COLLUVIAL WEDGE IMAGING USING
TRAVELTIME TOMOGRAPHY ALONG THE
WASATCH FAULT NEAR MAPLETON, UTAH (MS Thesis)

Maike-L. Buddensiek, University of Utah


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ABSTRACT

Four high-resolution seismic surveys were conducted across the Wasatch Fault Zone near Mapleton, Utah. The objective was twofold: 1. Use traveltime tomography to delineate fault structures and colluvial wedges to more than twice the depth of the Mapleton Megatrench excavated by USGS personal, 2. Assess the strengths and limitations of traveltime tomography by comparing the tomogram to the ground truth seen in the Megatrench log.

Approximately, four out of the five faults within the trench area are recovered accurately in the tomograms and the main fault's dip angle is estimated to be between 71 to 80 degrees. Two additional faults are interpreted outside the trench. The faults can be delineated down to 35 m below the surface, which is 20 m deeper than the excavated trench. Four out of six colluvial wedges found in the trench log were seen as low-velocity zones (LVZ) in the tomogram. Waveform tomography prevailed over ray-based traveltime tomography by recovering 4 instead of 2 of the 6 colluvial wedges. A newly discovered LVZ at a depth of 21 m possibly represents a colluvial wedge and is estimated to be less than 21,000 years old. If this LVZ is a colluvial wedge, the earthquake history obtained by trenching can be extended from 11,000 years to 21,000 years with seismic tomography.

However, the results prove that seismic tomography is able to recover some, but not all colluvial wedges, so that this conjectured extension of the earthquake history is not certain. My results demonstrate the ability of tomography to accurately recover faults, and show that waveform tomography more accurately resolves colluvial wedges compared to traveltime tomography. A future task is to apply tomography to the Nephi fault segment and identify drilling sites for locating and dating colluvial wedges.