Strong Motion Generation Areas Research Articles

Broadband source model of the 2021 MW 7.1 Fukushima‐ken Oki earthquake in Japan based on seafloor and onshore strong‐motion records

AbstractThe 2021 MW 7.1 Fukushima‐ken Oki earthquake was the largest down‐dip compressional intraslab earthquake since August 2016, which is when S‐net records from the seafloor started to be provided. In this study, the empirical Green's function method was used with strong‐motion records from S‐net on the seafloor and KiK‐net onshore to estimate the broadband (0.2–10 Hz) source model of this earthquake. Three strong‐motion generation areas (SMGAs) with very high stress drops were estimated around the edge of the fault. One SMGA with a stress drop of 125 MPa was located in the oceanic crust in the shallow part of the Pacific Plate. The other two SMGAs with stress drops of 313 and 188 MPa were located in the deep part where previous source models without S‐net estimated a small slip. The two deep SMGAs increased the contribution of S‐net to more than that of KiK‐net. The short‐period spectral level was as high as that of the 2011 MW 7.1 Miyagi‐ken Oki earthquake, which was also a down‐dip compressional intraslab earthquake. The estimated broadband source model simulated the horizontal and vertical motions well at stations within 200 km of the hypocenter, except for surface waves at a few S‐net stations.

Broadband Source Model of the 2023 Mw 7.8 Türkiye Earthquake from Strong-Motion Records by Isochrone Backprojection and Empirical Green’s Function Method

Abstract The 2023 Mw 7.8 Türkiye earthquake caused severe damage in near-fault regions. The broadband source model, which is important for predicting strong motions in near-fault regions, was estimated. First, high-frequency (3–10 Hz) source imaging was performed through isochrone backprojection using near-field strong-motion records. Four segments were set, consisting of three segments along the East Anatolian fault and one segment along the splay fault where the rupture started. The estimated rupture velocities at the four segments were 2.6–3.3 km/s. The broadband (0.2–10 Hz) source model was then estimated using the empirical Green’s function method. The locations of eight strong-motion generation areas (SMGAs) of the broadband source model were searched with reference to the large brightness area estimated by isochrone backprojection. The source parameters of the SMGAs were estimated to fit the calculated acceleration and velocity envelopes at 21 strong-motion stations to the observed ones. The locations of the SMGAs were mostly consistent with the large slip area estimated by previous studies from long-period waveforms or static data, except for one SMGA with the highest Brune’s stress drop on the splay fault. The highest stress drop caused large ground motions near the splay fault, for which the supershear rupture has been suggested by previous studies. Ground motions were reproduced except for some stations affected by the fling-steps or nonlinear site effects. Although the SMGAs were not located near the southern side of the southwestern segment in Hatay Province, the large ground motions at shorter than about 2 s were mostly simulated. Large empirical site amplification factors estimated in this study must play a role on the large ground motions. The forward rupture directivity effects, with a rupture velocity of 3.3 km/s as large as the S-wave velocity, were also responsible for the large ground motions there.

Simulating Strong Ground Motion from the Great 1923 Kanto Earthquake in the Tokyo Metropolitan Area Based on Source Model Derived from Seismic Intensity Data

ABSTRACT This study simulates strong ground motions in the Tokyo metropolitan area during the 1923 Kanto earthquake using the stochastic Green’s function method. Source characteristics were modeled using seismic intensity inversion analysis, and path and site characteristics were modeled using inhomogeneous attenuation structure and empirical amplification factors. The results of these simulations were consistent with seismic intensities estimated based on the collapse rate of wooden houses. The distribution of pseudovelocity response spectra averaged at periods of 1–2 s was large: ∼200 cm/s in southern Kanagawa and southern Chiba prefectures, ∼100–200 cm/s in eastern Tokyo, and ∼50–100 cm/s in eastern Saitama prefecture despite its distance from strong-motion generation areas (SMGAs). The simulation results were regressed on site characteristics and fault distance, and the residuals were interpolated using the Kriging method to estimate detailed maps of seismic intensity and response spectra on an ∼250 m mesh reflecting site-specific characteristics. The following conclusions can be made: (1) all SMGAs, other than those in northern Tokyo Bay, were located near large slip areas based on coseismic geodetic and seismic waveform data. Although the SMGAs in the northern part of Tokyo Bay exerted little influence on the southern part of the Kanto region, their consideration was required to reproduce the seismic intensity at the northwest Tokyo and Saitama; (2) intense strong motion in central Tokyo occurred at the back marsh, delta, coastal lowlands, and filled lands, whereas low levels of strong motion were determined at terraces covered with volcanic ash soil. Combined with building distribution, this indicates areas of high seismic risk; (3) the seismic intensity and response spectra in the Tokyo metropolitan area obtained through this simulation were larger than those obtained from seismic records of the 2011 Tohoku earthquake—the most recent megathrust earthquake.

