Showing papers on "Hypocenter published in 2019"

The 2018 Mw 7.5 Palu Earthquake: A Supershear Rupture Event Constrained by InSAR and Broadband Regional Seismograms

Abstract: The 28 September 2018 Mw 7.5 Palu earthquake occurred at a triple junction zone where the Philippine Sea, Australian, and Sunda plates are convergent. Here, we utilized Advanced Land Observing Satellite-2 (ALOS-2) interferometry synthetic aperture radar (InSAR) data together with broadband regional seismograms to investigate the source geometry and rupture kinematics of this earthquake. Results showed that the 2018 Palu earthquake ruptured a fault plane with a relatively steep dip angle of ~85°. The preferred rupture model demonstrated that the earthquake was a supershear event from early on, with an average rupture speed of 4.1 km/s, which is different from the common supershear events that typically show an initial subshear rupture. The rupture expanded rapidly (~4.1 km/s) from the hypocenter and propagated bilaterally towards the north and south along the strike direction during the first 8 s, and then to the south. Four visible asperities were ruptured during the slip pulse propagation, which resulted in four significant deformation lobes in the coseismic interferogram. The maximum slip of 6.5 m was observed to the south of the city of Palu, and the total seismic moment released within 40 s was 2.64 × 1020 N·m, which was equivalent to Mw 7.55. Our results shed some light on the transtensional tectonism in Sulawesi, given that the 2018 Palu earthquake was dominated by left-lateral strike slip (slip maxima is 6.2 m) and that some significant normal faulting components (slip maxima is ~3 m) were resolved as well.

The 2018 Hokkaido Eastern Iburi earthquake ( M JMA = 6.7) was triggered by a strike-slip faulting in a stepover segment: insights from the aftershock distribution and the focal mechanism solution of the main shock

Abstract: The Hokkaido Eastern Iburi earthquake (MJMA = 6.7) occurred on September 6, 2018, in the Hokkaido corner region where the Kurile and northeastern Japan island arcs meet. We relocated aftershocks of this intraplate earthquake immediately after the main shock by using data from a permanent local seismic network and found that aftershock depths were concentrated from 20 to 40 km, which is extraordinarily deep compared with other shallow intraplate earthquakes in the inland area of Honshu and Kyushu, Japan. Further, we found that the aftershock area consists of three segments. The first segment is located in the northern part of the aftershock area, the second segment lies in the southern part, and the third segment forms a stepover between the other two segments. The hypocenter of the main shock, from which the rupture initiated, is located on the stepover segment. The centroid moment tensor solution for the main shock indicates a reverse faulting, whereas the focal mechanism solution determined by using the first-motion polarity of the P wave indicates strike-slip faulting. To explain this discrepancy qualitatively, we present a model in which the rupture started as a small strike-slip fault in the stepover segment of the aftershock area, followed by two large reverse faulting ruptures in the northern and southern segments.

Slip distribution of the 2017 M w 6.6 Bodrum-Kos earthquake: resolving the ambiguity of fault geometry

Abstract: Author(s): Konca, A Ozgun; Guvercin, Sezim Ezgi; Ozarpaci, Seda; Ozdemir, Alpay; Funning, Gareth J; Dogan, Ugur; Ergintav, Semih; Floyd, Michael; Karabulut, Hayrullah; Reilinger, Robert | Abstract: SUMMARY The 2017 July 20, Mw6.6 Bodrum–Kos earthquake occurred in the Gulf of Gokova in the SE Aegean, a region characterized by N–S extension in the backarc of the easternmost Hellenic Trench. The dip direction of the fault that ruptured during the earthquake has been a matter of controversy where both north- and south-dipping fault planes were used to model the coseismic slip in previous studies. Here, we use seismic (seismicity, main shock modelling, aftershock relocations and aftershock mechanisms using regional body and surface waves), geodetic (GPS, InSAR) and structural observations to estimate the location, and the dip direction of the fault that ruptured during the 2017 earthquake, and the relationship of this event to regional tectonics. We consider both dip directions and systematically search for the best-fitting locations for the north- and south-dipping fault planes. Comparing the best-fitting planes for both dip directions in terms of their misfit to the geodetic data, proximity to the hypocenter location and Coulomb stress changes at the aftershock locations, we conclude that the 2017 earthquake ruptured a north-dipping fault. We find that the earthquake occurred on a 20–25 km long, ∼E–W striking, 40° north-dipping, pure normal fault with slip primarily confined between 3 and 15 km depth, and the largest slip exceeding 2 m between depths of 4 and 10 km. The coseismic fault, not mapped previously, projects to the surface within the western Gulf, and partly serves both to widen the Gulf and separate Kos Island from the Bodrum Peninsula of SW Anatolia. The coseismic fault may be an extension of a mapped, north-dipping normal fault along the south side of the Gulf of Gokova. While all of the larger aftershocks are consistent with N–S extension, their spatially dispersed pattern attests to the high degree of crustal fracturing within the basin, due to rapid trenchward extension and anticlockwise rotation within the southeastern Aegean.

