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Seismicity, Seismotectonics and Preliminary Earthquake Hazard Analysis of the Teton Region, WY

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Seismicity, Seismotectonics and Preliminary Earthquake Hazard Analysis of the Teton Region, WY FINAL TECHNICAL REPORT DEVELOPMENT OF EARTHQUAKE GROUND SHAKING HAZARD MAPS FOR THE YELLOWSTONE- JACKSON HOLE-STAR VALLEY, WYOMING Submitted to the U.S. Geological Survey Under the National Earthquake Hazards Reduction Program Program Element II Evaluate Urban Hazard and Risk USGS Award 05HQGR0026 Prepared by Bonnie Jean Pickering White Department of Geology and Geophysics The University of Utah Salt Lake City, UT 94112 and Robert B. Smith Department of Geology and Geophysics The University of Utah Salt Lake City, UT 94112 Principal Investigator Ivan Wong Seismic Hazards Group URS Corporation 1333 Broadway, Suite 800, Oakland, CA 94612 Phone: (510) 874-3014, Fax: (510) 874-3268 E-mail: [email protected] 26 September 2006 __________________________ This research was supported by the U. S. Geological Survey (USGS), Department of the Interior, under USGS Award Number 05HQGR0026. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied of the U.S. Government. PREFACE The Yellowstone-Jackson Hole-Star Valley corridor is located within the seismically and tectonically active Intermountain Seismic Belt in westernmost Wyoming and eastern Idaho. The corridor has the highest seismic hazard in the Intermountain U.S. based on the U.S. Geological Survey’s National Hazard Maps. The region contains the heavily visited Yellowstone and Teton National Parks and the rapidly growing areas of Jackson Hole and Star Valley. Although there has only been one large earthquake in this region in historical times (1959 moment magnitude [M] 7.5 Hebgen Lake), abundant geologic evidence exists for the past occurrence of surface-faulting earthquakes of M 7 or greater. In addition, background seismicity not associated with known faults and whose maximum magnitude is about M 6½ is relatively abundant within this portion of the Intermountain Seismic Belt and must be considered in seismic hazard evaluations. A Project Team of URS Corporation, the University of Utah, the U.S. Geological Survey (USGS), and Pacific Engineering & Analysis has proposed to develop a series of 12 deterministic earthquake scenario and probabilistic ground shaking maps for the Yellowstone-Jackson Hole-Star Valley corridor. Ground motions will be estimated based on the most up-to-date information on seismic sources, crustal attenuation and near-surface geology. Given the location of the corridor adjacent to major faults such as the Teton and Grand Valley faults, seismic hazard evaluations need to not only consider the hazards from numerous seismic sources, but also address the near-source effects on ground motions such as hanging wall and rupture directivity effects, as well as soil amplification effects. In December 2004, URS and the University of Utah received funding to support an initial task of the whole project, the analysis of the Jackson Lake seismographic data. The work was performed by Ms. Bonnie Jean Pickering White, a graduate student in the Department of Geology and Geophysics at the University of Utah. She was supervised by Dr. Robert Smith. The enclosed Master of Science Thesis by Ms. Pickering White describes the analysis of the Jackson Lake network as well as additional analyses of the seismotectonic setting of the Teton region. SEISMICITY, SEISMOTECTONICS AND PRELIMINARY EARTHQUAKE HAZARD ANALYSIS OF THE TETON REGION, WYOMING by Bonnie Jean Pickering White A thesis submitted to the faculty of The University of Utah in partial fulfillment of the requirements for the degree of Master of Science in Geophysics Department of Geology and Geophysics The University of Utah August 2006 Copyright © Bonnie Jean Pickering White 2006 All Rights Reserved ABSTRACT The U.S. Bureau of Reclamation (BoR) released their recorded earthquakes in the Teton region from 1986 to 2002 that focused on the seismic safety of Jackson Lake dam. The historic seismically quiescent Teton fault zone was the main focus of the seismic monitoring, but the data provided broad coverage of the general Teton Range, Jackson Hole and surrounding areas. The main objectives of this study were to generate an accurate hypocenter and magnitude list of regional earthquakes, analyze tomographic imaging to determine a three-dimensional P-wave velocity model, and integrate these data with GPS measurements of ground motion to better understand the seismotectonics of the Teton Region. These data provide key information for the seismic hazard analysis of the area. Seismicity was recorded on up to 20 short-period and 5 broadband seismographs focusing on the effects of earthquake hazards of the BoR operated Jackson Lake, Palisades