Characterized source model of the M7.3 2016 Kumamoto earthquake by the 3D reciprocity GFs inversion with special reference to the velocity pulse at KMMH16

The 2016 Kumamoto earthquakes caused severe damage centering on the Mashiki residential area. The velocity waveforms at station KMMH16 in Mashiki, during the M7.3 mainshock, show large pulses. We found that severe damage in Mashiki may be the result of the strong westward velocity pulse. The question raised is how the near-fault ground motions with strong velocity pulse at KMMH16 were generated during the mainshock. We focus on the characterized source model with Strong Motion Generation Areas (SMGA). Empirical Green’s function (EGF) method is widely used for source modeling in this case. However, in case that the target site is located just near the fault in nodal plane of source mechanism (like KMMH16), mechanism of the EGF event should perfectly fit mechanism of the mainshock, which is a rare case. Therefore, instead of using EGFs, we used theoretical 3D Green’s functions. Our approach is a nonlinear source inversion. This method requires calculation of waveforms and comparison with observations for many source models. To accelerate these calculations, we use pre-calculated GFs by the reciprocity method in the JIVSM velocity structure model. By comparison with aftershock records, we validated this structure for periods as short as 1.5 s. Target sites are limited to sites close to the fault: KMM005, KMM006, KMMH14, and KMMH16. First, we look for an initial SMGA source model by the grid search method applied to relatively long-period (> 3 s) waveforms and coarse grid of source parameters. Then, we tune that source model to fit observed short-period waveforms with the simplex search method. Necessary physical constraints for the range of the source parameters are applied here. The important point in our inversion scheme is to describe the Kostrov-like slip velocity functions inside each SMGAs by using two triangles. The resulting source model agrees well with other inversion results. We found that the observed westward pulse at KMMH16 is the result of the constructive interference of two pulses from SMGA1 and SMGA2, located in Hinagu fault and southwestern segment of Futagawa fault.Graphical

Modeling of rupture using strong motion generation area: a case study of Hualien earthquake (Mw 6.1) occurred on April 18, 2019

Modeling of rupture using strong motion generation area: a case study of Hualien earthquake (Mw 6.1) occurred on April 18, 2019

Attenuation Characteristics of High-Frequency Ground Motions from Local Sources Caused by Great Subduction Zone Earthquakes in Northeast Japan

AbstractGround-motion attenuation characteristics are examined for the peak ground accelerations (PGAs) and peak ground velocities (PGVs) from strong-motion generation areas (SMGAs), which emit strong high-frequency waves during great subduction zone earthquakes. A conventional ground-motion prediction equation (GMPE) for earthquakes is designed based on the magnitude and distance from the source fault to predict the peak ground motion amplitude. For great subduction zone earthquakes, significant wavetrains of high-frequency ground motions are often separately observed in seismograms, and the corresponding rupture areas are estimated as SMGAs along the plate interface. In this case, although the advantages of using the shortest distances measured from the closest SMGAs rather than the shortest fault distance have been confirmed in previous studies, it is more physically reasonable to examine the ground-motion attenuation characteristics of individual SMGAs based on their magnitudes and locations. Therefore, we examine the attenuation characteristics of the PGAs and PGVs of individual SMGAs of the 2003 Mw 8.2 Tokachi–Oki earthquake and 2011 Mw 9.1 Tohoku earthquake in the Japan subduction zone considering data at stations within an SMGA distance of 300 km and the SMGA moment magnitude. The model, exhibiting the same functional form as a conventional GMPE, is fitted to the PGA and PGV data pertaining to each SMGA that are normalized to a bedrock site with VS30=760 m/s at any distance. The uncertainties in the obtained PGAs and PGVs are 3.6% and 13.5% lower, respectively, in terms of the residual in logarithmic units than those in the results of previous approaches considering only the SMGA distance. This result could help develop GMPEs for SMGAs to more appropriately predict the strong motions generated during great subduction zone earthquakes.

