Abstract—We discuss the water!level variations in the E!1 well for the time period between May 2006 and 2010, inclusive. A trend towards an increasing level at an abnormally high rate occurred from mid!2006 to December 2009. This increase is regarded as the response of the aquifer of gas!saturated ground water that exists in the volcanogenic–sedimentary deposits of the Avacha volcano!tectonic depression to volumetric strain changes during the precursory period and the occurrence of a swarm of small earthquakes ( = 8.3) in the area of Koryakskii Volcano and to its phreatic eruption. We estimated the volumetric compression as Δε = –(4.1 × 10–6–1.5 × 10–5) from the amplitude of water!level rise using the elastic parameters of the wa! ter!saturated rocks. While the strain source was active, we observed a decreasing sensitivity of the hydrologic regime in the well to the precursory processes before large (M ≥ 5.0) tectonic earthquakes.