Joint estimation of afterslip rate and postseismic relaxation following the 1989 Loma Prieta earthquake

Abstract

Abstract. Global Positioning System (GPS) data from campaigns carried out over the 5 years following the 1989 Loma Prieta earthquake and leveling data measured in 1990 and 1992 define the postseismic velocity field around the Loma Prieta rupture zone. Subtraction of a background velocity field yields a residual velocity pattern which we interpret as the product of two physical processes: (1) slow afterslip along distinct planes in the upper cru, st and (2) viscoelastic relaxation of the lower crust and upper mantle. Biirgmann et al. [1997] previously derived an afterslip model involving uniform afterslip on two optimally determined planes, including oblique reverse slip on the coseismic rupture and reverse slip on a shallow thrust fault to the northeast of the San Andreas fault. We further consider models of distributed slip on these two fault planes plus a viscoelastic relaxation pattern which depends on a suitable coseismic rupture model and crust and mantle viscosities. Several fault models from the literature were considered for the 1989 coseismic rupture, with nearly identical impact on the results. Simultaneous maximum likelihood inversion of the GPS and leveling data for afterslip distribution and viscosity yields the following results: (1) A good fit to the data is obtained by

A model of deep crustal fluid flow following the Mw = 8.0 Antofagasta, Chile, earthquake

“…For example, a Maxwell time t = h/G, with t = 50 days and a shear modulus for lower continental crust of G = 40 GPa, leads to a dynamic viscosity h of around 1 Â 10 17 Pa s for the lower crust. This is 1 -2 orders of magnitude smaller than expected for the lithosphere [Pollitz and Bürgmann, 1998;Viti et al, 2003]. That is, the timescale of our problem (50 days) is 1 or 2 orders of magnitude too short to be related to viscous behavior of a relatively cold, lower continental crust.…”

“…[31] Postseismic viscous relaxation processes [Pollitz and Bürgmann, 1998;Viti et al, 2003] can be excluded because of the short timescale of the observation. For example, a Maxwell time t = h/G, with t = 50 days and a shear modulus for lower continental crust of G = 40 GPa, leads to a dynamic viscosity h of around 1 Â 10 17 Pa s for the lower crust.…”

Seismic cycle on a strike-slip fault with rate- and state-dependent strength in an elastic layer overlying a viscoelastic half-space

“…Analyzing GPS and leveling data following the 1989 Loma Prieta, California, earthquake (M = 7.1), Pollitz et al (1998) found that both postseismic sliding and viscoelastic relaxation of the lower crust and upper mantle occurs and the former effect is predominant in postseismic surface deformation. Similar conclusion was obtained for the 1994 Northridge earthquake, California, of M = 6.7 by Donnellan and Lyzenga (1998).…”

Mountain Building: From Earthquakes to Geologic Deformation

“…Indeed, there should be some time variation in crustal straining in the period between two large recurring earthquakes due to both frictional afterslip and ductile flow (e.g., Cohen, 1999;Pollitz et al, 1998;Smith and Wyss, 1968). In the case of the Himalayas, a strong asymmetry of the postseismic relaxation should be expected due to the presumably much lower viscosity beneath southern Tibet.…”