The study focuses on determining the position of the southern boundary of the Okhotsk Plate based on the analysis of seismicity distribution in the Hokkaido and Honshu regions as well as adjacent territories according to the Japan Meteorological Agency (JMA) data for a period of 1998-2022. The seismicity distribution data are compared with regional seismic tomography models and the distributions of the directions of principal seismotectonic deformation axes according to data on the focal mechanisms of strong (M-w > 4.7) earthquakes using the International Seismological Center (ISC) data for a period of 1976-2022 and other recent geological-geophysical characteristics, such as gravity field heterogeneities, crustal thickness, volcanic manifestations, etc. It is revealed that the southern boundary of the Okhotsk Plate actually passes along the southern tip of Hokkaido Island (through the Oshima Peninsula and Uchiura Bay) rather than along the Hidaka Ridge or through Honshu Island, as previously assumed by other authors.

Различные подходы к тектоническому районированию и разные типы (комплексы) используемых данных, на базе которых районирование производится, являют множественность прежде полученных различными авторами тектонических схем Камчатки и Камчатского региона. В данной работе для изучения пространственных связей тектонических проявлений геологических процессов использовался совместный анализ аномалий гравитационного поля, трансформант гравитационного поля и рельефа, магнитного поля и современных сейсмотомографических моделей аномалий скоростей. В результате проведенного исследования была построена тектоническая схема линейно-блокового типа. Полученные линейные структурные элементы хорошо согласуются с данными потенциальных полей, сейсмическими построениями и ранее опубликованными материалами. Показана продуктивность приведенного способа комплексного анализа геофизических полей для выявления их пространственных связей и блоковых структур среды, которые могут послужить основой для конструирования пространственных ограничений для источников гравитационного поля при решении обратных задач, задач разделения потенциальных полей и их углубленной геологической интерпретации. Different approaches to tectonic zoning and different types (complexes) of used data, on the basis of which the zoning is performed, cause the plurality of tectonic schemes of Kamchatka and the Kamchatka region previously obtained by different authors. In this work, a joint analysis of gravity field anomalies, gravity field and relief transforms, magnetic field and modern seismotomographic models of velocity anomalies was used to study spatial relationships of tectonic manifestations of geological processes. As a result of this study, a linear-block type tectonic scheme was constructed. The obtained linear structural elements are in good agreement with the data of potential fields, results and previously published materials. The productivity of the presented method of complex analysis of geophysical fields for revealing their spatial relations and block structures of the environment is shown. The analysis outcomes can be used to establish spatial constraints for gravity field sources when solving inverse problems, potential field separation problems and their detailed geological interpretation.

approaches to tectonic zoning and different types (complexes) of used data, on the basis of which the zoning is performed, are the plurality of tectonic schemes of Kamchatka and the Kamchatka region previously obtained by different authors. In this work, a joint analysis of gravity field anomalies, gravity field and relief transforms, magnetic field and modern seismotomographic models of velocity anomalies was used to study spatial relationships of tectonic manifestations of geological processes. As a result of this study, a linear -block type tectonic scheme was constructed. The obtained linear structural elements are in good agreement with the data of potential fields, seismic constructions and previously published materials. The productivity of the presented method of complex analysis of geophysical fields for revealing their spatial relations and block structures of the environment is shown. The analysis outcomes can be used to establish spatial constraints for gravity field sources when solving inverse problems, potential field separation problems and their detailed geological interpretation.

