Source Rupture Process Research Articles

Comparison between rupture parameters of intermediate and deep earthquakes at the Peru–Brazil–Bolivia border and northern Chile

We determined the main parameters of the source rupture process of intermediate- and deep-depth earthquakes occurring in the Peru–Brazil–Bolivia border region and northern Chile. The parameters of depth, fault-plane orientation, scalar seismic moment, slip distribution, and radiated seismic energy are obtained from seismograms. We selected 15 intermediate-depth earthquakes (100 km < h < 300 km) and 10 very deep earthquakes (h > 500 km) with magnitudes MW ≥ 6.0. For most events, the slip distribution over the rupture plane shows a single asperity, and the source time function presents a simple pulse. There are differences between intermediate-depth and deep earthquakes. The rupture areas, maximum slip and source time function (STF) duration are larger for intermediate-depth events than for deep events. Additionally, the STF’s show a sharper increase for deep earthquakes. The scaled radiated seismic energy shows larger values for deep depth events. The stress regime pattern derived from the obtained focal mechanism agrees with the geometry of the subduction of the Nazca plate. At intermediate depths, in the northern area up to 12°S, the stress pattern corresponds to a horizontal extension, while in the southern area, the tension axes dip at an angle of 30°. At deep depths, the stress regime corresponds to vertical compression in the north and dips of approximately 45° in the south.

Source rupture process of the March 18th, 2021, Mw6.0 Béjaia (Algeria) earthquake associated with the Western segment – A link with the August 1856 Djidjelli earthquakes (Io = VIII-IX, M ≥ 6)

A strong offshore earthquake (Mw6.0) struck Béjaia city (eastern Algeria) on March 18th, 2021. This earthquake was followed by several aftershocks among the Mw5.2 that occurred 13 min after the main shock. Moreover, another earthquake (Mw5.0) occurred in the same zone one year later on March 19th, 2022. Near-field digital accelerograph records were used to study the earthquake and its related aftershocks. First, the March 2021 (Mw6.0) main shock, six of its main aftershocks, and the March 19th, 2022 (Mw5.0) earthquake were located. These epicentres are distributed in a 10 km-long and 2 to 3 km-wide NE–SW-trending area, with depths ranging between 8 km and 14 km. Second, using waveform inversion, the seismic moment and the focal mechanism of the three events (the March 18th, 2021, main shock and its strongest aftershock (Mw5.2) that occurred 13 min after the main shock and the March 19th, 2022 (Mw5.0) earthquake) were determined. These focal mechanisms exhibit reverse faulting with a short lateral component. Third, the source rupture process of the March 18th, 2021 (Mw6.0), earthquake was calculated from waveform inversion to obtain the moment–release distribution on a finite fault. The nodal plane oriented N74E seems to be associated with the activated fault plane. Considering the seismotectonic framework of the region, the fault that activated during the 2021 earthquake sequence is offshore. This fault, called the Western Segment, which is situated in the western part of the reverse fault system, is also at the origin of the Djidjelli historical earthquakes of August 21st, and 22nd, 1856 (Io = VIII-IX, M ≥ 6.6).

Rupture process of the January 8, 2022, Menyuan M 6.9 earthquake

After the occurrence of destructively strong earthquakes, rapid acquisition of the source rupture process can provide important reference information for post-earthquake disaster relief and aftershock trend determination. An M 6.9 earthquake occurred in Menyuan County, Qinghai Province on January 8, 2022. The epicenter is located in the seismic gap in the middle section of the Haiyuan fault belt. Such a typical strong earthquake was taken as an example to investigate the rupture process of strong earthquakes. Three days after the earthquake, the InSAR (Interferometric Synthetic Aperture Radar) coseismic deformation field was obtained by Sentinel radar, indicating that the surface ruptured obviously. The southern block of the earthquake faces towards the satellite about 95 ​cm along the LOS (line of sight) direction, and the northern block is away from the satellite by ​∼ ​74 ​cm, consistent with the characteristic of left-lateral strike-slip motion. In this study, InSAR coseismic deformation data and far-field waveform data were used to jointly invert the earthquake rupture process, and a four-segment finite fault model was constructed by referring to the surface deformation. The inversion results show that the focal depth of the Menyuan earthquake is about 7 ​km, and the strike of the seismogenic fault is 89.0°, 104.0°, 119.0° and 131.0° from west to east, respectively. It is a high-dip left-lateral strike-slip earthquake event lasting about 14 ​s. The rupture propagation mode is a bilateral extension. The maximum slip along the fault is about 380 ​cm, and the seismic moment magnitude is 6.7. The surface rupture length is about 24 ​km, which is consistent with that measured in the field survey. The detailed seismic source model can provide basic data for the aftershock trend determination and seismic risk analysis of the adjacent active faults.

