Responses of a 234 U/ 238 U activity ratio in groundwater to earthquakes in the South Baikal Basin, Siberia

In the western part of the South Baikal Basin, spatial-temporal distribution of earthquake epicenters shows quasi-periodic seismic reactivation. The largest earthquakes that occurred in 1999 (MW = 6.0) and 2008 (MW = 6.3) fall within seismic intervals of 1994–2003 and 2003–2012, respectively. In the seismic interval that began in 2013, the 234U/238U activity ratio (AR) in groundwater was monitored assuming its dependence on crack opening/closing that facilitated/prevented water circulation in an active boundary fault of the basin. Transitions from disordered, high-amplitude fluctuations of AR values to consistent, low-amplitude fluctuations in different monitoring sites were found to be sensitive indicators of both small seismic events occurring directly on the observation area, and of a large remote earthquake. The hydroisotopic responses to seismic events were consistent with monitoring data on deformation and temperature variations of rocks. The hydroisotopic effects can be applied for detecting a seismically dangerous state of an active fault and prediction of a large future earthquake.