The 2016 Kumamoto-Oita earthquake sequence involving three large events (M w ≥ 6) in the central Kyushu Island, southwest Japan, activated seismicities in two volcanic areas with unusual and puzzling spatial gaps after the largest earthquake (M w 7.0) of April 16, 2016. We attempt to reveal the seismic process during the sequence by following seismological data analyses. Our hypocenter relocation result implies that the large events ruptured different faults of a complex fault system. A slip inversion analysis of the largest event indicates a large slip in the seismicity gap (Aso gap) in the caldera of Mt. Aso, which probably released accumulated stress and resulted in little aftershock production. We identified that the largest event dynamically triggered a mid-M6 event at Yufuin (80 km northeast of the epicenter), which is consistent with existence of the 20-km long zone where seismicity was activated and surface offset was observed. These findings will help us study the contribution of the identified complexity in fault geometries and the geotherm in the volcanic areas to the revealed seismic process and consequently improve our understanding of the seismo-volcano tectonics.
We constructed a crustal stress map of the Kanto region, central Japan, from earthquake focal mechanism solutions. In order to increase the spatial resolution of the stress map, we included more data than the routine catalog by determining focal mechanisms of small earthquakes down to magnitude 1.5 in approximately the past 14 years. We obtained 1142 well-constrained solutions using P-wave polarity data as well as body wave amplitudes, which successfully filled the gap in the stress fields left by previous studies. We merged our focal mechanism catalog with the Japan Meteorological Agency earthquake catalog and that of our previous studies, which have become a source of information on the stress map. For each earthquake, we estimated the direction of the maximum horizontal compressive stress (SHmax) based on plunge angle of the P-, Band nd Taxes. The type of stress field was also determined using rake angles, which provide a single scalar value on a continuous scale varying from −1 (normal faulting) to 0 (strike-slip faulting) to +1 (reverse faulting). We then computed the mean SHmax and type of stress field on a mesh interval of 10 km, which we refer to as a stress map. Compared with previous stress maps in the present study area, our map succeeds in reducing the blank area of stress information and provides higher spatial resolution in stress fields. The stress map reveals a complex stress pattern, which includes sudden changes in the SHmax direction, clockwise SHmax rotation from the Izu Peninsula to its north area, the existence of multiple tectonic stress provinces in the spatial scale of a few 10 km, and normal-faulting stress fields prevailing in the coastal region of the Pacific Ocean. These features are important for understanding local tectonics and evaluating future earthquake risk in this area.
A crustal stress map of the Chugoku region, western Japan, was constructed from earthquake focal mechanism solutions. In order to increase the spatial resolution of the stress map, we included more data than the routine catalog by determining focal mechanisms of small earthquakes down to magnitude 1.5 in approximately the past 12 years. We obtained 2988 well-constrained solutions using P-wave polarity data and body wave amplitudes. We merged our focal mechanism catalog with the Japan Meteorological Agency earthquake catalog, which have become a source of information on the stress map. For each earthquake, we determined the type of stress field using rake angles and the direction of the maximum horizontal compressive stress (S Hmax ) based on P-, B-, and T-axes. We then computed the mean SHmax and type of stress field on a mesh interval of 10 km. Compared with previous stress maps in the present study area, our 10-km mesh stress map provides higher spatial resolution in stress fields. Our stress map shows that the area is predominantly strike-slip stress field with E-W compression but the stress orientation rotates clockwise by about 20°in the Sea of Japan side of Shimane and Tottori prefectures. Based on our stress map, we evaluated the fault reactivation potential of 30 active faults targeted by the Headquarters for Earthquake Research Promotion Investigation Committee (2016), revealing that 28 active faults satisfy the condition for reactivation under the present-day stress field and typical friction coefficient. The remaining two active faults are unfavorably oriented to the present-day stress field, requiring external factors such as a development of anomalous high fluid pressure and a stress triggering associated with the rupture of adjacent active faults for reactivation.
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