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A number of moderate earthquakes occurred in the South China continental edge or coastal area. Several large earthquakes are sparsely distributed along the seismic belt with the northeast direction between the South China and the continental shelf of the South China Sea. Coulomb stress changes of the 1969 Yangjiang main shock (Ms 6.4) with the following events of the 1986 ML 5.4, the 1987 ML 5.4 and the 2004 ML 5.2 in the Yangbian Bay area are studied. The complex stress patterns of the main shock caused by the dextral Pinggang strike-slip fault imported on the planes of the normal fault, strike-slip fault and thrust fault of three events indicate that the distribution of aftershocks mainly located in the areas of positive stress changes and part of aftershocks fall into the stress shadow area due to the reactivity of the local faults. The 1969 Yangjiang event occurred between the intraplate and the broad plate boundary zone setting when we apply on the empirical relationship between the aftershock durations and the fault-loading rate. The intersection fault model is proposed and could explain the stress changes of the Yangjiang earthquake sequences in South China coastal area. The future hazard assessment requires to take account of the stress increase area along the tip of the main shock source fault and off-fault stress increase area.

An Ms 6.0 earthquake occurred in Yao'an County, Yunnan Province, on 9 June 2009. According to the results of relocated aftershocks, we find that the distribution of aftershocks has differences in the strike direction and depth in local areas. Therefore, the fault is divided into two segments. Combined with the surface deformation information obtained from InSAR Data as the constraint condition of the slip model, we inverse the three-dimensional coseismic slip distribution of the seismogenic fault of the Yao'an earthquake and discussed the influence of the Yao'an earthquake on the surrounding faults and the rupture direction of the event. The main research results are as follows: 1) we quickly obtain reasonable fault parameters, model, and slip distributions through relocated aftershocks and InSAR data, and the results obtained after inversion are consistent with the results of the geological structure and scholars' research. At the same time, the abnormal deep slip is found, which proves that there is a complex structure underground in the Yao'an area. 2) The results of static Coulomb stress change show that the aftershocks are basically distributed in the area where the static Coulomb stress increases. Faults with a relatively significant increase of Coulomb stress include the northwest section of Honghai Fault, the middle of Chenghai Fault, the middle of Hongge Fault, the middle of Nanhua-Chuxiong Fault, and the north of Yuanmou Fault. The future seismic risk of these faults should be considered. 3) The main direction of the Yao'an earthquake rupture is controlled by regional stress, and the abnormal rupture direction is affected by the complex underground structure.

Continental earthquakes, especially shallow earthquakes, can cause disasters in populated areas. On 20 May 2016, a moderate magnitude earthquake (Mw 6.0) ruptured on Northern Territory of Australia. With the aim of seismic hazard assessment, coseismic and postsesmic deformation of the Petermann earthquake has been determined using ALOS-2 ascending and Sentinel-1A descending images. The coseismic deformation field suggests that the earthquake ruptured in the NNW direction with a length of about 20 km, a rake angle of 57.1° and a moment magnitude of Mw 6.06. The maximum slip of 1.31 m can be observed at the depth of 1.25 km. The small baseline subset (SBAS) InSAR was utilized to obtain the postseismic deformation from 69 Sentinel-1A images in a descending orbit. The spatiotemporal evolution of the postseismic deformation presents a gradually decaying trend, with a maximum displacement of 5.4 cm within 858 days after the mainshock. Our afterslip model suggests that the maximum slip of 0.28 m occurred at the depth of 0.25 km with a slight shift to the northwest compared to the coseismic slip. The static Coulomb failure stress (CFS) changes at various depths were calculated, indicating the consistency between high loaded stress zones and aftershock distribution.

This paper presents the results of a study focused on evaluating the seismic performance of school buildings located in a region of high seismicity in the Pacific Coast of Mexico. For this purpose, a family of three archetype school buildings having typical plan view with one-, two-, and three-story height were designed with out-of-date 1990s seismic requirements. Detailed analytical models representative of the archetype school buildings included modelling the nonlinear behaviour of captive reinforced concrete (RC) columns and the adjacent partial-height infill masonry walls commonly found in this type of structures. The analytical models were subjected to a set of 22 as-recorded mainshock-aftershock seismic sequences gathered at stations placed on the subduction zone of the Mexican Pacific Coast. Drift-based fragility curves for captive RC columns failing in shear, which represent the probability of reaching or exceeding four damage states, based on experimental tests are introduced in this study for evaluating their seismic performance. This investigation revealed that the two- and three-story school buildings exhibit larger inter drift demands at the ground story than those exhibited by the one-storey school building, which led to a weak first-storey mechanism under some mainshock earthquake ground motions. Furthermore, it was noted that aftershocks increased the state of damage at the ground storey due to the increase of damage in the captive RC columns. Findings of this investigation highlight the priority of implementing retrofit strategies, mainly for three-storey school buildings.

Long-span infrastructures are usually taken over for public shelters, in which single-layer reticulated dome is a kind of popular structure. But there was less research for the damage assessment of single-layer reticulated domes subjected to mainshock-aftershock sequences. How to accurately assess the damage by mainshock and the stability redundancy for aftershock becomes an important problem. In order to explore the effects on the structural damage, this paper processes the dynamic time-history analyses for single-layer reticulated domes subjected to mainshock-aftershock sequences. The finite element model of Kiewitt8 single-layer spherical reticulated shell structure is established in ANSYS. Ten pairs of ground motion sequences in horizontal direction are generated with modulating amplitude to simulate the rare and extreme earthquakes. The results demonstrated that the structure even suffered from the extreme earthquake mainshock still has loading capacity and structural safety with the local plastic damage. But the effect on structural damage of aftershock ground motion with greater earthquake intensity cannot be ignored for single-layer reticulated domes. A structural damage assessment method is utilized with combining the damage factor by performing static stability analysis twice. It can effectively reflect the remaining loading capacity and structural safety factor for the practical seismic engineering applications.

This research attempts to investigate the efficacy of different passive vibration control systems, from both technical and economic points of view, to mitigate the lifetime seismic risk of typical buildings under probable mainshock-aftershocks sequences via a novel probabilistic framework. In this regard, three separate common building models are considered, including 5-, 10-, and 15-story intermediate steel moment-resisting frame buildings. These models are designed optimally with/without passive vibration control systems, including hysteretic damping, linear viscous damping, and base isolation systems as independent alternatives. Next, for the hazard modeling, all active faults located in the greater Tehran area are taken into account to simulate the earthquake scenario. Afterward, probable earthquake scenarios during the 50 years of buildings’ life-spans are generated randomly, including the events, as well as their corresponding synthetic stochastic accelerograms. In the next step, responses of buildings and their cumulative losses are calculated under the whole scenarios considering various damage types such as structural, non-structural, and content-related. In the end, by using the Monte-Carlo simulation method, the seismic risk of different building alternatives is estimated. Interestingly, the application of each of the considered vibration control devices leads to a significant reduction in lifetime losses (both physical and economic). It is revealed that the viscous damper has the highest efficacy among different systems. Importantly, by neglecting aftershocks' effects, the estimated lifetime risk will be underestimated by about 27 to 57% below the expected value. Finally, among the active faults in the greater area of Tehran, the North Tehran, Kahrizak, and Mosha faults have the highest contribution in the estimated lifetime risk of buildings.

This study evaluates the effect of earthquake shock sequences on the collapse mechanism of Special Steel Moment Frames (SSMFs) by studying the distribution of plasticity and hysteretic energy parameters. For this purpose, four SSMFs with 4–20 stories are numerically modelled in OpenSees and subjected to incremental dynamic analysis (IDA). These analyses aim to capture the collapse state of structures under aftershock events preceded by various mainshock levels. The distribution of Maximum Ductility Demand (MDD) and Energy Absorption Contribution (EAC) at the collapse state is then evaluated in presence of sequential shocks. The results show that the EAC parameter provides better metric for studying the collapse mechanism which is remarkably affected by the mainshock-aftershock sequence. Accordingly, the role of elements experiencing damage under the mainshock becomes less prominent in absorbing the aftershock energy and is substituted by contribution of elements with less deterioration under the mainshock. The members’ EAC is also shown to be affected by the percentage of the energy imposed by each individual shock as a fraction of the total energy imposed by the mainshock-aftershock sequence. In overall, for low-rise structures subjected to aftershock-only excitation, almost equal EACs are provided by the beams and columns while beams’ EAC is shown to exceed 60% for high-rise structures due to their flexural load bearing mechanism. With introduction of the mainshock damage, however, the dependency of the EACs to the structures’ height becomes less significant.

