Seismogenic structure of 1935 Hsinchu- (MGR=7.1) earthquake, , western 1935 年新竹台中烈震(MGR=7.1)的發震構造

Y.N. Nina Lin; Y.G. Chen; Y.M. Wu (Inst. of Geosciences, NTU); K.M. Yang (Exploration and Development Res. Inst., CPC); Yoko Ota (Yokohama Nat’l Univ., Japan)

Abstract

A large earthquake (MGR 7.1) took place in Miaoli on April 21, 1935 and caused severe damage in surrounding area. The associated surface ruptures daylighted the Tuntzuchiao Fault, trending NE between the Tachia and Taan River, and the Chihhu Fault, a back thrust trending N-S in the Shihtan area. In this study, we try to clarify the structural geometry and further identify the seismogenic structure for this event. The fold-and-thrust belt in Miaoli region is characterized by three major styles of structural interactions of thrusts and folds: the reactivated pre-existing normal faults, the low angle thrust cutting through the shallow strata, and the regional décollement at the base of the sedimentary strata in depth (Yang et al., 1994). The hypocenter of the 1935 mainshock is located right beneath the middle reach of the Taan River. By subsurface geology, we consider that the reactivation of the pre-existing normal fault preserved in the footwall of the Sanyi Fault (low angle thrust faulting, Figs. 1 & 2) may be the seismogenic structure of this earthquake. It currently performs strike-slip in character and extends to the ground surface as the Tuntzuchiao Fault in the south. This reactivated system may have disturbed the strata in the hanging wall of the Sanyi Fault. Structurally, the northern surface rupture, the Chihhu Fault, is a back thrust, which may also be related to the reactivation of the pre-existing normal fault or transfer fault beneath. As we know, preexisting normal fault beneath the décollement is usually a jog to bring about a ramp. In association with a décollement ramp a wedge back thrust will be occasionally developed, such as the case of the Chihhu fault. After the mainshock took place on the MNF, stress might have been transferred northward, triggering the Chihhu wedge back thrust to move. Introduction

On April 21, 1935, a large earthquake with MGR=7.1 (Gutenberg & Richter, 1949; Richter, 1958) took place in Miaoli area (Fig. 1). The epicenter of the mainshock was located to 24.30ºN, 120.75ºE (the middle reach of the Taan River) with a focal depth of 3 km. A strike-slip focal mechanism was determined from first P polarities (Cheng,

1995). Twelve seconds later, a ML=6.0 aftershock (Lin, 1987) took place approximately 40 km to the north of the mainshock. It was located at 24.70ºN, 121.00ºE with a focal depth of 9 km (Cheng, 1995). Two surface ruptures were reported: the Tuntzuchiao Fault (with strike N67ºE, dip 80ºE, and rake 0º) in the south and the Chihhu Fault (with strike N23ºE, dip 50ºW, and rake 90º) in the north of the mainshock epicenter (Sheu et al., 1982). A number of studies have been done on this event (CWB, 1985; Huang, 1992); however, the question of seismogenic structure remains unsolved. In general, a large earthquake is caused by a major fault. In 1935, two separate faults were found with different attitudes and fault plane solutions; thus, it is interesting to understand its seismogenic structure. In this study, we investigate geological data published by Chinese Petroleum Company and integrate other literatures to determine the subsurface structure of the source area and to give a kinematical answer for the question above.

The Tuntzuchiao Fault Judging the published pre-existing normal and transfer faults in this region (Fig. 1), the mainshock epicenter is located at the abrupt bend of the southern segment of the main normal fault (MNF). Here the strike of the MNF turns from N60ºE to N25ºE with direction change about 35º. It can be further extended southwestward and upward to the Tuntzuchiao Fault. Figure 2 shows 3-D geometry of the top surface of Talu Formation and the Sanyi Fault plane. Since the epicenter is located right on the bend of southern segment of the MNF, we suggest that this bend is a geometric singularity as an asperity to generate earthquakes.

The Chihhu Fault In previous studies, the Chihhu Fault is determined as a back thrust dipping to the west with fault width 10~11 km by using leveling and triangulation data (Shue, 1982; Huang, 1992). Using finite-element method, Huang (1992) further modeled the fault plane as a downward-curved surface with dips continuously decreasing from 55º to 30º downward from shallow depth. Based on these results, we proposed a conceptual cross-section to illustrate the subsurface geometry of the fault plane (Fig. 3). Therein, the Chihhu Fault is a wedge back thrust developed above a décollement ramp. This ramp may be formed due to a pre-existing structure. After the mainshock took place in 1935, stress might have been transferred northward and triggered the Chihhu Fault to move.

Conclusions After integrating subsurface geologic data and some published results, we reach these two following conclusions: 1. The seismogenic structure of 1935 Hsinchu-Taichung earthquake may be attributed to the action of the reactivated pre-existing normal fault beneath the Sanyi Fault plane. The abrupt bend of the southern segment of this main normal fault (MNF) forms a geometric singularity and causes earthquake to happen. The Tuntzuchiao Fault is the southwestern ground surface extension of this MNF. 2. We interpret the Chihhu Fault as a wedge back thrust with a ramp developed along a pre-existing structure. During 1935 Hisnchu-Taichung earthquake, stress might have been transferred northward from the mainshock and triggered the Chihhu Fault to move. References CWB (1985) The Symposium of the 1935 Hsinchu-Taichung Earthquake. (in Chinese) , ，232p. CPC (1994) Geological map of Miaoli (1:100,000). Taipei, Chinese Petroleum Company. Cheng, S.N. (1995) The study of stress distribution in and around Taiwan. (in Chinese) Ph. D. Thesis, National Central University, 215p. Huang, B.S. and Yeh, Y.T. (1992) Source geometry and slip distribution of the April 21, 1935 Hsinchu-Taichung, Taiwan earthquake. Tectonophysics 210, 77-90. Hung, J.H. and Wiltschko, D.V. (1993) Structure and kinematics of arcuate thrust faults in the Miaoli-Cholan area of Western Taiwan. Petroleum Geology of Taiwan 28, 59-96. Hung, J.H. (1994) Analysis of deformation fabrics in the Sanyi thrust sheet and the Chuhuangkeng anticline of western Taiwan. Petroleum Geology of Taiwan 29, 105-126. Lin, D.H. (1987) Mechanism of the Hsinchu-Taichung, Taiwan, earthquake of 1935. (in Chinese) M.S. Thesis, National Central University, 88p. Sheu, H.C., Kosuga, M., and Sato, H. (1982) Mechanism and fault model of the Hsinchu-Taichung (Taiwan) earthquake of 1935. (in Japanese) Zisin, Ser. II, Vol. 35, 567-574. Yang, K.M., Wu, J.C., Ting, H.H., Wang, J.B., Chi, W.R., and Kuo, C.L. (1994) Sequential deformation in foothills belt, Hsinchu and Miaoli areas: implications in hydrocarbon accumulation. Petroleum Geology of Taiwan 29, 47-74. Yang, K.M., Ting, H.H., Wu, J.C., and W.R. (1997) Geological model for complex structures and its implications for hydrocarbon exploration in northwestern Taiwan. Petroleum Geology of Taiwan 31, 1-42. Yang, K.M., Wu, J.C., Ting, H.H., Mei, W.W., Tsai, C.T., and Yeh, M.G. (2001) Subsurface geometry of the San-I thrust system, northwestern Taiwan. Symposium of the 2001 Annual Meeting, The Geological Society of China, 15-18.

Figure 1. The tectonic and geomorphic map of the study area. Orange and blue lines: main geologic structures in this area (CPC, 1994). Black lines: the published pre-existing normal faults and transfer faults (Yang et al., 1997; 2001). Yellow line: the main normal fault (MNF). See text for details.

Figure 2. The 3-D model of the Sanyi Fault plane and the top surface of Talu Formation. Original data are from Yang et al. (2001). The solid yellow lines represents the MNF while the dashed part means covered by the Sanyi Fault plane.

Figure 3. The conceptual model of the reactivated structure, the relative ramp and the wedge back thrust (modified from Hung, 1994). See Fig. 1 for location of the cross-section.