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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.