Characterized source model of the 2013 Lushan earthquake (Mw 6.6) by the empirical Green\u2019s function method

Ground motions near the source area of the mainshock of the 2013 Lushan earthquake (Mw 6.6) in Sichuan Province in China were reproduced using the characterized source model and the empirical Green’s function method (EGFM). The best-fit characterized source model consisted of one strong motion generation area (SMGA) and a background area. The synthesized ground motions of the characterized source model were in fairly good agreement with the observed ground motions in the frequency range from 0.5 to 30.0 Hz at ten strong motion stations. For the 2013 Lushan earthquake (Mw 6.6), both the relationships between the SMGA and the seismic moment, and those between the flat amplitude of the acceleration source spectrum in the short period and the seismic moment almost followed the empirical scaling relationships of inner fault parameters developed for crustal earthquakes. The reasons for the largest peak ground acceleration (PGA) (> 1 g) in the strong-motion observation history of China recorded at the 51BXD strong motion station were investigated from the source and site effects. We found that the directivity effect did not contribute to the largest record by comparing the effect of different positions of the rupture starting point on the synthesized ground motions. The nonlinear effect of shallow layers was negligible, as indicated by the similarity of the earthquake H/V spectral ratios between the mainshock and EGF events. A large shear-wave velocity contrast might not exist in the shallow layers as the station was situated on the slope of a small rock hill. Finally, we agreed with previous studies that the hanging-wall effect and topographic effect might be the reasons for generating the largest record at Station 51BXD.Graphical

Source properties of the 2019ML6.3 Hualien, Taiwan, earthquake, determined by the local strong motion networks

SUMMARYThe 2019 ML 6.3 Hualien earthquake struck the northern Longitudinal Valley (LV) and generated not only large strong motions (intensity of 7, as defined by the Central Weather Bureau, Taiwan) locally but also widespread strong shaking in metropolises in northern Taiwan. In this study, we analyse strong motion records from local seismic networks to understand the source properties of the 2019 event. We first obtain the centroid location of the 2019 event using the source-scanning algorithm (SSA) technique by applying the unfiltered records. The determined centroid location is 121.55°E, 24.10°N, with a depth of 22.5 km. This location is 5.5 km north–northwest of and 3.8 km deeper than the Central Weather Bureau hypocentre, suggesting that the 2019 event occurred on the high-angle west-dipping plane of the focal solution. The centroid time delay is 3.35 s. Then, we obtain strong motion generation areas (SMGAs) of the 2019 event using the empirical Green's function method by considering the broad-band waveforms (0.4–10 Hz). Unlike other moderate-sized earthquakes in Taiwan, which have one SMGA, we determine that there were two SMGAs in the 2019 event. SMGA1 initiated at the CWB hypocentre with a size of 4.00 km2, and SMGA2 initiated at the centroid location determined by the SSA approach with a size of 3.63 km2. Such small areas cause high stress drops of 13.7 and 27.4 MPa for SMGA1 and SMGA2, respectively. We infer that the localized high stress drop of SMGAs is one of the important factors responsible for high peak-ground accelerations (PGAs) in Taiwan in addition to a strong directivity effect coupled with the radiation pattern reported by the previous study. Furthermore, previous moderate-sized earthquakes on an active structure called the Xiulin segment revealed similar source properties with a high stress drop and generated large PGA locally as well as in the metropolises of northern Taiwan. Considering the stored moment deficit, the probability of a future large earthquake in the northern LV region remains high. It is essential to consider seismic hazard assessment and mitigation for this not-well-known but high-seismic-potential region.

