Expanding afterslip on the Chihshang fault, eastern Taiwan, inferred from postseismic GPS displacements following the 2003 Mw 6.8 Chengkung earthquake

We analyze postseismic deformation following the 2003 Mw 6.8 Chengkung earthquake to infer the slow, up-dip propagation of afterslip on the fault, which is important for understanding the slip behavior on the fault throughout the earthquake cycle. The Chengkung earthquake occurred in southeastern Taiwan, resulting from the rupture of the Chihshang fault, a high-angle suture boundary reverse fault between the Eurasian and Philippine Sea plates. In this study, we adopt the daily coordinate time series of 17 stations relative to the Chinese continental margin, Paisha, Penghu in southeastern Taiwan evaluated in the period of 1.5 years after the earthquake to estimate the postseismic deformation. We seek unsmoothed representations of afterslip using bounded least squares inversion techniques with the fully relaxed afterslip as the upper bound. We first compare fully relaxed afterslip with cumulative 1.5-year displacements and find that a more compact coseismic slip distribution, and correspondingly higher coseismic stress perturbation, than those inferred from standard coseismic slip inversions are required to fit the postseismic GPS data. Next, we divided the 1.5-year displacement time series into fifteen increments and invert for afterslip at each increment to investigate the temporal variation of accumulative afterslip distribution. We find that the afterslip slowly propagates outward from the coseismic source area, especially in the upper 10 km of the crust, up-dip of the coseismic source region. It takes more than 9 months for the afterslip front to propagate from a depth of 10 km to the ground surface. This inference of slowly propagating afterslip is different from the majority of afterslip studies that infer nearly stationary areas of decelerating afterslip on the fault.