Aftershock Events Research Articles

A New Smoothed Seismicity Approach to Include Aftershocks and Foreshocks in Spatial Earthquake Forecasting: Application to the Global Mw ≥ 5.5 Seismicity

Seismicity-based earthquake forecasting models have been primarily studied and developed over the past twenty years. These models mainly rely on seismicity catalogs as their data source and provide forecasts in time, space, and magnitude in a quantifiable manner. In this study, we presented a technique to better determine future earthquakes in space based on spatially smoothed seismicity. The improvement’s main objective is to use foreshock and aftershock events together with their mainshocks. Time-independent earthquake forecast models are often developed using declustered catalogs, where smaller-magnitude events regarding their mainshocks are removed from the catalog. Declustered catalogs are required in the probabilistic seismic hazard analysis (PSHA) to hold the Poisson assumption that the events are independent in time and space. However, as highlighted and presented by many recent studies, removing such events from seismic catalogs may lead to underestimating seismicity rates and, consequently, the final seismic hazard in terms of ground shaking. Our study also demonstrated that considering the complete catalog may improve future earthquakes’ spatial forecast. To do so, we adopted two different smoothed seismicity methods: (1) the fixed smoothing method, which uses spatially uniform smoothing parameters, and (2) the adaptive smoothing method, which relates an individual smoothing distance for each earthquake. The smoothed seismicity models are constructed by using the global earthquake catalog with Mw ≥ 5.5 events. We reported progress on comparing smoothed seismicity models developed by calculating and evaluating the joint log-likelihoods. Our resulting forecast shows a significant information gain concerning both fixed and adaptive smoothing model forecasts. Our findings indicate that complete catalogs are a notable feature for increasing the spatial variation skill of seismicity forecasts.

Bayesian Inference of Centroid Moment Tensors of the 2019 Ambon (Mw 6.5) Aftershock Earthquake Sequence, Indonesia: A Preliminary Result

The Ambon Mw 6.5 earthquake on September 26th, 2019, had contributed to give severe damages and significantly increased seismicity around Ambon Island and surrounding areas. Mainshock was followed by aftershocks with spatial distribution added to the impact of destructions in this region. We investigated aftershocks sequences to reveal the effect of mainshock toward the change in the in-situ stress field, including the possibility of the existing faults reactivation and the generation of aftershocks. We inferred centroid moment tensor (CMT) for significant aftershock events with Mw more than 4.0 using waveform data recorded from October 18th to December 15th, 2019. The aftershock focal mechanism was determined using the Bayesian full-waveform inversion code ISOLA-Obspy. This approach provides the uncertainty of the CMT model parameters. From ten CMT solution we had inferred in three seismic clusters, we found that majority of events have a strike-slip mechanism. Four events located on the south of the N-S trendings have a dextral strike-slip fault type, reflected the rupture of the mainshocks fault plane. Three events in the cluster of Ambon Island are dextral strike-slip, confirming the presence of the fault reactivation. Meanwhile, three CMT solutions in the north show the dextral strike-slip faulting and may belong to the mainshock main fault, connected with the cluster in the south.

Analysis of the Mw 6.5 Ambon Earthquake (September 26, 2019) based on the aftershocks hypocenter relocation

On September 26, 2019, an earthquake (Mw 6.5) occurred in the northern part of Ambon Island, Molucca, East Indonesia. The National Authority show this earthquake caused infrastructures damage and 28 fatalities. Up to October 31, 2019, the Indonesian Agency for Meteorology, Climatology, and Geophysics (BMKG) seismic network had recorded 479 aftershock events. We have relocated 463 out of the 479 aftershocks using the double-difference method. Our results show that some improvements in the hypocenter locations, where the focal depths of initial earthquakes fixed at 10 km have been updated to ~9.8 km depth on average. There are two aftershock clusters in a North-South direction of ~35 km length between Ambon Island and Haruku Island and in a West-East direction of ~30 km length in the Ambon Island, each with a width of ~8 km.

