.org) in the Sonitpur district of Assam valley, North East (NE) India occurred at the Kopili fault (KF) zone, a long transverse structure across the valley. The earthquake caused a few casualties and injuries and some damage to buildings, ground fractures, fountains and liquefactions, landslides and other such manifestations in several parts of the valley, with a maximum intensity of MMI VI. The KF is identified as a seismically most active fault that caused two (1869 and 1943) large earthquakes (Mw > 7.0) in the past and several severely felt earthquakes (Mw ≥ 5.0) in recent years. Fault plane solutions of the 28 April 2021 mainshock and six aftershocks (Mw > 3.5) are determined by waveform inversion using broadband seismograms of the nearest observatory at Tezpur. The solutions are comparable to the GCMT solution of the mainshock and correlated with the known right-lateral strike-slip motion at the KF zone. The low-stress drops of the earthquake sequence indicate relatively long source duration and small co-seismic slip of the mainshock and the aftershocks. The low-stress drop in terms of lower energy release may be due to reactivation of the KF. The low-stress drop also indicates a pre-existing fault zone at greater depth. A stress inversion analysis of the 17 available fault plane solutions indicates a NNE–SSW compressional and NNW–SSE tensional stress regime in the KF zone. The Coulomb stress change map, on the other hand, identifies the zone of triggered aftershocks. [ABSTRACT FROM AUTHOR]

The Kachchh Rift Basin (KRB) is a very peculiar region in the Indian subcontinent mostly due to the intense seismic activity and intricate tectonics that is located far from the active plate boundaries. The KRB region is located in the Deccan Volcanic Province of Western India. Recently, a moderate earthquake (Mw5.3) occurred in this region on June 14, 2020, that was followed by ~ 22 earthquakes (M ≤ 4.6) until June 23, 2020. This moderate-size event falls about 12 km SSE from the 2001 Bhuj earthquake (Mw7.6). The mainshock and aftershocks are found to be located in an area of ~ 20 × 20 km2 that is orienting NE-SW indicating the rupture area of the mainshock. Several pertinent questions arose for the recent moderate event as it was located in the vicinity of the 2001 mainshock. The estimated focal mechanism solution of the current event depicts reverse faulting with a small strike-slip component. The current seismic activity matches well with the south-dipping nodal plane that is akin to the South Wagad Fault (SWF) in the region. The Coulomb failure stress change (∆CFS) caused due to the 2020 mainshock, considering the source as a receiver fault, clearly demonstrates the locations of aftershocks in the zone of compression that corroborates well with the south-dipping source. Such a strong correlation between the static stress changes and the aftershocks indicated SWF as the causative source. Interestingly, we found that the recent earthquake sequence has occurred in the shadow zone of the 2001 Bhuj mainshock. The coseismic deformation due to the 2020 mainshock was also obtained using an InSAR interferogram that demonstrated coseismic subsidence of ~ 3.6 cm close to the source zone, which is found to be consistent with the activity along with the south-dipping SWF. In addition, the low seismic velocities (Vp, Vs), high Vp/Vs ratio, and positive Coulomb stress changes suggest entrapped magma fluid in the seismogenic rock matrix, which probably played a significant role in triggering earthquakes in the study region. We validated our observations with the earlier seismological, geophysical, and geological shreds of evidence. [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.

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]

The existence of aftershocks in an earthquake sequence can impact the analysis of the mainshock. In this study, we present a method for deleting an aftershock sequence based on the spatial relationship between earthquakes and faults. This method improves the performance of space window selection in the classical K-K method by eliminating aftershocks with an ideal fault buffer zone. The determination of fault buffer zones is based on a trial-and-error analysis of 69,714 earthquake records from the China Seismic Network Center (CENC) collected between 1980 and 2020. We selected 20 typical big earthquakes (ML7.0–8.0 or ~Ms6.6–8.0; for earthquakes above magnitude Ms7 or ML7.2, ML is approximately equal to Ms) as the mainshocks to establish the fault buffer zones. We also propose an empirical formula to determine the distance of the fault buffer zone by counting the aftershock deletion effect at different buffer distances. Compared with the classical K-K method, our method considers the correlation between the spatial distribution of aftershocks and faults, eliminates earthquake groups that are not related to the mainshock, greatly reduces the spatial range of aftershocks, improves the performance of deleting aftershocks of different magnitudes, and provides a new rule and reference for aftershock deletion.

