关键词: 福建地区, 双差层析成像, 速度结构, 福安-南靖断裂带

Abstract:

The Fujian area is located tectonically at the southeastern margin of the South China continent, which consists of three sub-blocks, the northwest Fujian block, the southwest Fujian block and the east Fujian block. This region is the forefront of the interaction between the Eurasian plate and the Philippine Sea plate. Geologically, the Fujian area has undergone a complex tectonic evolution process, and the huge intrusive-volcanic rocks formed by multi-stage tectonic changes were widely exposed in this region. Since the inversion of the crustal three-dimensional P-wave velocity structure was important for understanding the tectonic evolution process and the deep seismogenic environment in the region, a lot of research work has been carried out in Fujian area, including seismic body wave tomography, ambient noise surface wave tomography and artificial seismic profiles. Although some important features of the crustal velocity structure in this region had been obtained by natural seismic body wave or ambient noise surface wave imaging, the grid lateral resolution was relatively poor(generally above 0.5° horizontally), which made it difficult to constrain effectively the detailed features of the fault zone velocity structure in this region. For example, the Fu'an-Nanjingfault zone, as an important fault zone in the region, which controlled the magmatic intrusion activities before the Mesozoic, the features of its deep velocity structure have been rarely revealed. Although the resolution of artificial seismic profiles was high, it covered a relatively limited detection range in this region.

In this paper, 3203 natural local earthquakes were selected using the observation reports of Fujian seismic network from 1999 to 2021 and integrating some data from neighboring provinces, which includes both 76423 absolute arrival time data and 389021 P-wave relative arrival time data from131 seismic stations. The test results of checkboard showed that the northwest Fujian block had poor recovery at all depths due to the limited internal seismic ray coverage, most areas of the southwest Fujian block had good recovery at all depths, and the east Fujian block could been recovered at all depths except for its northern region which had poor recovery at 0km, 25km and 30km depth. Under this resolution condition, the three-dimensional crustal P-wave fine velocity structure in Fujian region was obtained. The arrival time residual conforms to a Gaussian distribution before and after the inversion. The travel time residuals of the seismic phases were mainly distributed in the range of -1.5 to 1.5s before the inversion, and these travel time residuals of the seismic phases were mainly distributed in the range of -0.5 to 0.5s after this inversion. The travel time residuals were reduced significantly and were more concentrated around 0. Using the velocity structure obtained from the inversion and combining with the geological structure and geophysical field characteristics of this region, the tectonic implications which may be related to these features of velocity structure in the region were discussed. The main results are as follows:

(1)In the near-surface shallow layer, the P-wave low-velocity feature is mainly correlated better with the NW-trending faults, such as the Nanri island fault, Meizhou bay fault, Yong'an-Jinjiang fault and Jiulong river fault. This may be related to the relatively young activity age and more fragmented shallow parts of the NW-trending faults. The lateral variation of velocity is small in the middle and upper crust at 5km and 10km depths relative to other depths, but there is a relatively high velocity zone of P velocity in northeastern Fujian area.

(2)The P-wave velocity structure shows generally a relatively low velocity feature at 15~25km depth within the southwest Fujian block, especially in the south of the Yong'an-Jinjiang fault zone. Although the range distribution of this low velocity anomaly is relatively large, the magnitude of the anomaly is not large, and the upper crust and the bottom of the lower crust in the southwest Fujian block do not show this anomalous feature. On the other hand, the magnetotelluric sounding of the middle and lower crust of this block shows a high resistivity and the receiver function shows a low Poisson's ratio, this suggests that the low-velocity feature of this block is not caused by partial melt or ductile shear zone, but may be mainly caused by the more quartz-rich composition of the regional crust.

(3)There exist two P-wave low velocity anomalies in the middle-lower crust of the East Fujian block, which are below the two high thermal anomalous area of the geothermal heat flow in this region. It may suggest that the formation of these two relative low velocity anomalies may be related to the transformation of the coastal area into an extensional environment and the upwelling of deep mantle materials caused by the high-angle retraction of the Paleo-Pacific plate in the late Yanshanian period.

(4)The P-wave velocity features show that the velocity at the two sides of the Fuan-Nanjing fault zone is different obviously in the middle and lower crustal depths. This may imply the Fu'an-Nanjing fault has a certain control on the distribution of crustal velocity structure in the region, which is consistent with its deep characteristics of cutting the Moho interface which reflected by the Bourg gravity anomaly and aeromagnetic anomaly, which further confirms that it is a major deep fault zone in the region.

Key words: Fujian area, double-difference tomography, velocity structure, Fu'an-Nanjing fault zone