On determining the orientation and amount of simple-shear: Implications for detecting shear zones and faults with GPS

Jake Griffiths, Purdue University

Abstract

This dissertation introduces the MODES "method of detecting simple-shear," which is specifically designed for extracting the amount and preferred orientation of simple-shear from the deformation-gradient and velocity-gradient tensors. Because simple-shear is diagnostic of shear zones and faults, and the Global Positioning System (GPS) is used in tectonic studies to determine the deformation-gradient or velocity-gradient tensor, this study provides improvements and extensions to existing analytical methods used in active tectonics studies, e.g., strain analysis and dislocation theory. Furthermore, methods and procedures developed in this study have implications for detecting and analyzing shear zones and faults that develop at the ground surface. The derivation of MODES is based on one definition and two assumptions: by definition, simple-shear deformation becomes localized in some way; by assumption, the twirl within the deformation-gradient (or the spin within the velocity-gradient) is due to a combination of simple-shear and twist, and coupled with the simple-shear and twist is a dilatation (or contraction). The preferred orientation is thus the orientation of the plane containing the simple-shear and satisfying the mechanical and kinematical boundary conditions. The dilatation coupled to the simple-shear is normal to the preferred orientation. Shear zones and faults can shift obliquely, and their walls can dilate and rotate accommodating strike- and dip-shift, opening and closing, and a scissors-like action. Therefore, any method of detecting the simple-shear associated with such shear zones must be capable of dealing with two- and three-dimensional deformations. MODES deals with such deformations and this dissertation illustrates how by challenging it with displacements computed from theoretical models of slip on buried faults, displacements measured for earthquake ruptures in Taiwan and Turkey, and secular velocities measured near the Marmara Sea region of Anatolia. Taken together, the chapters in this dissertation illustrate how MODES works and provide new insights---e.g., the orientation of a shear zone at the ground surface is determined by the existence and amount of dilatation normal to the zone---into the orientation and kinematics of shear zones that developed at the earth’s surface in association with ground-rupturing earthquakes.

Degree

Ph.D.

Advisors

Granger, Purdue University.

Subject Area

Geophysics

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