Seismic Focal Zone Research Articles

RECONSTRUCTION OF THE TECTONIC STRESS FIELD IN THE DEEP PARTS OF THE SOUTHERN KURIL-KAMCHATKA AND NORTHERN JAPAN SUBDUCTION ZONES

Earthquake focal mechanisms in the Southern Kuril-Kamchatka and Northern Japan subduction zones were analysed to investigate the features of the tectonic stress field inside the Pacific lithospheric plate subducting into the upper mantle. Earthquake focal mechanism (hypocenter depths of more than 200 km) were taken from the 1966– 2018 NIED, IMGiG FEB RAS and GlobalCMT catalogues. The tectonic stress field was reconstructed by the cataclastic analysis method, using a coordinate system related to the subducting plate. In most parts of the studied seismic focal zone, the axis of the principal compression stress approximately coincides with the direction of the Pacific lithospheric plate subduction beneath the Sea of Okhotsk. It slightly deviates towards the hinge zone separating the studied regions. The principal tension stress axis is most often perpendicular to the plate movement, but less stable in direction. This leads to compression relative to the slab in some parts of the studied regions, and causes shearing in others. The hinge zone is marked by the unstable position of the tension axis and high values of the Lode–Nadai coefficient, corresponding to the conditions of uniaxial compression, while the compression direction remains the same, towards the slab movement. Two more areas of uniaxial compression are located below the Sea of Japan at depths of 400–500 km.

The Ore and Petroleum Regions of the South Okhotsk Province and Deep Geodynamics

The South Okhotsk Sea province, which includes the Sakhalin, Kunashir, Iturup, Urup islands and surrounding sea areas, comprises numerous occurrences of rare, noble metals and other mineralization, as well as petroleum fields, gas hydrate accumulations, and, locally, active emission of water–hydrocarbon gases. The occurrences and deposits of solid, liquid, and gaseous mineral resources are controlled by hidden deep-seated transform-type fault zones: Nosappu (Tuscarora), Iturup, and Urup. These long-lived extended (over 1000 km) zones are distinguished on the NW Pacific megaplate margin, near the SE flank of the Kuril–Kamchatka trough. Using seismotomographic methods we have established their extension to the west from the seismic focal zone in the oceanic slab subsided into mantle transition zone. In the areas of pull-apart extension, the faults provided active formation of drainage channels for seawater penetration in the lithosphere with subsequent serpentinization of its ultramafic rocks, and for decompressional generation of ascending mantle-derived abiogenic fluid flows. The latter penetrated from the subslab asthenosphere into the suprasubduction mantle wedge and sublithospheric mantle and caused metasomatism. The subsequent migration of the flows led to the generation of the primary magma reservoirs in the lower parts of the continental lithosphere, and intermediate and peripheral chambers in the Earth’s crust. The injection of melts from the chambers in the consolidated crust led to the formation of abyssal and hypabyssal intrusive massifs, arch-dome uplifts, and magmatogenic-ore systems predominantly among the rocks of the pre-Pliocene basement. The concentration of oil and gas accumulations from the mantle-derived abiogenic hydrocarbons bearing mercury, gold, rhenium, and PGE in reservoirs beneath fluid-impermeable beds among Cenozoic sedimentary basins also was related to deep sub- and supraslab fluid flows.

