Sobolev,G.A., and Tyupkin,Yu.S., 1997. Low-seismicity precursors of large earthquakes on Kamchatka, Volc. Seis., .18: 433-446.

Sobolev,G.A., and Tyupkin,Yu.S., 1999. Phases of preparation, seismological precursors and prediction of earthquakes on Kamchatka. Volc. Seis., 20: 615-627