Method of Detecting Simple-shear

We have derived a method of detecting simple-shear (MODES), which is characteristic of faults and shear zones, using three-dimensional displacements or velocities. In this poster, we present the theory of MODES and illustrate how it works by analyzing a set of displacements measured with the Global Positioning System in a quadrilateral of stations across the 1999 Chi-Chi earthquake ground rupture south of T'ai-chung City, Taiwan [Yu et al., 2001]. The results are illustrated by means of a three-dimensional diagram, the spherical hamburger, which is reminiscent of the seismologist's ``beach-ball.'' The basic assumption of MODES is that the components of a deformation tensor are continuous within a domain of the earth's surface containing survey stations where three components of displacement have been measured. There are no assumptions made about the styles of deformation or the orientation of shear zones and faults. Instead, these quantities are determined by MODES, which consists of three parts: (1) analysis of a deformation tensor to determine whether it contains simple shear and if so determine the orientation of the simple-shear zone in terms of coordinates where S is the direction, ST is the plane, and N is the normal to the plane of simple-shear, (2) calculation of the deformation tensor in the (S, N, T) coordinates, and (3) determination of the importance of the simple shear by comparison of the amount of simple-shear to the amount of pure-shear.