The paper shows that the abnormally low values of longitudinal wave velocities, typical even for dense differences, are an important feature of Holocene effusive rocks of medium-basic composition. In order to explain this phenomenon, a series of experiments was conducted to saturate the samples with a phosphor followed by a study of the structure of the pore-fractured space on a fluorescent microscope. The experiments showed that low P-wave velocities are caused by the network of the finest microcracks formed in the rock during cooling and crystallization of the magmatic melt.

This paper is concerned with the eruptions that began simultaneously in the Karymsky volcanic center, Kamchatka, in 1996 and associated phenomena. A significant earthquake swarm started occurring there on January 1, 1996 with magnitudes as high as 6.9. A monoton. summit eruption of Karymsky Volcano followed on January 2, which has continued discharging andesite-dacite lava at a rate of 0.8 t/s until March 1997 and later. Basalts were emplaced along a fissure after 28 000 years of repose producing a phreatomagmatic eruption in the Akademia Nauk caldera at pyroclastic discharge rates of over 800 t/s. The distance between the erupting vents is 6 km. Basic tnunamt. waves resulting from underwater explosions and a high tsunami were observed, a new peninsula developed in the caldera lake, the ground surface experienced an extension of more than 2,3 m, and the fresh-water caldera lake of volume 0.47 km3 was transformed into an acid one (pH 3,2). Brief information is provided on the state of the volcanic center by the late 1995, a successful prediction, and the eruptions themselves. We estimate the depth to the pressure center in the primary magma chamber (18,3 ± 0,8 km), the volume of the crustal magma chambers (400 km3), possible dimensions of the emplaced dike in the stronger crustal layers (thickness 0,7 m, length 4700 m). We discuss a likely mechanism for and relations between the observed processes and the eruptions.