Rupture processes and Coulomb stress changes of the 2017 Mw 6.5 Jiuzhaigou and 2013 Mw 6.6 Lushan earthquakes

Abstract: Since the 2008 great Mw 7.9 Wenchuan earthquake, two destructive earthquakes, the 2013 Mw 6.6 Lushan earthquake and the 2017 Mw 6.5 Jiuzhaigou earthquake, struck the eastern margin of the Tibetan Plateau, causing many casualties and significant property damage. The rupture processes and Coulomb stress change of the Lushan and Jiuzhaigou earthquakes are investigated in this study. The general patterns of the slip models of the two events are similar, where the slip is concentrated around the hypocenter and the primary ruptured zone extends about 20 km along strike. The rupture zone of the 2017 Jiuzhaigou earthquake spans a depth range of 4–16 km with a peak slip of ~ 115 cm, whereas the rupture zone of the 2013 Lushan earthquake is concentrated at 8 to 20 km depth with a peak slip of 125 cm. The coseismic static Coulomb stress changes induced by the two events are computed with the obtained slip models. The Tazang fault and the northern extremities of the Minjiang and Huya faults were strongly loaded by the Jiuzhaigou earthquake, whereas the Lushan earthquake mainly affected its surrounding faults. Therefore, we infer that the seismic hazard potential in these regions has probably been increased further, and the Lushan earthquake did not contribute significantly the occurrence of the Jiuzhaigou earthquake. Additionally, we also compute the stress changes imparted by the 2008 Wenchuan earthquake. The computed stress changes in both events’ hypocenters exceed the trigger threshold (0.1 bar), which suggests that the Wenchuan earthquake played a pivotal role in the occurrence of these two earthquakes.

Hypocenter relocation of the aftershocks of the Mw 7.5 Palu earthquake (September 28, 2018) and swarm earthquakes of Mamasa, Sulawesi, Indonesia, using the BMKG network data

Abstract: On September 28, 2018, the Mw 7.5 earthquake occurred in Palu, Central Sulawesi, Indonesia. This earthquake produced strong tremors, landslides, liquefaction and a tsunami and caused thousands of fatalities and damaged houses and infrastructure. We have relocated 386 of the 554 Palu aftershocks by using the double-difference relocation method (hypoDD) from September 28 to November 22, 2018. The aftershock pattern is consistent with the crustal deformation in the area and generally shows that the events have a NW–SE trending of ~ 200 km in length and ~ 50 km in width. Most of the aftershocks are located to the east of the Palu-Koro Fault Line. Since November 2, 2018, there have been hundreds of swarm earthquakes in the area of Mamasa, West Sulawesi, which is about 230 km south of the city of Palu. Some of these earthquakes were felt, and houses were even damaged. We have relocated 535 of the 556 swarm earthquakes having a magnitude of M 2 to M 5.4. Our results show that the seismicity pattern has a dip that becomes shallower to the west (dipping at a ~ 45° angle) and extends from north to south for a length of ~ 50 km. We also conducted a focal mechanism analysis to estimate the type of fault slip for selected events of an M > 4.5 magnitude. Most of the solutions of the focal mechanism analysis show a normal fault type. This swarm earthquake probably corresponds to the activity of the fault in the local area.