Dynamic Rupture Study of Near-Field Velocity Pulses during the 2016 Kumamoto Earthquake, Japan

ABSTRACT Simulating the ground motions of future earthquakes requires a proper understanding and modeling of source, path, and site effects. Ground motions recorded during recent earthquakes very close to their ruptured faults provide new evidence of the importance of source effects and suggest that physics-based rupture modeling is critical to account for them. Here, we develop dynamic rupture models to simulate the near-fault ground motions generated by the 2016 Kumamoto, Japan, earthquake (Mw 7.0) at Nishihara village, which feature a large-amplitude velocity pulse. Comparison of mainshock and foreshock waveforms suggests that the source of the velocity pulse is on the Futagawa fault segment located very close to the site. Our dynamic models use the spectral element method and are built upon a previous kinematic description of the event via a so-called “characterized source model,” with three strong-motion generation areas (SMGAs) on the assumed fault plane. We first develop a reference model that reproduces the main features of the rupture process in agreement with previous results of kinematic source inversion. We then examine the sensitivity of the simulated near-fault ground motions to the frictional parameters (critical slip-weakening distance and stress drop) in the shallow part of the fault and to the geometrical properties of the shallow SMGA. Even assuming drastically different frictional properties in the shallow part of the fault, the amplitude of the simulated ground motions was affected little. On the other hand, changes of geometrical properties of the shallow SMGA generated large differences in simulated ground motions. The results indicate that geometrical features of the shallow SMGA played a more important role in generating near-fault ground motions with velocity pulses as observed at Nishihara village during the 2016 Kumamoto earthquake.

グリーン関数の震源位置依存性と強震動生成域の配置が計算地震動に与える影響

When we create input ground motions for building design, it is necessary to consider multiple cases for source parameters such as strong motion generation area (SMGA) arrangement and its area size, and multiple faults. Presently, it is difficult to cover all the possible cases of faults and source parameters Therefore, we often considered the safety side ground motions with the faults and SMGA placement close to the planned site. However, it has been reported that long period ground motions do not necessarily directly relate to the distance between the epicenter and the site, and the ground motions vary with the direction of the source and the depth of the hypocenter.

Questionnaire surveys on damages at high-rise buildings and synthesized near-fault long-period strong ground motions of the 2018 Hualien, Taiwan, earthquake

Long-period pulse-like strong ground motions were recorded near the Milun fault and caused serious building damages in the Hualien City during the main shock of the 2018 Hualien, Taiwan, earthquake. In this paper, questionnaire survey on feeling of shaking, indoor damages, structural, and nonstructural damages to high-rise reinforced concrete residential buildings were analyzed by three building groups according to the positions to the Milun fault and different site conditions. Relationships between the results of questionnaire survey on building damages and characteristics of strong ground motions were studied. Furthermore, a characterized seismic source model was proposed to elucidate the generation mechanism of near-fault long-period strong ground motions. In this model, a shallow strong motion generation area (SMGA7) in the south part of Milun fault with rise-time of 2.6 s contributed to the long-period pulse-like strong ground motions. Besides, a deep SMGA (SMGA8) under the SMGA7 with rise-time of 1.2 s also contributed to the strong ground motions. Using this model, strong ground motions at the sites of buildings for questionnaire survey were synthesized. Because of the directivity effects, amplitudes of ground motions were increased from north to south along the Milun fault, which can explain the serious building damages at the buildings located at the southwest to the Milun fault. The reasons of building damages were also examined from the aspect of predominant periods of ground sites and fundamental periods of buildings, which were extracted from microtremor measurement. Damages at the buildings located on the central part of the hanging wall with relatively good site condition were the lightest for almost all the surveyed items. Pulses with period in the range of 1 s–2 s which were generated by the SMGA8 and amplified by the near surface layers are considered to be responsible for building damages.

Source and strong-motion characteristics of two M\u2009>\u20096 buried earthquakes in southwest Taiwan

We used near-field strong-motion data to investigate the complex combination of source effect and site response for two recent disastrous earthquakes in southwest Taiwan. We estimated strong-motion generation areas (SMGAs) of 2.8 km × 2.8 km and 6.0 km × 4.2 km in a frequency band of 0.4–10 Hz for the 2010 Jiashian and 2016 Meinong earthquakes, respectively. The high-stress drops of 26.2 and 17.0 MPa for these two buried events were potentially related to the small dimension and deep rupture. Our results revealed that both earthquakes exhibited westward rupture directivity, whereas the 2016 Meinong event exhibited a stronger directivity effect because of the consistency between the propagation and slip directions. The localized high peak ground velocity (PGV) patch and the nonlinear site response could be attributed to the soft sediment with high pore fluid pressure and low-velocity structure beneath this region. However, the greater seismic moment and closer faulting location to the thick-mudstone-layer region for the 2016 Meinong event reinforced the strong ground shaking and serious damage over the broad area. This implies that this thick-mudstone-layer region in southern Taiwan plays a crucial role in earthquake response, and an investigation of characteristic site effects should be conducted for seismic hazard mitigation.Graphic abstract

Strong Ground Motion Simulation of 2016 ML6.6 Meinong, Taiwan, Earthquake Using the Empirical Green's Function Method

AbstractThe Meinong mainshock (ML6.6) occurred in southern Taiwan at 19:57:26.08 (UTC) on 5 February 2016. To study source characteristics of the mainshock, ground motions of the mainshock are simulated using the empirical Green's function method to construct a strong motion generation area (SMGA) source model. Records of the event occurred on 2 May 2010 (ML4.6) are chosen as the empirical Green's functions along with six strong motion stations surrounding the source area. Waveforms of the mainshock at all used stations are simulated utilizing the empirical Green's function method to construct a best‐fit source model by comparing the synthetic waveforms with the observed ones in the frequency range 0.2–10 Hz. The estimated SMGA is located on the NS‐striking fault plane. The size of SMGA is 7.2 km in length along the strike direction by 7.2 km in width along the dip direction. The rupture starts at the bottom left‐hand corner (i.e., southern bottom) of the SMGA and then extends radially toward the upper right‐hand corner with a rupture velocity of 2.6 km/s. The size of the SMGA and the rise time of the mainshock follow the self‐similar empirical relationship by Miyake et al. (2003, https://doi.org/10.1785/0120020183).

Large Stress Release During Normal-Faulting Earthquakes in Western Turkey Supported by Broadband Ground Motion Simulations

This article investigates the stress drop variability of shallow normal-faulting earthquakes in western Anatolia through strong-motion simulations. For this purpose, source characteristics of three moderate to large magnitude events are constrained by the empirical Green’s function simulation in a broadband frequency range. Recordings of ten strong-motion stations in 78-km epicentral distance range are utilized for the simulation. Estimated strong-motion generation areas (SMGAs) are 22 km2, 66 km2, and 110 km2 where rise times are 0.6 s, 0.7 s, and 0.6 s, respectively, for the 2011 Simav (Mw 5.8), 2017 Lesvos (Mw 6.3), and 2017 Bodrum-Kos (Mw 6.6) earthquakes. Those values are found to be consistent with global source-scaling relationships. One particular observation is that stress drop ratios between the mainshock and aftershock for all events are relatively large compared with those previously calculated for strike-slip events in Turkey. Stress drops of SMGAs for the Simav and Bodrum earthquakes are in the range of 25 MPa, and this value drops to 19 MPa for the Lesvos earthquake. To further investigate the stress drop variation of earthquakes in Western Anatolia, an earthquake source database (fc-Mo) offered by Yamanaka et al. (IAG-IASPEI 2017, S07-1-03, 2017) is utilized. Brune (Journal of Geophysical Research, 76:5002, 1971) stress drop values of > 360 small to moderate earthquakes (Mw 3.0–6.0) are calculated with the given corner frequency and seismic moment information assuming a constant shear wave velocity. Results indicate that the majority of the earthquakes have a stress drop value < 5 MPa. This value changes to between 5 and 57 MPa for the remaining earthquakes. These high stress drop values support the former findings stating that normal-faulting earthquakes may release higher stress than strike-slip earthquakes. This indicates that the regional stress regime in western Turkey may cause relatively larger stress release during the normal-faulting mainshocks.

