An inconvenient geological truth is staring at the Intermountain West. One of its most densely populated metropolitan area happens to also be its most seismically unstable, raising the possibility — some would say inevitability — of catastrophic loss of life and economic upheaval in the event of a major earthquake.
A network of fissures, known as the Wasatch fault zone, runs 220 miles from central Utah north along the Wasatch Front through Salt Lake City into Idaho, a region with as many as 185,000 homes and other structures built of unreinforced masonry, a construction type prone to crumbling when the earth moves. Last month’s magnitude 5.7 temblor was but a mild taste of what could happen.
Now the Utah Geological Survey has released a new report after four years of study, mapping the myriad Wasatch Front faults capable of surface rupture, the kind of seismic movements that can damage buildings and infrastructure.
The Wasatch fault zone is comprised of 10 segments — from Malad, Idaho, to Fayette, Utah. The new study expands the length of surface-rupture faults that are mapped by 64% — from 451 miles to 739 miles, with the biggest gains notched for the Provo, Salt Lake City and Weber segments. Researchers emphasized their findings do not indicate an increase in earthquake risk.
“What has changed is our resolution of the faults and our ability to more closely see the smaller portions of the fault zones or systems,” said Nathan Schwebach, spokesman for the Department of Natural Resources. “It’s kind of like our human circulatory system. The circulatory system includes large arteries and smaller veins. Yet it’s one system. Because of this research, we have now been able to zoom in more closely on smaller sections of the [Wasatch Front] fault systems, which significantly improves our understanding of the entire zone.”
The study harnessed airborne light detection and ranging (or lidar) elevation data to improve the accuracy, resolution and detail of current fault maps.
“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, a co-author of the report. “This new study provides detailed fault mapping and delineates ‘special study zones’ for the Wasatch fault zone from southern Idaho to central Utah.”
(Christopher Cherrington | The Salt Lake Tribune)
A fairly recent technology, lidar is a surveying method that illuminates a target with laser light and measures the reflected light, recording differences in laser return times and wavelengths. These differences are used to render digital three-dimensional representations of the ground.
“We can wipe away all the trees and buildings from the data so we can just see the surface without distractions,” Kleber said. “We can calculate the slope and see subtle changes in the slope.”
The new study also helped locate previously unmapped fault traces and find potential sites for future investigations into earthquakes that occurred in the distant past, known as paleoseismic events. These investigations should be pursued “in areas that are being rapidly developed or are in areas lacking good earthquake timing and recurrence information,” the report recommends.
The special study zones and fault mapping will soon be available on a hazards app that the UGS is beta testing. The agency has already posted an interactive map illustrating faults and folds that produced earthquakes exceeding magnitude 6.5 over the Quaternary Period, which covered the past 2.6 million years. These geological structures are those that are most likely to produce large quakes in the future.
Not covered in the study is the fault that shifted near Magna on March 18, damaging dozens of homes and scores of historic buildings. That quake and a previous temblor in the same area in 1962 were not strong enough to breach the surface, so that fault remains hidden underground.
University of Utah seismologists have deployed dozens of portable seismometers around the northern parts of the Salt Lake Valley to record hundreds of aftershocks stemming from that magnitude 5.7 jolt. This research is hoped to help map the lesser faults underlying the valley with great precision.
While the new lidar-based study helps pin down fault locations, it lacks the precision necessary to determine where it’s best to safely site buildings on specific lots.
(Images courtesy of the Utah Geological Survey) These images compare an aerial photograph, left, with recently acquired lidar slope-shade images, center and right, from the Levan segment of the Wasatch fault zone. The fault trace is faintly visible in the aerial photo, but far more visible on the slope-shade images. The right image shows the interpreted fault trace.
“While we recommend additional site-specific investigation prior to building," UGS hazards geologist Greg McDonald said, “it’s up to local agencies to regulate development within our delineated special study zones.”
The study proposes special study areas around fault traces to look for ground cracking, tilting, and minor faulting that usually accompany surface ruptures, according the UGS. This zone of deformation occurs adjacent to the main fault scarp and can extend hundreds of feet, mostly on the “downthrown,” or valley side of the fault.
“Those are red flag areas," Kleber said, “that we suggest cities ask for extra studies be done.”
Researchers also tapped an archive of historical aerial photographs. Many of these pictures were shot in the early 1970s, predating much of the residential and commercial development now filling the fault zone. Aerials of the Salt Lake Valley date back to 1936.
“These photographs were most useful for mapping in urban areas, where surface fault traces have been obscured by modern ground disturbance,” the report states.
To complete the study, Salt Lake County and it cities, along with the Utah Division of Emergency Management, helped pay for a massive data-acquisition effort in 2014. The Utah Geological Survey teamed with the U.S. Geological Survey to hire a plane and conduct overflights, photographing the fault zones with a laser camera. Kleber said they covered 3,000 square miles and recorded 40 billion lidar points.
