Crustal stress field in Ilan Plain, NE Taiwan and the westernmost Okinawa trough-deduced from seismic stress inversion

Abstract

The tectonic stress in the Ilan Plain (IP), NE Taiwan region is complex. Because the Philippine Sea plate converges and subducts beneath the Eurasian plate, and the Okinawa trough is rifting westward. We adopt the focal mechanisms collection during the period 1996 - 2016 to invert the stress field in the crust with grid of 0.1° by 0.1°. Most of the focal mechanisms in the crust occurred in the shallow 15-km depth. The stress inversion result shows that the dominant stress in this region is of extension (σ3) in the NW-SE direction and with a clockwise rotation in the southeast part. On the contrary, the compressional stress (σ1) is relatively complex in the same IP region and can essentially be divided into three parts: (1) offshore the IP, mostly east of 121.95°E, the σ1 is quite stable in the about N-S direction; (2) it behaves unstable and large uncertainty in the NW-SE direction in the northern flank of the IP; and (3) the σ1 remains stable in the southern wing of the IP and is oriented in the NE-SW direction, except for two W- and E-end. The different states and stability distributions of the σ1 stress axis between the north and south IP is clear, which implies that this could be caused by some potential structures or stress divisions. The left-lateral strike-slip Choshui crosscutting through the middle IP may be the corresponding structure in line with the preceding stress barrier.