GPS observations of fault afterslip and upper crustal deformation following the Northridge earthquake

Abstract

Global Positioning System (GPS) observations indicate that significant aseismic deformation occurred in the year following the January 17, 1994, Northridge earthquake. The postseismic observations show the same sense of motion as the coseismic. Compared with the coseismic displacements, the far‐field postseismic motions (1–2 fault dimensions away) are proportionally larger than those seen in the near field. The postseismic data are best modeled with two faults: one on the rupture plane and one located in the shallow crust. The upper crustal fault may represent an actual fault or may be indicative of viscous relaxation occurring in the upper crust. The inferred afterslip and/or relaxation moment is approximately 2.9×1018 N m or 22% of the mainshock moment release. We expect that the moment release due to the afterslip and relaxation effectively reduce the earthquake hazard locally. It is not clear from this study how the postseismic deformation loads the surrounding faults or alters the state of stress on those faults.