BOISE – A cluster of earthquakes south of Cascade has been shaking items off shelves and jolting residents out of their sleep this week.
“The dog starts barking every single time,” said Armen Mannschreck, a high school English teacher who lives near Clear Creek. A temblor Tuesday night – the strongest so far – sent a bowl crashing to the floor from a dresser in Mannschreck’s house.
It also set the cabinet doors swinging at the home of Linda Jarvis, another Clear Creek resident who is regularly awakened by middle-of-the-night quakes these days. Jarvis said she’s often felt tremors in the 19 years she’s lived at her home – but only in the last week have they been strong enough to get her up in the night.
“The cat wakes up; my cabinet doors in the kitchen go open and closed and I have mortar coming out of the brick on my fireplace outside,” Jarvis said.
Residents of the Clear Creek area, about 10 miles south and southeast of Cascade, have been calling seismologists to report feeling several earthquakes a day, said Idaho Geological Survey geologist Bill Phillips. IGS instruments have been recording from 50 to 100 earthquakes per day around Clear Creek in the last week.
Earthquake clusters – or swarms, as seismologists describe them – are common in Idaho. The last one was in Donnelly in 2001, said Jim Zollweg, a seismologist at Boise State University. There have been several other swarms in the last few decades.
The swarm in the Clear Creek area, which started around Sept. 22, is unusual in that residents are feeling the earthquakes, the largest of which, Tuesday night, had a magnitude of about 3.7. Usually an earthquake measuring 3.7 isn’t felt and causes no damage at all – only at about 4.7 magnitude do people notice the shaking, or do earthquakes crack plaster or leave other signs, Zollweg said.
He thinks the Clear Creek temblors might be closer to the surface – only one to two miles underground, instead of the more common three to seven miles below. That would make them easier to feel, he said.
“The swarm is a little unusual, but we don’t know that it’s unusual enough for people to be worried,” said Zollweg, who has been deflecting a lot of requests for predictions on how big, and how frequent, the earthquakes might get. He’s willing to speculate that the swarm will go on for weeks or months – but he can’t say how strong future earthquakes might be. There is a fault in the earth near the Clear Creek area, but it’s not clear if the swarm at Clear Creek is related to that fault or to any other, he said.
“I’m kind of thinking there is some potential for a somewhat larger quake,” Zollweg said. “What that means is really hard to say. I’m not comfortable thinking we’ve seen the biggest one yet, but if history is any guide, it’s not going to get much bigger.”
That’s comforting news to Jarvis.
“It’s unnerving, and it’s scary while it’s going on,” she said. “But I don’t worry about dying. I’m not ready to move.”
Earthquakes, many detected only by instruments, are common in Idaho. The Idaho Geological Survey at the University of Idaho in Moscow said the state is ranked fifth in the nation on a scale of earthquake risk. The risk is greater only in California, Nevada, Utah and Alaska.
Idaho felt two large earthquakes in recent memory; both caused deaths and millions of dollars in damage. One was the 1959 Hebgen Lake earthquake in Montana, with a magnitude of 7.1; the other was the 1983 Borah Peak earthquake, with a magnitude of 7.
Sean Christian, an eighth grader at Cascade High School, was woken up by a temblor that shook his Clear Creek home Tuesday night.
“It was a big shaking and it almost knocked my dad’s guns out of his gun cabinet,” said Christian, who is 13. He said he used to be scared, but he’s used to earthquakes now.
“I just go back to sleep,” Christian said.
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