Co-Seismic Landslides Triggered by the 2014 Mw 6.2 Ludian Earthquake, Yunnan, China: Spatial Distribution, Directional Effect, and Controlling Factors

The 2014 Mw 6.2 Ludian earthquake exhibited a structurally complex source rupture process and an unusual spatial distribution pattern of co-seismic landslides. In this study, we constructed a spatial database consisting of 1470 co-seismic landslides, each exceeding 500 m2. These landslides covered a total area of 8.43 km2 and were identified through a comprehensive interpretation of high-resolution satellite images taken before and after the earthquake. It is noteworthy that the co-seismic landslides do not exhibit a linear concentration along the seismogenic fault; instead, they predominantly extend along major river systems with an NE–SW trend. Moreover, the southwest-facing slopes have the highest landslide area ratio of 1.41. To evaluate the susceptibility of the Ludian earthquake-triggered landslides, we performed a random forest model that considered topographic factors (elevation, slope, aspect, distance to rivers), geological factors (lithology), and seismic factors (ground motion parameters, epicentral distance, distance to the seismogenic fault). Our analysis revealed that the distance to rivers and elevation were the primary factors influencing the spatial distribution of the Ludian earthquake-triggered landslides. When we considered the directional variation in ground motion parameters, the AUC of the model slightly decreased. However, incorporating this variation led to a significant reduction in the proportion of areas classified as “high” and “very high” landslide susceptibility. Moreover, SEDd emerged as the most effective ground motion parameter for interpreting the distribution of the co-seismic landslides when compared to PGAd, PGVd, and Iad.

FocMech-Flow: Automatic Determination of P-Wave First-Motion Polarity and Focal Mechanism Inversion and Application to the 2021 Yangbi Earthquake Sequence

P-wave first-motion polarity is important for the inversion of earthquake focal mechanism solutions. The focal mechanism solution can further contribute to our understanding of the source rupture process, the fault structure, and the regional stress field characteristics. By using the abundant focal mechanism solutions of small and moderate earthquakes, we can deepen our understanding of fault geometry and the seismogenic environment. In this paper, we propose an automatic workflow, FocMech-Flow (Focal Mechanism-Flow), for identifying P-wave first-motion polarity and focal mechanism inversion with deep learning and applied it to the 2021 Yangbi earthquake sequence. We use a deep learning model named DiTingMotion to detect the P-wave first-motion polarity of 2389 waveforms, resulting in 98.49% accuracy of polarity discrimination compared with human experts. The focal mechanisms of 112 earthquakes are obtained by using the CHNYTX program, which is 3.7 times more than that of the waveform inversion method, and the results are highly consistent. The analysis shows that the focal mechanisms of the foreshock sequence of the Yangbi earthquake are highly consistent and are all of the strike-slip type; the focal mechanisms of the aftershock sequence are complex, mainly the strike-slip type, but there are also reverse and normal fault types. This study shows that the deep learning method has high reliability in determining the P-wave first-motion polarity, and FocMech-Flow can obtain a large number of focal mechanism solutions from small and moderate earthquakes, having promising application in fine-scale stress inversion.