A sequence of main and aftershocks is a complex natural hazard, which may introduce severe damage to critical infrastructure systems. Managing the risk caused by sequential earthquake events is challenging due to the uncertainties of aftershocks and their dependency on the preceding mainshock. Moreover, the dynamic behavior of a structural system under aftershocks must be evaluated with consideration of possible structural damage caused by the mainshock. To overcome these difficulties, this paper proposes a new framework based on a Bayesian network (BN) for risk modelling and inference for structures under a sequence of main and aftershocks. The stochastic mechanisms of sequential earthquakes and the hysteretic behavior of structures are described by BN models, which are then integrated for comprehensive modeling of the risk. The developed BN model facilitates probabilistic inference to obtain fragility curves given various circumstances regarding sequential earthquake events and observations. Numerical examples demonstrate fragility assessment and inference with the proposed BN‐based framework. [ABSTRACT FROM AUTHOR]

On September 21, 2004, the series of perceptible earthquakes occurred near the city of Kaliningrad. This series of seismic events affected not only the Sambia Peninsula, but also the entire Baltic region up to southern Sweden and eastern Denmark. The following events were recorded instrumentally: the foreshock with a magnitude of 5.0, the main shock with a magnitude of 5.3, and several aftershocks. The above magnitude estimates correspond to the highest values provided by various agencies. The magnitude estimates are conservative, but in this case it is even more difficult to explain the abnormally high intensities in some locations. Abnormally high seismic effects can be explained in only two ways: either the influence of the geological medium structure, or the occurrence of new earthquakes, which were triggered by the fore- and mainshock. The first assumption is not supported by the results of seismic microzoning, because the maximum amplification of seismic intensity was only 0.6 at two sites. To explain the observed intensity, such amplification is too weak. We attempted to find factual material confirming the possibility of earthquakes triggered by the fore- and mainshock. The article analyzes the distribution of seismic effects from the September 21, 2004 Kaliningrad earthquakes within Kaliningrad oblast. The observed intensities are compared with the calculated seismic effects in terms of the seismic intensity scale. As a result, it was found that the observed level of seismic effects significantly exceeds the calculated one and cannot be explained by the effects from the fore- and mainshock because of ground conditions. The macroseismic surveys in the territory of Kaliningrad oblast revealed excessively large cracks in the ground with significant horizontal movements along them and an anomalous lowering of the water level in a pond near the village of Veselovka, despite the torrential rains the day before. According to the macroseismic data, the average value of the attenuation coefficient for the region under consideration was b ≈ 2.7, which is significantly lower than the world average. The acquired data gave us grounds to hypothesize about certain earthquakes that accounted for the anomalously high seismic effects in Kaliningrad oblast: these seismic events were triggered by the fore- and mainshock of the 2004 Kaliningrad earthquakes, but were not recorded by seismic stations. A conclusion is drawn about the stress state of the crust beneath the Sambia Peninsula. Therefore, trigger earthquakes are quite possible here. [ABSTRACT FROM AUTHOR]

This research attempts to propose a new simulating model capable of generating synthetic stochastic earthquake scenario including both seismological, and acceleration signal properties. The Iranian plateau is selected as a mainland for developing the model due to the existence of high-quality earthquake databases. This model is composed of two sub-models. The first one generates random seismic event scenarios including mainshocks and aftershocks. The second one procreates 3-dimensional synthetic stochastic accelerograms for each of the previously simulated events. This scenario-based predictive model offers a powerful tool to the risk modelers interested in lifetime seismic risk assessment through more realistic earthquake scenarios. [ABSTRACT FROM AUTHOR]

One of the efficient methods for seismic design of structures is Performance-based Plastic Design (PBPD) in which by choosing the desired yield mechanism and the target drift at the beginning of the design, unlike that in conventional design methods, there is no need for a time-consuming process of trial and error to achieve the desired performance. On the other hand, recent earthquakes have shown that aftershocks can increase the damage to a building after the main earthquake and thereby threaten the safety of the inhabitants. However, the seismic hazard of aftershocks is still not explicitly included in the current design codes. In this study, attempts have been made to develop the PBPD method for considering the effects of aftershocks. For this purpose, by performing an incremental dynamic analysis (IDA), the ductility factor μ and the ductility reduction factor Rμ of six PBPD frames (2-, 4-, 6-, 8-, 10-, and 12-story) were calculated under mainshock and mainshock-aftershock sequences. Then, the energy modification factor γ, which is related to the two parameters μ and Rμ, was calculated for both the mainshock and the mainshock-aftershock sequences. An equation is presented in order to calculate the ratio of the energy modification factor of the mainshock-aftershock sequence to the mainshock energy modification factor ( γ m s + a s γ m s ) according to the design period, and the factor obtained is multiplied by the energy modification factor γ so that the effect of aftershock can be considered for the frame design, and by using this modification, the performance of the frame under the mainshock-aftershock sequence can be improved. The results of the time-history analysis performed on the modified PBPD frames show better performance by the frames designed on the basis of modified PBPD than the frames designed on the basis of the original PBPD method under MS-AS sequences in terms of inter-story drift and rotation of plastic hinges. [ABSTRACT FROM AUTHOR]

An earthquake of small magnitude (ML3.5) occurred on 12 April 2020 near the east district boundary of NCT, Delhi with maximum PGA for the event observed to be 14.13 gals. A few smaller aftershocks also occurred in the area. The estimated fault plane solution of the mainshock suggests normal faulting with some strike slip component. The focal mechanism corroborates with the NE - SW orienting lineaments mapped in the region near the epicenter. The source parameters of the event, namely, seismic moment, stress drop, corner frequency, and source radius are estimated to be 1.15 x 1014 N-m, 25.7 bars, 5.7 Hz and 300 m, respectively. The decay rate of acceleration with epicentral distance suggests a regression relation PGA = 474 D −1.347, which may be useful for understanding the ground motion in the region. A noise analyses at NDI rock and UJWA soil sites clearly suggest a significant reduction in ambient noise by ∼10 dB in the frequency band (1.0–10.0) Hz at the respective sites, during the COVID-19 lockdown situation. The reductions of the noise level improve the signal to noise ratio substantially at all the seismic stations located in the urban agglomerations, which enabled the recording of clear phases of the event and hence improved the analysis.

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.

Timber structures are important components of the architectural and cultural heritage. To enrich literature on reconnaissance and heritage preservation, this paper provides a comprehensive damage survey and assessment of the Shuanghe Confucian temple after the 2019 Changning earthquake Series. Confucian temple is a temple for the veneration of Confucius and the sages and philosophers of Confucianism in Chinese folk religion and other East Asian religions. The Shuanghe Confucian temple, which sits in southwestern China, was first built at about 1241, but have been gradually deteriorating and refurbished for 600 years. It has the supreme structural grade in ancient Chinese architecture and delicate local characteristics. On June 17th, 2019, an Ms 6.0 earthquake stroke Changning County, and another four earthquakes (aftershocks) with magnitude larger than Ms 5.0 took place in the next 20 days. The Shuanghe Confucian temple experienced very distinctive damage during the earthquake series. The paper first present sufficient ground motion information. Characteristics of the all the major structural components are also introduced along with their mechanical functions. Next, the damage of all the typical timber structural components, like roof overhangs drop, column rupture, column bottom lateral residual displacement, and non-structural components failure are carefully documented and assessed. The paper reveals two important aspects that some major damages of the Shuanghe Confucian temple occurred in aftershocks, and that the columns bottom lateral residual displacement was caused by eternal impact induced by adjacent wall collapse, instead of structural inner forces. Thus, this paper helps better understand the seismic performance of traditional timber structures and identify potential weaknesses which might have adverse influences on timber structures. Lessons learnt from the Shuanghe Confucian temple can be used in future to better protect other historical timber structures from earthquakes. [ABSTRACT FROM AUTHOR]