Проведен анализ проявления магматизма в зоне перехода от континентальной окраины Камчатки к Курильской островной дуге и геофизических параметров погружающейся под них океанической плиты Северо-Западной Пацифики. Наличие прибрежного вулканического комплекса миоценового возраста в основании вулканического пояса Южной Камчатки способствует интенсивным коровым процессам, которые, вероятно, привели к формированию кальдерообразующих извержений в голоцене. Для Северных Курил характерно формирование ареального вулканизма, приуроченного к разрывным нарушениям земной коры. Аномальным является отсутствие вулканизма на о. Шумшу, расположенном непосредственно вблизи Камчатки. Отсутствие вулканизма на этом острове связывается с высокоскоростной аномалией, фиксируемой по данным сейсмотомографии. На основе представленных данных выдвигается предположение о наличии океанического поднятия на слэбе, погружение которого привело к дезинтеграции зон плавления, отвечающих за формирование вулканизма. Magmatism manifestations in the transition zone from the continental margin of Kamchatka to the Kurile island arc and some geophysical parameters of the subducted oceanic plate of the Northwestern Pacific are considered. The presence of the Miocene coastal volcanic (Pribrezhny) complex at the base of the South Kamchatka volcanic belt contributes to intense crustal processes causing caldera-forming eruptions in the Holocene. The Northern Kuriles are characterized by initiation of areal volcanism associated with crustal fault zones. Anomalous is the absence of volcanism on Shumshu island proximal to Kamchatka. Seismic tomography data reveal a high seismic velocity anomaly below it, which can explain this phenomenon. Based on the presented data an assumption is made about existence of a seafloor elevation on the slab, whose submergence led to disintegration of the melting regions responsible for generation of volcanism.

Avacha and Koryaksky are active volcanoes located in the vicinity of Petropavlovsk-Kamchatsky, the main city of Kamchatka, and they represent a serious hazard to the surrounding population and infrastructure. Here we investigate the upper-crustal structure beneath these volcanoes by implementing local earthquake tomography based on data from permanent seismic stations and from a temporary network installed in 2018–2019. Although the total amount of seismic rays recorded by the temporary network dataset is much lower, adding this data and attributing an appropriate weight lead to considerable increase of the resolution compared to the case of using merely the permanent network data. The resulting distributions of the P and S wave velocities, and especially the Vp/Vs ratio, reveal two magma reservoirs located below Avacha and Koryaksky volcanoes. Different depths of their upper limits explain differences in the eruption activity styles of these volcanoes. Below the Avacha Pass, we observe a thick layer of low-velocity soft volcaniclastic sediments formed due to the activity of both volcanoes. A conservative numerical estimate of a steady-state conductive temperature field around the magma source below Avacha demonstrates that the high temperature zone can be reached by drilling a well to a reasonable depth, thus suggesting that the area might be exploited as a source of geothermal energy.

The area of Central Kamchatka limited by latitudes of 52.5 and 54 degrees includes six active volcanoes (Avacha, Koryaksky, Zhupanovsky, Mutnovsky, Gorely and Opala), as well as a number of dormant and extinct strato-volcanoes, monogenic cones and large calderas. Furthermore, it contains the Malko-Petropavlovsk fracture zone (MPZ), which marks the boundary between two distinct subduction regimes to the south and to the north. We present a new seismic tomography model for this area, which was constructed based on the joint use of data of the Kamchatkan permanent seismic stations and a temporary network installed in the region in 2019-2020. A series of synthetic tests have demonstrated fair resolution of the derived seismic velocity structures in the crust and in the mantle wedge down to-150 km. The distributions of the P and S wave velocities, and especially the Vp/Vs ratio, clearly highlight the connection between the volcanic centers in Central Kamchatka and the sub -ducting slab. At depths below 40 km depth, we observe two large low-velocity anomalies centered below Zhupanovsky and Mutnovsky volcanoes and covering all other volcanoes in the area. In the vertical sections, the corresponding anomalies of high Vp/Vs ratio have mushroom shapes with the heads spreading along the bottom of the crust, which probably represent the underplating of magma material that feeds the volcanoes of the groups. The tomography results also reveal some important tectonic features, such as a V-shaped fault system in the Avacha Graben, which is the part of the MPZ.

Сейсмотектонические деформации в районе Японских островов1 Институт нефтегазовой геологии и геофизики им.А. А. Трофимука СО РАН,

