Postseismic relaxation following the 1992 M7.3 Landers and 1999 M7.1 Hector Mine earthquakes, southern California

[1] Postseismic relaxation (measured postseismic deformation less the deformation that would have occurred at the preseismic rate) has been monitored at the same 10 monuments over ∼6 years following both the 1992 Landers and the 1999 Hector Mine earthquakes. For both earthquakes the displacement components of the observed relaxation are well described by γi + αif1(t), where γi and αi are constants peculiar to each component at each monument, t is the time after the earthquake, and f1(t) is a temporal function common to all components and all monuments for that earthquake. The temporal function f1(t) can be approximated by bt + c loge(1 + t/τ), where τ = 38.7 ± 15.2 days and 25.6 ± 7.7 days for the Landers and Hector Mine relaxations, respectively. Because the estimated values of τ do not differ significantly, the transient term loge(1 + t/τ) in the temporal function may be the same for both earthquakes. The asymptotic (t → ∞) relaxation rates αib are only a few mm/a and do not appear to be significantly different following the two earthquakes. The asymptotic deformation rates appear to be slightly greater than the preseismic deformation rates, but the preseismic rates were not measured directly. Thus, the deformations of the Landers array measured over the first 5.6 years following the Landers earthquake and over the first 6.4 years following the Hector Mine earthquake are generally consistent with a simple model in which the transient postearthquake relaxation depends upon time as loge(1 + t/τ) with τ = 28 ± 5 days and the asymptotic postseismic speeds of the monuments exceed the preseismic speeds by at most only a few millimeters per annum.