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Debris Flows in Utah UTAH GEOLOGICAL SURVEY SURVEY NOTES Volume 35, Number 1 January 2003 URBAN GROWTH AND INCREASED HAZARD VULNERABILITY Table of Contents Maps Show Potential Geologic Effects of The a Magnitude 7 Earthquake . 1 Debris Flows in Utah . 4 Director’s What About Great Salt Lake? . 5 Fire-Related Debris Flows Damage Perspective Houses . 6 New Utah Minerals . 7 Survey News . 7 by Richard G. Allis Energy News . 8 Glad You Asked . 9 GeoSights . 10 Triggered Seismicity in Utah from the everal articles in this issue of Sur- north of Provo), and at Santaquin (just Denali Fault Earthquake . 11 vey Notes highlight the rapid south of Provo), demonstrated that people are most receptive to messages Teacher’s Corner . 13 Surban growth along the Wasatch Front and the increased vulnerability about geologic hazard vulnerability Design by Vicky Clarke from geologic hazards. Utah had the when damaging events actually occur. Cover: Salt Lake City skyline. Photograph fourth highest population growth in Provo City is now taking advantage of by Frank Jensen, courtesy of the Utah Travel the country last decade (29%), with a grant from the Emergency Water- Council. most growth occurring along the Inter- shed Protection program of the NRCS State of Utah mountain seismic belt. Because of to reduce the debris-flow hazard from Michael O. Leavitt, Governor this, the state has the seventh highest Buckley Draw which also burned last Department of Natural Resources annualized earthquake loss rate ($51 summer, but fortunately missed expe- Robert Morgan, Executive Director million). The growth statistics for the riencing any severe summer down- UGS Board 1990s highlight another trend – that of pours. The Santaquin debris flows Robert Robison, Chair rapid urbanization occurring within were not triggered by the storm of the Geoff Bedell Craig Nelson E.H. Deedee O’Brien Charles Semborski the Wasatch Range, compared to the century; the 0.27 inches of rain that fell Steve Church Ron Bruhn traditional growth areas along the val- in probably less than 15 minutes could Kevin Carter (Trust Lands Administration-ex officio) ley floors and benches adjacent to the occur anywhere along Wasatch Front UGS Staff mountain fronts in northern and once every few years. The UGS is cur- Administration Richard G. Allis, Director southern Utah. The county with the rently working with the National Kimm Harty, Deputy Director highest growth rate during the 1990s Weather Service on compiling a pack- John Kingsley, Assoc. Director was Summit County (92%), with Park age of educational materials based on Daniel Kelly, Acct. Officer Becky Wilford, Secretary/Receptionist City being its largest city. This trend the Santaquin debris flows to capital- Jo Lynn Campbell, Admin. Secretary towards building in steeper terrain has ize on the window of opportunity and Linda Bennett, Accounting Tech. increased the risk of forest fire haz- increase the awareness of city planners Michael Hylland, Tech. Reviewer ards, and the associated subsequent and the public. Survey Notes Staff risks of debris flows. Repeated Editor: Jim Stringfellow On another topic, starting January 1, Editorial Staff: Vicky Clarke, Sharon Hamre droughts in recent years have com- 2003, anyone practicing geology before Cartographers: James Parker, Lori Douglas pounded these risks, with several the public in Utah requires a profes- Geologic Hazards Gary Christenson damaging debris flows occurring this William Lund, Barry Solomon, sional geologist license. The “grandfa- year (see Rich Giraud’s article on the Francis Ashland, Richard Giraud, ther” period when the ASBOG exam is Greg McDonald, Justin Johnson Santaquin debris flow). not required extends until January 1, Energy and Mineral Resources David Tabet Robert Gloyn, Robert Blackett, Roger Bon, The UGS endeavors to warn and 2004. During this time applicants still Thomas Chidsey, Bryce T. Tripp, Craig Morgan, advise local governments and their must meet educational and work J. Wallace Gwynn, Jeff Quick, Kevin McClure, city planners on the risks of geologic experience standards to receive a Sharon Wakefield, Carolyn Olsen, Cheryl Gustin, Tom Dempster, Mike Kirschbaum, hazards as new subdivisions are being license. Information about applying Brigitte Hucka planned. Unfortunately geologic haz- for licensure can be found at the State Geologic Mapping Grant Willis ards are not always a high priority Division of Occupational and Profes- Hellmut Doelling, Jon King, Bob Biek, Kent Brown, Michael Wright, when all the other growth pressures sional Licensure website: Basia Matyjasik, Douglas Sprinkel are also taken into consideration. This http://www.dopl.utah.gov/licens- Geologic Information and Outreach year’s debris flows near Alpine (just ing/geologist.html. Sandra Eldredge, William Case Christine Wilkerson, Mage Yonetani, Patricia Stokes, Mark Milligan, Carl Ege, Rob Nielson, Jeff Campbell Survey Notes is published three times yearly by Utah Geological Survey, 1594 W. North Temple, Suite 3110, Salt Lake City, Utah 84116; (801) 537-3300. The UGS is an applied scientific agency that creates, evaluates, and