[SSS11-P02] S-wave reflectors beneath the earthquake swarm in the Yonezawa-Aizu area, NE Japan
Keywords:S-wave reflectors, crustal fluid, earthquake swarm
The objective of this study is to estimate the areas of fluid by investigating distribution of S wave reflectors, which are considered to indicate the localized (sill-like) distribution of fluid. As a previous study, Hasemi et al., [2016JpGU] found reflected S waves in seismograms recorded at the Hi-net stations of NIED. In this study, we use 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 the normal moveout (NMO) correction to these waveform data, and we made some reflection profiles of seismograms along various lines for each station. These profiles were set passing above each cluster and intervals of 0.005 degrees in both the north-south direction and east-west direction.
As a result, we confirmed continuous S wave reflectors beneath the earthquake swarm for each profile; in the northeastern part of this area, reflector is found at a depth of 13-14 km beneath the station, in the southwest part at 20-21 km for the southern station, in the central part at 13-15 km for the eastern station, and in the western part at 12-14 km for the northwest station. In either case, S wave reflectors can be found directly below or around the earthquake, suggesting that crustal fluid is involved in the swarm activity in this area. However, there were cases where the reflecting surface could not be clearly identified depending on the location of the station, or conversely, a few of reflectors could be confirmed. It is conceivable that the crustal fluid that causes the S wave reflectors is locally distributed rather than uniformly distributed widely at a certain depth.