Lower Crustal Structure in Northern California: Implications From Strain-Rate Variations Following the 1906 San Francisco Earthquake

S. J. Kenner and P. Segall, Stanford University

Abstract

       It is well known that geodetic data alone cannot uniquely characterize structure or rheology beneath active seismogenic zones.  Nevertheless, comparison of spatial and temporal variations in deformation rate with naturally time dependent mechanical models can place valuable constraints on fault zone geometry and rheology.  We consider postseismic strain-rate transients by comparing geodetic data from north of San Francisco bay obtained between 1906 and 1995 to predictions from viscoelastic finite element models.  Models include (1) an elastic plate over a viscoelastic half-space, (2) distributed shear within a viscoelastic layer, (3) discrete shear zones within an otherwise elastic layer, (4) discrete shear zones in combination with distributed viscoelastic shear, and (5) mid-crustal detachment surfaces.  We vary, as applicable, locking depth, elastic thickness, depth to the top and bottom of the distributed shear layer, distributed shear relaxation time, discrete shear zone relaxation time, and shear zone width.
        Physically reasonable elastic plate over viscoelastic half-space models (1) do a poor job predicting both spatial and temporal variations in the data.  The best-fitting distributed shear models (2) do a poor job predicting spatial variations in the deformation rate.  Although they fit the geodetic data, recent findings from seismic reflection-refraction studies in northern California argue against models with sub-horizontal detachments (5).  Models incorporating discrete shear zones (3, 4) fit the geodetic data and are consistent with seismic studies which argue for discrete shear zones extending through the entire crust.

Reference: Kenner, S.J., and P. Segall, Lower Crustal Structure in Northern California: Implications From Strain Rate Variations Following the 1906 San Francisco Earthquake, in press, Journal of Geophysical Research, April 2002.