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Davis Journal

One year later – the Magna Earthquake

Mar 18, 2021 11:20AM ● By Tom Haraldsen

It’s referred to as the Magna Earthquake, even though what shook us all a year ago on March 18 was actually centered in a portion of West Valley City, and certainly affected us all. The 5.7 magnitude earthquake struck as many of us were getting ready to go to work or already enroute. A friend of mine in West Jordan said his car started swerving while he drove along 7800 South. More than 30,000 “felt” reports were received by the U.S. Geological Survey and the University of Utah Seismograph Stations. It was the largest quake ever recorded in the area, higher than the previous 5.2 quake in 1962.

No major injuries were reported from the mainshock or aftershocks. Damage occurred throughout the Wasatch Front, including at a few homes and businesses in Davis County, with the most severe damage in Magna. HAZUS, software used by the state to estimate potential losses, shows there could be upwards of $62 million in building-related damages, contributing to $629 million in total economic loss. This does not include damages to public infrastructure.

Over 2,500 aftershocks have been recorded since, as the Magna earthquake sequence is still ongoing but has seen a decrease in activity over the last year. As of Feb. 28, 2021, the UUSS has recorded 2,590 earthquakes as part of the sequence, including 1 in the M5 – 5.9 range; 6 in the M4 – 4.9 range; 30 in the M3 – 3.9 range; 137 in the M2 – 2.9 range; 698 in the M1 – 1.9 range; 1,507 in the M0 – 0.9 range and 83 < M0. There were also 28 earthquakes where no magnitude could be determined.

“The Magna earthquake sequence is one of the best recorded earthquake sequences in the entire Basin and Range province,” said Keith Koper, director of UUSS and chair of the Utah Seismic Safety Commission (USSC), in a release about the earthquake. “Our network of permanent and temporary seismometers created a wealth of data that led to several research opportunities both published and ongoing. This research has impacted our understanding about faults, earthquakes, and seismic hazard in Utah and the Intermountain West.”

Prior to the Magna earthquake, the exact location of the Wasatch fault was unknown, but many scientists thought it dipped at a steep angle deep beneath the Salt Lake Valley. Based on data from the Magna sequence, several research papers now suggest the sequence actually occurred on the Salt Lake City segment of the Wasatch fault. Evidence also suggests that the fault curves to a shallower angle and is not as deep beneath the surface as previously thought. 

Researchers conclude that because of the likely shallower dip of the fault, which would put it closer to the surface, ground shaking estimates may be higher than previously thought for future earthquakes on the Salt Lake City segment of the Wasatch fault. Based upon these findings, ground-motion scenarios for future Wasatch fault earthquakes should be reevaluated to account for increased shaking. 

The M5.7 was a moderate earthquake and a reminder that Utah is earthquake country. Utah is at risk of a major earthquake of M7 to 7.5, and the M5.7 Magna earthquake did not diminish that risk. Utahns can reduce their risk in an earthquake by being prepared beforehand. During an earthquake, drop, cover and hold on. Have on hand a minimum of a two-week supply of water, food, medicine and other necessities. Be prepared for long disruptions to utilities like power, water and sewer. 

For more on earthquake preparedness, we urge Utahns to visit bereadyutah.gov and participate in the Great Utah ShakeOut on April 15, 2021. Register at shakeout.org/utah. For more on the 2020 Magna earthquake sequence or general earthquake information for Utah, visit earthquakes.utah.gov.