distributes information about Utah’s Environmental Sciences Michael Lowe geologic environment, resources, and hazards to promote safe, beneficial, and wise use of land. The UGS is a division of the James Kirkland, Charles Bishop, Janae Wallace, Department of Natural Resources. Single copies of Survey Notes are distributed free of charge to residents within the United Martha Hayden, Hugh Hurlow, Kim Nay, States and Canada and reproduction is encouraged with recognition of source. Don DeBlieux, Matt Butler ISSN 1061-7930 S URVEY N OTES 1 New Maps Show Potential Geologic Effects of a Magnitude 7 Earthquake in the Salt Lake City Area by Barry J. Solomon Introduction using HAZUS computer software, In 1860, essayist and poet Ralph which was developed for the Federal Waldo Emerson wrote, “We learn Emergency Management Agency for geology the morning after the earth- use in estimating losses and planning quake, on ghastly diagrams of cloven for emergency preparedness, mountains, upheaved plains, and the response, and recovery. dry bed of the sea.” Human nature The earthquake hazards we mapped remains the same now as then—we include surface fault rupture, tectonic often learn our geologic lesson after subsidence, earthquake ground shak- an earthquake, rather than plan in ing, liquefaction, and earthquake- advance. The seismically active cen- induced landsliding. Most hazards tral Wasatch Front of Utah, with a were mapped by considering the A strong earthquake in the Salt Lake City population of about 1.7 million cen- thickness of unconsolidated deposits area may create a fault scarp similar to this, tered upon Salt Lake City, has the formed during the 1983 Borah Peak earth- (“soil” to geologists and engineers), potential to be shaken by a strong quake in Idaho. rock and soil properties, and the rela- (magnitude 7) earthquake. However, tionship of thickness and properties to the region has not experienced a Mapped Earthquake Hazards from effects observed in historical earth- strong earthquake in historical time. the Scenario Earthquake quakes worldwide. To help understand earthquake risks Surface fault rupture: Movement and estimate losses in the region, we along faults deep within the earth The Scenario Earthquake mapped geologic hazards posed by a generates earthquakes. In Utah, if the The Wasatch fault zone trends north- magnitude 7 earthquake along the earthquake is strong enough, com- south through the Wasatch Front and Salt Lake City segment of the Wasatch monly greater than magnitude 6.5, the is divided into 10 segments, including fault zone, a major active zone of nor- fault movement will break to the the Salt Lake City segment. Geologic mal faulting. ground surface. Along the Wasatch evidence indicates that the Salt Lake fault zone, this surface fault rupture Federal, state, and private-industry City segment generates large earth- will form a near-vertical scarp as one partners cooperated in this study, par- quakes (approximately magnitude 7) side of the fault is uplifted and the tially funded by the U.S. Geological on average every 1,350 years, the other side is downdropped. We esti- Survey’s National Earthquake Haz- most recent having been about 1,300 mate an average scarp height of 6.1 ards Reduction Program and with years ago. Because a large earthquake feet where faulting occurs along the additional support and technical on the Salt Lake City segment will East Bench of Salt Lake Valley. This assistance from the Utah Division of affect the greatest number of people amount of displacement is capable of Emergency Services and Homeland and probably produce the greatest causing irreparable damage to any Security, URS Corporation, and Pacific losses along the central Wasatch structure built across the scarp. A Engineering & Analysis. Our hazard Front, we selected a magnitude 7 zone of additional deformation will maps will provide the geologic basis event on this segment as the scenario likely accompany the main scarp on for a comprehensive loss estimate earthquake. its downthrown side. 2 S URVEY N OTES Peak horizontal ground accelerations resulting from a M 7 scenario earthquake along the Salt Lake City segment of the Wasatch fault zone. S URVEY N OTES 3 Tectonic subsidence: When earth- rated, cohesion- quake faults break the ground surface less soils (com- in a geologic setting like the Wasatch monly sand) to Front, the adjacent valley floor may lose their strength drop down and tilt towards the fault, and ability to creating a subsidence trough. The support the extent of tilting is controlled by the weight of overly- amount and length of surface fault ing soil and struc- displacement. tures. Liquefac- tion is one of the When the Salt Lake Valley floor tilts major causes of during the scenario earthquake, the earthquake dam- shoreline of Farmington Bay in Great age. Salt Lake will shift to the southeast. Developed areas near the lake shore During our sce- and in the northern Jordan River nario earthquake, flood plain may be flooded if the level much of Salt Lake of Great Salt Lake is high when shift- Valley and nearby A landslide caused by the 1992 St.
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