Postseismic horizontal strain and displacement following the
28 June 1992 Landers, California earthquake is broad scale
and cannot be explained solely by delayed afterslip located
at the rupturing fault trace. Both the observed strain at
Pinon Flat Observatory (PFO) and observed Global Positioning
System (GPS) receiver velocities evolve rapidly after the
Landers-Big Bear earthquake sequence. The observed
exponential decay of these motions, with time scales of 4 -
34 days, may reflect a soft creep rheology in the lower
crust and brittle-ductile transition zone, or, even within
the seismogenic crust itself. Here a simple model of a
two-dimensional screw dislocation in a layered Maxwell
viscoelastic Earth is employed in conjunction with a
composite rheology to demonstrate that the short time-scale
transient response modes (4 - 34 days) are consistent with
the behavior of a bi-viscous lower crust. The lowest
viscosity of this system is derivable from laboratory
experimental data on the long-term creep of natural
quarztites and the highest viscosity is consistent with
isostasy-related lower crustal flow in a continental
extensional tectonic environment. The model predicts
significant stress relaxation at the base of the seismogenic
crust. Near the base of the seismogenic zone, and about 4 km
away from the mainshock, the rate of predicted relaxation is
on the order of 0.01 MPa per day during the first twenty days
of postseismic flow. Oblate spheroidal inclusions at 5concentration levels that are both aligned and fairly flat in
shape, and that have a viscosity of 3-4 x 
Pa sec, are
consistent with both the amplitude and decay time of
horizontal crustal strain observed at PFO after the Landers
mainshock. It is speculated that the structures exposed in
cross-sections and in seismic reflection profiles of the
lower crust that have mylonitic associations are, in part,
the cause of such rapid postseismic evolution in southeastern
California. Unmylonitized quartz-rich rock at sufficiently
elevated temperatures could also contribute to the rapid
decay modes.
AGU Index Terms: 1200 Geodesy and gravity; 1239 Rotational variations; 1243 Space geodetic surveys; 3902 Creep and deformation
Keywords/Free Terms: Postseismic deformation, lower crust, mylonitic rocks, crustal stress diffusion
JGR-Solid Earth 96JB02846
Vol. 101
, No. B12
, p. 28,005