[SSS04-P12] Estimation of subsurface structure based on microtremor and gravity observation in Shigaku area, Mt. Sanbe, Shimane Prefecture, Japan
Keywords:microtremor observation, gravity observation, subsurface structure, Shigaku area, Mt. Sanbe, 2018 western Shimane Prefecture earthquake
An earthquake (Mj6.1) occurred in the vicinity Ohda city western Shimane Prefecture in Japan on April 9, 2018. Characteristics of site amplification effect of the temporary sites were understood from analysis of seismic data. We need to investigate the causes of the local site effects for the prevention or mitigation of earthquake disasters in the area. The purpose of our study was to estimate the subsuface structures of the target area by using microtremor and gravity survey. Therefore, densely microtremor and gravity observations were carried out to estimate the characteristic of ground motion in the damage area, Shigaku area, Mt. Sanbe, Shimane Prefecture. 3-compoent single-site observations at 80 points and array observations at 1 site were made to observe microtremor, and 51 points were made to observe gravity in the area.
There are two clear peaks in H/V spectrum in the short-period band (0.2 - 0.5 s) and the long-period band (1 - 3 s). The feature of the distribution of the predominant period is thought to be influenced by the ground structure of the sedimentary layer due to the volcanic deposits of Mt. Sanbe. From the estimation results of the S-wave velocity structure, the thickness of soft soil with S-wave velocity of 170m/s to 250m/s was about 40m. The S-wave velocities of 700m/s and 1300m/s corresponding to hard sedimentary rocks were about 200m and 500m, respectively.
Gravity anomalies were corresponded characteristic of the caldera topography of Mt. Sanbe, and there were low and high anomalies reflecting the complex structure near the summit. The basement altitude was obtained by performing a three-dimensional analysis using the gravity anomaly with a surface layer of 2.2g/m^3 and a base layer of 2.5g/m^3. As a result, the feature of the shape of the basin by the caldera topography were able to be grasped. It was found that the altitude of the basement surface was about -200m on a flat part (surface thickness of about 700 m) and about -1500m at the deepest part (surface thickness of about 1500m).
There are two clear peaks in H/V spectrum in the short-period band (0.2 - 0.5 s) and the long-period band (1 - 3 s). The feature of the distribution of the predominant period is thought to be influenced by the ground structure of the sedimentary layer due to the volcanic deposits of Mt. Sanbe. From the estimation results of the S-wave velocity structure, the thickness of soft soil with S-wave velocity of 170m/s to 250m/s was about 40m. The S-wave velocities of 700m/s and 1300m/s corresponding to hard sedimentary rocks were about 200m and 500m, respectively.
Gravity anomalies were corresponded characteristic of the caldera topography of Mt. Sanbe, and there were low and high anomalies reflecting the complex structure near the summit. The basement altitude was obtained by performing a three-dimensional analysis using the gravity anomaly with a surface layer of 2.2g/m^3 and a base layer of 2.5g/m^3. As a result, the feature of the shape of the basin by the caldera topography were able to be grasped. It was found that the altitude of the basement surface was about -200m on a flat part (surface thickness of about 700 m) and about -1500m at the deepest part (surface thickness of about 1500m).