This study is aimed at constructing a 3D subsurface geometry of the Chelungpu thrust and its associated structures, as well as examining the implications of the studies results for active tectonics in the area. Nine balanced cross-sections were constructed across the foothills belt in the study area to delineate the subsurface geometry of the major thrusts in the foreland of the fold-and-thrust belt.

The Chelungpu thrust cuts down to the subsurface invariably along the base of the Chinshui Shale and is merged with the Changhua thrust into a common decollement at a depth of 5 to 7 kilometers below the sea level. There is a pre-existing normal fault underneath the common decollement of the Changhua and Chelungpu thrusts which accommodates the thickened strata in the hanging wall of the Chelungpu thrust.

The restored cross-sections indicate that during its propagation toward the foreland the Chelungpu thrust originally was a low-angle thrust before it met a pre-existing high-angle normal fault, which was then reactivated and became the frontal ramp of the thrust. In the latest stage, displacement along the Changhua thrust left the normal fault behind and kept it underneath the common decollement.

The subsurface geometry of the Chelungpu thrust is a uniform curved plane striking N-S, with some local wavy features and a ramp striking E-W in the northern part of the thrust. To the north of the ramp, the fault plane transforms into a spoon-shaped geometry. In the southern part of the study area, the southern end of the Chelungpu thrust is cut off, and its displacement is transferred into a splay thrust that strikes NE-SW and connects the Chelungpu and Shuangtung-Hsiaomao thrusts.

At the hypocenter of the Chi-Chi earthquake, not only is the dip angle of the décollement of the Chelungpu thrust gentler, but the depth much shallower than that of the mainshock. As the hypocenter of the mainshock is very close to the pre-existing normal fault underneath the décollement, a connection between them is highly implied. We also suggest that the ramp in the northern part of the Chelungpu thrust provides a stronger strain guide during the subsurface rupture propagation thereby creating the bended surface rupture.