[S03-P] Poster

[S03-P-09] Temporal change of subsurface structure near Mt. Aso inferred from seismic interferometry using V-net vertical array data

^○Yuta Mizutani¹, Kiwamu Nishida², Yosuke Aoki² (1.The University of Tokyo, Tokyo, Japan, 2.Earthquake Research Institute, The University of Tokyo, Tokyo, Japan)

It is important to monitor the temporal change of the seismic velocity related to the volcanic event. In this study, we estimated the temporal change of subsurface structure in the Aso region with seismic interferometry (SI). In this study, we analyzed the data recorded from Jan. 1, 2015, to Oct. 31, 2016, at 4 V-net stations deployed by National Research Institute for Earth Science and Disaster Resilience (NIED) around Mt. Aso. Each station is composed of a broadband seismometer at the surface and a high-frequency seismometer (1 Hz) at the bottom of a borehole (depth ~200 m). First, the records were bandpass-filtered from 2 to 8 Hz. After 1-bit normalization and spectral whitening, a daily CCF between the same component of the bottom and surface sensors was calculated. Then, we made a reference CCF to stack CCFs from Oct. 25 to 31, 2016. Second, we measured the delay times in time windows of 2.56 s whose center time was increased from -5 s to 5 s every 0.2 s. When the temporal change is homogeneous, the slope characterizes the bulk velocity change within a spatial scale of about 2 km, and the intercept characterizes the localized velocity change between the sensor pair (~200 m). After the 2016 Kumamoto Earthquake, in the NS component at Takamori station and the ES station at other stations, the estimated slope showed the velocity reduction of approximately 0.2 %. On the local velocity change in the borehole, nearly 5-8 % velocity drop was observed at 3 stations except for Nagakusa station. Otherwise, there was an approximately 20 % drop at Nagakusa station. This could attribute the serious damage on the station of the subsurface structure by the earthquake. At Ichinomiya station, the velocity change with the time scale of a few weeks was detected. This change was well agreed with a simple model of the ground water level inferred from the data, and could be localized near the borehole because the station is in a volcanic alluvial fan.