Earthquakes in Oklahoma

ENID, Okla. — How many earthquakes have occurred in Oklahoma this year?

It depends on the source.

In 2015, Oklahoma has sustained more than 550 significant earthquakes, according to U.S. Geological Survey and Oklahoma Geological Survey. However, the surveys can’t agree on a number.

“We’ve had more 3.0-plus earthquakes this year than we had in all of last year,” new OGS Director Jeremy Boak said. “There was some imprecision in some of the things. I think if they happen close enough together, sometimes it’s hard to be sure.”

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As of Aug. 25, OGS recorded 596 significant earthquakes; OGS has the 2014 earthquake record at 585 magnitude 3.0 or greater quakes.

In contrast, the USGS showed 568 significant earthquakes, which still was below last year’s record. (As of Sept. 10, USGS was reporting 598 quakes — above last year’s total.)

“Magnitude is calculated by a formula that corrects amplitude for the distance between an epicenter and a station. At different distances, the seismogram characteristic changes rapidly or completely as body waves, normal modes and surface waves disperse. Because of these differences, different formulas are required at different distances. The USGS and OGS use different formulas for magnitude because our networks are designed differently and emphasize analysis of data from stations at different distances,” USGS Geophysicist George Choy said.

“Ideally, magnitudes computed by different formulas should be identical. In the real world, the earth is heterogeneous, so empirically derived corrections for amplitude can be off. The different formulas also emphasize different parts of the waveform. These different parts of the waveform could well be sampling different parts of the earthquake source.”

When OGS data is eventually integrated into the Advanced National Seismic System Com­prehensive Catalog — which contains information, including magnitudes, hypocenters, tectonic summaries and maps — the magnitudes are reconciled, he said.

Jessica Miller contributed to this report.

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