РУДНЫЕ, НЕФТЕГАЗОВЫЕ РАЙОНЫ ЮЖНО-ОХОТСКОЙ ПРОВИНЦИИ И ГЛУБИННАЯ ГЕОДИНАМИКА

In the South Okhotsk Sea province – on the islands of Sakhalin, Kunashir, Iturup, Urup and surrounding sea areas – many occurrences of rare, noble metal and other mineralizations as well as of oil-and-gas fields, gas hydrate accumulations, and isolated areas of active emission of water-hydrocarbon gases are known. Occurrences and deposits of solid, liquid and gaseous mineral resources are controlled by hidden deep fault transform zones: Nosappu (Tuscarora), Iturup, and Urup. These long-lived extended (more than 1000 km) zones are distinguished at the N-W Pacific megaplate margin near the S-E flank of the Kuril-Kamchatka trogue. Using the seismotomographic methods we have established their extension to the west from the seismic focal zone in the oceanic slab that subducted into the transition zone of the mantle. In the areas of strike-slip extension the faults accounted for the active formation of the drainage channels for the penetration of the sea water in the lithosphere with the following serpentinization of its ultramafites, and for decompressional generation of ascending mantle-derived abiogenic fluid flows. The latter penetrated from the underslab asthenosphere in the oversubduction mantle wedge and beneath the lithospheric mantle, where they accounted for the development of the processes of metasomatism. The subsequent migration of flows initiated the creation of primary magma reservoirs in the lower parts of the continental lithosphere, and intermediate and peripheral chambers in the Earth’s crust. The injection of melts from the chambers in the consolidated Earth's crust led to the formation of abyssal, hypabyssal intrusive massifs, arch-dome uplifts and magmatogenic-ore (ore-magmatic) systems predominantly among the rocks of the pre-Pliocene basement. The concentration of oil and gas accumulations mainly from the mantle-derived abiogenic hydrocarbons containing mercury, gold, rhenium, and PGE in the Cenozoic sedimentary basins amidst the reservoirs under the impermeable beds also resulted from deep under- and overslab fluid flows.

ЗЕМЛЕТРЯСЕНИЕ 28 февраля 2013 г. с Mw(GCMT)=6.8, I0=5–6 (Юго-восточное побережье Камчатки)

The February 28, 2013 earthquake (MwGCMT=6.8), which occurred on the southeast coast of Kamchatka, is discussed. The instrumental hypocenter location reported by the KB GS RAS is φ=50.67N, λ=157.77E, h=61 km. Three magnitudes were obtained: local magnitude ML=6.9, code-wave magnitude Mc=6.6, mo-ment magnitude Mw=6.7. The focal mechanisms of the February 28, 2013 earthquake and its aftershocks with ML≥6 correspond to the tectonic conditions of subhorizontal compression in the NW–SE direction. For most of these mechanisms, the shallow plane falls under Kamchatka, which corresponds to the geometry of the subduction zone. The small-focus aftershocks cloud of the February 28, 2013 earthquake has the 5828 km size and is extended in the north-east – south-west direction. The aftershock process has the three-phase character and can be considered as a stream of seismic events decreasing in time with three successive stages with different attenuation modes. The change in attenuation modes is associated with strong after-shocks. The duration of the decaying aftershock process is ~ 100 days. The February 28, 2013 earthquake was felt with intensity ranged from 2 to 5–6 on the MSK-64 scale in 33 localities (Δ=81–493 km). The mac-roseismic impact area was about 56∙103 km2. The mainshock and its four strong aftershocks have a similar picture of macroseismic manifestations: the macroseismic effect is larger on the east coast of Kamchatka; the strongest shakes has been observed in Severo-Kurilsk (Paramushir Island). The elongation of the isoseist along the Kamchatka eastern coast has been observed, which is typical for Kamchatka earthquakes. The strong ground motions from the February 28, 2008 earthquake has been recorded using the Kamchatka net-work of digital accelerographs. The level of peak amplitudes for this earthquake is consistent with average trends in the Kamchatka region. Attenuation parameters are typical for Kamchatka. The shallow layer of the seismic focal zone of the Kuriles and South Kamchatka, in which the February 28, 2013 earthquake was recorded, is one of the most active seismic areas in the North-West Pacific. Earthquakes with M>8 were re-peatedly occurred here, causing a tsunami and intensity of shakes up to 9 on the MSK-64 scale in the south of Kamchatka.

The state of stress in the aftershock area of the March 11, 2011 Tohoku earthquake

This paper presents the results of the stress inversion in the Earth’s crust in the Japanese seismic focal zone after the Tohoku earthquake of March 11, 2011. The stress tensor parameters are determined using the method of cataclastic analysis (MCA) of discontinuous displacements. The reconstruction of the stress state is based on earthquake focal mechanism data for magnitude range (Mw) 3.0–6.0. The calculations are performed at six depths. The 2011 Tohoku earthquake brought significant stress changes in the upper layers of the lithosphere of the continental slope (0-30 km). These changes have led to the emerging of extensive areas of horizontal extension here. The areas with changed geodynamic regime were not reduced over the past few years. The magnitude 9.0 Tohoku earthquake was a catastrophic mega earthquake and in this case the presence of an anomalous state of stress may take a long time.

