1935年新竹臺中烈震（MGR=7.1）之地表變形與發震構造

Abstract

1935年4月21日清晨，苗栗地區發生規模7.1的強震，並由日人命之為「新竹臺中烈震」。該次地震產生屯子腳斷層（TTCF）與紙湖斷層（CHF）兩條地表破裂。前人研究中，曾以三角測量資料進行發震構造的模擬，然其所採用的斷層長度遠超過地表破裂實際長度，部份斷層亦不具地質証據，因此本研究整合許多當前已發表之陸海地下地質資料，同時搭配地震以重建發震構造模型。我們將研究區分為屯子腳、紙湖與濱海三個地區並個別討論之：在屯子腳地區，由空間關係看來，中新世所發育的古正斷層可能是屯子腳斷層的地下延伸，其往東北方於三義—小東勢逆斷層系統下有一走向轉折。當其由正斷層再活動為走向斷層的過程中，此轉折可能形成所謂的「asperity」，進而產生災害性強震。在紙湖地區，1935年餘震重定位的結果顯示，紙湖斷層極可能為一發育於區域滑脫面上的「深座」背衝斷層。在濱海地區，本研究推論斗煥坪斷層（THPF）可能與1935年7月17日的強烈餘震有關，而此斷層向西入海亦可與一反轉之古正斷層相連。我們由以上證據來建立發震構造的基本模型，重新進行三角測量的地表變形模擬，所得之斷層面解亦具較高之彌合度與可信度。結果顯示，古正斷層復活在苗栗地區的新期活動構造扮演關鍵性的角色，其中屯子腳斷層、紙湖斷層與斗煥坪斷層均可能與古正斷層系統有關。由此，一個新期活動構造區塊亦透過此1935年的事件而具體展現。不過，屯子腳斷層與紙湖斷層並未在階地上留下太明顯的累積變形量，尤其屯子腳斷層甚至可能仍未截切較老的三義—小東勢斷層系統，這均指示了相對晚近發育或是活動週期較長的可能性。相對斗煥坪斷層則有累積性的應變，顯示其應為具較長活動史之活動構造。

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.