Purpose: This study investigates the seismic performance of vertical irregular steel moment-resisting frames subjected to mainshock–aftershock (MS–AS) seismic sequence. The building structures situated in a seismically active region experience sequence of earthquakes during their lifetime. These seismic sequences have the potential to increase the damage state of the structures. The irregular building structures are present due to availability of space, functional and architectural requirements etc. behave differently as compared to the regular buildings during earthquakes. Methods: In this paper, three steel-moment resisting frames having different configuration i.e. 3, 6, and 9-storey were considered as a reference regular frame. Three vertical irregularities-stiffness, strength and combined stiffness and strength are introduced in three different locations (bottom, mid-height, and topmost storey) along the height of each building frames. These regular and vertically irregular frames are subjected to mainshocks alone and artificial MS–AS seismic sequences that are generated by using the repeated and randomized approach. The nonlinear time-history analysis is performed using computer software SAP 2000 on regular and irregular building frames subjected to mainshocks and MS–AS seismic sequence. Results: The comparison between the seismic performance of vertically irregular and regular frames subjected to mainshocks alone and MS–AS seismic sequences shows that the vertical irregular frames have a significant effect on the distribution of mean interstorey drift demands along the height of building frames. Conclusion: The effect of MS–AS seismic sequence on seismic demands of vertical irregular and regular frames have been increased compared to mainshocks. However, the repeated seismic sequences show smaller structural responses relative to the randomized seismic sequences. [ABSTRACT FROM AUTHOR]

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]

An earthquake of small magnitude (ML3.5) occurred on 12 April 2020 near the east district boundary of NCT, Delhi with maximum PGA for the event observed to be 14.13 gals. A few smaller aftershocks also occurred in the area. The estimated fault plane solution of the mainshock suggests normal faulting with some strike slip component. The focal mechanism corroborates with the NE - SW orienting lineaments mapped in the region near the epicenter. The source parameters of the event, namely, seismic moment, stress drop, corner frequency, and source radius are estimated to be 1.15 x 1014 N-m, 25.7 bars, 5.7 Hz and 300 m, respectively. The decay rate of acceleration with epicentral distance suggests a regression relation PGA = 474 D −1.347, which may be useful for understanding the ground motion in the region. A noise analyses at NDI rock and UJWA soil sites clearly suggest a significant reduction in ambient noise by ∼10 dB in the frequency band (1.0–10.0) Hz at the respective sites, during the COVID-19 lockdown situation. The reductions of the noise level improve the signal to noise ratio substantially at all the seismic stations located in the urban agglomerations, which enabled the recording of clear phases of the event and hence improved the analysis. [ABSTRACT FROM AUTHOR]

This study investigates the effects of aftershocks on the potential damage of reinforced concrete (RC) structures retrofitted by fibre-reinforced polymer (FRP). An eight-storey RC frame that was poorly confined due to deficient transverse reinforcement was selected and then retrofitted by FRP wraps to provide external confinement. Inelastic time history and damage analyses of the frame under single earthquakes and mainshock–aftershock sequences were performed. FRP retrofit did not ensure a damage-free structure, and it minimised the damage though. The results indicated different contributions of aftershocks to the damage of the retrofitted frame, and this finding illustrates the limitations of current seismic codes. The damage caused by single earthquakes is proposed to be used as a criterion for dealing with the effects of aftershocks as follows. If the mainshocks cause minor or light damage, the effect of aftershocks is negligible, whereas the effect of aftershocks needs to be considered if the mainshocks cause moderate or severe damage. The outcomes of this study can be used in making decisions about whether the effects of aftershocks should be considered. [ABSTRACT FROM AUTHOR]

