March 18, 2020 · 7:09 a.m.
Magna was a warning shot
On a Wednesday morning in the first week of pandemic shutdowns, a magnitude 5.7 earthquake struck about 7 miles below the ground just northeast of Magna, on the west side of the Salt Lake Valley. It was Utah's strongest earthquake in nearly 30 years.
Older brick buildings and chimneys took the worst of it. Power went out across the valley. Nobody died, but it was a small, close look at what a much larger Wasatch earthquake would mean.
M5.7
Magnitude, about 12 km (7 mi) deep.
26,376
People who told the USGS "Did You Feel It?" what they felt.
887
Aftershocks of magnitude 1 or larger recorded nearby by the end of 2020, including two M4.6 the same day.
See what Magna did at your address
What it revealed about the fault
The Wasatch fault is easy to see at the surface: it's the straight, steep edge where the mountains meet the valley. What it does deep underground is harder to know.
Magna's mainshock didn't break the surface. Its seismic waves showed it slipped on a fault dipping west at a gentle angle of about 30 to 35 degrees, 9 to 12 km down. That fits a long-standing idea that the Wasatch fault flattens as it goes deeper, curving under the valley like the inside of a spoon. Geologists call that shape listric.
If that's right, more of the valley sits above the fault surface than a steeper fault would put there, and some research suggests shaking in Salt Lake City could be stronger than earlier estimates. This is an active research question, not settled science.
Pang, G., et al. (2020). Geophysical Research Letters. doi:10.1029/2020GL089798
What it revealed about the basin
The Salt Lake Valley is a deep bowl of rock filled with thousands of feet of softer sediment: old lake beds from Lake Bonneville, sand, gravel, and clay. Seismic waves slow down and grow taller when they move from hard rock into soft sediment, and they can bounce around inside the bowl.
After Magna, USGS scientists studied more than 3,000 recordings from 59 Utah earthquakes, Magna among them. Sites sitting on deep basin sediment shook roughly 3 to 10 times more than thin-sediment sites at long periods, about one second and slower. That's the slow, rolling motion that sways taller buildings, very roughly ten stories and up, along with bridges and pipelines.
The 2023 USGS National Seismic Hazard Model now includes basin-specific data for Salt Lake City. Building-code values usually lag the newest hazard model by several years.
What we won't do: multiply your address's shaking by "3 to 10." That range compares groups of sites across many recordings. It isn't a forecast for any single building. We're working with seismologists on an honest way to show basin depth by address, and we won't publish it until they've reviewed it.
Moschetti, M. P., et al. (2021). Seismological Research Letters 92(6). doi:10.1785/0220200449 · USGS, 2023 50-State National Seismic Hazard Model
Magna vs. the big one
Each step up the magnitude scale releases about 32 times more energy. A magnitude 7 on the Salt Lake City section of the Wasatch fault, the kind of earthquake geologists find evidence for in trenches across the valley, would release roughly 90 times more energy than Magna. It could break the ground surface along the fault, and it would shake much longer and harder across the whole valley.
Scientists estimate a 43% chance of a magnitude 6.75 or larger earthquake in the Wasatch Front region in the next 50 years. That's not a countdown. It's a reason to do the inexpensive things now.
Working Group on Utah Earthquake Probabilities (2016), Utah Geological Survey Miscellaneous Publication 16-3.
What to do with this
- Look up your address to see what Magna did there and what the ground is like.
- Make a plan. The first tier takes a weekend and costs almost nothing.
- Check whether your building is brick.
Event facts: USGS M5.7 Magna event page and the USGS ComCat earthquake catalog (aftershocks within 20 km, through December 31, 2020).