Results Of Deep Seismic Sounding Research Articles

The Earth’s Crust Structure of the Persian Gulf According the Results of Deep Seismic Sounding

The article presents the results of seismic work performed by the DSS method in the Persian Gulf. Bottom analog type seismographs and seismic airgun sources were used in the works. The bottom seismographs were placed and the shooting was carried out according to three regional profiles with a length from 100 to 250 km. The main result of these studies was to obtain velocity-depth model of the sedimentary cover and the Earth’s crust up to Moxo boundary, which was revealed at a depth of about 43 km in the water area. Given that the thickness of the upper crust is only 4–5 km, and the speed characteristics of the remaining layers, the crust type can be attributed to the subcontinental Archean type. This situation (complete absence or drowning of the upper layer of the Earth’s crust) is typical for the waters closest to the Persian Gulf: the Black Sea, the Caspian Sea, the Mediterranean and the Red Seas. A structure was found in the Earth’s crust of the studied area, which may be a brachianticline having an isometric dome-shaped shape, which corresponds to the folding of the platform type in the areas of salt-dome tectonics. No faults have been found in the Earth’s crust of the Persian Gulf water area adjacent to the Bushehr Peninsula.

PRISM3D: a 3-D reference seismic model for Iberia and adjacent areas

SUMMARY We present PRISM3D, a 3-D reference seismic model of P- and S-wave velocities for Iberia and adjacent areas. PRISM3D results from the combination of the most up-to-date earth models available for the region. It extends horizontally from 15°W to 5°E in longitude, 34°N to 46°N in latitude and vertically from 3.5 km above to 200 km below sea level, and is modelled on a regular grid with 10 and 0.5 km of grid node spacing in the horizontal and vertical directions, respectively. It was designed using models inferred from local and teleseismic body-wave tomography, earthquake and ambient noise surface wave tomography, receiver function analysis and active source experiments. It includes two interfaces, namely the topography/bathymetry and the Mohorovičić (Moho) discontinuity. The Moho was modelled from previously published receiver function analysis and deep seismic sounding results. To that end we used a probabilistic surface reconstruction algorithm that allowed to extract the mean of the Moho depth surface along with its associated standard deviation, which provides a depth uncertainty estimate. The Moho depth model is in good agreement with previously published models, although it presents slightly sharper gardients in orogenic areas such as the Pyrenees or the Betic-Rif system. Crustal and mantle P- and S-wave wave speed grids were built separately on each side of the Moho depth surface by weighted average of existing models, thus allowing to realistically render the speed gradients across that interface. The associated weighted standard deviation was also calculated, which provides an uncertainty estimation on the average wave speed values at any point of the grid. At shallow depths (<10 km), low P and S wave speeds and high VP/VS are observed in offshore basins, while the Iberian Massif, which covers a large part of western Iberia, appears characterized by a rather flat Moho, higher than average VP and VS and low VP/VS. Conversely, the Betic-Rif system seems to be associated with low VP and VS, combined with high VP/VS in comparison to the rest of the study area. The most prominent feature of the mantle is the well known high wave speed anomaly related to the Alboran slab imaged in various mantle tomography studies. The consistency of PRISM3D with previous work is verified by comparing it with two recent studies, with which it shows a good general agreement.The impact of the new 3-D model is illustrated through a simple synthetic experiment, which shows that the lateral variations of the wave speed can produce traveltime differences ranging from –1.5 and 1.5 s for P waves and from –2.5 and 2.5 s for S waves at local to regional distances. Such values are far larger than phase picking uncertainties and would likely affect earthquake hypocentral parameter estimations. The new 3-D model thus provides a basis for regional studies including earthquake source studies, Earth structure investigations and geodynamic modelling of Iberia and its surroundings.

Structure of the Sediment and Crust in the Northeast North China Craton from Improved Sequential H-k Stacking Method

AbstractH-k stacking method is a standard receiver-function method to detect crustal thickness. But this method can not be applied in low-velocity sedimentary basins. To solve this problem, we propose an improved sequential H-k stacking method. The improved method needs two sequential stacks. Firstly, sediment structure is calculated using converted waves and multiples on the bottom boundary of sediments. Secondly, the sedimentary results are applied to calculate the crustal structure. Theoretical calculations and “recovery tests” indicate that the improved method can obtain accurate estimates in sedimentary basins. With the teleseismic data of North China Craton, the structure of sediments is thick in the depression and thin in the uplifted area, which is consistent with Deep Seismic Sounding results. The crust to the west of the North-South Gravity Lineament is relatively thick and has a low average Poisson ratio, whereas the east is relatively thin and has a high average Poisson ratio. This result and the structural feature from data regression imply that the eastern crust of the North China Craton has experienced wide extension, which reflect the crustal response to the severe destruction and deformation in that area compared to the western crust.

