• Abstract and Figures
• Public Full-text

U.S. Department of the Interior U.S. Geological Survey Assessment of Regional Earthquake Hazards and Risk Along the Wasatch Front, Utah FRONT COVER. View to the southeast of downtown Salt Lake City, Utah, circa 1980. Wasatch Range with prominent Lake Bonneville shorelines in the background. Assessment of Regional Earthquake Hazards and Risk Along the Wasatch Front, Utah PAULA L. GORI and WALTER W. HAYS, Editors U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-K-R Contributions from Utah Geological Survey, University of Utah, and Utah State University Chapters K R are issued as a single volume and are not available separately. Chapter titles are listed in the volume table of contents. U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Charles G. Groat, Director Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. Author affiliations given in this volume were correct at the time the report was approved for publication. Published in the Eastern Region, Reston, Va. Manuscript approved for publication October 5,1995. Library of Congress Cataloging in Publication Data Assessing regional earthquake hazards and risk along the Wasatch Front, Utah / edited by Paula L. Gori and Walter W. Hays. p. cm. (U.S. Geological Survey professional paper : 1500K-R) Includes bibliographical references. Supt.ofDocs.no.: 119.16: 1500KZ. 1. Earthquakes Wasatch Range (Utah and Idaho) 2. Earthquakes Utah. I. Gori, Paula. II. Hays, Walter W. III. Series: U.S. Geological Survey professional paper; 1500. QE535.2.U6A84 2000 551.2'2'097922 dc20 92-33618 CIP For sale by U.S. Geological Survey, Information Services Box 25286, Federal Center, Denver, CO 80225 CONTENTS [Letters designate the chapters] Introduction, by Paula L. Gori and Walter W. Hays (K) Site Amplification in the Salt Lake City-Ogden-Provo Urban Corridor and the Implications for Earthquake- Resistant Design, by Walter W. Hays (L) Predicting Strong Ground Motion in Utah, by Kenneth W. Campbell (M) Probabilistic Analysis of Earthquake Ground-Shaking Hazards Along the Wasatch Front, Utah, by R.R. Youngs, F.H. Swan, M.S. Power, D.P. Schwartz, and R.K. Green (N) Relative Ground Response in Salt Lake City and Areas of Springville-Spanish Fork, Utah, by Kenneth W King, Robert A. Williams, and David L. Carver (O) In Situ Poisson's Ratio Measurements Near Prove, Utah, by Richard D. Miller, Don W. Steeples, Kenneth W. King, and Ralph W Knapp (P) Earthquake Losses in Central Utah, by S.T. Algermissen, E.P. Arnold, K.V. Steinbrugge, Margaret G. Hopper, and PS. Powers (Q) Isoseismals of Some Historical Earthquakes Affecting the Wasatch Front Area, Utah, by Margaret G. Hopper (R) Seismic-Risk Methods and Estimates for Utility Systems and State-Owned Buildings Along the Wasatch Front, by Craig E. Taylor and Delbert B. Ward III ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT, UTAH INTRODUCTION By Paula L. Gori and Walter W. Hays THE EARTHQUAKE THREAT of normal faulting that extends approximately 370 km at the western foot of the Wasatch Range. Earthquakes have There are many geologists who are very wise, but even they do not understand the forces which produce mountains. And yet it been reported since the arrival of the Mormon pioneers in must be admitted, not only that mountains have been made, but 1847. During the last two centuries, eight earthquakes of that some mountains are still rising. The mysterious forces magnitude1 greater than or equal to 6 have occurred in appear to act in different ways in different places, and it is pos­ Utah. The two largest were the magnitude (M L 7.0) 6.6 sible that their nature is not universally the same. Suffice it to Hansel Valley earthquake of 1934 (fig. 2) and the M L 6.5 say that in the Great Basin the movements they cause are verti­ cal. It is as though something beneath each mountain was Richfield earthquake of 1901. slowly, steadily, and irresistibly rising, carrying the mountain The historical record of seismicity in Utah has been with it. broadened by incorporating geologic evidence and moni­ So began an article in 1883 by Grove Karl Gilbert, the first toring small earthquakes. Although no large earthquakes Chief Geologist of the U.S. Geological Survey (USGS), (M>7.0) have occurred since Utah was settled, clear geo­ warning the citizens of the Great Basin about the poten­ logic and geomorphic evidence (Hamblin, 1976) demon­ tial for damaging earthquakes in their region. Describing strates that large earthquakes have occurred repeatedly the damage and loss of life in a small town in California throughout the late Pleistocene (2 million years before that he had just visited after an earthquake, Gilbert cau­ present) and Holocene (last 10,000 years) on segments tioned that a similar earthquake could occur in Salt Lake within the Wasatch fault zone. For this reason and City. "It is useless to ask when this disaster will occur. Our because of the continuing low level of seismicity, scien­ occupation of the country has been too brief for