The advancements in analysis and design approaches considering the seismic loads have helped safeguard buildings against structural failure and collapse. However, design objectives should also consider the associated effects and performance evaluation incorporating the sequential effects. Improving the performance of buildings at both the asset and community levels is now of major interest in the field of structural engineering. This research addresses the advancement toward this goal by incorporating post-earthquake fire and aftershock to multi-hazard design methodology. A set of mid-rise buildings – 3-, 9-, and 20-story moment resisting steel frames, characteristic of those found in rapidly growing metropolitan areas of US, are selected for the study considering the relative vulnerability to post-earthquake fire. The analyses of the responses are based on advanced finite element simulation methods/techniques and corresponding approaches. ABAQUS/CAE is primarily employed for the simulation and analysis. The effects of fire are thoroughly studied from the literature and considered explicitly, and the analysis is performed for a complete cycle of main shock (full-intensity) –fire- aftershock (reduced intensity) earthquake. The research commences with the investigation of the effects of modeling approach-- using beam element and using solid element with slab, on Post Earthquake Fire (PEF) study of a single room model. At the end of a complete sequence, the model developed using 3D solid elements are found to underestimate the stiffness loss by around 15% compared to those developed using 2D beam elements. The behaviors of selected steel moment resisting frame buildings then simulated with PEF are investigated to quantify the overall performance behavior of the building system. Inter-story Drift Ratio (IDR) and Stiffness Degradation over the complete sequence of events are chosen as the damage measures. The same approach is applied to a real building in California and the performance behavior is studied. All the building systems studied show the significant damages and change in behavior after incorporating the fire and aftershock. In all building sets, the stiffness loss (global) due to mainshock alone (2% to 10% in the cases studied) is very less as compared to the stiffness loss due to fire alone (6% to 15%) and aftershock (10% to 25%) alone.

Following an earthquake event, the safety of continued occupancy in terms of whether a mainshock-damaged building could withstand upcoming strong aftershocks is a key question for stakeholders. To answer this question, the residual collapse capacity of mainshock-damaged buildings should be evaluated, and in this way, an accurate assessment of the structural damage inflicted during a mainshock-aftershock sequence is of great importance. Recent studies have shown that energy-based damage measures, which calculate the dissipated energy due to cyclic-plastic deformations, can more accurately reflect the secondary damage induced by aftershocks than deformation-based damage measures. Park-Ang damage index is a combined seismic damage measure that integrates the damage caused by excessive deformations with the damage caused by the cumulative hysteretic energy dissipation. This study presents a framework for assessing the post-mainshock structural safety, in which the quantification of the damage accumulated during a mainshock and the determination of the remaining capacity to sustain strong aftershocks are made based on the Park-Ang damage index. First, residual collapse capacity diagrams are produced by performing incremental dynamic analysis under a suite of selected sequential ground motions. Next, an average diagram is made to find the maximum safe damage index, so that values above this threshold would represent the buildings that are deemed to be unsafe. The proposed framework is applied to a reference four-story steel moment-frame building to investigate its collapse potential and evaluates its structural safety in a hypothetical earthquake scenario. The results of the case study indicate that the proposed framework can effectively help structural engineers to decide on whether a mainshock-damaged building is safe to reoccupy and communicate the final decision in a simple diagram to stakeholders.

Although structures are generally subjected to mainshock-aftershock sequences, the current structural design process merely considers the main seismic event. This study aims to compare the performance of Reinforced Concrete Moment Resisting Frames (RC-MRFs) with Shape Memory Alloy (SMA) and Ultra-High-Performance Steel Fiber Reinforced Concrete (UHPSFRC) under the influences of mainshock and aftershock sequence. To achieve this aim, 3, 6, and 8-story five-span frames were simulated considering three different approaches: (i) RC frames; (ii) RC frames with SMA rebars in the plastic hinge length of the beams; (iii) RC frames with UHPSFRC and SMA in the plastic hinge length of the beams. Each frame was analyzed under two different scenarios, with and without considering seismic sequences to assess the post-earthquake performance of damaged RC frames. Time history analyses were conducted using seven real accelerograms. Maximum transient inter-story and roof drifts, as well as residual inter-story drifts, were determined and compared in undamaged and damaged structures to assess the effect of seismic sequence. As the performance evaluation of the damaged structures appears to be essential to make repair policy decisions and rescue operations, damage states were defined according to FEMA-356 based on maximum transient and residual drifts. Numerical results illustrate the salient effect of using SMA and UHPSFRC synergistically in reducing transient and residual drifts and enhancing the functionality of structures in both mainshock and aftershock.

The observations from recent seismic events, shake table tests and large-scale experimental studies showed that mass timber structures demonstrate acceptable seismic performance. For these structures, the connectors are designed to demonstrate a ductile behaviour during the earthquake which is accompanied by inelastic deformation of the fasteners and crushing of the wood fibres. Despite the observed satisfactory seismic performance, the significant lateral stiffness degradation, as a result of irrecoverable damage in the connectors, makes these structures vulnerable to aftershocks and future events. Nowadays, a low damage seismic performance where the damage is localised in the replicable fuse and the inter-story drifts are capped within the desired range may be the preferred performance criteria for new timber buildings. This paper aims to assess the seismic performance of conventional mass timber buildings upgraded with resilient low damage braces. A design procedure for the strengthening of the lateral load resisting system is proposed and applied to a case study structure. The results showed that while the original structure failed to meet the defined seismic performance criteria, the strengthened structure demonstrated superior seismic performance. Furthermore, the lateral performance of the original and strengthened systems under subsequent earthquakes is investigated. The results of this study show the significant potential of the introduced bracing system for the strengthening of mass timber structures.

The Manchester Courts building was a heritage building located in central Christchurch (New Zealand) that was damaged in the Mw 7.1 Darfield earthquake on 4 September 2010 and subsequently demolished as a risk reduction exercise. Because the building was heritage listed, the decision to demolish the building resulted in strong objections from heritage supporters who were of the opinion that the building had sufficient residual strength to survive possible aftershock earthquakes. On 22 February 2011 Christchurch was struck by a severe aftershock, leading to the question of whether building demolition had proven to be the correct risk reduction strategy. Finite element analysis was used to undertake a performance-based assessment, validating the accuracy of the model using the damage observed in the building before its collapse. In addition, soil-structure interaction was introduced into the research due to the comparatively low shear wave velocity of the soil. The demolition of a landmark heritage building was a tragedy that Christchurch will never recover from, but the decision was made considering safety, societal, economic and psychological aspects in order to protect the city and its citizens. The analytical results suggest that the Manchester Courts building would have collapsed during the 2011 Christchurch earthquake, and that the collapse of the building would have resulted in significant fatalities.

s In this study, we present the variations of electron density (Ne) and electron temperature (Te) in the nighttime ionosphere above the areas of seismic activities by the statistical method. The Ne and Te were observed by the CSES over ~2.5 years from May 2018 to December 2020. In the statistical analysis, the Ne and Te closed to the epicenters both in space and time are compared with their background values. The data associated with aftershocks, at high latitudes, and during high magnetic activity periods are carefully checked and finally removed from the statistics, to avoid spurious effects and the high variability of the ionosphere on the statistics. The results show that: (1) During nighttime, anomalous ionospheric perturbations related to earthquakes with magnitudes larger than 5 are in evidence; (2) According to the histogram distributions of Ne and Te perturbations, the positive variations of Ne and the negative variations of Te are related to seismic activities; (3)The significant variations of positive Ne and negative Te related to earthquakes mainly occurred ~1–7 days and ~13–15 days before the earthquakes respectively, and within 200 ​km from the epicenters.

