HR: 17:15h
AN: H34A-06    [Abstracts]
TI: Evolution of fault scarp knickpoints following 1999 Chi-Chi earthquake in West-Central Taiwan
AU: * Sklar, L S
EM: leonard@sfsu.edu
AF: San Francisco State University, Department of Geosciences, San Francisco, CA 94132 United States
AU: Stock, J D
H34A-06 AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Roering, J J
H34A-06 AF: University of Oregon, Department of Geological Sciences, Eugene, OR 97403 United States
AU: Kirchner, J W
H34A-06 AF: University of California, Berkeley, Department of Earth and Planetary Science, Berkeley, CA 94120 United States
AU: Dietrich, W E
H34A-06 AF: University of California, Berkeley, Department of Earth and Planetary Science, Berkeley, CA 94120 United States
AU: Chi, W
H34A-06 AF: Academia Sinica, Institute of Earth Sciences, Taipei, 115 Taiwan
AU: Hsu, L
H34A-06 AF: University of California, Berkeley, Department of Earth and Planetary Science, Berkeley, CA 94120 United States
AU: Hsieh, M
H34A-06 AF: National Taiwan University, Department of Geosciences, Taipei, 106 Taiwan
AU: Tsao, S
H34A-06 AF: Central Geological Survey of Taiwan, MOEA POBOX 968, Taipei, 235 Taiwan
AU: Chen, M
H34A-06 AF: Central Geological Survey of Taiwan, MOEA POBOX 968, Taipei, 235 Taiwan
AB: The 1999 Chi-Chi earthquake in West-Central Taiwan caused vertical offset of 2-8 meters along the Chelungpu fault, creating numerous knickpoints on rivers draining the Western Foothills Range. This event offers an excellent opportunity to study the erosional mechanisms which control the style and rate of knickpoint migration and the role of sediment in mediating landscape response to tectonic forcing along fault boundaries. Here we report results from three field surveying campaigns four, 17 and 41 months after the coseismic surface rupture. We focus on seven channels spanning a range of offset magnitude, drainage area and degree of alluviation. Erosional response was rapid due to the weak underlying mudstone and several typhoons. Plunge pool scour and block toppling, the principal fluvial erosional processes which tend to preserve and translate the scarp morphology upstream, are enhanced by large vertical offset, low sediment load and wide joint spacing. In contrast, bedrock wear by bedload abrasion, hydraulic plucking and hydration fracturing result in scarp destruction by notching and upstream diffusion of the tectonic signal, and are favored by high sediment load, low vertical offset and weaker rock. The most rapid upstream propagation of the earthquake-induced pulse of accelerated bedrock erosion occurred on channels where a thin pre-rupture alluvial mantle was rapidly stripped for several hundred meters upstream exposing the bedrock to extensive wear. Alluvial stripping upstream of the knickpoints may be due to flow acceleration approaching the free fall over the scarp, and to a reduction in the effective friction angle as the wave of sediment removal sweeps upstream. In some cases, incision upstream of the scarps created narrow inner channels leaving behind remnants of the pre-rupture bedrock bed as nascent strath surfaces. Where alluvial cover thickness was equal or greater than scarp offset, knickpoints diffused symmetrically by progradation of a downstream deposit and upstream migration of an alluvial headcut. These results suggest that the rate of coarse sediment supply to the river network may strongly influences the rate of upstream migration of the signal of tectonic uplift and thus affects the time scale of landscape adjustment to changes in rates of rock uplift.
DE: 1815 Erosion
DE: 1825 Geomorphology: fluvial (1625)
DE: 1856 River channels (0483, 0744)
DE: 1862 Sediment transport (4558)
DE: 8175 Tectonics and landscape evolution
SC: Hydrology [H]
MN: Fall Meeting 2005