Structure of the 1999 Chi-Chi earthquake rupture and interaction of thrust faults in the active fold belt of western Taiwan

We summarize the structural characteristics of the surface ruptures of the 1999 Mw 7.6 Chi-Chi earthquake in western Taiwan and discuss the geological interaction of the Chelungpu fault with adjacent faults. Based on geological investigations, seismological analyses, and GPS measurements of surface co-seismic displacements, we describe the regional 3-D fault plane and slip distribution of the Chi-Chi earthquake and compare these to the geological features of the Chelungpu fault. We find that one key feature of the Chelungpu fault is the stratigraphy-controlled slip surface: at the level of the uppermost few kilometers, the Chelungpu fault slip plane generally follows the bedding plane of the Pliocene Chinshui shale. The second key feature of the Chelungpu fault is the difference in structurally geometric configuration between its northern and southern segments. The northern Fengyuan segment shows a bedding-parallel thrust fault within east-dipping strata in both footwall and hanging wall. In contrast, the southern Tsaotun segment exhibits east-dipping strata are overthrust onto flat-lying recent alluvial deposits. These two features not only explain a hinterland imbricate thrusting on the hanging-wall of the Fengyuan segment, but also explain the change in strike of the Chi-Chi surface ruptures at the northern end. The southern end of the 1999 Chi-Chi rupture is interpreted to be linked to a series of NW-trending strike-slip faults. In particular, we propose that the Luliao strike-slip fault served as the lateral ramp of the Chelungpu fault, and the Gukeng strike-slip fault acted as a barrier to end the southern propagation of the 1999 rupture. Geomorphic features and paleoseismological data indicate that the range-front Chelungpu fault has generated large earthquakes during the last several thousand years. Alternatively, in the Miaoli area to the north and the Chiayi area to south, historical earthquakes as well as active geomorphic features are not restricted on the range-front thrust faults. Instead, more complicated structures, including tightly spaced folds, duplex structures, and strike-slip faults are involved in seismogenic processes. A more detailed investigation of regional structural characteristics is needed for mitigation against the seismic hazards in the 300-km-long active fold belt in western Taiwan, where several damaging large earthquakes have been documented during the last century.