Noble-gas geochemistry of the M7.2 earthquake close to Lake Van (Turkey)

Terrigenic He release and changes in the He isotope ratio in response to tectonic activity are well known geochemical responses to tectonic activity. However, the very local nature of the He release from the solid earth implies that every system considered has to be addressed as a single and unique entity. Only such case-specific assessment allows to infer possible links between geochemistry and major seismic events. Our research in Lake Van (Turkey) during the last two decades sets a solid experimental basis to evaluate and interpret possible changes in the noble gas isotope composition in the water column induced by the devastating earthquake of magnitude 7.2 occurred on Oct. 23rd 2011 close to the city of Van. Lake Van is one of the largest terminal lakes and the largest soda lake on Earth. The lake basin is situated in a tectonically active region characterized by the presence of major faults and volcanoes and is known to accumulate mantle fluids. As during the last years the deep-water mixing of Lake Van is suppressed by a salinity-driven stabilization due to a lake level rise, the geochemical signature of fluids released in relation to this major earthquake is expected to accumulate and to be preserved in the water body for a certain period of time being defined by the slow but still ongoing water exchange due to turbulent mixing. In this work we present the noble-gas concentrations of water samples from Lake Van acquired before and after the earthquake. For the first time the accumulation of terrigenic He with a strong crustal He isotope signature coupled with warm fluids is observed in Lake Van. The injection of crustal He has to be considered as a sudden event that started at least in 2010. The observed anomalies can be produced only by forcing crustal fluids into the lake whereby the recent high tectonic activity is most likely the key process that triggered such a release of geogenic fluids. Our findings indicate that noble gases could be promising precursors for major seismic events in the region of Lake Van, as shown for the case of the recent magnitude 7.2 earthquake, with a potential forecast time of one year.