Featured Application: The proposed mainshock integrated aftershock fragility function model can be used to more effectively evaluate the seismic vulnerability of bridges in seismically active regions where mainshock-aftershock sequences pose a high threat. Seismic fragility analysis is often conducted to quantify the vulnerability of civil structures under earthquake excitation. In recent years, besides mainshocks, strong aftershocks have been often witnessed to induce structural damage to engineered structures, including bridges. How to accurately and straightforwardly quantify the vulnerability of bridges due to sequential mainshocks and aftershocks is essential for an efficient assessment of bridge performance. While recognizing the limitation of existing methods, this paper proposes a mainshock integrated aftershock fragility function model, which empirically encodes the effects of mainshocks and retains the simple form of traditional fragility curves. A pile foundation-supported bridge system is modeled considering seismic soil-structure interaction to demonstrate the proposed fragility model. Numerical examples show that the resulting fragility curves incorporate the initial value for the probability of collapse of the bridge system due to a mainshock and the effects of the variable aftershocks conditional on the mainshock. Statistical analysis confirms that the proposed model fits the simulated vulnerability data (e.g., seismic intensities of aftershocks and the response demands conditional a select mainshock ground motion) both accurately and robustly. [ABSTRACT FROM AUTHOR]

Aftershock probabilistic seismic hazard analysis (APSHA) has a key role in risk management after a major earthquake. The main goal of the current study is to assess aftershock hazard in a strategic and earthquake-prone region of Iran (Bushehr province). Bushehr province is a strategic region in the Middle East due to its major petroleum export facilities, industrial corridors and the Bushehr nuclear power plant. To prepare APSHA for Bushehr province, a seismic source is selected which surrounds the active faults in the study area. A uniform earthquake catalog is collected which contains information on a total of 1143 earthquakes (Mw > 4) occurred in the study area from 1900 to 2018. Aftershock parameters are calculated using the epidemic-type aftershock sequence model. Aftershock sequences follow a non-homogenous Poisson's process, and their magnitude and location depend on the size and location of the mainshock. In this study, APSHA is performed for the intervals of 1, 7 and 30 days after the mainshock, by assuming occurrence of mainshocks with return periods of 475 and 2475 years. The results show that the aftershock hazard curve is greater than that of the mainshock hazard curve. [ABSTRACT FROM AUTHOR]

Aftershocks can increase building vulnerability and decrease life-safety of the residents; nevertheless, no practical design method has yet been considered the seismic risk of aftershock. In this study, a new design method known as performance-based plastic design (PBPD) has been utilized instead of conventional elastic design (ED) for steel moment frames which considers nonlinear behavior of structures directly in the design process. Result of time-history analysis shows that drift increase due to aftershock for ED-frame is more than PBPD-frame. In addition, plastic hinge and hysteretic energy distributions for PBPD-frame when subjected to mainshock–aftershock sequence is more desirable than ED-frame. [ABSTRACT FROM AUTHOR]

This work proposes comprehensive empirical predictive equations for generating stochastic synthetic 3-dimensional accelerograms for the Iranian plateau based on the existing database. First, the databank of Iranian accelerograms is collected, sorted, processed, declustered and categorized into the pulse-like and non-pulse-like data. To simulate the artificial accelerograms, a stochastic model capable of handling both the temporal and spectral non-stationarity of accelerograms is adopted. By implementing nonlinear curve fitting, parameters of the stochastic model are estimated. Then, the recorded events are categorized into eight distinct groups based on the existence of pulse-like components, and whether the recorded accelerogram is a mainshock or aftershock. Next, for each group, a Bayesian linear regression analysis is performed to obtain empirical predictive equations with probabilistic perspective. This proposed set of equations can estimate parameters of the stochastic model by using input seismological properties including event magnitude, focal depth, epicentral distance and average shear wave velocity of top 30 m of the subsurface profile. Finally, the validity of model is successfully assessed using the existing ground motion prediction equations for the Iranian plateau. [ABSTRACT FROM AUTHOR]

In this paper, a comprehensive accelerogram database of the Iranian plateau containing 3585 data with all three components is gathered. The raw data are processed by the wavelet-based denoising method, and results are compared with the contaminated data. All the data are classified into mainshock and aftershock categories using the time and spatial window method. Afterward, the data are categorized into the pulse-like and non-pulse-like events based on the detection of velocity pulse in any of horizontal and/or vertical directions. Eventually, among 3585 data, the ones with an average shear wave velocity of top 30 m of subsurface soil profile are selected and their important ground motion parameters such as peak ground acceleration, velocity, and displacement, Arias intensity, ground motion duration, and acceleration response spectra are studied. It is observed that the intensity of vertical component of the accelerograms is almost at the same level as the strong horizontal direction or even more, in some cases. Near-field events cause more severe responses than far-field earthquakes. Generally, the mainshocks responses are slightly tougher than the aftershocks. By comparing the acceleration response spectra of the accelerograms with the ones obtained from the building design codes, the necessity of revising the existing standard spectra is revealed for considering the effects of mainshocks, aftershocks, near-field effects, as well as separate vertical component of the ground motions directly. Finally, the presented database, as a powerful tool, will give an interesting opportunity to the scientists and engineers who are enthusiastic to work on the Iranian plateau accelerograms. [ABSTRACT FROM AUTHOR]

With the vast amounts of data recorded by the CWB seismic networks, a fast and precise algorithm for near real-time earthquake locations as well as fundamentally urgent source and hazard studies of large damaging earthquakes in Taiwan becomes increasingly important. Here we present a novel method for automatic phase identification and event detection, modified from a Python-based PhasePApy package to be more computationally efficient and suitable for the high-rate seismicity and large-scale dense network in Taiwan. The performance efficiency is enhanced by substituting the Python code with a Fortran subroutine to calculate the characteristic function for phase picker. It is further improved by amalgamating the picks close enough in time but on different components, associating pairs of the amalgamated picks with the origin times of candidate events through an empirical linear relation between the P and S-P times reported in the CWB phase catalog, and isolating clustered candidate events and corresponding phase picks to determine the locations of real earthquakes. The algorithm is applied to detecting in almost real time over two thousands of events that occurred within four days accompanying the 4 February 2018 ML 5.89 foreshock and 6 February 2018 ML 6.25 Hualien mainshock, which far surpasses the capability of experienced human analysts. The temporal and spatial distributions of the detected aftershocks provide the timely and first-hand information to estimate the aftershock decay rate and verify the orientation of the fault plane and rupture extent of the foreshock and disastrous mainshock as a guide for fast risk assessment and source characteristics analysis. [ABSTRACT FROM AUTHOR]

Knowledge of what governs the interaction between slow‐ and fast‐slip earthquakes is essential to understanding the nature of the earthquake cycle. In southern Taiwan, four major earthquakes (2008 Mw 5.2 Taoyuan, 2010 Mw 6.4 Jiashian, 2012 Mw 5.9 Wutai, and 2016 Mw 6.4 Meinong events) that occurred near the active tremor areas provide a unique opportunity to analyze their spatiotemporal association. With the declustered tremor catalog built in this study, we were able to statistically evaluate the possible association between tremor and mainshock‐aftershock sequences in space and time. We found that close‐by mainshocks influences tremor's timing in a matter of less than 5 days by short‐term triggering, while the 2010 ML 6.4 Jiashian earthquake appears to be most responsible for such triggering. Assuming a low dip angle thrust faulting mechanism, tremors coincided with small static stress increases (8 kPa) in the 2010 ML 6.4 Jiashian earthquake. The other three major events, however, caused either negative or neglectable stress changes in the tremor zone. The different responses of the tremors can be explained by coseismic slip‐induced static stress change. In this complex tectonic region as a boundary between continental subduction and collision, the interaction between slow‐ and fast‐slip phenomena could be facilitated by a commonly high fluid pressure environment. Key Points: We explore the spatiotemporal association between tremors and earthquakes using 7‐year tremor and earthquake catalogIn a confined distance of 5 km, a greater percentage of earthquakes tend to occur in 5 days prior to the tremorsA commonly high fluid pressure environment may be needed, to facilitate their interaction [ABSTRACT FROM AUTHOR]

