Oklahoma Earthquake Swarms: Gotta’ blame somebody or something
Update 3 September 2016: 5.6 magnitude earthquake near Pawnee, Oklahoma, United States
The September 3, 2016 M 5.6 Oklahoma earthquake occurred as the result of shallow strike-slip faulting about 15 km northwest of the town of Pawnee. The earthquake occurred within the interior of the North America plate, far from any plate boundaries. The preliminary focal mechanism solution for the earthquake indicates rupture occurred on either a left-lateral fault striking east-southeast, or on a right lateral fault striking north-northeast. The mainshock location aligns with a major regional SW-NE trending fault. In general, it is very difficult to correlate earthquakes to specific faults in the region and identification of the fault responsible for this event will require further study and fieldwork.
Prior to the September 3, 2016 earthquake, the surrounding region of Oklahoma and Kansas has hosted close to 80 other M 4 or larger events over the preceding decade. Two of these were larger than M 5: a M 5.6 earthquake in November 2011, near Prague, Oklahoma, and a M 5.1 in February 2016 near Fairview, Oklahoma. The Prague earthquake resulted in 2 injuries, and over a dozen homes destroyed; it was felt in 17 states. The Fairview event was also broadly felt but did not cause significant damage. In the immediate vicinity of the September 3 event, a M 3.2 earthquake occurred on September 1, 2016, just to the southwest. Within an hour of the September 3 M 5.6 earthquake, 4 aftershocks have been located, the largest being a M3.6 event 56 minutes after the mainshock. 3 of the 4 aftershocks align on the NW-SE left-lateral plane of the focal mechanism solution.
Locations across the central and eastern United States (CEUS) have been experiencing a rapid increase in the number of induced earthquakes over the past 7 years. Since 2009 rates in some areas, such as Oklahoma, have increased by more than an order of magnitude. Scientific studies have linked the majority of this increased activity to wastewater injection in deep disposal wells in several locations. However, other mechanisms such as fluid withdrawal, enhanced oil recovery, or hydraulic fracturing processes can also result in induced earthquakes. In addition, regions with frequent induced earthquakes may also be subject to damaging earthquakes that would have occurred independently of human activity. Making a strong scientific case for a causative link between a particular human activity and a particular sequence of earthquakes typically involves special studies devoted specifically to the question. Such investigations usually address the process by which the suspected triggering activity might have significantly altered stresses in the bedrock at the earthquake source, and they commonly address the ways in which the characteristics of the suspected human triggered earthquakes differ from the characteristics of natural earthquakes in the region.
Magnitude uncertainty 5.6 mwr ± 0.0 Location uncertainty 36.431°N 96.931°W ± 1.0 km Depth uncertainty 4.5 km ± 4.1 Origin Time 2016-09-03 12:02:44.560 UTC
This Enid News & Eagle series explores the science behind efforts in determining why there was a sudden upswing in earthquakes in the Sooner State beginning in 2009.
Posted: Thursday, January 7, 2016
Posted: Sunday, September 6, 2015
ENID, Okla. — In northwest Oklahoma, some scientists are digging for seismic answers as fervently as wildcatters have drilled for black gold during boom times.
Published literature left in dust-covered library bins for decades now is considered invaluable to scientists studying Oklahoma’s earthquakes. Meanwhile, oil companies now are providing proprietary data on faults gathered through exploration, according to U.S. Geological Survey geophysicist George Choy.
In turn, the data is being used by Oklahoma Corporation Commission to regulate the oil and gas industry after wastewater injection was pinpointed as a cause for the increased seismicity. Scientists now know there is a link between injection wells and faults but have yet to get to the bottom of why some injection wells are linked to earthquakes and others are not.
Who’s at fault?
“We’ve put ourselves on record saying that there was relationship to the injection of wastewater in deep wells and into the basement, and that’s about as far as we can go right now, because we haven’t been able to really localize that and say, ‘It’s these faults, it’s these faults,’” Oklahoma Geological Survey Director Jeremy Boak said. “We know it’s certain faults in certain orientations that are properly aligned with the stress field occurring in Oklahoma, and they’re the ones that move. Most of those are small faults.”
‘Fits and spurts, little bursts’
One certainty is the increase in earthquakes in the state.
