Detection of a segment boundary of lateral faults through gravity anomaly: a case study of the Median Tectonic Line fault zone, Japan

Recent developments of analyses of gravity anomaly and gravity gradient tensor have provided information on subsurface fault structures [e.g. Kusumoto et al., 2016; Matsumoto et al., 2017]. In this study we elucidate features of a lateral fault through analyses of gravity anomaly and gravity gradient tensor.

The survey area is around the boundary between the eastern and the western sections of the southern boundary of the Sanuki Mountains of the Median Tectonic Line fault zone, which is one of the major right-lateral faults in Japan. We conducted gravity survey on July-September 2018 and obtained 101 gravity data in the survey area. We compiled those data with those of Geological Survey of Japan [2013]. We applied ordinal corrections to obtain Bouguer anomaly and applied a low-pass filter with a cutoff wavelength of 5 km and removed a linear trend. From this gravity anomaly, we calculated directional derivatives (DD) along the strike and the dip of the fault zone, the horizontal first derivative (HD), the vertical first derivative (VD), the normalized total horizontal derivative (TDX), dimensionality index (Di), and dip angle. In addition, we calculated theoretical gravity anomalies caused by a right-lateral fault based on the formula of Okubo (1992).

For DD along the strike, we recognize a similar pattern between the observed one and the theoretical one. A similar pattern is also found locally for DD along the dip. The zero isoline of VD, high HD and high TDX cross the fault zone at the middle part of the faults in the survey area. Di shows 3D-like structures at the middle part of the faults. We consider that these features reflect a structural boundary of the faults. This part corresponds to the location of the segment boundary on the fault zone [HERP, 2017]. We conclude that spatial variations in VD, HD, TDX, and Di, can illuminate the segment boundary.

This study is supported by JSPS KAKENHI Grant Number 17K05629.