Models of Lower Crustal Strain Cycles for Strike-Slip Faults: Implications for Distinguishing Distributed vs. Localized Strain Beneath the Seismogenic Zone

Authors : Lynch, J C, Richards, M A

Transient crustal deformation has been observed after large earthquakes on continental transform faults. Based on earthquake rupture depths, we understand the brittle thickness of the crust to be 15-20 km, and that post- and interseismic deformation is accommodated by creep below this depth. However, the distribution of lower crustal strain is not well understood. Two models are generally suggested: (1) strain in the lower crust is highly localized within a narrow, weakened zone, approximated by a fault that cuts the entire lithosphere, with continuous slip occurring below the seismogenic zone, and (2) strain in the lower crust is distributed, approximated by a ductile layer that deforms continuously. Here, we use 3D finite element models to examine the effect of strain localization in the lower crust on surface observables (displacement and velocity profiles, principle stress orientations and magnitudes) during the earthquake cycle. Our model consists of a domain 300 km wide, 400 km long and 50 km deep, with a 15 km deep throughgoing fault. We prescribe constant velocity boundary conditions along both edges consistent with relative plate motions. The fault fails according to a Coulomb-type law, with the failure criterion such that earthquakes occur roughly every 250 years. Beneath the fault (from a depth of 15 km to 50 km), we define a "shear zone" of visco- elastic elements. By varying the width (5 km-80 km) and viscosity (10**18 to 10**20 Pa s) of this zone, we vary the degree of strain localization in the lower crust. The effects of postseismic relaxation are amplified with wider shear zones as well as with lower viscosity within the shear zones. Specifically, fault parallel velocity profiles at the surface show a greater acceleration near the fault immediately after a slip event for models with lower shear zone viscosity and/or wider shear zones. Model principle stress orientations throughout the earthquake cycle suggest that slip events cause greater rotation of the principle axes with lower shear zone viscosity and/or wider shear zones. Our results suggest that regional observations of stress orientation changes during the earthquake cycle may be used to discriminate between the two end-member model types.