In this paper to improve the seismic behavior of structures and decrease structural earthquake damages, a new hybrid control system is proposed consisting of Friction Damper and Vertical Shear Panel (HFD-VSP) in series. The new system benefits of two-level energy dissipation such that in moderate earthquakes friction damper (FD) as the first fuse operates while in strong earthquake both FD and Vertical Shear Panel (VSP) dissipate energy. FD can eliminate the shortcoming of VSP in terms of ductility and low cycle fatigue. FD and VSP are modeled in ABAQUS and related numerical results are compared to those of previous experimental studies for verification purposes. Pushover and cyclic loadings are conducted for two models. The hysteresis curves show symmetric two-level behavior with stable strength and stiffness by increasing the ductility ratio of 87.3%. To study the performance of the HFD-VSP, seismic analysis and incremental dynamic analysis, IDA, are applied to two 5-story buildings with and without hybrid dampers. Maximum drift and maximum roof displacement decrease by 17.4% and 17.1% on average under earthquakes. Average IDA results show that first mode spectral acceleration at HFD-VSP is 1.14 times that of the frame with just VSP. The drift and spectral acceleration of the collapse prevention level increase 33.3% and 28%. The proposed system using FD and VSP performed better than the vertical shear panel against an extensive range of earthquakes as well as the mainshock with strong foreshocks and aftershocks.

On April 25, 2015, a M8.1 earthquake happened near Pokhara, Nepal, and aftershocks occurred continuously which caused significant losses. In November 2015, the authors made an in-depth investigation on the earthquake damages of various engineering structures in Nepal. As the post-disaster reconstruction work has not been completed, up to this day, the current reconstruction work is still worthy of continuous attention. Firstly, the earthquake damage of typical frame and masonry buildings in Nepal was described and the causes of damage were analyzed. The reinforcement measures and suggestions were putted forward for post-earthquake reconstruction, which focused on the load transmission path of the structure, the enclosure structure, the beam-column joints, the integrity of the structure, house site selection and structural stiffness. With the help of the finite element software, the numerical modeling analysis of the wall reinforcement measures and a typical reconstruction house was carried out. The results show that the seismic performance of masonry wall can be greatly improved after being wrapped by reinforced concrete or seismic band, the use of seismic band in reconstruction of typical houses in Nepal can effectively improve the seismic performance of buildings. The surrounding areas of door and window openings are still the most vulnerable places to be damaged. Reinforcement should be strengthened in these locations.

There was a shallow earthquake in the Central Sulawesi province of Sulawesi island of Indonesia with a moment magnitude (Mw) 7.5 on 28th September 2018 at 18:02:44 local time. The event was preceded by major foreshocks and followed by aftershocks of significant magnitude. The epicenter of the main shock was in the Donggala regency of Minahasa peninsula of Central Sulawesi, approximately 70 km from the provincial capital of Palu. The earthquake was caused by the tectonic movement of the left lateral Palu- Koro fault within the Molucca Sea microplate, triggering major geotechnical failure and structural damage in Palu city and Sigi regency. Thousands of people died or are still unaccounted for, and countless others were injured. Balaroa, Petobo, Jono-Oge and Sibalaya were the worst hit mainly due to large-scale flow-slides and mud flows. It was the first time that such large-scale flow failures were triggered by an earthquake, and that the failure of very gentle sloping ground swept away whole localities. The objective of this research was to provide insight into the scale of ground failure and other infrastructural damage caused by the event, especially in Jono-Oge area, where the flow distance was longest. The authors performed preliminary and detailed surveys in the area twice by conducting Portable Dynamics Cone Penetration Test (PDCPT), collecting disturbed and undisturbed samples and using aerial drone (UAV) photography. The findings of the reconnaissance survey are described here along with subsequent data interpretation. Finally, the mechanism of the flow-slides is discussed.

Gas densities were measured by x-ray absorption spectroscopy to examine spatial inhomogeneity in density behind incident and reflected shock waves in a miniature (12.7 mm bore) high repetition rate shock tube. Shock waves were generated in argon. The pressure and temperature behind the incident shock were P2 ~ 1.72 bar and T2 ~ 800 K, while those behind the reflected shock were P5 ~ 6.5 bar and T5 ~ 1480 K. Time-resolved line-of-sight transmission measurements using x-rays at 9 keV photon energy were made along various axial and transverse locations covering the entire shock tube cross section. The x-ray transmission at each location was converted to pathlength-integrated densities, which in some conditions could be further converted to pathlength-averaged densities. The measured gas densities were compared with those calculated by the normal shock wave relations, and good agreement was found in the pre-shock and immediately behind the incident shock regions. Similar agreement was found for reflected shock conditions very near the endwall. However, increasingly large differences between the measured and calculated gas densities were found as distance from the endwall increased. Reconstruction by Abel inversion allowed time and radially resolved densities to be obtained from the transverse measurements. While the profiles measured in this experiment are of insufficient signal-to noise level and resolution to definitively assign boundary layer thicknesses, future improvements may be able to do this. The radially resolved densities reveal growing sidewall thermal boundary layers in gases behind the reflected shock. These reconstructions were compared with a model of the thermal boundary layer and are consistent with values of the thermal boundary layer thickness being between 0.25 mm and 1 mm at 0.7 ms after shock reflection.

This study investigated into the effect of infill walls on behavior of reinforced-concrete (RC) special moment frames subjected to multiple earthquakes. For this purpose, 4, 8 and 12-storey RC frames in three cases of inclusion and exclusion of infill walls as well as inclusion of infill walls with openings are analyzed using OpenSees software. The nonlinear static and dynamic analyses are carried out on the models and for dynamic analyses, 20 main and aftershocks are taken into account. Accordingly, strength and stiffness of the structures are determined using nonlinear static (pushover) analysis and structural responses such as displacement ductility, rotational ductility and energy absorption are obtained from nonlinear dynamic analyses. The results indicate that inclusion of infill walls, increases stiffness and ultimate strength of the structure, but to the contrary, ductility and energy absorption are reduced. Among all models, the structures including infill walls, incurred intense strength loss owing to cracking and destruction of the infill walls. Comparison of results of the main shocks and aftershocks illustrates that contribution of the aftershocks, has raised the responses by 20–100% highlighting importance of aftershocks particularly for the structures with infill walls.

In this article, the effect of mainshock-aftershock sequences on the seismic performance of ordinary reinforced concrete frames (ORCFs) retrofitted by fiber reinforced polymers (FRPs) is studied. Various mainshock damage levels are used to precede aftershocks that are incrementally scaled to drive the studied structures into the collapse state. The mainshock damage is represented by the maximum inter-story drift (MID) parameter, and aftershock scaling is performed following incremental dynamic analysis (IDA) procedure. The derived collapse capacities are evaluated to study the effect of the mainshock and the retrofit action. The obtained results indicate that while the retrofit program is effective in elevating the collapse capacities of the structures, the capacity reductions caused by various mainshock damage levels in the studied structures are independent of the structures strength and ductility capacities. Rather, these capacity reductions were attributed to structures’ inherent plastic behavior mechanisms, including concrete cracking and reinforcement yielding.

Modern seismic-resistant timber structures are designed in a way that the massive timber elements remain elastic, and the earthquake-induced damage is localised in the connections. However, these connections may experience large plastic deformations which usually is accompanied by crushing of the surrounding wood. These plastic deformations make the structure vulnerable to aftershocks and future events. Furthermore, the expected residual deformation in the connections and members could make the structure unusable after a large seismic event. As a result, researchers and engineers are developing new concepts and structural systems that can provide life-safety performance while minimize the earthquake damage. This paper investigates the seismic performance of timber structures with rocking walls that use low damage re-centring hold-down connections. These friction-based connections can dissipate the earthquake energy without significant damage while providing a re-centring potential for the structure. The proposed structural system is described, and then a numerical model for a case study structure is developed and subjected to non-linear analyses. Furthermore, the performance of the designed structure is compared with similar structures that use simple friction dampers and conventional fasteners. The results showed that the proposed structural system has a superior seismic performance and can potentially be considered as an option for the use in modern seismic-resistant timber structures.