This study continues analysis of the new seismic tomographic structure of the suprasubduction complex of the central zone of Kamchatka, obtained from the dense local networks data of 2018-2020, and is devoted to the analysis of the velocity structure in the Malko-Petropavlovsk fracture zone margins and around them. The seismic tomographic model involves about 98,000 P- and S-wave travel times from 2963 local earthquakes from August 2018 to July 2020. The resolution of this model makes it possible to trace the feeding systems of volcanoes of the South Kamchatka and East Volcanic Belt to the slab surface, as well as to identify subvertical structural faults. To construct the orientations of the compression and extension axes we used the foci mechanisms of 41 earthquakes with M >= 4.5 from the catalog of the International Seismological Center for the period 1979-2019. Along the Malko-Petropavlovsk fracture zone, the Avacha transform fault is clearly traced in the geometry and mutual arrangement of velocity anomalies almost throughout the entire depth of the model. Comparison of seismic anomalies with a map of the directions of the compression and extension axes distribution from the earthquake foci mechanisms showed the correlation between the change in the value of the velocity anomalies along the Avacha transform fault with the axes direction change by almost 180 degrees. A near-surface low-velocity anomaly to the depths of 25-35 km was found along the western border of the Malko-Petropavlovsk zone under the southern tip of the Sredinny Ridge. This anomaly probably marks the axes junction zone boundary of the ancient volcanic front along the Sredinny Ridge and the modern active Eastern Volcanic Belt, which formed as a result of the Kronotsky paleoarc accretion. To the west from the Sredinny Ridge southern tip, another low-velocity anomaly was revealed. This anomaly was traced to a depth of similar to 150 km, has a contrasting southern boundary confirmed by the distribution of the compression and extension axes directions by the earthquake foci mechanisms and apparently marks the southern boundary of the West Kamchatka block.

Magmatism manifestations in the transition zone from the continental margin of Kamchatka to the Kurile island arc and some geophysical parameters of the subducted oceanic plate of the Northwestern Pacific are considered. The presence of the Miocene coastal volcanic (Pribrezhny) complex at the base of the South Kamchatka volcanic belt contributes to intense crustal processes causing caldera-forming eruptions in the Holocene. The Northern Kuriles are characterized by initiation of areal volcanism associated with crustal fault zones. Anomalous is the absence of volcanism on Shumshu Island proximal to Kamchatka. Seismic tomography data reveal a high seismic velocity anomaly below it, which can explain this phenomenon. Based on the presented data an assumption is made about existence of a seafloor elevation on the slab, whose submergence led to disintegration of the melting regions responsible for generation of volcanism.

Настоящее исследование продолжает работы по новой сейсмотомографической структуре надсубдукционного комплекса центральной зоны Камчатки, построенной по данным плотных локальных сетей станций 2018-2020 гг., и посвящено анализу скоростной структуры в пограничных областях и окружении Малко-Петропавловской разломной зоны. В сейсмотомографической модели задействовано около 98 тыс. времен пробега P- и S -волн от 2963 локальных землетрясений за период времени с августа 2018 по июль 2020 г. Разрешающая способность этой модели дает возможность проследить системы питания вулканов Восточного вулканического пояса и Южной Камчатки до поверхности слэба, а также позволяет выявить субвертикальные структурные нарушения. Для построения ориентаций осей сжатия и растяжения использованы механизмы очагов 41 землетрясения с М ≥ 4.5 из каталога Международного сейсмологического центра за период 1979-2019 гг. Вдоль Малко-Петропавловской зоны поперечных дислокаций практически на всю глубину модели в геометрии и взаимном расположении скоростных аномалий отчетливо прослеживается Авачинский трансформный разлом. При сопоставлении сейсмических аномалий с картой распределения направленностей осей сжатия и растяжения по механизмам очагов землетрясений наблюдается явная корреляция смены знака аномалий скорости вдоль Авачинского трансформного разлома с изменением направленностей осей практически на 180°. Вдоль западной границы Малко-Петропавловской зоны под южной оконечностью Срединного хребта обнаружена близповерхностная низкоскоростная аномалия, прослеживающаяся до глубин 25-35 км. Эта аномалия, вероятно, маркирует границу зоны сочленения осей древнего вулканического фронта по Срединному хребту и современного активного Восточного вулканического пояса, образовавшегося в результате аккреции Кроноцкой палеодуги. Западнее южной оконечности Срединного хребта выявлена еще одна низкоскоростная аномалия, прослеживающаяся до глубины ~150 км и имеющая контрастную южную границу, которая подтверждается распределением направленностей осей сжатия и растяжения по механизмам очагов землетрясений и, по всей видимости, маркирует южную границу Западно-Камчатского блока. This study continues analysis of the new seismic tomographic structure of the suprasubduction complex of the central zone of Kamchatka, obtained from the dense local networks data of 2018-2020, and is devoted to the analysis of the velocity structure in the Malko-Petropavlovsk fracture zone margins and around them. The seismic tomographic model involves about 98,000 P - and S -wave travel times from 2963 local earthquakes from August 2018 to July 2020. The resolution of this model makes it possible to trace the feeding systems of volcanoes of the South Kamchatka and East Volcanic Belt to the slab surface, as well as to identify subvertical structural faults. To construct the orientations of the compression and extension axes we used the foci mechanisms of 41 earthquakes with М ≥ 4.5 from the catalog of the International Seismological Center for the period 1979-2019. Along the Malko-Petropavlovsk fracture zone, the Avacha transform fault is clearly traced in the geometry and mutual arrangement of velocity anomalies almost throughout the entire depth of the model. Comparison of seismic anomalies with a map of the directions of the compression and extension axes distribution from the earthquake foci mechanisms showed the correlation between the change in the value of the velocity anomalies along the Avacha transform fault with the axes direction change by almost 180°. A near-surface low-velocity anomaly to the depths of 25-35 km was found along the western border of the Malko-Petropavlovsk zone under the southern tip of the Sredinny Ridge. This anomaly probably marks the axes junction zone boundary of the ancient volcanic front along the Sredinny Ridge and the modern active Eastern Volcanic Belt, which formed as a result of the Kronotsky paleoarc accretion. To the west from the Sredinny Ridge southern tip, another low-velocity anomaly was revealed. This anomaly was traced to a depth of ~150 km, has a contrasting southern boundary confirmed by the distribution of the compression and extension axes directions by the earthquake foci mechanisms and apparently marks the southern boundary of the West Kamchatka block.

