The present proposal is aimed at understanding relationships between geochemical tracers such as helium and carbon dioxide and the occurrence of seismicity along the North Anatolian Fault Zone (NAFZ) in Turkey. In collaboration with Turkish colleagues, who are monitoring major and minor element geochemistry and other isotope systems (18O, D/H and tritium), we plan to sample fluids for helium and carbon (from production wells and surface springs) from nine previously-monitored geothermal localities sited along an 800-km active segment of the NAFZ. These two geothermal tracers have great utility in their ability to distinguish crustal and mantle inputs to the volatile inventory. We want to test the hypothesis - seen in other studies of earthquake-prone regions - that changes in the balance between mantle- and crustal-derived volatiles have potential as a precursor of impeding seismic activity. The western segment of the NAFZ region experienced two catastrophic earthquakes in 1999 entailing more than 25,000 fatalities: it continues to be seismically active. This work, aimed at understanding how crustal fluids respond to seismic events, has clear societal relevance. In addition, prior work carried out through collaborative efforts with our Turkish colleagues has discovered that mantle-derived 3He occurs in fluids along the entire strike (~800 km) of the active western segment of the NAFZ. The presence of a mantle-derived fluid component indicates leakage across the ductile-brittle transition region of the seismogenic zone. The same observation has been made for the San Andreas Fault System (SAFS) in California. Indeed there are many other parallels between the NAFZ and the SAFS. Information gained in the present study, particularly on the distribution of seismicity along the NAFZ and the westward progression of epicenters (towards the metropolis of Istanbul), may be applicable to the SAFZ.
