SURVEY NOTES

Hazard News-Putting Down Roots in Utah’s Earthquake Country Second Edition Provides Updated and New Information

by Emily Kleber, John Good, Adam Hiscock, and Steve Bowman


Putting Down Roots in Earthquake Country

When the ground starts shaking from an earthquake, do you know what to do? Do you know why we have earthquakes in Utah, how we monitor them, and how we mediate their effects? The Utah Seismic Safety Commission (USSC) recently released the second edition of the booklet, Putting Down Roots in Earthquake Country—Your Handbook for Earthquakes in Utah, to help Utahns understand earthquake hazards, and prepare their family, friends, and community for a disaster. This booklet reminds Utahns that a major earthquake does not have to ruin life as we know it—we can take steps as individuals, families, and entire communities to be ready. 

Putting Down Roots in Earthquake Country (a.k.a. “Roots”) was first published for Utah in 2008 and is based on the successful booklets of the same name published by the U.S. Geological Survey for the San Francisco Bay area, northern, and southern California. Similar publications are available for Oregon, Idaho, Alaska, Nevada, and the central United States, and some states also include translated versions for non-English speakers. The last time Roots for Utah was updated was in 2014, when additional scientific data was added. Since then, there have been multiple scientific, preparedness, and engineering advances, as well as several notable earthquakes in Utah, including the March 18, 2020, magnitude 5.7 Magna, Utah, earthquake. In 2021, the USSC decided that the time was right to update Roots with the latest earthquake information available for Utah, with the Utah Geological Survey (UGS) leading the effort. This edition of Roots benefited immensely from strong partnerships with individuals representing organizations making up the USSC. The effort to update Roots was led by the UGS and the Utah Division of Emergency Management (DEM), with input on content from the experts at the University of Utah Seismograph Stations, Be Ready Utah, Envision Utah, the Structural Engineers Association of Utah, the U.S. Geological Survey, and the Federal Emergency Management Agency. This second edition represents countless hours of discussion, editing, and care to bring the best information to the people of Utah. 

This map shows the relative hazard from earthquake ground shaking in Utah. Areas that have experienced several historical earthquakes felt by Utahns, like the Wasatch Front, have the highest hazard. Data source: U.S. Geological Survey

The second edition of Roots contains several new pages that address important topics for the growing state of Utah. Utah’s population is growing on the Wasatch Front, but more people are also moving to areas like southern Utah and Cache Valley, which also have seismic hazards. New pages in Roots address the location of faults, history of past earthquakes in southern Utah and Cache Valley, and note special seismic hazard considerations for each area. Additionally, there is an expanded page on the hazard of liquefaction, which will affect areas having high groundwater levels and could cause an immense amount of damage to critical infrastructure in Utah, like water, sewer, and energy. 

Key updates addressing earthquake probability and response in Utah have been added to the second edition of Roots. A 2016 report from the Working Group on Utah Earthquake Probabilities compiled scientific data to create an “earthquake forecast” for the Wasatch Front. This group determined that there is a 1-in-2 chance (essentially a coin flip) of one or more earthquakes of magnitude 6.0 or larger in the Wasatch Front region in the next 50 years. Additionally, a 2015 report led by the Earthquake Engineering Research Institute and partners indicates the potential losses from a magnitude 7.0 earthquake on the Salt Lake City segment of the Wasatch fault, including economic losses, casualties, and impacts to infrastructure. This report has chilling implications, and the potential impacts have only worsened over time. These facts about our earthquake hazard in Utah are jarring, but knowledge is power, and with proper knowledge, we can address these big issues.

A significant problem for Utah with regards to earthquakes is our history of constructing buildings and homes using unreinforced masonry, mostly as brick. The second edition of Roots includes an in-depth explanation of what unreinforced masonry construction is, how to identify it, how it performs poorly when shaken by earthquakes, and how Utah came to have so much of this dangerous construction for an area with high earthquake hazard. This issue is important in Utah, and one that the USSC has been working hard on for decades. Two recent reports highlighted in the second edition of Roots include the Wasatch Front Unreinforced Masonry Risk Reduction Strategy and the Utah K-12 Public Schools Unreinforced Masonry Inventory. This new information aims to educate readers, inspire them to take proactive steps for themselves and their communities, and to improve unreinforced homes and buildings for all. 