Crustal P wave velocity structure beneath the SE margin of the Tibetan Plateau from Deep Seismic Sounding results

The crust and uppermost mantle beneath the southeastern (SE) margin of Tibetan Plateau record the lateral expansion of Tibet and far-field effects from the ongoing continental collision and convergence. In order to obtain constraints on the deep structure and the eastward expansion of the plateau, we synthesized Deep Seismic Sounding (DSS) results in this region and constructed the P wave velocity models for each terrane. The principal characteristics of the crustal velocity model and tectonic significance are: (1) Crustal thickness decreases from 53 km beneath the Songpan-Ganzi terrane (SGT) to 34 km in the south direction beneath Simao terrane (SMT), and 42 km in the east direction beneath Chengdu area. The crust is dominantly felsic in upper crust, with a small percentage of mafic composition in the lower crust. (2) The crustal structure is very heterogeneous in this region. The Indochina Block is characterized by low seismic velocities in the upper crust, while the west Yangtze terrane (WYZT) and SGT show higher velocities in the upper crust. The lower crust beneath WYZT is relatively thicker than other terranes. (3) We deduce that the high velocity in the upper crust and relatively thick lower crust beneath the SGT and WYZT can be associated with magmatic processes that generated the Emeishan flood basalts which may have been triggered by rifting or mantle plume. The lateral variations of the crustal thickness in SE Tibet may be due to lower crustal flow from the Tibetan Plateau to the SE direction after blocked by the cold and rigid Yangtze craton in the east direction.

A comparative analysis of the seismic and density models for the Earth’s Crust of the Central Kurils

The seismic and density models for the Earth’s crust of the central region of the Kuril Island arc are compared. The seismic data are based on the results of deep seismic soundings that were performed here in the 1980s. The density models were constructed by using marine and satellite gravimetric data obtained in recent years. Comparison of the seismic and density models reveals their overall consistency, with a few discrepancies. The possible sources of these discrepancies are discussed.

Gravity inversion of deep-crust and mantle interfaces in the Three Gorges area

Abstract: To better understand the heterogeneity of deep-crust and mantle interfaces in the region of the Three Gorges, China, we used the Parker-Oldenburg iterative inversion method to invert existing Bouguer gravity data from the Three Gorges area (1 : 500000), a new gravity map of the Three Gorges Dam (1 : 200000), and the results of deep seismic soundings. The inversion results show a Moho depth of 42 km between Badong and Zigui and the depth of the B2 lower-crustal interface beneath the Jianghan Plain and surrounding areas at 21–25 km. The morphology of crustal interfaces and the surface geology present an overpass structure. The mid-crust beneath the Three Gorges Dam is approximately 9 km thick, which is the thinnest in the Three Gorges area and may be related to the shallow low-density body near the Huangling anticline. The upper crust is seismogenic, and there is a close relationship between seismicity and the deep-crust and mantle interfaces. For example, the M 5. 1 Zigui earthquake occurred where the gradients of the Moho and the B2 interface are the steepest, showing that deep structure has a very important effect on regional seismicity.

Finite-difference migration of the field of refracted waves in studies of the deep structure of the Earth’s crust and the upper mantle based on the DSS (on the example of the DOBRE profile)

The main results of deep seismic sounding (DSS) are usually presented in the form of high-velocity models of the medium. Some model examples and the international DOBRE profile have shown that the informativeness of the data obtained can be significantly enhanced by the construction of wave images of the Earth’s crust, based on the migration of refracted and wide-angle reflected waves. The Donets Basin Refraction/Reflection Experiment (DOBRE) profile crosses the Dnieper-Donets paleorift zone in the Donbas region. Along the profile, refracted waves from the basement and the upper mantle and the reflections from the crust basement (the M boundary) are reliably traced. This wave migration has been used to construct a wave image of the structure of the Earth’s crust. As a result, a clear seismic image of the basement surface, whose depth changes along the profile from 0 to 20 km, was obtained. In near-slope parts of the basin, several major faults were identified that had not been identified previously during standard kinematic data processing. It is shown that the crust-upper mantle transition zone is a clearly reflective horizon only within the crystalline massifs; under a depression, it is represented by a lens-shaped highly-heterogeneous area. As shown in the model examples, the images obtained using such a migration accurately reflect the structural features of the medium, in spite of its complicated structure.

Pripyat Trough: Tectonics, geodynamics, and evolution

The Pripyat Trough is situated in the East-European Platform and is a part of the more extensive system of the Sarmatian-Turanian lineament. It is one of the most thoroughly drilled and seismically investigated oil-bearing basins of the paleorift type that are known in the ancient platforms. Its geological structure reflects the typical features of ancient rift basins and can be interpreted as the tectonic type of an oil-bearing paleorift. The tectonic style of the paleorift can be reconstructed from the structure of some markers, such as the surfaces of the basement, of the subsalt deposits, of the intersalt rocks, of the upper salt-bearing rock sequence, and of some other rocks residing in the upper parts of the sequence. The Pripyat Trough is characterized by the block tectonics of its subsalt deposits, by the block-fold tectonics of its intersalt deposits, and by the essentially fold tectonics of its upper salt-bearing and overlying deposits. The specific styles of the tectonic deformation of the subsalt, intrasalt, and suprasalt deposits suggest three structural zones, namely, the Northern, Central, and Southern zones. Proceeding from the results of deep seismic sounding, the Pripyat rift is interpreted as a zone of listric breaking which had enveloped not only the Pripyat Graben but also the adjacent areas of the Ukrainian Shield and Belorussian Anteclise, forming its shoulders.

A Plausible Model for Evolution of Schist Belts and Granite Plutons of Dharwar Craton, India and Madagascar During 3.0-2.5 Ga: Insight from Gravity Modeling Constrained in Part from Seismic Studies

Gravity modeling of an E-W profile across Dharwar Craton, India and Madagascar, integrated with the results of Deep Seismic Sounding (DSS) across the Dharwar Craton suggest a thick crust of 40-42 km under the eastern part of Eastern Dharwar Craton (EDC), the Western Dharwar Craton (WDC) and the central part of the Madagascar. Towards east of these blocks, the crustal thickness is reduced to 36-38 km along the Eastern Ghat Fold Belt (EGFB), shear zone between the EDC and the WDC and the east coast of Madagascar, respectively. These zones of thin crust are also characterized by high density lower crustal rocks associated with thrusts. The seismic section across Dharwar Craton shows domal- shaped reflectors in the lower crust and upper mantle under the WDC which may be related to asthenopheric upwelling during an extension phase. The occurrences of large schist belts with volcano-sedimentary sequences of marine origin of late Archean period (3.0-2.7 Ga) as rift basins in the WDC and Madagascar also suggest an extensional phase in this region during that period. It is followed by a convergence between the WDC and the EDC giving rise to collision-related shear and thrust zones between the WDC and the EDC associated with high density lower crustal rocks. The seismic section shows upwarped reflectors in the upper crust which may be related to this convergence. Eastward dipping reflectors under WDC and EDC and west verging thrusts suggest convergence from the west to the east which resulted in easterly subduction giving rise to subduction-related K-granite plutons of the EDC of 2.6-2.5 Ga. In this regard, the Closepet granite in the EDC which extends almost parallel to the shear zone between the WDC and EDC and shows an I-type calk-alkaline composition may represent relict of an island arc and the linear schist belts with bimodal volcanics of the EDC east of it might have developed as back arc rift basins. Subsequent collision between India and Antarctica along the EGFB during Middle Proterozoic, indicated by eastward dipping reflectors in the crust and the upper mantle and west verging thrust gave rise to contemporary high-grade rocks of the EGFB (1.6-1.0 Ga) and associated mafic and felsic intrusives of this belt. The part of adjoining Cuddapah basin contemporary to the EGFB towards the west consisting of marine shelf type of sediments which are highly disturbed and thickest at its contact with the EGFB may represent a peripheral foreland basin. Gravity modeling provides thickest crust of 42 km in the southern part of the WDC and does not support sharp increase in crustal thickness of 50-60 km with high velocity upper mantle as suggested from receiver function analysis. It may represent some foreign material of high density trapped in this section such as part of oceanic crust during convergence and subduction that is referred to above. It is supported from eastward dipping reflectors in lower crust and upper mantle in adjoining region.

