Seismogenic Structure of the Chiayi-Tainan Area and the Long-term Slip Rates of Frontal Thrusts in Southwestern Taiwan
|Keywords:||發震構造;滑移速率;褶皺逆衝斷層帶;平衡剖面;層序地層;seismogenic structure;slip rate;fold-and-thrust belt;balanced cross-section;sequence stratigraphy||Issue Date:||2007||Abstract:||
分析晚第四紀岩層的垂直變形及近期大地測量的位移型態，嘉南地區造山帶前緣的斷層具有較高的活動潛能。在嘉義區塊為嘉義盲斷層及九芎坑斷層，台南區塊為新營盲斷層及六甲斷層。透過平原區鑽井資料的層序地層分析及定年資料顯示，新營盲斷層開始活動的年代為39,245±215 yr BP，其長期的滑移速率為2.4±0.6 mm/yr，而六甲斷層的長期的滑移速率為14.3±0.3 mm/yr。整合發震構造及長期滑移速率的計算，可推估新營盲斷層發生大地震的規模約為7.26 (Mw)，再現週期約為742年；六甲斷層大地震的規模約為7.01 (Mw)，再現週期約為100年。
Taiwan, the young orogen that resulted from the collision of the Luzon arc and the Eurasian continental margin, exhibits strong seismic activity. Since the 1999 Chi-Chi earthquake, the government has conducted many projects on active fault study in Taiwan, documenting more than fifty active faults around the island. Recently, a neotectonic map of Taiwan was developed, showing major active deformation belts around the island and 11 domains that are distinguished by geomorphic markers, geodetic and seismologic data. A case study of the 1999 Chi-Chi earthquake shows that the frontal thrust belt of central Taiwan exhibits as a imbricate thrust system that is locked and rooted into an aseismic décollement creeping at about 27~42 mm/yr. In addition, based on foreland basin retrieval and river terraces investigation, the sum of the frontal active Changhua blind thrust and the Chelungpu thrust was found to have about ~32 mm/yr of long-term shortening rates. These results indicated that there is a highly crustal strain accommodation at the frontal thrust belt where disastrous earthquakes would occur in near future.
Similar to that of the Central Taiwan, the Chiayi-Tainan area also has a locking frontal thrust belt, but the traces of active faults in this area are mainly less than 20 km, such as the Chiunchungkeng fault, the Muchiliou fault and the Liouchia fault. Indeed, the Mesozoic basement (the so-called Peikeng High) is underneath the Chiayi area at a shllow depth of nearly ~ 2 km, and suddenly drops deeper to ~ 6 km southward. A pre-existing normal fault, the Yichu Fault, a major boundary trends ENE and serves as that accounts for the separation of the basement. Given these observations, several structural models have been proposed for the frontal thrust belt in the Chiayi-Tainan area, such as a shallow décollement of a thin-skinned collision model, a deep décollement of a thick-skinned collision model, and an inversion of pre-existing normal faults model. As the subsurface geometry of the faults plays a key role in future earthquake hazard assessment, the first chapter of this thesis uses geological and geophysical data from both the Western Foothills and the Coastal Plain to reconstruct the fault geometry underneath the Chiayi-Tainan area.
Chapter two of this thesis focuses on the morphotectonic division in the Chiayi-Tainan area by drainage pattern analysis and Holocene vertical movement rates measurement. Analysis of borehole data and drainage patterns of the southwestern part of the Coastal Plain indicate that is the Taiwan orogenic belt. This is interpreted as being caused by a NNE-trending blind thrust beneath the Coastal Plain, and it represents the deformation front of the Taiwan fold-and-thrust belt. The deformation front is thus located between the plain area and tilted tableland area of western Taiwan, westwards of where it had previously been defined. The frontal thrust is proposed to be segmented by several E-W trending strike-slip faults. For example, the Kukeng fault and the Hsinhua fault bound the Chaiyi block to the north and the south respectively, delineating a segment of the fold-and-thrust belt in southwestern Taiwan.
Chapter three of this thesis presents the evaluation of slip rates of two frontal thrusts, namely the Hsinyin blind thrust underneath the Coastal Plain and the Liouchia fault along the mountain front. The calculation of the slip rates is based on the borehole data and subsurface geometry of the faults that reconstructed in chapter one. These results indicate that the two faults have about ~18 mm/yr long-term shortening rates regarding the arc-continental margin collision. As the highly crustal stain has been accommodated in the frontal orogenic belt, the final part of this thesis contains present the implications for earthquake hazards in the study area.
The following is the list of chapters
Chapter 1 A basement impinged frontal orogenic belt of southwestern Taiwan and its implication for the subsurface seismogenic structure
Chapter 2 Active deformation front delineated by drainage pattern analysis and vertical movement rates, southwestern Coastal Plain of Taiwan
Chapter 3 Timing and slip rates of the frontal thrusts from late Pleistocene-Holocene borehole data analysis, southwestern Taiwan
|Appears in Collections:||地質科學系|
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