Page 215 - Geothermal Energy Systems Exploration, Development, and Utilization

4.6 Application (Practical) 191
the actual injection of fluid has ceased. At the Berlin Geothermal field, a magnitude
4.4 seismic event has been observed about two weeks after the end of the injection,
although there are discussions whether this particular event can undoubtedly be
connected to the injection activity. Since the 4.6 earthquake at The Geysers field
in Northern California, in the 1980’s when its fluid production was at its peak,
there have been only few magnitude 4 events. Almost all other induced seismicity
at other geothermal fields has been in the range of magnitude 3 or less (e.g., Majer
et al., 2007; Rybach, 2006).
Although induced seismic events are inherent to EGS development, because
hydraulic stimulation, by definition and design is a form of induced seismicity,
there have been recent EGS development projects as well, at Groß Sch¨ onebeck
(Kwiatek et al., 2008), for example, where no microseismic activity strong enough
to be perceptible at the surface was induced.

Structural Damage and Perceptibility Concerning any potential structural damage,
it is important to notice, that within the frame of a probabilistic seismic hazard
analysis for engineering purposes, it is common practice to specify a lower bound
of magnitude 5.0, on the basis that smaller events are not likely to be of engineering
significance (Bommer, Georgallides, and Tromans, 2001). Humans, nevertheless,
can feel seismic events of lower magnitude already. According to the modified
Mercalli intensity scale, most people indoors can feel the ground shaking caused by
a seismic event of magnitude 3. A few people might even feel the movement caused
by magnitude 2 events. These felt vibrations, though they may not be causing any
structural damage, may give rise to disturbance or distress to the people living close
to the source location. Complaints and protests may result, which can ultimately
have the potential to jeopardize the entire project.

Traffic Light Approach Acknowledging the fact that a certain degree of induced
seismicity is inevitably associated to hydraulic stimulation measures, and recog-
nizing that hydraulic stimulations should not produce levels of ground shaking
at the surface, which present a serious disturbance or threat to the local pop-
ulation, an induced seismicity warning system has been developed by Bommer
et al. (2006). It is based on pre-defined thresholds of both intensity of induced
ground movements and frequency of occurrence of any episodes of shaking, in
the style of standards relating to acceptable levels of vibrations in buildings caused
by construction-related activities. The parameter most indicative of any damage
potential has turned out to be the peak ground Velocity (PGV). Using correlations
between PGV and macroseismic intensity, and considering local attenuation rela-
tions, PGV thresholds are converted into equivalent magnitudes of seismic events
at the depth in which the induced seismicity is most likely to occur. Incorporating
also the frequency of occurrence, the microseismic events are finally classified into
three classes (Figure 4.9) with associated operational directions: Green – Continue
injection as planned; Amber – Continue injection with caution, be ready to act;
Red – stop injection immediately.