Horizontal coseismic deformation of the 1999 Chi-Chi earthquake measured from SPOT satellite images: Implications for the seismic cycle along the western foothills of central Taiwan

The 1999 Chi-Chi earthquake, M_w = 7.6, broke a major thrust fault along the western foothills of the Central Range of Taiwan. We have measured the horizontal coseismic displacement field by correlating optical satellite images acquired before and after the earthquake. These data reveal the fault trace and a clockwise rotation of surface displacements toward the north with much larger displacements and strain in the hanging wall. At the surface, coseismic slip increases from 5-6 m near the epicenter to 10-11 m to the north. In the epicentral area, we observe a left-lateral strike-slip zone trending N125°E, and farther north, a fault zone trending N-S with a right-lateral component. The data were modeled using elastic dislocations. The fault geometry consists of a shallow 20-35° east dipping ramp, which soles out into a low dipping décollement at a depth of ∼6 to 8 km. Surface displacements can be satisfactorily modeled, assuming a constant slip azimuth on the main fault plane, close to the azimuth of plate convergence (N305°E ± 5°). At depth, slips along the fault plane evolve from 5-6 m in the south to 7 to 12 m to the north. Our model suggests that the deeper portion of the fault was not activated during the Chi-Chi earthquake. This zone of slip deficit must break during large earthquakes or be activated during transient episodes of aseismic slip. On the basis of these observations, the western front of the central Taiwan should produce a M = 7 to 7.5 event, about every 150 to 250 years.