The Iran earthquake of November 12, 2017 is one of the most significant earthquakes throughout the history of Iran, which shook the regions of NW Zagros Mountains, Sarpol-e Zahab Region, and Northwest of Iran with a sheer magnitude of 7.3 Mw. The earthquake triggered many succeeding landslides, causing the largest seismic landslide in Iran (Maleh-Kabood) over the past few centuries. The seismic landslide was detected using coherence change detection techniques. Nine Single Look Complex (SLC) of Sentinel-1A, B imageries were processed to build interferometric coherence products. The coherence results enabled the detection of the surface area for the Maleh-Kabood landslide caused by the Azgleh earthquake. Its instability was recognized over time. The results also indicated that the surface deformation of Maleh-Kabood landslide continued on owing to aftershocks of a size greater than 4.5 Mw following the Azgleh earthquake. The results are indicative of the robustness of the InSAR coherence technique in detecting surface deformation as well as seismic landslide studies and environmental monitoring. The results also stipulated the potential of Sentinel-1A, B C-band coherence maps as suitable tool for the detection of active landslides.

Aftershocks usually come after a mainshock. However, controversy of the impact of aftershocks on structures arises in previous studies. To further interpret the effect of sequence-type ground motions on reinforced concrete (RC) structures, an incremental seismic damage (ISD) model is proposed based on the modified Park-Ang model. Based on the ISD model, an attempt is made to identify the coupling mechanism between ISD and recorded peak response, the transition in periods and the characteristics of sequence-type ground motions. The analogous resonance-type aftershocks defined herein provides some assistance in addressing such mechanism. Smaller post-yielding stiffness implies more sensitivity of damage to excitations with long periods. The mean period of a sequence-type motion is found to have a distinct impact on the global damage development tendency. The coupling effect on ISD development is comprehensively investigated by several single-degree-of-freedom systems and two RC frames. A mainshock-induced damage tolerance or “threshold” is observed, implying no obvious ISD is developed unless a significant increment in recorded peak response is caused. The reduced earthquake resistance reserve caused by mainshocks is not always consistent with ISD development.

Based on the analysis of the field data of nonstationary electromagnetic soundings with different installations, the evolution of the stressed state of the geological massif in the epicentral region of the destructive 2003 Мw7.3 Chuya earthquake is investigated in the paper. The measurements by these methods were executed in a lot of regular points located at two sites of the western part of the Chuya basin of Gorny Altai (Russian Federation). The field works have been performed for 13 years. We developed a measurement technique using data from the three Transient Electromagnetic Method (TEM) method settings for determination of the resistivity anisotropy coefficient. Interpretation of the field data is carried out based on the inverse problem solution, and further, taking into account the geological structure and seismological data, variations of true electrophysical characteristics of the cross-section (electrical resistivity and resistivity anisotropy coefficient) are analyzed. According to the TEM data acquired in the 1980s, the geoelectric parameters of rocks in the period of low seismic activity were estimated. Comparison of the results of quantitative interpretation of the modern measurements (2004–2016) with those taken before the Chuya event allows tracing a change in the geoelectric parameters of the cross-section in the aftershock period, and on this basis tracking the evolution of the rock massif after the destructive seismic impact. Keywords: Transient electromagnetic, Resistivity anisotropy coefficient, Time-lapse survey, Gorny Altai, Chuya basin

This paper presents the seismic vulnerability assessment of existing RC buildings designed according to previous non-seismic codes that are exposed to various levels of corrosion and mainshock-aftershock sequences. A refined finite element model of an existing four-storey reinforced concrete building is adopted. Twenty as-recorded natural ground motions are collected from international databases to perform incremental dynamic analyses for the inelastic response of the testbed building. The effects of corrosion are applied on external beams and columns to simulate a realistic exposure. The robust fragility assessment is conducted for a range of seismic intensity measures. Moreover, a new intensity measure, based on the modified acceleration spectrum intensity, is proposed. Such seismic parameter accounts for the elongation period experienced by structures during earthquake events and appears to be more reliable and accurate for corroded RC structures than the most adopted peak ground acceleration and spectral acceleration at the first natural period. The results of the comprehensive numerical simulations contribute to providing relevant indications on the non-linear response of existing corroded buildings under multiple excitations and, highlight that current seismic codes are no longer conservative for such detrimental phenomena.

Historical records indicate that strong earthquakes are usually accompanied by aftershocks with large peak ground accelerations. Structural damages caused by the mainshock can be further aggravated by aftershocks, which can lead to structural collapse. Current structure seismic design practices generally consider the mainshock effects only. Performance of buildings subjected to mainshock-aftershock sequential ground motions, should always be fully investigated. Previous studies on sequential ground motion focus mainly on frame structures. There is a paucity of publications addressing other type of structures, especially high-rise buildings. The reasons for this omission have been identified as the complexity of high-rise buildings and unaffordable computational costs related to the nonlinear dynamic analysis of long duration sequential ground motions. Thus, the authors used Tianhe-2, once known as the fastest supercomputer in the world, to conduct nonlinear dynamic analysis of a typical 20-story frame-core tube building subjected to sequential mainshock-aftershock ground motions. This study focuses on 104 mainshock and aftershock ground motions from four different sites. Thus, the effects of mainshocks only and sequential ground motions of a frame-core tube structure at four different sites were analyzed in this study's finite element model. The Performance of these structures under mainshocks and sequential ground motions are compared in terms of inter-story displacement ratio, hysteretic energy and damage index based on the Park-Ang model. The results prove that a supercomputer can be used to solve the computational cost issue in structural engineering and emphasize that the effects of sequential earthquakes need to be considered in structural design, even for frame-core tube structures with lateral resisting members.

The Lenglongling fault, located in the north Qilian fold zone along the northeastern margin of the Tibetan Plateau, plays an important role in accommodating the tectonic deformation between the Tibetan Plateau and the Alxa block. On 21 January 2016, the Ms 6.4 Menyuan earthquake occurred near the Lenglongling fault. In this study, we utilize the waveform matched filter technique and double-difference earthquake location algorithm to examine the spatial-temporal evolution of aftershocks following the Menyuan mainshock. Events from both China Earthquake Networks Center (CENC) routine catalog and relocated catalog are used as templates to scan through the continuous waveform data (21 January 2016 to 31 December 2016). We identify three and eleven times more events than listed in CENC routine catalog and relocated catalog, respectively. We find that the earliest aftershocks (e.g., in the first 10 min or so) mostly illuminated a steeply dipping plane, while aftershocks at later times outlined a near vertical dipping plane with mostly strike-slip focal mechanisms. The relocated aftershocks also suggest that the Menyuan mainshock occurred in a secondary fault rather than the major Lenglongling fault, which is consistent with geodetic and geological observations. Moreover, we find that aftershocks systematically expand with time along the fault strike and 16 repeating earthquake clusters occur in the rupture zone, which are likely driven by afterslip of the mainshock.

We investigate significant earthquake sequences in the observation period between 2000 and 2018 along the Tuz Golu Fault Zone in Central Anatolia. These sequences include a series of strong events with moment magnitudes (M-w) > 5.0, e.g. 30 July 2005 Bala-Ankara M-w 5.2, 20 December 2007 Bala-Ankara M-w 5J and 26 December 2007 M-w 5.6 Afar-Ankara earthquakes. The mainshocks are shallow focus strike-slip events with minor normal component at depths of 12-15 km. Focal depths of aftershocks range from 2 to 30 km. The focal mechanisms of the aftershocks are mainly normal faulting with variable strike-slip components. The geometry of focal mechanisms reveals a normal faulting regime with E-W and also NE-SW trending directions of T-axis in the entire activated region. A stress tensor inversion of focal mechanism data is performed to acquire a more accurate picture of the Tuz Gou Fault Zone stress field. The stress tensor inversion results display a predominant normal stress regime with a NNE SSW oriented maximum horizontal compressive stress (S-H). According to high-resolution hypocenter relocations of the Tuz Golu Fault Zone seismic sequences, eight clusters are revealed. The aftershock activities in the observation period between 5 March 2000 and 2 June 2018 extend from NW to SE direction. Seismic cross-sections show that a complex pattern of the hypocenter distribution with the activation of Tuz Gold Fault Zone segments. The western clusters are associated with a fault plane trending mainly N-S and dipping towards almost vertical, while the eastern clusters are related to a fault plane trending NW and dipping towards SE direction. The best constrained focal depths are mainly confined in the crust (depth < 30 km) and range from 2 to 30 km. In this study, we compare the results of an analysis of b-values from seismicity and stress tensors of focal mechanism solutions to understand their coupling in terms of faulting and earthquake hazard implications. In particular, this comparison allows to investigate the spatiotemporal correlation between b-values and stress distributions and it is therefore able to locate fault segments that have a high potential of generating large earthquakes. According to recent seismicity, b-values range from 0.5 to 1.5 along the Tuz Golu Fault Zone in Central Anatolia. The maximum horizontal compressive stress (S-H) indicates N-S and NE-SW directions while the minimum horizontal compressive stress (Sh) displays E-W. These results represent that low stress variances are correlated with high b-values. Low b-values are predominant in areas with high stress rates, consistent with a high heterogeneity of focal mechanisms.

