Subsurface Wastewater Injection Sites Shown to be More Susceptible to Remote Earthquake Triggering

Underground fluid injection can induce earthquakes by increasing pressure on the geologic features deep below the surface of the site. This induced seismicity has affected a growing number of sites in the past decade due to an increase in subsurface wastewater disposal associated with natural gas extraction. The increased seismicity of a place may not appear for a few days or up to many years, and there currently is no way to diagnose underground damage near fluid injection sites that might lead to increased seismicity. Van der Elst et al. (2013) investigate the possibility that dynamic triggering from large, distant earthquakes may be an indicator of stress in sites of wastewater injection. The triggers in this study are three large earthquakes (over 8 moment magnitude, or Mw) between 2010 and 2012. The authors find three sites with a history of fluid injection that respond to these triggers with patterns of seismicity indicative of underground damage.—Shannon Julius
Van der Elst, Nicholas J., et al. “Enhanced remote earthquake triggering at fluid-injection sites in the midwestern United States.” Science 341.6142 (2013): 164-167.

Van der Elst et al. (2013) examined earthquake data to look for sites responsive to three large trigger earthquakes. The earthquakes were: a February 2010 8.8 Mw in in Maule, Chile; a March 2011 9.1 Mw earthquake in Tohoku-oki; and April 2012 8.6 Mw in Sumatra. In order to judge whether induced seismicity was promoted by anthropogenic activity, the authors found data for all earthquakes 3 Mw and higher in the central U.S. within 10 days of each event. For sites that had a clear response to the trigger earthquake, the authors studied earthquake records for months and years following the triggering event. In particular, the authors looked for sites that had low seismicity before the initial trigger and uncharacteristic seismic events afterwards.  
When the authors mapped all 3 Mw and higher earthquakes that happened within 10 days of a trigger earthquake, they found that triggering happened almost exclusively at three sites: Prague, Oklahoma; Snyder, Texas; and Trinidad, Colorado. Each of these sites had relatively low seismicity before the first trigger, at least one fairly large earthquake in response to the trigger, and a delayed earthquake “swarm” months after the trigger. Additionally, each site had a strong history of wastewater injection within 10 kilometers of the induced earthquake activity.
The first site was the Cogdell oil field near Snyder, Texas. This site had a number of earthquakes in response to the March 2011 event in Tohoku-oki. The largest earthquake was a 3.8 Mw event that happened two and a half days after smaller events. A few months later, in September 2011, the site had a seismic “swarm” that included a 4.3 Mw main shock. The rate of earthquakes was higher at the site in the 10 days after the earthquake at Tohoku-oki and immediately after the September swarm than at other any time from February 2009 to the present.
The 2010 Maule event triggered a series of earthquakes in an area near a fluid injection site in Prague, Oklahoma. The largest event was 4.6 Mw, and it occurred only 16 hours after the Maule quake. There was a very low rate of earthquakes at the site prior to the trigger and no activity measured in the 4 months before the Maule event. These triggered earthquakes were suggestively located near the epicenter of a 5.0 Mw earthquake that occurred the following year, in November 2011. This event led to two more earthquakes with a magnitude greater than 5.0 Mw. The largest of these, a 5.7 Mw in November, could possibly be the largest earthquake associated with wastewater disposal. As a sign of continued seismicity, the 2012 earthquake in Sumatra also caused a small amount of activity near the edge of the swarm in 2011. 
Trinidad, Colorado also experienced seismic activity as a result of the Maule earthquake, again a small response near the site of future activity. There were only four events in the day after the earthquake in Maule, but the site only had five earthquakes in the entire previous year, so this result is significant. The delayed swarm occurred in August 2011 and included a 5.3 Mw main shock. As in Oklahoma, the Sumatra earthquake caused a small boost in earthquake activity near the site of the previous earthquake swarm.

The authors are concerned that these findings indicate the likelihood of future induced seismic activity at these damaged sites, and they suggest that improved seismic monitoring should occur in areas of subsurface wastewater injection.

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