Velocity structure of sedimentary formation in the South Yellow Sea Basin based on OBS data

After many years of geological and geophysical studies, the South Yellow Sea (SYS) is believed to be the front of South China Block when colliding with the North China Block. The SYS basin is filled by the Mesozoic-Cenozoic terrigenous sediments overlying the Pre-Paleozoic and Mesozoic-Paleozoic marine sediments. As the results of the multi-stage tectonic activity and sedimentation events, the SYS basin has very complex structure. Many geophysical surveys have been conducted to uncover the detailed basin structures. However, the connection of the shallow and deep structures remain to be studied, either due to the low resolution of crustal structure for the magnetic and gravity methods or due to the lack of deep structure information in the shortoffset marine seismic data. Here, we use the first-arrival time data of a wide-angle OBS array (OBS2013) and the multiscale seismic tomography (MST) to invert the P-wave velocity structure of the SYS. The MST method is devised to cope with the poor determinacy caused by the uneven raypaths coverage. By decomposing the inverted area with the sub-models with different cell sizes, the MST method can invert the velocity model on the meshes with correct scale and provide a smoother velocity model that is suitable for the geologic interpretation. The accuracy of the first-arrival times is improved after adopting a five-station moving average method, which suppresses the random noise and enhances the first-arrival waveforms by averaging the nearby traces. The selection of data is based on the angle between the array and the shot-receiver line. Here, the picks with angle less than two degrees are adopted to avoid the off-line influence. To show the resolution of our MST algorithm, we conducted a checker-board test using the same inversion parameters and shotreceiver geometry with the case of field-data inversion. The resulted velocity profile shows that the velocity anomalies shallower than 6 km depth is properly recovered. By comparing the inverted velocity model with multi-channel seismic profile, the sedimentary formation is interpreted, referring to gravity and magnetic data. The final inverted velocity model shows strong lateral and vertical velocity variations. In the vertical, the sedimentary formation is divided into two parts by the Indosinian surface. The top one is the extensional formation with low P-wave velocity, and the other is the extrusion formation carrying a high speed feature. In the transverse, six boundaries of lateral velocity (F1-F6) were interpreted as six faults, which control the development of the SYS basin. Based on the research results, we infer F1 to be the boundary of Jiaolai basin and F4 to be a strike-slip fault, which is a normal fault on its top and the reverse fault at its bottom. The faults divide the sedimentary formation beneath the survey line into low or high P-wave velocity zones. Four low velocity zones (LVZs, 5 km . s (1)) are identified beneath 2013OBS line with the prior gravity, magnetic and conventional seismic data. The LVZs, namely I, II, III and IV, have the similar gravity and magnetic anomaly characteristics, and are demonstrated to be formed in the faulted period. In the HVZs, area V consists of the metamorphic rocks formed during the collision orogeny in the Qianliyan Uplift. The other four HVZs show as high-value gravity anomaly and low-value magnetic anomaly in the Northern Depression, which correlate with the feature of the marine sedimentary strata. However, sandstone is confirmed located at the strata with 2 km thickness in VIII and IX (buried depth <6 km) with a lower velocity (5 km . s P-wave), referring to the V-P/V-S. Based on the short wavelength magnetic anomaly, there might be igneous rocks in the HVZ IX of the Central Uplift.