Aftershocks of the 16 September 2015 M8.3 Illapel earthquake in central Chile were recorded for a period of one year by the Chile-Illapel Aftershock Experiment (CHILLAX) seismic network. An initial catalog of about 100,000 events was generated by a novel automated picking algorithm that combines an auto-regressive detection/onset estimation method with some recently developed windowing techniques. We combine arrival times of P and S waves from about 9,000 of the best recorded of these events with relative arrival times of teleseismic P waves and phase delays of Rayleigh waves recovered from ambient noise to generate a three-dimensional image of P and S wavespeeds in that part of the Andean margin located beneath the network. Hypocenters of aftershocks located in the final model appear to be grouped into four distinct zones: two lower parallel zones that dip about 20° to the east, an upper, highly active zone that dips about 30° to the east, and a diffuse zone of smaller events located between this upper zone and the surface. Nearly all of the events with M > 4 are located at the bottom edge of the highly active zone and define a plane dipping about 26° to the east. Most of these events have almost identical thrust mechanisms and are separated from the lower zones by a steep gradient in increasing wavespeeds. The lower end of the upper zone, at about 55 km depth, is marked by another velocity gradient dipping to the west that bounds the diffuse shallow activity and projects to the surface west of the high Andes. We postulate that the triangular region delimited by these two high wavespeed gradients is a subduction wedge, generated by the removal of material from the leading edge of the upper plate by subduction erosion, and subsequently underplated back on to the overriding plate at depth. This model explains many of the features we observe in the seismicity and wavespeeds and is consistent with geologic observations of uplift and extension in this part of the Chilean coastal margin. Characteristics of this wedge, such as its shape and the lack of correlation with the coeval back-arc tectonic setting, suggest that high Andes are not directly coupled with the behavior of the subduction interface.

Actively deforming crust and upper mantle around the obliquely convergent Pacific-Australia plate boundary in the northern South Island of New Zealand have been investigated by seismic tomography using data from a temporary c. 50 station network plus GEONET. The Alpine-Wairau Fault (principal component of the plate boundary at the surface) transects the study area, separating the Buller-Nelson (BN) province of active compressional inversion involving steep reverse faulting to the northwest, from the Marlborough fault system (MA) dominated by dextral strike-slip faulting to the southeast. In the course of this study, MA hosted the 2013 Seddon earthquake sequence (M5.9, 6.6) and the 2016 M7.8 Kaikoura earthquake. Active fault structures and present seismicity are associated with a heterogeneous distribution of low-velocity and Vp/Vs anomalies. Elsewhere, there is a general association between crustal seismicity and low-velocity zones along major fault structures within both the MA and BN seismotectonic provinces along which high Vp/Vs anomalies are locally conspicuous. Areas of active crustal seismicity are also generally characterized by high Vp/Vs, for instance the hypocentre and aftershocks of the 2016 M7.8 Kaikoura earthquake overlie a low-velocity region with high Vp/Vs. The coincidence of anomalously low Vp and Vs and anomalously high Vp/Vs with zones of high electrical conductivity defined by a previous MT transect is consistent with the notion that upward migration of overpressured hydrothermal fluid from the subducting slab at depth leads to a heterogeneous distribution of overpressured fluid in and around the base of the crustal seismogenic zone, weakening the overlying crust and promoting seismic rupture along major fault systems. It seems possible that the Association of mid-crustal low-velocity zones with anomalously high Vp/Vs may diagnose rupture preparation zones where frictional strength is being lowered by the build-up of fluid overpressure concurrent with accumulating shear stress, so that eventual fault failure is ‘dual-driven’.

We estimate the fault model of the 2 September 2009 Tasikmalaya intraslab earthquake based on the GPS data available in western Java, Indonesia. The focal mechanism of the earthquake was used to help construct two possible fault models: a west-dipping fault with a strike of 160.8° and an east-dipping fault with a strike of 34.0°. In this study, vertical information from GPS data is crucial for constructing the top depth of the fault. The subsidence information from GPS data located near the epicenter suggests that the earthquake involved a deeper fault model. While the amount of the moment release of the east-dipping fault (Model dipE) is equivalent to Mw 6.9, the moment release of the west-dipping fault (Model dipW) is equivalent to Mw 6.8. The GPS data inversion indicates that Model dipW produces a better fit than Model dipE. The tsunami simulation indicates that the tsunami height generated by the east-dipping fault is smaller than that generated by the west-dipping fault, implying that the maximum tsunami height of the latter is closer to agreement with the reported one. Unlike Model dipE, the stress transfer analysis of Model dipW indicates that most of the aftershocks were located in the region where ΔCFF is positive, suggesting positive stress from the ruptured triggered aftershocks. The combined analysis of GPS data, tsunami simulation, and stress transfer suggests that the fault ruptured during the 2009 earthquake was dipping westward with a steep dip angle.

Due to the unique subsoil conditions prevailing in the Mexico City basin, seismic risk has been strongly correlated to site effects. Thus, during the Mw 8.1 09/19/1985 subduction fault earthquake, and its strong aftershock Mw 7.5 09/21/1985, extensive damage was observed in the area, along with the loss of thousands of lives, despite these events had an epicentral distance of around 430 km from Mexico City. The observed damage was mostly due to site affects originated by the high plastiCity clay deposits found in the basin, which lead to large amplifications, and duration elongation of the ground motions coming from the epicenter. In addition, a frequency content modification occurs, which in turn, leads to a double resonance effect between the incoming ground motions, soil deposits, and the damaged buildings. Exactly 32 years after this devastating event, the Mw 7.1 09/19/2017 normal fault earthquake, reminded us of the importance of accounting for site effects, and most importantly the need to carry out a proper characterization of basin geometry, soil profile configuration, hydraulic conditions, and maintenance-structure periodic assessments of the building stock in Mexico City. This is required to reduce uncertainties of seismic vulnerability studies for extreme-event seismic hazard scenarios. In this paper, the role of site response and seismic soil-structure interaction as key factors responsible of the observed damage in the City is revisited, through series of 3D finite difference models of typical structure-foundation-soil typologies found at the areas where most of the damage was observed, highlighting its clear impact in the final damage distribution observed around the City.

Increasingly frequent earthquakes in the Longmenshan (LMS) tectonic belt are linked to the ongoing eastward growth of the Tibetan Plateau during the Late Cenozoic. Following the devastating 2008 Ms 8.0 Wenchuan earthquake in the central LMS and the 2013 Ms 7.0 Lushan earthquake in the southern segment, an unexpected Ms 5.4 Qingchuan earthquake occurred on September 30, 2017 in the northern LMS. We synthesize focal mechanism solutions, relocated aftershocks, regional geology, and seismic reflection profiles in order to determine the pre-existing faults and active fault system in the northern LMS, thus to better assess potential seismic hazards. At a seismically resolvable scale, we identify a large structural wedge at the basement that is cross-cut by numerous multiphase and multi-behavior active faults. Our seismotectonic model indicates that the seismogenic fault responsible for the 2017 Ms 5.4 Qingchuan earthquake is a pre-existing thrust fault with a dip of ∼34° in the frontal thrust ramp. Reactivation of this fault then triggered the activity of the co-seismic strike-slip fault that was formed in the 2008 Wenchuan event. The seismic interpretation, focal mechanism solutions, and aftershock distribution indicate that two sub-segment faults with different geometries are distinguished along the northern LMS. Accordingly, a transformation of the coseismic fault with different geometry and kinematics is formed. With the constraints that the mainshock and aftershocks occurred on two different faults, we propose that both the frontal ramp thrust fault and the recently-formed strike-slip fault are tectonically active. This study highlights the importance of the influence of the pre-existing fault system on the evolution of the northern LMS during the Late Cenozoic. The rapid uplift of the eastern Tibetan Plateau and the complex fault system indicate the potential for seismic risks in the northern LMS.