This new edition of Roots includes a new page discussing a technology called “earthquake early warning.” Earthquakes cannot be predicted, but earthquake early warning technology can detect earthquakes quickly and broadcast a warning of the predicted arrival times of ground motion (shaking) and the severity (intensity) of shaking in the general region of the earthquake epicenter. Even if only seconds before strong shaking arrives, alerts can prompt critical actions to protect life and property. The technology has been used for decades in countries like Mexico, Japan, and Chile, and is currently being implemented in California, Oregon, and Washington. In the 2022 Utah legislative session, the UGS, University of Utah Seismograph Stations, and the DEM were funded to do a feasibility study for an earthquake early warning system in Utah to determine how this technology could be most effectively used in the Beehive State. The informational page in the second edition of Roots aims to educate the public about this technology and what it could mean for Utah. 

The second edition of Roots is available as a print copy for free at the Natural Resources Map & Bookstore, and online as a PDF document, as well as in an online interactive version. Copies will be distributed among agencies of the Utah Seismic Safety Commission, including the Division of Emergency Management and the University of Utah Seismograph Stations. The UGS is currently working on creating a Spanish language version, with the goal of translating into other languages in the future. The USSC plans to update Roots as new scientific data is gathered and analyzed, best practices change, and the public asks for more information. Please take some time to read through Roots, and also visit earthquakes.utah.gov for any additional questions or information about earthquake hazards in Utah. 


How to get a copy of Roots:

Department of Natural Resources Building
1594 W. North Temple Salt Lake City, Utah 84116-3154
Phone: 801-537-3320 (local)
Store Hours: Monday-Friday, 10 a.m. to 5 p.m.


How to get a copy of Roots:

Department of Natural Resources Building
1594 W. North Temple Salt Lake City, Utah 84116-3154
Phone: 801-537-3320 (local)
Store Hours: Monday-Friday, 10 a.m. to 5 p.m.


SURVEY NOTES

Glad You Asked: What are the Oldest Rocks in Utah?

by Stephanie Carney


Location of select Precambrian-age outcrops (red) in Utah discussed in this article.

Utah’s oldest rocks formed during the Precambrian, a time in Earth’s history that occurred 4,600 to 540 million years ago and was characterized by simple, single-celled organisms before the “Cambrian Explosion” of complex organisms around 540 million years ago. Many of Utah’s Precambrian outcrops are well studied and have been radiometrically dated. The results of these studies and analyses indicate that the oldest known rocks in Utah belong to the Green Creek Complex in northwestern Utah. 

The Raft River and Grouse Creek Mountains in Box Elder County are home to the Green Creek Complex, which comprises a 2.7-billion-year-old (Ga) schist (metamorphosed sedimentary rock) and a 2.5 Ga monzogranite (an igneous pluton that intruded the schist). These rocks were deposited and formed during the late Archean Eon of the Precambrian. The schist originated as sediments deposited on a passive margin (where virtually no tectonic activity is taking place, like the present-day Atlantic Coast) of the North American craton (the ancient core of the North American continent). The 2.7-Ga age records the maximum time of deposition of these sedimentary rocks. The monzogranite began as a magma body that intruded into the older sedimentary rocks from below, therefore sedimentation took place between 2.7 and 2.5 Ga. The minerals that make up the granite slowly crystalized as the magma body cooled in the subsurface, and the 2.5-Ga age records when the minerals formed. 

The rocks of the Green Creek Complex were subjected to multiple metamorphic events since 2.5 Ga. The first of these events was when the Grouse Creek block, a tectonic terrane which the Green Creek rocks are a part of, collided with the North American craton around 1.7 billion years ago during the early Proterozoic Eon of the Precambrian. The rocks also record later metamorphic events that affected the complex during the Late Cretaceous Period, around 95 and 82 million years ago. These later events would have been related to the Sevier orogeny, which was a mountain building event that lasted from about 160 to 80 million years ago.