Cascading rupture process of the 2021 Maduo, China earthquake revealed by the joint inversion of seismic and geodetic data

The 2021 Mw 7.4 Maduo earthquake occurred in the north-central Bayan Har block, which has been the most seismically active block of the Tibetan Plateau during recent decades. The Maduo earthquake provides an opportunity to study the rupture kinematics and seismogenic environment associated with subsidiary faults inside the block. We investigate the source rupture process of the Maduo earthquake by combining the seismological analyses and the joint inversion of geodetic and seismic data. We find that the source rupture propagates bilaterally on a splay fault geometry consisting of two steeply dipping segments. The slip distribution is dominated by sinistral slips and contains several asperities. The maximum slip is approximately 4.2 m, and the rupture duration is about 40 s. The spatiotemporal evolution of Coulomb failure stress (CFS) changes on the fault demonstrates the interactions of stress triggering between multiple asperities, indicating the cascading rupture process of the Maduo earthquake. Moreover, this event produces significant CFS increases on the subsidiary faults of the Bayan Har block, among which the Maduo-Gande and Tibet Dagou-Changmahe faults receive the largest stress loading. The occurrences of the 1947 Dari and 2021 Maduo earthquakes suggest that the seismic hazard of the subsidiary faults should not be neglected. Because the CFS on the Tuosuo Lake and Maqin-Maqu segments of the East Kunlun fault is also increased, the enhanced seismic hazard deserves further attention. In addition, the slip distribution illustrates that the source rupture is mainly concentrated in the brittle upper crust. We propose that the Maduo earthquake arises from the continuous loading on the internal fault of the Bayan Har block caused by the collision between the Eurasian and Indian plates, and the eastward extrusion of the middle-to-lower crustal flow facilitates the occurrence of this event. These factors constitute the seismogenic environment of the Maduo earthquake.

Strong ground motion data of the 2015 Gorkha Nepal earthquake sequence in the Kathmandu Valley

Strong-motion records of earthquakes are used not only to evaluate the source rupture process, seismic wave propagation and strong ground motion characteristics, but also to provide valuable data for earthquake disaster mitigation. The Kathmandu Valley, Nepal, which is characterised by having soft sediments that have been deposited in an earthquake-prone zone, has experienced numerous earthquakes. We have operated four strong-motion stations in the Kathmandu Valley since 2011. These stations recorded the 2015 magnitude 7.8 Gorkha Nepal earthquake that occurred in the Himalayan continental collision zone. For several months after the mainshock, we deployed four additional temporary stations. Here, we describe the seismic data for 18 earthquakes over magnitude 5.0 collected by this array, including the 2015 magnitude 7.3 Dolakha earthquake of maximum aftershock and three large aftershocks of magnitude 6-class. These data are essential for validating the sedimentary structure of the basin and for evaluating the hazard and risk of future earthquakes in the Kathmandu Valley.

A procedure for 3D simulation of seismic wave propagation considering source‐path‐site effects: Theory, verification and application

AbstractThis paper aims at obtaining a semi‐analytical and semi‐numerical 3D model of source‐to‐site seismic wave propagation due to kinematic finite‐fault sources. To this end, a two‐step procedure integrating the frequency‐wavenumber (FK) approach with the spectral element method (SEM) is proposed based on the concept of domain reduction. First, the broadband responses of a stratified crust are accurately calculated by using a novel FK approach and are converted into effective seismic inputs around the region of interest. After that, the seismic wavefields at local and regional scales arising from complex geological and topographical conditions are finely simulated using the SEM, and a perfectly matched layer absorbing boundary condition is simultaneously applied to realize the absorption of outgoing waves. In this procedure, a hybrid source modelling scheme that combines the low‐wavenumber deterministic and high‐wavenumber stochastic components on the fault plane is introduced, effectively addressing the high‐frequency motion radiated from the source rupture process. Subsequently, the proposed FK‐SEM procedure is verified step‐by‐step using the point source and finite‐fault source models. To illustrate the feasibility of the procedure, 3D physics‐based numerical simulations (PBSs) of two seismic events, including a historical Sanhe‐Pinggu earthquake and a well‐recorded Yangbi earthquake, are performed. The case studies validate that the proposed FK‐SEM procedure allows a significant reduction in computational effort and a substantial improvement in modelling resolution and can be applied to the source‐to‐site broadband synthetics of earthquake scenarios with limited resources. In addition, this coupled geophysics‐engineering simulation meets the requirements of time‐history analysis for engineering structures, which facilitates the study of soil‐structure interactions and regional‐scale building damage assessment.

