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Show UTILIZING NEW SEISMOLOGICAL TECHNIQUES TO ANALYZE THE DECEMBER 27, 2003, LEVAN-NEPHI EARTHQUAKE SEQUENCE, CENTRAL UTAH J. Mark Hale (Kristine Pankow) Department of Geology and Geophysics Between December 27th and 29th, 2003, a sequence of 36 locat-able earthquakes occurred in the West Hills area near Levan, Utah (largest Richter local magnitude, ML, 3.7). The sequence began with three main earthquakes within a four-minute time span which had similar magnitudes (ML 3.6-3.7) and locations, as determined by the University of Utah Seismograph Stations (UUSS). These three earthquakes produced dissimilar peak ground accelerations and velocities at the same recording stations, which implies differences in focal mechanisms, focal depths, and/or other source properties. The routine locations for the earthquake sequence do not correspond to any known fault and appear to show artifacts caused by the velocity model used in the locations. The primary objective of this study is to use the Levan sequence as a test case for evaluating two new techniques developed in the last 4-5 years, as implemented in the computer codes HypoDD (Waldhauser & Ellsworth, 2000) and HASH (Hardebeck and Shearer, 2002). The double-difference HypoDD relocation technique makes it possible to compute high-precision relative relocations which minimize artifacts and errors caused by the velocity model. The HASH code computes the most probable double-couple focal mechanism from P-wave first motion data along with realistic uncertainty limits on the mechanism. The similarity of the P-wave first motion data from the three largest earthquakes in the December 2003 Levan sequence suggests that they have similar focal mechanisms. A composite focal mechanism determined using data from two of these earthquakes shows predominantly normal slip along a northerly-striking fault. The relocated epicenters from HypoDD show less scatter than the routinely-determined epicenters and form a north-northeast trending zone that is four kilometers long. With further analysis of the Levan data it may be possible to determine the geometry of the fault or faults on which the earthquakes occurred. During the process of studying the Levan data, it was found that the degree to which focal mechanisms can be constrained from first motion data is sensitive to the focal depth and the station distribution. Consequently, one goal of our study will be to determine criteria for when HASH can be successfully applied to determine focal mechanisms of Utah earthquakes. Initial work suggests that acceptable results can be obtained for earthquakes with foci deeper than 10 km in areas with good seismic station coverage. |