Estimation of Strong Motion Generation Area for the 2004 Parkfield Earthquake Using Empirical Green’s Function Method

The empirical Green’s function (EGF) method is used to simulate the 2004 Parkfield earthquake (Mw = 6.1). The strong motion generation areas (SMGA) are estimated to reproduce near-source ground motions in a broadband frequency range of 0.25–10 Hz. In this simulation, the EGF uses the small events that occurred in 2005 (Mw = 4.5), 2012 (Mw = 4.4), 2010 (Mw = 4), and 2007 (Mw = 4). The source spectral ratio estimates the seismic moment ratio between large and small events whose corner frequencies obey the omega-squared scaling law. The estimated SMGA is 12 km in length by 8 km in width and is located near the large slip area of the waveform inversion. The result shows that the rupture propagates radially from the southeastern bottom part of the SMGA toward the northwestern shallow direction. In addition, a characterized source model comprising two asperities and a background slip area is proposed. The rupture propagates from asperity1 toward asperity2 with 2 s time delay. The characterized source model simulates more accurately than the single SMGA. The comparison between the 5% damped pseudo acceleration response spectra of our simulations verifies a better compatibility for more than 2 Hz.

Extension of Characterized Source Model for Long-Period Ground Motions in Near-Fault Area

Strong ground motions from the 2016 Mw 7.0 Kumamoto earthquake (Japan) can be well simulated based on a characterized source model consisting of strong-motion generation areas (SMGAs) with high stress drop and a background area with low stress drop, except at very near-fault stations (Irikura et al. in Earth Planets Sp 69:10, 2017). Strong ground motions observed at very near-fault stations less than 3 km away from the surface traces along the Futagawa fault zone have long-period motions including maximum permanent displacements beyond 2 m. To reproduce such long-period ground motions at those very near-fault stations, one must place SMGAs in the seismogenic zone but also add long-period-motion generation areas (LMGAs) in the weak shallow layer (SL) zone between the top of the seismogenic zone and the free surface. During the 2010 Mw 7.0 Darfield (New Zealand) earthquake, surface breaks caused by the mainshock were found associated with active faults by field surveys. Strong ground motions from the 2010 Darfield earthquake can also be simulated well using the conventional characterized source model, except for ground motions at very near-fault stations. Reproduction of very near-fault motions with permanent components for the 2010 Darfield earthquake also requires consideration of LMGAs in the SL zone. We thus propose an extension of the characterized source model by adding LMGAs in the SL zone. The parameters for the SMGAs are given following the recipe of Irikura and Miyake (Pure Appl Geophys 168:85–104, 2011), while the parameters of the LMGAs are estimated from two scaling relationships, viz. the surface displacement versus the average slip in the rupture area from the source inversion, and the surface displacement versus the rise time of the slip velocity time function in the LMGA from the forward simulation.

Strong motion generation area modelling of the 2008 Iwate earthquake, Japan using modified semi-empirical technique

The Iwate–Miyagi earthquake (Mw 6.9) of 14 June 2008 is one of the largest intraplate earthquakes that struck north-east Japan. This earthquake has produced the largest peak ground acceleration (PGA) ever recorded. The acceleration values 4022 and 1036 gal were observed at the surface and borehole accelerometers of IWTH25. To understand the cause of this extremely large acceleration, it is highly essential to obtain the detailed rupture process of Iwate–Miyagi earthquake. The present paper estimates the rupture model for this earthquake using the modified semi-empirical technique (MSET). The detailed analysis proposes one strong motion generation area (SMGA) in the rupture plane and nucleation point in the extreme western corner of the SMGA. Using this estimated source model, a satisfactory match is observed between the simulated and actual records. The quantitative analysis of these waveforms provides an almost 1:1 match for PGA values. Furthermore, the variation of these PGA values with epicentral distance shows similar attenuation rate. These results confirm the reliability of MSET and the estimated source model of this earthquake. To the best of our knowledge, this study is the first to model SMGAs in the rupture model using MSET and provides sufficiently reliable information which will be useful for seismic hazard prevention management.