A Method to Determine Moment Magnitudes of Large Earthquakes Based on the Long‐Period Coda

AbstractWe present a novel and robust method for estimating moment magnitudes (Mw) of large earthquakes with long‐period and long‐lasting coda energy. Fitting the energy with a simple decay model, we derive a straightforward relationship between the coda energy and the Mw. Tests with both real and synthetic data of 10 globally distributed large earthquakes (Mw &gt; 8) verify the method and the results are stable and reliable even with a fast calculation of synthetics for a 1D model. Tests also show that the method is applicable for earthquakes with Mw above 7.5–8.0 with energy sufficiently greater than the ambient noise. The method removes or reduces the effects of geometric spreading, focal mechanism, source rupture process, and the actual Earth structure, making it advantageous for estimating the magnitudes of large earthquakes. The new long‐period coda moment magnitude (Mwo) estimations are similar to the conventional solutions but are slightly larger (by 0.04 on average).

Effect of Source Rupture Process on Surface Wave Group Velocity: An Example Using 2015 Nepal Earthquake Data

Effect of Source Rupture Process on Surface Wave Group Velocity: An Example Using 2015 Nepal Earthquake Data

Source Process Featuring Asymmetric Rupture Velocities of the 2021 Mw 7.4 Maduo, China, Earthquake from Teleseismic and Geodetic Data

AbstractOn 21 May 2021, an Mw 7.4 left-lateral strike-slip earthquake occurred within the Bayan Har block in the Tibetan plateau. To learn about the source rupture process, we collected the teleseismic waveforms and utilized the backprojection method to investigate the rupture kinematics of the earthquake. The results indicate that the earthquake was a bilateral rupture event with asymmetric rupture velocities. The rupture velocity in the east of the epicenter was uniform and in the range of 2.72–3.67 km/s, whereas, in the west, it was in the range of 1.39–1.78 km/s in the first 20 km and then increased to 2.82–3.17 km/s. The slip distribution constrained by the Interferometric Synthetic Aperture Radar and Global Positioning System displacements clearly reveals kinematic coseismic slip in greater detail, which makes up for the limitations of the backprojection method. Two main asperities in the east verify the results of the backprojection method. The rupture depth in the west was slightly shallower than that in the east, which may be the reason for the asymmetry of rupture velocities. The initial rupture point was updated based on the asymmetric velocities and geodetic slip distribution. The multiple-point-source moment tensors based on the rupture velocities and new initial rupture point not only match the fault geometries determined by relocated aftershocks but also fit well with the released energy distribution, which proves the asymmetry of rupture velocities.

The Source Characteristics of the Mw6.4, 2016 Meinong Taiwan Earthquake from Teleseismic Data Using the Hybrid Homomorphic Deconvolution Method

The kinematic source rupture process of the 2016 Meinong earthquake (Mw = 6.4) in Taiwan was derived from apparent source time functions retrieved from teleseismic S-waves by using a refined homomorphic deconvolution method. The total duration of the rupture process was approximately 15 s, and one slip-concentrated area can be represented as the source model based on images representing static slip distribution. The rupture process began in a down-dip direction from the fault toward Tainan City, strongly suggesting that the rupture had a unilateral northwestern direction. The asperity with an area of approximately 15 × 15 km2 and the maximum slip of approximately 2 m were centered 12.8 km northwest of the hypocenter. Coseismic vertical deformation was calculated based on the source model. Compared with the results derived from InSAR (Interferometric Synthetic Aperture Radar) data, our results demonstrated that the location with maximum uplift was accurately well detected, but our maximum value was just approximately 0.4 times of the InSAR-derived value. It reveals that there are the other mechanisms to affect the vertical deformation, rather than only depending on the source model. At different depths, areas west, east, and north of the hypocenter maintained high values of Coulomb stress changes. This explains the mechanism behind aftershocks being triggered and provides a reference for predicting aftershock locations after a large earthquake. The estimated seismic spectral intensities, including spectral acceleration and velocity intensity (SIa and SIv), were derived. Source directivity effects caused damage to buildings, and we concluded that all damaged buildings were located within a SIa value of 400 gal. Destroyed buildings taller than seven floors were located in an area with a SIv value of 30 cm/s. These observations agree with those on damages caused by the 2010 Jiasian earthquake (ML 6.4) in Tainan, Taiwan.

