HR: 1340h
AN: G13A-0799    [Abstracts]
TI: Temporal and Spatial Variations of Afterslip Following the 1999 Chi-Chi, Taiwan Earthquake
AU: * Hsu, Y
EM: yaru@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, No.128, Sec.2, Academia Rd., Nankang, Taipei, 115 Taiwan
AU: Segall, P
EM: segall@pangea.stanford.edu
AF: Geophysics Department, Stanford University, Mitchell Building, Suite 101 397 Panama Mall , Stanford, CA 94305 United States
AU: Yu, S
EM: yusb@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, No.128, Sec.2, Academia Rd., Nankang, Taipei, 115 Taiwan
AB: GPS data collected following the 1999 Chi-Chi, Taiwan, earthquake (M$_{w}$ 7.6) reveal noticeable postseismic deformation, with maximum displacements of 25 and 23 cm in horizontal and vertical components during the 15 months after mainshock. Postseismic transients mainly occurred on the hanging wall causing east-west extension, similar to the deformation pattern during mainshock. Distributed afterslip reproduces the overall features in the GPS observations (Hsu et al., 2002, GRL; Yu et al., 2003, JGR). In this study, we try to understand to what extent space-time inversion of afterslip alone can explain the postseismic deformation. The Extended Network Inversion Filter (McGuire and Segall, 2003, GJI) is employed to reveal the fault-slip history. This approach is based on a Kalman filter and capable of extracting spatially coherent signals, such as fault slip transients, and separating these from incoherent signals, such as local benchmark motions. We assume a two-segment fault geometry with a 100 km long, 24$\deg$ dipping and N3$\deg$E trending shallow rupture segment connected to a 40 km wide, horizontal decollement at a depth of 10 km. Our results show that the slip-rate distribution remains roughly stationary over the 15 month period, while the magnitude of the slip rates decay rapidly with time. The average slip rates on the shallow fault decay more rapidly than the deep slip. The slip rate on the shallow dipping fault began at about 0.45 m/yr and decreased to 0.1 m/yr in 200 days, while that on the decollement started at 0.55 m/yr and took about 450 days to decay to 0.1 m/yr. The cumulative slip on lower decollement over 15 months is 1.5 times larger than that on the dipping fault. The total accumulated afterslip moment is 5.78$\times$10$^{19}$ N-m, much more than seismic moment, 2.0$\times$10$^{19}$ N-m, released by aftershocks. In contrast with the limited coseismic slip on the decollement, afterslip is prominent there. The deep postseismic slip is consistent with models including velocity-strengthening friction below the rupture zone.
DE: 7205 Continental crust (1242)
DE: 1709 Geodesy
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting