(LiveScience) – Mile for mile, there are almost as many earthquakes rattling Oklahoma as California this year. This major increase in seismic shaking led to a rare earthquake warning today (May 5) from the U.S. Geological Survey and the Oklahoma Geological Survey.
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." [Watch 2500+ Oklahoma Earthquakes Since 2012 (Video)]
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.
This is the first time the USGS has issued an earthquake warning for a state east of the Rockies, Williams said. Such seismic hazard assessments are more typically issued for Western states following large quakes, to warn residents of the risk of damaging aftershocks, he said.
The geological agencies took action after the rate of earthquakes in Oklahoma outpaced that of even California for the first few months of 2014. (California regained the lead in April.)