Ground motion of the Ms7.0 Jiuzhaigou earthquake

Abstract Eighty-eight free-field strong ground motion observation datasets obtained by the National Strong Motion Observation Network System of China for the Jiuzhaigou Ms7.0 earthquake were used to study the characteristics of the strong ground motion and the site response. In this study, we calculated the VS30 of the near field station and compared the observed values of the horizontal peak acceleration and peak velocity with the CB2014 ground motion prediction models developed by Campbell et al. (2014). The results indicate that the observed values of the horizontal peak acceleration and peak velocity of the Jiuzhaigou earthquake are both smaller than the predicted values obtained from the ground motion prediction equations. By regressing the spatial variation curves of the D SR (5–95%) and D SR (5–75%) ground motion durations, and comparing them with Bommer’s (2009) ground motion duration prediction curve, it was found that the duration of the Jiuzhaigou earthquake was greater than that obtained from the global empirical prediction equation. The scope of the source duration corresponding to the D SR (5–75%) duration is 2.76–4.28 s, and the scope of the source duration corresponding to the D SR (5–95%) duration is 8.88–10.36 s, which are close to the peak time and completion time of the seismic moment release during the source rupture process. The linear elastic acceleration response spectra of 11 stations within 100 km of the fault were calculated for comparison with the design spectrum. It was concluded that the range of the predominant period of the response spectrum was 0.05–0.26 s, which is less than the natural vibration period of the local multistory building. However, the response spectrum value recorded by station 51JZZ is greater than the design spectrum of the 8° rare earthquake and the observation values of the nearby strong ground motion stations. Through analysis of the H/V spectrum ratio and the case of station 51JZZ, which is closest to the epicenter, this phenomenon was concluded to be related to the magnification effect of the near-surface soil layer and the nonlinear response of the site under the action of strong earthquakes.

Coseismic deformation of the 2021 MW7.4 Maduo earthquake from joint inversion of InSAR, GPS, and teleseismic data

The MW7.4 Maduo earthquake occurred on 22 May 2021 at 02:04 CST with a large-expansion surface rupture. This earthquake was located in the Bayan Har block at the eastern Tibetan Plateau, where eight earthquakes of MS >7.0 have occurred in the past 25 years. Here, we combined interferometric synthetic aperture radar, GPS, and teleseismic data to study the coseismic slip distribution, fault geometry, and dynamic source rupture process of the Maduo earthquake. We found that the overall coseismic deformation field of the Maduo earthquake is distributed in the NWW-SEE direction along 285°. There was slight bending at the western end and two branches at the eastern end. The maximum slip is located near the eastern bending area on the northern branch of the fault system. The rupture nucleated on the Jiangcuo fault and propagated approximately 160 km along-strike in both the NWW and SEE directions. The characteristic source rupture process of the Maduo earthquake is similar to that of the 2010 MW6.8 Yushu earthquake, indicating that similar earthquakes with large-expansion surface ruptures and small shallow slip deficits can occur on both the internal fault and boundary fault of the Bayan Har block.

Revisiting the Source Rupture Process of the Mainshock of the 2016 Kumamoto Earthquake and Implications for the Generation of Near-Fault Ground Motions and Forward-Directivity Pulse

