Partitioning of shortening across the western foothills of central Taiwan: implications for seismic hazards
Résumé
The devastating 1999 Mw 7.6 ChiChi earthquake, west-central Taiwan, has immediately drawn attention to the potential for major seismic hazards in this highly populated region of Taiwan. Numerous efforts have contributed since then to better estimate these hazards, however some key questions such as whether or not shortening is fully released during major seismic events are yet poorly resolved. To address these issues, we have conducted morphotectonic investigations on the active structures of the western foothills to estimate their contribution to crustal shortening across the range over time scales of ~10s to ~100s kyr. Deformed fluvial terraces and geomorphic markers were surveyed in the field and dated using Optically Stimulated Luminescence (OSL) and radiocarbon. From there, the slip rate on the Chelungpu fault, which broke during the ChiChi earthquake, is estimated to ~12.9 mm/yr, and may be up to ~15.8 mm/yr towards the north of this region. A recently-proposed analytical model of fault-tip folding is used to derive the kinematics of shortening of the Pakuashan anticline, which is the frontal most structure growing at the tip of the Changhua blind thrust. We get that slip on this fault has been of 16.3 +/- 4.1 mm/yr since ~62 kyr ago. It is possible that further east the Shuangtung fault absorbs ~10 mm/yr. These results suggest that the faults of the western foothills absorb most of (if not all) the ~42 mm/yr total shortening across the range, and that they stand as major sources of seismic hazards. If we assume that all these faults are fully locked and that they release their strain in events such as the Mw 7.6 ChiChi earthquake, the recurrence time of such events would be of ~64 kyr. Comparison with historical seismicity over the last ~150 years reveals that episodic aseismic deformation might also play a role in accommodating shortening