Vol 13, 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 (М_w = 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 of deformation in water-saturated rocks. This can be useful in geophysical monitoring and intermediate-term prediction of strong earthquakes for the Kamchatka region.

The common geochemical features of Kamchatka near-surface ashes (ashes that underlie surface organogenic soil horizons) include their depletion in most chemical elements compared with the average figures for volcanic rocks of the same composition and their relative enrichment in a narrow range of elements, mostly those which are typomorphic for basic rocks (Р, Mg, V, Cr, Cu, Zn, Zr, and Sb). Viewed upon the background of small variations in the concentrations of chemical elements in ashes from different areas in the peninsula, the range of their dominant elements is controlled by their petrochemical composition. The vigorous activity of Kamchatka’s Northern Group of Volcanoes has produced relatively higher concentrations of mobile forms of elements in the near-surface ashes of the Northern Province compared with the ashes found in the southern part of the peninsula.

This paper discusses the tectonic setting, seismological characteristics, and the aftershock process for the great (M_W = 7.8) Near-Islands Aleutian earthquake of July 17, 2017 off the Commander Islands. Our analysis showed that the rupture zone as inferred from the distribution of aftershock epicenters in the shape of an elongate narrow zone approximately 400 km long completely covered the north slope of the Commander Islands uplift and was confined to the Bering Fault. The rupture zone involved this entire earthquake-generating area as far as a transverse structure west of the Near Islands group (Attu Island). The fault-plane solutions and the inferred displacements in the rupture zones of the main shock and of the largest foreshocks and aftershocks revealed the rupture to be a practically pure right lateral movement. The aftershock process of the July 17 earthquake was occurring at rather a sluggish pace for such a large earthquake. In addition, the process involved two other features to distinguish it from the aftershock processes of most Kuril–Kamchatka earthquakes: (1) a low release of the cumulative scalar seismic moment (M_{0cum aft}), which amounted to between 0.75% and 1.0% of the mainshock seismic moment (M_0me), according to several estimates; and (2) a very slow increase in the deficit of seismic moment (M₀) release. At the same time, the duration of the quasi-stationary phase in the release of M_0cum by aftershocks, which is estimated to have lasted approximately half a year and which occupied much of the entire duration of the aftershock process for this earthquake seems to be unusually long. These features in the aftershock process of the Near-Islands Aleutian earthquake of July 17, 2017 distinguish the process from those common to most large Kuril–Kamchatka earthquakes. Overall, the rupture zone can be regarded as a transform feature between two Benioff zones (The Aleutian and the Kuril–Kamchatka ones) rather than as a subduction event, as is typical of the two latter arcs.