ABSTRACT Strong near-fault ground motions associated with the MJMA 7.3 mainshock of the 2016 Kumamoto, central Kyushu, Japan, earthquake sequence have received attention by seismological and engineering communities. In this study, the kinematic source rupture process was reanalyzed based on an improved approach for the representation of source faults. The slips at densely distributed point sources were defined via the bilinear interpolation of those at surrounding control points. The result shows that the rupture started on the Hinagu fault with a small initial rupture and propagated beyond the junction to the Futagawa fault. The rupture on the Futagawa fault mainly propagated up and northeastward. A large slip area with a peak slip of 4.9 m and peak slip velocity of 3.1 m/s was detected at depths ranging from 3 to 15 km in the central part of the Futagawa fault. This asperity spatially coincides with a body with moderate-seismic velocity (VP∼6 km/s) and low-seismic attenuation. The slips on the Futagawa fault have significant normal-slip components, whereas the slip vectors of the Hinagu fault represent almost pure right-lateral strike slip. The shallower part of the fault segments in the western Aso caldera is characterized by relatively large normal slips. The estimated slip-velocity functions at shallower depths (&amp;lt;3 km) are almost temporally symmetric and relatively long. The shallower portion of the source fault significantly contributes to the velocity and displacement waveforms at near-fault sites. On the contrary, the slip-velocity functions at deeper depths (&amp;gt;3 km) are temporally asymmetric and have a sharp peak. The simulation of the ground-motion evolution suggests that the lateral flow in the Aso Valley was primarily triggered by the strong forward-directivity pulse generated from the asperity on the Futagawa fault.

Present‐Day Deformation Mechanism of the Northeastern Mina Deflection Revealed by the 2020 Mw 6.5 Monte Cristo Range Earthquake

AbstractThe 15 May 2020 Monte Cristo Range Mw 6.5 earthquake occurred in the northeast of the Mina deflection, which accommodates approximately a quarter of the relatively dextral motion between the Pacific and North American plates. The Monte Cristo Range event provides an opportunity to study the present‐day regional deformation mechanism and active tectonics. In this study, we investigate the source rupture process of the event using joint inversion of interferometric synthetic aperture radar and broadband seismic data. We find that the rupture propagates almost simultaneously on two main segments. The fault motion changes from the predominantly sinistral slip near the epicenter on the eastern segment to the oblique slip on the western segment, with a maximum coseismic slip of 0.8 m. Our results suggest that the accommodation of slip transfer localized in the northeastern Mina deflection tends to transform from the wrench‐ to extension‐dominated transtension.

A hybrid source mechanism of the 2017 Mw 6.5 Jiuzhaigou earthquake revealed by the joint inversion of strong-motion, teleseismic and InSAR data

As occurred during the tourist season, the 2017 Mw 6.5 Jiuzhaigou (China) earthquake led to destructive damages. The seismogenic fault of this event was merely speculated to be the northern extension of the Huya fault, while no apparent surface ruptures were discovered in the field investigation. Previous studies and released moment tensor solutions indicated the Jiuzhaigou earthquake was dominated by left-lateral strike slip with partial normal-slip component. It seems unintelligible that the normal slip occurred in this event as the epicenter was located near the boundary of convergent blocks. Hence, a reasonable and elaborate source rupture model is necessary to investigate such a blind fault and the role it plays in the complex fault system of this region. Combining space-based geodetic, teleseismic or regional seismic observations can provide detailed information about earthquake ruptures. We first attempted to determine the fault geometry using the Bayesian approach with synthetic aperture radar interferograms (InSAR). And then the fault geometry was refined based on the relocated aftershock distribution, and a two-segment fault model was constructed. Based on the two-segment model, we resolved the source rupture process of the 2017 Jiuzhaigou earthquake through the joint inversion of strong-motion, teleseismic body-wave and InSAR data. The inversion results reveal a hybrid source mechanism, in which normal and thrust slips coexist besides the strike-slip component. We suggest that the eastward motion with the extrusion of the lower crustal flow in the northeastern margin of the Bayan Har block is responsible for such a faulting behavior. The co-seismic Coulomb stress changes show a significant stress loading in the western segment of the Tazang fault, increasing its seismic hazard. Due to the lack of aftershocks to the southeast of the seismogenic fault, the probably enhanced seismicity in the northern segment of the Huya fault is also worth further attention.

