Geometry and structure of northern surface ruptures of the 1999 Mw = 7.6 Chi-Chi Taiwan earthquake: Influence from inherited fold belt structures

Surface ruptures associated with the 1999 Mw = 7.6 Chi-Chi earthquake in central western Taiwan have been characterised by mapping along the northern fault-segment. The earthquake occured on the reactivated Chelungpu fault in the frontal portion of the thin-skinned Taiwan fold-and-thrust belt. The N-S trending Chelungpu fault is a 90-km-long major west-verging thrust, which principally slips within, and parallel to, bedding of the Pliocene Chinshui Shale. In the northern segment of the earthquake fault trace, that we name the Shihkang-Shangchi fault zone, the surface ruptures turn to an E-W strike and produce a series of thrust-and-backthrust pop-ups, about 15 km long, forming several discontinuous subsegments distributed within a broad regional Pliocene syncline. The northern fault segments activated during the Chi-Chi earthquake, in the area where the displacement is largest, not only display anomalous trends and a variety of mechanisms, but also raise a major problem of structural inheritance. Detailed field investigation and kinematic analysis indicate that the surface ruptures in the Shihkang-Shangchi fault zone are the result of dip-slip thrusting, occasionally with a minor strike-slip component. The surface ruptures emerge at the surface from bedding-parallel thrusts on both limbs of the regional south-plunging syncline. In the middle part of the syncline, bedding-parallel thrusts are connected by thrusts that cross-cut beds. The surface ruptures also reactivate a NE-SW trending anticline (Diaoshenshan), with west-vergent thrust on the backlimb and east-vergent backthrust on the forelimb. This anticline is undergoing uplift by breaking through the regional N-S trending syncline. Combining GPS measurements, seismological data, and geological analyses, we propose a kinematic model with a 3-D fault surface for the 1999 earthquake in the area of the northern termination. We highlight the influence of both the local and regional structures (bedding parallel slip, pre-existing faults and folds) on the development of the earthquake rupture, and hence the role of the structural inheritance. We conclude that the south-plunging regional Pliocene syncline in fact acts as a slip/strain guide for the northern termination of the Chelungpu thrust. The Pliocene Chinshui Shale, as a major source of weakness within the syncline, has strongly influenced the pattern of slip surface during the 1999 earthquake rupture. We interpret the large vertical displacement along the northern segment as the fault rupture occuring over a surface whose radius of curvature tightens toward the north around the core of the syncline.