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Hi,

Magnitudes for Oklahoma Earthquakes Shift Upward
https://www.usgs.gov/news/magnitudes-oklahoma-earthquakes-shift-upward

M5.8 - 15km NW of Pawnee, Oklahoma
http://earthquake.usgs.gov/earthquakes/eventpage/us10006jxs#executive

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Revisions follow standard USGS re-analysis

*The U.S. Geological Survey is updating the official magnitude of
the September 3, 2016 Pawnee, Oklahoma earthquake
<http://earthquake.usgs.gov/earthquakes/eventpage/us10006jxs#executive> 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
<http://earthquake.usgs.gov/earthquakes/eventpage/usp000jadn#executive> 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.*



*For more background on differences in magnitude, see “Why do USGS
magnitudes differ from those published by other agencies?
<https://www2.usgs.gov/faq/categories/9826/8155>”*


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Enjoy the day,
Emily


Emily C. Wild
Librarian (Physical Scientist)
U.S. Geological Survey, Denver Library
Denver Federal Center
ph: (303) 236-1003
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USGS Libraries: http://library.usgs.gov
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Profile: https://profile.usgs.gov/ecwild