Prestate of Stress and Fault Behavior During the 2016 Kumamoto Earthquake (M7.3)

Fault behavior during an earthquake is controlled by the state of stress on the fault. Complex coseismic fault slip on large earthquake faults has recently been observed by dense seismic networks, which complicates strong motion evaluations for potential faults. Here we show the three‐dimensional prestress field related to the 2016 Kumamoto earthquake. The estimated stress field reveals a spatially variable state of stress that forced the fault to slip in a direction predicted by the “Wallace and Bott Hypothesis.” The stress field also exposes the pre‐condition of pore fluid pressure on the fault. Large coseismic slip occurred in the low‐pressure part of the fault. However, areas with highly pressured fluid also showed large displacement, indicating that the seismic moment of the earthquake was magnified by fluid pressure. These prerupture data could contribute to improved seismic hazard evaluations

Prestate of Stress and Fault Behavior During the 2016 Kumamoto Earthquake (M7.3)

Fault behavior during an earthquake is controlled by the state of stress on the fault. Complex coseismic fault slip on large earthquake faults has recently been observed by dense seismic networks, which complicates strong motion evaluations for potential faults. Here we show the three‐dimensional prestress field related to the 2016 Kumamoto earthquake. The estimated stress field reveals a spatially variable state of stress that forced the fault to slip in a direction predicted by the “Wallace and Bott Hypothesis.” The stress field also exposes the pre‐condition of pore fluid pressure on the fault. Large coseismic slip occurred in the low‐pressure part of the fault. However, areas with highly pressured fluid also showed large displacement, indicating that the seismic moment of the earthquake was magnified by fluid pressure. These prerupture data could contribute to improved seismic hazard evaluations

Potential for crustal deformation monitoring using a dense cell phone carrier Global Navigation Satellite System network

Monitoring of crustal deformation provides essential information for seismology and volcanology. For such earth science fields and other purposes, various Global Navigation Satellite System (GNSS) networks have been constructed at the national and regional levels. In Japan, the continuous nationwide GNSS network, the GNSS Earth Observation Network System (GEONET), is operated by the Geospatial Information Authority ofJapan. Although GEONET has made a substantial contribution to earth science research, the large spacing of GEONET sites makes it difficult to accurately understand crustal deformation phenomena in some cases. However, cell phone carriers in Japan have constructed independent GNSS networks to improve their positioning services in recent years. In this study, we examine the performance of a GNSS network operated by SoftBank Corp. for crustal deformation monitoring. The network has more than 3300 sites throughout Japan, which is approximately 2.5 times the number of the GEONET sites. To assess the quality of SoftBank's GNSS data, we first analyzed data from Miyagi Prefecture and evaluated the stability of the coordinate time series for nine consecutive days during a quiet (interseismic) period. The calculated standard deviations were approximately the same for both networks. Furthermore, we calculated the displacement between September 2020 and March 2021. The results reveal that almost all SoftBank sites showed a consistent displacement with their surrounding GEONET sites. Next, we analyzed the coseismic deformation associated with the off-Fukushima earthquake (MJMA 7.3) on February 13, 2021, in both static and kinematic modes. We obtained a westward coherent displacement along the coastline in both networks, although several outliers were observed for the SoftBank sites. Based on these initial assessments, we conclude that these private sector GNSS sites are useful for crustal deformation monitoring with appropriate data quality control

Spatiotemporal crustal strain distribution around the Ishikari-Teichi-Toen fault zone estimated from global navigation satellite system data

Abstract 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

Estimation of convergence boundary location and velocity between tectonic plates in northern Hokkaido inferred by GNSS velocity data

The present location of the tectonic boundary and the convergence rate between the Amur and Okhotsk plates in northern Hokkaido, Japan, were herein estimated from the velocity field using data from a continuous GNSS network. The observed velocity profiles are in agreement with the theoretical ones calculated from a tectonic block collision model. The estimated kinematic boundary agrees with both geological and seismic boundaries. Overall, this indicates that the geological boundary acts like a mechanical one. The calculated convergence velocity of 14.0-16.5 mm/year is consistent with predictions from regional plate motion models and suggests that a considerable amount of interplate convergence is in progress along this boundary. Deep crustal seismicity is also in agreement with the estimated elastic thickness of 20.5-25.5 km. The non-occurrence of large earthquakes during the past several centuries, and the estimated convergence velocity suggest that there is a high potential for a large event in the near future