High-resolution shallow structure along the Anninghe fault zone, Sichuan, China, constrained by active source tomography

The Anninghe fault (ANHF), located in southwest China, was a major block boundary that hosted M 7.5 earthquakes historically. For seismic hazard assessment, it is critical to investigate fault properties before future earthquakes. To investigate the fault structure, we deployed three linear dense arrays with an aperture of approximately 8-9 km across different segments of the ANHF from October 2019 to March 2020. More importantly, we detonated a new methane source to generate seismic waves, which is environmentally friendly and can be used in different regions such as mountainous and urban areas. After data acquisition, we first removed the noise to accurately pick up the first arrivals of seismic waves. Then, we conducted the first-arrival seismic tomography, a method commonly used in the petroleum industry, to obtain the high-resolution P-wave velocity structure. The tomographic results showed distinct low-velocity zones (LVZs) of approximately 1000-1500 m in width and approximately 300-400 m in depth along the fault, well consistent with the lateral distribution of site amplification that was derived from regional earthquake waveforms. These LVZs may have formed as a combined result of the fault damage zone and ANHF-controlled sediments. As the Anning River Valley is densely populated, our newly identified LVZs shed lights on earthquake hazard in the region. In addition, we demonstrate that using a combination of methane detonation sources, linear dense arrays, and active source tomography can effectively determine the shallow P-wave velocity model in complex environments (i.e., mountains and urban areas).