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

This paper analyzes bore-hole wall breakouts of the 5 wells located in southern part of the East China Sea regionwith logging sections deeper than 3 000 m and logging intervals longer than 1 000 m or even near 2 000 m. Bymaking a combined study of the breakout characteristics of these 5 wells and 11 other wells analyzed in Xu andWu (1997). it is inferred that the maximum horizontal principal compressive stress in the East China Sea 'region isgenerally in NE-SW direction, and the horizontal differential stress there is possibly weak. P- and T-axisorientations of focal mechanism of earthquakes occurred from 1977 to 1998 in the region of O kinawa trough andRyukyu 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 thatthe 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 EastChina Sea region. The result shows that the stress there is possibly attributable to both the strong northwestwardcompression in Taiwan region and the back-arc extension of the Ryukyu island arc. Less strong ear thquakes in theEast China Sea region is possibly related to the weak horizontal shear stress there.