Seismic fragility of structures with energy dissipation devices for mainshock-aftershock events

This paper presents a mainshock-aftershock seismic fragility and collapse capacity assessment of reinforced concrete (RC) structures retrofitted with a hybrid damper composed of a steel slit plate and friction pads. Three and eight-story RC buildings are designed and assessed before and after retrofit considering the aftershocks effect. Non-linear time-history response analysis (NLTHA) using twelve natural earthquake sequences are used to produce incremental dynamic analysis (IDA) curves to obtain the median collapse capacity of the structures. Three different damage state (DS) levels are used for the mainshock ground excitation to quantify the scale factors required for conducting the aftershock IDAs. The maximum inter-story drift ratio (MIDR) is used as the main engineering demand parameter. The study shows the importance of considering the aftershock in the seismic assessment process of RC structures. The un-retrofitted structures are found to experience a high level of deterioration under aftershock event which is not considered in the design stage. The findings of the study reveal that the mainshock-aftershock sequence responses of the retrofitted structures show better performance in terms of the median collapse capacity and the seismic fragility compared to the un-retrofitted ones.

Energy absorption and inelasticity distribution mechanisms in steel moment frames affected by mainshock-aftershock sequences

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.

Seismic performance assessment of tunnel form concrete structures under earthquake sequences using endurance time analysis

A low-computational cost method is developed for seismic performance assessment of tunnel-form concrete structures under earthquake sequences. Using incremental dynamic analysis (IDA), a structure already damaged under a code spectrum-compatible mainshock is subjected to a set of aftershock events. The seismic reliability of the structure under successive earthquakes is then evaluated by comparing the fragility curves of the intact and the damaged structures. To achieve this, the newly developed endurance time approach is used to obtain the residual capacity of the structures to withstand aftershocks. The efficiency of the proposed method is demonstrated by using 5 and 10-storey 3D tunnel-form concrete structures as case study examples. It is shown that using endurance time method for seismic performance evaluation under mainshock-aftershock sequences generally leads to accurate results (less than 15% error), while it can significantly reduce (up to 90%) the computational costs of the conventional IDA method. The results also indicate that the application of the code design basis earthquake as the mainshock has no significant effect on the seismic performance of the tunnel form concrete structures under aftershocks, especially for the shorter systems. This highlights the excellent seismic resistance of tunnel-form structures under earthquake sequences.

Comparing responses of special and intermediate moment frames under repeated earthquakes

Steel moment-resisting frames (MRFs) are designed to resist a design-level single seismic event. However, every earthquake may be accompanied by several foreshocks or aftershocks. The cyclic deterioration of structural elements during early earthquakes may cause the lateral-load-resisting system to collapse under the mainshock or aftershocks. The differences in the seismic performance of special and intermediate MRFs (here called SMFs and IMFs, respectively) subjected to single and multiple earthquake events were investigated. For this purpose, four perimeter steel MRFs with four and eight storeys were designed and subjected to 22 synthesised multi-record earthquake events. The non-linear modelling technique adopted accounts for the cyclic deterioration of structural elements, which is a key parameter in the evaluation of seismic performance under sequential earthquakes. The results showed that structural collapse capacity may be reduced considerably if the structure is subjected to foreshock and aftershock events in addition to the mainshock. In addition, it was found that, for the IMF models, the switch from the life safety (LS) limit state to the collapse prevention limit state was immediate. Therefore, the design of IMFs for the LS performance level may result in a substantial risk of collapse when the structure is exposed to repeated earthquakes.

Compilation of the seismic hazard maps in Bosnia and Herzegovina

Bosnia and Herzegovina is located in the center of the Western Balkans. Until 2018 Bosnia and Herzegovina used the hazard maps based on the macroseismic intensity. The first generation of probabilistic seismic hazard maps of Bosnia and Herzegovina is compiled and presented. Two seismotectonic models were created, the areal and linear (fault) together with the earthquake catalogue filtered out from foreshock and aftershock events. Hazard maps were constructed in terms of the peak ground acceleration, and presented in two maps, with an exceedance on average once in 95 or 475 years. The hazard map used for the design of earthquake-resistant buildings (return period of 475 years with a probability of exceedance of 10% in 50 years) is created for the ground type A with the velocity of the seismic wave propagations of vs,30 = 800 m/s. These maps have been accepted as a part of the National Annex in BAS EN 1998–1:2018.