and Grassley Lake dams. Data available for the study included seismic station coordinates, P-wave picked arrival times, and hypocenters of 8,000+ earthquakes located by a one-dimensional velocity model from 1986 to 2002 of magnitude 0.1 < M < 4.7. The P-wave data that initially had been determined by automated first arrival picks were repicked manually with an average picking error of < 0.12s for 10% of all the events including M > 2.0. This data quality check was vital to ascertain that the uncertainties were acceptable for the tomographic algorithm. The P-wave arrival time data were then used to produce a new catalog of 8,537 precisely located earthquakes with hypocenter location uncertainties using a nonlinear probabilistic method and tomography-determined three-dimensional velocity model. Hypocenter residuals in time were improved by 59%. The resulting stress-field orientations derived from accurate focal mechanisms revealed dominantly east-west extension across the Teton fault with a northeast-southwest stress orientation along the northern Teton fault area and southern Yellowstone region. We noted that there was no correlation of hypocenters with the down dip projection of the Teton fault. On the other hand, ground deformation data from GPS and leveling data revealed an unexpected result suggesting a reverse loading of east-west compression in the vicinity of on the Teton fault. Combined results from the 1987-2003 campaign GPS surveys, showed an average valley floor uplift of ~0.5-1.5 mm/yr and a valley floor west motion of ~1 mm/yr with respect to the mountain block. These observations may reveal a cause of the seismic quiescence of the Teton fault compared to its Late Quaternary fault loading rate ~2-3 mm/yr. The three-dimensional tomography image revealed two notable large low velocity zones at 0 km (sea level) to 5 km depth, centered beneath the Jackson Lake dam and beneath the southern end of Jackson Hole. P-wave velocities of 4.2 km/s to 4.6 km/s correlate with low gravity anomalies of -245 mgal centered near the Jackson Lake Dam and -225 mgals located near the town of Jackson, WY. The combination of the accurate hypocenter and accurate magnitudes along with the seismotectonic analysis helped to refine location and geometry of the background seismicity as key input for a probabilistic earthquake hazards analysis. The preliminary probabilistic seismic hazard analysis was conducted for four sites in the Jackson Hole valley along with regional hazard maps for the greater Teton and Yellowstone area. Preliminary results show the largest peak ground acceleration hazard is located around the Teton fault due to its large average 1.3 mm/yr slip rate, which is the largest slip rate in the greater Teton and Yellowstone region. TABLE OF CONTENTS ABSTRACT....................................................................................................................... iv LIST OF FIGURES ........................................................................................................... ix LIST OF TABLES........................................................................................................... xiii ACKNOWLEDGEMENTS............................................................................................. xiv Chapters 1. INTRODUCTION ...................................................................................................1 2. SEISMOTECTONICS OF THE TETON REGION................................................8 2.1. Geology and Tectonic History...................................................................8 2.2. Historic and Regional Seismicity.............................................................16 2.3. Jackson Lake Seismic Network Data.......................................................20 3. METHODOLOGY AND RESULTS .....................................................................25 3.1. Cross-Correlation of Seismic Waveforms ................................................26 3.1.1. Theory..........................................................................................27 3.1.2. Master Station Selection and Clustering Results.........................29 3.2. Minimum One-Dimensional Velocity Model...........................................34 3.2.1. Theory..........................................................................................35 3.2.2. Calculation of Minimum 1D Model: Results and Performance Tests........................................................................36 3.2.3. Discussion....................................................................................50 3.3. Three-Dimensional P-wave Tomography ................................................54 3.3.1. Theory..........................................................................................55 3.3.2. Calculation of
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