Postseismic relaxation following the 1994 Mw6.7 Northridge earthquake, southern California

We have reexamined the postearthquake deformation of a 65 km long linear array of 11 geodetic monuments extending north-south across the rupture (reverse slip on a blind thrust dipping 40 degrees S-20 degrees W) associated with the 1994 M(w)6.7 Northridge earthquake. That array was surveyed frequently in the interval from 4 to 2650 days after the earthquake. The velocity of each of the monuments over the interval 100-2650 days postearthquake appears to be constant. Moreover, the profile of those velocities along the length of the array is very similar to a preearthquake velocity profile for a nearby, similarly oriented array. We take this to indicate that significant postseismic relaxation is evident only in the first 100 days postseismic and that the subsequent linear trend is typical of the interseismic interval. The postseismic relaxation (postseismic displacement less displacement that would have occurred at the preseismic velocity) is found to be almost wholly parallel (N70 degrees W) to the nearby (40 km) San Andreas Fault with only negligible relaxation in the direction of coseismic slip (N20 degrees E) on the Northridge rupture. We suggest that the N70 degrees W relaxation is caused by aseismic, right-lateral slip at depth on the San Andreas Fault, excess slip presumably triggered by the Northridge rupture. Finally, using the Dieterich (1994) stress-seismicity relation, we show that return to the preseismic deformation rate within 100 days following the earthquake could be consistent with the cumulative number of M > 2.5 earthquakes observed following the main shock.