Detailed sedimentary structure of the Mianning segment of the Anninghe fault zone revealed by H/V spectral ratio

Abstract

The Anninghe fault is a major left-lateral strike-slip fault in southwest China and a seismic gap with a potential earthquake larger than M_W 7.0 lies in the Mianning-Xichang segment according to recent observations. The shallow structure of this region can offer a glimpse into the geometry of the fault, which plays an important role in earthquake hazard mitigation. To further investigate the sedimentary structure of the Anninghe fault zone, two dense linear arrays with a station spacing of around 80 ​m were deployed across the fault. In this study, the H/V spectral ratio (HVSR), together with its peak frequency at each station site, was obtained by applying the Nakamura method. Our findings demonstrate that the peak frequency behaves in high correlation with lithology and is controlled by topography. HVSR in foothills or regions with magmatic intrusion shows a single peak at about 2–3 ​Hz. In locations with abundant Quaternary sedimentation, such as Anninghe valleys and fracture zones, another low-frequency peak around 0.4 ​Hz can be noticed in HVSR. By using the empirical relationship, the thickness of the sedimentary layer around the fault fracture zone is estimated to be 300–600 ​m. Furthermore, the sedimentary interface shows a downward dip to the east, possibly influenced by the east-west extrusion stress. Considering the resonance effect, buildings with 6–9 stories in the valley area of the Anninghe require additional attention in earthquake hazard prevention.