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We assessed the seismic potential around the northeastern edge of the Longmenshan fault zone (LFZ) by analyzing the aftershocks of the 2008 Mw 7.9 Wenchuan earthquake, focusing on the right-lateral strike-slip Qingchuan fault (QCF). The data used in the present study were collected from a dense network of broadband seismographs, installed after the Mw 7.9 mainshock by the China Earthquake Administration. We analyzed 440 aftershocks that occurred around the northeastern part of the aftershock area between December 2008 and June 2009, the local magnitudes (ML) of which ranged from 2.0 to 5.2. We determined 127 well-constrained focal mechanism solutions for these aftershocks down to ML 2.5 by using P-wave polarity data in conjunction with body-wave amplitudes. An analysis of the stress tensor inversion revealed that, upon approaching the QCF from the LFZ, the stress field changes from a reverse faulting regime to a mixed regime containing strike-slip components and the orientation of the maximum compressional stress changes from a NW–SE to a WNW–ESE direction. This stress field favors a right-lateral strike-slip movement along the fault. The slip tendency analysis revealed that the QCF is regarded as being favorably oriented to the present-day stress field. The coseismic Coulomb stress changes induced by the mainshock increased along the preferred fault plane in the northeastern half of the QCF, which is further loaded by viscoelastic relaxation. This evidence suggests that the potential for a future earthquake in the QCF is high. [ABSTRACT FROM AUTHOR]

Present-day stress fields in and around metropolitan Osaka, Japan, have been investigated based on 238 microearthquake focal mechanism solutions from approximately the past 10 years. The successful retrieval of the focal mechanisms in the noisy urban area owed much to the clean seismic data recorded at four borehole observatories deeper than 500 m depth in Osaka prefecture, in which P-wave onset polarity of the microearthquake could be clearly identified. We found many microearthquakes with a pure reverse-faulting mechanism, which is consistent with the faulting style of major active faults within the area (42-km long east-dipping Uemachi and 38-km long east-dipping Ikoma fault zones); however, a considerable number of microearthquakes with strike-slip components were also found to occur. Most of the P-axes are oriented in the ESE–WNW to ENE–WSW directions sub-horizontally, conforming to the general tectonic trend of the area. The analysis of a stress tensor inversion delineated small but noticeable differences in stress regime, on a scale of at least 10 km, where strike-slip faulting prevails throughout the region but the middle and southern sections of the Uemachi fault zone contain some reverse-faulting components. Based on the estimated stress fields, together with assumptions regarding the deep geometry of the Uemachi fault zone, we assessed the reactivation potential of the fault zone through slip-tendency analysis. The results indicate higher potential in the middle and southern sections of the fault zone, which is related to the locally increased reverse component detected there. [ABSTRACT FROM AUTHOR]

The 2016 Kumamoto-Oita earthquake sequence involving three large events ( M ≥ 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 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. [ABSTRACT FROM AUTHOR]