Matthew Hornbach, Southern Methodist University
If you’ve been following the news lately, chances are you’ve heard about – or even felt – earthquakes in the central United States. During the past five years, there has been an unprecedented increase in earthquakes in the North American mid-continent, a region previously considered one of the most stable on Earth.
According to a recent report by the Oklahoma Geological Survey, Oklahoma alone has seen seismicity rates increase 600 times compared to historic levels.
The state has gone from experiencing fewer than two magnitude-three earthquakes per year to greater than two per day, the report found. Similarly, my home state of Texas has experienced a near 10-fold increase in magnitude-three earthquakes or greater in the past five years.
The recent uptick in earthquakes in Texas, Oklahoma and several other central US states raises an obvious question: What is causing all of this seismicity?
Earthquake Causes
Several factors can promote the occurrence of earthquakes. There are natural changes caused by the shifting of Earth’s plates, the advance and retreat of glaciers, the addition or removal of surface water or ground water, and the injection or removal of fluids due to industrial activity.
Studies including two reports issued in April, indicate that human activities, including activities related to oil and gas extraction, are beginning to play a significant role in triggering earthquakes in the central US.
Extracting oil and gas from shale rock involves cracking, or fracturing, a layer of underground rock with a high-pressure mix of water, sand, and chemicals. As the oil and gas are released, those injection fluids and briny water also come up. That wastewater is later disposed of in what are called injection wells, or sometimes disposal wells.
It is important to note that it is not the fracking process itself that usually causes these earthquakes; it is the rapid injection of fluid during wastewater disposal that sometimes pumps hundreds of millions of gallons of brine deep into the earth each year.
Hundreds of studies
So do injection wells cause earthquakes?
A recent peer-reviewed scientific study I co-authored concludes human-activities, specifically water production and wastewater injection, represent the most likely cause of earthquakes in the Azle/Reno, Texas region, where significant gas production and wastewater injection began five years ago.
But this is not a fundamentally new discovery. For nearly a century, industry and academic researchers have recognized that human activities can and do sometimes trigger earthquakes.
Indeed, entire books – including many standard texts used in advanced petroleum geology, geomechanics, and petroleum engineering classes – are dedicated to understanding fault reactivation, rock mechanics, and the ways humans can facilitate these processes for the betterment of humanity.
Damage from a 2011 earthquake in Oklahoma.Brian Sherrod, US Geological Survey, CC BY
Additionally, multiple studies and reports, including hundreds of peer-reviewed scientific studies – and independent studies conducted by the National Research Council of the United States National Academy of Science and Engineering – confirm that the injection or removal of fluids can and indeed do trigger earthquakes.
What is unique and exciting about our Azle/Reno study is the unprecedented support and cooperation of the energy industry, which in many instances provided mission-critical data, technical support, and constructive scientific reviews to allow scientists to better assess, model, and understand earthquakes in the Azle/Reno area and across Texas.
In our instance, industry researchers went far beyond state regulatory requirements by providing insight into the location and orientation of regional faults, injection reservoir pressures, and subsurface flow.
The Azle/Reno study highlights how cooperation, transparency, and mutual respect between, industry, academia, and regulators can improve our understanding of seismicity, and help mitigate risk for all parties working, living, and conducting business in Texas.
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