M4.2 (12 July) and M3.9 (14 July), Oklahoma: the Earthquake Series Continues

More Oklahoma quakes – what do these tremors mean?

The occurrence of earthquakes of magnitude 4.2 (M4.2) on Saturday 12 July, and M3.9 on 14 July in central Oklahoma indicate that the series of earthquakes which has persisted in the state over recent months – and years – shows no signs of abating.

The past week has seen 25 tremors of at least M2.5 along a band trending broadly north-south across the centre of the state and extending up into Kansas.

The Oklahoma Earthquake Swarm

Repeated and regular earthquakes are mainly associated with tectonic margins (such as the San Andreas Fault Zone in California) or areas of hotspot volcanism (such as Yellowstone) where the causes are clear and associated with major-scale earth movements.

In Oklahoma, however, there is no such source for earth movement. Too far east of the Rockies to be affected by uplift, and too far east of the known New Madrid seismic zone of Missouri-Tennessee to be influenced by it, Oklahoma is geologically part of the United States’ stable continental interior. As such, this region is not an area where we would expect regular seismic activity.

Yet there has been significant, and increasing, movement.

The weekend’s tremors, and the many smaller ones around them, represent an unusual and worrying trend.

The United States Geological Survey, in a press release issued in October 2013, said that “Studies show one to three magnitude 3.0 earthquakes or larger occurred yearly from 1975 to 2008, while the average grew to around 40 earthquakes per year from 2009 to mid-2013”.

An update issued months later, in May 2014, confirmed what residents may well have been thinking – that the earthquakes seem to be coming more regularly.

And they sounded a note of alarm. “The rate of earthquakes in Oklahoma has increased remarkably since October 2013 – by about 50 percent – significantly increasing the chance for a damaging magnitude 5.5 or greater quake in central Oklahoma”, they warned.

Oklahoma Earthquakes: Pointing the Finger at Fracking

It’s beyond dispute that the number of small tremors (and possibly also of larger ones) in Oklahoma is increasing. The USGS analysis considered the cause and reached the conclusion that these tremors are not natural hazards but are the result of human activity.

Induced seismicity, as it is known, has many potential sources, from mining to building large dams (the weight of water ponded behind these dams increases pressure). In the case of the Oklahoma quakes, the USGS concluded that the most likely source was wastewater injection into deep geological formations, a by-product of energy extraction methods such as fracking (though not, it should be noted, a direct result of the fracking process itself).

Oklahoma Fracking Tremors: Implications

The USGS and Oklahoma Geological Survey have increased monitoring in the state, and continue to study the patterns of seismic activity and consider the implications – in particular, the possibility that future tremors might include something of a magnitude similar to the M5.7 which shook the Prague area of the state in 2011.

In the meantime the shaking in Oklahoma continues – and there are no signs of it slowing down.

© Copyright 2014 Jennifer Young, All rights Reserved. Written For: Decoded Science