BENEATH THE TRENCH: HIGH-RESOLUTION SEISMIC TOMOGRAMS AND SEDIMENT CORES APPLIED TO PALEOSEISMOLOGY, MERCUR, UTAH

Sediment coring combined with seismic tomography has the potential to extend the paleoseismic record beyond the limits of trenching. We test these methods at Mercur, Utah, by drilling through vertically-stacked, suspected colluvial packages that record multiple earthquakes to look for evidence of earthquake clustering. Two high-resolution seismic tomograms have successfully imaged a low-velocity zone (LVZ) corresponding to a colluvial package that records four earthquake events in an adjacent paleoseismic trench. The tomograms also indicate a deeper LVZ that we interpret as a second colluvial package associated with significantly older earthquakes than those logged in the trench. A dual-wall percussion hammer drill rig is used to recover 4 in. diameter, relatively undisturbed, sediment cores through the deeper colluvial package on each of the two tomograms. Four additional cores include two cores drilled basinward of the fault scarp, and two cores drilled in a channel along strike. Fine-grained sediments interpreted as loess are found at various depths in these cores. Loess accumulates preferentially on the lower third of the fault scarp, and therefore, our cores penetrate the distal portion of the colluvial package. A thicker fine-grained zone that correlates between all six cores is interpreted as the remnants of a buried soil horizon and the depth of that zone corresponds to the base of the deeper LVZ on the seismic tomograms. To bracket the timing of surface-rupturing earthquakes that pre-date the events logged in the trench, samples from the fine-grained deposits are currently being analyzed with stimulated luminescence techniques.