[S-SS10] Active faults and paleoseismology

[SSS10-P05] Estimation of the subsurface fault structure of the Bijosan II fault from gravity anomaly analysis

*Akihiro Sawada¹, Yoshihiro Hiramatsu¹, Wataru Kobayashi², Masaaki Hamada² (1.School of Geosciences and Civil Engineering, College of Science and Engineering, Kanazawa University, 2.Hokuriku Electric Power Company)

Keywords:density structure, gravity gradient tensor, 2-D Talwani method, reverse fault

Introduction

The Bijosan fault zone, located in the central part of the Noto Peninsula, is a reverse fault extending in a northeast-southwest direction along the northwestern margin of the Ohchi Plain and consists of the Bijosan II fault and the fault north of Tokuda (HERP, 2006). A seismic reflection survey (Shimokawa et al., 2002) and a trench survey (Azuma et al., 2005) were conducted on the Bijosan II fault, and the steep gradient zone of gravity anomaly along the fault was observed (Sutou et al., 2004). Gravity anomaly analysis is effective for estimating subsurface fault structure, and recently, gravity gradient analysis has revealed subsurface fault structure (e.g., Kusumoto, 2016, 2017; Matsumoto et al., 2016; Hiramatsu et al., 2019). In this study, we present the subsurface fault structure of the Bijosan II fault estimated from gravity anomaly with dense gravity measurements and gravity gradient tensor analysis.



Data

In this study, we used the gravity data measured previously by Kanazawa University, Yamamoto et al. (2011), and Geological Survey of Japan, AIST (2013). In addition, we conducted gravity measurements around the Bijosan II fault from March to December 2020, mainly in areas where there were relatively few measurement points, to obtain more detailed gravity measurement data. We applied Bouguer and topographic corrections to the gravity data with a correction density of 2300 kg/m³. We also removed a plain trend from the gravity data and applied a low-pass filter with a cutoff wavelength of 3 km to those to obtain gravity anomaly data. We calculated the gravity gradient tensor by using the Fourier transform from the gravity anomaly data (Mickus and Hinojosa, 2001). From the gravity gradient tensor, the spatial distributions of structural indices, such as horizontal first derivative (HD), vertical first derivative (VD), horizontal first derivative normalized by vertical first derivative (TDX), dimensionality index, and dip angle, were estimated.



Results and Discussion

Compared with the Ohchi Plain, the gravity anomaly in the Bijosan area, which is the hanging wall of the Bijosan II fault, is about 4–9 mGal higher. This high gravity anomaly is limited to a zone of about 1.5 km width along the Bijosan II fault. Analysis using the gravity gradient tensor shows that the HD and TDX maxima and VD=0 contours extend parallel to the surface trace of the Bijosan II fault (Imaizumi et al., eds., 2018). Comparison of the distribution of the structural indices along the strike of the Bijosan II fault with the surface trace shows that the northeastern edge of the subsurface fault structure is roughly consistent with that of the surface trace (Imaizumi et al., eds., 2018) and the subsurface fault structure does not extend northeastward beyond it. The HD and TDX maxima and VD=0 contours are located slightly to the southeast of the surface trace, suggesting that the subsurface fault structure is located southeast of the surface trace, which is consistent with the location of the fault inferred from seismic reflection survey (Shimokawa et al., 2002) and the fault that forms the boundary between granite and sedimentary layers (Azuma et al., 2005). We also confirmed that the subsurface fault structure is located southeast of the surface trace from the density structure analysis using the 2-D Talwani method.



Acknowledgments: We used the gravity data from Yamamoto et al. (2011), and Geological Survey of Japan, AIST (2013).