Report 10 Seismic CAT-Scan of an Ancient Earthquake

Two-dimensional and three-dimensional seismic surveys were conducted across the Oquirrh fault, Utah with the purpose of imaging the fault structure to a depth of about 15 m. Results show that the 3-D tomogram clearly delineates the fault zone and a colluvial wedge, both of which correlate extremely well with the geologic cross-section interpreted from an adjacent trench. The thickness of the colluvial wedge image is used to give a moment magnitude estimate of 6.8 for the most recent earthquake on this fault, which is in close agreement with the 7.0 estimate based on a nearby trenching study. This study demonstrates, for the first time, that seismic imaging methods can be used to accurately estimate the size of prehistoric earthquakes. Thus, seismic tomography has the possibility of providing cheaper, deeper and wider, but less resolved, images of fault systems than the intrusive excavation of trenches across faults.