What’s Shaking in Ohio? Using Earthquake Waveform Similarity to Investigate Whether They Are Caused by Deep Wastewater Injection Wells
February 10, 2014
From March to December 2011, the Ohio Department of Natural Resources Seismic Network (OSN) recorded 11 earthquakes in Youngstown, leading to speculation that the earthquakes were caused by wastewater injection.
In fact, this occurred during a dramatic rise in small to moderate seismicity in the central and eastern United States over the past decade that is thought to be due primarily to an increase in natural gas and oil production activities that has led to large volumes of wastewater. Pumping near Youngstown stopped after a local network was installed in December 2011 and showed earthquakes near the injection point. To further investigate whether a causal relationship exists between injection and the earthquakes, we developed a multiple station template matching (waveform cross correlation) algorithm, which can detect events ~10x smaller than traditional techniques. Utilizing only existing regional broadband seismometers 50-250 km from the earthquake source, we have now detected over 400 earthquakes in the sequence. Nearly all earthquakes occur during the commercial injection, and the cumulative number of earthquakes is remarkably proportional to the cumulative injection volume. A combination of relative and absolute relocation of earthquake hypocenters shows the first events occurring close to the injection well and propagating WSW (consistent with the focal mechanism of the largest earthquake in the sequence) at a rate of 1-2 km/y. Individual families of events show delay times of 1-6 days between injection volume fluctuations and seismicity rate variations, proportional to distance from the well, which we interpret to represent diffusivity in the now saturated fault zone. We envision that injection-related porefluid pressure increased in discontinuous permeable zones of an ancient basement fault system, reduced effective normal stress and permitted fault slip to occur. Key to this interpretation is fact that the Youngstown injection well was drilled ~200 feet into the Proterozoic crystalline basement and was completed “open hole” thus providing a direct pathway for injection fluids into the basement fault zone.
While state records indicate there are currently over 210 wastewater injection wells in Ohio, only 34 have been drilled into Precambrian igneous/metamorphic rocks (23 completed “open hole”). The locations of the 34 deep wells were compared with locations of recent seismic events catalogued by OSN. Other than the Youngstown sequence, we have identified only four earthquakes located within 10 km of a currently permitted deep injection well. Using our waveform matching algorithm, we can confirm that these are essentially isolated earthquakes that do not form a sequence similar to Youngstown. However, a potential earthquake sequence of interest is a series of 6 earthquakes near Marietta detected by OSN between November 2010 and September 2012. While there is no well injecting into the basement in this region, there are several nearby injection wells including that among the largest volumes in the state. Preliminary investigation with our waveform matching algorithm reveals over 50 earthquakes in this sequence, suggesting another potential relationship between seismicity and wastewater injection.