Coulomb stress changes due to main earthquakes in Southeast Iran during 1981 to 2011

Abstract: Southeast of Iran experienced eight destructive earthquakes during 30 years from 1981 to 2011. Six of these events with M > 6.5 were fatal and caused great human and financial losses in the region. The 1981 July 28 (Mw 7.2) Sirch earthquake with 65 km surface rupture was the largest event in this region since 1877 and with other three earthquakes occurred in Golbaf-Sirch region during 17 years. The 26 December 2003 (Mw 6.6) Bam earthquake was one of the most destructive events in the recorded history of Iran. There were more than 26,000 killed, 30,000 to 50,000 injured people, and more than 100,000 were homeless. We calculated the static coulomb stress changes due to this earthquake sequence (four earthquakes) between 1981 and 1998 on the Golbaf-Sirch right-lateral fault and the Shahdad reverse fault and a slow slip on the Shahdad fault. Our calculations showed positive stress changes due to previous events on the ruptured plane of next earthquake. For example, the rupture plane of the 14 March 1998 (Mw 6.6) Fandoqa earthquake received a maximum positive stress change about 2.3 MPa. Also, some parts of the surrounding faults received positive stress changes due to these events. Stress changes on the planes of other four events until 2011 were calculated in this study. The 26 December 2003 (Mw 6.6) Bam earthquake and the 20 December 2010 (Mw 6.5) first Rigan earthquake received negligible (about thousandth (0.001)) negative stress changes in this sequence. The last event in our study area, the 27 January 2011 (Mw 6.2) second Rigan earthquake, experienced more than 0.5 MPa coseismic coulomb stress changes especially in its hypocenter and according to our calculations, it is mostly due to the first Rigan event. By using well-located aftershocks of the Rigan earthquake, we investigated the correlation between coulomb stress changes and aftershocks distribution. Calculated coulomb stress changes due to these two events on the optimally oriented strike-slip faults for the first event showed that most of the well-located seismicity occurred in regions of stress increase and majority of them concentrated near the ruptured plane where the stress changes are in the highest value. Based on our computation for the second event, it would be concluded that most of the aftershocks located in the places that imposed stress are positive and some of them are in places where the imposed stress changes are zero or very small. So, there is a good correlation between coulomb stress changes and aftershocks distribution for both Rigan events. Calculating imparted coulomb stress changes that resolved on the nodal planes of the Rigan first event aftershocks has also been considered to examine whether they were brought closer to failure or not by using different fault friction. Various values of effective coefficient of friction (0.2, 0.4, and 0.8) were used to find the best value of fault friction that produces the highest gain in positively stressed aftershocks. Based on these calculations, majority of aftershocks received positive stress changes by increasing the effective coefficient of friction.

Rupture process of the 2018 Hokkaido Eastern Iburi earthquake derived from strong motion and geodetic data

Abstract: The source rupture process of the 2018 Hokkaido Eastern Iburi earthquake was investigated by performing a joint inversion analysis using the strong motion and geodetic data. A fault model that consists of two fault planes was constructed by considering the relocated aftershock distribution and the focal mechanisms. The inversion result showed that the large slip occurred at approximately 22 km depth, which was much shallower than the hypocentral depth. Our results showed that the rupture initiated on the minor fault plane around the hypocenter and the major fault plane started to rupture 4–6 s after the rupture initiation. Although the shape of the minor fault plane has not been clearly determined, the major fault plane appears to be high-angle, east-dipping. An additional inversion with only the major fault plane showed that the strong ground motion near the source was mainly generated from the major fault plane. The total seismic moment estimated by the inversion with the two fault-plane model was 1.1 × 1019 Nm, which yielded an Mw of 6.6. Using the inversion result of the two fault-plane model, we simulated the ground surface and borehole waveforms at strong motion station IBUH03 where the large velocity pulse was observed on the ground surface. The simulations suggest that this large velocity pulse was generated from the combination of the large slip of the source and large site amplification of the velocity structure between the ground surface and borehole seismometers.

