State of stress before and after the 1994 Northridge earthquake

The state of tectonic stress in the epicentral area of the 17 January 1994, Northridge earthquake (Mw, 6.7) is investigated by applying a stress inversion method to P-wave polarity, data from earthquakes in Northridge from July 1981 to January 1994 and from the Northridge aftershocks during January 1994 to December 1995. A 3-D crustal model is used to trace the rays taking off from the hypocenter, which reduced the effects of large structural heterogeneities on the determination of the stress tensor. We found significant temporal changes of stress orientations induced by the Northridge earthquake. The principal pressure (P) axis is oriented N32 degrees E from 1981 to June 1992, and N30 degrees E from 28 June 1992 to 16 January 1994, suggesting that the stress field in Northridge was not affected by the 1992 Landers earthquake. During two weeks following the Northridge mainshock, the P-axis is oriented N13 degrees E, which is a significant (17 degrees) change from that before the earthquake (N30 degrees E). Between February 1994 and August 1995 the P-axis orientation changes from N18 degrees E to N26 degrees E, and finally ends up at N34 degrees E by the end of 1995, which is close to that before the Northridge earthquake. These results suggest that the stresses rotated coseismically, then rotated more slowly back to their original orientation. The aftershocks caused by the mainshock changed the stress distribution in the crust, which showed up as a regional stress change. The stress recovery appears to have completed within two years after the mainshock, which is very short compared to the time scale of the earthquake cycle.