What can central Utah earthquake swarms reveal about western earthquakes?

Flowchart illustrating the workflow followed in this study. Credit: Geochemistry, Geophysics, and Geosystems (2023). doi: 10.1029/2023GC010867

Most of the earthquakes that rock beneath the Great Western Basin come in storm waves, bunched together in time and space. Scientists call these seismic groups “swarms,” a category distinct from the many aftershocks that follow a large tremor, such as the 2020 magnitude 5.7 Magna earthquake on the Wasatch fault.

Instead of spreading evenly over time, many of these small, often imperceptible earthquakes strike an area in a short period of time, say a few days or weeks.

Central Utah has been the scene of dozens of earthquake swarms that have been recorded over the past 40 years by an ever-expanding network of seismic arrays operated by the University of Utah.

Now seismologists in Utah are analyzing decades of seismic data in hopes of figuring out the importance of these swarms to a geologically complex region known as a geothermal hotspot and to recent volcanic activity — in geological terms.

“In central Utah, seismic swarms are more common than any other type of sequence. We’ve looked at all kinds of sequences, but 80% of the sequences are swarms. That’s amazing,” said Postdoctoral Research Fellow Geza Petersen. “We’ve also seen that these elements are very heterogeneous. So one site in central Utah can have very, very different behavior than others just 30, 40, 50 kilometers away.”

The results were published July 13 in the journal Geochemistry, Geophysics, Geosystems.

Geothermal hot spot

FORGE is located outside Milford, within a research area that spans Beaver, Iron, Sevier, and Paiute counties. The research area is home to three geothermal power plants and includes the cities of Circleville, Beaver and Richfield.

Researchers believe that earthquakes are caused by hot water, powered by geothermal activity, pouring through cracks in the Earth’s crust.

During the past 40 years, the University of Utah’s seismometric stations have detected seismic sequences that feature earthquakes of magnitude 1.5 or greater. But on further examination of the data, Peterson and Banquo were able to identify hundreds of additional, smaller, 0.5-magnitude earthquakes recorded in 50 distinct sequences.

They conclude that 40 qualifies as squadrons. A lot can be learned from these small earthquakes, but they’re hard to study, according to Panko, associate director of Seismograph Stations.

“We’re all worried about the Wasatch Front, but the other thing to know is we’ve got earthquakes all over Utah,” Pankoff said. “We’ve recorded a certain level, but in some of these places there are probably earthquakes that happen all the time that we don’t see. That’s a really important thing to understand.”

Thousands of earthquakes

All told, the research analyzed 2,300 earthquakes, most of which were between 1 and 3 on the Richter scale, the largest of which was a magnitude 5.1 that struck east of Richfield in 1989. This quake wasn’t part of a swarm, but was a mainshock followed by several aftershocks. The full catalog of the study area contains 20,000 events between 1981 and 2023, according to Petersen.

“However, we cannot determine exactly how many of them are part of a sequence because we limit the study of sequences that contain at least 20 earthquakes within 10 days. We do not look at smaller sequences because we need a lower bound to look at the statistical parameters and compare the characteristic patterns of the sequences,” she said.

“However, in the 1980s and 1990s, the earthquake network in Utah was not as dense as it is today. There were far fewer stations. So we can only study larger sequences with larger magnitudes than these times. There are likely many more seismic swarms or sequences.”

The study greatly expands on another recent study that focused exclusively on a swarm of hundreds of small earthquakes around Milford in the spring of 2021. This area did not see much seismic activity during the seismograph’s 40-year data window. Meanwhile, Petersen said, earthquakes were occurring more frequently every few months in the nearby mineral mountains to the west during the same time period.

“So it’s a very heterogeneous system,” she said. “You have a group of earthquakes in the same place and you can start to learn about the structures that are activating in place. If you only have one earthquake, you can’t learn that much.”

Swarms of metallic mountains were first discovered a few years ago when new seismometers were installed for the FORGE geothermal research project.

“Before we didn’t have the decision, but now we can see that there are always events coming up, and that’s fast,” Petersen said. “Within a couple of hours, you suddenly have 30, 40, 50 events and then it stops again. You have this over and over again, you have so much activity. You can’t really feel it. It’s so small for that, but we can see it on the seismometers.”

The title of the paper is “Small-Scale Seismic Swarms in Central Utah (USA): Regional Tectonic Interactions, Local Structures and Hydrothermal Systems.”

Further information: GM Petersen et al, Small-Scale Seismic Swarms in Central Utah (USA): Regional Tectonic Interactions, Local Structures and Hydrothermal Systems, Geochemistry, Geophysics, Geosystems (2023). doi: 10.1029/2023GC010867

Provided by the University of Utah

Citation: What can central Utah earthquake swarms reveal about western earthquakes? (2023, July 25) Retrieved July 25, 2023 from https://phys.org/news/2023-07-central-utah-earthquake-swarms-reveal.html

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