Most of the earthquakes that rumble beneath the Great Western Basin come in bursts, which are coordinated in time and place. Scientists call these seismic groups “swarms,” which are a different category from the many aftershocks that occur after a large tremor, such as the 5.7 magnitude Magna earthquake in 2020 on the Wasatch Fault.
Rather than being spread out evenly over time, many of these small, usually imperceptible earthquakes strike a region over a short period of time, say a few days or weeks.
Central Utah has been the stage for dozens of earthquake swarms recorded over the past 40 years by an expanding network of seismic arrays managed by the University of Utah.
Now seismologists in Utah are analyzing decades of seismic data in the hope of discovering the importance of these masses in a geologically complex region known as a geothermal hotspot and for recent-geologically speaking-volcanism.
“In central Utah, seismic swarms are more common than any other type of sequence. We looked at all types of sequences, but 80% of the sequences are swarms. That’s amazing,” said Gesa Petersen, a post-doctoral research fellow. “We also found that they are very heterogeneous. So one location in central Utah can have very different behavior than other locations that are only 30, 40, 50 kilometers away.”
The findings were published on July 13 in the journal Geochemistry, Geophysics, Geosystems.
A geothermal hot spot
Located outside of Milford, FORGE is within a research area that includes Beaver, Iron, Sevier and Paiute counties. The research area is home to three geothermal power plants and includes the towns of Circleville, Beaver and Richfield.
Researchers suspect that the earthquakes are caused by hot water, powered by geothermal activity, seeping through cracks in the earth.
Over the past 40 years, the University of Utah Seismograph Stations have detected earthquake sequences that indicate earthquakes of magnitude 1.5 or greater. But upon further examination of the data, Peterson and Pankow were able to identify hundreds of additional smaller earthquakes, as small as magnitude 0.5 recorded in 50 separate sequences.
They ended up 40 qualified as herds. Much can be learned from these small earthquakes, but they are difficult to study, according to Pankow, who is the Seismograph Stations associate director.
“We’re all concerned about the Wasatch Front, but the other thing to know is that we have earthquakes all over Utah,” Pankow said. “We’re recording at a certain level, but in some of these places there’s probably earthquakes that happen all the time that we don’t see. That’s something that’s really important to understand.”
Thousands of earthquakes
All told the research analyzed 2,300 earthquakes, most of which were between magnitude 1 and 3. The largest was a magnitude 5.1 that struck east of Richfield in 1989. That one was not part of a swarm, but a mainshock followed by several aftershocks. The entire catalog for the study area contains 20,000 events between 1981 and 2023, according to Petersen.
“However, we do not really say how many of them are part of a sequence because we limit the study for sequences with at least 20 earthquakes in 10 days. We do not look at smaller sequences because we need some minimum to look at the statistical parameters and to compare the characteristic patterns of the sequences,” he said.
“However, in the 1980s and 1990s, the seismic network in Utah was not as dense as it is today. There were fewer stations. So we can only study larger sequences with larger magnitudes from these times. There are probably more swarms or seismic sequences.”
The study expands greatly on another recent study that focused only on a cluster of hundreds of small earthquakes around Milford in the spring of 2021. That area did not experience much earthquake activity throughout the 40-year window of seismograph data. On the other hand, earthquakes have occurred as often as every few months in the nearby Mineral Mountains to the west during the same time period, Petersen said.
“So it’s a heterogeneous system there,” he said. “There’s a bunch of earthquakes in the same area and you can start learning about the structures that are activated in that area.
The swarms in the Mineral Mountains were first noticed a few years ago when new seismometers were installed for the FORGE geothermal research project.
“Before we didn’t have a resolution, but now we can see that there are events coming up more often, and they’re fast,” Petersen said. “Within a few hours, suddenly there are 30, 40, 50 events and then it stops again. You have it all the time, you have a lot of activity. You can’t feel it. It’s too small for that, but we can see it on the seismometers.”
The paper is titled “Small-Magnitude Seismic Swarms in Central Utah (US): Interactions of Regional Tectonics, Local Structures and Hydrothermal Systems.”
GM Petersen et al, Small-Magnitude Seismic Swarms in Central Utah (US): Interaction of Regional Tectonics, Local Structures and Hydrothermal Systems, Geochemistry, Geophysics, Geosystems (2023). DOI: 10.1029/2023GC010867
Provided by the University of Utah
Citation: What can earthquake ‘swarms’ in central Utah reveal about earthquakes in the West? (2023, July 25) retrieved 25 July 2023 from https://phys.org/news/2023-07-central-utah-earthquake-swarms-reveal.html
This document is subject to copyright. Except for any fair dealing for the purpose of private study or research, no part may be reproduced without written permission. Content is provided for informational purposes only.