ГЛУБИННАЯ ГЕОДИНАМИКА И ОСОБЕННОСТИ МЕТАЛЛОГЕНИИ КУНАШИР-УРУПСКОГО ЗВЕНА КУРИЛЬСКОЙ ОСТРОВОДУЖНОЙ СИСТЕМЫ

The southern part of the Kuril island-arc system (from Kunashir to Urup islands) differs from the northern part (from Simushir to Paramushir islands) in its spatial arrangement relative to the subducting Pacific megaplate’s motion vector, greater number of volcanoes, and endogenic mineralization diversity (Au, Ag, PGE, Re, IN, Sn, Cu, Zn, Pb and others). The location of deep northwest-trending zones of strike-slip faulting on the opposite oceanic slope of the Kuril Trench is another peculiarity of the Southern Kurils. Significant extent (up to 1000 km), great depth (more than 100 km) and permanent (J-Q) activity of the strike-slip faults allowed thermal-fluid flows from the undersubduction asthenosphere to penetrate through the seismic focal zone, above the mantle wedge and oversubduction asthenosphere and reach subcontinental lithosphere. The upwelling of the flows was accompanied by metasomatic processes, formation of initial magma reservoirs and both the transitional and peripheral reservoirs in the Kuril island-arc crust, as well as by the evolution of volcanic activity and ore genesis. The synthesis of the Southern Kurils’ metallogeny and geodynamics data allows us to recognize the islands as a natural phenomenon intended for study of geological problems and mineral resources exploitation.

АРТЕФАКТ В КАМЧАТСКОМ КАТАЛОГЕ ЗЕМЛЕТРЯСЕНИЙ: ИССЛЕДОВАНИЕ И УСТРАНЕНИЕ

According to the available data from the Kamchatka Regional Catalog, in the southwestern part of the Kamchatka region, deep earthquakes systematically occurred away from the main seismic focal zone. The paper shows that these anomalous events in the catalog are an artifact caused by the ambiguity of solutions for the hypocenters of earthquakes, which occur in the region located away from the seismic network. The results can be improved by eliminating the station data with the maximum residuals of the arrival times of the waves. Hypocenters were relocated for 102 deep earthquakes from an extended area covering anomalous events. Most of the new solutions lie within the Kamchatka seismic focal zone. The corresponding changes have been added to the Kamchatka regional catalog.

Spatial distribution of earthquake hypocenters in the Crimea—Black Sea region

Some aspects of the seismicity the Crime—Black Sea region are considered on the basis of the catalogued data on earthquakes that have occurred between 1970 and 2012. The complete list of the Crimean earthquakes for this period contains about 2140 events with magnitude ranging from −1.5 to 5.5. Bulletins contain information about compressional and shear waves arrival times regarding nearly 2000 earthquakes. A new approach to the definition of the coordinates of all of the events was applied to re-establish the hypocenters of the catalogued earthquakes. The obtained results indicate that the bulk of the earthquakes’ foci in the region are located in the crust. However, some 2.5% of the foci are located at the depths ranging from 50 to 250 km. The new distribution of foci of earthquakes shows the concentration of foci in the form of two inclined branches, the center of which is located under the Yalto-Alushta seismic focal zone. The whole distribution of foci in depth corresponds to the relief of the lithosphere.

