A late Miocene to Pleistocene, shallow marine succession of 2003 m in measured depth was drilled and cored at the TCDP (Taiwan Chelungpufault Drilling Project)-A borehole. We established a lithostratigraphic column for the TCDP-A well and correlated the well-bore rock succession to its surface equivalents and rock successions drilled at nearby oil exploration wells. Our results find that the lithostratigraphy for the TCDP-A borehole is as follows (numbers are in measured depth with reference to wireline logs): (1) 0 - 1013 m: lower Cholan Formation; (2) 1013 - 1300 m: Chinshui Shale; (3) 1300 - 1707 m: Kueichulin Formation; (4) 1707 - 2003 m: Cholan Formation, a repeated formation in the footwall of the Sanyi Fault. Our data show that the Chinshui Shale and Kueichulin Formation are not exposed to the west of the TCDP-A well and the Chi-Chi surface rupture occurs near the base of the Cholan Formation. At TCDP-A borehole the most likely Chi-Chi rupture lies at 1111 m in the Chinshui Shale while its surface equivalent occurs near the base of the Cholan Formation, thereby indicating that the Chi-Chi rupture cuts stratigraphic upsection of some 170 meters in the direction offaulting transport.

Shallow drilling prior to the Chi-Chi earthquake at where later the Chi-Chi rupture had occurred indicates that the Chi-Chi rupture occurs on a pre-existing fault plane. Another shallow borehole located some 240 m to the west of the Chi-Chi surface rupture encountered a previously unknown thrust fault. The hangingwall material of this fault is intensively weathered in contrast to the fresh hangingwall rocks of the Chi-Chi rupture encountered further east noted above. The extent of weathering in the above two fault zones indicates the thrust fault encountered at the west is older than the currently active Chi-Chi rupture in the east. Noting that the Sanyi fault is previously reported as inactive since late Pleistocene above observations suggest a hinterlandward (i.e., eastward) migration of splay faults within the thrust-sheet bounding fault zone.