HR: 0800h
AN: T11C-1268    [Abstracts]
TI: Paleoseismological Evidence of Large Pre-historical Earthquakes on the Western Haiyuan Fault Near Tianzhu, Gansu Province
AU: * Liu, J
EM: liu@ipgp.jussieu.fr
AF: Tectonique - IPGP, UMR7578 - BP89 4, place Jussieu, Paris, 75010 France
AU: Klinger, Y
EM: klinger@ipgp.jussieu.fr
AF: Tectonique - IPGP, UMR7578 - BP89 4, place Jussieu, Paris, 75010 France
AU: Xu, X
EM: xiweixu@vip.sina.com
AF: Institut of Geology, China earthquake Administration, Beijing, 100029 China
AU: Lasserre, C
EM: lasserre@geologie.ens.fr
AF: Geology dpt - ENS, 24,rue Lhomond, Paris, 75231 France
AU: Chen, G
AF: Institut of Geology, China earthquake Administration, Beijing, 100029 China
AU: Chen, W
AF: Institut of Geology, China earthquake Administration, Beijing, 100029 China
AU: Tapponnier, P
EM: tappon@ipgp.jussieu.fr
AF: Tectonique - IPGP, UMR7578 - BP89 4, place Jussieu, Paris, 75010 France
AB: The Haiyuan fault is a major active left-lateral fault that bounds the Tibetan Plateau to the northeast. Two great earthquakes have ruptured the fault system in the past century: in 1920 (M=8.7) along the eastern Haiyuan fault and in 1927 (M=8-8.3) on a thrust fault system north of the western Haiyuan fault. Studying this active fault has profound implications on the framework of deformation of the Tibetan Plateau, and on the mechanics of continental deformation in general. Previous studies have mainly focused on the slip rate of the fault, which remains debated. Paleoseismological investigations on the fault are sparse and mainly in the reach of the 1920 earthquake rupture, or along the eastern half of the Haiyuan fault. The seismic history of the 260-km-long western Haiyuan fault is largely unknown. Despite clear evidence in the field of Holocene activity, this segment of the fault bears no trace of a large earthquake in the past several centuries. It has therefore been reported as a major seismic gap (the `Tianzhu gap') with high potential seismic hazard. We have opened two trenches in a small pull-apart basin along the fault near longitude 103ø30', north of Songshan in the Gansu province. The trenches are about 3 m deep, 4-5 m wide, and 20-30 m long. The excavation exposes sedimentary layers of alternating colors: black and yellowish white. The black layers are composed of organic-rich silty to clayey deposit, and yellowish white layers of coarser-grained sandy deposit. The main fault zone is readily recognizable in the exposures, by the disruption and tilting of the layers. The stratigraphy shows no obvious hiatus in deposition. 5-6 paleoseismic events are identified in the 2-3 m deposit below the ground surface. Charcoal is abundant in exposures and has been sampled. The initial batch of 10 samples has been selected for C14 dating, currently in process. The dates will help us to quantify the frequency of large ground-breaking earthquakes. If combined with the amounts of slip-per-event of these events, it will shed light on the slip rate in the last several millenniums.
DE: 9320 Asia
DE: 7221 Paleoseismology
DE: 7230 Seismicity and seismotectonics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting