Co- and Post-seismic Deformation of the 1999 Chi-Chi Earthquake Revealed by Earth Observations

Abstract

The Mw 7.6 Chi-Chi earthquake struck Central Western Taiwan in 1999. The rupture was complex with several dislocations along the 100-km long Chelungpu thrust fault. Revisiting this earthquake is a challenge, as the precision and coverage of the Earth observations available are quite poor, but it allows better understanding of regional fault properties. Furthermore, the topographic and vegetation coverage complexity of the area prevents coherent radar images. In this thesis, radar and optical images, and terrestrial geodetic measurements, were utilised to study the Chelungpu fault and more specifically the impact of the Chi-Chi earthquake. First, five types of Earth observations were processed to investigate the co-seismic deformation. The Particle Swarm Optimization and Okada Inversion (PSOKINV) geodetic inversion package was used with the generalized Akaike’s Bayesian Information Criterion (gABIC) to precisely determine the coseismic slip distribution and relative weighting of datasets. Differences in results using the data sets jointly or separately (e.g., under-estimation due to InSAR, inconsistencies in SPOT offsets, smoother slip distribution with gABIC weighting) are observable. Most of the energy was released in the northern part of the fault, where the strike veers toward the east, and mainly at depths of less than 4 km. The PSOKINV-gABIC approach is viable for the study of complicated cases such as the Chi-Chi earthquake (complex fault geometry and multi-source observations) and can substantially benefit the weight determination and physical (no overlap or gap) realism of the fault geometry. Second, the post-seismic deformation of this event was observed using 20 years of time series from InSAR and GPS. Then, a model of the afterslip and viscoelastic relaxation was built. 11 years of ERS, 3 years of EnviSAT, 6 years of Sentinel-1A/B descending and 3 years of Sentinel-1A/B ascending images were processed using GAMMA-Remote Sensing software and atmospherically corrected using GACOS (Generic Atmospheric Correction Online Service for InSAR). Finally, InSAR time series were extracted and validated using GPS data. The modelling of the postseismic deformation following the Chi-Chi earthquake was done using a power-law Burgers rheology to analyse the interplay between afterslip and viscoelastic flow. From the time series, a change in the ground displacement can be observed in 2004 showing a slow-down of afterslip deformation. The forward modelling of the postseismic deformation showed that the maximum afterslip is observed on the southern part of the fault, on the decollement in depth (while the maximum slip was happening at shallow depth on the north section of the fault during the co-seismic deformation). Furthermore, the study of the strain enabled me to conclude that the interseismic phase has started over most of the area, most segments of the Chelungpu fault are now getting locked again. More data coverage and a longer monitoring of the Chelungpu fault will be needed to observe the entire earthquake cycle