Mechanical deformation model of the western United States instantaneous strain-rate field

Fred F Pollitz,Mathilde Vergnolle

doi:10.1111/j.1365-246x.2006.03019.x

Abstract

Summary We present a relationship between the long-term fault slip rates and instantaneous velocities as measured by Global Positioning System (GPS) or other geodetic measurements over a short time span. The main elements are the secularly increasing forces imposed by the bounding Pacific and Juan de Fuca (JdF) plates on the North American plate, viscoelastic relaxation following selected large earthquakes occurring on faults that are locked during their respective interseismic periods, and steady slip along creeping portions of faults in the context of a thin-plate system. In detail, the physical model allows separate treatments of faults with known geometry and slip history, faults with incomplete characterization (i.e. fault geometry but not necessarily slip history is available), creeping faults, and dislocation sources distributed between the faults. We model the western United States strain-rate field, derived from 746 GPS velocity vectors, in order to test the importance of the relaxation from historic events and characterize the tectonic forces imposed by the bounding Pacific and JdF plates. Relaxation following major earthquakes (Mγ 8.0) strongly shapes the present strain-rate field over most of the plate boundary zone. Equally important are lateral shear transmitted across the Pacific–North America plate boundary along ∼1000 km of the continental shelf, downdip forces distributed along the Cascadia subduction interface, and distributed slip in the lower lithosphere. Post-earthquake relaxation and tectonic forcing, combined with distributed deep slip, constructively interfere near the western margin of the plate boundary zone, producing locally large strain accumulation along the San Andreas fault (SAF) system. However, they destructively interfere further into the plate interior, resulting in smaller and more variable strain accumulation patterns in the eastern part of the plate boundary zone. Much of the right-lateral strain accumulation along the SAF system is systematically underpredicted by models which account only for relaxation from known large earthquakes. This strongly suggests that in addition to viscoelastic-cycle effects, steady deep slip in the lower lithosphere is needed to explain the observed strain-rate field.

Highlights

Deformation in continental regions is commonly interpreted in terms of two end-member models (King et al 1994; Thatcher 2003)
(3) Evaluating the consequent effect on the deformation fields. We find that this approach allows us to capture the relative rigidity of the Sierra Nevada/Great Valley (SNGV) block (e.g. Fig. 3) with a reasonably realistic model of the relatively thick SNGV lithosphere
The Model 4 rheology fits the entire data set slightly better than the Models 1 and 2 rheologies, and it better fits most of the subregional data sets, western Nevada and southern California around the Landers, Kern County, Owens Valley rupture zones

Summary

Introduction

Deformation in continental regions is commonly interpreted in terms of two end-member models (King et al 1994; Thatcher 2003). The second end-member model, known as the ‘thin sheet model’ (England & McKenzie 1982) accommodates the view that lithospheric deformation over length scales longer than the lithospheric thickness is essentially continuous and that over long time periods the lithosphere behaves as a viscous fluid. This model is generally applied to the thermally defined lithosphere, to which an effective viscosity can be derived that depends on the variation of temperature with depth and an assumed rheology of the lithosphere

Objectives

Findings

Discussion

Conclusion