Deciphering cumulative fault slip vectors from fold scarps: Relationships between long-term and coseismic deformations in central Western Taiwan

We document the 30 ka cumulative slip history and long-termslip vector azimuth on the northern Chelungpu fault based on a series of fault-bend folded alluvial terraces and draw quantitative relationships between geological structure, deformation observed from the geomorphology, and coseismic displacements during the 1999 M-w = 7.6 Chi-Chi earthquake. In our study area, three main terrace levels show progressive folding by kink band migration in relation to the underlying fault geometry, forming a main N-S fold scarp up to similar to 193 m high and secondary E-W scarps. Detailed analysis using 5 m resolution digital elevation model allows us to characterize the scarp morphology and quantify the deformation parameters, namely, terrace heights, fold scarp relief, and fold limb width and slope angle. The 3-D deformation of the highest terrace, dated by optically stimulated luminescence at 30.2 +/- 4.0 ka, enables to simultaneously determine amplitude and azimuth of the long-term slip vector based on scarp relief. The long-term slip vector, oriented N338 degrees +/- 6 degrees, is found to parallel the Chi-Chi coseismic displacements in this area. Cumulative slip and dating results yield a constant slip rate of 17.7 +/- 2.2mm/a in the direction N338 degrees +/- 6 degrees. Late Quaternary shortening rates observed at four sites vary along strike in a similar way to Chi-Chi coseismic displacements. Together with the collinearity of long-term and coseismic slip vectors at our study site, this suggests that Chi-Chi earthquake is a characteristic earthquake for the Chelungpu thrust with recurrence interval similar to 470 years. We also discuss implications for the regional and long-term distribution of shortening in the central Western Foothills.