Abstract: It is important to detect the fine velocity structures of the crust and uppermost mantle to understand the regional tectonic evolution, earthquake generation processes, and to conduct earthquake risk assessment. The inversion of uppermost mantle velocity and Moho depth are strongly influenced by crustal velocity heterogeneity. In this study, we collected first arrivals of Pg and Pn and secondary arrivals of Pg wave from the seismograms recorded at Fujian provincial seismic network stations. New 3-D P-wave velocities were inverted by multi-phase joint inversion method in Fujian Province. Our results show that the fault zones in Fujian Province have various velocity patterns. The shallow crust is characterized by high velocity that represents mountains, while the mid-lower crust shows low velocities. The anomalous velocities are correlated closely with tectonic faults in Fujian Province. Velocity anomalies mainly show NE-trending distribution, especially in the mid-lower crust and uppermost mantle, which is consistent with the NE-trending of the regional main fault zones. Meanwhile, a part of velocity patterns show NW trending, which is related to the secondary NW-oriented faults. Such velocity distribution also shows a geological structural pattern of "zoning in east-west direction and blocking in north-south direction" in Fujian area.
In the crust, a low velocity zone is found along Zhenghe-Dapu fault zone as mentioned by previous study, however our result shows the low velocity exists at depth of 20~30km in mid-lower crust. Compared with previous study, this low velocity zone is larger and deeper both in range and depth.
The crustal thickness of 28~35km from our joint inversion is similar to the results from the receiver functions of previous studies. The thinnest crust(28km)is observed at offshore in the north of Quanzhou; while the thickest crust(35km)is located west of Zhangzhou near the Zhenghe-Dapu fault zone. Generally, thinner crustal thickness is found in offshore of Fujian Province, and thicker crustal thickness is in the mainland. However, we also found that crustal thickness becomes thinner along the east side of Yongan-Jinjiang Fault.
The values of Pn velocities in the region vary from 7.71 to 8.26km/s. The velocity distribution of the uppermost mantle presents a large inhomogeneity, which is correlated with the distribution of the fault zone. High Pn velocity anomalies are found mainly along the west side of the Zhenghe-Dapu fault zone(F₂), and the east side of the Shaowu-Heyuan fault zone(F₁), which is strip-shaped throughout the central part of Fujian. Low Pn velocity anomalies are observed along the coast and Taiwan Straits, including the Changle-Zhaoan fault zone, the coastal fault zone, and the Fuzhou Basin. We also found a low Pn velocity anomaly zone, which extends to the coast, in the Shaowu-Heyuan fault zone at the junction of the Fujian, Guangdong and Jiangxi Provinces. In the west of Taiwan Straits, both high and low Pn velocity anomalies are observed.
Our results show that the historical strong earthquakes(larger than magnitude 6.0) are mainly distributed between positive and negative anomaly zones at different depth profiles of the crust, and similar anomalies distribution also exists at the uppermost mantle, suggesting that the occurrence of strong earthquakes in the region is not only related to the anomalous crustal velocity structure, but also affected by the velocity anomaly structure from the uppermost mantle.

Key words: Fujian Province, joint inversion, 3-D P-wave velocity structure, crustal thickness

摘要： 地壳介质的非均一性对反演上地幔顶部速度和地壳厚度具有重要影响。文中对福建地区地壳上地幔顶部的速度结构进行联合反演，结果显示福建地区地壳浅层主要以高速异常特征为主，与区域内山脉特征相对应，而在中下地壳主要以低速异常为主，区域内速度异常与区域断裂构造密切相关。该结论进一步证实沿政和-大埔断裂带存在低速带，但低速异常主要出现在深20~30km的中下地壳部分，与前人所得的低速异常区相比范围更大，出现深度更深。联合反演结果显示福建陆域地壳厚度为28~35km，地壳在沿海地区偏薄，向内陆逐渐变厚，与接收函数的结果较为一致，且发现沿永安-晋江断裂东侧存在显著的地壳减薄特征。上地幔顶部速度结果显示在政和-大埔断裂带西侧以高速异常为主，在福州盆地及沿海区域表现为低速异常。

关键词: 福建地区, 联合反演, 三维速度结构, 地壳厚度