[S-CG56] 変動帯ダイナミクス

[SCG56-15] Possibility of shallow repeating aseismic slip associated with the 2011 Mw5.8 and 2016 Mw5.9 crustal earthquakes in northern Ibaraki Prefecture, Japan

*吉田 圭佑¹、福島 洋² (1.東北大学理学研究科附属地震噴火予知研究観測センター、2.東北大学 災害科学国際研究所)

After the 2011 M9 Tohoku earthquake in Japan, the largest number of earthquakes occurred in northern Ibaraki Prefecture in the inland area of Japan. Previous geodetic analyses suggest that earthquakes of Mw 5.8 and 5.9 repeatedly occurred in 2011 and 2016, respectively, in the same area on the same fault with an extremely short interval for earthquakes that occur in the crust. We here examine the fault structure and rupture processes of these two earthquakes in detail to help elucidate the mechanism of earthquake occurrence on crustal faults. Our earthquake relocation results show that the two M~6 events and their aftershocks are located on a common sharp planar structure, supporting that the two M~6 events occurred on the same fault. However, the hypocenters of the two M~6 events are widely separated (~10 km); the 2011 event was initiated near the top of the seismogenic zone (z=~6 km), whereas the 2016 event was initiated at the bottom (z=~12 km). Our waveform inversion results show that while the 2011 Mw5.8 rupture occurred near the hypocenter, the 2016 Mw5.9 rupture first propagated to the shallow side near the 2011 hypocenter with a small slip and grew rapidly there. Our result confirms that their main rupture areas were close, though it does not deny the possibility that the two rupture areas are a few kilometres apart. The estimated coseismic slip area of the 2011 event is mainly distributed from z=~7 km to the deeper side, while the geodetic estimate suggested a shallower depth range from 0 to 7 km. The difference may indicate that what repeated in the 2011 and 2016 events was aseismic slip rather than coseismic slip.