The structure of the upper crust under the Kambalny volcano (Southern Kamchatka) according to the results of seismic tomography

In this study we present the results of tomography studies for seismic velocity in the area of Kambalny volcano (Southern Kamchatka). After a long repose stage, on March 24, 2017, it produced a strong phreatic eruption, which ejected an ash cloud to the distance of up to 1000 km. We have obtained the first 3D model of seismic velocities beneath the area of Kambalny based on the data recorded by a temporal network of ten seismic stations installed for one year in 2018-2019. The distributions of velocities of the P and S seismic waves, and especially the Vp/Vs ratio, provide the information on the geometry of the plumbing system beneath the volcano in the upper crust down to ~10 km, which makes it possible to build a scenario of preparation and occurrence of the explosive eruption in 2017. We clearly identify an anomaly of high Vp/Vs ratio in the depth interval of 7-10 km, which is interpreted as a magma reservoir responsible for Holocene activity of Kambalny. This reservoir appears to be connected with the volcano edifice by a linear zone of high Vp/Vs ratio, which may represent a system of fractures originated during the eruption in March 2017 and served as a pathway for magma ascent. We propose that the interaction of hot magma with meteoric fluids in shallow layers caused active boiling and steam formation in a closed reservoir below the volcano. After exceeding a critical pressure, the steam escaped to the surface causing an explosive eruption. We also found evidence that geothermal fields located to the north and northwest of Kambalny might be fed from separate deep sources. The area of Kambalny is characterized by strong geothermal activity, most of which is located to the north and to the west of the volcano. The northern geothermal manifestations mostly occur on the northern part of the Kambalny Ridge and in the Pauzhetka depression. We found that the geothermal activity in these areas is likely associated with a deep source, which appears to be isolated from the magma reservoir below Kambalny volcano. A similar isolated anomaly is observed below geothermal fields in the area of the Koshelev volcano to the west, which may indicate that the geothermal activity appears to be independent of the magmatic system of Kambalny volcano, at least for its upper-crustal part.

This study was partially supported by the Russian Science Foundation project # 20-17-00075.