Statistical Regularity and Relation Between Small Earthquakes and Crustal Velocity Structure Beneath Bohai Sea and Its Surrounding Areas in China

The Bohai Sea is a seismically active region in China, and the studying of the velocity structure of Bohai Sea and its relationship with earthquake may be helpful to the analysis and prediction of earthquakes. Now, ambient noise tomography is an effective tool to obtain crust and upmost mantle structure. In this paper, the S-wave velocity model of the crust beneath the Bohai Sea and its surrouding areas in China was constructed applying ambient noise tomography method, with steps of calculating cross correlations of all possible vertical-component data recorded in 2013 year, retrieving Empirical Green’s Functions (EGFs) for Rayleigh wave, measuring and assessing phase velocity-dispersion curves, creating the phase-velocity maps for the 8-35 s period of the Rayleigh wave, constructing the phase velocity maps and inverting the S-wave velocity structure. We work on the statistics and analysis of the characteristic relationship between crustal S-wave velocity and the temporal and spatial distribution of small earthquakes. The results from all available vertical profiles along different directions reveal that small earthquakes usually occur at the edge of the low-velocity anomaly within the shallow crust in a certain velocity range and, under special conditions, even in a distinct velocity contour. The locations of occurrence are closely related to undulating changes of the corresponding Moho morphology and the locally high Vs anomalies within the middle crust.