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 an alysis of Global Positioning System (GPS) and leveling data suggests afters lip on the Loma Prieta rupture as well as aseismic slip on a thrust fault n ortheast of the San Andreas fault which we identify with the Foothills thru st 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 estim ated by maximum likelihood. Conditional on this estimate, reverse slip on t he Foothills thrust decays from 45 +/- 12 mm/yr immediately, after the eart hquake to zero by 1992. Reverse slip on the Loma Prieta rupture surface dec ays from 57 +/- 11 mm/yr to zero by 1994. Right-lateral slip on the Loma Pr ieta rupture surface decays monotonically from 30 +/- 10 mm/yr to zero by 1 994. These results suggest that (1) triggered afterslip can occur off the m ain rupture zone on adjacent faults, (2) shallow afterslip dominated the po stseismic deformation for the 8 years following the earth-quake, and (3) po stseismic slip on the Foothills thrust may account for a significant portio n of its total slip budget.