Deformation features of the subducting Pacific lithospheric plate are considered according to the data on earthquake focal mechanisms. The territory includes the convergent boundaries between the Pacific Plate and the North American (in the Aleutian arc region), the Okhotsk, the Eurasian and the Philippine plates.It has been shown that the angle of subducting Pacific Plate in the Aleutian subduction zone affects the focal mechanisms of earthquakes that occurred in the upper, 35 km part of the oceanic plate in the zone of its bending. There occur normal-fault earthquakes at a steep-angle subduction and rare thrust earthquakes at a shallow-angle subduction. The azimuthal orientation of P-axes of the focal mechanism solutions in the upper (1–70 km) contact zone corresponds to the Pacific Plate displacement vector when the plate fragments are subducting west-northwestwards. There occurs a change in azimuthal orientation of the compression axes in the subducting plate at a depth of more than 70 km: the axes occupy different azimuthal sectors showing difference in the orientation of their slope, with the orientations of the T-axes become multidirectional.The calculation of seismotectonic deformations was carried out based on the data on focal mechanisms of 7768 earthquakes. It was revealed that the Exx and Ezz deformation fields are the most homogeneous at depths of 1–70 km. The pattern of seismotectonic deformations changes abruptly for deep parts of the subducting plate (105–200, 200–400, and 400–700 km), there are observed heterogeneous deformation fields Exx, Eyy and Еzz with alternating episodes of extension and shortening.There has been proposed the author’s scheme of the influence of the upper mantle convection structure on the geometry of the subducting plate (slab) as a potential catalyst for the processes responsible for the separation of seismic activity zones and the change of earthquake types with depth and in different parts of the extended subduction zone.

