Layered 1-D velocity models are widely used in seismic network routine locations and in seismological studies, such as earthquake relocation, focal mechanism inversion, synthetic seismogram calculation and geodynamics simulation. It’s also used as a reference model for 2-D and 3-D tomographic inversions. Therefore, obtaining a more reliable 1-D velocity model is an extremely important work for the study of earthquake source parameters and seismic tomography. The onshore-offshore area of the Pearl River Estuary is located at the transition zone between South China block and South China Sea bock, the special ocean-land transitional crustal type and the littoral fault zone, which is the regional seismic control structure passing through it, makes it a potential seismic source. Meanwhile, the Pearl River Delta has the most developed economy and dense population in South China. However, the 1-D velocity model used in seismic network routine operations has not been updated since 1990. To investigate the seismic structure and potential strong earthquake risk in this area, we conducted a 3-D active source seismic experiment in 2015, which incorporated sea-based airgun sources and land-based dynamite sources, and seismic recorders both at the onshore and offshore area in the Pearl River Estuary. A high quality subset of the data was used to derive an improved 1-D seismic V_P model for seismological studies. The model is constructed using the VELEST program with first arrival P-wave travel time data, together with station corrections, which account for shallow velocity anomalies from the true velocity model. The reliability of our new model is assessed by good fitting of the travel time data of airguns and dynamites and better earthquake relocation results.
The final 1-D model provides a good fit for travel time data. After iterative inversion, the root-mean-square travel-time error is 0.07s in the onshore area and 0.21s in the offshore area. Within 6km top of the model, the P-wave velocity of onshore area is 5.22~5.99km/s, and the offshore area is obviously lower, which is 2.11~6.03km/s. The retrieved values are in agreement with the thick sedimentary basins in the Pearl River Estuary Basin whose velocity is obvious lower. Then the velocity smoothly increases with depth, within the depth range of 6~15km, the P-wave velocity of onshore area is slightly lower than the offshore area, which may be due to the wide-spread low velocity layer at the middle crust depth in South China Block. Below the depth of 15km, the P-wave velocity of offshore area is greater than that of the onshore area, which is consistent with the high velocity layer in the base of the thinned continental crust and the gradually uplifting of Moho depth seaward as reported in the previous studies.
The spatial distribution of station corrections correlates well with the near-surface structure and geological features. In the area onshore of the Pearl River Estuary, positive values of station corrections are mostly observed in correspondence with the Pearl River Delta sedimentary basins due to its lower velocity values, such as Sanshui Sag, Shunde Sag and Dongguan Sag, etc. While stations located in granite, limestone and metamorphic rocks outcropping area show early P-wave arrivals(negative station corrections). In the area offshore of the Pearl River Estuary, the spatial distribution of station corrections shows a significant lateral variation and 80%larger than the onshore area. It has a good spatial correlation with the buried depth of the sedimentary basement inverted by reflection seismic survey, where the deposits are thicker, the station corrections are positive, the underground medium presents a low velocity, and vice versa. Negative values of station corrections are observed northwest of the NE-trending littoral fault zone, while positive values correspond to the thick sedimentary basins in the Pearl River Estuary Basin southeast of the littoral fault zone.
At last, we relocated 425 earthquakes in the onshore area and 234 earthquakes in the offshore area with M_L≥0.0 using simul2000 algorithm. The result shows that our new model is better than the South China model, the seismic travel time residual after relocation is greatly reduced, the land P wave residual is reduced by 22.6%, and the S wave is reduced by 21.2%. The sea P wave residual is reduced by 25.7%, and the S wave is reduced by 15.6%. The new model is better for regional earthquake location.
We provide a more reliable V_P velocity model, which can be used to earthquake location, earthquake source parameter inversion and 3-D velocity model studies in the Pearl River Estuary.

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.

The neotectonics in Zhanjiang Bay area is almost the inferred faults and there are not any active faults seen on the ground surface. So it is difficult for research on the seismogenic structure. This paper analyzes and interpretes the gravity data that can reflect the feature of deep faults and then discusses the seismogenic structure of Zhanjiang Bay area in combination with its geology and earthquake activity. There is a huge NEE-trending high gravity gradient belt lying in the coastal region among Guangdong, Guangxi, and Hainan, and Zhanjiang Bay is located in this gravity gradient belt. We analyzed and interpreted more than eighty images obtained with many different methods one by one, then, got the result that Zhanjiang Bay area is embraced by two giant fault belts trending in the NEE and NW direction respectively, and its interior is crossed over by the NE-trending fault belt. These three fault belts are well shown in the gravity images, especially the NEE-trending fault belt and NW one. The gravity isolines and gradient belts or the thick black stripes of the NEE-and NW-trending fault belts are displayed apparently. Also, these gravity structures are good in continuity, extend vastly and cut deeply. What is more, the NEE-trending fault belt plays a leading and region-controlling part. It shows good continuity, and cuts off the NW-and NE-trending faults frequently and intensively. The NW-trending fault belt also is good in continuity and cuts the NEE-and NE-trending faults relatively frequently and strongly, but it is restricted by the NEE-trending one. Last, the continuity of the NE-trending fault is worse and the strength cutting off NE-and NW-trending faults is significantly weak, just in some segments and in the shallow positions. According to the characteristics above and combined with the analyses of geological structure and earthquake activity, the conclusion can be drawn that the NEE-trending fault is the controlling structure and the main seismogenic structure in Zhanjiang Bay area, and the NW-trending fault is the second one. They conjugate and act together. Therefore, Zhanjiang Bay has the tectonic condition for generating M_S=6.5 earthquakes.