Twenty-Five Years of Postseismic Viscous Relaxation Following the Mw 6.9 Loma Prieta Earthquake

Abstract:

The 1989 M_w 6.9 Loma Prieta earthquake provides the first opportunity of probing the crustal and upper mantle rheology in the San Francisco Bay Area since the 1906 M_w 7.9 San Francisco earthquake. Here we use geodetic observations including GPS and InSAR to characterize the 1989 M_w 6.9 Loma Prieta earthquake postseismic displacement from 1989.8 to 2013. Pre-earthquake deformation rates are constrained by nearly 20 years of USGS trilateration measurements and removed from the postseismic measurements prior to the analysis. We observe 1-4 mm/yr GPS horizontal displacement toward the Loma Prieta epicenter until 2000, and ~2 mm/yr surface subsidence of the northern Santa Cruz Mountains between 1992 and 2002, which is not associated with the seasonal and longer-term hydrological deformation in the adjoining Santa Clara Valley. Previous work indicates afterslip dominated in the early (1989-1994) postseismic period, so we focus on modeling the postseismic viscoelastic relaxation between 1994 and 2010 based on geodetic observations. The best fitting result predicts an 11-km-thick viscoelastic lower crust with viscosity of about 10¹⁹ Pa s below a 19-km-thick elastic upper crust, underlain by a bi-viscous upper mantle with transient viscosity of 10^16.5-17.5 and steady-state viscosity of 10^17.5-18.5 Pa s. The millimeter scale postseismic deformation does not resolve the viscosity in different layers very well, but the inferred lithospheric rheology is consistent with estimates in southern California. The viscoelastic relaxation may also contribute to the enduring increase of aseismic slip and repeating earthquake activity on the San Andreas Fault near San Juan Bautista, which continued for at least a decade after the Loma Prieta event.