Dynamic Source Model for the 2011 Tohoku Earthquake in a Wide Period Range Combining Slip Reactivation with the Short-Period Ground Motion Generation Process

This paper describes a validated dynamic rupture model of the 2011 Tohoku earthquake that reproduces both long-period (20–100 s) and short-period (3–20 s) ground motions. In order to reproduce the observed large slip area (slip asperity), we assign a large Dc (slip critical distance) area following kinematic source inversion results. Sufficiently large slip is achieved through rupture reactivation by the double-slip-weakening friction model. In order to reproduce the strong-motion generation areas (SMGAs), we assign short Dc and large stress-drop areas following empirical Green’s function (EGF) simulation results, which indicate that, although more distant from the hypocenter, SMGA1 ruptured earlier than SMGA2 or SMGA3, which are closer to the hypocenter. This observation is confirmed by the backprojection method. In order to reproduce this important feature in dynamic simulation results, we introduce a chain of small high stress-drop patches between the hypocenter and SMGA1. By systematic adjustment of stress drops and Dc, the rupture reproduces the observed sequence and timing of SMGA ruptures and the final slip derived by kinematic models. This model also reproduces the multiseismic wavefront observed from strong ground motion data recorded along the Pacific coast of the Tohoku region. We compare the velocity waveforms recorded at rock sites along the coastline with one-dimensional (1D) synthetic seismograms for periods of 20–100 s. The fit is very good at stations in the northern and central areas of Tohoku. We also perform finite-difference method (FDM) simulations for periods of 3–20 s, and confirm that our dynamic model also reproduces wave envelopes. Overall, we are able to validate the rupture process of the Tohoku earthquake.

Modelling of strong motion generation areas for a great earthquake in central seismic gap region of Himalayas using the modified semi-empirical approach

Over the past decades, strong motion generation areas (SMGAs) have received significant attention in the modelling of high-frequency records. Herein, we propose the source model for a scenario earthquake (\(M_{\mathrm{w}}\) 8.5) in the central seismic gap region of Himalayas. On the rupture plane, three SMGAs have been identified. Further, SMGA parameters are evaluated using available empirical relations. The spatiotemporal distribution of aftershocks is utilised to locate these SMGAs on the rupture plane. Further, the modified semi-empirical technique (MSET) is used to simulate the strong motion records. It has been observed that the study area can expect peak ground acceleration of \({>}\hbox {100 cm/s}^{2}\) and its distribution is mainly affected by the location of nucleation point in the rupture plane. Furthermore, the estimated peak ground acceleration (PGA) values are comparable with the earlier studies in the region. This confirms the robustness of generated rupture model with three SMGAs and the reliability of MSET to simulate high-frequency records.

Near-Source Strong Pulses During Two Large MJMA 6.5 and MJMA 7.3 Events in the 2016 Kumamoto, Japan, Earthquakes

Extremely large ground motions with strong pulses were observed near source faults during two large MJMA 6.5 and MJMA 7.3 events of the 2016 Kumamoto earthquakes in Japan. To investigate the mechanisms for generation of near-source strong pulses during both events, we first performed strong ground motion simulations in a broadband frequency range between 0.2 and 10 Hz using the empirical Green’s function method. For both the M6.5 and M7.3 events, strong motion generation area (SMGA) source models were prepared to simulate the ground motions, which were able to reproduce well the characteristics of the observed ground motions in and around the source areas. We also conducted ground motion simulations based on hypothetical simple source models to study the effect of rupture directivity on near-source strong ground motions. The principal findings regarding rupture directivity effects on near-source strong ground motions are as follows: (1) During the M6.5 event, the forward and upward rupture directivities from two strike-slip SMGAs caused two distinct strong pulses in the fault normal and parallel components, respectively. (2) During the M7.3 event, the upward rupture directivity along the fault dip direction from the strike-slip SMGA, including a small normal-slip, caused a strong pulse in both the fault parallel and normal components.