Applications of geophysical inverse theory to faulting and dike intrusion

Yijun Du, Purdue University

Abstract

Surface deformation in the form of length, angle and elevation changes recorded by geodetic networks are inverted to determine slip and opening at depth using geophysical inverse theory. The forward model is based on three-dimensional elastic dislocation theory in a homogeneous isotropic half space. Three problems are considered. (1) Various inversion techniques are compared to test factors that enter into the selection of the most suitable technique for a specific problem. The techniques are applied to the 1983, Borah Peak, Idaho, earthquake to determine a meaningful deformation model by using all available geological and geophysical information. It is shown that constraining information plays an important role in each level of solving an ill-posed inverse problem, and dictates the selection of a particular inversion technique. (2) Inversion of elevation changes associated with the January 1979 intrusion in Krafla, northern Iceland reveals three-dimensional characteristics of the dike and associated normal faults, and suggests that faulting occurred in advance of the laterally-propagating dike. The distribution of dike opening displacements provides an estimate of driving force and information about the dynamics of lateral dike propagation. (3) The coseismic geodetic data associated with three consecutive moderate earthquakes along the central Calaveras fault are inverted for slip at depth. The perturbed stress field corresponding to each displacement field is calculated. These results together with the seismic information are used to examine how tractions on fault segments are transferred in space due to contributions by different loading processes. It is shown that the faulting processes along the central Calaveras fault are highly heterogeneous and that slip along a fault segment has a profound effect on the stress field around the neighboring segments. Coseismically-transferred shear stress only accounts for a few percent of the total stress drop of an earthquake. It is inferred that the most significant process of the stressing should be due to viscoelastic relaxation of the lithosphere and asthenosphere. These results demonstrate that inversion techniques together with constraining information provide a tool for determining fundamental characteristics of physical processes at depth.

Degree

Ph.D.

Advisors

Aydin, Purdue University.

Subject Area

Geology|Geophysics

COinS