1935年新竹臺中烈震（MGR=7.1）之地表變形與發震構造

Abstract

On April 21, 1935, a large earthquake (MGR 7.1) took place in Miaoli, northwestern Taiwan. The associated surface ruptures rocked the Tuntzuchiao Fault (TTCF) and the Chihhu Fault (CHF) in the south and north respectively. Numerical modeling using triangulation data has been done in the past, but the adopted faults either were over-sized or lacked geologic evidence. This study evaluates published on-land and off-shore subsurface geologic data as well as seismicities altogether in order to rebuild the structure model. Three sub-areas are discussed. They are, respectively: the TTCF area, the CHF area and the coastal area. In the TTCF area, a pre-existing normal fault is considered as the subsurface extension of the TTCF. It extends northeastward beneath the Sanyi-Hsiaotungshi Thrust System and runs into a change in strike direction. During the reactivation from a normal fault to a strike slip fault, this geometric singularity may be an asperity and cause large earthquakes to happen, such as the 1935 event. In the CHF area, the relocated aftershocks of 1935 show that the CHF may be a NW-dipping and deep-seated back-thrust, which was possibly generated through reverse movement detached along a fault ramp on the regional decollement. In the coastal area, one of the offshore inverted normal faults may connect to the dextral Touhuanping Fault (THPF) and be responsible for the July 17th aftershock. Incorporating all these evidence, a new geologically-based model is reconstructed and the result shows a better fit than before. Our result reveals that reactivation of pre-existing normal fault controls the neotectonic activities in the Miaoli area. This neotectonic domain is actually wrapped by the TTCF, CHF, and THPF. However, the TTCF and CHF may have a relatively short history of activity due to lack of obvious terrace deformation, while the activity of the THPF may be longer by showing cumulative strain. The TTCF may not even have truncated the older Sanyi-Hsiaotungshi Thrust System around the middle section of the Taan River.