Residual displacement demand is a key indicator of seismic damage and plays a pivotal role in post-earthquake decision making on damaged structures. This parameter can be effective as an estimator of the residual capacity of structural systems damaged under seismic sequences. Residual displacement ratio (Cr) is a conventional approach to predict the residual displacement demand from maximum linear displacement. This study investigates the residual displacement ratios of structures as bi-linear SDOF systems subjected to mainshock-aftershock sequences. Constant-strength spectra based on Cr are developed taking into account the effects of ground motion parameters (such as site condition, magnitude, epicentral distance, and duration), and of structural characteristics (such as strength modification factor and post-yield stiffness ratio). Several analytical equations are proposed to predict the residual displacement ratio as function of the elastic vibration period and the strength modification factor for different levels of the post-yield stiffness ratio and also various aftershock intensities. The proposed equations can be employed for seismic assessment of structures against mainshock-aftershock sequences.

As an important aspect in disaster operations management, relief distribution has been challenged by lots of factors, such as unpredictable occurrence time, intensity and location of secondary disasters (e.g. aftershocks and landslides, which usually occur after an earthquake), and availability of vehicles. A multi-stage stochastic programming model is developed for disaster relief distribution with consideration of multiple types of vehicles, fluctuation of rental, and the state of road network. The state of road network is characterized using uncertain and dynamic road capacity. The scenario tree is employed to represent the uncertain and dynamic road capacity, and demonstrate the decision process of relief distribution. A progressive hedging algorithm (PHA) is proposed for solving the proposed model in large-scale size. Based on a real-world case of Yaan earthquake in China, numerical experiments are presented to study the applicability of the proposed model and demonstrate the effectiveness of the proposed PHA. Useful managerial insights are provided by conducting numerical analysis.

Knowledge of seismic hazard is an integral part of strategic planning for critical infrastructural development. This is lacking in Nigeria that is seriously considering the construction of nuclear power plants, dams, bridges etc. to meet increasing need by its exploding population. This study therefore, is aimed at computing critical earthquake hazard parameters and peak ground accelerations (PGAs) for the country using the probabilistic seismic hazard assessment (PSHA) approach. Earthquake catalog (1933–2018) was used to carry out the PSHA. The data were sourced from seismic stations in Nigeria, institutes in neighboring countries and earthquake catalogs published by international Agencies. The data were screened to remove artificial events, fore- and aftershocks, etc. Different magnitude scales were harmonized to moment magnitude (Mw) scale. The selection of the earthquakes to calculate the PGAs was carefully carried out to include events on land (by specifying area of interest on the seismicity map with maximum latitude and longitude; and minimum latitude and longitude respectively using excel), and the Cornell-McGuire method was used to compute the PGAs for Nigeria. The results indicate low activity rate of earthquakes in Nigeria, but high probability of yearly occurrence of earthquakes of magnitudes 2.0–3.0. In the same vein, the probability of occurrence of earthquakes of magnitudes 4.0–6.0 is low. The b-value and activity rate (λ) are 0.69 ± 0.07 and 1.684 ± 0.462 respectively; while the expected maximum magnitude in the region is 6.7 ± 0.34 within the 1–1000 years’ period. A magnitude 6.0 can be expected to occur approximately every 143 years in Nigeria, and the probabilities of this event occurring annually, 50, 100 and 1000 years is 6.7%, 95.81%, 99.70% and 100% respectively. The PGAs for a 10% probability of exceedance in 50 years range from 0.01–0.08g. Findings from PSHA will contribute to planning, with the PGAs useful for engineering design and as baseline parameters for the establishment of seismic building codes for Nigeria.

Current afterslip models of the Mw 7.8 Gorkha earthquake contradict with the depth ranges of relocated aftershock distribution and inter-seismic locked-creeping transition zone. We use both GPS and interferometric synthetic aperture radar (InSAR) data to image the afterslip evolution in the first 189 post-seismic days, i.e., from 25 April to 1 November 2015. In the study region, 51 GPS stations are divided into three categories based on their observation time: (1) 25 stations with both pre- and post-seismic observations, (2) 16 stations with only post-seismic observations, and (3) 10 stations with only pre-seismic observations. The secular tectonic motion and seasonal variations are corrected carefully and directly using the fitted functions from the pre-seismic position time series. During the inversions, we adopt the widely accepted flat–ramp–flat geometrical model of the Main Frontal Thrust fault (MFT)–Main Himalayan Thrust fault (MHT). The preferred weight ratios between GPS and InSAR observations are elaborately determined through comprehensive analysis of model resolution and fitting residuals. In contrast to previous afterslip models, our preferred afterslip distributes on the crustal ramp of the MHT from depths of 11 to 29 km, which is verified by further inversions using only GPS data and checkerboard resolution tests. The seismic moment released by the afterslip in the first six months is 3.42 × 1019 N m, only approximately 4% of the mainshock moment. The rupture gap between the mainshock and the 12 May aftershock is filled up with post-seismic aseismic slip.

The Mw6.9 Yushu earthquake occurred on 14 April 2010, in Qinghai, China; the largest aftershock, a Mw5.8 event, occurred west of the mainshock on 29 May 2010 (˜40 days later). The aftershock had a different focal mechanism from the mainshock. Furthermore, seismicity after 29 May showed different spatial characteristics in terms of focal depth and distribution direction. To better understand the faulting and the relationship between these two events, we derived the whole displacement field caused by the Yushu mainshock and the Mw5.8 aftershock based on multi-perspective Interferometric Synthetic Aperture Radar (InSAR) data. We then conducted a robust inversion of the slip distribution jointly constrained by InSAR and GPS data. The results indicate that the Mw5.8 aftershock produced a separated deformation field with significant displacement changes of up to ˜4–6 cm, which indicates another intersecting ruptured fault at the west end of the Yushu fault. The slip distribution shows a 75-km NW rupture with a maximum slip of ˜2.1 m at a depth of ˜0–10 km on the main Yushu fault, and a 20 km NE rupture with peak slip of ˜0.4 m at a depth of ˜5–15 km on a vertical hidden fault. Both events showed a dominant left-lateral component. The total rupture length associated with the 2010 Yushu earthquake sequence reached ˜95 km. By calculating Coulomb stress changes, we confirmed that the mainshock triggered the Mw5.8 aftershock. Our results imply that the increased stress at the western end of the Yushu fault caused by the mainshock rupture may have played an important role in transferring the rupture plane from the Yushu fault to the NE hidden fault.

In this study, a detailed space-time-magnitude assessment on the aftershock sequence of July 21th, 2017, MW 6.5, Bodrum-Kos earthquake (Turkey) is carried out by focusing on the analyses of aftershock parameters: b-value, p-value, Dc-value and Mamax. b-value is estimated as 0.90 ± 0.05 with a completeness magnitude Mc = 1.6 and this relatively small b-value may be resulted from the abundance of aftershocks with magnitude ML ≥ 4.0. However, it is well represented by the Gutenberg-Richter law with a typically b ≈ 1.0. p-value is estimated as 1.04 ± 0.02 with a c-value = 0.224 ± 0.039 and is well characterized nearly close to the global p ≈ 1.0. This relatively high p-value may be a result of the relative fast decay rate of the aftershock activity. Dc-value is calculated as 1.74 ± 0.09 and it means that aftershocks are homogeneously distributed. Temporal distribution of b-value shows that small b-values may be due to a stepwise increase in effective stress before the occurrence of larger aftershocks. Regional changes range from 0.5 to 1.2 in b-value, from 0.4 to 1.3 in p-value and from 3.0 to 5.2 in Mamax. The smallest b-values are found in and around mainshock, including Karaada, Bodrum, Akyarlar and Turgutreis, and these regions have high stress as well as coseismic deformation. The largest p-values are found in and around the mainshock epicenter including Karaada, and it is interpreted that this situation may be caused by coseismic deformation. Mamax values larger than 4.4 are observed in and around mainshock epicenter, including Bodrum and Akyarlar, and there is a clear correlation between spatial variations of b-values and Mamax. These results show that stress changes and coseismic deformation seem highly effective on b and p-value variations. Therefore, a special interest needs to be given to these anomaly regions since aftershock hazard is highly associated with these parameters and may be developed considering their space-time-magnitude distribution.