Configuration and structure of the Philippine Sea Plate off Boso, Japan: constraints on the shallow subduction kinematics, seismicity, and slow slip events

Abstract: We applied tomographic inversion and receiver function analysis to seismic data from ocean-bottom seismometers and land-based stations to understand the structure and its relationship with slow slip events off Boso, Japan. First, we delineated the upper boundary of the Philippine Sea Plate based on both the velocity structure and the locations of the low-angle thrust-faulting earthquakes. The upper boundary of the Philippine Sea Plate is distorted upward by a few kilometers between 140.5 and 141.0°E. We also determined the eastern edge of the Philippine Sea Plate based on the delineated upper boundary and the results of the receiver function analysis. The eastern edge has a northwest–southeast trend between the triple junction and 141.6°E, which changes to a north–south trend north of 34.7°N. The change in the subduction direction at 1–3 Ma might have resulted in the inflection of the eastern edge of the subducted Philippine Sea Plate. Second, we compared the subduction zone structure and hypocenter locations and the area of the Boso slow slip events. Most of the low-angle thrust-faulting earthquakes identified in this study occurred outside the areas of recurrent Boso slow slip events, which indicates that the slow slip area and regular low-angle thrust earthquakes are spatially separated in the offshore area. In addition, the slow slip areas are located only at the contact zone between the crustal parts of the North American Plate and the subducting Philippine Sea Plate. The localization of the slow slip events in the crust–crust contact zone off Boso is examined for the first time in this study. Finally, we detected a relatively low-velocity region in the mantle of the Philippine Sea Plate. The low-velocity mantle can be interpreted as serpentinized peridotite, which is also found in the Philippine Sea Plate prior to subduction. The serpentinized peridotite zone remains after the subduction of the Philippine Sea Plate and is likely distributed over a wide area along the subducted slab.

Tracking the development of seismic fracture network from The Geysers geothermal field

Abstract: Underground fluid injections result in rock mass fracturing. The associated environmental hazards in a significant part stem from a possibility for linking these fractures. The resultant crevices may allow for an undesired and hazardous fluid migration. We studied the fracture linking problem on data from a part of The Geysers geothermal field in California, USA. We parameterized seismic events by the distance between hypocenter and injecting well, by the angle between the position vector of hypocentre and the maximum horizontal stress direction and by the angle of rotation required to turn the event’s double-couple mechanism into the prevailing in this area faults’ orientation. To make these parameters comparable, we transformed them to equivalent dimensions. Based on distances between events in the transformed parameter space, we divided the seismic events into clusters. The percentage of potentially linked fractures in clusters was greater at low than at high injection rate.

Structural heterogeneity in and around the fold-and-thrust belt of the Hidaka Collision zone, Hokkaido, Japan and its relationship to the aftershock activity of the 2018 Hokkaido Eastern Iburi Earthquake

Abstract: The Hokkaido Eastern Iburi Earthquake (M = 6.7) occurred on Sep. 6, 2018 in the southern part of Central Hokkaido, Japan. Since Paleogene, this region has experienced the dextral oblique transpression between the Eurasia and North American (Okhotsk) Plates and the subsequent collision between the Northeast Japan Arc and the Kuril Arc due to the oblique subduction of the Pacific Plate. This earthquake occurred beneath the foreland fold-and-thrust belt of the Hidaka Collision zone developed by the collision process, and is characterized by its deep focal depth (~ 37 km) and complicated rupture process. The reanalyses of controlled source seismic data collected in the 1998–2000 Hokkaido Transect Project revealed the detailed structure beneath the fold-and-thrust belt, and its relationship with the aftershock activity of this earthquake. Our reflection processing using the CRS/MDRS stacking method imaged for the first time the lower crust and uppermost mantle structures of the Northeast Japan Arc underthrust beneath a thick (~ 5–10 km) sedimentary package of the fold-and-thrust belt. Based on the analysis of the refraction/wide-angle reflection data, the total thickness of this Northeast Japan Arc crust is only 16–22 km. The Moho is at depths of 26–28 km in the source region of the Hokkaido Eastern Iburi Earthquake. Our hypocenter determination using a 3D structure model shows that most of the aftershocks are distributed in a depth range of 7–45 km with steep geometry facing to the east. The seismic activity is quite low within the thick sediments of the fold–thrust belt, from which we find no indication on the relationship of this event with the shallow (< 10–15 km) and rather flat active faults developed in the fold-and-thrust belt. On the other hand, a number of aftershocks are distributed below the Moho. This high activity may be caused by the cold crust delaminated from the Kuril Arc side by the arc–arc collision, which prevents the thermal circulation and cools the forearc uppermost mantle to generate an environment more favorable for brittle fracture.

