TRANSIENT UPPER CRUSTAL KINEMATIC COMPATIBILITY STRUCTURE ILLUMINATED BY THE CHI CHI EARTHQUAKE: RESULTS FROM STRAIN INVERSIONS IN THE LULIAO REGION, TAIWAN

Modeling of earthquake focal mechanisms and coseismic GPS data from the Luliao region, at the southern tip of the 1999 Chi-Chi Earthquake rupture, suggests the existence of an evolving km-scale zone of diffuse brittle strain that enables the differential advance of the foreland fold and thrust belt (FFTB) in west-central Taiwan. The earthquakes define a steeply northeast-dipping tabular volume extending to ~11 km depth. The focal mechanisms from the six-month period following the earthquake yield best-fitting strain tensors suggest a dominance of strike-slip faulting. Our strain inversions, using a micropolar continuum model, reveal orogen-perpendicular (NW-SE) shortening and orogen-parallel (NE-SW) maximum stretching. Our inversions also indicate plane strain deformation with positive, non-zero relative vorticity values, suggestive of CCW (map view) block rotations. Published coseismic GPS data provide further evidence that this site is the locus of strike-slip faulting accompanied by block rotation. Preliminary 2D strain inversions for GPS stations that span the inverted focal mechanisms reveal negative (CCW) rotation values and principal strain axes generally consistent with our focal mechanism inversions. We interpret our findings to reflect a zone of kinematic compatibility that is activated by differential westward expansion of the foreland FFTB. In particular, this zone separates an area of greater westward propagation near Taichung from an area of lesser propagation to the south. Differential expansion of the orogen and fanning of the geologic units within the FFTB appear to be influenced by an eastward pointing, lower-plate promontory south of the Sanyi-Puli seismic zone. Unlike the Luliao events, the Sanyi-Puli seismic zone extends to ~30 km depth and is interpreted as a reactivated continental margin fracture zone inherited from South China Sea rifting. This promontory is coincident with the widest, most distal portion of the FFTB. The Luliao seismic zone lies south of the promontory and links the southern tip of the Chi Chi rupture with the area where the Hsuehshan slate belt pinches out southward against the Backbone Range. These relations coupled with our inverse models suggest that lower plate architecture plays a role in exhumation patterns and the evolution of this upper plate, kinematic compatibility zone.