POSTSEISMIC STRESS EVOLUTION FOR A STRIKE-SLIP-FAULT IN THE PRESENCE OF A VISCOELASTIC ASTHENOSPHERE

A model for post-seismic, near-field effects on transcurrent vertical faults in the presence of a viscoelastic asthenosphere is presented. T he lithosphere is modelled as an elastic layer welded to a viscoelasti c half-space (the asthenosphere). The earthquake is modelled as a stre ss-drop event (crack), whereas in previous similar papers it has usual ly been modelled as a 'constant-slip event'. Unlike crack models, cons tant slip dislocations are not suitable for describing the stress held in the proximity of the fault, which is the main concern of the prese nt paper. The sliding section is supposed to have a fixed extension an d to reach the free surface. Two cases are considered: a fault which b ecomes welded immediately after an earthquake ('locked' problem), and the case in which the slip distribution evolves in the post-seismic ph ase ('unlocked' problem), following the stress relaxation of the asthe nosphere. In both cases, the singular integral equation of crack equil ibrium is solved in the Laplace transform domain. The locked problem i s assumed to reproduce the post-seismic stress recovery following an e arthquake on a fault, which preserves the initial strength threshold a fter the earthquake. Solutions show that major aftershocks in the deep section of the fault can occur if the fault cuts most of the lithosph ere owing to relaxation of the asthenosphere. The unlocked problem con cerns a fault on which the post-seismic strength is equal to the resid ual stress left after the earthquake, and post-seismic creep occurs; o wing to relaxation of the asthenosphere, the fault keeps sliding after the earthquake, and afterslip can be observed at the ground surface. We compare the results obtained with the advanced phases of afterslip observed in some Californian faults. The stress evolution is a slower process in this case, with respect to the locked model, owing to the t wo-way relaxation, i.e. through fault creep and asthenosphere flow. Th e comparison of the unlocked model with afterslip data seems to requir e a fault cutting most of the lithosphere if the relaxation time for t he asthenosphere is greater than similar to 10 yr.