High‐Resolution 3D Shallow S Wave Velocity Structure of Tongzhou, Subcenter of Beijing, Inferred From Multimode Rayleigh Waves by Beamforming Seismic Noise at a Dense Array

The 3D S wave velocity of shallow structures, especially the quaternary sediments at 0–1 km near the surface, is an important issue of concern in urban planning and construction for seismic hazard assessment and disaster mitigation. Due to the facility and less dependence on the site environment, noise‐based technique is an ideal way to acquire the fine structure of urban sedimentary basin. Based on the dense array composed of more than 900 stations deployed in Tongzhou at a local scale of 20 × 40 km2, we prove that the lateral variation of the phase velocity of multimode surface waves can be estimated directly with sufficient accuracy by beamforming seismic noise with moving subarrays. Rayleigh wave phase velocity maps, at frequencies between 0.3 and 2.5 Hz for the fundamental mode as well as 0.8 and 3.0 Hz for the first overtone, are obtained. The 3D S wave velocity model at 0–1‐km depth with lateral resolution of 1 km is then established by inverting phase velocity maps of the two modes. The sediment thickness is delineated by the impedance interface given by microtremor H/V (horizontal‐to‐vertical) spectral ratio. The proposed model is in good agreement with the distribution of tectonic units. The sediment thickness of Daxing high and two sags around Gantang and Xiadian are, respectively, 100–400 and 400–600 m, which correlates well with the 1 km/s isosurface of S wave velocity. The model presents some evidence on the extension of Daxing fault along NE direction. Plain Language Summary Large cities are usually built on sedimentary basins which are prone to capture and amplify seismic energy with the result of larger damage. The main factors to determine the site amplification are the depth to basement and S wave velocity of the sedimentary layer. The combination of beamforming (BF) and microtremor H/V is suggested to build the high‐resolution 3D S wave velocity model of the basin. We prove that the lateral variation of the Rayleigh wave phase velocity of the fundamental mode as well as the first overtone can be estimated directly with sufficient accuracy by BF seismic noise, without tomographic inversion. The 3D S wave velocity model can thereby be established with high resolution by inverting the phase velocity maps of multimode surface waves. The depth to basement is delineated by the impedance interface given by microtremor H/V. The 3D S wave velocity and the depth to basement of Tongzhou, the subcenter of Beijing, is established using the data at a