Research: Sharp increase in central Oklahoma seismicity

Oklahoma has had more earthquakes over magnitude 3 in 2014 than California.

“Seismic swarms” of earthquake activity have been recorded in Oklahoma, according to just-released research led by Dr. Katie Keranen at the Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York.

Oklahoma earthquakes in areas of high-rate water disposal constitute nearly half of all central and eastern US seismicity from 2008 to 2013.

Unconventional oil and gas production provides a rapidly growing energy source; however high production states in the United States, such as Oklahoma, face sharply rising numbers of earthquakes.

In the research findings, reported in SCIENCE, Dr. Keranen writes, “Subsurface pressure data required to unequivocally link earthquakes to injection are rarely accessible. Here we use seismicity and hydrogeological models to show that distant fluid migration from high-rate disposal wells in Oklahoma is likely responsible for the largest swarm. Earthquake hypocenters occur within disposal formations and upper-basement, between 2-5 km depth.

Seismicity in the United States midcontinent surged beginning in 2008, predominantly within regions of active unconventional hydrocarbon production. In Arkansas, Texas, Ohio, and near Prague, Oklahoma, recent earthquakes have been linked to wastewater injection although alternative interpretations have been proposed, writes Dr. Keranen.

Sharp increase in Oklahoma magnitude 3+ earthquakes.

Sharp increase in Oklahoma magnitude 3+ earthquakes.

Conclusively distinguishing human-induced earthquakes based solely on seismological data remains challenging. Seismic swarms within Oklahoma dominate the recent seismicity in the central and eastern United States, contributing 45% of M3 and larger earthquakes between 2008-2013. No other state contributed more than 11%.

Our work demonstrates that a very small number of wastewater disposal wells, operating at exceedingly high volumes, create substantial anthropogenic seismic hazard, Dr. Keranen writes. Four of the highest-volume disposal wells in Oklahoma (~0.04% of wells) are capable of triggering ~20% of recent central US earthquakes in a swarm covering nearly 2000 square kilometers, as shown by our analysis of modeled pore pressure increase at relocated earthquake hypocenters. This massive Jones seismic swarm is in sharp contrast to the oft-cited aseismic behavior of the majority of active disposal wells. The area of increased pressure related to these wells continually expands, increasing the probability of encountering a larger fault and thus increasing the risk of triggering a higher-magnitude earthquake.

These results from Oklahoma, where the rate of M3+ earthquakes is now double that of California, are broadly relevant for understanding fluid-related earthquake triggering and rapid fluid transmission in the shallow crust to great distances. Our results illuminate the sharp response of faults in critical equilibrium to the strong forcing mechanism of massive subsurface fluid injection.

With growing development of unconventional reservoirs, the number of new disposal wells drilled and the volume of water disposed at individual wells have abruptly increased. The burgeoning hydrocarbon production from unconventional reservoirs has sharply changed energy and energy politics, and states from California to New York are currently debating permitting and regulation of hydraulic fracturing and wastewater disposal.

Read the story in SCIENCE.

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Keranen explores motion behind earthquakes

When earthquakes happen, Prof. Katie Keranen of Cornell’s Earth and Atmospheric Systems, watches … measures … and, monitors.

Keranen images the Earth’s crust by studying the propagation of energy waves through rock, a field known as seismology. The sources of these waves are subdivided into two categories: natural or passive sources, such as movement caused by earthquakes, and active sources, which are manmade.

Dr. Katie Keranen on location.

Dr. Katie Keranen on location.

Seismologists are interested in the ground motion that results from both types To generate an active source, Keranen drills a shallow hole in the Earth’s surface, loads it with explosives and detonates them, causing ground motion.

The shaking is picked up by the web of sensors, or seismometers, deployed around the origin point of the explosives. The data from the wave pattern yields high resolution images of the geological features of Earth’s interior several kilometers deep, Keranen said.

Using waves to image the interior a solid object by breaking it into multiple slices is known as tomography. This technique has a variety of applications, including medical X-ray imaging.

“Just like we can get a medical tomogram to look inside the human body, we can look inside the earth using seismology,” Keranen said.

Seismometers pick up on a variety of wave-generating events – explosions or nuclear blasts; implosions such as collapsing underground caves; earthquakes; and volcanoes. Seismologists distinguish these events by analyzing each unique wave behavior, according to Keranen. The patterns yield valuable information about the source, magnitude and other particulars of an event.

A typical earthquake pattern consists of three wave fronts, Keranen said. The P wave is the fastest, has the lowest amplitude and is the least dangerous. It often triggers earthquake early warning systems, alerting people that the next two (and more destructive) waves are on their way. The S wave causes sideways shearing movement. Surface waves are responsible for general motion close to Earth’s surface.

(Story credit: Jacqueline Carozza, Cornell Daily Sun)