No 2 (2019)

This paper is concerned with the main stages in the setting-up and technical development of a system specializing in physical and chemical parameters of groundwater at a network of wells and springs in the Petropavlovsk Geodynamic Test Area, Kamchatka. The focus is on a description of hydrogeochemical and hydrogeodynamic precursors to Kamchatka earthquakes (Мw = 6.6‒7.8) that occur a few weeks to a few months before a seismic event, manifesting themselves in anomalous changes in chemical composition and groundwater level. The precursors are discussed in application to their use at specialized councils on earthquake prediction. It is shown that the system of automated observation of groundwater parameters at wells as developed at the Kamchatka Branch of the Geophysical Survey of the Russian Academy of Sciences (KB GS RAS) is capable of identifying hydrogeodynamic precursors of water-level in near real time and of providing, in some particular cases, quantitative estimates of pre-seismic and coseismic deformation of water-saturated rocks. This can be useful in geophysical monitoring and intermediate-term prediction of strong earthquakes for the Kamchatka region.

Common geochemical features of volcanic near-surface ashes of Kamchatka (ashes located under the surface organogenic soil horizons) are their depletion by most chemical elements in comparison with average indicators for volcanic rocks of corresponding composition and relative enrichment of a limited range of elements, mainly typomorphic for main rocks (P, Mg V, Cr, Cu, Zn, Zr, Sb). Against the background of small variations in the contents of chemical elements in the ashes of different areas of the peninsula, the range of their priority elements is determined by their petrochemical composition. The active activity of the Northern group of volcanoes of Kamchatka causes a relatively high content of mobile forms of elements in the near-surface ashes of the Northern province in comparison with the ashes of the southern part of the peninsula.

The tectonic position, seismological characteristics and features of the aftershock process of the source of the strongest Near-Aleutian earthquake on July 17, 2017 on the Commander Islands with М_w = 7.8 are considered. The analysis showed that the seismic source according to the distribution of aftershock epicenters in the form of a linearly elongated narrow zone with a length of about 400 km almost completely occupied the northern slope of the Commander island elevation and was located in the Bering fault zone. It covered the whole of this seismic-generating zone up to the transverse structure to the west of the Near Islands (Attu is.). In accordance with the focal mechanisms solution and the nature of the displacements in the foci of the main shock, the strongest foreshocks and aftershocks, the shift in the source was an almost pure right-sided shift. The aftershock process of the July 17 earthquake developed quite enough inertly for an earthquake of such strength. In addition, it has two features in comparison with the aftershock processes of most of the Kuril-Kamchatka earthquakes: 1) low release of the cumulative scalar seismic moment (M_{0cum aft}), which according to various estimates was from 0.75% to 1.0% of the seismic moment of the main shock (M_0me); 2) a very slow increase in the deficit in the release of the seismic moment (M₀). At the same time, the duration of the quasi-stationary phase of M0cum release in aftershocks, estimated at about ½ year and covering a significant part of the duration of the entire aftershock process of this earthquake, seems unusually long. These features of the aftershock process of the Middle Aleutian earthquake on July 17, 2017 distinguish it from the aftershock processes characteristic of most strong Kuril-Kamchatka earthquakes. In general, its source can be considered as a transform between the two Benioff zones – Aleutian and Kuril-Kamchatka, and not subduction, that is characterise the last two.