A More Complete Catalog of the 2011 Youngstown, Ohio Earthquake Sequence from Template Matching Reveals a Strong Correlation to Pumping at a Wastewater Injection Well
Holtkamp, Stephen; Currie, Brian; Brudzinski, Michael R.
From March to December 2011, the Ohio Department of Natural Resources Ohio Seismic Network (ODNR OSN) recorded 11 earthquakes in Youngstown OH. Although small earthquakes aren't uncommon to intraplate regions, the persistence of events led to speculation that the earthquakes were being caused by a nearby wastewater injection well. The well stopped pumping after a local seismic network showed the earthquakes were nucleating near the tip of the injection well. Unfortunately, the 11 recorded events recorded by the ODNR do not represent a good characterization of the seismic sequence, making it difficult to firmly establish a direct causal relationship between the injection well and the earthquakes. This is a natural limitation of traditional seismic techniques, which in this case requires an earthquake to be at least ~M2.0. To address this limitation, we have developed a multiple station template matching (waveform cross correlation) algorithm, which is able to detect events several orders of magnitude smaller than traditional techniques. Our technique utilizes highly sensitive broadband seismometers located within 200km of the earthquakes, eliminating the need for costly local seismic deployments. In applying this technique, we detect ~250 seismic events occurring between January 2011 and January 2012, allowing us to directly test the correlation between seismic events and injected volumes. The seismic sequence started two weeks after injection began and ended 2 weeks after injection ended. The first months of pumping are characterized by a higher rate of seismicity than the final 8 months, which we interpret to be a consequence of initial reactivation of an ancient basement fault system which is near-optimally oriented with regional stress fields. Initially, we envision that injection-related pore-fluid pressure increased in discontinuous permeable zones of the fault system, reduced effective normal stress and permitted fault slip to occur. Early displacement likely promoted fluid infiltration into adjacent, initially inaccessible regions of the fault, iteratively increasing the area of potential failure. After the initial stage there is a direct relationship between the volume of injected wastewater and the number of earthquakes, suggesting development of more fully integrated fault zone permeability though time. This hypothesis is reinforced by the observation that the largest magnitude events (M>~2) were uncommon until after the midpoint of the sequence.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013