On the choice and implications of rheologies that maintain kinematic and
dynamic consistency over the entire earthquake cycle
Abstract
Viscoelastic processes in the upper mantle redistribute seismically
generated stresses and modulate crustal deformation throughout the
earthquake cycle. Geodetic observations of these motions at the Earth’s
surface offer the possibility of constraining the rheology of the upper
mantle. Parsimonious representations of viscoelastically modulated
deformation should simultaneously be able to explain geodetic
observations of rapid postseismic deformation and near-fault strain
localization late in the earthquake cycle. We compare predictions from
time-dependent forward models of deformation over the entire earthquake
cycle on and surrounding an idealized vertical strike-slip fault in a
homogeneous elastic crust underlain by a homogeneous viscoelastic upper
mantle. We explore three different rheologies as inferred from
laboratory experiments: 1) linear-Maxwell, 2) linear-Burgers, 3)
power-law. Both the linear Burgers and power-law rheological models can
be made consistent with fast and slow deformation phenomenology from
across the entire earthquake cycle, while the single-layer linear
Maxwell model cannot. The kinematic similarity of linear Burgers and
power-law models suggests that geodetic observations alone are
insufficient to distinguish between them, but indicate that one may
serve as a proxy for the other. However, the power-law rheology model
displays a postseismic response that is strongly earthquake magnitude
dependent, which may offer a partial explanation for observations of
limited postseismic deformation near magnitude 6.5-7.0 earthquakes. We
discuss the role of mechanical coupling between frictional slip and
viscous creep in controlling the time-dependence of regional stress
transfer following large earthquakes and how this may affect the seismic
hazard and risk to communities living close to fault networks.