Induced Seismicity in the Krafla High Temperature Field

Abstract

A local network of six seismic stations has been operated in shallow boreholes in the Krafla high temperature area in Iceland since the autumn of 2006. This paper reports the results of the recordings in 2009 and 2010. Considerable micro-seismicity is observed; most of it seems to be either associated with re-injection of geothermal fluid in the reservoir or circulation losses during drilling. The depth distribution of the earthquakes shows that 95% of all quakes originate above 2.7 km depth, indicating that the brittle ductile boundary is at that depth. This is further supported by recent drilling of two wells that entered silicic rich magma just below 2 km depth. Waves from distant earthquakes show a zone of high S-wave attenuation below parts of the geothermal field, confirming earlier results of an underlying magma chamber.