The Taiwan range results from the collision between the Chinese Passive Margin and the Luzon volcanic arc. This mountain range is characterized by extreme rates of erosion and deformation, that are amongst the highest in the world. The long-term shortening rate across the range is of ~40 mm/yr over the long-term (~2 Myr), that is twice the shortening rate across the Himalayan orogen! Because of the obliquity between the Chinese margin, the Luzon arc and the direction of convergence, the collision has propagated southwards over time. Also, to the north of the island, this collision stops because of the reversal in the polarity of the subduction. The Taiwanese range therefore shows strong lateral variations in terms of structure and style of deformation, illustrating the transition from an oceanic subduction to a mature collision from southern to central Taiwan, and the collapse of a mountain range to the north.
This particular context explains why Taiwan is amongst the most seismically active regions on Earth. Since the 1999 Mw 7.6 Chi-Chi earthquake, several studies have focused on better assessing seismic hazards, by identifying and characterizing the active faults of Taiwan. Here we propose to contribute to this understanding by focusing on the central and southern regions of the western foothills.
-> Central western foothills of Taiwan: Quantifying lateral variations in the long-term slip rate of the Chelungpu fault.
The Chelungpu fault ruptured during the September 1999 Mw 7.6 Chi-Chi earthquake, Central Taiwan. This event is amongst one of the most documented continental earthquakes, and its study may therefore hemp better constrain the mechanics of active faults. The ChiChi earthquake was characterized by coseismic displacements that increase along-strike and updip, from south to north. This variation in coseismic slip appears correlated to the geometry and segmentation of the thrust fault. Previous studies suggested that such variations may also exist in the long-term fault slip rate, but this aspect has not yet been clearly documented. To address this issue, we investigate deformed fluvial terraces along the Choushui and Tatu-Wu rivers, within the southern and central segments of the thrust fault. Optical ages (OSL) of ~13 ka to ~38 ka are obtained for these terraces, and these ages enable estimation of fault slip rates. We combine our findings with the fault slip rate determined by other authors for the northern segment. Statistical analysis of the data indicates the existence of lateral variations in the long-term slip rate of the Chelungpu fault, with increasing slip rate towards the north. This pattern in the slip rate averaged over a time scale of ~10s kyr is therefore similar to that observed for coseismic displacements during the 1999 Chi-Chi earthquake. These similarities in the deformation pattern during 1 earthquake or cumulated over several seismic cycles may indicate that the ChiChi earthquake could be characteristic in terms of slip distribution of the earthquakes breaking the Chelungpu fault. The similarities between coseismic and long-term deformation are probably controlled by the physical properties of the fault zone. Our results also allow for discussing the plausible evolution of major rivers draining the foothills of central Taiwan over the last ~40-60 kyrs.
-> Southwestern foothills of Taiwan: investigating the kinematics of active faults from the analysis of deformed fluvial terraces along the Tsengwen river.
The active faults of the foothills in central Taiwan have been extensively investigated after the 1999 ChiChi earthquake. Within the southwestern foothills, other active faults have been mapped, but their kinematics remains unconstrained. Only two historical earthquakes on strike-slip faults at the northern and southern limits of this region are known (1906 Meishan; 1946 Hsinhua), and it has been proposed that a Mw 7.5 earthquake could occur there if the mountain front ruptured during a single event.
We aim at quantifying the kinematics of the active within the southwestern foothills of Taiwan. For that we investigate deformed fluvial terraces along the Tsengwen river, and combine our findings with borehole date within the coastal plain.
Finally, we propose to re-analyze interseismic deformation in the region by integrating the obtained long-term kinematics of active faults. Our modeling will be based on a thermo-mechanical approach (ADELI), where a temperature-dependent crustal rheology will be considered, together with the effect of surface processes on the observed short-term deformation.
Funding: INSU, ORCHID program, IPGP