The Oklahoma earthquake swarms are an ongoing series of human activity-induced earthquakes affecting central Oklahoma southern Kansas, northern Texas since 2009. Beginning in 2009, the frequency of earthquakes in the U.S. state of Oklahoma rapidly increased from an average of fewer than two 3.0+ magnitude earthquakes per year since 1978 to hundreds each year in the 2014–17 period. Thousands of earthquakes have occurred in Oklahoma and surrounding areas in southern Kansas and North Texas since 2009. Scientific studies attribute the rise in earthquakes to the disposal of wastewater produced during oil extraction that has been injected more deeply into the ground. Two most notable cases include water injection, geothermal-induced earthquakes in areas previously without any seismic activity - in Pohang, South Korea and St. Gallen, Switzerland. Two of the most significant earthquakes in these swarms were the November 5, 2011 Prague earthquake east of the Oklahoma City area and the September 3, 2016 earthquake near Pawnee, north of Prague. The 2011 Prague earthquake, at reported magnitude 5.6, was at the time the strongest recorded earthquake in the history of Oklahoma. The 2016 earthquake was initially reported to be an identical 5.6 magnitude, but this was later upgraded to 5.8, making it the strongest earthquake on record. Simultaneously, the USGS upgraded the magnitude of the Prague earthquake to 5.7. Numerous seismologists had advised local residents of an even greater risk of earthquakes in 2014, by which time the number of earthquakes had increased to a dangerously high level. In response to the major increase in earthquakes in the Central United States, the United States Geological Survey (USGS) began developing a new seismic hazard model to account for risk associated with induced seismicity. By June 26, 2014, no fewer than six individual earthquake sequences in Oklahoma had been identified and named by the Oklahoma Geological Survey (OGS). Other swarms have been observed in south-central Kansas and North Texas. In March 2013, a peer-reviewed paper published by a research team led by seismologist Katie Keranen at the University of Oklahoma in the scientific journal Geology reported that "the volume of fluid injected into the subsurface related to the production of unconventional resources continues to rise" and that there was a link between the "zone of injection and the seismicity" potentially triggering the Prague earthquake. On March 28, 2016 the USGS released the USGS National Seismic Hazard Map which concluded that the primary cause of the earthquake in Oklahoma in 2011 was pressure on fault lines from cumulative effects of injecting oil drilling wastewater under high pressure into the underground. Although the 2011 earthquake was the largest on record until that time, the USGS reported that the central and eastern U.S. (CEUS) had undergone the most dramatic increase in seismic activity in the United States since 2009 with an average of 318 earthquakes of magnitude 3.0 a year up from 24 a year from 1973 to 2008. In 2015 there were 1,010 earthquakes in the CEUS region. By mid-March 2016, there were already 226 earthquakes of magnitude 3.0 and larger in the CEUS.
Background The first earthquake known to have occurred within the boundaries of Oklahoma since the state was established occurred in September 1918 when a series of shocks was felt in El Reno, Oklahoma, the strongest of which was an estimated intensity V on September 10. On September 27, 1929, another tremor of intensity VI was centered in the same area and was felt in central and western Oklahoma; minor damage occurred in nearby areas and one chimney fell. The total affected area was approximately 20,000 km2 (7,700 sq mi). A magnitude 5.5 earthquake had occurred near El Reno on April 9, 1952 at 10:29 a.m. CST (16:29 UTC). At the time, it was the strongest earthquake ever recorded in Oklahoma. Most of Oklahoma was affected, as were parts of Arkansas, Iowa, Kansas, Missouri, Nebraska, and Texas. Damage was not extensive, but local residents were alarmed, and several thousands of dollars in damage occurred. Chimneys were toppled, walls were cracked, windows were broken, and bricks were loosened from buildings. In Oklahoma City, a crack measuring 15 meters (49 ft) was found in the State Capitol following the earthquake. The earthquake, which occurred along the Nemaha Fault, had a maximum intensity of VII near the epicenter. Scattered earthquakes occurred in Oklahoma between 1952 and 1969 with intensities as high as VII. Between 1978 and 2008, the average long-term rate of earthquakes was approximately two earthquakes of magnitude 3.0 or greater per year. However, in 2009, this number jumped to 20 with the beginning of several swarms of earthquakes in Oklahoma. Research suggests that most of the significant earthquakes in Oklahoma since the 1930s may have been induced by oil production activities, primarily waste-water disposal.
Earthquakes
2009–2010 The average number of earthquakes between 1978 and 1999 was 1.6 earthquakes of magnitude 3.0 or higher per year. This average dropped even lower between 2000 and 2008, with no year ever having more than three earthquakes of greater than or equal to magnitude 3.0 during the period. However, totals well above the 1978–2008 averages were recorded in 2009, with 20 earthquakes with magnitudes of at least 3.0 recorded. By the end of the year, Oklahoma County had seen its most active earthquake year since 1889, as recorded by the Oklahoma Geological Survey. Earthquakes continued in 2010, with numerous earthquakes occurring in Central Oklahoma; among these is an earthquake which took place on October 13 in the vicinity of Norman. With a USGS-assigned magnitude of 4.4 Mw, it resulted in two fall-related injuries, and the USGS received felt reports from seven states. The year ended with an annual statewide total of 43 earthquakes of magnitude 3.0 or greater.
2011–2012
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