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The tectonic stress field was investigated in and around the aftershock area of the Hokkaido Eastern Iburi earthquake (MJMA = 6.7) occurred on 6 September 2018. We deployed 26 temporary seismic stations in the aftershock area for approximately 2 months and located 1785 aftershocks precisely. Among these aftershocks, 894 focal mechanism solutions were determined using the first-motion polarity of P wave from the temporary observation and the permanent seismic networks of Hokkaido University, Japan Meteorological Agency (JMA), and High Sensitivity Seismograph Network Japan (Hi-net). We found that (1) the reverse faulting and the strike-slip faulting are dominant in the aftershock area, (2) the average trend of P- and T-axes is 78° ± 33° and 352° ± 51°, respectively, and (3) the average plunge of P- and T-axes is 25° ± 16° and 44° ± 20°, respectively: the P-axis is close to be horizontal and the T-axis is more vertical than the average of the P-axes. We applied a stress inversion method to the focal mechanism solutions to estimate a stress field in the aftershock area. As a result, we found that the reverse fault type stress field is dominant in the aftershock area. An axis of the maximum principal stress (σ1) has the trend of 72° ± 7° and the dipping eastward of 19° ± 4° and an axis of the intermediate principal stress (σ2) has the trend of 131° ± 73° and the dipping southward of 10° ± 9°, indicating that both of σ1- and σ2-axes are close to be horizontal. An axis of the minimum principal stress (σ3) has the dipping westward of 67° ± 6° that is close to be vertical. The results strongly suggest that the reverse-fault-type stress field is predominant as an average over the aftershock area which is in the western boundary of the Hidaka Collision Zone. The average of the stress ratio R = (σ1 − σ2)/(σ1 − σ3) is 0.61 ± 0.13 in the whole aftershock area. Although not statistically significant, we suggest that R decreases systematically as the depth is getting deep, which is modeled by a quadratic polynomial of depth. [ABSTRACT FROM AUTHOR]

Six wells were continuously monitored in Hokkaido, northern Japan, to relate groundwater level changes to regional seismic activity. Groundwater level changes following the 2018 Hokkaido Eastern Iburi earthquake were detected in three of the six wells, even though they are located hundreds of kilometers from the epicenter. The groundwater level changes are qualitatively consistent with the volumetric strain induced by the earthquake. We analyzed groundwater level responses to the M2 tidal constituent before and after the earthquake, but related changes in amplitude and phase shifts remained within the usual variation. Observed coseismic change was explained by the response to the M2 tidal constituent component and the calculated volumetric strain for one of the wells, where groundwater level decreased. The observed change in the other two was found to be much greater than the corresponding estimates of the volumetric strain. [ABSTRACT FROM AUTHOR]

The 2018 Hokkaido Iburi Eastern Earthquake on 9 September 2018 was an inland moderate-sized Mw6.6 earthquake. This Mw6.6 earthquake demonstrated that even a moderate-sized earthquake can halt social activity in Japan, which is equipped with advanced earthquake disaster countermeasures. JMA Japan Metrological Agency GNSS Global Navigation Satellite System ETAS Epidemic type aftershock sequences PGA Peak ground acceleration PGV Peak ground velocity Qs S-wave attenuation Qp P-wave attenuation SAR Synthetic aperture radar InSAR Interferometric SAR Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. [Extracted from the article]

