The generally placid, well-mannered state of Oklahoma has developed a very West Coast habit of late: the state, it seems, has got the shakes. Oklahoma has been hit by nine earthquakes since last Monday, most of them concentrated in the area just east of Oklahoma City, according to the National Earthquake Information Center, a division of the United States Geological Survey.
The recent tremors have all been slight — the strongest, which hit Thursday, had a magnitude of 3.7 — the sort of event that makes a water glass tremble, but will not knock the wedding china to the floor. Nonetheless, residents accustomed to more stable ground beneath their feet have been startled.
“It’s certainly getting a lot of people’s attention,” said Austin Holland, a research seismologist with the Oklahoma Geological Survey. Oklahoma lies in the middle of the North American tectonic plate, as opposed to more quake-prone areas like Japan or California, where plates rub together. But there are fault lines that run through the state.
Last week’s quakes came as part of a larger period of increased activity that stretches back at least six weeks, the National Earthquake Information Center said. This month, at least 23 earthquakes were recorded in a single weekend, one with a magnitude of 5.6. That Sunday, Oklahoma residents awoke to collapsed chimneys and sections of buckled highway.
“It’s been going on for quite a while,” said Don Blakeman, a geophysicist at the National Earthquake Information Center. “We don’t know exactly why it occurs, but it doesn’t indicate that anything huge and terrible is going to happen.”
But Mr. Holland said that increased earthquake activity in Oklahoma could be traced back as far as two years. Possible causes, including the process of extracting oil and gas from the ground below, are still under investigation, he said. “It keeps me up at night,” Mr. Holland said. But Oklahomans should not be nervous, he added. “It’s because I’m curious.”