Spatial distribution of seismic wave reflectors beneath the Yonezawa-Aizu region, northeastern Japan

In order to understand the mechanism of inland earthquake occurrence in northeastern Japan, it is necessary to consider the influence of crustal fluid. It is interpreted that crustal fluid forms low-velocity areas and/or seismic reflectors beneath volcanic area or active fault [e.g., Hori et al., 2004; Hasegawa et al., 2005]. In prefectural border between Yamagata and Fukushima prefectures, where the seismic activity was low before the 2011 off the Pacific coast of the Tohoku Earthquake, triggered shallow microearthquake swarm started at 7 days after the Tohoku-Oki earthquake. Some previous studies, suggest that the triggered seismicity occurred due to increase of fluid pressure [e.g., Okada et al., 2015]. In this study, we will estimate the distribution of S wave reflectors and consider spatial relationship with hypocenter distribution, seismic low-velocity area and Neogene caldera.

We use data from the temporary seismic network deployed by Kochi University, Chiba University, and Tohoku University, to investigate more detailed distribution of reflectors in addition to the Hi-net stations. We used waveforms observed at these stations from 4798 events in the period from May 2011 to February 2012. We used hypocenter locations obtained by the double-difference method [Waldhauser and Ellsworth, 2000]. We applied the automatic amplitude control (AAC) correction and 4-16 Hz band pass filler to these waveform data. Next, in order to estimate the depth of the S wave reflectors, we also made two-dimensional reflection profiles by converting lapse time to depth [Inamori et al., 1992]. As a result, we find continuous S wave reflectors beneath the earthquake swarm for each profile; in the northeastern part of this area, reflector is located at a depth of 13-14 km, in the southwest part at 20-21 km, and in the central part at 13-15km.In the northeastern part, S wave reflectors looked more clearly at the three stations located directly beneath the hypocenter, and it is located apparently deeper in the northeast direction. We will determine the strike and dip of the S wave reflectors that explain travel times of reflected waves to obtain three dimensional distribution of reflectors.