Seismic fragility analysis is often conducted to quantify the vulnerability of civil structures under earthquake excitation. In recent years, besides mainshocks, strong aftershocks have been often witnessed to induce structural damage to engineered structures, including bridges. How to accurately and straightforwardly quantify the vulnerability of bridges due to sequential mainshocks and aftershocks is essential for an efficient assessment of bridge performance. While recognizing the limitation of existing methods, this paper proposes a mainshock integrated aftershock fragility function model, which empirically encodes the effects of mainshocks and retains the simple form of traditional fragility curves. A pile foundation-supported bridge system is modeled considering seismic soil-structure interaction to demonstrate the proposed fragility model. Numerical examples show that the resulting fragility curves incorporate the initial value for the probability of collapse of the bridge system due to a mainshock and the effects of the variable aftershocks conditional on the mainshock. Statistical analysis confirms that the proposed model fits the simulated vulnerability data (e.g., seismic intensities of aftershocks and the response demands conditional a select mainshock ground motion) both accurately and robustly.

The influences of initial damage paths and aftershock (AS) spectral shape on the assessment of AS collapse fragility are investigated. To do this, a four-story ductile reinforced concrete (RC) frame structure is employed as the study case. The farfield earthquake records recommended by FEMA P695 are used as AS ground motions. The AS incremental dynamic analyses are performed for the damaged structure. To examine the effect of initial damage paths, a total of six kinds of initial damage paths are adopted to simulate different initial damage states of the structure by pushover analysis and dynamic analysis. For the pushover-based initial damage paths, the structure is "pushed" using either uniform or triangle lateral load pattern to a specified damage state quantified by the maximum inter-story drift ratio. Among the dynamic initial damage paths, one single mainshock ground motion or a suite of mainshock ground motions are used in the incremental dynamic analyses to generate a specified initial damage state to the structure. The results show that the structure collapse capacity is reduced as the increase of initial damage, and the initial damage paths show a significant effect on the calculated collapse capacities of the damaged structure (especially at severe damage states). To account for the effect of AS spectral shape, the AS collapse fragility can be adjusted at different target values of ε by using the linear correlation model between the collapse capacity (in term of spectral intensity) and the AS ε values, and coefficients of this linear model is found to be associated with the initial damage states. [ABSTRACT FROM AUTHOR]

The present research focuses on the statistical evaluation of Iranian plateau aftershocks from an engineering perspective and presents probabilistic models applicable for generating random earthquake scenarios. Accordingly, a comprehensive earthquake data catalog including the period from 1964 to 2016 is prepared. Data are declustered into 37 separate mainshock-aftershock sequences by considering the completeness moment magnitude of the database. The well-known modified Omori occurrence rate formula is adopted to determine the recurrence time of the events, considering the effect of secondary aftershocks. In addition to computing the probability density functions of the parameters of the Omori formula, the joint probability distribution of the aftershock occurrence versus magnitude and occurrence time is obtained for modeling their magnitude sequences. The obtained results are applicable for producing randomly generated mainshock-aftershock scenarios. [ABSTRACT FROM AUTHOR]

Although the use of segmental bridge decks has increased around the world, their use in seismic regions under vertical earthquake motions is yet to be fully understood. The asymmetric behavior of segment joints under vertical motions as well as the low strain ductility of cables passing through the joints raise questions on postearthquake performance of decks following a disastrous earthquake and during aftershocks. Because strong aftershocks can expedite the collapse of structures damaged in the mainshock, assessing the residual capacity of mainshock-damaged bridges becomes important with regard to postearthquake performance. In the current research, two medium- and long-span case study bridges were investigated to study the residual capacity of bridges of such type. To this end, a twodimensional model for segment joints was developed and verified via experimental results. The results related to nonlinear time-history analysis under the design earthquakes showed that midspan segment joints were bound to suffer spalling concretes at the bottom and yielding tendons at the top. The findings revealed that the residual capacity of the mainshock-damaged bridges decreases with the increase in the damage level caused by mainshock, yet not more than 50% in comparison to the intact bridge, even at the worst damage level. [ABSTRACT FROM AUTHOR]