Analysis of H/V Ratio Curve to Estimate Seismic Hazard Vulnerability in Lombok Island, West Nusa Tenggara, Indonesia


 
 
 In 2018, Lombok Island was hit by a series of destructive earthquakes. According to Indonesian Meteo- rological, Climatological, and Geophysical Agency data, about 1,973 felt earthquakes (M > 3) which shaken Lombok were recorded during August 2018 with three earthquakes with the largest magnitude of 6.9 Mw, 6.8 Mw, and 6.2 Mw. National Board for Disaster Management reported about 555 deaths, 1,833 people injured, and 186,010 houses damaged as a result of Lombok earthquake on August 5th, 2018. A number of seismometers were placed on Lombok Island from August 3rd, 2018 to October 19th, 2018 to monitor the aftershock events. There are 17 stations that record seismicity in Lombok consisting of 10 broadband sensors and 7 short period sensors. In this study, we used the Horizontal-to-Vertical Spectral Ratio (HVSR) method to analyze the risk of earthquake in the Lombok region. The basic concept of this method is to do a comparison between the horizontal component spectrum and the vertical component spectrum of a wave, where theoretically the particle movement of the horizontal component is greater than the particle movement of the vertical component on soft ground, whereas on the hard ground both components (horizontal and vertical) will be similar. H/V curve obtained from earthquake record- ings (Earthquake Horizontal-to-Vertical Ratio/EHVR) and H/V curve obtained from microtremor recordings (Microtremor Horizontal-to-Vertical Ratio/MHVR) shows good agreement and high correlation. Empirical correction of EMR (Earthquake-to-Microtremor Ratio) managed to decrease the difference of estimation of predominant frequency and amplification factor between EHVR and MHVR. Predominant frequency, amplification, and seismic vulnerability map agree with the geological condition of Lombok Island, where high value of amplification and seismic vulnerability was found on soft and thick ground. This study conclude that the maximum ground acceleration and the construction of the building should also considered when one wants to investigate the effect of an earthquake to the damage occurred, beside the site effects. 
 
 

Conditioning criteria based on multiple intensity measures for selecting hazard-consistent aftershock ground motion records

A mainshock earthquake is typically followed by a sequence of aftershocks, normally with a higher probability of occurrence within the first few days after the mainshock event. Quantifying the subsequent damage due to an aftershock event for a given structure that is already at some state of damage due to the mainshock event is an important task in seismic hazard and loss assessment. This task requires a framework to quantify the aftershock hazard in addition to the mainshock hazard that is already well defined. The present study aims to develop such a framework that facilitates the quantification of aftershock hazard given two sequential fault rupture scenarios—an approach that ultimately serves the basis for a hazard-consistent aftershock ground motion selection. To this end, conditioning criteria based on a single, two, and average-of-multiple intensity measures are utilized in order to select hazard-consistent aftershock records from databases comprising both mainshock and aftershock events. The primary outcome of this study is, thus, a stochastic framework that defines the aftershock seismicity of a given site and enables the aftershock ground motion selection for performance-based seismic assessment and design of structures.

An Earthquake Source Spectrum Model Considering Rupture Directivity and Its Application for Obtaining Green’s Functions

