High resolution structure of Hualong fault revealed by ambient noise double beamforming imaging with dense seismic array

Double beamforming tomography (abbreviated as DBF) is a new method to image underground velocity structure using seismic ambient noise records. DBF can improve the signalto-noise ratio by superposing the coherent energy of ambient noise field, which is helpful to identify and extract different types of seismic waves and their incident azimuth (weak body wave, reflected surface wave, etc.), so as to conduct imaging research on underground velocity structure. Particularly for ambient noise surface wave imaging ,DBF does not require tomographic inversion, as it ensures directly finding the appropriate phase velocities from local slowness (azimuthal) searching. With a dense seismic nodal array, we image the shallow structure of Hualong fault in Panyu, Guangzhou based on ambient noise records. Fundamental Rayleigh wave phase velocities are extracted for periods of 0. 6 similar to 1. 2 s using DBF method. Then, a 2-D V, profile model from the surface to 1200 m depth is derived by shear wave inversion. V, results show that the shallow low-velocity anomalies in the northeast side of the array (near the Pearl River Delta) extend to the depth of 600 m, reflecting differences in sediment thickness, compaction rate and lithologic composition in different blocks on both sides of Hualong fault. In addition, the relative high-velocity anomalies below depth of 900 m correspond to the bedrock formed by Tertiary magmatic activities. The near-surface structural characteristics of Hualong fault effectively improve our cognition of the fault system in the Pearl River Delta, which also provide valuable references for further studies on regional structural characteristics,focal mechanisms and seismic hazard assessment.