installed approximately 300 new earthquake-monitoring instruments in the highest-risk areas. Full implementation of ANSS will result in 6,000 new instruments on the ground and in structures in at-risk urban areas (Box 4.2).
Seismic events that are thought to be induced are flagged in the USGS earthquake database. However, many or most events that USGS scientists suspect may be induced are not labeled as such, due to lack of confirmation or evidence that those events were in fact induced by human activity.10 This is often true with events in regions that have experienced natural earthquakes before any mining or extraction operations were established. The earthquake location accuracy provided by the NEIC depends primarily on the number and location of seismic stations recording the event. During the 2008-2009 Dallas–Fort Worth earthquake swarm, the accuracy of the initial NEIC locations was on the order of 10 km (6 miles), which made the events difficult to assign to a particular injection well (Frohlich et al., 2011). In areas of low historical seismicity, the NEIC network coverage tends to be sparser than in more seismically active areas, making the detection of small events (< M 3) and accurate hypocenter locations difficult (Box 4.3).
Although the concerns surrounding induced seismicity are relatively new, at least two states have now adopted, or are in the process of adopting, regulations or approval procedures to address the issue. Colorado and Arkansas are currently reviewing underground injection permits for possible problems with induced seismicity in the Raton Basin, Colorado, and Guy-Greenbrier area, Arkansas (Box 4.4). Recent seismic activity in the Raton Basin near a large coalbed methane field with active injection has prompted the Colorado Oil and Gas Conservation Commission (COGCC) to initiate a policy requiring the Colorado Geologic Survey to review all Class II injection permits for geologic features that could result in seismicity due to injection. According to a statement released by the COGCC, “if historical seismicity has been identified in the vicinity of a proposed Class II UIC well, COGCC requires an operator to define the seismicity potential and the proximity to faults through geologic and geophysical data prior to any permit approval” (COGCC, 2011). Due to apparent instances of induced seismicity in Arkansas, the Arkansas Oil and Gas Commission (AOGC) proposed regulations to establish a “Moratorium Zone” covering over 1,000 square miles where no permit for a Class II well will be granted without a hearing by the Commission (AOGC, 2012). The proposed regulations also require no Class II permit will be issued within 5 miles of a “Moratorium Zone Deep Fault” without a hearing by the Commission.
10 Bruce W. Presgrave, USGS, personal communication, March 3, 2011.