The present-day seismicity of the White Sea region

Abstract: A revised earthquake catalog has been compiled for the White Sea region for the period between 2005 and 2016. The earthquake parameters were revised using a single velocity model (BARENTS), a single methodological approach (Generalized Beamforming), and all available raw data and bulletins of Russian and foreign seismic stations. The location of two nuclear explosions detonated on July 18, 1985 and September 6, 1988 in northern European Russia for civilian purposes showed that the algorithm for calculating hypocenter parameters combined with the BARENTS velocity model is an effective tool. The resulting earthquake catalog enabled us to reveal the leading patterns in the distribution of recent seismicity in the White Sea region.

Probabilistic estimates of hypocenters from the data of Kamchatka seismic network stations

Abstract: A new approach is proposed for determining earthquake hypocenters aimed at a more comprehensive characterization of its uncertainty and ambiguity. Application of the new approach to study the seismic focal subduction zones and volcanic seismicity is discussed by the example of the data of the Kamchatka Branch of the Geophysical Survey of the Russian Academy of Sciences.

Lower Bounds on Ground Motion at Point Reyes during the 1906 San Francisco Earthquake from Train Toppling Analysis

Abstract: Independent constraints on the ground motions experienced at Point Reyes station during the 1906 San Francisco earthquake are obtained by analyzing the dynamic response of a train that overturned during the earthquake. The train is modeled as a rigid rectangular block for this study. From this analysis, we conclude that the peak ground acceleration (PGA) and peak ground velocity (PGV) at Point Reyes station would have been at least 4 m/s^2 and 0.5 m/s⁠, respectively. This lower bound is then used to perform simple checks on the synthetic ground‐motion simulations of the 1906 San Francisco earthquake. It is also shown that the hypocenter of the earthquake should be located to the south of Point Reyes station for the overturning of the train to match an eyewitness description of the event.

Focal depth of the Yunnan Jinggu M w 6.1 earthquake: Discussion on depth of the brittle-ductile transition zone of a young fault

Abstract: Since most continental earthquakes occur in the brittle layer instead of ductile layers, the focal depth of an earthquake provides an important constraint on the rheology structure of fault zone. According to the results of previous studies, the typical depth of brittle-ductile transition zone on mature faults is about 10 km. However, several moderate earthquakes occurred on young faults in stable cratons, and focal depths of those events are very shallow (e.g. the 1993 M w6.1 India Killari earthquake and a series of shallow earthquakes in Australia). Those observations suggest that depth of the brittle- ductile zone on young faults in cratons is shallower than that on mature faults. In this study, we report a case on a young fault in a tectonically active region that has not been well studied and discuss implications for the rheology structure. On October 7, 2014, an M w6.1 earthquake occurred in Jinggu, Yunnan (China), the southeastern part of the tectonically active Yunnan-Myanmar block, followed by two aftershocks of M >5 in December. According to historical seismicity and geologic studies in this region, it is suggested that the seimogenic fault of the Jinggu earthquake is a young fault. We use data from global and regional seismic networks, as well as aftershock data from the Lozhadu reservoir network and temporary stations, to study focal depths of the mainshock and aftershocks. The hypocenter of a reference event ( M w4.3) that was recorded with two close temporary stations is determined, and then the hypocenters of the mainshock and two moderate aftershocks are relocated with a relative method based on Pn/Pg arrival times. The relocated hypocenter of the mainshock is 9.5 km deep, whereas the hypocenters of the two aftershocks are about 10 km deep. The centroid depth is investigated with the CAP method based on waveform modeling. Both teleseismic and regional waveforms are used to invert for the centroid depth of the mainshock. The optimal waveform fit is obtained with a depth of 5 km. The two aftershocks are studied with regional waveforms only and the results are close to the hypocenter depths. Since the hypocenter indicates the initial point of rupture whereas the centroid location is close to the center of the main rupture patch, we propose the mainshock initiated at 10 km depth and expanded to the brittle layer at shallower depth. For two aftershocks, the consistency of hypocenter depth and centroid depth suggests circular rupture patterns. Therefore, we propose the bottom of the seismogenic layer and brittle-ductile transition zone is about 10 km deep. This hypothesis is also supported by other observations. Magnetotelluric study in this region reveals a low conductivity layer above 10 km that is regarded as brittle rock, whereas the high conductivity layer beneath 15 km is regarded as ductile rock. Furthermore, a regional rheology model, which is obtained with surface temperature, seismic velocity, and GPS data, also suggests the brittle-ductile transition zone is at about 9 km depth. In summary, the depth of the brittle-ductile transition zone on the young fault in the tectonic active region is similar to that of mature faults, which is useful information for lithosphere geodynamics studies.

