© USGSOklahoma earthquakes.
In a joint statement, the agencies said the risk of a damaging earthquake - one larger than magnitude 5.0 - has significantly increased in central Oklahoma.
Geologists don't know when or where the state's next big earthquake will strike, nor will they put a number on the increased risk. "We haven't seen this before in Oklahoma, so we had some concerns about putting a specific number on the chances of it," Robert Williams, a research geophysicist with the USGS Earthquake Hazards Program in Golden, Colorado, told Live Science. "But we know from other cases around the world that if you have an increasing number of small earthquakes, the chances of a larger one will go up."
That's why earthquakes of magnitude 5 and larger are more frequent in states such as California and Alaska, where thousands of smaller temblors hit every year.
Comment: Yearly summary of earthquake data.