Structural geology of a classic thrust belt earthquake: the 1999 Chi-Chi earthquake Taiwan (Mw=7.6)

We document the structural context of the 1999 Chi-Chi earthquake (Mw=7.6) in western Taiwan, which is one of the best-instrumented thrust-belt earthquakes. The main surface break and large slip (3–10 m) is on two segments of the shallow otherwise aseismic bedding-parallel Chelungpu–Sanyi thrust system, which shows nearly classic ramp-flat geometry with shallow detachments (1–6 km) in the Pliocene Chinshui Shale and Mio-Pliocene Kueichulin/Tungkeng Formations. However, rupture is complex, involving at least six faults, including a previously unknown deeper thrust (8–10 km) on which the rupture began. We compare the coseismic displacements with a new 3D map of the Chelungpu–Sanyi system. The displacements are spatially and temporally heterogeneous and well correlated with discrete geometric segments of the 3D shape of the fault system. Geodetic displacement vectors are statistically parallel to the nearest adjacent fault segment and are parallel to large-scale oblique fault corrugations. The displacement magnitudes are heterogeneous at several scales, which requires in the long term other non-Chi-Chi events or significant aseismic deformation. The Chelungpu thrust has a total displacement of ∼14 km but the area of largest Chi-Chi slip (∼10 m) is on a newly propagated North Chelungpu Chinshui detachment (∼0.3 km total slip) which shows abnormally smooth rupture dynamics.