We estimate the velocity structures of P (Vp) and S waves (Vs) in southern Hokkaido, Japan, where a large number of crustal low‐frequency earthquakes (LFEs) have occurred. To investigate the relationship between structure and crustal seismicity, we compare the obtained velocity heterogeneity between the crustal LFE regions, which are characterized by active volcanoes, Quaternary volcanoes, and seismic swarms. The tomographic results show that low‐velocity zones (LVZs) with high Vp/Vs are located around the Moho discontinuity beneath active volcanoes, and the LVZs extend horizontally from beneath the active volcanoes to other LFE regions. In southern Hokkaido, crustal LFEs also appear inside and below the upper crust as well as around the Moho discontinuity. These intermediate‐depth and deep LFEs are located around the LVZs with a high Vp/Vs ratio, indicating the contribution of partial melt to the LFEs genesis, even at a distance from the active volcanoes. At intermediate depths of the crust, LFEs are distributed even in the high‐velocity zones, adjacent to the LVZs. This indicates that the partial melt migrations within the crust affect the activity of LFE at the intermediate depths. Below the seismic swarms in the upper crust, LVZs with crustal LFEs are distributed, probably owing to upward migrations of fluid originating from the solidification of the partial melt; this indicates a link between the shallow and deep crustal seismicity in southern Hokkaido. Key Points: Constructing detailed P and S wave velocity structures in southern Hokkaido, JapanPartial melt and their movement are related to the genesis of crustal low‐frequency earthquakesShallow and deep crustal seismicity are linked through fluid and melt migrations [ABSTRACT FROM AUTHOR]

Abstract: We installed and operated a low-cost groundwater level observation system at intermittent hot spring wells in order to monitor volcanic strain signals from the active Meakan-dake volcano in eastern Hokkaido, Japan. Data are sampled at 1Hz and are transmitted to the data center in real time. Evaluation of the water level time series with theoretical predictive tidal strain and coseismic static strain changes has suggested that the wells penetrate to the artesian aquifer and act as a volumetric strain sensor. An active earthquake swarm with more than 400 events occurred at the shallower part of the volcano from January 9 to 11, 2008. Three independent wells recorded pre- to co-swarm groundwater drops simultaneously, which represented a decrease in volumetric strain. The total volumetric strain change during the three active days was estimated to be from 6 to 7×10−7. The observed data, including changes in volumetric strain, absence of deformation in the GPS coordinates, and activation of deep low-frequency earthquakes, might imply possible deflation of a source deeper than 10km, and these preceding deeper activities might induce an earthquake swarm in a shallower part of the Meakan-dake volcano. [Copyright &y& Elsevier]

Abstract: Japan Marine Science and Technology Center installed a cabled geophysical observatory system off Kushiro, Hokkaido Island in July 1999. This observatory system comprises three ocean bottom seismographs (OBSs), two tsunami gauges, and a geophysical/geochemical monitoring system. 4 years and 2 months after the installation, a megathrust earthquake (the 2003 Tokachi-Oki earthquake, 26th September in Japan Standard Time (JST), M JMA 8.0) occurred along a plate boundary underneath a forearc basin where the system is located. The system recorded clear unsaturated seismograms just at 28.6 km from the epicenter. This paper demonstrates advantages brought by the cabled observatory to record the megathrust earthquake showing how earthquake detectability is improved dramatically combining permanent OBS and land-based observations around the region, and importance of the in situ monitoring on the seismogenic zone. In the present study, processing OBSs and land-based network together, and comparing magnitudes of common observed earthquakes with national authorized network, event detection level improved down to M 1.5, which is much lower than the previously designed as down to M ∼2. Comparing detection level before and after installing OBSs, we found dramatic improvement of the earthquake detection level in the interesting region. Real-time continuous observations of microearthquakes since 1999 have brought us tremendous findings. First, a seismic quiescence started about 10 days before the 2003 Tokachi-Oki earthquake. Second, aftershock distribution is not uniform over the focal area and can be divided into several sub-regions, which might indicate an existence of several asperities. We think that the geophysical observations helped to understand the initiation process of the rupture of the 2003 Tokachi-Oki earthquake and that observations including seismological, geodynamic, hydrogeological, and the other multidisciplinary observations would provide a clue to future understanding of seismogenic processes at subduction zones. [Copyright &y& Elsevier]

