Investigating Active Mechanisms of the Tuntzuchiao Fault by Multiple-scaled Reflection Seismic Methods

• Main Authors: Yi-Heng Li, 李奕亨
• Other Authors: Chien-Ying Wang
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/k247ae

博士 === 國立中央大學 === 地球物理研究所 === 97 === The 1935 Hsinchu-Taichung Earthquake (M=7.1) in northwestern Taiwan that induced the greatest damage in the Taiwan’s earthquake history, was marked by a 15 km surface rupture, the Tuntzuchiao Earthquake Fault. Although the present topography dose not indicate much active evidence, the structural variation underneath the Holocene terrace deposit is relatively wild. The Tuntzuchiao Fault is not only a strike-slip faults of pure shear, but also grows with a complex structure developing under the transitional tectonic stress fields. This study provides insights of regional structures and seismic characters near the fault’s shear zone by combining the middle-scale shallow reflection seismic method and the large-scale petroleum seismic survey. After detailed seismic profile analysis, we propose that the Tuntzuchiao Fault is a strike-slip fault which develops as a “positive flower structure” under the compressional condition. The formations near the fault were up-rised and formed an arched zone. Shear stresses resulted in a duplex fault within the deformed area. The fault faded to the southwest, but was greatly deformed when the fault colliding with the hanging-wall of the San-I Fault in its northwestern corner. The Tuntzuchiao fault strongly related with the reactivation of the regional normal fault system. The pre-existing normal faults in northwestern Taiwan developed in neogene and extended into the onshore area. The stress field changed into compression while the Pliocene orogeny occurred. The pre-existing normal faults were reactivated to form a strike-slip fault circumstance under compressional stresses, which developed positive flower structures. The pre-existing “paleo-normal fault” can be identified as a precursor. The reactivation of pre-existing normal faults and stress field transition control the main neotectonic activities in northwestern Taiwan. In the study region, the trajectories of maximum stresses converge toward, and perpendicular to, the front of the Peikang Basement. Model results suggest that the chance of forming large-scale wrench faults is much less than that of forming conjugate strike-slip faults of pure shear. All the evidences including terrace generation, GPS crustal deformation and seismic profiles indicate that the Tuntzuchiao Fault is still quite active and should not be ignored in the earthquake mitigation studies.