关键词: 阳江地区, 双差地震层析成像, 速度结构, 低速层, 地壳减薄, 平岗断裂隐伏段

Abstract: This paper collects 43 225 absolute first arrival P wave arrival times and 422 956 high quality relative P arrival times of 6 390 events occurring in Yangjiang and its adjacent area from Jan. 1990 to Aug, 2019. These seismic data is recorded by 49 stations from Guangdong seismic network, Guangxi seismic network and Hainan seismic network. Based on the seismic data above, we simultaneously determine the crustal 3-D P wave velocity structure and the hypocenter parameters of 6 255 events in Yangjiang and its adjacent area by applying double-difference seismic tomography. The result shows that shallow P wave velocity in Yangjiang area is higher due to the thinner sedimentary layer and widely exposed Yanshanian granite, Indosinian granite and Cambrian metamorphic rocks. There are obvious correspondences between the distribution of shallow velocity and fault structure as well as geological structure. The velocity transfer zone along Mashui-Pubai correlates with the NE strike of Yangchun-Zhilong Fault, and the low velocity anomaly on the west corresponds to the Yanshanian granite system, while the high velocity anomaly on the east corresponds to the Cambrian epimetamorphic rock system. The Yangjiang M6.4 earthquake locates at the high velocity seismogenic body among the low velocity anomalies due to Yanshanian granite system. Besides, there is a low velocity anomaly existing below the high velocity seismogenic body as mentioned above, we speculate the low velocity anomaly is a ductile shear zone due to partial melting of lower and middle crust caused by mantle wedge melting and basaltic underplating. Moreover, a wide range of low velocity anomaly exists in 20km depth, which verifies the low velocity layer in the middle crust at Yangjiang area of South China continent. The velocity image from land to ocean in 30km depth shows low velocity in NW side and high velocity in SE side, which verifies the characteristic of crust thinning in South China coastal continent. The NEE seismic belt from Yangbianhai to Pinggang is speculated to locate in a buried fault in the southwest segment of Pinggang Fault. The buried thrust fault is a N78°E strike fault, dipping to NW with a dip angle of 85°. In addition, the buried fault locates in the abnormal junction of high velocity on the NW side and low velocity on the SE side, which reflects the tectonic activity characteristic of NW plate uplifting and SE plate declining from Miocene period. The characteristic of activity in the buried fault shows thrust movement with a small strike-slip component, which is consistent with the focal mechanism of the M4.9 earthquake occurring in 2004. Finally, there is a large difference of formation occurrence between the southwest buried fault of Pinggang Fault and the northeast segment of Pinggang Fault. We speculate that, the formation occurrence of Pinggang Fault changes near Pinggang area in the form of “dough-twist”, which causes different velocity structure and movement characteristic.

Key words: Yangjiang area, double-difference seismic tomography, velocity structure, low velocity layer, crust thinning, the buried segment of Pinggang Fault