Spatial variations in slip deficit on the central San Andreas Fault from InSAR

We use ERS InSAR measurements to record spatial variations in creep rate along the creeping segment of the San Andreas Fault (SAF), California, between 1992 and 2001. Inversion of geodetic data yields a slip rate distribution along the creeping segment, which is used for first-order moment release and deficit calculations. We present a time-averaged spatial picture of surface deformation and associated subsurface creep. An interferometric stack is constructed from 12 interferograms that show good coherence. For the decade of observation, the total right-lateral offset spanned by the data is similar to 34 mm yr(-1). Along most of the length of the creeping segment, this offset occurs within a narrow (< 2 km) zone close to the fault trace. In the northern part, a minor part of the offset is taken up by the nearby Calaveras-Paicines Fault. In general, the observed rates of surface creep are consistent with those obtained by several other studies for a longer and/or earlier period of time, using different geodetic methods. This suggests that the average creep rate has been constant over a period of almost four decades. A joint GPS-InSAR inversion implies that the shallow creep rate is variable along strike, reaching up to 31.5 +/- 1 mm yr(-1) in the central section of the creeping segment, tapering off along-strike to the south and becoming partitioned across two subparallel faults in the north. The deep slip rate beneath the seismogenic layer is 33 +/- 3 mm yr(-1). The difference between shallow and deep slip rates suggests that there is a shallow slip deficit on the creeping segment of the SAF (CSAF). Moment release rate due to aseismic slip is approximately three orders of magnitude greater than seismic moment release. The annual creep on the CSAF is equivalent to the moment released in a M 6 earthquake. The equivalent moment of the slip deficit relative to the deep slip rate is between 4.1 x 10(17) and 8.4 x 10(17) N m yr(-1), which is equivalent to a magnitude 5.7-5.9 earthquake. Over a 150 yr period, the deficit is equivalent to a M 7.2-7.4 earthquake. This slip deficit may be compensated by intermittent moderate sized events or by accelerated afterslip following large events on nearby fault segments.