[S-SS10] Active faults and paleoseismology

[SSS10-03] Microtremor Array Observation across Active Faults in Ogasawara Iwo-To Island

*Takumi Murakoshi¹, Takayuki Kouno² (1.National Defense Academy, 2.Japan Air Self-Defense Force)

Keywords:Ogasawara Iwo-To Island, Active fault , Microtremor, H/V Spectral Ratio

Iwo-To is a seismic and tectonically active volcanic island located in Ogasawara, Tokyo, Japan. In 2020, more than 100 volcanic earthquakes occurred per day during a three-day observation period. Currently, seismic activity is still observed on this island. The Motoyama area, located on the east side of Iwo-To island is experiencing rapid crustal movement with uplift measuring 50– 100 cm per year (The handout of 147th Coordinating Committee for Prediction of Volcanic Eruption, 2020). Kaizuka et al. (1983) reported the presence of active faults at several locations in the western and northeastern parts of Iwo-To island. In the present study, a microtremor array survey was conducted from September 30–October 2, 2019, to investigate the ground properties and subsurface geometry near active faults in the northeastern part of Iwo-To island. Using two lines with lengths of 1,000 and 550 m that crossed the presumed active faults, 37 and 15 observations were made respectively. The intervals between the observation points were set at 10 m or 25 m near active faults and at 50 m in other locations. Using a seismometer (JU410, Hakusan Corporation, Japan) as the observation device, three components of horizontal and vertical directions were measured continuously for more than 15 min at each observation point at a sampling frequency of 100 Hz. The H/V spectral ratios of the acceleration waveform records of the three components of microtremors were used for analysis. To understand the characteristics of the ground properties and the spatial changes in the subsurface faults, we analyzed the spatial changes in the dominance period, amplitude, and shape of the H/V spectral ratio of the microtremors. The results were used to effectively detect the horizontal changes occurring during the predominant period of the microtremor H/V spectral ratio across the surface faults estimated from the geological map data.

Acknowledgments:
This observation in Iwo-to island was supported by Japan Maritime Self-Defense Force Atsugi Air Base and Iwoto Air Base.