Oklahoma, U.S. Differ in Earthquake Calculations

The Oklahoma Geological Survey reported fewer than half the number of 4.0 magnitude and higher earthquakes in Oklahoma last year as the U.S. Geological Survey, according to agency records.

While OGS reported six earthquakes of magnitude 4.0 and higher last year, the USGS reported 14 earthquakes in that range. This year, 13 earthquakes in that range have been reported by OGS while 21 were reported by the USGS.

The differences are due to the way the agencies calculate earthquake magnitudes, the Tulsa World reported. An analysis by the newspaper shows that since 2010, OGS reported a smaller number than USGS more than half the time for earthquakes of magnitude 3.8 and higher.

The difference between a magnitude 3.9 and a magnitude 4.0 earthquake is largely academic to seismologists because most people experience about the same amount of shaking. There’s also a margin of error based on locations of seismometers and different formulas to calculate earthquakes.

George Choy, seismologist for the USGS’ National Earthquake Information Center in Golden, Colo., said the USGS does multiple calculations on earthquakes.

“We try to be conservative. If we overestimate it and bring it down, then media types ask us what happened. … Rarely do people complain if it (magnitude) gets bigger.”

Austin Holland, chief seismologist for the Oklahoma Geological Survey, said the agency is releasing a report soon that will summarize state earthquake data for the past year. He said the report will also explain how OGS figures earthquake magnitude “because the magnitudes are such an issue that some people get hung up on.”

Last year the state experienced 585 earthquakes of magnitude 3.0 or higher, more than in the past 35 years combined. That figure earned Oklahoma the title of the most seismically active among the contiguous 48 states.

Numerous scientific studies since the 1960s have found the practice of injecting wastewater produced by oil and gas drilling into areas on fault lines can trigger earthquakes. Several studies and the USGS have attributed Oklahoma’s increase in earthquakes to wastewater disposal.