Mechanics of Post Seismic Deformation of the 1999 Chi-Chi, Taiwan, Earthquake

GPS measurements of 450 days after the 1999 Chi-Chi, Taiwan, earthquake revealed large postseismic displacement, from dozens to hundreds of millimeters, especially on the hanging wall of the fault. Two possible mechanisms responsible for the postseismic deformation of the Chi-Chi earthquakes are the viscoelastic relaxation of the lower crust and the afterslip in the regions with coseismic slip deficit on the fault plane, respectively. In this study, the postseismic deformation process of the 1999 Chi-Chi, Taiwan, earthquake is modeled dynamically by the revised LDDA method with the GPS data as constraints and the possible postseismic deformation mechanisms, viscoelastic relaxation and afterslip, are discussed. The results indicate that the postseismic deformation observed by GPS can be well explained by the viscoelastic relaxation mechanism. The viscosity of the lower crust below the Taiwan area is estimated as 1017 Pa· s by applying the trial and error method. The slips on the fault focus mainly on the part near the ground surface, and the maximum and minimum values are on the ground surface and at the fault tip, respectively. The slips decrease with depth, postseismic slips on the fault increase with time, but the speed of increase diminishes. Observable slip of the fault on the ground surface due to postseismic stress relaxation will last about 13 years. The westward and upward components of the total slips accumulated during this period are about 25.5% and 14.7% of the coseismic slip, respectively. Shear stress drops on the fault are heterogeneous. The effect of afterslip on the postseismic deformation of the Chi-Chi earthquake can be investigated by the LDDA method with rate and state dependent frictional law. Supposing that the fault is a rate-strengthening material, the results obtained suggest that the GPS data cannot be explained by the afterslip mechanism alone, which can only affect the postseismic deformation in the early 50 days after the mainshock.