Time-dependent triggered afterslip following the 1989 Loma Prieta earthquake

We employ a modified version of the Network Inversion Filter to investigate time-dependent slip following the 1989 Loma Prieta earthquake. Previous analysis of Global Positioning System (GPS) and leveling data suggests afterslip on the Loma Prieta rupture as well as aseismic slip on a thrust fault northeast of the San Andreas fault which we identify with the Foothills thrust belt. We analyzed 173 daily GPS solution files at 62 stations collected from 1989.8 to 1998.3 (a total of 1,134 three-dimensional relative baseline determinations). The observed position changes are assumed to result from secular deformation, random benchmark motions, and temporally varying fault slip. The data reveal temporal variations in slip rate but poorly resolve spatial variations in fault slip. The amount of temporal smoothing is estimated by maximum likelihood. Conditional on this estimate, reverse slip on the Foothills thrust decays from 45 +/- 12 mm/yr immediately, after the earthquake to zero by 1992. Reverse slip on the Loma Prieta rupture surface decays from 57 +/- 11 mm/yr to zero by 1994. Right-lateral slip on the Loma Prieta rupture surface decays monotonically from 30 +/- 10 mm/yr to zero by 1994. These results suggest that (1) triggered afterslip can occur off the main rupture zone on adjacent faults, (2) shallow afterslip dominated the postseismic deformation for the 8 years following the earth-quake, and (3) postseismic slip on the Foothills thrust may account for a significant portion of its total slip budget.