Eastern Anatolia is a ∼2 km high plateau shaped by the continent-continent collision of the Arabian and Eurasian plates. The left lateral East Anatolian Fault Zone, the right lateral North Anatolian Fault Zone and the Bitlis fold-thrust belt are the major tectonic boundaries of this convergence zone. The Arabian-Eurasian collision has resulted in high volcanism and well recorded seismic activity in Eastern Anatolia and its surroundings. Karliova Junction is located at the intersection of these major fault systems and contains secondary faults such as the Karakocan fault and the Sancak-Uzunpinar fault. We processed high quality waveform data collected from a recent seismic activity in the proximity of the city of Bingol where the largest event is the moderate-size earthquake (Mw = 5.3) occurred on 2 December 2015. This event is located to the west of Karliova Junction and to the northwest of Sancak-Uzunpinar fault. The spatial distribution of the aftershocks points out unmapped faults with NNW-SSE alignment towards the west of Sancak-Uzunpinar fault. Aftershock depth distribution indicates a nearly 15 km deep brittle seismogenic zone. The relocated aftershock distributions and seismic moment calculations yield a rupture area 9 km in length and 5 km in width with an average 8 cm of slip. The latest four earthquakes preceding the 2015 Eq with magnitudes larger than Mw > 6 enhanced the Coulomb stress failure in the 2015 Bingol Earthquake rupture area. The ruptured fault plane is in-line with the optimally oriented right-lateral strike-slip faults.

Strong earthquakes can disturb the regional stress field. The variation in the static Coulomb stress changes induced by earthquakes can explain the spatial distribution of certain aftershocks, and cascade earthquake sequences. However, the coseismic Coulomb stress change is not the only stress perturbation, interseismic and postseismic viscoelastic relaxation can also affect the evolution of the stress field. This study focuses on how stress perturbation from four strong earthquakes (the 1933 M7.5 Diexi, 1955 M7.5 Kangding, 1976 M7.2 Songpan and 2008 M8.0 Wenchuan) occurred around the Longmen Shan area in western China affect the occurrence of the 2013 Lushan M7.0 earthquake. In addition, we examine the stress evolution along the Longmen Shan fault zone and its adjacent faults to assess the regional seismic hazard. We construct a 3D viscoelastic finite element model that includes the fault structure, inhomogeneous medium, tectonic stress, gravity to simulate the evolutionary stress field when using observed Global Positioning System (GPS) vectors as the boundary conditions. The simulated results show that the coseismic Coulomb stress change induced by the Kangding, Songpan, and Wenchuan earthquakes is positive at the hypocenter of Lushan earthquake. In particular, the coseismic Coulomb stress change imparted by the Wenchuan earthquake exceeds the earthquake-triggering threshold of 0.01MPa. Additionally, the tectonic stress at the hypocenter of the Lushan earthquake increases with time, indicating the stress cumulates gradually at this location. We also compare the distribution of the earthquakes (from 1933 to 2013) with the effective and Coulomb evolutionary stress fields. Almost 70% of the earthquakes in the research region are located in the effective and Coulomb stress-enhanced areas. Our study helps to better understand the origin of Lushan earthquake and provide a theroetical basis for assesing regional seismic hazard.

The 2014 Lloja earthquake with 5.1 Mw occurred in the Eastern Cordillera of Bolivia between the Subandean belt (characterized by reverse and thrust faulting) and the Altiplano (where extensional stresses due to gravitational collapse dominate). Determination of the Lloja focal mechanism, therefore, is important to sample the transition between these two Andean stress provinces. In addition, two different moment tensor inversions had been published: a strike-slip solution with a normal component (NEIC-USGS) and a purely strike-slip solution (GCMT). A detailed relocation of the epicenter was carried out using two data sets: a) only the local stations of the La Paz network with a 1D model appropriate for the Central Andean plateau, and b) local + regional stations with the 3D RSTT model. Both epicenters agree within their uncertainties of about +- 5 km each. P-wave polarity of the local and regional stations, together with the S-wave polarity at LPAZ station, allowed the focal mechanism to be determined as a strike-slip solution with a small normal component, similar to the USGS solution. One nodal plane strikes NNE-SSW and was interpreted as the seismogenic fault because of its consistency with a) the distribution of the relocated aftershocks, and b) the NE-SW trending local Caracato - Rio La Paz geological fault near the epicenter. The strike-slip mechanism with a small component of normal faulting is consistent with a gradual transition between compressional stresses in the sub-Andean belt and extensional stresses in the Altiplano.

The Tian Shan Range, which trends E-W along the southern margin of the Junggar Basin, is one of the longest and most active intracontinental orogenic belts in central Asia. On 8 December 2016 (05:15:04 UTC), a Mw 6.0 earthquake ruptured the northern Tian Shan front. Here, we use Sentinel-1 radar imagery to investigate the deformation and source parameters related to this event. The co-seismic surface deformation was predominated by uplift without surface rupture. Ascending and descending interferograms indicate that the event triggered small co-seismic deformations with maximum line-of-sight displacements of 22 mm and 24 mm, respectively. Although the north-dipping and south-dipping plane solutions can both fit the observations well, the north-dipping solution with a dip of 58° is preferred in consideration of the relocated aftershocks and regional geological structure. Significant slip is located between depths of 12 km and 17 km, suggesting that the event was caused by a completely blind thrust fault. This blind rupture is characterized largely by a compact thrusting patch with a peak slip of 56 cm at a depth of 13 km. The source model generates a geodetic moment of 6.678 × 1017 N m corresponding to a Mw 5.85 event. Both the interferometric synthetic aperture radar modeling and the aftershock locations indicate that the rupture plane is linked to the Huoerguosi-Manas-Tugulu fault at a depth of ∼16 km, a typical locking depth in the Tian Shan. We suggest that the 2016 Hutubi earthquake more likely occurred on a back-thrust of the Huoerguosi-Manas-Tugulu fault, and the back-thrust is interpreted to represent a preexisting normal fault beneath the Qigu anticline belt that was tectonically inverted during the Cenozoic.

A framework for building-portfolio risk and loss assessment under mainshock-aftershock earthquake sequences is presented. The temporary increase in seismic hazard after a large-magnitude mainshock coupled with the reduction in the structural capacity under sequential ground motions are considered in the framework. For the inventory of reinforced concrete frames located in Los Angeles County, the annual rate of “complete” (HAZUS) damage and expected annual losses are, on average, 10% and 25% higher, respectively, when aftershocks are incorporated in the risk assessment procedure. A short-term risk and loss analysis after a hypothetical scenario mainshock on the Puente Hills fault showed a substantial increase in the seismic risk within one week after the mainshock. The aftershock-induced losses during the same period were estimated at approximately 30% of the losses due to the mainshock.

On 8 September 2017, a great earthquake ( M w 8.2) took place in the Mexican subduction zone in the Tehuantepec gap, where no large subduction earthquakes have taken place since 1902. However, the 8 September earthquake did not occur on the contact between the Cocos and North American plates. The centroidal hypocentral depths reported by different agencies, including the National Seismological Service, range from 45 to 47.4 km, placing it immediately beneath the down-dip limit of the interplate locked zone. The source mechanism reflects down-dip tensional faulting. The inversion of the fault process shows a rupture that initiated at the bottom of the lithosphere and propagated upward, breaking through the entire subducted lithosphere. An unusually long, complex and copious aftershock sequence followed the main event. The relocated aftershocks, in the first 20 days following the main event, delineate a 160-km-long fault, sub-parallel to the oceanic trench, immediately beneath the interplate contact. The aftershocks also concentrate in secondary intraslab faults ∼50 km down-dip from the mainshock rupture, revealing that pervasive tensional stresses are present within the subducted plate. This deformation pattern suggests a large-scale tensional regime in the slab, apparently induced by its own gravitational weight that pulls it away from the strongly coupled interplate contact. Other large intraplate earthquakes in the vicinity, in 1931 ( M w 7.8) and 1999 ( M w 7.5), also reflect this detachment of the downgoing Cocos plate that sinks into the mantle under its own gravitational weight. The slab detachment suggests that the up-dip segment of the plate interface near the Isthmus of Tehuantepec is locked and possibly primed for a megathrust earthquake, such as the M w 8.6 that took place in 1787, on an adjacent segment of the Mexican subduction zone.

