In a week when the United States Geological Survey has reported its ongoing research on the Oklahoma earthquake swarm, the centre of the Sooner State experienced yet another significant (for its location) tremor – this time a magnitude 3.6 event which had its epicentre just 27km from Oklahoma City.
According to information from the USGS, the earthquake, which occurred at 8.30am local time, shook the ground over an area roughly bounded by Guthrie, Norma, Shawnee and Oklahoma City.
In theory, places like Oklahoma should not be unduly troubled by seismic activity. Most earthquakes occur at plate boundaries and the central and eastern united States are far from such boundaries. But over several years Oklahoma has been troubled by a swarm of minor earthquakes – and the occasional larger one – which has attracted the attention of seismologists.
Stable continental areas do experience earthquakes as a result of movement along existing, often long-buried faults and in some areas (such as the New Madrid Seismic Zone) these tremors can associated with major fault zones and potentially significant earthquake events. And as seismologist Robert Yeats notes, eastern North America is old and has many zones of underlying weakness.
In general, however, earthquakes in these area are small, shallow and infrequent, although they tend to be felt over larger distances than those close to plate margins because of the age and density of the rock).
The Oklahoma Earthquake Swarm: Induced Seismicity
Oklahoma, however, has produced a pattern of minor earthquakes. In late 2013 the USGS reported that “since January 2009, more than 200 magnitude 3.0 or greater earthquakes have rattled Central Oklahoma, marking a significant rise in the frequency of these seismic events.”
The number continues to increase: a more recent press release indicated that “183 earthquakes of magnitude 3.0 or greater occurred in Oklahoma from October 2013 through April 14, 2014. This compares with a long-term average from 1978 to 2008 of only two magnitude 3.0 or larger earthquakes per year.”
May 20’s M3.6 (and an M3.0) are just the latest in a week in which the USGS map shows 24 seismic events of at least M2.5 in the state.
This anomalous pattern requires explanation and the finger seems to be pointing firmly in the direction of human activity. Industrial processes such as wastewater injection – associated with oil and gas extraction – can effectively lubricate existing faults and increase levels of seismic activity.
The Prague Earthquake of 2011
The largest earthquake in the sequence, and the largest in the recorded history of the state, took place at Prague on 6 November 2011. Measuring M5.6, this quake was was 100 times stronger than the M3.6 of 20 May and proved deadly, killing two people, damaging or destroying many homes and buckling Highway 62.
Oklahoma Quakes: Human Activity and Damage
The Prague earthquake reminds us that even apparently stable areas aren’t immune to reactivation of ancient faults; and the ongoing sequence of notable earthquakes raises questions about how human activity – and about how authorities should reassess and manage earthquake hazard in areas that were previously considered ‘safe’.