Seismicity supports the theory of incipient rifting in the western Ionian sea, central Mediterranean

Abstract: The present work focuses on earthquake locations and seismogenic stress in the eastern offshore of Sicily, a sector of the central Mediterranean region where geophysical information available is not good enough, yet, for proper geodynamic modeling. I have applied to an updated seismic database of the study area a Bayesian non-linear hypocenter location method already proven to be more effective than linear methods when the recording network geometry is poor, like in the present case. Then, I have selected from literature and official catalogs the local earthquake focal mechanisms computed by waveform inversion, and inverted them for stress tensor orientations. The results confirm the main finding of the previous investigations, i.e. that NW-trending convergence between Africa and Eurasia is a main source of tectonic stress in this area, however they also furnish evidence of additional tectonic factors locally acting together with convergence. In particular, extensional dynamics are detected inside the convergence-related compressional domain: these are characterized by a minimum compressive stress oriented SW-NE (perpendicular to convergence) and can be related to the rifting process (opening SW-NE) detected by previous investigators at the southwestern edge of the Ionian subduction slab. The findings of the present study may also concur to answer several open questions left by previous investigators.

О генезисе Бачатского землетрясения 2013 года

Abstract: The M L 6.1 earthquake that occurred on June 18, 2013 in Kuzbass is the strongest seismic event related to mining operations in this region . O pinions about its genesis differ . On the one hand, its hypocenter and most aftershocks occurred directly underneath the Bachat open-pit mine, which suggests that this seismic event was due to anthropogenic impacts . On the other hand, the earthquake focus was located at a depth of several kilometers, which, according to some authors, argues against th e anthropogenic factor – the technogenic change in the parameters of the stress field was insignificant against the lithostatic pressure and, especially, the rock strength ( e.g. [Lovchikov, 2016]) . Our study aims to discover and assess an impact of the mining operations in the near-surface areas of the crust , investigate whether the Bachat earthquake was caused by the mining operations, and clarify which processes in particular were the most probable trigger s of dynamic movement in the Bachat earthquake source . The probable geometrical parameters of the fault plane were estimated from the structural and tectonic conditions of the study area and the published locations of the aftershock s [Emanov et al., 2017]. It is established that seimic events of magnitudes similar to that of the Bachat earthquake cannot be caused by the overall anthropogenic load on the area, and it is unlikely that such a strong earthquake may occur due to the direct effect s of seismic vibrations resulting from mass explosions during the mining operations . Our a nalytical models and numerical simulations , as well as the analysis of seismological observation data show that the most probable factor that initiat ed dynamic movement s in the earthquake source was the extraction of the huge rock volume and its transportation from the Bachat open-pit mine . It should be noted that the size of the zone , wherein the geomechanical initiation criteria are met , is considerably larger than the critical size of a nucleation zone for a M 6 earthquake. However , open -pit mining operations can hardly affect the localization of strong earthquake s ources. Mining operations can only trigger a seismic event that has been already prepared by the natural evolution of the crust.

