Page 214 - Geothermal Energy Systems Exploration, Development, and Utilization

190 4 Enhancing Geothermal Reservoirs

−3.5

−4

Depth (km) −4.5

−5

−5.5 Dorbarth 2006
1 0.5 0 −0.5
−1
−1.5
−2 −2.5 −1 −0.5 0 0.5 1
Northing Easting
Figure 4.8 Relocation of induced seismicity event related
to a stimulation treatment in Soultz sous Forets (Dorbath,
2006). (Please find a color version of this figure on the color
plates.)

It has to be emphasized, that the aforementioned mechanisms give a qualitative
descriptionof potentialsource mechanisms of induced seismicevents.Butalthough
a relationship between stimulation and induced seismicity appears to be obvious,
no quantifiable correlations could be identified up to now, relating pressure, flow
rate, volume of injected water, stress field and rock mechanical properties of the
reservoir, and the number and magnitude of induced events (H¨ aring, 2007).

Natural and Induced Seismicity After Brune and Thatcher (2002), the size of an
earthquake generally depends on how much slip occurs along the fault, how much
stress there is on the fault before slipping, how fast it fails, and over how large
an area the failure occurs. This indicates, that the occurrence of induced seismic
events to a great deal depends on local conditions, namely, and most importantly,
the magnitude and orientation of the local stress field and the extent and orientation
of local faults and fractures in relation to this local stress field. The actual ground
motion, on the other hand, which might be induced by a seismic event of a certain
magnitude, not only depends on the source distance, but also on the local soil
conditions.

Historical Cases and Magnitudes Cases of induced seismicity have been reported
from various EGSand conventionalgeothermalprojects:Soultz-sous-Forˆ ets,France
(magnitudes M < 2.9), the Geysers, USA (M < 4.6), Cooper Basin, Australia, (M <
3.0), Berlin, El Salvador (M < 4.4), and most recently Basel, Switzerland (M < 3.4).
It has been observed in various occasions that induced seismic events associated to
hydraulic stimulation activities often occur also after shut-in, which means, after