us to tists believe that some parts of segments of the Wasatch learn how fast the Wasatch grows; and indeed, it is only fault zone are overdue for a damaging earthquake by such disasters that we can learn. By the time experi­ (Schwartz and Coppersmith, 1984). Machette and others ence has taught us this, Salt Lake City will have been (this report) have identified 10 segments of the Wasatch shaken down...." (Gilbert, 1883, p. 4). fault that have produced large earthquakes on the average After more than a century of research, geologists and of every several hundred to 1,000 years. other scientists have increased their understanding of the tectonic processes at work in the Wasatch Front and are now beginning to answer the questions about the seismic- Magnitude is a "number that characterizes the size of an earth­ ity and tectonics of the Wasatch Front that Gilbert and quake, based on measurement of the maximum motions recorded by a seismograph for earthquake waves of a particular frequency. Scales others first posed. Scientists, engineers, architects, urban most commonly used in the Western United States are (1) local magni­ planners, and emergency managers are not waiting for a tude (ML) (commonly referred to as "Richter magnitude"), (2) surface- major earthquake disaster to learn that measures must be wave magnitude (Ms), and (3) body-wave magnitude (ms). None of these implemented to mitigate an earthquake's effects. They are scales satisfactorily measures the largest possible earthquakes because taking actions now to prepare for and to mitigate the each relates to only certain frequencies of seismic waves and because the spectrum of radiated seismic energy changes with earthquake size. physical effects of such an earthquake. The recently devised moment magnitude (M) scale, based on the con­ The majority of Utah's population lives adjacent to the cept of seismic moment, is uniformly applicable to all sizes of earth­ Wasatch fault zone (fig. 1), an active, north-trending zone quakes" (Ziony, 1985, p. 18). REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT 113° IDAHO 112 41 h- /' 0 50 100 KILOMETERS FIGURE 1 Location map of the Wasatch Front study area (shaded), northern Utah. Generalized fault locations are also shown. INTRODUCTION The USGS earthquake hazards program in the Wasatch Front area was conducted under the auspices of the "Regional Earthquake Hazards Assessment" ele­ ment, one of five elements in the USGS' Earthquake Hazards Reduction Program. The element was estab­ lished to provide concentrated and coordinated attention to geographic regions containing large urban areas at risk from earthquakes by utilizing past research and fostering partnerships with universities, the private sector, local government, and State and Federal agen­ cies. The effort provides a technical basis for devising and implementing mitigation and loss reduction meas­ ures. The five interrelated goals in the Utah comprehensive research and implementation program were: Information systems. To produce quality data along FIGURE 2. Fault scarp formed in Utah's Hansel Valley north of Great Salt with a comprehensive information system, available to Lake in the 1934 Hansel Valley earthquake. both USGS and non-USGS users, for use in earthquake hazards evaluations, risk assessment, and implementation Utah's largest urban centers, where more than 80 of loss reduction measures. percent of the State's 2 million people live, are located Synthesis of geological and geophysical data. To along the Wasatch Front. A large earthquake centered prepare synthesis reports describing the nature, extent, near Salt Lake City has the potential to cause extensive frequency of occurrence, and physical effects of the damage to buildings, lifelines, and public facilities. The earthquake hazards of ground shaking, surface faulting, expected level of peak ground acceleration would be in earthquake-induced ground failure, and tectonic deforma­ the range of 0.2 to 0.4 g or greater. Surface-fault rupture tion and recommend future research to increase the and tectonic deformation (fig. 3) as well as landslides and knowledge base required for the creation and implemen­ liquefaction would be expected to occur in many areas tation of mitigation and loss-reduction measures. (fig. 4). Death and injury rates could be high, depending Ground-motion modeling. To produce deterministic on the time of day that the earthquake struck and the and probabilistic ground-motion models and maps of the nature and extent of pre-event mitigation actions. ground-shaking hazard and commentaries on their use in building codes and land use regulations. The October 28, 1983, Borah Peak, Idaho, magnitude Loss estimation models. To devise economical meth­ (Ms) 7.3, earthquake reminded scientists and policymak- ods for acquiring inventories of structures and lifeline sys­ ers that large earthquakes will recur in the Intermountain tems in urban areas, to create a standard model for loss Seismic Belt (fig.

Load more

  289

Copy Link