HR: 0800h
AN: G21B-1278    [Abstracts]
TI: Using MODES to Detect Zones of Simple-shear in Slumgullion Landslide and along the Chi-Chi Rupture
AU: Johnson, A M
EM: gotesson@purdue.edu
AF: Purdue University, Department of Earth and Atmospheric Sciences 550 Stadium Mall Drive, West Lafayette, IN 47907 United States
AU: * Griffiths, J H
EM: griffiths@purdue.edu
AF: Purdue University, Department of Earth and Atmospheric Sciences 550 Stadium Mall Drive, West Lafayette, IN 47907 United States
AB: Active faults and shear zones generally shift obliquely, accommodating components of both dip- and strike-shift so any method of detecting such shift must be capable of dealing with horizontal and vertical displacements or velocities. MODES, our method of detecting shear zones, uses three-dimensional GPS or other geodetic survey measurements to determine the style of deformation and the orientation of shear zones or faults; it determines the strike, dip and sense of shift of a shear zone. Data provided by mapping and surveying by Fleming et al. [1999] on the huge Slumgullion landslide in SW Colorado are a uniquely fine opportunity to relate the results of the MODES kinematic analysis of displacements on the landslide with the mapped patterns of structures in the landslide. For example, in one part of the ``hopper'' area of the landslide where the width of the slide narrows about 50% in the down-slope direction, the faults were interpreted to be oblique, right-lateral, normal faults. The results of the MODES analysis of displacements in the area are consistent with the strike and relative vertical displacements accommodated by the mapped faults, but they indicate that the faulting almost certainly was predominantly right-lateral/reverse rather than right-lateral/normal as suggested on the map. In most places on the landslide, though, the kinematics of the faulting shown on the map are completely consistent with the results of MODES. In a place near the toe of the active slide MODES detected a strike-shift shear zone that had not developed into a fault at the ground surface at the time of mapping. A through-going strike-slip fault was anticipated though because of a pattern of en-echelon tension cracks and oblique normal faults that was mapped in the area. Such structures are characteristically developed at the surface above a strike-slip fault propagating to the ground surface. The orientation of the zone of en-echelon cracks was N60° to 70°E. With the three-dimensional displacement data, MODES detected a strike-shift shear zone striking N65°E and dipping 89°, so the detected shear zone is essentially vertical and trends approximately parallel to the fault zone mapped by Fleming et al. [1999]. We have used MODES also to analyze the three-dimensional displacements measured with GPS across the 1999 Chi-Chi rupture in Taiwan and to determine the orientation and direction of shift of a shear zone representing the fault within the upper several hundred or thousand meters of ground below the surface. One value of the MODES kinematic analysis in this case is to provide boundary conditions for dislocation solutions for the subsurface shape of the main rupture during the earthquake. The near surface observations, of course, are limited to a few meters depth. In the process of performing the MODES analysis of GPS data for a few areas, we found examples of quadrilaterals along overall straight segments of the rupture with remarkably homogeneous deformations and a few examples of quadrilaterals at turns in the overall trend of the rupture with markedly inhomogeneous deformation.
DE: 5194 Instruments and techniques
DE: 8010 Fractures and faults
DE: 8011 Kinematics of crustal and mantle deformation
DE: 8012 High strain deformation zones
SC: Geodesy [G]
MN: Fall Meeting 2005