195 6. The use of passive seismic methods for Geothermal exploration and monitoring Hence, a seismological array can be set-up in the exploration area for a given period to try and record multiple seismic or microseismic events for which specific information such as location, magnitude and focal mechanisms can be inferred and compiled into a catalog. Such an approach will typically provide information about active fault zone geometry, location of fractured zones that facilitate hydrothermal fluid flow (Simiyu, 2009; Faulds and Hinz, 2015). Figure 6.5 shows an example of microseismic analysis carried out at the Menengai geothermal prospect in Kenya (Simiyu, 2009). The location of microseismic event epicenters underlines the existence of two different trends that could be associated with fault zones, which intersect at the Menengai crater. Such observation suggests that these faults are likely still active, and at their intersection magma and thermalfluid flow are occurring. Figure 6.5 Map indicating a passive seismic network (blue triangles) and the location of microseismic events epicenters (red dots) in the area of the Menengai Crater. Modified after Simiyu (2009). Another potential insight from seismic catalogs relates to the detection and location of zones where the brittle/ductile boundary is characterized by a relatively high elevation. Close to the surface, the rocks of the crust tend to have a brittle behavior which can relate to significant seismic activity. But progressing in depth, the increase of temperature induces a transition from brittle to ductile behavior (e.g. Tanaka, 2004; Suzuki et al., 2014), which leads to a drastic decrease in seismic events activity and occurrence. Figure 6.6 shows an illustration of such an observation
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