Oklahoma earthquake: State survey lowers magnitude to 4.7

The Oklahoma Geological Survey reduced its rating Thursday of Wednesday's earthquake near Norman to magnitude 4.7 and said the source of the quake was likely natural.
BY MICHAEL KIMBALL Modified: October 15, 2010 at 8:55 am •  Published: October 15, 2010

The Oklahoma Geological Survey said continued analysis of Wednesday's earthquake data shows the source likely was natural and occurred on a previously unknown fault line.

The survey also reduced its rating of the earthquake to magnitude 4.7 from the latest Wednesday estimate of 5.1, research seismologist Austin Holland said. Officials had anticipated the reduction based on the stark difference between its rating and the magnitude 4.3 rating given by the U.S. Geological Survey.

Holland said dealing with numerous calls from government officials, the media and state residents also limited the amount of time seismologists had to review the data Wednesday.

The Oklahoma survey and the U.S. survey use different equipment at different locations to measure different data sets, resulting in numbers that can vary from each other and don't mean the same thing, Holland said. The measurement the U.S. survey used for its rating, moment magnitude (Mw), records the low-frequency energy released in the earthquake. The Oklahoma survey rating (mbLg) measures the amplitude of surface waves that people can feel.

"They both provide relative measures of the earthquake," Holland said. "They're just making different assumptions about the earth."

Holland said the depth of Wednesday's quake, about eight miles, points to a natural source.

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