Using global positioning system (GPS) observations of northeastern China and the southeast of the Russian Far East over the period 2012–2017, we derived an ITRF2014-referenced velocity field by fitting GPS time series with a functional model incorporating yearly and semiannual signals, linear trends, and offsets. We subsequently rotated the velocity field into a Eurasia-fixed velocity field and analyzed its spatial characteristics. Taking an improved multiscale spherical wavelet algorithm, we computed strain rate tensors and analyzed their spatial distribution at multiple scales. The derived Eurasia-referenced velocity field shows that northeastern China generally moved southeastward. Extensional deformation was identified at the Yilan–Yitong Fault (YYF) and the Dunhua–Mishan Fault (DMF), with negligible strike–slip rates. The principal strain rates were characterized by NE–SW compression and NW–SE extension. The dilation rates show compressional deformation in the southern segment of the YYF, northern end of the Nenjiang Fault (NJF), and southeast of the Russian Far East. We also investigated the impact of the 2011 Tohoku Mw 9.0 earthquake on the crustal deformation of northeastern China, generated by its post-seismic viscoelastic relaxation. The velocities generated by the post-seismic viscoelastic relaxation of the giant earthquake are generally orientated southeast, with magnitudes inversely proportional with the epicentral distances. The principal strain rates caused by the viscoelastic relaxation were also characterized by NW–SE stretching and NE–SW compression. The dilation rates show that compressional deformation appeared in the southern segment of the DMF and the YYF and southeast of the Russian Far East. Significant maximum shear rates were identified around the southern borderland between northeastern China and the southeast of the Russian Far East. Finally, we compared the multiple strain rates and the seismicity of northeastern China after the 2011 Tohoku earthquake. Our finding shows that the ML ≥ 4.0 earthquakes were mostly concentrated around the zones of high areal strain rates and shear rates at scales of 4 and 5, in particular, at the DMF and YYF fault zones. [ABSTRACT FROM AUTHOR]

We investigated the cause of destructive ground motion during the 2018 Hokkaido eastern Iburi earthquake. We conducted strong motion observations of aftershocks and microtremors and the surface wave method in the damaged areas of the town of Mukawa, Hokkaido prefecture, Japan. The ground accelerations were continuously recorded during a period of approximately 3 months after the main shock on September 6, 2018. The heavily damaged buildings were mainly situated around the strong motion station (HKD126) in Mukawa town. Such concentration of damage can be explained by the strong power that was observed in the 1–2 s period of the response spectrum at this station. We estimated the S-wave velocity profiles of this station site and a temporary station site that was installed on a nearby hill. The estimated S-wave velocity, which was inverted from phase velocity structures with the microtremor array and the surface wave method observations explained the difference in the SH-wave amplification characteristics between the two sites. An analysis of HKD126 and the temporarily observed records clearly indicates the strong effects of the local geological conditions on the heavily damaged area of Mukawa. The strong ground motion power generated during the main shock in Mukawa for 1–2 s period was mainly amplified by this shallow underground velocity structure. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Based on analyses of global navigation satellite system data since 1996, we investigate the spatiotemporal strain field around the Ishikari-Teichi-Toen fault zone, which is a major active fault zone close to the epicenter of the 2018 Eastern Iburi earthquake in Hokkaido, Japan. Strain rates during almost whole periods, except for the timings of two distant large interplate earthquakes and following several years show an E–W to ESE–WNW contraction of ~ 0.1 ppm/year. This strain rate is approximately an order of magnitude larger than that of the surrounding area. Strain rate disturbances due to large earthquakes diminish within several years and return to the original level, suggesting that there is a uniform strain accumulation along this fault zone. Strain rate profiles that traverse the fault zone are characterized by a major contraction, corresponding to the Ishikari lowlands where a significantly thick low seismic velocity layer exists. A relatively high strain rate around this fault zone may reflect some amount of inelastic strain accumulation in addition to the elastic strain accumulation along the faults originating from complex fault and crustal structures. [ABSTRACT FROM AUTHOR]