[J08-P] Poster

[J08-P-08] Velocity structure and Earthquake Distribution in Nagaoka Region

^○Shutaro Sekine, Yoshihiro Sawada, Keiji Kasahara, Shunji Sasaki, Yoshihiro Tazawa (Association for the Development of Earthquake Prediction (ADEP), Tokyo, Japan)

The Nagaoka region is located on the high strain rate zone at eastern margin of Japan Sea, and it is also the area where the Chuetsu Earthquake and the Chuetsu-oki Earthquake have occurred. Between the two large faults, another faults are confirmed on the western margin of the fault zone of the Nagaoka plain. To investigate the activity of faults, the Association for the Development of Earthquake Prediction (ADEP) determined to newly construct a high-density seismic observation network (AN-net) in the region from 2010. In this study, we introduce the network and estimate the distribution of the earthquakes, and velocity structures in this region. AN-net consists of the 40 stations in the Nagaoka plane from November 2010. The size of the network is roughly 20km wide and 40 km long, covering the Nagaoka plain. In each station, the velocity seismometer and accelerometer at the bottom of a borehole　which depth is about 100m, and another accelerometer is set on the ground. Half of the stations has GNSS stations. These data from each observation station are sent in real time. In this study, we calculate P- and S- velocity structure by Double Difference tomography. This method is used both absolute and relative arrival times. After 2010 when the AN-net was constructed, arrival times of each earthquakes is picked manually in the AN-net region. Before 2010 and region of the Surrounding AN-net, we get the arrival data from JMA unified earthquake catalog. The number of absolute P- and S-wave arrival times used in the tomography is 241,626 and 216,802, respectively, with the relative arrival times for the manually picked P- and S- waves reaching 993,390 and 858,086, respectively, from 13,353 earthquakes which occurred from October 1997 to 2016. The weighted root mean square (RMS) travel time residual was reduced 0.53s to 0.08s after 45 iterations. Based on the velocity structure, most of the earthquakes occurred at the depths where P-wave velocity is greater than 5.5km/s.