Upper crustal velocity structure of Pearl River Delta, China, derived from dense-array observations of ambient noise

The Pearl River Delta, located in the middle of the southeast coast of south China, is a graben basin. Although this region is considered tectonically relatively inactive, many small earthquakes still occur, and multi groups of faults with different directions are well developed. To better understand the geological structures in this region, we use about 30 days of ambient noise data from 88 portable stations and 38 permanent broadband stations to obtain a high-resolution 3D upper crustal S-wave velocity model. Over 3700 Inter-station group-velocity curves were measured in the 1-10 s period range and tomographically inverted by a direct surface wave inversion method in a 0.05°×0.05°grid. The checkerboard test shows that the tomographic final resolution is 0.1°×0.1°. Our results show that in the shallow crust of the study area, the velocity distribution corresponds to surface geology and geological features. The Huizhou-Dongguan depression and the Pearl River mouth exhibit low S-wave velocity feature, while the high S-wave velocity zone corresponds to the distribution of Mesozoic granite. Some faults are almost between low velocity and high velocity zone, which may play an important role of the channel of magmatic activity. The upper crustal structure in this area is closely related to the intense magmatic tectonic activity and crustal extension since Mesozoic.