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In tectonically active mountain belts, earthquake-triggered landslides deliver large amounts of sediment to rivers. We quantify the geomorphic impact of the 1999 [M.sub.w] 7.6 Chi-Chi earthquake in Taiwan, which triggered >20,000 landslides. Coseismic weakening of substrate material caused increased landsliding during subsequent typhoons. Most coseismic landslides remained confined to hillslopes. Downslope transport of sediment into the channel network occurred during later storms. The sequential processes have led to a factor-of-four increase in unit sediment concentration in rivers draining the epicentral area and increased the magnitude and frequency of hyperpycnal sediment delivery to the ocean. Four years after the earthquake, rates of hillslope mass wasting remain elevated in the epicentral area. Keywords: earthquakes, landslides, typhoons, sediment supply, fluvial erosion rates, Taiwan.

Author(s): Simon J. Dadson (corresponding author) [1]; Niels Hovius [1]; Hongey Chen [2]; W. Brian Dade [4]; Meng-Long Hsieh [2]; Sean D. Willett [5]; Jyr-Ching Hu [2]; Ming-Jame Horng [6]; [...]

The authors demonstrate that a simple geomorphic model can be based on the parameterization measure they call 'channelization' and resolution-invariant topographic measures. Research with this model suggests channel disequilibrium significantly affects mountain landscape dynamics.

The emergence of an orogen is generally characterized by an early phase of rapid surface uplift and the concomitant evolution of montane topography, leading in some cases to a steady state in which tectonic mass flux is balanced by erosion. This early phase of mountain growth is exemplified in the Finisterre Mountains of Papua New Guinea, a propagating growth fold whose catchments can be observed at a range of stages in their temporal evolution. Watersheds appear to initiate by isolated gorge incision, to expand by large-scale landsliding in a manner controlled by ground-water seepage, and to entrench by fluvial incision of landslide scars and deposits. Once a montane system of ridges and valleys is established, only rare, major landslides can modify the drainage pattern. The steady-state morphology of a mountain belt is therefore intimately related to its initial phase of growth.

In humid uplands landsliding is the dominant mass wasting process. In the western Southern Alps of New Zealand landslides are scale invariant and have a power-law magnitude frequency distribution. Independent studies from other regions suggest that this is a general property of landsliding. This observation is of critical importance to the evaluation of the impact of events of different length scales over different time intervals on landscape evolution. It is particularly useful when estimating regional geomorphic rates, because it constrains the frequency and overall significance of extreme events, which cannot otherwise be evaluated. By integrating the complete response of the system, we estimate the regional denudation rate due to landsliding to be 9 [+ or -] 4 mm [yr.sup.-1]. Sediment discharge from the western Southern Alps is dominated by landslide-derived material.