Focal mechanisms of three earthquakes in Finland and their relation to surface faults

Focal mechanisms for three recent earthquakes in Finland are determined using P-wave polarities together with SV/P and SH/P phase amplitude ratios. The events occurred on May 11, 2000 in Toivakka, Central Finland ( M L=2.4), on September 15, 2000 in Kuusamo, northeastern Finland ( M L=3.5), and on May 2, 2001 in Kolari, western Finnish Lapland ( M L=2.9). In order to obtain reliable estimates of the source parameters, one-dimensional crust and upper mantle velocity models are derived for the epicenter areas from deep-seismic sounding results. The starting models are modified by one-dimensional ray tracing using the earthquake observations. The events are relocated by employing P- and S-phase arrival times from the nearest seismic stations and the final velocity models. Synthetic waveforms, calculated with the reflectivity method, are used to further constrain and verify the source and structural parameters. The Toivakka earthquake indicates thrust- or reverse-faulting mechanism at a depth of 5 km. After comparison with aeromagnetic and topographic data we suggest the eastward dipping nodal plane (358°/42°) was the fault plane. The best-fitting fault plane solution of the Kolari earthquake suggests pure thrust-faulting at a depth of 5 km. The nodal plane striking 035°/30° correlates well with surface observations of the postglacial, possibly listric fault systems in the source area. The Kuusamo earthquake (focal depth 14 km) has a normal-faulting mechanism with the nodal planes trending 133°/47° or 284°/47°. Preference is given to the SW-dipping nodal plane, as it seems to coincide with topographic and magnetic lineament directions that have been active after the last ice age. The three earthquakes have occurred in old Precambrian faults and shear zones, which have been reactivated. The reactivated faults are favourably oriented in the local stress field.

A preliminary seismic model for the region of the west-Bohemian earthquake swarms

A layered model of the crustal and upper mantle structure for the region of the West-Bohemian earthquake swarms is proposed. This model represents a compilation of the results of deep seismic soundings, surface-wave investigations, and some local seismic measurements in the western regions of the Bohemian Massif.

Study of Curie isothermal surface in Sichuan Basin and the seismic area on its western margin

Using the aeromagnetic data and the method of 3—D magnetic stratum inversion, the data of aeromagnetic anomalies in the investigated region have been processed; and thereby the quantitative buried depth of the Curie isothermal surface in Sichuan Basin and its western marginal area has been obtained. The reliability of the inversion results has been confirmed by the features of regional tectonics, the distribution of geothermal gradient and the results of deep seismic sounding. By analyzing the deep geothermal field, it is concluded that the followings are the environmental conditions for the development and occurrence of earthquakes in the western marginal area of Sichuan Basin: (1) The collision of the Yangtze block with the Sichuan-Qinghai block and Kangdian rhombic block has produced strong stress accumulation; and (2) Plastic creep in the middle-lower crust owing to the up-welling of mantle flux has provided an adjusting unit for rock rupture and stress release.

Dispersion of Rayleigh waves along the Prague-Warsaw profile

Phase velocities of Rayleigh waves propagating along the Prague-Warsaw profile have been determined. A layered model of the Earth's crust and upper mantle, satisfying the observed dispersion of Rayleigh waves, has been derived. In constructing this model, the results of deep seismic soundings along international profile VII were also taken into account.

Investigations of the East Greenland continental margin between 70° and 72° N by deep seismic sounding and gravity studies

Results are presented from a deep seismic sounding experiment with the research vessel POLARSTERN in the Scoresby Sund area, East Greenland. For this continental margin study 9 seismic recording landstations were placed in Scoresby Sund and at the southeast end of Kong Oscars Fjord, and ocean bottom seismographs (OBS) were deployed at 26 positions in and out of Scoresby Sund offshore East Greenland between 70° and 72° N and on the west flank of the Kolbeinsey Ridge. The landstations were established using helicopters from RV POLARSTERN. Explosives, a 321 airgun and 81 airguns were used as seismic sources in the open sea. Gravity data were recorded in addition to the seismic measurements. A free-air gravity map is presented. The sea operations — shooting and OBS recording — were strongly influenced by varying ice conditions. Crustal structure 2-D models have been calculated from the deep seismic sounding results. Free-air gravity anomalies have been calculated from these models and compared to the observed gravity. In the inner Scoresby Sund — the Caledonian fold belt region — the crustal thickness is about 35 km, and thins seaward to 10 km. Sediments more than 10 km thick on Jameson Land are of mainly Mesozoic age. In the outer shelf region and deep sea a ‘Moho’ cannot clearly be identified by our data. There are only weak indications for the existence of a ‘Moho’ west of the Kolbeinsey Ridge. Inside and offshore Scoresby Sund there is clear evidence for a lower crust refractor characterised byp-velocities of 6.8–7.3 km s−1 at depths between 6 and 10 km. We believe these velocities are related to magmatic processes of rifting and first drifting controlled by different scale mantle updoming during Paleocene to Eocene and Late Oligocene to Miocene times: the separation of Greenland/Norway and the separation of the Jan Mayen Ridge/Greenland, respectively. A thin igneous upper crust, interpreted to be of oceanic origin, begins about 50 km seaward of the Liverpool Land Escarpment and thickens oceanward. In the escarpment zone the crustal composition is not clear. Probably it is stretched and attenuated continental crust interspersed with basaltic intrusions. The great depth of the basement (about 5000 m) points to a high subsidence rate of about 0.25 mm yr−1 due to sediment loading and cooling of the crust and upper mantle, mainly since Miocene time. The igneous upper crust thickens eastward under the Kolbeinsey Ridge to about 2.5 km; the thickening is likely caused by higher production of extrusives. The basementp-velocity of 5.8–6.0 km s−1 is rather high. Such velocities are associated with young basalts and may also be caused by a higher percentage of dykes. Tertiary to recent sediments, about 5000 m thick, form most of the shelf east of Scoresby Sund, Liverpool Land and Kong Oscars Fjord. This points to a high sedimentation rate mainly since the Miocene. The deeper sediments have a rather high meanp-velocity of 4.5 km s−1, perhaps due to pre-Cambrian to Caledonian deposits of continental origin. The upper sediments offshore Scoresby Sund are thick and have a rather low velocity. They are interpreted as eroded material transported from inside the Sund into the shelf region. Offshore Kong Oscars Fjord the upper sediments, likely Jurassic to Devonian deposits, are thin in the shelf region but thicken to more than 3000 m in the slope area. The crust and upper mantle structure in the ocean-continent transition zone is interpreted to be the result of the superposition of the activities of three rifting phases related to mantle plumes of different dimensions: During period 2 and 3 only a few masses of extrusives were produced, but large volumes of intrusives were emplaced. So the margin between Scoresby Sund and Jan Mayen Fracture Zone is interpreted to be a stretched margin with low volcanic activity.

Characteristic of crustal structure in the shulu fault basin and its vicinity

The deep seismic reflection profiling carried out in Xingtai earthquake area provides a new knowledge of the crustal structure of the Shulu fault basin and its vicinity. In the Ningjin-Xinhe and Lincheng-Julu deep seismic reflection profiles trending in NWW, CDP stack profiles respectively show a one-side fault basin (i. e. Shulu fault basin) within TWT 4.0 s. The width of the basin is about 15 km (Eogene system boundary), and Xinhe fault extends to below TWT 4.0 s (i.e. 8 km deep) with listric shape as a main boundary fault. These profiles also display distinctly a detachment in mid-crust. The Xinhe fault extends downward and converges to the detachment. The results of deep seismic sounding and magnetotelluric sounding indicate the low-velocity and high-conductive zone beneath the detachment, which is beneficial to the detach between upper and lower plates. The Renxian-Ningjin deep seismic reflection profile trending in NNE lies within the fault basin, which shows the complicated structure of the basin. The shallow part of the profile is divided into three sub-basins by three lateral uplifts. In the mid-lower crust from Gengzhuangqiao to Xiaohezhuang of the profile, there are a lot of strong reflection events with laminae structure, which have been deformed strongly. Two NWW-trending profiles also have similar reflection feature. This may indicate that there is a relative large region where the magma upwell into mid-lower crust. The abnormal low velocity zone in lower crust indicates that the magmatism is still strong at present. The magmatism may be an important factor of the tectonic active region.

Deep structure, volcanism and Wadati-Benioff zone of the northwestern pacific convergent margin

The morphology of the Wadati-Benioff zone in the region of Kamchatka, the Kurile Islands and Hokkaido, based on the distribution of 6319 earthquake foci, has verified the existence of an intermediate-depth aseismic gap and its relation to active andesitic volcanism. It appeared that deep-focus earthquakes in this region belong to a paleosubduction zone activated by an intermediate-depth collision with the active subduction zone in the area of Hokkaido. A system of deep seismically active fracture zones was delineated in the continental plate and confirmed by the results of deep seismic sounding. Two of these fractures, dipping toward the subduction zone, may be considered as the principal feeding channels for active and Holocene volcanoes of the continental volcanic bels of Kamchatka.