ABSTRACTA small earthquake with strong rupture directivity is inappropriate to use as a point source when estimating ground motion. A source spectrum model is proposed to remove the effects of rupture directivity and finite fault size from observed earthquakes. This model is developed by using a kinematic source model of a rectangular fault with bilateral-bidirectional rupture propagation. Its amplitude spectrum is modeled to decay as ω−2 at high frequency and is approximated by its envelope to make deconvolution a stable operation. The effectiveness of the proposed spectrum model is demonstrated through application to the two aftershocks of Mw 4.0 and Mw 5.5 observed following the 2016 Kumamoto earthquake. These aftershocks had similar focal mechanisms and were located near to each other. A method to estimate the source parameters by using the proposed spectrum model is presented and its effectiveness is demonstrated. The deconvolution of the ground-motion records using a suitable source spectrum model gives us the Green’s function. The amplitude spectra of the Green’s functions obtained from both observed aftershock events are shown to be consistent. The far-field ground motions are simulated by using the Green’s functions. The simulated ground motions match well with the observed ones. Simulation of the ground motions by using the Green’s functions obtained from the deconvolution of ground-motion records by the proposed spectrum model has an advantage compared to the use of small earthquake records as empirical Green’s functions (EGFs). Specifically, this approach reduces the variation in ground-motion simulation results due to the choice of different EGFs.

Some Foreshocks and Aftershocks Associated with the Moiyabana Earthquake of 2017 in Botswana

On the 3rd of April 2017, an earthquake of moment magnitude 6.5 occurred near Moiyabana in central Botswana. This paper is aimed at studying the spatial distribution of the foreshock and aftershock sequences associated with the Moiyabana earthquake. The foreshocks and aftershocks data used were from the Botswana Geoscience Institute (BGI) and the Seisan software was used to analyze the foreshock and aftershock events. The analyses revealed nine epicenter locations of foreshocks which are spread out across the country and most of them are located in the central and southern parts of Botswana, while the aftershocks are clustered around the mainshock. Although five of the nine foreshocks occurred far from the boundaries of major tectonic units, the other four occurred near key features such as the Zoetfontein, Lecha and Chobe faults. The spatial distribution of aftershocks indicates that the stress released by the mainshock, re-activated the planes of weakness in the vicinity of the mainshock and farther away from the mainshock. Hence, this affected the Zoetfontein fault, the boundary between Passarge basin and Magondi belt and the boundary between the Kaapvaal craton and Limpopo mobile belt. The aftershocks also show a northwest-southeast trend, which probably indicates the rupture plane; and mainly lie within the Limpopo mobile belt that is sandwiched between the Kaapvaal craton to the south and Zimbabwe craton to the north. Furthermore, the aftershocks concentration to the south reveals a close relation in demarcating the boundary of the Kaapvaal craton and the Limpopo mobile belt.

ART THERAPY BENCANA SEBAGAI UPAYA PENANGANAN TRAUMA ANAK PASCA GEMPA LOMBOK

This activity aims to treat psychologically the victims of earthquakes in Lombok which occurred in mid-2018. The targets were chosen based on the location of the earthquake affected in severe status, namely in SDN 1 Sembalun. Children become important victims to be given care so as not to experience prolonged trauma. Art Therapy is the right method to be applied to children. The disaster art therapy carried out in this activity is, (1) Free drawing on the clouds. (2) Drawing feelings. (3) Free writing techniques. (4) Free storytelling techniques. Based on the results of the evaluation of activities it was found that children became calmer. It is expected that through this activity elementary teachers are able to have skills in overcoming students' anxiety and trauma in the event of aftershocks. First, post-earthquake trauma in children is easy to return to if aftershocks occur. Secondly, a continuous psychological assistance program is needed from all teachers and the community. Third, it is necessary to improve physical facilities and play environments in the community and schools.

Implementation of Filter Picker Algorithm For Aftershock Identification of Lombok Earthquake 2018

The ability to identify earthquake events that are consistent, efficient and accurate is increasingly needed along with the increase in the amount of data analyzed. In this paper a filter picker algorithm is implemented to identify aftershock events and determination of arrival time automatically, especially for the P wave phase. Here modifications are made in determining the uncertainty of arrival time and there are additional criteria in determining the time of arrival used. The additional criteria are that in a certain time span, there are at least 5 stations determined by the time the filter picker arrives. This is done to minimize identification errors due to local noise and other practical reasons, namely the minimum number of stations to determine the location and other seismological analysis. To test the filter picker algorithm, aftershock data from the Lombok earthquake occurred on July 29 (M 6.4), August 5 (M 7), and August 19 (M 6.3 and M 6.9) 2018. The aftershock data were used for 30 days, from August 4, 2018 to September 4, 2018 using local seismic station in Lombok Island. The results of the filter picker algorithm were evaluated by comparing the number of earthquake events detected and the accuracy of determining the P wave arrival time automatically to the results of manually arriving time. In addition, a comparison of the results obtained from a broadband type seismometer with a short period is used to find out how much influence the type of tool has on its performance results. The results of the comparison with the manual arrival time show that more than 85 percent of the results of the automatic arrival time have a difference below 0.2 seconds. Therefore, it can be said that the filter picker algorithm is quite effective for identifying events and determining the arrival time of P waves. In this paper it is also shown that this algorithm can be used for broad band and short period seismometer sensor, even without the prior correction of instruments.