The strongest earthquakes and the largest explosive eruptions are confined to plate convergent boundaries. Many geodynamics aspects attract the scientific community's attention since answers to the most important questions cannot be obtained without reliable information about the deep structure. Geophysical studies of the crust and mantle provide essential information for lithospheric blocks interactions, mantle convection and fluid migration. This data is necessary to identify reliable criteria for assessing volcanic and seismic risk. The studied area is central Kamchatka, where the cities of Petropavlovsk-Kamchatsky, Elizovo, and Vilyuchinks are located. It includes territory from the Gorely and Mutnovsky volcanoes in the south to the Bakening volcano and the Verkhneavachinskaya caldera in the north. It extends from the eastern to the western peninsula coasts. The study area includes the Avachinskaya group of volcanoes, the Vilyuchinsky and Zhupanovsky volcanoes, Karymshina caldera and a number of monogenic cinder cones. This region is assumed to be located at a transition between two principle different subduction regimes in the north and south of Kamchatka. Previous studies are sparse and have poor resolution due to the low density and uneven distribution of seismic stations. In this study, we used a large dataset recorded by a new dense temporary network deployed in 2019-2020, which was specially designed for performing high-quality seismic tomographic studies of the suprasubduction complex structure (crust and upper mantle) beneath central Kamchatka. This dataset was supplemented by data recorded by (1) the temporary network operated on the Avachinskaya group of volcanoes in 2018-2019 and (2) the permanent stations Kamchatka branch of the Federal Research Center of the GS RAS. The seismic model is based on the data from 2687 local earthquakes that occurred during the operation of the mentioned temporary networks and were recorded by 134 regional stationary and temporary stations. In the tomographic inversion we used 59088 travel times of P-waves and 34697 of S-waves. The new model makes it possible to trace zones of fluid and melt release from the slab, their migration in the mantle wedge and crust, and allows assessing their role in feeding the magmatic systems. Volcanoes of the Avachinskaya group have a common magma plumbing system at a depth more than 50 km, which could be traced from the slab. The Vilyuchinsky volcano feds through an intermediate large magma chamber located at a depth of 30-55 km, which is also related to the feeding of the Bolshebannaya hydrothermal system situated to the west. This large chamber fed from a conduit originated on the slab at more than 70 km depth. The feeding system of the Gorely and Mutnovsky volcanoes is traced to the slab at depths of more than 100 km. This work was supported by the Russian Science Foundation (project No. 22-27-00215) and the Ministry of Education and Science of the Russian Federation (megagrant No. 14.W03.31.0033).

The seismotectonic deformations were determined in the Pacific and Okhotsk (Eurasian) lithospheric plates subduction zone based on 2458 mechanisms of earthquake foci data for 1977-2019. The deformation features of medium in different deep layers are shown. The deformation field uniformity in the depth range of 1-70 km and the deformation field inhomogeneity for the submerged plate deep parts (105-200, 200-400 and 400-700 km) are revealed. One of the deformations field change reasons is the influence of ascending and, especially, descending currents of thermogravitational convection in the upper mantle sublithospheric part, in particular, the convection structure influence on the subducting plate geometry

Strong explosive eruptions of volcanoes throw out mixtures of gases and ash from high-pressure underground reservoirs. Investigating these subsurface reservoirs may help to forecast and characterize an eruption. In this study, we compare seismic tomography results with remote sensing and petrology data to identify deep and subaerial manifestations of pre-eruptive processes at Bezymianny volcano in Kamchatka shortly before its violent explosion on December 20, 2017. Based on camera networks we identify precursory rockfalls, and based on satellite radar data we find pre-eruptive summit inflation. Our seismic network recorded the P and S wave data from over 500 local earthquakes used to invert for a 3D seismic velocity distribution beneath Bezymianny illuminating its eruptive state days before the eruption. The derived tomography model, in conjunction with the presence of the high-temperature-stable SiO 2 polymorph Tridymite in juvenile rock samples , allowed us to infer the coexistence of magma and gas reservoirs revealed as anomalies of low (1.5) and high (2.0) Vp/Vs ratios, respectively, located at depths of 2–3 km and only 2 km apart. The reservoirs both control the current eruptive activity: while the magma reservoir is responsible for episodic dome growth and lava flow emplacements, the spatially separated gas reservoir may control short but powerful explosive eruptions of Bezymianny.

In modern concepts, the upper mantle of the Earth is a highly viscous incompressible liquid, and its flow is described using the Navier – Stokes equations in the Oberbeck – Boussinesq and geodynamic approximations. Convective flows in the upper mantle play a decisive role in the kinematics of lithospheric plates and the geological history of continental regions. Mathematical modeling is a basic method for studying convective processes in the mantle. Our paper presents a numerical model of convection, which is based on the implicit artificial compressibility method. This model is tested in detail by comparing our calculation results with the results of a well-known international test. It is demonstrated that the Fedorenko grids sequence method is highly efficient and reduces the computing time almost by a factor of eight. The numerical model is generalized in order to state the problem in a spherical system of coordinates. It is used to analyse the distribution of convective flows in the upper mantle underneath the Eurasian continent. The analysis shows that the thickness and geometrical parameters of the lithospheric blocks are the factors of significant influence on the distribution of convective flows in the upper mantle. The resulting structure of convective flows is manifested in the surface topography of large platform areas wherein the lithosphere thickness is increased. Thus, the locations of extended downward convection flows under the East European and Siberian platforms are clearly comparable to syneclises observed in the study area.

