Slip rates on the Chelungpu and Chushiang thrust faults inferred from a deformed strath terrace along the Dungpuna river, West central Taïwan

International audienceThe Chelungpu fault produced the September 1999 Mw=7.6 ChiChi earthquake, central Taiwan. The shortening rate accommodated by this structure, integrated over several seismic cycles, as well as its contribution to crustal shortening across the Taiwanese range has remained unresolved. To address this issue, we focus our study on the Chelungpu and Chushiang thrust faults within the southernmost portion of the ChiChi rupture area. Structural measurements and available seismic profiles are used to infer subsurface geometry of structures. The Chushiang and Chelungpu faults appear as two splay faults branching onto a common ramp that further north connects only to the Chelungpu surface trace. We survey a deformed strath terrace along the Dungpuna river, buried under a 11,540 +/- 309 cal yr old fill deposit. Given this age, the dip angles of the faults and the vertical throw determined from the offset of the strath across the surface fault traces, we estimate slip rates of 12.9 +/- 4.8 mm/yr and 2.9 +/- 1.6 mm/yr on the Chelungpu and Chushiang faults respectively. These yield a total shortening rate of 15.8 +/- 5.1 mm/yr to be absorbed on the common 2 decollement at depth within our study area. This total value is an upper bound on the slip rate on the Chelungpu fault further north, where the Chushiang fault disappears and transfers shortening to adjacent faults. Combining these results with the recently constrained shortening rates on the most frontal Changhua blind thrust reveals that all these frontal faults presently absorb most of the long-term horizontal shortening across the Taiwanese range. They thus stand as the major sources of seismic hazard in this heavily populated area. The return period of earthquakes similar to the ChiChi event over a ~80 km long stretch of the Western Foothills is estimated to be ~64 yr. This value is an underestimate because it assumes that all the faults locked during the interseismic period slip only during such large events. Comparison with historical seismicity suggests that episodic aseismic deformation might also play a major role in accommodating shortenin