1) Introduction

The Chelungpu thrust is one of the most important active faults in the western foothills of central Taiwan, which by preliminary estimate consumes together with the adjacent Changhua thrust ~45% of the ~80mm/y plate convergence rate between the Philippine Sea Plate and the Eurasian Plate. Its activity is well documented by the ~80km long surface rupture of the 1999 Magnitude 7.6 Chi-Chi earthquake. In addition, there was substantial coseismic fold growth of the Neiwan/Tungshi anticline within the thrust sheet above fault-bends and at the northern termination of the rupture. This project is focused on the northern termination of the Chelungpu thrust, where the N-S striking surface rupture of the 1999 earthquake merges into a NE-SW to E-W trending surface fold. The project consists of two parts. In a first step, the long-term fault-slip rate of the Chelungpu thrust system is studied. The area investigated for this purpose is Hsinshe, which is located in the hanging wall of the northern section of the Chelungpu thrust. Three distinct levels of fluvial terraces deposited by the northward running Ta Chia river are discernible in the area. The highest and supposedly oldest terrace has been OSL dated to 55.0+/-2.6 ka (Chen et al. 2003). All three terraces show westward tilting and are displaced vertically up to 100 m by a N-S trending fold scarp. The highest and oldest terrace shows the greatest amount of deformation whereas the lowermost and supposedly youngest terrace is only slightly deformed. The deformation is caused by the underlying thrust geometry, which ramps from ~3km depth in the east of the study area to the surface ~5km west of Hsinshe (Yue et al. 2005). Motion of the thrust sheet over these fault bends produces a hanging wall syncline with a northward bifurcating axial surface (Lai et al. 2006). These axial surfaces are the cause of the large-scale kink-bands and displacements in the terraces. The aim of the ongoing study is a detailed understanding of the deformation history of the Chelungpu thrust in this area by combining field data, subsurface data and geochronological data. We therefore are dating the different terrace levels using OSL and 14C methods. The different deformation amount of the individual terrace levels together with the ages will thus allow us to determine the long-term fault-slip rate of the Chelungpu thrust system over a ~55 ka timescale. The dating of the Hsinshe terraces is a first step in establishing the long-term slip history and rate (last 50-110 ka) of the various active thrusts of western Taiwan (including the Changhua, Chelungpu and Shuangtung thrusts). This is important to understand the long-term kinematics of active thrust belts and for assessing the seismic hazards of these active faults. The post-earthquake river incision observed along Ta An and Ta Chia rivers suggests that terrace dating is an appropriate method of dating earthquakes. The second part of this project concentrates on the very recent history of the Chelungpu thrust, focusing on the co-seismic folding during the 1999 Chi-Chi earthquake, which occurred in the Neiwan/Tungshi anticline in the area of Cholan and the Ta An river. The Neiwan/Tungshi anticline is likely the best-constrained case of large coseismic folding in the world and needs in-depth study. Only preliminary work has been done so far. It is now recognized that fault related folds grow by slip on faults, especially in large earthquakes. This is best studied in areas where there is subsurface control from petroleum seismic data, in addition to good surface exposures and deformed geomorphic surfaces. In the case of the Neiwan/Tungshi anticline we not only have excellent data from theses three mentioned fields, but also observe ~10m coseismic fold growth during the Chi-Chi earthquake. Only a few other examples of coseismic fold growth are known worldwide in the last decades: e.g. 1980 Magnitude 7.3 El Asnam in Algeria (King & Vita-Finzi 1981), or the 1983 Magnitude 6.5 Coalinga earthquake under Kettleman Hills anticline, California (Stein & King 1984). Both these earthquakes occurred before the current era of excellent geodetic control and GPS surveying technology. The Chi-Chi is by far the best-instrumented earthquake. The coincidence of recent large earthquakes, the availability of subsurface petroleum data, and excellent geomorphic, geodetic and seismological control is extremely rare. Hence, this is a great opportunity for Taiwan. A detailed understanding of the coseismic folding mechanisms as well as of the amplitude of folding and the amount of surface uplift in the area are the final aims of this part. Our results will also give important support for studies that are beginning on the river incision event that is taking place in the nine years since the earthquake (e.g. the formation of the canyon south of Cholan). Additionally, this work will be important worldwide for establishing the still underestimated risks of coseismic fold growth. For example, many active fold scarps are known within the city of Los Angeles/USA, but their seismic hazard is not yet fully assessed. The Chi-Chi earthquake is the first historic instance of coseismic fold growth (fold-scarp growth). We are going to document the damage, injury and loss of life in the coseismic folding. In this sense, these studies hold importance both from a basic science (fault-related fold growth and post-earthquake incision event) and an earthquake hazard point of view, and will help establishing Taiwan as a center for studies of active structural processes.

2) Data acquisition

For both parts of the project, a great amount of data has already been collected from literature and reports, geological maps (Central Geological Survey (CGS) maps and Chinese Petroleum Corporation Taiwan (CPC) maps), digital elevation models, pre- and after-Chi-Chi aerial photographs, and numerous CPC seismic profiles, well data and shot point tracks. The data acquisition is intended to be continued and expanded onto damage reports of affected townships (Fengyuan, Cholan, Neiwan, …).

3) Field work

a) Structural fieldwork

Structural fieldwork was carried out along the Ta An and the Ta Chia rivers, in order to collect dip and strike data from outcrops, which formed in the last 9 years due to the river incisions following the 1999 co-seismic surface uplift. These data together with data from the CPC seismic profiles are intended to be incorporated into the construction of geologic cross-sections, in order to get a better knowledge of the deeper structures, especially the Neiwan Anticline. In the course of the construction of these profiles it will emerge whether further structural fieldwork is needed.

b) Sample collections for OSL dating

The luminescence dating method provides a possibility for direct dating of the depositional event of sediments using their constituent minerals quartz and feldspar. This method is an ideal tool for dating the terraces at Hsinshe. Two field inspections of the Hsinshe terraces gave insight of the possibility of sample collections for OSL dating. One sample has been already taken from a sand lens in a natural outcrop. As further suitable outcrops are lacking, we intend to dig trenches for the purpose of sampling for OSL (and possibly also for 14C dating if charcoal material is encountered). Various appropriate trench locations have been selected. After the acquisition of trenching permits, we intend to dig 3 to 6 trenches.

c) RTK measurements and laser range-finding surveying

The method of real time kinematics (RTK) is used to gain high precision surface profiles, from which the amount of vertical uplift due to the 1999 earthquake can be measured by comparing the RTK data with available CPC shot point tracks. First quality tests have been carried out and yielded satisfying results. Several CPC shot point tracks are planned to be remeasured using RTK. Deformed terraces will also be measured by means of laser range-finding surveys.

4) Lab work

An introduction into the OSL method (theory and lab work) was given by research colleagues of our group at the National Taiwan University. The first OSL sample, which was collected recently, is being processed at the moment. After sampling the terraces at Hsinshe, an intense time of lab work is planned in order to date the age of the deposition of the material.

5) Collaborations

Various collaborations were initiated. Intense co-operations exist with members of the Department of Geosciences at the National Taiwan University (faculty members as well as PhD and master students). A close collaboration with various scientists at CPC was established. Numerous visits to Miaoli to the CPC Exploration & Development Research Institute as well as to the CPC Exploration & Production Business Division were paid, during which data collection and scientific discussions were carried out. This close co-operation is planned to continue, especially concerning the construction of geological profiles using CPC seismic data. The applicant is invited to give a guest lecture at CPC end of June 2008.

References:

Chen, Y.G., Chen, Y.W., Chen, W.S., Zhang, J.F., Zhao, H., Zhou, L.P., Li, S.H., 2003. Preliminary results of long-term slip rates of 1999 earthquake fault by luminescence and radiocarbon dating. Quaternary Science Reviews 22, 1213–1221.

King, G.C.P. & Vita-Finzi, C. 1981: Active folding in the Algerian earthquake of 10 October 1980. Nature 292, 22 – 26; doi:10.1038/292022a0.

Lai, K.Y., Chen, Y.G., Hung, J.H., Suppe, J., Yue, L.F., Chen, Y.W. 2006. Surface deformation related to kink-folding above an active fault: Evidence from geomorphic features and co-seismic slips. Quaternary International 147, 44–54.

Stein, R.S. & King, G.C.P. 1984: Seismic Potential Revealed by Surface Folding: 1983 Coalinga, California, Earthquake. Science 224/4651, 869 – 872, DOI: 10.1126/science.224.4651.869.

Yue, L.F., Suppe, J., Hung, J.H. 2005. Structural geology of a classic thrust belt earthquake: the 1999 Chi-Chi earthquake Taiwan (Mw 7.6). Journal of Structural Geology 27, 2058–2083.