HR: 1340h
AN: G13A-0785    [Abstracts]
TI: Local Postseismic Relaxation Observed After the 1992 Landers (M=7.3), 1999 Hector Mine (M=7.1), 2002 Denali (M=7.9), and 2003 San Simeon (M=6.5) Earthquakes
AU: * Svarc, J L
EM: jsvarc@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS/977, Menlo Park, CA 94025 United States
AU: Savage, J C
EM: jasavage@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS/977, Menlo Park, CA 94025 United States
AB: The U. S. Geological Survey has observed the local postseismic deformation following the 1992 Landers (M=7.3), 1999 Hector Mine (M=7.1), 2002 Denali (M=7.9), and 2003 San Simeon (M=6.5) earthquakes. The observations consist of repeated campaign-style GPS surveys of geodetic arrays (aperture $\sim$ 50 km) in the epicentral area of each earthquake. The data span the intervals from 0.037 to 5.6, 0.0025 to 4.5, 0.022 to 1.6, and 0.005 to 0.55 yr postearthquake for the Landers, Hector Mine, Denali, and San Simeon earthquakes, respectively. We have reduced the observations to positions of the monuments measured relative to another monument within the array. The temporal dependence of the relative displacements for each monument can be approximated by {\it a}+{\it bt}+{\it c(1-exp[-t/d])} where {\it a}, {\it b}, {\it c}, and {\it d} are constants particular to that monument and {\it t} is the time after the earthquake. The relaxation times {\it d} were found to be 0.367ñ0.062, 0.274ñ0.024, 0.145ñ0.017, and 0.032ñ0.002 yr for the Landers, Hector Mine, Denali, and San Simeon earthquakes, respectively. The observed increase in {\it d} with the duration of the time series fit suggests that the relaxation process involves more than a single relaxation time. An alternative function {\it a'}+{\it b't}+{\it c'log(1+t/d')} where {\it a'}, {\it b'}, {\it c'}, and {\it d'} are constants particular to each monument furnishes a better fit to the data. This logarithmic form of the relaxation (Lomnitz creep function), identical to the calculated response of a simple spring-slider system subject to rate-state friction [{\it Marone et al.}, 1991], contains a continuous spectrum of relaxation times. In fitting data the time constant {\it d'} is determined by observations within the first few days postseismic and consequently is poorly defined. Adequate fits to the data are found by simply setting {\it d'}=0.001 yr and determining {\it a'}, {\it b'}, and {\it c'} by linear least squares. That the temporal dependence is so readily fit by both exponential and logarithmic functions suggests that the temporal dependence by itself may not be very diagnostic of the causative mechanism.
DE: 1208 Crustal movements--intraplate (8110)
DE: 1243 Space geodetic surveys
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting