May 2 (UPI) -- A "radioactive clock" trapped in New Mexican rock has revealed the longest earthquake record, a timeline stretching more than 400,000 years.
The clock isn't a clock, but a series of radioactive elements trapped in creamy, crystalized veins of the mineral calcite running through rock fragments along the Loma Blanca fault in the Rio Grande rift, just south of Albuquerque, New Mexico.
Researchers didn't set out to break a record. They were just trying to better understand how fault slips trigger earthquakes.
"We weren't expecting any of this," Laurel Goodwin, a professor of geosciences at the University of Wisconsin, said in a news release. "It's been quite the odyssey for us."
Calcite veins are deposited by pressurized fluids that are squeezed through fault lines during earthquakes. During subsequent earthquakes, the veins become fractured and heal creating a history of seismic activity.
The calcite veins contain uranium. Because uranium decays into thorium, researchers can use ratios of the radioactive elements to date the veins.
By dating multiple generations of calcite deposits, researchers were able to document a historic sequence of earthquakes along the Loma Blanca fault line, each measuring between 6.2 and 6.9 magnitude.
Researchers identified two mechanisms response for two different types of earthquakes. The scientists found evidence of regularly occurring earthquakes, separated by intervals of between 40,000 and 50,000 years. They also found evidence of an earthquake cluster, a string of slips separated by just 5,000 to 11,000 years.
The release of accumulated stress explained the more regularly spaced quakes, while an uptick in fluid pressure deep beneath the surface caused the earthquake cluster.
Veins linked with the cluster events also showed evidence of rapid carbon dioxide degassing, the release of highly pressurized fluids through cracks in Earth's crust.
"When pore pressure increases far enough over the background level, the fault fails and cracks form, releasing fluid from the basin," Goodwin said.
Researchers suggest wastewater fluid injections -- employed by hydraulic fracturing operations and linked with small earthquakes throughout the United States -- could trigger quakes in a similar but accelerated manner.