Seismic CAT Scan of an Ancient Earthquake along the Oquirrh Fault, Utah

Dave Morey's 1997 MS Thesis

  • Introduction and Chapter 1 (PDF), (PS) and latex.
  • Chapter 2 (PDF), (PS) and latex.
  • Chapter 3 (PDF), (PS) and latex.
  • Chapter 4 (PDF), (PS) and latex.
  • Biblio latex, title latex, Abstract latex and gzip figures are in directory "/uufs/geophys.utah.edu/common/tomofs/www/htdocs/UTAMtheses/morey/figs".

    Abstract

    Two-dimensional and three-dimensional seismic surveys were conducted across the Oquirrh fault, of Central Utah with the purpose of imaging the fault structure to a depth of 40-60 ft. The scientific objective was to use the resulting reflectivity and tomographic images to deduce the paleoseismic history of this fault zone. The tomographic images were generated by conducting a seismic CAT (computer axial tomography) scan of the fault zone by inverting first-arrival traveltimes of 3-D seismic data. \

    The 3-D seismic data (112,896 traces) were collected over a 145 ft x 30 ft patch of ground along the Oquirrh fault Their first arrival traveltimes were then picked, and these traveltimes were inverted by a 3-D tomographic technique. To complement the 3-D tomogram, a 2-D reflectivity image was obtained by a 570 ft long reflection survey that cut across the fault. Results show that the 3-D tomogram clearly delineates the fault and a colluvial wedge, both of which correlate extremely well with the geologic cross-section taken from the adjacent trench. The thickness of the colluvial-wedge image provided an estimate of 22.8 ft for the surface displacement of the last surface-rupturing event on the Oquirrh fault. This thickness measurement was also used with the layer dip angles measured from the 2-D reflectivity section to give a moment magnitude estimate of 6.8 for the most recent earthquake. This is in very good agreement with the magnitude estimate of 7.0 based on a nearby trenching study.\

    This study demonstrates, for the first time, that seismic imaging methods can clearly delineate the shape and depth of a colluvial wedge associated with a normal fault earthquake. It also shows, for the first time, that information from reflectivity images and tomographic images can be combined to accurately estimate the size of prehistoric earthquakes. It is conjectured that a wider recording aperture could have enabled the imaging of multiple colluvial wedges at depth, and so provided the necessary information to estimate recurrence intervals of large magnitude earthquakes. This assumes that the timing of the colluvial wedge formation can be obtained by dating of cores drilled through the wedges. Thus, a new tool in paleoseismology is established where 3-D tomography combined with 2-D reflection imaging has the possibility of providing cheaper, deeper and wider, but less resolved, images of fault systems than the expensive excavation of trenches across faults. In some cases, this may provide a viable alternative to the intrusive task of fault trenching.