Steps Toward a “Best Practices” Protocol - Induced Seismicity Potential in Energy Technologies - page 135

energy technologies. The committee has done this exercise for induced seismicity associated
with injection wells used for oil and gas development (Environmental Protection Agency
[EPA] Underground Injection Control [UIC] Class II wells) or with carbon storage (EPA
UIC Class VI wells) and has developed an example of the primary elements that might be
included in a best practices protocol matrix (Table 6.4).
THE USE OF A TRAFFIC LIGHT CONTROL SYSTEM
The protocols described in Box 6.1 and Tables 6.3 and 6.4 refer to a “traffic light” con-
trol system for responding to an instance of induced seismicity. Such a system, although
rarely employed in energy technology projects with active cases of induced seismicity, 2
allows for low levels of seismicity but adds additional monitoring and mitigation require-
ments when seismic events are of sufficient intensity to result in a concern for public health
and safety. The preferred criterion to be used for such a control system has been the level of
ground motion observed at the site of the sensitive receptor, be it a public or private facility.
Seismic event magnitude alone is generally insufficient as the only criterion because of the
nature of attenuation (absorption or loss of energy) with increasing distance from an event
location to a sensitive receptor site. Zoback (2012) provides a summary of a traffic light
system for the purpose of managing potential induced seismicity from wastewater disposal.
As an example, the Bureau of Land Management (BLM) recently issued as its “Con-
ditions of Approval” 3 for a proposed EGS project the specific procedures to be followed
in the event that induced seismicity is observed to be caused by the proposed stimulation
(hydraulic fracturing) operation. The specific procedures included the use of the traffic
light control system that allows hydraulic fracturing to proceed as planned (green light) if
it does not result in an intensity of ground motion in excess of Mercalli IV (“light” shaking
with an acceleration of less than 3.9%g), as recorded by an instrument located at the site of
public concern. However, if ground motion accelerations in the range of 3.9%g to 9.2%g
are repeatedly recorded within one week, equivalent to Mercalli V (“moderate” shaking),
then the operation is required to be scaled back (yellow light) to reduce the potential for
the further occurrence of such events. And finally, if the operation results in a recorded
acceleration of greater than 9.2%g, resulting in “strong” Mercalli VI or greater shaking, then
the active operation is to immediately cease (red light).
The authority for the permitting of Class II injection well location varies by state and is
discussed in Chapter 4. Well permits of Class II injection wells in Colorado, for example, are
reviewed by the Colorado Geological Survey (COGCC, 2011). During a geologic review,
2 To the committee's knowledge, the traffic light system has been applied only at the Berlin geothermal field in El
Salvador (Majer et al., 2007) and at Basel, Switzerland.
3 R.M. Estabrook, BLM, Conditions of Approval for GSN-340-09-06, Work Authorized: Hydroshear, The Geysers,
January 31, 2012.