The Canterbury Earthquake Sequence (CES) began with the Darfield earthquake on 4 September 2010. Continual large and small aftershocks since that time have meant communities have cycled through repeated periods of impact, response and recovery. Scientific communication about aftershocks during such a prolonged sequence has faced distinct challenges. We conducted research to better understand aftershock information needs for agencies and the public, and how people interpreted and responded to such information. We found that a wide range of information was needed from basic facts about aftershocks through to more technical information, and in different formats (e.g. maps, tables, graphs, text, analogies). Information needs also evolved throughout the sequence, and differed depending on people's roles and experiences, and the phase of impact, response and recovery communities were in. Interpretation of aftershock information was influenced by a variety of factors including how understandable and relevant the information was, whether people had prior knowledge or experience of aftershocks, whether the information was personalised or contextualised, emotions and feelings, credibility and trust, and external influences. Given that such a diversity of evolving information is required, it is imperative that geoscientists strategize how to provide such information before a significant earthquake occurs.

This study aims at reconstructing Pleistocene slope erosion processes and transformation of relief in well-developed Pleistocene (Tyrrhenian) marine terraces along the western Peloponnese. The investigations are based on existing geological data, field observations, a selected seismic reflection profile, sedimentological analysis and radiometric dating. Aftershock rupture segment extends for 6 km in a WNW direction. Each fault movement in an earthquake results in a sudden increase in the size of the scarp, by an amount ranging from a few centimeters to several meters. Thrust faulting occurs on shallow dipping faults and does not commonly produce a true fault scarp, because the tip of the upper part, over the thrust block, tends to collapse, leaving a sinuous scarp that does not coincide with the fault plane. Strike-slip faulting may result in a scarp if there is a significant extensional or thrust component of motion, or if rock-types with different weathering characteristics and thickness are compared by faulting. Failing this, both active and inactive faults are often marked by a fault-line, a line of preferential erosion along the fault. The combined effects of fault motion, erosion, and sediment deposition affect the form of the scarp and the latter two will eventually eliminate it after movement on the fault ceases. If the rock erosion rates on both fault sides are known or can be assumed to be negligible, and the sediments deposited contain material that can be dated, then the magnitude of the scarp can be used to determine the age at which the scarp began to form and the long-term rate of movement on the fault. More information regarding the relative significance and rate of movement of these faults can be obtained by observing their effects on the drainage pattern in the area. In particular, morphotectonic data indicate that the observed rupture segment may reflect a fault-line scarp corresponding to an old fault scarp, which belongs to the isotopic stage 7 or prior, i.e. isotopic stage 11, during the evolution of NW Peloponnese basin. Moreover, morphotectonic data clearly shows that NW Peloponnese is characterized by a complex structural pattern and erosion process phenomena aided by human activity. This data is well-supported by seismic reflection profiles information.

The 2013 Craig, Alaska MW 7.5 earthquake ruptured along ∼150 km of the Queen Charlotte Fault (QCF), a right-lateral strike-slip plate boundary fault separating the Pacific and North American plates. Regional shear wave analyses suggest that the Craig earthquake rupture propagated in the northward direction faster than the S-wave (supershear). Theoretical studies suggest that a bimaterial interface, such as that along the QCF, which separates oceanic and continental crust with differing elastic properties, can promote supershear rupture propagation. We deployed short-period ocean-bottom seismometers (OBS) as a part of a rapid-response effort less than four months after the Craig earthquake mainshock. During a 21-day period, 1,133 aftershocks were recorded by 8 OBS instruments. Aftershock spatial distribution indicates that the base of the seismogenic zone along the QCF approaches ∼25 km depth, consistent with a thermally-controlled fault rheology expected for igneous rocks at oceanic transform faults. The spatial distribution also provides supporting evidence for a previously hypothesized active strand of the QCF system within the Pacific Plate. Tomographic traveltime inversion for velocity structure indicates a low-velocity (VP and VS) zone on the Pacific side of the plate boundary at 5–20 km depths, where Neogene Pacific crust and upper mantle seismic velocities average ∼3–11% slower than the North American side, where the Paleozoic North American crust is seismically faster. Our results suggest that elastic properties along the studied portion of the QCF are different than those of a simple oceanic–continental plate boundary fault. In our study region, velocity structure across the QCF, while bimaterial, does not support faster material on the west side of the fault, which has been proposed as one possible explanation for northward supershear propagation during the Craig earthquake. Instead, we image low-velocity material on the west side of the fault. Explanations could include that part of the rupture was subshear, or that fault damage zone properties or fault smoothness are more important controls on supershear rupture than a bimaterial contrast.

Due to the heavy damages caused by the 2017 Ms 7.0 Jiuzhaigou earthquake that occurred in a complex tectonic environment, investigations on the seismogenic structure and tectonic environment are very important for understanding the geodynamic mechanism and mitigating earthquake hazards in the region. In this work, we present a high-resolution 3-D crustal seismic model for the source region of the Jiuzhaigou earthquake by exploiting the arrival times of aftershocks based on double difference tomography. Our results revealed that zones of Jiuzhaigou's rupture fault and aftershocks predominantly coincide with the high-velocity anomalies that are surrounded by the low-velocity anomalies, from the western territory to the aftershock zone. An area with a high Poisson's ratio ~0.28 is primarily the cradle for most of the aftershocks, and the lithology is interpreted as consisting of mafic and calcium-rich rocks. In the mid-crust, tomographic images exhibit a low-velocity zone in and around the source region. This low-velocity zone originates from the interior of the Tibetan Plateau, extends across the thrust faults (the Longriba fault and the Minjiang fault and the Jiuzhaigou's causative fault) and then finally encounters the strike-slip faults (the Tazang fault and East Kunlun fault). The counter belt of both the Tazang fault and East Kunlun fault features high-velocity anomalies. The mid-crust low anomalous zone is characterized by a high Poisson's ratio and low shear wave velocity, which indicates a weaker crust that easily flows and deforms. Therefore, we propose that this low velocity is a crustal channel flow in the vicinity of seismic sources, which causes the transformation from thrusting to strike-slip motion in the study area.

We herein use the joint inversion of P-receiver functions and fundamental mode group velocity dispersion data (9–70 s) of Rayleigh and Love waves to estimate crustal and lithospheric thicknesses at twenty three-component mobile broadband stations in Kachchh, Gujarat. Modeled Moho depths range from 35 to 43 km, while lithospheric thicknesses vary from 64 to 106 km. The main result of our modelling is the delineation of a marked crustal (∼2–4 km) as well as lithospheric (∼10–20 km) thinning and a 2–6% drop in Vs across the lithosphere-asthenosphere-boundary (LAB), within the Samkhiali graben (associated with gravity high) underlying the central Kachchh rift zone (KRZ), where 95% of the continued aftershock activity took place since 2001. Such a large drop in Vs could be attributed to the presence of carbonatite melts in the upper mantle. Our modeling reveals a 4 km crustal thinning below the central KRZ and a 4 km crustal thickening below the surrounding riftless regions. This kind of crustal structure is inferred to induce large flexural deviatoric stresses (∼50–100 MPa) in the upper crust, thereby, these stresses in the presence of regional plate tectonic stresses can bring the Samkhiali graben below the central KRZ near to the critical stress level. While stress-transfer, meteoric water (in the upper crust), and metamorphic fluid as well volatile CO2 flows (in the lower crust) provide the required triggering effect to the critically stressed graben structure (down to 35 km depth) for generating continued aftershock activity in the Kachchh rift zone, since 2001. We also propose that the deeper circulation of volatile CO2 through the inferred conduit (related to the 65 Ma Deccan Plume activity) extending from lower crust down to asthenosphere plays a key role in the generation of uninterrupted occurrence of earthquakes in the Kachchh rift zone, Gujarat, India.