1:45 PM - 3:15 PM
[SSS09-P03] Estimation of Subsurface Structures in Areas with Rapidly Changing Sedimentary layers Based on Dense Microtremor Observation
Keywords:microtremor, rapidly changing sedimentary layers, subsurface structure, The 2016 Central Tottori Earthquake
The 2016 Central Tottori Earthquake caused minor building damage in the Yura district of Hokuei-cho, Tottori Prefecture, but the damage area showed regional characteristics. In addition to the earthquake resistance of the buildings, differences in the subsurface structure may have had a significant impact on the damage. Microtremor observation has been conducted in this area, and there were areas where the spectral ratio of horizontal to vertical motion (H/V) showed a sudden change in the period of excitability and periodic characteristics. In this study, more detailed microtremor observations were conducted in areas where the H/V characteristics of microtremors change rapidly.
For microtremor observation, 11 seismometers (3-component accelerometers, JU410) were irregularly placed along the road at 4 to 128 m in a straight line in the target area, and were observed simultaneously. As a result of the analysis, H/V of 58 single points and phase velocity dispersion curves of 9 linear array observation points were obtained.
The H/V characteristics from west to east were A: two peaks near 1 sec and 0.3 sec period, B: a single peak near 1 sec period, and C: a region where the peak becomes indistinct and multiple peaks appear, in that order. Between the characteristics periods, from A to B, the short-period peak gradually moved to the long-period side and became a distinct peak, and from B to C, the distinct peak became flattened. In the area with a distinct peak in H/V at B, the top layer has a thick low-velocity layer with a thickness of 12 to 15 m and Vs=90 m/s. The uppermost layers on both sides are Vs=200 m/s at A and Vs=130 to 200 m/s at C. These characteristics indicate that the depositional environment differs within a small area and that the subsurface structure changes drastically. We plan to study the effects of the seismic characteristics in the future.
For microtremor observation, 11 seismometers (3-component accelerometers, JU410) were irregularly placed along the road at 4 to 128 m in a straight line in the target area, and were observed simultaneously. As a result of the analysis, H/V of 58 single points and phase velocity dispersion curves of 9 linear array observation points were obtained.
The H/V characteristics from west to east were A: two peaks near 1 sec and 0.3 sec period, B: a single peak near 1 sec period, and C: a region where the peak becomes indistinct and multiple peaks appear, in that order. Between the characteristics periods, from A to B, the short-period peak gradually moved to the long-period side and became a distinct peak, and from B to C, the distinct peak became flattened. In the area with a distinct peak in H/V at B, the top layer has a thick low-velocity layer with a thickness of 12 to 15 m and Vs=90 m/s. The uppermost layers on both sides are Vs=200 m/s at A and Vs=130 to 200 m/s at C. These characteristics indicate that the depositional environment differs within a small area and that the subsurface structure changes drastically. We plan to study the effects of the seismic characteristics in the future.