The U.S. Geological Survey is updating the official magnitude of the September 3, 2016 Pawnee, Oklahoma earthquake to Mw 5.8 (from 5.6), making it Oklahoma’s largest recorded earthquake to date.
The magnitude revision is based on further in-depth analysis of seismic recordings. 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.
Concurrently, the USGS is also updating the official magnitude of the November 6, 2011 Prague, Oklahoma earthquake to Mw 5.7 (from 5.6). Questions regarding their relative size prompted a re-analysis of both earthquakes. Both updates are the result of comprehensive studies of the long-period, globally-recorded seismic data for these earthquakes, using consistent approaches and datasets for each event.
“USGS analyses indicate that the two earthquakes are very similar in size – to within typically-cited uncertainties of 0.1 magnitude units,” said Gavin Hayes, USGS research geophysicist. “However, the 2016 Pawnee event is slightly larger than the Prague earthquake in 2011,” noted Hayes.
“While the difference in size between the two events is less than 0.1 magnitude units,” Hayes continued, “rounding magnitudes to one decimal place means that the magnitude of the Prague earthquake is Mw 5.7, and the Pawnee earthquake is Mw 5.8.”
Precisely ranking the largest earthquakes in Oklahoma is difficult because seismic instrumentation has vastly improved over the last several decades. Other large, documented and felt earthquakes in Oklahoma include an instrumentally recorded 1952 event centered near El Reno, to which magnitudes of 4.9 to 5.7 have been assigned. Before the instrumental era, an 1882 earthquake in southern Oklahoma has magnitude estimates ranging from 4.8 to 5.7, based on the area over which it was felt.
Magnitude estimates can vary for a variety of reasons, including differences in methods used to compute magnitude, differences in data used, uncertainties in that data, differences in how that data is processed, and differences in our assumptions about the Earth structure through which seismic waves travel.