2022 年 75 巻 p. 29-41
A magnitude 7.1 earthquake occurred west off Satsuma Peninsula, Japan in 2015, which was the largest earthquake ever recorded in the northern Okinawa Trough (OT). The northern OT is assumed to be in a beginning stage of a back-arc rifting that drives crustal extension, and the occurrence of the 2015 mainshock-aftershock sequence might be associated with such tectonics. In order to understand the rifting process that controls the seismicity, we precisely determined hypocenters and focal mechanisms of the aftershocks listed in the Japan Meteorological Agency catalog using both offshore and onshore seismic data. Initial hypocenters were determined using manual picks of P and S phases and a 1-D velocity structure. We then applied station correction terms to the P- and S-arrival times so as to consider low-velocity sedimentary layers beneath each station, especially beneath the offshore stations. Finally, the relative hypocenters were refined by the double-difference location method. The average root-mean-square residuals of the double differences decreased from 489 ms to 39 ms. The relocated hypocenter distribution clearly shows three linear alignments: one in the N95°E direction, consisting of left-lateral strike-slip fault events, and two in the N15-20°E direction, consisting of normal fault events. The tension axes for almost all the solutions lie in a common direction (NW-SE). The well-determined aftershocks mainly located in the upper crust with focal depths of 0-15 km. The results from our ocean-bottom seismic observation indicate that the two alignments consisting of normal fault events might be caused by crustal extension related to the back-arc rifting in the northern OT because the direction of the hypocenter distribution is consistent with the strike of the existing normal faults imaged by the multichannel seismic reflection survey. The left-lateral strike-slip fault alignment could be explained by along-axis variations in the back-arc spreading rate and presence of a transcurrent fault.