Abstract
It is proposed that a preseismic fault slip process provides one of the possible mechanisms that explain observations of relatively long-term, anomalous surface deformations preceding large crustal earthquakes.
The process in a two-dimensional dip-slip fault zone is numerically analyzed by using the finite element method with a new element model. The process is modeled in terms of (1) constitutive laws for the fault zone and surrounding material and (2) a nonuniform distribution of the shear stress relative to yield strength along the fault zone. The constitutive law for the fault zone is assumed to be expressed by a mechanical model of elastic-elastoplasticity, which is composed of two kinds of springs and a slider. The law draws support from the data obtained by the recent stick-slip experiments with pre-faulted rock samples.
Preseismic slip in a certain portion of the fault zone, which is caused by remotely applied displacements acting parallel to the fault, is represented by a crack-like solution. The portion of the fault zone undergoing slip elongates in response to the remotely applied, successively increasing displacements, controlled by pre-existing nonuniformity of the shear stress relative to yield strength along the fault zone. Therefore, various types of the change of reseismic, anomalous surface deformation with time can be explained by assuming suitable variations of shear stress and/or yield strength along an earthquake fault.
