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

摘要： 文中基于广东、 广西和海南地震台网共49个台站于1990年1月—2019年8月记录的6 390个地震的43 225条P波绝对到时数据和422 956条相对到时数据, 利用双差地震层析成像方法获得了阳江地区0.1°×0.1°的三维地壳P波速度结构及6 255个地震的精定位结果。 结果表明, 阳江地区的浅层P波速度较高, 可能与该区沉积层较薄且普遍出露的燕山期花岗岩、 印支期花岗岩和寒武纪变质岩有关。 浅层速度的分布与断裂构造及地质构造具有明显的对应关系, 马水—蒲白一带的高、 低速转换带与阳春-织篢断裂的NE走向有明显的一致性, 西侧的低速体多对应燕山期花岗岩系的分布区, 而东侧的高速体多对应寒武纪变质岩系。 阳江6.4级地震位于燕山期花岗岩系低速异常所夹的高速孕震层内, 其下方为低速异常, 推测下方的低速层属于延性剪切层, 地幔楔熔融以及玄武岩的底侵作用使得该层岩石部分熔融,表现出低速异常。 20km深度附近存在大范围的低速异常, 验证了阳江地区存在华南大陆的中地壳低速层; 30km深度处陆区到沿海呈现出NW低速、 SE高速的特征, 验证了华南沿海大陆的地壳减薄特征。 推测NEE向的洋边海—平岗地震条带位于平岗断裂西南的逆冲隐伏段上, 该隐伏段与洋边海断裂交会处发生的3次5级以上地震可能均为洋边海断裂和平岗断裂共同作用的结果。 隐伏段在地下4~13km深度处延伸, 走向为N78°E, 倾向NW, 倾角约为85°, 位于高、 低速异常交界处并偏向于高速一侧, 其NW侧为高速区, SE侧为低速区, 反映了中新世以来NW盘抬升、 SE盘下降的构造活动特征, 亦与2004年阳江4.9级地震的逆断兼具少量左旋走滑的性质吻合。 平岗断裂隐伏段与NE段的产状差异较大, 推测平岗断裂可能在平岗附近扭为 “麻花”状, 两侧表现出不同的构造和运动性质。

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