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aNote that in several cases the causal relationship between the technology and the event was suspected but not confirmed. Determining whether a particular earthquake was caused by human activity is often very difficult. The references for the events in this table and the ways causality may be determined are discussed in the report. Also important is the fact that the well numbers are those wells in operation today, while the numbers of events listed extend over a total period of decades.

bAlthough seismic events M > 2.0 can be felt by some people in the vicinity of the event, events M ≥ 4.0 can be felt by most people and may be accompanied by more significant ground shaking, potentially causing greater public concern.

cOne event of M 4.1 was recorded at Coso, but the committee did not obtain enough information to determine whether or not the event was induced.

dM 4.8 is a moment magnitude. Earlier studies reported magnitudes up to M 5.3 on an unspecified scale; those magnitudes were derived from local instruments.

the Earth’s crust suggest that the theoretical threshold for frictional sliding along favorably oriented preexisting fractures was exceeded (see also Chapter 2).

Figure 3.15 shows histograms of the maximum magnitudes reported for induced seismicity associated with different energy technologies: geothermal energy, hydrocarbon extraction, fluid injection for secondary and tertiary oil and gas recovery, hydraulic fracturing associated with unconventional oil and gas production, and wastewater disposal from any of the energy technologies (injection wells) (see Appendix C for data sources for this figure); note that CCS is not included in this figure due to the absence of any known significant induced seismic events associated with this technology.

The largest seismic events and most numerous reports of induced seismicity are associated with extraction activities, with magnitudes up to 7 associated with extraction of gas at the Gazli field. The next largest set of seismic events (two sites in the world, one with an event of M 5.1 and another site with an event of M 6) is associated with injection activities related to waterflooding for secondary recovery in oil and gas production. Waste and wastewater disposal activities have produced some moderate earthquakes (M ~ 4.5), notably in Denver in 1967 at the Rocky Mountain Arsenal, but these are rare. Oklahoma, Colorado, and Arkansas have experienced a recent increase in seismic activity; these events are being examined for potential links to injection (Ellsworth et al., 2012). In the New Mexico–Colorado border area, the Raton Basin is an active coalbed methane field that has experienced several swarms of seismic events, including a M 5.3 in August 2010. In light of the seismicity in the Raton Basin, the Colorado Geological Survey (CGS) is now reviewing all permit applications for water disposal wells in Colorado in regard to the possibility of induced seismicity, assisting the Colorado Oil and Gas Conservation Commis-

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