Observation of a medium-term earthquake precursor in Irkutsk

Complex geophysical investigations of the earth dam of the Irkutsk hydroelectric power plant (HEP) revealed strong negative anomalies of the natural electric field (NEF). These anomalies are associated with focal mechanisms of earthquakes, in which subhorizontal compressive stresses prevail. The sources of such earthquakes are located in the south of the Baikal rift zone. Their epicentral distances are comparable with the range of the action of precursors of earthquakes with M > 5. NEF anomalies correlate with an increase in the filtration at the earth’s dam base in the Angara fault zone along the Angara River bed. In the shape of their plots, manifestations of these NEF anomalies are similar to variations in the ratios of velocities of longitudinal and transverse waves before earthquakes in the Garm region.