Crustal structures and anisotropy in southeastern China: Continental collision and intraplate earthquakes

This study investigates the role of crustal structures and anisotropy in southeastern China regarding intraplate earthquakes within the context of concentrated and perturbed tectonic stresses. The region is characterized by a complex multiphase tectonic history,including the ongoing collision between the Philippine Plate and the South China Sea margin. Utilizing the teleseismic receiver function method, we offer a comprehensive depiction of the crustal structures and layered anisotropy in Guangdong and Fujian, two earthquake-prone areas. Our results reveal that the crustal anisotropy is stratified into two layers, with a slow axis perpendicular to the orogenic belt in the upper-middle crust, whereas the fast axis aligns perpendicularly at lower crust levels. The lateral heterogeneity and anisotropy of the crustal structure emphasize both the boundary delineation and differential movement between continental blocks. The observed increase in density and intensity of intraplate earthquakes across this boundary supports our conclusion regarding a connection between decoupled crustal deformation and seismicity. Specifically, the foliation of metamorphic rocks during the Mesozoic collision generated orogen-parallel fabrics in the upper-middle crust, while ductile shearing resulted in orogen-perpendicular fast directions at lower crust levels. In contrast, mismatch evolution between continental blocks reactivated by present collision provides stress conditions conducive to earthquake occurrence in southeastern China. Our findings provide valuable insights into mechanisms behind intraplate earthquakes as well as tectonic evolution within continental collision zones.

Plain Language Summary

The mechanism of intraplate earthquakes is a topic of great interest in the study of continental collision and regional tectonic evolution. The controlling factors behind these earthquakes are highly complex and uncertain. In this study, we aim to investigate the role of crustal structures and anisotropy in southeastern China within the context of intraplate seismic activity. This region, which is currently experiencing a collision between the Philippine Plate and the South China Sea Margin, has a multiphase tectonic history that includes remnants of paleo-Pacific subduction. By utilizing the receiver function method, we provide a detailed picture of crustal structures and layered anisotropy in Guangdong and Fujian, two earthquake-prone regions in southeastern China. Our findings reveal that anisotropy in the crust can be stratified into two layers: an upper-middle crust and a lower crust with a slow axis and a fast axis, both perpendicular to the orogeny, respectively. The lateral heterogeneity and anisotropy of the crustal structure highlight the boundary and differential movement between the continental blocks. The enhanced density and intensity of the intraplate earthquakes across this boundary provide evidence for a connection between decoupled crustal deformation and seismicity. Specifically, our observations suggest that (1) the Mesozoic continental collision induced metamorphic foliation in the upper-middle crust, (2) ductile shearing is characteristic of the lower crust, and (3) ongoing collision has led to mismatch evolution between continental blocks, which creates stress conditions favorable for earthquake occurrence.