A rockslide on Highway 21 near Lowman, Idaho, after a 6.5-magnitude earthquake struck Tuesday. (Tyler Beyer via AP)

Popular songwriter Carole King, known for “You’ve Got a Friend” and “I Feel the Earth Move,” was sitting in Idaho shortly before 6 p.m. Tuesday. All of a sudden, the earth moved under her feet.

It was an earthquake. And not a little one — a 6.5 magnitude. It was centered below the Sawtooth Mountains about 80 miles northeast of Boise, but it rattled the state capital for a fraction of a minute. It was even felt in northern Utah, which was jostled by a 5.7-magnitude tremor near Salt Lake City less than two weeks ago. The quakes are unrelated.

It was the most powerful earthquake to strike the Gem State since a 6.9-magnitude earthquake in 1983, during which two children in Challis were killed by falling debris. There were no reports of serious damage or injury as a result of Tuesday’s quake, which was centered in a sparsely populated area.

And while thousands were caught off guard, bewildered by the out-of-the-blue earthquake, some scientists — such as Glenn Thackray, a geoscientist at Idaho State University — had already encouraged people near the epicenter to prepare for an earthquake a decade ago.

“We knew quickly that that area was likely to have earthquakes,” said Thackray, who discovered the Sawtooth Fault in 2010. That fault did not produce Tuesday’s quake, which instead occurred on a boundary intersecting that fault. Regardless, a decade ago, Thackray set about notifying residents of the town of Stanley about the potential for a major earthquake.

“I was able to tell people up there [about] the common-sense things to do,” he said. “In terms of houses and such, telling people do the common-sense things in earthquake country, like mounting bookshelves to the wall … taking heavy things off high shelves, especially near their bed.”

Tuesday’s earthquake was centered just outside of Stanley.

Stanley Mayor Steve Botti told the Idaho Statesman that the shaking was so intense he was unable to walk down the stairs. “Stuff was flying all over the place,” he said.

Discovering a fault

Discovering a previously unknown fault is no easy task. Faults are features within the earth’s crust, sometimes extending tens of miles below ground. Faults don’t only occur at the interface of two tectonic plates; instead, faulting occurs any time the bending, twisting, stretching or compression of a plate results in finer boundaries that press up against each other.

But in the late 2000s, Thackray used lidar, or light detection and ranging, which employs laser lights, to map the earth’s surface. His team was able to digitally “remove” trees, leaving behind a three-dimensional model of the surface at a resolution of two or three feet. That’s when he noticed something rather unusual.

“We found a surface feature called the fault scarp,” said Thackray. “That … step in the landscape represented faulting. As soon as we saw that, we knew that fault was active and we suddenly have confirmation of that.”

How big is this fault?

“If we were walking towards the mountain front and came to that scarp, we’d have to climb 20 to 25 feet [at a 45 degree slope],” he said.

Quakes are few and far between — but they’re big

Thackray believes that 25-foot gap “represents three earthquakes in [the past] 15,000 years,” with a roughly eight-foot slip during each.

Rocks fall from the north side of the Snake River Canyon during an earthquake Tuesday near Twin Falls, Idaho. (Israel Bravo via AP)

“It’s not going very frequently,” he said.

That’s a theme of Idaho’s earthquakes, according to Thackray. There are fewer of the day-to-day rumbles that are ubiquitous in places such as California or Oklahoma, and instead periodic higher-end quakes.

“There are a lot of active faults that break very infrequently,” said Thackray. “Many of them … seem to be quite locked. Nothing that suggests the block is slipping at all; it’s very quiet.”

But then, suddenly, there’s an earthquake.

In studying sediment layers in the region, Thackray eventually concluded that the fault generates big quakes every 4,000 years or so.

“These faults don’t rupture that frequently,” said Thackray. “But the simple math is sometime it’s going to rupture again. There’s no reason to think these faults are dying out.”

Where was Tuesday’s quake?

Tuesday’s quake wasn’t exactly on the fault itself.

“This looks like it was on a fault that actually cuts across the end of the Sawtooth Fault,” he explained. “The fault we looked at is sort of like a tilted sheet of plywood that extends underground … this one is at an angle to the [Sawtooth Fault].

“[The earthquake] had a different kind of motion, and it looks like it [was] very close to or on a fault. There were two earthquakes in 1944 and 1945, and this looks like it may have been on the fault that created those earthquakes.”

Thackray lamented that the region remains somewhat poorly understood seismically, but he said he is optimistic that Tuesday’s quake will shed some light on the complex processes at work below the surface.

The takeaway though, he says, is that Idaho isn’t immune to higher-end earthquakes. And while the aftershocks of Tuesday’s quake will peter out in the days and weeks ahead, he cautions that residents should be aware the Sawtooth Fault is active. But when will the next big one strike? That’s the ever-important question.

“I would tell people in that area that sometime between an hour from now and 1,000 years from now,” he said, joking.

Tuesday’s events, however, offer a piece of advice to Idahoans far and wide: Always be ready.