Time-dependence of the stress shadowing effect and its relation to the structure of the lower crust

Great earthquakes, like the 1906 San Francisco earthquake, perturb the regi onal stress held and can generate "stress shadows" that delay the occurrenc e of subsequent M greater than or equal to 5.5 events, We investigate how t ime-dependent postseismic stress transfer between the three subparallel str ike-slip faults in northern California-the San Andreas, Hayward, and Calave ras faults, and their northern extensions-depends on the presence acid geom etry of lower crustal shear zones. Models incorporating Maxwell viscoelasti c relaxation of the mantle, a lower crustal detachment surface, and vertica l shear zones beneath the seismogenic faults are compared. Short-term relax ation of lower crustal shear zones enhances the amplitude and lateral exten t of the stress shadow Conversely, longer period mantle relaxation reloads the entire crust, As a consequence, after a 1906 type event on the San Andr eas fault, the stress shadow on the Hayward fault will be greater in magnit ude and duration if the faults are connected via lower crustal structures w ith time-dependent rheologies. In addition, our results demonstrate that, f or certain geometries, the total stress (tectonic plus postseismic perturba tion) may actually continue to decrease during the decades immediately foll owing a great earthquake. This effect has not been documented in prior time -dependent models of stress accumulation during the earthquake cycle.