Seismogenic Structure of the Chiayi-Tainan Area and the Long-term Slip Rates of Frontal Thrusts in Southwestern Taiwan
Date Issued
2007
Date
2007
Author(s)
Yang, Chih-Cheng
DOI
en-US
Abstract
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
Subjects
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
SDGs
Type
thesis
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