GREEN RIVER -- Three earthquakes shook parts of northern Lincoln County on Sunday afternoon and evening, according to the Wyoming State Geological Survey.
The tremblers happened within hours of each other and were felt by residents in Jackson, Lander, Dubois, Thayne, Bedford, Afton, Alpine and other small communities in western Wyoming, officials said.
Survey geologists said one injury was reported when a retired firefighter was thrown from his horse when it spooked during a trembler. He was taken to the Jackson hospital. There was no word on his condition.
Geologist Seth Wittke said agency personnel are monitoring the area to gather any additional information about Sunday's quakes.
"This wasn't a real big (shaker) ... compared to the ones that happened in the Upper Gros Ventre Valley about a month ago in the area," Wittke said in a phone interview Monday.
A series of quakes struck the Jackson area in Teton County in early August. Those quakes also occurred within hours of each other.
"The last couple of events including Sunday's have been fairly closely spaced, but that's not necessarily indicative of earthquakes in Wyoming," he said.
"They can happen one at a time ... or eight or nine in a row like in Kelly or like the three we had Sunday," said Wittke.
The first quake Sunday, of 4.1 magnitude, occurred at approximately 2:10 p.m. at a depth of 3.1 miles.
The event was located 25 miles south-southeast of Jackson and 30 miles north-northeast of Afton, near Willow Creek.
Wittke said a second earthquake with a magnitude of 4.0 struck around 4:20 p.m. at a depth of approximately 4.3 miles. The quake was located about 25 miles south of Jackson and 30 miles north-northeast of Afton along the Little Greys River.
At 8:43 p.m., a third earthquake measuring 3.6 was located 25 miles south of Jackson and 30 miles northeast of Afton, near the McCain Guard Station.
The area is in the heart of the seismically active "Wyoming Overthrust Belt," where earthquakes are common occurrences. The area includes entire formations of rock that are slowly being altered and deformed, resulting in numerous active faults.
Wittke said it is likely that similar earthquakes could happen again in the area.
The earthquakes marked the second significant seismic event that has occurred in western Wyoming this summer.
On Aug. 4, a series of nine rapid earthquakes, the strongest measuring 4.8, occurred in the Teton County area near Kelly, at a site between Jackson and Dubois.
The main quake in that event was felt in Teton, Park, Sweetwater, Sublette and Lincoln counties.
Wittke said Sunday's earthquakes were not related to the earlier Teton County events. "They're in a completely different area and they're unrelated for the most part," he said.
Numerous magnitude 2.0 or greater earthquakes have been recorded in Lincoln County dating back to 1915, according to U.S. Geological Survey data. Most were relatively small magnitude events and subsequently not felt.
Prior to Sunday, the county's most recent seismic activity occurred on March 23, 2002, when a magnitude 3.4 earthquake was reported approximately 5 miles northeast of Alpine.
Wittke said the first 4.1 magnitude event Sunday was felt as intensity level 3 in Thayne, Bedford and Alpine, and as intensity level 2 in numerous other locations.
He said the intensity levels -- which run from 1-10, with 10 being the most extreme -- are based on "felt" reports submitted to the agency through its online reporting program.
"People fill out a form on the site and based on how they answer the questions, we assign it an intensity ... so those numbers come from the people that experience the shaking themselves reporting online to us," said Wittke.
By late Monday afternoon, 179 people had filled out response surveys from as far away as Blackfoot, Idaho.
Intensities from 2-3 are listed as including weak shaking, but no damage.
Contact southwest Wyoming bureau reporter Jeff Gearino at (307) 875-5359 or firstname.lastname@example.org
The Wyoming State Geological Survey asks residents to participate in the "Did You Feel It" program if they felt ground motions and/or experienced damage related to Sunday's earthquakes in northern Lincoln County. Go to http://earthquake.usgs.gov/earthquakes/dyfi/ for more information.