New evidence from shallow seismic surveys for Quaternary activity of the Benchahe fault

In order to redefine the activity of the Benchahe fault, according to target layers with different burial depths, we followed the work rationale of 'from known to unknown, from deep to shallow and step by step'. We adopted not only the multi-level seismic detection method but also the joint exploration of P- and SH-waves. We obtained high-resolution images of the stratum structure in the depth range from 1350 m to near the surface and clear image results of the Benchahe fault. This exploration research accurately locates the Benchahe fault, and further shows that it is not a single fault but rather a fault-terrace belt consisting of two N-dipping faults within the effective detection depth. We clarify that the up-breakpoint of the southern section (F1-1) of the Benchahe fault is shallower and more active. The Benchahe fault is a near EW-trending fault, dipping north with an apparent dip angle of around 60 degrees-75 degrees. Subsequently, through geologic borehole section verification, we show that the results of the multi-level and multi-wave shallow seismic reflection method, determining the relevant parameters and crossing the location of the fractures, are reliable and accurate. We can observe that the reflective layers and fault characteristics revealed by the seismic profiles correspond well with the geologic borehole section. The depth of the upper breakpoint discernible from the borehole section is 93.8 m, which belongs to the top of the middle Pleistocene strata. Therefore, we may safely conclude that the newest active time of the Benchahe fault is late mid-Pleistocene to early late Pleistocene.