SHEAR CONCENTRATION IN A COLLISION ZONE - KINEMATICS OF THE CHIHSHANGFAULT AS REVEALED BY OUTCROP-SCALE QUANTIFICATION OF ACTIVE FAULTING,LONGITUDINAL VALLEY, EASTERN TAIWAN

Repeated measurements of active deformation were carried out at three sites along the active Chihshang Fault, a segment of the Longitudinal Valley Fault zone of eastern Taiwan (the present-day plate boundary be tween the Philippine Sea Plate and Eurasia). Reliable annual records o f displacement along an active fault, were obtained based on detailed surveys of faulted concrete structures. Along the active Chihshang Fau lt striking N18 degrees E, we determined average motion vectors trendi ng N37 degrees W with an average shortening of 2.2 cm/yr. Thus, the tr ansverse component of motion related to westward thrusting is 1.8 cm/y r, whereas the left-lateral strike-slip component of motion is 1.3 cm/ yr. The fault dips 39-45 degrees to the east, so that the vertical dis placement is 1.5-3 cm/yr and the actual oblique offset of the fault in creases at a rate of 2.7-3.7 cm/yr. This is in good agreement with the results of regional geodetic and tectonic analyses in Taiwan, and con sistent with the N54 degrees W trend of convergence between the northe rnmost Luzon Are and South China revealed by GPS studies. Our study pr ovides an example of extreme shear concentration in an oblique collisi on zone. At Chihshang, the whole horizontal shortening of the Longitud inal Valley Fault, 2.2 cm/yr on average, occurs across a single, narro w fault zone, so that the whole reverse slip (about 2.7-3.7 cm/yr depe nding on fault dip) was entirely recorded by walls 20-200 m long where faults are tightly localized. This active faulting accounts for more than one fourth (27%) of the total shortening between the Luzon Arc an d South China recorded through GPS analyses. Further surveys should in dicate whether the decreasing shortening velocity across the fault is significant (revealing increasing earthquake risk due to stress accumu lation) or not (revealing continuing fault creep and 'weak' behaviour of the Chihshang Fault).