SGER: Geomorphic Response to the September1999 Taiwan Earthquake: Evolution of River Profiles Across the Scarp and Inference about Earthquake Recurrence Intervals

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0000893Dietrich Tectonic forcing of the evolution of mountainous landscapes is widely thought to occur through the upstream propagation through the river network of knickpoints created by faulting along range boundaries. The September 21, 1999, magnitude 7.6 earthquake in Taiwan offers a rare opportunity to study how the signal of tectonic uplift is transmitted through an evolving landscape. The 60 km long surface rupture, along the base of the western foothills of the Central Range, created waterfalls from 2 to 8 m high across several major rivers. In this initial exploratory phase of our work PIs will survey the initial scarp profiles and channel geometry, measure bed sediment size distributions, collect bedrock samples for strength and abrasion resistance testing, and document geomorphic features from past uplift events. They anticipate rapid evolution of the channel-crossing fault scarps because of the weak underlying mudstones and sandstones, the high annual rainfall and high coarse sediment load. They will use the results of the field surveys and laboratory testing in a numerical model of bedrock channel incision, to predict the rate and style of upstream knickpoint propagation. In subsequent field campaigns they will re-survey the evolving fault scarps and channel conditions to compare with model predictions. In addition to providing insight into how an uplift event propagates through a landscape, this work may help constrain the recurrence interval for earthquakes of this magnitude, information of considerable practical importance to engineers studying this disaster.