Between 1978 and 1998, Oklahoma averaged fewer than two earthquakes a year, according to U.S. Geological Survey records. The following decade, fewer than three magnitude 3.0 or greater earthquakes were recorded each year, until numbers climbed significantly in 2009.
Since 2009, numbers have elevated, with the state surpassing a 2014 record of earthquakes larger than 3.0 magnitude in August this year, OGS records indicate.
“I think the growth of the rise of earthquakes over the past few years has been something close to exponential, that is, it increases by a certain percentage every month or so,” Boak said. “But, actually, what it has tended to do is to go in fits and spurts, little bursts, and then slows down and then accelerates.”
Boak said he believes the state will go well past the 2014 record, unless the situation changes “surprisingly” in the next few months.
Choy noted a sizable body of research correlates the onset of massive injection well activity with the onset of earthquakes.
“There are also a number of papers describing the feasible mechanisms by which injected fluids can cause earthquakes. These mechanisms involve changes in stress caused by injected fluids on pre-existing but previously inactive faults,” Choy said.
Most geological faults in Oklahoma have been inactive for millions of years, Choy said.
“Only a few faults near active injection wells have become active,” he said. “The majority of wells have not been associated with seismic activity. Scientists are trying to identify the factors that resulted in induced activity for some wells.”
Boak said some of the densest areas of faulting are not really seismically active, while some of the seismically active areas do not have a lot of mapped faults.
USGS can only ascribe an earthquake to a fault if the earthquake is accurately located, Choy said.
“The more stations we have, the better the location. Assuming we have a good location, a nearby fault can be associated with the earthquake,” he said. “However, most faults are still not known. Sometimes, if earthquake locations line up, we can identify a fault by the linear pattern. The spatial distribution of earthquakes also gives us an idea of the rupture length. This would be a ‘new’ fault in the sense of being newly discovered.”
Choy said it may be an exaggeration to say more faults are being discovered in Oklahoma.
First, scientists are studying literature and geological maps published decades ago, he said.
“It is one example of the value of pure research; we never know how data gathered today will be used in the future,” Choy said. “Second, oil companies — which have had proprietary data on faults from exploration — have slowly started making some of their data available.
“Earthquake patterns can be used to identify a fault that has not yet been mapped. So in that sense, it has been discovered.”
When it comes to new faults, Choy said intact rock will not break on its own unless intense stresses build up and exceed the strength of the rock.
“Faults develop naturally over hundreds of thousands of years in tectonically active regions,” he said. “Oklahoma is not near a plate boundary, so tectonic forces are insignificant. Occasionally, for reasons we are still investigating, in the mid-continent, stress may build up locally and result in an earthquake. This is rare but does happen in the mid-continent, (for example) the New Madrid region.”
Boak noted four of the strongest earthquakes in American history were in the New Madrid region, in Missouri and southern Illinois, in 1811 and 1812.
“So, there are these areas inside the major plates that can be seismically active. And these are more related to those kind of faults than to the major plate boundaries,” Boak said.
Smaller earthquakes produce smaller signals, Choy said.
“That means in order to record it, an instrument must have been deployed fortuitously close to the event. This requires increasing the number or density of stations,” Choy said. “However, due to realistic budgetary and limited human resources, we cannot instrument every single square mile of the state. We could prioritize deploying instruments about wells and active faults rather than at any random spot.”
There has been a fair amount of discussion about if there could be a cap on the size of earthquakes the moving faults might produce, Boak said.
“It’s probably bigger than most of the ones we’re getting,” Boak said. “But there’s still an open question. An awful lot of good seismologists really think we may never have an answer to the question of whether the Prague earthquake, which was a fairly substantial earthquake, whether that was a natural earthquake or whether that was part of this pattern of induced seismicity. We haven’t developed a good model that says, ‘This is how you would tell them apart.’”
The Prague earthquake in November 2011 measured magnitude 5.6 and was the largest earthquake in recorded history in Oklahoma.
