Energy Technologies: How They Work and Their Induced Seismicity Potential - Induced Seismicity Potential in Energy Technologies - page 69

BOX 3.7
Dallas-Fort Worth Earthquake Swarm October 2008 to May 2009
A series of M 2.5 to M 3.3 earthquakes occurred in the Dallas-Fort Worth (DFW) area of Texas,
where earthquakes were felt and reported by local residents in October 2008 and May 2009. The National
Earthquake Information Center (NEIC) located the earthquakes in the vicinity of the DFW airport.
The state of Texas historically experienced a low rate of natural seismicity at the time of these earthquakes
and the entire state has only two permanent seismographic stations operated by the NEIC. Because of the
sparse seismographic station coverage, the NEIC can only locate events in Texas that are greater than about
M 2.5 with location accuracy of plus or minus 6 miles or 10 km. Researchers from the University of Texas
(UT) and Southern Methodist University (SMU) deployed a temporary network of six seismographic stations
in the DFW area to locate seismic events more precisely. The UT-SMU seismic array ran from November 9,
2008, to January 2, 2009, and located 11 earthquakes that spanned a 1-km-long, north-south trending
zone in close proximity to a saltwater disposal (SWD) well used for wastewater injection by Chesapeake
Oil and Gas Company. The wastewater originated from wells in the vicinity of the DFW airport producing
from the Barnett Shale (Figure 3.11). The first felt DFW earthquakes started about 6 weeks after injection
into the disposal well was initiated. The close correspondence of the earthquakes with the location and depth
of the well, together with the close timing of the start of injection and the start of seismic activity, strongly
suggest that injection was the cause of the seismic activity.
A state tectonic map compiled by the Texas Bureau of Economic Geology shows a northeast trending
normal fault in the subsurface in close proximity to the SWD injection well. The earthquake swarm continues
in the DFW area to this date, with M 2.6 or less events occurring prior to August 2011, over 2 years after
shutdown of the injection well (Eisner, 2011). The persistent seismicity after the nearby injection wells were
shut in demonstrates the difficulty in assessing whether the seismic activity is induced or natural. Similar to the
post-shut-in events that have occurred in relation to EGS projects in France and Switzerland, understanding
the cause and magnitude of these events through time requires further research that combines field observa-
tions and data with fluid flow and geomechanical simulation codes.
Box continues