HR: 10:20h
AN: G12A-01 INVITED     [Abstracts]
TI: Postseismic Deformation: Different mechanisms in different times and places.
AU: * Segall, P
EM: segall@stanford.edu
AF: Geophysics, Stanford University, Stanford, CA 94305 United States
AB: Improved understanding of postseismic deformation may elucidate time dependent stress transfer and triggered seismicity following large earthquakes. Afterslip, distributed viscoelastic flow, and poroelastic relaxation alter crustal stress and pore pressure distributions and in many cases lead to distinctive surface deformation patterns. Delayed triggering, due to rate and state dependent friction, on the other hand need not lead to detectable surface deformation. Postseismic deformation recorded following the 1999 ChiChi, Taiwan, 2003 Tokachi-Oki, Japan, and 2000 south Iceland earthquakes can be used to test for the effects of these processes. Horizontal displacements of 10 cm accumulated in the first year following the Chi-Chi quake. These are best explained with continued slip on the Chelungpu fault (Hsu et al, G.R.L. 2002). Inversions indicate the afterslip was roughly localized in a ring around the locus of maximum coseismic slip. The observed displacement pattern is inconsistent with predictions from viscoelastic and poroelastic models. Viscoelastic relaxation of the lower crust produces shortening of the hanging wall instead of the observed extension. The fully drained poroelastic response predicts deformation concentrated near the fault ends, which was not observed. Fully time dependent calculations, however, are still required because poroelastic displacements need not be monotonic. Afterslip following the M 8 Tokachi Oki earthquake is also localized around the area of high mainshock slip (Miyazaki et al, GRL, 2004). Surprisingly, the slip is not located downdip of the mainshock, but along strike of the source region. This indicates that the transient deformation is not caused by deceleration of the earthquake instability, but rather by stress increases due to the mainshock. A major question is whether intermediate depth afterslip following the Tokachi Oki and ChiChi earthquakes occurs in stable (steady state velocity strengthening) areas which will never initiate fast earthquake slip, or on unstable (velocity weakening) parts of the fault that slipped in a stable fashion following the earthquakes due to the pre-earthquake fault state and stress. Resolution of this question has important implications for future earthquakes in these areas. Postseismic deformation in the month following the South Iceland earthquakes was clearly detected by InSAR. The spatial and temporal patterns are inconsistent with both afterslip and viscoelastic deformation. The InSAR data are, however, well explained by a rapid poroelastic response. This was confirmed by water level changes with the same spatial and temporal scales as the deformation (JĒnsson et al, Nature, 2003). The decay of aftershocks is substantially longer that the poroelastic relaxation, suggesting that poroelastic effects do not control the timing of triggered earthquakes. The InsAR data, however, are insensitive to pore pressure changes at the depths of most aftershocks. At longer time scales, other processes dominate the observed deformation (Arnadottir, this meeting). An inescapable conclusion of these studies is that different physical processes dominate postseismic deformation in different geologic environments at different time scales.
DE: 8155 Plate motions--general
DE: 8159 Rheology--crust and lithosphere
DE: 7209 Earthquake dynamics and mechanics
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting