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A Vertical Exposure of the 1999 Surface Rupture of the Chelungpu Fault at Wufeng, Western Taiwan: Structural and Paleoseismic Implications for an Active Thrust Fault

Lee, Jian-Cheng and Chen, Yue-Gau and Sieh, Kerry and Mueller, Karl and Chen, Wen-Shan and Chu, Hao-Tsu and Chan, Yu-Chang and Rubin, Charles and Yeats, Robert (2001) A Vertical Exposure of the 1999 Surface Rupture of the Chelungpu Fault at Wufeng, Western Taiwan: Structural and Paleoseismic Implications for an Active Thrust Fault. Bulletin of the Seismological Society of America, 91 (5). pp. 914-929. ISSN 0037-1106. doi:10.1785/0120000742.

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We mapped and analyzed two vertical exposures—exposed on the walls of a 3- to 5-m-deep, 70-m-long excavation and a smaller 3-m-deep, 10-m-long excavation—across the 1999 rupture of the Chelungpu fault. The primary exposure revealed a broad anticlinal fold with a 2.5-m-high west-facing principal thrust scarp contained in fluvial cobbly gravel beds and overlying fine-grained overbank deposits. Sequential restoration of the principal rupture requires initial failure on the basal, east-dipping thrust plane, followed by wedge thrusting and pop-up of an overlying symmetrical anticline between two opposing secondary thrust faults. Net vertical offset is about 2.2 m across the principal fault zone. From line-length changes, we estimate about 3.3 m of horizontal shortening normal to fault strike. The ratio of these values yields a total slip of 4.0 m and an estimate of about 34° for the dip of the fault plane below the excavation. This value is nearly the same as the 35° average dip of the fault plane from the surface to the hypocenter. Restoration of the exposed gravelly strata and adjacent overbank sediments deposited prior to the 1999 event around the principal rupture suggests the possible existence of a prior event. A buried 30-m-wide anticlinal warp beneath the uplifted crest of the 1999 event is associated with three buried reverse faults that we interpret as evidence for an earlier episode of folding and faulting in the site. The prior event is also recorded in the smaller excavation, which is located 40 m south and is oriented parallel to the larger excavation. Radiocarbon dating of samples within the exposed section did not place tight constraints on the date of the previous event. Available data are interpreted as indicating that the previous event occurred before the deposition of the less than 200 ^(14)C yr B.P. overbank sands and after the deposition of the much older fluvial gravels. We interpret the previous event as the penultimate event relative to the 1999 Chi-Chi earthquake. We estimated the long-term slip rate of the Chelungpu fault to be 10-15 mm/yr during the last 1 Ma, based on previously published retrodeformable cross sections. This rate is, however, significantly higher than geodetic rates of shortening across the Chelungpu thrust where two pairs of permanent Global Positioning System stations suggest 7-10 mm/yr of shortening across the fault. Given the 4 m of average slip, the long-term slip rate yields an interseismic interval of between 267 and 400 yr for the Chelungpu fault.

Item Type:Article
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URLURL TypeDescription
Chen, Yue-Gau0000-0002-8693-583X
Sieh, Kerry0000-0002-7311-2447
Mueller, Karl0000-0002-3516-2259
Additional Information:© 2001 Seismological Society of America. Manuscript received 1 March 2001. This study was supported by Institute of Earth Sciences, Academia Sinica, National Taiwan University, Central Geological Survey, and National Science Council Grants NSC88-2116-M001-037 and NSC89-2116-M001-037. Helpful reviews and comments by P. Knuepfer and an anonymous reviewer greatly improved the manuscript. This is a contribution of the Institute of Earth Sciences, Academia Sinica, IESEP2001-013, and of Caltech’s Seismological Laboratory 8829.
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Institute of Earth SciencesUNSPECIFIED
Academia SinicaUNSPECIFIED
National Taiwan UniversityUNSPECIFIED
Central Geological SurveyUNSPECIFIED
National Science CouncilNSC88-2116-M001-037
National Science CouncilNSC89-2116-M001-037
Academia SinicaIESEP2001-013
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Caltech Division of Geological and Planetary Sciences8829
Issue or Number:5
Record Number:CaltechAUTHORS:20140801-142629035
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Official Citation:Jian-Cheng Lee, Yue-Gau Chen, Kerry Sieh, Karl Mueller, Wen-Shan Chen, Hao-Tsu Chu, Yu-Chang Chan, Charles Rubin, and Robert Yeats A Vertical Exposure of the 1999 Surface Rupture of the Chelungpu Fault at Wufeng, Western Taiwan: Structural and Paleoseismic Implications for an Active Thrust Fault Bulletin of the Seismological Society of America, October 2001, v. 91, p. 914-929, doi:10.1785/0120000742
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:47828
Deposited By: Ruth Sustaita
Deposited On:01 Aug 2014 21:49
Last Modified:10 Nov 2021 17:50

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