The results of the cruise of R/V Akademik M.A. Lavrentiev conducted by the Pacific Institute of Oceanology, Far East Division of the Russian Academy of Sciences and the Shirshov Institute of Oceanology, Russian Academy of Sciences in August to September 2005 are considered. The aim of the works was to specify the tectonic structure, seismogenic potential, and tsunamigenic hazard of the central segment of the Kuril Island Arc. The complex studies involved single-channel seismic profiling, gravimetry, magnetometry, detailed bathymetry, dredge sampling of sea-floor rocks and sediments, and gas geochemistry. Geophysical and geological data are reported. It was demonstrated that the target area is an active tectonic destruction zone, the zone boundaries were outlined, and the main internal structural and compositional heterogeneities were identified.

The paper presents data obtained during the 69th and 72nd expeditions of the research vessel Akademik Mstislav Keldysh(2017, 2018). A mechanism of methane discharge that explains the...

Original results of igneous rock studies are presented. The rocks were dredged during a marine expedition (cruise 37 of R/V Akademik M.A. Lavrent’ev in August–September, 2005) in the region of the submarine Vityaz Ridge and Kuril Arc outer slope. Several age complexes (Late Cretaceous, Eocene, Late Oligocene, Miocene, and Pliocene-Pleistocene) are recognizable on the Vityaz Ridge. These complexes are characterized by a number of common geochemical features since all of them represent the formations of island arc calc-alkali series. At the same time, they also have individual features reflecting different geodynamic settings. The outer slope of the Kuril Arc demonstrates submarine volcanism. The Pliocene-Pleistocene volcanic rocks dredged here are similar to the volcanites of the Kuril-Kamchatka Arc frontal zone.

Because of the probability of the occurrence of a strong earthquake in the central part of the Kuril‐Kamchatka Arc, which corresponds to the seismic gap (SG) region [1], it is necessary to take into account the possibility of tsunami wave generation and the risk of its occurrence not only at the Kuril Islands but also on the coast of the Sea of Okhotsk. The necessity of the prediction of such event became especially urgent after the catastrophic tsunami on the coasts of several countries in Southeast Asia caused by the strong earthquake on December 26, 2004, whose epicenter was located near the northwestern edge of Sumatra Island (Indonesia). The latter event occurred in the SG region in the Sunda subduction zone after an almost 200-yr-long silence. The source of this earthquake appeared unusually long (more than 1300 km) and very nonuniform. Taking into account the strong indentation and block structure of the near-island slope of the trench, the above fact indicates the keyboard mechanism of the displacement of the ocean bottom [2, 3]. In this process, the initial perturbation in the source is transferred horizontally to the neighboring blocks unevenly with jumps and stops [4]. Coastal services in the countries of Southeast Asia were unprepared for the arrival of a tsunami. These countries lack systems of tsunami warning and notification of the population about the approaching hazard. Therefore, the number of casualties and material losses from the breaking wave were very high. In principle, if timely information about the localization of the tsunami source is provided, modern computer capabilities will allow us to perform real-time numerical modeling of the possible spreading of the tsunami wave, the expected arrival time of the wave at the given coast, the altitude of its runup, and so on. However, the calculation results generally depend on the source model used in the calculations. Usually, a monoblock is chosen, the uplift of which at the bottom triggers the perturbation of the ocean surface (see, for example, [5]). Application of the keyboard model [2, 3] allows us to consider various versions of the motion of block-keys at the ocean bottom and determine the optimal version in terms of correspondence to the observed data. In numerical modeling of the characteristics of potential tsunami related to the occurrence of a strong earthquake in the SG region in the Middle Kuril Islands, it is reasonable to start from the structure of the possible source, the geometry of which was distinguished during the Kurils-2005 marine hydrophysical expeditions (cruise 37 of the R/V Akademik Lavrent’ev in summer 2005) [6]. In this paper, we present the results of numerical modeling of the propagation and rolling of a tsunami wave on the coast. The generation of a tsunami wave by this potential source in the SG zone was based on the keyboard subduction model. A nonlinear system of equations for shallow water was used in the numerical modeling for the description of the generation and propagation of the tsunami wave (see, for example, [7]). The calculations took into consideration the Coriolis force and bottom friction. The modeling yielded wave fields of displacement and velocity fields for the entire basin of the Sea of Okhotsk along all coasts of the continent and islands. A calculation area in the square 40 ° ‐60 ° N and 140 ° ‐160 ° E with a grid of 1201 × 1201 points was chosen for modeling.

Here we present the first radiometric age and geochemical (major and trace element and isotope) data for samples from the Hydrographer Ridge, a back arc volcano of the Kurile Island Arc, and a newly discovered chain of volcanoes (“Sonne Volcanoes”) on the northwestern continental slope of the Kurile Basin on the opposite side of the arc. The ⁴⁰Ar/³⁹Ar age and geochemical data show that Hydrographer Ridge (3.2–3.3 Ma) and the “Sonne Volcanoes” (25.3–25.9 Ma) have very similar trace element and isotope characteristics to those of the Kurile Island Arc, indicating derivation from a common magma source. We conclude that the age of the “Sonne Volcanoes” marks the time of opening of the Kurile Basin, implying slow back arc spreading rates of 1.3–1.8 cm/y. Combined with published data from the Kurile fore arc, our data suggest that the processes of subduction, Kurile Basin opening and frontal arc extension occurred synchronously and that extension in the rear part and in the frontal part of the Kurile Island Arc must have been triggered by the same mechanism.