The source motion of 2003 Bam (Iran) earthquake constrained by satellite and ground-based geodetic data

Abstract

The interpretation of coseismic surface deformation measurements through inversion techniques is of major importance to understand the mechanical behaviour of a seismic fault. Dense geodetic data sets in the vicinity of the ruptured fault provide unique constraints on detailed fault geometry and slip distribution at depth, making them complementary to seismological data.

Bam earthquake (Mw 6.6, 2003 December 26) induced surface deformation has been precisely mapped by Envisat ASAR interferometry and by subpixel correlation techniques applied to Spot-5 and ASAR amplitude images. These oblique and horizontal estimations of deformation have been completed with one levelling profile along the main road crossing the rupture from west to east.

We process these data (separately and jointly) in a two-step inversion technique, within the elastic half-space theory framework. Our objective is to determine the dislocation model at depth that satisfies simultaneously all the geodetic constraints. Also, we estimate the relative contribution of each geodetic data set to this inversion process. We first use a stochastic direct approach called neighbourhood algorithm in order to estimate the average characteristics of the rupture, and their relative uncertainty. Constraining in this way the geometry of the ruptured fault, we then linearize the inverse problem and compute the slip distribution on the fault using a standard weighted least-square technique, assuming the solution is smooth to some degree. At each step, we discuss the optimal models, their stability as well as the relative influence of each data set on the derived models parameters.

Our preferred model reveals a shallow dislocation on a quasi-vertical fault, slightly dipping towards east. The slip vector has a strike-slip component as high as 2 m, while the dip-slip component seems negligible. However, the estimation of the resolution matrices emphasizes the fact that the details of deep fault slip distribution remain out of the scope of this ill-conditioned inverse problem. Yet, our preferred model suggests a main dislocation limited at depth between 1 and 6 km. By contrast, the aftershocks observed in the months following the earthquake are located just beneath the estimated main shock.