UGS RI-280: Fault Trace Mapping and Surface-Fault-Rupture Special Study Zone Delineation of the Wasatch Fault Zone, Utah and Idaho
By Greg N. McDonald, Emily J. Kleber, Adam I. Hiscock, Scott E.K. Bennett, and Steve D. Bowman
Mark Milligan, Geologist, Utah Geological Survey, (801) 537-3326 – MarkMilligan@utah.gov
PRESS RELEASE: New study provides crucial information for reducing earthquake risk in densely populated and rapidly developing areas adjacent to active Wasatch Front faults
Salt Lake City (April 28, 2020) – A recently released four-year study by the Utah Geological Survey (UGS) provides new detailed mapping of faults along the Wasatch Front that are capable of surface rupture. In a large earthquake, surface rupture could cause significant damage to homes, schools, businesses, and other buildings and infrastructure along the Wasatch Front.
Researchers used recently acquired high-resolution elevation surveys (lidar) derived from laser sensors mounted on aircraft. This fault mapping and the new delineation of surrounding “special study zones” provide critical tools to local governments for land use planning and regulation, and will be of great use to potential homebuyers.
Last month’s Magna earthquake that shook the Salt Lake Valley serves as a wakeup call to those living along the Wasatch Front. The moderate Magna earthquake measured magnitude 5.7. Faults in the area are capable of generating earthquakes of up to about magnitude 7.6 and releasing 700 times more energy! Earthquakes larger than about magnitude 6.5 can rupture the ground surface, producing fault scarps from a couple inches high to up to 20 feet high and 40 miles long on the Wasatch Front. Fault scarps are hazardous building locations because they can reactivate during subsequent earthquakes.
“As a result of this research, we better understand where we’ve had surface-rupturing earthquakes in the geologic past, and where we may have them in the future. Knowing where fault scarps are present helps us make better land use decisions now and in the future,” said UGS hazards geologist Emily Kleber. “This new study provides detailed fault mapping and delineates “special study zones” for the Wasatch fault zone from southern Idaho to central Utah.”
While the study’s increased accuracy and detail helps to pin down fault locations, it is not precise enough to safely locate specific buildings on individual lots. Furthermore, ground cracking, tilting, and minor faulting usually accompany surface fault rupture. This zone of deformation occurs adjacent to the main fault scarp and can extend hundreds of feet, mostly on the downthrown (valley) side of the main scarp. Based upon such concerns, this report delineates “special study zones” around fault traces. “While we recommend additional site specific investigation prior to building, it’s up to local agencies to regulate development within our delineated special study zones,” said UGS Hazards Geologist Greg McDonald.
This study was a collaborative effort between the UGS and the U.S. Geological Survey. Along with new lidar imagery, mappers utilized previous geologic mapping and studies of ancient earthquakes, historical aerial photography, and field investigations. In addition to increased detail of previously mapped faults, the study identified new fault traces and potential sites for future field investigations of Wasatch Front faults.
UGS Report of Investigation (RI) 280, Fault Trace Mapping and Surface-fault-rupture Special Study Zone Delineation of the Wasatch Fault Zone, Utah and Idaho, is available (PDF) for free from the UGS website. Print-on-demand copies are available for purchase from the Natural Resources Map & Bookstore, 1-888-UTAHMAP, www.mapstore.utah.gov.
RI 280 summarizes the background, approach, and results of the mapping that was completed, and describes how the special study zones were delineated, but does not include fault trace mapping, which is currently available in an interactive map on the UGS website at https://geology.utah.gov/apps/qfaults/index.html. Special study zone delineations and the fault mapping will both be available through a soon-to-be-launched UGS hazards app that is currently in beta testing.
This work was made possible through the financial support of the U.S. Geological Survey Earthquake Hazards Program, Salt Lake County and their local city partners, the Utah Division of Emergency Management, the UGS, the U.S. Environmental Protection Agency, and the Utah Automated Geographic Reference Center. The Utah Geological Survey, a division of the Utah Department of Natural Resources, provides timely scientific information about Utah’s geologic environment, resources, and hazards.