Recent research has shown that for the larger earthquakes recorded (M ≥ 5.2) within the global centroid moment tensor (gCMT) there is a positive correlation between the magnitudes of earthquakes and the magnitudes of their aftershocks [13]. Through a modification of model independent stochastic de-clustering [12] and a more localized catalog provided by the Southern California Earthquake Data Center (SCEDC), the methodologies of Nichols and Schoenberg can be extended to catalogs complete with a much lower minimum magnitude of completeness (M ≥ 2.2). Results indicate that the positive correlation observed between larger earthquakes in the gCMT catalog and their aftershocks is also evident in the relationship between the magnitudes of earthquakes in the SCEDC data and their aftershocks. However, with the lower minimum magnitude of completeness found in the SCEDC catalog and with short periods of extreme earthquake activity evident within the data, the statistical power of the stochastic de-clustering algorithms to distinguish between mainshocks and aftershocks is diminished. [ABSTRACT FROM AUTHOR]

This study proposes a framework for the seismic fragility assessment of steel moment-resisting frames exposed to different rates of corrosion and subjected to real mainshock-aftershock (MS-AS) sequences. The inherent uncertainties in the structural characteristics may significantly affect the structural performance. Thus, in addition to the demand parameters, the framework considers randomness in the capacity parameters using the Latin hypercube sampling. Three frames are modeled in OpenSees and then subjected to nine real as-recorded MS-AS sequences to illustrate the application of the framework. Four limit states are defined based on the maximum interstory drift ratio and the multiple stripe analysis is chosen for the fragility assessment. The presented framework allows for the plot of the fragility surfaces for each corrosion level conditioned on both the mainshock and the aftershock intensity. The results indicate that corrosion and aftershock may significantly affect the seismic fragility especially for the extreme limit states. • Proposes a novel fragility framework for steel frames subjected to corrosion and real mainshock-aftershock sequences. • Evaluates the corrosion vulnerability based on the lifetime of the building rather than corrosion percentages. • Supports not only demand-driven random variables but also capacity-driven random variables. • Develops MS-AS fragility surfaces to reflect the characteristics of MS-AS sequences. • Compares the fragility surfaces for low-, medium-, and high-rise steel moment-resisting frame buildings. [ABSTRACT FROM AUTHOR]

We reviewed studies on aftershocks in Taiwan for the following topics: the spatial-temporal distributions and focal-plane solutions of aftershocks from of thirty larger earthquakes with magnitudes > 5; the correlations between the main shock and the largest aftershock based on dependence of the differences in magnitudes (ΔM), occurrence times (ΔT) epicenters (ΔH) and focal depths (ΔD) upon the main shock magnitude, Mm; magnitude-dependence of p-value of Omori's law of aftershocks; the correlation between the b-value of the Gutenberg-Richter's frequency-magnitude law and the p-value; application of the epidemic-type aftershock sequences (ETΔS) model to describe the aftershock sequence; the mechanisms of triggering aftershocks; and dynamic modeling of aftershocks. The main results are: (1) The spatial distribution of aftershocks for some earthquakes is consistent with the recognized fault; (2) Unlike Bath's law, ΔM slightly increases with Mm; (3) ΔT does not correlate with Mm; (4) ΔD does not correlate with Mm; (5) ΔT somewhat increases with ΔD; (6) The p-value slightly increases with Mm; (7) There is a negative correlation between the b- and p-values. (8) There was seismic quiescence over a broader region of Taiwan before the 1999 Chi-Chi earthquake; (9) Both the static and dynamic stress changes trigger aftershocks; and (10) Dynamic modeling shows that a decrease in elastic modulus is a significant factor in triggering aftershocks. [ABSTRACT FROM AUTHOR]