Thousands of earthquakes kept Nevada seismologists busy in 2015 | SierraSun.com

Thousands of earthquakes kept Nevada seismologists busy in 2015

Special to the Bonanza

Cracks developed from the magnitude 7.3 Pleasant Valley earthquake in 1915 in alluvial soil near Lovelock, Nev. It was the state's largest earthquake, and it was centered in Pleasant Valley, south of Winnemucca.

More than 17,500 earthquakes were recorded in 2015 by the University of Nevada, Reno's Nevada Seismological Laboratory.

That includes the 231 recent quakes in south Reno and the magnitude 4.8 in Caliente that shook Las Vegas in January 2015.

"While the Reno shaking is fresh in our minds, what's really bumping the number up from the background rates is the energetic sequence in far northwestern Nevada, the Sheldon sequence," Graham Kent, director of the Nevada Seismological Lab said.

Through 2015 there have been 4,511 earthquakes recorded in the remote Sheldon Wildlife Refuge near Vya, east of Cedarville, Calif.

The earthquake activity that began there in July 2014 has produced 234 earthquakes greater than magnitude 3.0, and 24 earthquakes of magnitude 4.0 or larger, including the largest, a magnitude 4.8 on Sept. 14 and a magnitude 4.6 just before Christmas this year.

That leaves more than 13,000 other Nevada and eastern California earthquakes, including swarms as well as less prolonged mainshock-aftershock sequences in Caliente, Carson City and Virginia City.

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Outside of the persistent Sheldon sequence, Nevada was hit with 54 magnitude 3 or greater earthquakes or about one per week.

Overall, there have been 19 earthquakes of magnitude 4.0 and larger and the University's seismological lab has reported 160 earthquakes between magnitude 3.0 and 3.9.

"The south Reno swarm that just happened produced about 30 earthquakes in two days and about 200 more in the days that followed," Kent said. "For our entire network, we are averaging about 50 located earthquakes a day."

They aren't all part of identifiable swarms like Sheldon, South Reno or Virginia City.

"Many of the earthquake clusters in the Walker Lake area in 2015 were more typical sequences and not like Sheldon that has now continued for more than a year," said Ken Smith, associate director of the Nevada Seismological Lab. "Just as the Caliente 4.8 that startled Las Vegas in May or the 4.8 at Scotty's Castle at Death Valley in February with their fairly extensive aftershocks, in Nevada we wouldn't be surprised to see a significant event anywhere."

The Nevada Seismological Lab is a public service and research department in the University's College of Science tasked by the State of Nevada to operate a monitoring network and report earthquake information for activity throughout the state.

The Seismology Lab's data are provided to the USGS, other national data centers and shared with adjacent network operators in California, Utah, Washington and Oregon.

Worldwide Research HERE IN NEVADA

The Seismological Lab, a 24/7/365 operation, has about 30 faculty, researchers and graduate students who support the earthquake monitoring as well as conduct outreach and research in Nevada, nationally and internationally to better inform disaster managers, policymakers and the general public about seismic hazards.

In addition to numerous geophysical studies of faults in Nevada, the researchers have projects in California, the Pacific Northwest, Japan, New Zealand, Australia and the Himalayas.

They've tracked and studied fault lines throughout the west, including pioneering work defining underwater fault systems in Lake Tahoe, the Salton Sea and Pyramid Lake and along the entire Pacific coastline from Mexico to Canada using a high-resolution, state-of-the-art seismic imaging system.

In May, following the April 25 Gorka earthquake in Nepal, Nevada Seismological Lab researcher and professor Steve Wesnousky was one of the first scientists to enter the region to search for geophysical signs of the 7.8 earthquake.

He is one of the world's leading scientists on the hundreds-of miles-long Himalayan fault, having studied it for more than 15 years. College of Science Dean Jeff Thompson and Kent cobbled together some funding and sent Wesnousky's two graduate students to Nepal to assist him in the studies.

"This was a unique window to understand what a surface rupture might look like 'fresh' as compared to a trench looking at paleoearthquakes," Kent said. "The information they gathered helps us understand other thrust faults around the world, such as in the Los Angeles basin, and what the surface deformation associated with these large earthquakes might look like."

The seismological lab team met with colleagues from the local university and spent two weeks searching for surface ruptures and other geophysical signs. Their work and travels were chronicled in a daily blog on the College of Science website.

"We have a variety of research projects underway here in Nevada that helps inform the world as well," Kent said. "With the Sierra Nevada range extending away from us, and the unique structures we find in the Great Basin, it's not surprising Nevada is the third most seismically active state in the country."

Though the third most seismically active state behind California and Alaska, Nevada has not had a magnitude 7.0 earthquake in 60 years.

A CENTURY AGO: THE Great Nevada Earthquake

This year marks the 100-year anniversary of the largest recorded earthquake in Nevada, the Great Nevada Earthquake with a magnitude 7.3 that left a 35-mile-long surface scar with a vertical offset as high as 19 feet in one area and numerous large cracks in the earth.

"The Pleasant Valley earthquake caused damage to multiple communities that were as far as 50 miles away," Craig dePolo, an earthquake geologist with the University of Nevada, Reno's Nevada Bureau of Mines and Geology, said. "This illustrates that large earthquakes cause widespread damage and can affect many communities at the same time. It was felt throughout Nevada but the most damage was in Winnemucca, the closest town; with nearly half the chimneys damaged, walls collapsed and windows broken."

The Great Nevada Earthquake ushered in a period of significant quakes that hasn't been matched in 60 years.

"The first 54 years of the 20th century we had seven earthquakes of magnitude 6.5 or larger and haven't had one since," Kent said. "We haven't had the big one since the Dixie Valley/Fairview Peak sequence in 1954 east of Fallon, which was two magnitude 7s four minutes apart. Since then, Nevada's population has grown 13-fold."

The magnitude 6.0 Wells, Nevada, earthquake in 2008 has been the largest event in Nevada in 42 years. Historically, Nevada can expect to have three magnitude 7.0 earthquakes per century and one magnitude 6.0 or larger every decade, Kent said.

The lab also runs the Great Nevada ShakeOut, a statewide "drop, cover and hold-on" earthquake drill, and helped grow participation by more than 60,000 in 2015, with some 630,000 Nevadans participating in this year's sixth annual drill.

Updated earthquake activity information is available at http://www.seismo.unr.edu/Earthquake.

This article was submitted by the Nevada Seismological Laboratory at the University of Nevada, Reno.