[S-SS11] Strong Ground Motion and Earthquake Disaster

[SSS11-P13] Estimation of Subsurface Structure of near the Mino Fault, the Median Tectonic Line, Japan Based on Gravity Survey

*Mei Takayanagi¹, Tatsuya Noguchi¹, Eisuke Yoshida¹, Akihiro Siotsu¹ (1.Tottori university)

Keywords:gravity anomaly, density structure, the Mino Fault, the Median Tectonic Line

We conducted a gravity survey of the Mino Fault and its vicinity in the Median Tectonic Line fault system to understand the subsurface structure near the fault. The Mino Fault is 13.5 km long and extends along the northern margin of the Yoshino River in the northern part of Tokushima Prefecture. There is a point where gouge outcrops along the fault on the east bank of the Kawachiya River. The Mino Fault is inferred to be a lateral strike-slip fault, and a large amount of fault clay is distributed along the fault due to the wide fracture zone along the fault.Two Lacoste-Romberg gravimeters were used to measure gravity at 98 points so that the difference between two measurements at the same point was within 10 μGal. The Trimble R2 GNSS receiver was used to measure the position of the observation points, and the position determination accuracy was 10 mm horizontally and 20 mm vertically. The gravity anomaly was determined using an assumed density of 2.33 g/cm³ after various corrections. The gravity anomaly is characterized by the distribution of low gravity anomaly areas in the northern part of the Yoshino River and high gravity anomaly areas in the southern part of the river. The low gravity anomaly area in the northwestern part of the river is considered to be composed of sandstone-mudstone alternation (Izumi Group), and the basement is deep. The high gravity anomaly in the southeastern part of the study area may be caused by the high-density metamorphic rocks such as mafic schist and quartz schist (Sambagawa Metamorphic Belt) on the south side of the Yoshino River.Furthermore, we estimated the depth of the basement by analyzing a three-dimensional analysis of 2.00 g/cm³for the surface layer and 2.33 g/cm³ for the basement layer. The results indicate that the basement drop is 60 to 150 m near the fault line.This suggests that there are folds or step differences near the fault line. However, the Mino Fault is considered to be a lateral strike-slip type fault, and the presence of fracture zones may cause low density near the fault, which may reduce gravity anomalies. We plan to analyze the fault using a multilayer model and to evaluate the fault comprehensively including the results of microtremor surveys.