Page 216 - Geothermal Energy Systems Exploration, Development, and Utilization

192 4 Enhancing Geothermal Reservoirs
Just Clearly
perceptible perceptible
10000
Disturbing
V. light damage
Cumulative frequency of events 100 Green Amber weak structures
1000
Alarming
Damage in

Red

Damaging
10

1

0 2 4 6
PGV equivalent magnitude

Figure 4.9 Pseudo Gutenberg Richter plot (‘‘traffic light
plot’’) with thresholds of PGV equivalent magnitudes (after
Bommer et al., 2006).

Although the ‘‘traffic light approach’’ has been successfully applied during three
hydraulic injection experiments in the Berlin geothermal field, and in El Salvador,
in 2003 and 2004, it has to be kept in mind, that it is a reactive measure. Induced
seismic events have been reported by various authors (e.g., Bommer et al., 2006;
Hoover and Dietrich, 1969; Hsieh and Bredehoft, 1981; Baisch et al., 2006; Charl´ ety
et al., 2007; Cuenot, Dorbath, and Dorbath, 2008) well after shut-in. Also at Basel,
the injection was halted after the first felt event, but this did not prevent subsequent
perceptible events from happening (IEA-GIA, 2009).

Addressing Induced Seismicity In order for EGS to be successfully utilized, the
issue of induced seismic events should be proactively addressed on various levels,
as illustrated by IEA-GIA(2009) and Bromley and Mongillo (2008).

• Estimate local potential for natural seismic hazard and induced seismicity: When
considering sites for EGS development, it is judicious to consult geological
and seismological information in order to determine suitability in relation to
background natural seismicity, the state of stress, and the existence of superficial
deposits, with the potential of amplified ground shaking. Many of the necessary
information might be publicly accessible via governmental institutions like
geological surveys, for example.
• Technological innovation: Pioneering concepts for stimulation, and field man-
agement strategies for operation of EGS ought to be used and developed, aiming