Regional fault mechanics following the 1992 Landers earthquake (California)

This study investigated the transient regional mechanics following the 1992 Landers earthquake in the Mojave block of southern California. A finite element model (FEM) of the Mojave block consisting of a poroelastic upper crust overlying the viscoelastic lower crust decoupled from the upper mantle was constructed. The FEM was constrained by synthetic aperture radar interferograms and GPS data. FEM results determined the significance of poroelastic coupling between the 1992 Landers and Big Bear earthquakes. The homogeneous poroelastic model predicted a maximum increase in left lateral slip potential (change in shear stress less the change in effective fault normal stress scaled by a coefficient of friction) along the southwest part of the Big Bear fault, consistent with the epicentral location. In contrast, slip potential calculated for a weak fault zone in a state of isotropic stress for drained conditions indicated a maximum increase along the northeast part of the Big Bear fault. The FEM was capable of simulating the transient behavior of the crust as a mechanical system of poroelastic and viscoelastic layers. Two time-dependent parameters, upper crust diffusivity and lower crust viscosity, were calibrated to 10–2 m2·s –1 and 5 × 1018 Pa·s respectively. The diffusivity was resolved to within an order of magnitude and the lower crust viscosity was resolved to a single significant digit. The calibrated FEM was used to quantify the mechanical coupling between the 1992 Landers and 1999 Hector Mine earthquakes. Slip potential near the Hector Mine earthquake epicenter increased from 0.02 MPa to 0.05 MPa. This increase in slip potential was primarily due to the recovery of decreased fluid pressure from the 1992 Landers coseismic strain field. Slip potential predictions were insensitive to small variations of fault plane dip and hypocentral depth estimations. The most important result of this project was a model that could successfully capture the transient mechanical behavior of a regional system.