Determining Source Model and Aftershocks of 2006 Yogyakarta Earthquake, Indonesia using Coulomb Stress Change

On 26 May 2006 at 22:53:59 UTC, an earthquake with moment magnitude of 6.4 occurred in Yogyakarta, Indonesia. The source of the event is still debatable. Some believe the event was caused by the reactivation of the Opak Fault which has a left-lateral type movement. Previous studies indicated there are two possibilities to explain the mechanism of the Yogyakarta earthquake. First is based on the focal mechanism from NIED (National Research Institute for Earth Science and Disaster) Japan which indicated that the event occurred in an oblique reverse slip. This model states that the complex Opak fault is a flower structure (strike-slip) type. Second is based on NEIC (National Earthquake Information Center) US which indicated that the event was caused by a pure strike-slip fault (active Opak fault). The May 26th earthquake triggered many aftershock events around the old Opak fault. The majority of aftershock events on 3-6 June 2006 were located around 5 km east of Opak fault. It has a trendline of N45°E and lies parallel with the Opak fault. We use Coulomb Stress change to determine which type of source model fit better the aftershocks pattern. The target fault for Coulomb Stress analysis is a left lateral pure strike slip with an orientation of N45°E/90°SE.

Application of double-difference method for relocating aftershocks hypocenters in Opak Fault Zone

The 6,4 Mw earthquake occurred in Yogyakarta on 27th May 2007 has been renowned as a manifestation from Opak fault activity. Double-Difference (HypoDD) method was implemented to relocate hypocenters location, due to its poor accuracy caused by un-modeled velocity structure. The relocation was carried out on aftershocks data from 15th June to 19th June 2006. This method relocated 296 out of 303 aftershock events. Seismicity map after relocation showed concentrated epicenters by the east side of Opak fault. Distribution of the hypocenters which previously ranged from 0 to 40 km significantly shifted to the range of 10-20 km. RMS values obtained after relocation reduced close to 0 which indicated an outstanding improvement of hypocenters location in Opak fault zone.

State-dependent fragility curves using real and artificial earthquake sequences

Large mainshocks could trigger numerous aftershocks, exposing residents to a significant risk of life loss. Structures may be excited by different earthquake sequences in their efficient lifetime. It is thus essential to take into account the effects of accumulation damage on seismic behavior of structures. This paper investigates the influence of sequential earthquakes on the fragility curves for different damage states. Fragility curves obtained by real mainshock–aftershock (MSAS) events and artificial MSAS sequences generated based on (1) the generalized Omori’s law and (2) scaling of mainshock are compared and discussed. Results of analysis showed that the well-known Omori’s law could be considered as a suitable tool for aftershocks generation for obtaining of state-dependent fragility curves.

Evidence of strong long-period ground motions of engineering importance for Nankai Trough plate boundary earthquakes: comparison of ground motions of two moderate-magnitude earthquakes