Jessica Miller | Enid News & Eagle
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2011 November 06 03:53:10 UTC : Earthquake Details
|Depth||5 km (3.1 miles)|
|Distances||34 km (21 miles) NNE of Shawnee, Oklahoma
63 km (39 miles) SSE of Stillwater, Oklahoma
68 km (42 miles) ESE of Guthrie, Oklahoma
71 km (44 miles) ENE of OKLAHOMA CITY, Oklahoma
|Location Uncertainty||horizontal +/- 10.5 km (6.5 miles); depth +/- 2.8 km (1.7 miles)|
|Parameters||NST=189, Nph=192, Dmin=26.7 km, Rmss=1.06 sec, Gp= 29°,
M-type=centroid moment magnitude (Mw), Version=C
The magnitude 4.7 and 5.6 earthquakes that occurred on November 5, 2011, were situated in a region located about 50 km east of Oklahoma City, Oklahoma. Earthquakes are not unusual in Oklahoma, but they often are too small to be felt. From 1972-2008 about 2-6 earthquakes a year were recorded by the USGS National Earthquake Information Center; these earthquakes were scattered broadly across the east-central part of the state. In 2008 the rate of earthquakes began to rise, with over a dozen earthquakes occurring in the region east- northeast of Oklahoma City and southwest of Tulsa, Oklahoma. In 2009 the rate of seismicity continued to climb, with nearly 50 earthquakes recorded–many big enough to be felt. In 2010 this activity continued. The magnitude 4.7 and 5.6 earthquakes of November 5, 2011, are the largest events recorded during this period of increased seismicity. Additionally, the M5.6 quake is the largest quake to hit Oklahoma in modern times.
There have been dozens of aftershocks recorded following the shallow November 5, 2011 magnitude 5.6 earthquake and its magnitude 4.7 foreshock that occurred on the same day. These aftershocks will continue for weeks and potentially months but will likely decrease in frequency. This is not an unusual amount of aftershock activity for a magnitude 4.7 to 5.6 earthquake sequence. There is always a small possibility of an earthquake of larger magnitude following any earthquake, but the occurrence of the magnitude 5.6 earthquake, and the increase in activity in recent years does not necessarily indicate that a larger more damaging earthquake will occur.
In general, it is very difficult to correlate earthquakes to specific faults in the region and in eastern North America. The earthquake sequence that started yesterday occurred close to where a magnitude 4.1 earthquake occurred on February 27, 2010. From the location of the earthquake and the focal mechanism it is possible that this earthquake occurred on the Wilzetta fault. The Wilzetta fault is one of a series of small faults formed in the Pennsylvanian Epoch (approx. 300 million year ago) during the intraplate deformation known as the Ancestral Rocky Mountains mountain-building episode (orogeny). The relationship between the recent earthquakes and this older structure is still unknown and requires further investigation.
The Meers fault located in south-central Oklahoma, about 100 km southwest of Oklahoma City, is the only fault identified in the state with evidence of surface-rupturing earthquakes in the last 3000 years (prior to historical settlement of the region). Paleoseismology studies have identified a temporal clustering of a least three earthquakes on this fault, two of which are dated (1200-2900 years before present) and the third is believed to be older in age. An earthquake of magnitude 5.6 like the one that occurred yesterday east of Oklahoma City, are believed to be capable of striking anywhere in eastern North America at irregular intervals. Earthquakes east of the Rocky Mountains, although less frequent than in the West, are typically felt over a much broader region. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than a similar magnitude earthquake on the west coast. A magnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 100 km (60 mi) from where it occurred, and it infrequently causes damage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as far as 500 km (300 mi) from where it occurred, and sometimes causes damage as far away as 40 km (25 mi). According to felt reports submitted to the USGS’ Did You Feel It? Website, yesterday’s magnitude 5.6 was clearly felt from St. Louis, Missouri, to southwest of Dallas, Texas, an epicentral distance of about 500 km. More than 60,000 individuals from 14 states have reported their observations on this website.
Working together the USGS, Oklahoma Geological Survey (OGS) and the University of Oklahoma School of Geology and Geophysics have deployed about 35 portable seismograph stations after the M4.7 foreshock and the M5.6 mainshock to facilitate improved detection and location of earthquakes. These portable stations will remain deployed in their current configuration for several weeks to a few months. This work is being done in partnership with the USGS, and builds on earlier cooperative efforts with OGS in the past two years to expand seismic monitoring in the region.
Oklahoma Geological Survey – Mapping
Seismicity Map of Oklahoma
Seismic Hazard Map of Oklahoma
Animation of Oklahoma Seismicity
Animation of M5.6 Foreshocks and Aftershocks-Nov 5, 2011 to July 31, 2012
Image of the complete Bouguer gravity map of the United States, Showing regional tectonic features, provided by the USGS (annotations by Gibson).