The major part of the Northern group of volcanoes (NGV) in Kamchatka is occupied by the Klyuchevskoy group, which is a unique cluster of more than thirteen volcanos having exceptionally diverse eruption styles and compositions. The NGV also includes Shiveluch volcano to the north and Kizimen volcano to the south, both andesitic strongly explosive volcanoes. The crustal structure beneath the Klyuchevskoy group was previously explored using data of the permanent stations and several temporary networks; however, for studying the mantle structures, no high-quality data was available. To close this gap, a temporary seismic KISS network was installed throughout the NGV by an international consortium from August 2015 to July 2016. Together with 22 permanent stations, it included more than 100 simultaneously operating seismic stations. Based on the KISS data, we manually picked more than 43,000 arrival times of the P and S waves from 665 events (65 picks per event on average). Furthermore, this dataset was supplemented with the arrival times from the slab-related seismicity recorded by permanent stations during long-term observations. Several resolution tests have demonstrated that this dataset allows very high quality recoveries of the anomaly both laterally and in the vertical direction. The distributions of seismic anomalies in the uppermost mantle (50 km depth) show clear connection with the composition of the volcanoes. All the andesitic volcanoes (Kizimen, Udina, Zimina, Bezymyanny, Zarechny, Kharchenko and Shiveluch) are located above prominent low-velocity anomalies, whereas the basaltic volcanoes (Nikolka, Tolbachinsky Dol, Ostry and Plosky Tolbachik, Ushkovsky and numerous monogenic cones) are mostly associated with higher velocities in the mantle. This correlation might be explained by the effect of the mantle temperature to the rheological properties of the crust. Over the hot mantle, the crust becomes ductile, and it favors for forming intermediate crustal reservoirs, where magma is accumulated and separated for long time making it more felsic. Above the colder mantle, the crust is brittle and may be fractured by ascending mafic intrusions. In this case, mantle material quickly penetrates through the crust and reaches the surface producing fissure basaltic eruptions and shield volcanoes. Another important conclusion follows from the interpretation of the vertical section throughout the NGV from Kizimen to Shiveluch. Along this section, the only one deep low-velocity anomaly reaching depths of more than 100 km is located beneath Shiveluch, which perfectly coincides with the gap in the Pacific slab imaged by other studies. Further to the south, the low-velocity anomaly is observable in the uppermost mantle down to 60-70 km. This result shows that all the volcanoes of the NGV are fed from a single source associated with the ascent of the hot asthenosphere though the slab window beneath Shiveluch. Then the hot asthenospheric material spreads southward along the crust bottom. This flow heats the mantle wedge, which is highly contaminated with volatiles coming from the slab, and leads to active melting and forming magma sources. This may explain exceptional activity and diversity of the volcanoes in this zone.

