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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 experi-
enced 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).
STATE EFFORTS
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 injec-
tion 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 hear-
ing 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.
 
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