Present-day tectonic stress in the East China Sea region and its possible origin

Abstract

This paper analyzes bore-hole wall breakouts of the 5 wells located in southern part of the East China Sea region with logging sections deeper than 3 000 m and logging intervals longer than 1 000 m or even near 2 000 m. By making a combined study of the breakout characteristics of these 5 wells and 11 other wells analyzed in Xu and Wu (1997), it is inferred that the maximum horizontal principal compressive stress in the East China Sea region is generally in NE-SW direction, and the horizontal differential stress there is possibly weak. P- and T-axis orientations of focal mechanism of earthquakes occurred from 1977 to 1998 in the region of Okinawa trough and Ryukyu Islands are investigated. P-axes of earthquakes in Okinawa trough are generally oriented NE-SW, coinciding with the trough stretching trend, while T-axes are nearly perpendicular to the trend. This indicates that the horizontal action of the Philippine Sea Plate to the East China Sea region is not pushing but pulling. A two-dimensional finite element simulation is conducted to study the origin of the present-day tectonic stress in the East China Sea region. The result shows that the stress there is possibly attributable to both the strong northwestward compression in Taiwan region and the back-arc extension of the Ryukyu island arc. Less strong earthquakes in the East China Sea region is possibly related to the weak horizontal shear stress there.