We analyzed strong-motion and broadband recordings of two moderate-magnitude earthquakes that occurred in the Nankai Trough. The first event was the 2004 Mw 6.5 southeast off-Kii peninsula earthquake, an aftershock event inside the Philippine Sea Plate near the Nankai Trough axis. The second event was the 2016 Mw 5.8 southeast off-Mie Prefecture earthquake, an independent event in the rupture area of the 1944 Mw ~ 8 Tonankai earthquake. The centroid depths were 11 and 14 km for the 2004 and 2016 events, respectively. Despite a large difference in the moment magnitude between the two events, the JMA magnitude (Mj) was 6.5 for both the events. We found that the short-period ground motions (e.g., response spectra at periods < 1 s) as well as the much longer-period ground motions (> 20 s) for the 2016 event scaled generally well with the moment magnitude of the event. In contrast, the ground motions from the 2016 event were comparable to those for the larger-moment-magnitude 2004 event at equal distances at periods of about 2–20 s in wide areas and the observed acceleration response spectra at those periods were noticeably underestimated for the 2016 event by the ground motion prediction equation (GMPE) that employs Mw. An examination of the existing subsurface velocity model suggested that the difference in the relative location of the two events with respect to the thick accretionary prism of low seismic velocity most probably caused the comparable amplitude of the seismic waves at those periods. As a result, we posit that the values of Mj are equal for the two events because Mj is estimated using the displacement amplitude of ground motions at periods smaller than about 6 s. On the other hand, GMPE employing Mj generally described the observed data well. The results suggested that the plate boundary earthquakes in the Nankai Trough may excite strong long-period ground motions of engineering importance, and these ground motions appear to be better explained by Mj than by Mw in GMPEs for moderate-magnitude earthquakes in the Nankai Trough subduction zone.

3D crustal resistivity structure beneath the Wagad aftershock zone of the 2001 Bhuj earthquake, Kutch, India: Heterogeneous resistivity structure controlled by widespread fluid infiltration and clues to aftershocks pattern

The epicentral region of the 26 January 2001 Bhuj earthquake (7.7 Mw) in the Kutch region of India has been experiencing a vigorous and unique intraplate aftershock activity, which shows a V-shaped pattern and N/NE migration along the western margin of the Wagad uplift located to the northeast of the mainshock. This study presents 3D resistivity structure of the Wagad aftershock zone, imaged using magnetotelluric (MT) data, to understand the crustal geoelectric structure and subsurface geometry of faults/lineaments, which could provide critical inputs to evaluate and strengthen the earthquake generation mechanism and aftershocks pattern. The obtained 3D conductivity structure showed a thick (~3 km) and homogeneous conductive (1–30 Ωm) layer of Tertiary/Mesozoic sedimentary cover in the area. A highly heterogeneous upper and middle crust, characterized by a mix of conductive (<30 Ωm) and resistive to moderately resistive (>300 Ωm) structures, is imaged below the sedimentary cover. Enhanced conductivity zones showed spatial correlation with the surface trace of various faults/lineaments and are interpreted as fluid enriched zones in the crust, which play a critical role in earthquake triggering. Significantly higher fluid enrichment noticed in the crust below two minor faults, Khadir Transverse Fault (KTF) and Manfara Fault (MF), which are hitherto considered to have an insignificant role in the seismogenesis of the area. The aftershock events seem to occur mostly on the resistive parts of the crust, and its pattern shows good alignment with the anomalous conductive zones associated to the above two faults. The results provide a strong clue to infer that the KTF and MF have a vital role in the observed aftershock activity and control the V-shaped aftershocks pattern.

Probabilistic mainshock-aftershock collapse risk assessment of buckling restrained braced frames

Buckling-Restrained Braced Frames (BRBFs) are among the common seismic resistant systems with many beneficial characteristics such as stable cyclic behavior and high energy dissipation. However, recent studies have shown that BRBFs are susceptible to residual deformations during earthquakes which makes them vulnerable to aftershock events. The aim of the current study is to investigate the aftershock collapse capacity of BRBFs. In the first part of the paper, simplified procedures including IDA and collapse fragility analyses are carried out to gain more insight regarding the residual drift and collapse capacity of the intact frames. Then, aftershock fragility assessment is conducted for several damage states, to highlight the influence of post-mainshock residual drifts on the collapse of the structures. As for the second part, a detailed probabilistic framework is introduced and utilized to include the effects of upcoming aftershocks on the annual collapse probability of the structures. Results show that aftershock can highly intensify the structural response especially when the structure tolerates large residual drifts during the mainshock.