HOUSTON, Sept. 7 (Xinhua) -- The U.S. Geological Survey (USGS) on Wednesday upgraded Saturday's earthquake in the U.S. state of Oklahoma from magnitude 5.6 to magnitude 5.8, the largest in the state's history.
The USGS attributed the change to further in-depth analysis of seismic recordings, saying in a statement that "Changes in estimated magnitude for an earthquake are common in the hours-to-days following the event, as more data are analyzed in greater detail than is possible in the first minutes after the earthquake occurs."
The earthquake, which occurred at 7:02 a.m. local time (1202 GMT) on Saturday morning with the epicenter about 14.4 km northwest of Pawnee, a city in northern Oklahoma, was also felt in several surrounding states including Kansas, Missouri, Arkansas and Texas.
There was no damage found to bridges within a 48 km radius of the earthquake's epicenter; no buildings in the city had collapsed but windows were shattered and the facades of some downtown brick buildings from the late 1800s and early 1900s were damaged.
The quake happened in the state's north-central part, a key energy-producing region, and several aftershocks ranging from 3.4 to 3.6 magnitude were also reported to shake the area hours after the earthquake.
Oklahoma, a big oil and gas producer in the nation, has enjoyed an increase in earthquakes of magnitude 3.0 and greater that have been linked to underground disposal of wastewater from oil and natural gas production.
A number of earthquakes hit northwest Oklahoma this year, including a 5.1 magnitude earthquake. State regulators have asked producers to reduce wastewater disposal volumes in earthquake-prone regions of the state.