Rupture Process of the 2008 Wenchuan, China, Earthquake: A Review

Abstract: The May 12, 2008, Wenchuan earthquake (MW 7.9, MS 8.1) is the largest continental intraplate event to strike globally in the last 60 years. It caused great destruction and loss of life along the steep eastern margin of the Tibetan Plateau, adjacent to the Sichuan Basin. The event ruptured multiple faults with a mix of thrust- and right-lateral strike-slip faulting along the northeast-trending Longmen Shan thrust belt, with an overall oblique compressional deformation. Surface displacements of up to ~11 m, the distribution of thousands of aftershocks and landslides, geodetic observations, and seismic wave imaging indicate a total rupture extent of ~280 km, extending unilaterally northeastward from the hypocenter. The primary slip has a patchy distribution along the segmented out-of-sequence Beichuan fault, with large-slip patches in the region from Yingxiu to Xiaoyudong, near Beichuan, and near Nanba. The southwestern segment near Yingxiu, where the hanging wall is comprised of the high seismic velocity Pengguan massif, has primarily thrust displacement with minor right-lateral component. Strongly oblique slip occurred near Beichuan and progressively more steeply dipping right-lateral strike-slip dominates toward the northeast. The large-slip patches are less than 10 km deep, but slip extends deeper in the southwestern region and the fault appears to have listric extension into a mid-crustal decollement with shallow dip below 20 km depth. The rupture expanded with an average rupture velocity of ~2.8 ± 0.2 km/s along this segmented fault zone with a total rupture duration of ~110 s, with faster rupture speed in the northeastern region, possibly being supershear. Most aftershocks are concentrated from 10 to 20 km deep, below the large-slip zones. Predominantly thrust slip with vertical offset of ~3.5 m occurred at shallow depth along ~72 km of the imbricate Pengguan fault located 6–7 km to the southeast of the southern Beichuan fault. Shallower dip of the Pengguan fault may cause it to converge with the Beichuan fault at depth, and/or to flatten into the same mid-crustal decollement. Oblique compressional left-lateral slip occurred on the short conjugate southward-dipping Xiaoyudong fault, connecting the two range-parallel faults. The moment-scaled radiated energy of the Wenchuan event is higher than for typical interplate thrust faulting, likely contributing to the extensive damage.

Double seismic zone and seismicity in the mantle wedge beneath the Ogasawara Islands identified by an ocean bottom seismometer observation

Abstract: Around the Ogasawara Islands, only a few seismic stations in the area can be used to determine the hypocenters of regional earthquakes; thus, hypocenter location precision tends to be low. To more precisely determine hypocenter locations, we deployed a temporary seismic observation network of pop-up ocean bottom seismometers around the Ogasawara Islands from July to October 2015. We identified a double seismic zone in the 70–200 km depth range associated with the subducting Pacific slab. The slab-normal distance between the two planes of the double seismic zone is about 30–35 km, similar to such distances observed along the Japan and Mariana trenches. Furthermore, we found unusual seismicity in the mantle wedge at 20–50 km depth beneath the Ogasawara trough that might be related to structure formed at the onset of the oceanic slab subduction. The hypocenters determined from the ocean bottom seismometer observation were horizontally separated by a few tens of kilometers from hypocenters published by the Seismological Bulletin of Japan. USGS locations (Preliminary Determination of Epicenters) seem to be offset westward about 30 km compared with the locations determined in this study.

Electrical resistivity modeling around the Hidaka collision zone, northern Japan: regional structural background of the 2018 Hokkaido Eastern Iburi earthquake (Mw 6.6)

Abstract: The Hidaka collision zone, the collision boundary between the NE Japan and Kurile arcs, is known to be an ideal region to study the evolution of island arcs. The hypocenter of the 2018 Hokkaido Eastern Iburi earthquake (Mw 6.6) in the western part of the Hidaka collision zone was unusually deep for an inland earthquake, and the reverse fault that caused the earthquake has an uncharacteristically steep dip. In this study, we used three-dimensional inversion to reanalyze broadband magnetotelluric data acquired in the collision zone. The inverted resistivity model showed a significant area of high resistivity around the center of the collision boundary. We also identified a conductive zone beneath an area of serpentinite melange in a zone of high P–T metamorphic rocks west of the high-resistivity zone. The conductive zone possibly reflects areas rich in pore fluids related to the formation and elevation of the serpentinites. Sensitivity tests indicated the need for additional magnetotelluric survey data to delineate the resistivity distribution around the epicentral area of the 2018 earthquake although the resistivity model showed a conductive zone in this area.