The study is devoted to the analysis of seismotectonic deformations (STD) main axes directions distribution according to the mechanisms of earthquake foci and their complex comparison with the structure of the lithosphere based on the results of seismotomography and numerical modeling of the structure of convective flows in the upper mantle.The International Seismological Center (ISC) catalog for 570 seismic events with M=5.0–8.0 was used to calculate the STD [http://www.isc.ac.uk/iscbulletin/search/fmechanisms/] that occurred between 1976 and May 2019 with the addition of materials on 154 foci 1905-1975 from [Radziminovich et al, 2016, Geodynamics & Tectonophysics; Imaev et al., 2000; Kuchay, 2013].The STD field reconstruction was carried out for the region 38°-80° N and 63o-156o E using the technique described in [Bushenkova et al, 2018, Geodynamics & Tectonophysics; Kuchai, Kozina, 2015, Russian Geology and Geophysics]. The reconstructed STD field for each elementary volume of averaging shows that the predominant direction of the STD axes changes from West to East. The submeridional horizontal shortening, characteristic for the Tien Shan and Altai, turns to the NE, at ~ 93 мeridian and persist up to 105 meridian, where the shortening in the Baikal rift zone occurs in the near-vertical direction and then again takes the NE orientation in Yakutia. The northern part of the study area is characterized by a near-vertical shortening. The predominant subhorizontal elongation appears in the Earth's crust in the eastern part of the study region.The 3D seismotomographic model of the upper mantle velocity anomalies is based on ISC catalog data since 1964. When specifying boundary conditions in the 3D thermal convection numerical simulation, variations in the thickness of the lithosphere are taken into account (from geological and geophysical data, including seismotomographic data, specify the boundaries of the thickened lithosphere of plates and cratons surrounded by the thinned lithosphere of the northern Asia fold belts), according to the conclusions of our previous studies on the really significant effect of changes in lithosphere thickness on the structure of convective flows in the upper mantle [Bushenkova et al, 2018, Geodynamics & Tectonophysics; Chervov, Chernykh, 2014, Journal of Engineering Thermophysics].Comparing the orientations distribution of the STD main axes with the seismotomographic model of the region, we observe the areas of the STD axes directions turning coincide with the sharp boundaries of the seismic velocities anomalies sign change in the upper mantle.Comparing the numerical model of thermal convection with the distribution of the STD main axes orientations we observe an obvious correlation of the STD main axes directions with extended downflows in the upper mantle (elongations are aligned along the strike of the downflow in the plan and shortenings across it). The orientation change occurs mainly above the convection upflows. The most clear correlation is observed in the southern half of the study region, because the lithosphere here has a smaller thickness and block size and the crust is less consolidated, which makes it more exposed to mantle processes.

Klyuchevskoy and surrounding volcanoes in central Kamchatka form the Northern Group of Volcanoes (NGV), which is an area of particularly diverse and intensive Pleistocene‐Holocene volcanism. In this study, we present a new seismic tomographic model of the crust and uppermost mantle beneath NGV based on local earthquake data recorded by several permanent and temporary seismic networks including a large‐scale experiment that was conducted in 2015–2016 by an international scientific consortium. Having an unprecedented resolution for this part of Kamchatka, the new model reveals many features associated with the present and past volcanic activity within the NGV. In the upper crust, we found several prominent high‐velocity anomalies interpreted as traces of large basaltic shield volcanoes, which were hidden by more recent volcanic structures and sediments. We interpret the mantle structure to reflect asthenospheric flow up through a slab window below the Kamchatka‐Aleutian junction that feeds the entire NGV. The interaction of the hot asthenospheric material with fluids released from the slab determines the particular volcanic activity within the NGV. We argue that the eastern branch of the Central Kamchatka Depression, which is associated with a prominent low‐velocity anomaly in the uppermost mantle, was formed as a recent rift zone separating the NGV from the Kamchatka Eastern Ranges.

In this study, we have mapped for the first time robustly the 3D structure of two upper-crustal magmatic reservoirs beneath the active volcanoes Avacha and Koryaksky, which are called “home volcanoes” for Petropavlovsk-Kamchatsky, the main city of Kamchatka (~200,000 inhabitants). These volcanoes represent a serious potential hazard for the city, because they are located at a distance of 25–30 km from the populated areas. A new tomographic model (VP, VS, VP/VS ratio) was built, for which we used the arrival times of seismic P- and S-waves from almost 5,000 local events, recorded by a permanent network of seismic stations during 2009–2018.The resolution of the derived models was carefully tested by a series of synthetic simulations. Prominent anomalies with extremely high VP/VS ratios (up to 2.4) were retrieved directly beneath both volcanoes and interpreted as magma reservoirs containing high degrees of partial melt and/or fluids. Beneath Avacha, the upper limit of the anomaly is located at the depth of ~2 km below the surface. The reservoir appears to be connected to the surface by a neck-shaped anomaly of high VP/VS ratio associated with active seismicity, which is interpreted as a magma and fluid conduit. Beneath Koryaksky, the magma related anomaly is deeper: its upper limit is located at a depth of ~ 7 km below the surface. This anomaly is connected with the volcanic coneby a vertical seismicity cluster, which possibly marks the pathway of fluid ascent and degassing. Between the volcanoes, a 2–3 km thick layer of very low VP and VS is interpreted as deposits of volcanoclastic sediments. Generally low Vp/Vs ratios in the area between the volcanoes show that the magma reservoirs in the upper crust are not interconnected. This study was partially supported by the RFBR project # 18-55-52003.