Application of horizontal-to-vertical spectral ratio on prospecting urbansite response and subsurface active faults in Mingguang, Anhui province

A dense survey line consisting of 133 stations was deployed in Mingguang, Anhui Province, which spanned across two main faults, namely the Jiashan-Lujiang fault and the Chihe-Taihu fault, along the eastern boundary of the Tan-Lu fault zone. Based on the ambient noise data, we calculated the horizontal-to-vertical spectral ratios (HVSR) for the 133 stations, and the peak frequencies along the profile were obtained. With the obtained HVSR, we then inverted for the near-surface S-wave velocity structures using a particle swarm optimization algorithm, and the reliability of the result was verified by comparing the HVSR result with the geological, borehole drilling, active-source imaging and ambient noise dispersion results. The sedimentary structure along the profile, as well as the layouts and activities of the two main faults were further analyzed. It is found that: (1) though the HVSR profile varies greatly and presents complex dual and multiple peaks, the site responses were all classified as medium-hard soil of type II; attentions should be paid to the amplification effect of the western depression for strong ground motions; (2) the soil-rock interface revealed in the inverted S-wave velocity structure is relatively consistent with the drilling results, and is also highly consistent with the reflection profile using the active sources; in addition, the interface between the unconsolidated and consolidated bedrocks which could not be identified by the active-source profiling is also revealed; (3) the delineation of the two faults by the S-wave velocity structure using HVSR agrees well with the profiles using borehole data, active seismic data, ambient noise dispersion data, as well as geological data. The Chihe-Taihu fault may be an active fault, and it is more active than the Jiashan-Lujiang fault. Our study indicates that HVSR method can be used in prospecting the characteristics of site responses, shallow velocity structures, and hidden active faults in the urban areas.