Rate change observed at JPLM after the Northridge earthquake

Abstract

Geodetic time series determined with the Global Positioning System indicate that the geodetic rate of a permanent site in Pasadena, California (JPLM) changed significantly after the 17 January 1994 Northridge California earthquake. Subtracting the pre-quake rate and co-seismic offset leaves 30 ± 4 mm of integrated eastward excess motion observed in the three years following the earthquake. North and vertical components show excess motion of -11 ± 3 mm and 25 ± 11 mm respectively. Local surveys to three additional points near JPLM changed by no more than 6 mm E, 3 mm N, and 15 mm V during the two years after the earthquake, ruling out the possibility of a local effect at the JPLM monument. The direction and size of the post-seismic displacements at JPLM are not consistent with additional slip on the fault which ruptured. The most rapid accumulation of excess motion occurs immediately after the earthquake, suggesting a relationship between the two events.