3-D P-wave velocity structure of the crust beneath Hainan island and adjacent regions

Using over 3500 first P arrival times recorded by 9 digital seismic stations of the Hainan Digital Seismic Network during 1999-2005, a 3-D P-wave velocity model of the crust beneath Hainan Island and adjacent regions was determined. Our result shows that in shallow upper crust the spatial distribution of velocity anomalies is somewhat associated with surface geological structure in the region. A relatively low velocity anomaly appeas north of Wangwu-Wenjio fault, while a relatively high anomaly is found south of that fault. This spatial velocity variation is consistent with the following regional tectonic feature: northern Hainan Island is a depressional region with many active volcanoes and high geothermal activity, while central-southern Hainan is an uplifted and stable region. In middle and lower crust the velocity is lower in eastern Hainan than that in western region, possibly suggesting the existence of hot materials upwelling from the mantle in eastern Hainan. The pattern of velocity anomaly also indicates that the NW-SE trending faults, e.g., the Puqian-Qinglan fault, may be shallow, while the W-E trending Wangwu-Wenjiao fault may be deep, perhaps extending down to the Moho discontinuity.