[S-SS10] Strong Ground Motion and Earthquake Disaster

[SSS10-P17] Gravity survey around the Mino Fault, the Median Tectonic Line, Miyoshi City, Japan

*Mei Takayanagi¹, Tatsuya Noguchi¹, Asumi Nishikori¹, Masanori Kohno¹ (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 in the area of the fault. The Mino Fault is 13.5 km long and extends along the northern border 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 Kouchidani River. The Mino Fault is inferred to be a strike-slip fault, with a wide fracture zone along the fault, and a large amount of fault clay distribution. Gravity observations were conducted at 82 points using a Lacoste-Romberg gravimeter. GNSS surveying equipment was used to obtain elevations within 10 cm. Various corrections were made to the measured values, and the gravity anomaly was calculated with an assumed density of 2.33 g/cm^3. The gravity anomaly is characterized by a zone of high anomaly along the fault on the north side of the Mino fault. We also estimated the depth of the basement using a homogeneous two-layer model with a surface layer of 2.00 g/cm^3 and a basement layer of 2.33 g/cm^3 for two-dimensional and three-dimensional analyses. As a result, we found that the basement drop is about 100 to 200 m from the north side to the south side across the fault. This suggests that there is a fold or step difference on the north side of the fault. However, the Mino Fault is considered to be a strike-slip type fault, and the presence of fracture zones may result in low density on the south side of the fault, which may lower the gravity anomaly. We plan to analyze the fault using a multilayer model and to evaluate the fault comprehensively including the results of microtremor surveys.