REGIONAL-SCALE PATTERNS OF FAULT GEOMETRY AND KINEMATICS IN THE TAIWAN ARC-CONTINENT COLLISION ZONE: ANALYSIS OF A DECADE-LONG RECORD OF FOCAL MECHANISMS

More than ten years of earthquake data are now available in the Broadband Array in Taiwan for Seismology (BATS) centroid moment tensor catalog. Of these, focal mechanisms of 401 earthquakes that occurred in the Eurasian plate of the Taiwan arc-continent collision zone from July 1995 to December 2007 were analyzed to examine the regional-scale deformation patterns in order to better understand orogenic processes.

The focal mechanisms were classified by the triangle diagram. The geometry of the different classes was analyzed using the strike and dip distributions, and the kinematics was analyzed using the orientation of the incremental shortening and extension axes. The triangle diagram shows that the dataset consists of 44% thrust mechanisms, 36% “other” mechanisms, 15% strike-slip mechanisms, and 5% normal mechanisms. The nodal planes of the thrust mechanisms dominantly strike NNE to NE and dip between 35° and 60°. The nodal planes of the “other” and strike-slip mechanisms have similar strike distributions characterized by two peaks, one striking NNW and the other striking E to ENE. The dips of the nodal planes of the strike-slip mechanisms range from 60° to 90°, but nearly vertical nodal planes are not common. The mean incremental shortening axes of the different classes of focal mechanisms consistently plunge shallowly toward the foreland. The trends of the mean incremental shortening axes of the thrust mechanisms and the “other” mechanisms with a thrust-slip component are between the plate convergence vector and the normal to the plate boundary. The trends of the mean incremental shortening axes of the strike-slip mechanisms and the “other” mechanisms with a normal-slip component, in contrast, are parallel to the plate convergence vector.

The moderate dips of the nodal planes of the thrust mechanisms, the substantial amount of “other” mechanisms, and the less abundant vertical nodal planes of the strike-slip mechanisms are not consistent with the simple predictions of the Anderson theory of faulting. The observed patterns may be a result of complexities introduced by the reactivation of preexisting faults in the colliding Eurasian plate. The trends of the mean incremental shortening axes of the different classes of focal mechanisms suggest variations in strain partitioning across the obliquely convergent orogen.