Deep structure of the Earth's crust in the contact zone of the Palaeozoic and Precambrian Platforms in Poland (Tornquist-Teisseyre zone)

One of the major tectonic problems in Europe concerns the southwest margin of the East European Platform in the region of the so-called Polish-Danish trough. In general, this margin is assumed to be the Tornquist-Teisseyre (T-T) Line, running approximately from northwest to southeast in this part of Europe. Determination of deep crustal structure of the contact zone between the Precambrian Platform and the Palaeozoic Platform was the main aim of the deep seismic sounding (DSS) programme in Poland in 1965–1982. Deep seismic soundings of the Earth's crust have been made in the T-T Line zone along nine profiles with a total length of about 2600 km. The results of deep seismic soundings have shown that the crust in the marginal zone of the East European Platform has highly anomalous properties. The width of this zone ranges from 50 km in northwest Poland to about 90 km in southeast Poland. The crustal thickness of the Palaeozoic Platform in Poland is 30–35 km, and of the Precambrian Platform 42–47 km, while in the T-T tectonic zone it varies from 50 to 55 km. Above the Moho boundary, in the T-T zone, at a depth of 40–45 km, there is a seismic discontinuity with P-wave velocities of 7.5–7.7 km/s. Boundary velocities, mean velocities and stratification of the Earth's crust vary distinctly along the T-T zone. There are also observed high gravimetric and magnetic anomalies in the T-T zone. The T-T tectonic zone determined in this manner is a deep tectonic trough with rift properties. The deep fractures delineating the T-T tectonic zone are of fundamental importance for the localization of the plate edge of the Precambrian Platform of eastern Europe. In the light of DSS results, the northeastern margin of the T-T tectonic zone is a former plate boundary of the East European Platform.

Relocation of the Koyna earthquakes with a new velocity model

abstractHypocenter locations of 380 earthquakes of magnitude ≧ 3 in the Koyna region, recorded by the Central Water and Power Research Station Network of Seismological Observatories and the India Meteorological Department stations during the period December 1967 to December 1973, are determined after selection of an appropriate crustal model. The model is chosen from among the few available for the region on the basis of comparatively better hypocenter locations in terms of error parameters in the location of 32 events of magnitude ≧ 4 (master events). An epicenter map and hypocentral depth cross-section of the earthquakes in the region are presented. Least-squares fit through the best hypocentral locations along the line perpendicular to the strike of fault plane inferred by Tandon and Chaudhury (1968) gives a dip of 27° in the east-southeast direction and the surface manifestation of the fault 25 km to the west of the Koyna Dam. While the chosen strike direction of the fault is also in agreement with those obtained in a number of fault plane solutions and LANDSAT imagery (Lee and Rayleigh, 1969; Sykes, 1970; Banghar, 1972; Chandra, 1976; Rastogi and Talwani, 1980; Langston, 1981), the observed dip direction of the fault agrees only with the results of deep seismic sounding (Kaila et al., 1979) and the fault plane solution of Rastogi and Talwani (1980). The amount of dip is, however, much less than any of the values reported so far.

Earthquake distribution and volcanism in Kamchatka, Kurile Islands, and Hokkaido

The morphology of the Wadati-Benioff zone in the region of Central Kuriles, based on the distribution of 1202 earthquake foci, verified the existence of an intermediate depth aseismic gap and its relation to active andesitic volcanism. Several deep seismically active fracture zones of different tectonic function were delineated in the continental plate and confirmed by the results of deep seismic sounding. A seismically active fracture zone, found in front of the Kamchatka-Kurile trench, indicates the segmented nature of fracturing of the subducting Pacific plate.

Earthquake distribution and volcanism in Kamchatka, Kurile Islands, and Hokkaido Part 1: Kamchatka and northern Kuriles

The morphology of the Wadati-Benioff zone in the region of Kamchatka and Northern Kuriles, based on the distribution of 1102 earthquake foci, verified the existence of an intermediate depth aseismic gap and its relation to active andesitic volcanism. A system of deep seismically active fracture zones, genetically connected with the process of subduction, was delineated in the continental plate and confirmed by the results of deep seismic sounding. Two of these fractures, dipping toward the subduction zone, may be considered as the principal feeding channels for active and Holocene volcanoes of the continental volcanic belts of Kamchatka.