Estimated Seismic Source Parameters for 2019 Magnitude 7.6 Papua New Guinea Earthquake

On 14 May 2019, a major (Mw 7.6) earthquake struck eastern Papua New Guinea, causing a tsunami warning that was later canceled. The last major rupture in the region was a 2000 Mw 8.0 event, which resulted in massive horizontal land movements of up to several meters and a series of aftershocks with primarily thrust mechanisms, and caused a damaging tsunami. Seismic methods for characterizing the source process were applied. Vertical components of the long-period P-waves from 35 stations of the Global Seismic Network with even azimuthal coverage were adopted in the inversions. First, the focal mechanism solution was retrieved after the earthquake. From the inverted results as well as aftershock distribution, the causative fault of the great PNG earthquake was confirmed to be a fault of strike = 316°, dip = 84°, and rake = 0°, indicating that the earthquake was a left-lateral strike-slip event. Then, to clearly understand the spatiotemporal evolutionary process of the source rupture of the earthquake, far-field body wave data were collected, and the source rupture process of this earthquake was studied using the finite fault inversion method. A finite-fault model was estabished with length and width of 152 and 32 km, respectively, and we set the initial seismic source parameters referring to the center of the focal mechanism solution. This was estimated by the focal mechanism solutions of defferent studies referring to different focal mechanism solutions. We found that the focal mechanism solution determined by Global Centroid Moment Tensor Catalog was not appropriate. Inversion results indicated that the seismic moment was 3.63 × 1020 Nm (Mw 7.6), and the source duration was ~ 40 s. The rupture propagated mainly toward the northwest in an asymmetrical bilateral mode, with a maximum slip of ~ 13.2 m, and a large-scale slip patch strongly ruptured to the surface. The study of Coulomb stress changes suggested that the PNG earthquake may trigger a thrust-type rupture in the New Britain Trench, which has the potential to trigger a tsunami.

Delayed Subevents During the MW6.2 First Shock of the 2016 Kumamoto, Japan, Earthquake

AbstractEarthquakes have been considered mostly to generate concentric rupture starting from the hypocenter. On the other hand, delayed subevents occurring after the first passage of rupture front have been found in the recent earthquakes, for example, the 2011 Tohoku earthquake. They potentially cause a long duration of ground motions. We study a source rupture process during the Mw6.2 first shock of the 2016 Kumamoto, Japan, earthquake, which may include the delayed subevent. The source model with three segments (S1, S2, and S3) is estimated from the near source records by using Bayesian inference, Markov chain Monte Carlo method. The maximum a posteriori solution well explains the near source records, and its layout of S1 and S2 segments are consistent with the previous studies. The posterior distribution suggests that location of S1 and S3 segments spatially overlaps with a time interval of about 6–7 s. Therefore, the S3 segment is accepted as the delayed subevent in this event. The first rupture on S1 concentrically propagates to trigger the second rupture on S2, and the second rupture then causes the dynamic triggering of S3 segment. Physical background how the delayed subevents can release the stress is needed to investigate in the future.

Source rupture process of the 2018 Hokkaido Eastern Iburi earthquake deduced from strong-motion data considering seismic wave propagation in three-dimensional velocity structure

The source rupture process of the 2018 Hokkaido Eastern Iburi earthquake (MJMA 6.7) was analyzed by a kinematic waveform inversion method using strong-motion data in 0.04–0.5 Hz. This earthquake occurred close to the Hidaka Collision Zone and the Ishikari depression, where the crustal structure is rather complex. Thus, we used a three-dimensional velocity structure model to compute the theoretical Green’s functions by the finite difference method. A source fault model with strike-angle variation was set based on the spatial distribution of the early aftershocks. The strong-motion stations used for the source inversion were selected based on the result of forward ground motion simulation of a moderate aftershock. The slip in the first 5 s was relatively small, but an area of significant slip with peak slip of 1.7 m was found in the depth range from 22 to 32 km. The rupture propagated upward mainly in the southwest direction. Based on the regional crustal structure and the configuration of the Moho discontinuity, the large-slip area was thought to be located in the lower crust, and its rupture did not reach the upper part of the continental crust. Most of the early aftershocks occurred around the large-slip area. The later aftershocks at the depth shallower than 20 km occurred outside the causative source fault of the mainshock. Three-dimensional ground motion simulation demonstrated that the heterogeneous source process and the three-dimensional basin and crustal velocity structure brought a large velocity pulse to an area to the southwest of the source fault, where the largest PGV was observed during the mainshock. The spatial distribution of the simulated PGV resembled the observed PGV distribution except some sites located inside the Ishikari depression where thick Quaternary soft low-velocity sediments exist at the top of the basin.