Other Noteworthy Precambrian-age Rocks in Utah

Outcrop of the Farmington Canyon Complex gneiss in the Wasatch Range. Compass for scale. Photo courtesy of Zach Anderson, Utah Geological Survey.

The Farmington Canyon Complex is not quite as old as the Green Creek Complex, but is more accessible to Utahns, especially those living along the Wasatch Front. The Farmington Canyon Complex is exposed in the Wasatch Range east of Ogden, Layton, and Farmington, as well as on Antelope Island in Great Salt Lake, and consists of a suite of metamorphic rocks including schist, gneiss, and quartzite. Zircons from the Farmington Canyon Complex have yielded ages of about 2.4 Ga and the rocks were subjected to the same metamorphic event that affected the Green Creek Complex 1.7 billion years ago. The Frary Peak trail on Antelope Island winds through some of these impressively metamorphosed rocks.

Other notable Precambrian rocks are the Red Creek Quartzite and the Owiyukuts Complex, in the eastern Uinta Mountains; metamorphic rocks in the Beaver Dam Mountains in southwestern Utah; and metamorphic rocks in east-central Utah. The Red Creek Quartzite is in Daggett County near Clay Basin and the Wyoming and Colorado borders. It is composed of the metamorphic rocks quartzite, mica schist, and amphibolite and has yielded metamorphic ages around 1.7 Ga. A little south and east of the Red Creek outcrops is a relatively small exposure of highly metamorphosed gneiss called the Owiyukuts Complex. The gneiss there has yielded a metamorphic age of about 1.8 Ga. 

In southwestern Utah, ancient metamorphic gneiss, schist, and pegmatite are exposed in the Beaver Dam Mountains west of St. George. These rocks have a metamorphic age of about 1.8 to 1.7 Ga as well. Similarly, Precambrian gneiss, schist, and amphibolite rocks near Westwater Canyon of the Colorado River and the Coach Canyon area near the Utah-Colorado border in eastern Utah have metamorphic ages of about 1.7 Ga. 

How Are Ages Determined?

Example of zircon grains under cathodoluminescence microscope. Photo courtesy of Dr. Liz Balgord of Weber State University.

Radiometric or isotopic dating is one of the best ways to determine the age of an igneous or metamorphic rock. This dating technique measures how much of a radioactive isotope of an element has decayed over time. An isotope is a different form of the same chemical element; i.e., it is the same element but has a different mass (extra neutrons). An unstable or radioactive isotope (called a parent isotope) is one that decays over time to become a different stable elemental isotope (daughter isotope). The length of time it takes for one-half of the parent isotope to decay into its daughter isotope is called the half-life. For example, the unstable argon isotope 39Ar will decay to the stable form of 40Ar and has a half-life of 269 years, and the unstable samarium isotope 147Sm will decay to stable neodymium (143Nd) and has a half-life of 106 billion years!

One of the most reliable methods for determining the age of very old rocks is uranium-lead (U-Pb) zircon geochronology. Igneous rocks often contain the mineral zircon, which is an accessory mineral that crystallizes at high temperature inside a magma body. During crystallization, zircon incorporates uranium in its crystal structure but not lead. Lead is created and added to the crystal solely from the radioactive decay of uranium. Uranium has two radioactive isotopes, 238U and 235U. The isotope 238U decays to 206Pb and has a half-life of 4.47 billion years, and 235U decays to 237Pb and has a half-life of 710 million years. Because these two different decay series occur in the zircon, two ages can be determined by measuring the ratio of parent and daughter isotope for each decay track. The results of each measurement can then be compared to verify the age of the mineral and, therefore, the rock. 

An approximate age for sedimentary rocks can also be obtained using zircon geochronology. Zircon is a durable and hard mineral, and grains that have weathered out of igneous rocks are often deposited along with other minerals and sediments that eventually will form a sedimentary rock. A maximum age of the sedimentary rock can be determined from the age of the zircons within it. The age of the zircon grains can also point to where they came from, or their provenance, by comparing them to other known ages of local or regional igneous rocks.