The state of stress in the Earth's crust along the northwestern flank of the Pacific seismic focal zone before the Tohoku earthquake of 11 March 2011

This paper presents a detailed reconstruction of the state of stress in the Earth's crust in the Japanese seismic focal zone before the Tohoku earthquake of March 11, 2011. The stress parameters are determined using the method of cataclastic analysis (MCA) of discontinuous displacements. This method integrates some of the main principles of the theory of plastic deformation and a generalization of the results of rock failure experiments. The reconstruction of the state of stress is based on earthquake focal mechanism data for two magnitude ranges, Mw: 1) 3.0–5.5 and 2) 5.0–7.0. The calculations for the first data set are performed at six depths. Stress reconstruction for the second data set is performed from 0 to 50km. The focal mechanism data provides reasonably stable parameters of the stress field for magnitudes in this range. A large amount of data on the focal mechanisms of earthquakes with magnitudes between 3.0 and 5.5 has allowed us to obtain new data on changes in the stress field, which were missed in the previous calculation for the second data set. It is shown that in the upper part of the lithosphere (up to 90km) the axis of maximum compression is generally oriented slightly (25–35°) under the oceanic plate west of the Japan Trench, and it dips subvertically east of the trench. The upper part of the tectonosphere is divided into two geodynamic regimes: subhorizontal compression west of the trench axis and subhorizontal extension east of the trench axis. West of the Japan Trench, the axis of maximum horizontal compression (SHmax) is orthogonal to the trench axis, but east of the Japan Trench, its orientation has changed (~90°). The underthrusting shear stress that acts normal to the horizontal planes has a similar west and west-southwest direction in the upper layers (0–50km). Deviations in the orientation are observed at depths between 70 and 90km at the junction between the Japan Trench and the Nankai Trough and in the crust of the northeastern part of the Tohoku region. Obtained results are summarized in 2 D-pattern, which can be used to explain the loading mechanism in the region of the Tohoku earthquake. The results of the stress reconstruction also allowed us to obtain the new specific features of the field stress: the oceanic trench divides lithosphere into the areas of high effective pressure west of the trench axis and the areas of low effective pressure east of the trench axis. The rupture developed in the area of the maximum stress gradient and it stopped in the north-east and the south-west directions, where there were areas of high effective pressure.

The state of stresses of the lithosphere in Japan before the catastrophic Tohoku earthquake of 11 March 2011

The article presents results of tectonophysical methods applied to reconstruct tectonic stress field of the north-western flank of the Pacific Ocean seismic focal zone in the region wherein the 2011 Tohoku earthquake was prepared. The reconstructions are based on earthquake foci data for the time period before the catastrophic seismic event. The field of stresses, wherein the Tohoku earthquake focus was formed, had a high gradient along the dip of the seismic focal zone. It is revealed that the focus developed in the junction area of the crust segments with high and low levels of effective pressure. A wide area of lower effective pressure was located at depths close to 30 km, and it was the most susceptible to brittle fracture. In our opinion, the area impacted by the Tohoku earthquake is large due to a large length of the crustal segments with the high gradient of stresses, which are located along the eastern part of the crust of the Honshu Island. The stress reconstruction also shows that the axis of the Japan oceanic trough divides the seismic focal zone into areas of horizontal compression (westward) and horizontal extension (eastwards). According to our calculations, lateral compression is the highest at the crustal depths up to 20 km westward of the trough’s axis, where maximum lateral compression axes are oriented orthogonally to the troug h’s strike. Eastward of the trough’s axis, minimum horizontal compression axes are oriented orthogonally to the trough’s strike. At the crossing point of the Japan trough, a sharp changing of stress is by a factor of 5 to 8 of internal cohesion of rocks, τf. This sharp changing of stress is lower for the Izu-Bonin trough and varies from 3 to 5 τf.

10.1007/s11486-008-3004-8

Results of investigation of the lithosphere in the Kamchatka seismic focal zone from dynamic characteristics of earthquake records obtained at regional stations are presented. It is assumed that the specificity of the source zone can be estimated by the relation Cr = K P − bK S − c characterizing relative energies (energy classes, according to [Fedotov, 1972]) of short period transverse and longitudinal waves in the source. Azimuthal, spatial, and temporal variations in Cr and their relation to focal mechanisms are examined. Spatiotemporal variations in this parameter are shown to be caused by the influence of variations in the conditions in the source zone (its substance or process) on the radiation of P and S waves.

Creepex of Kamchatka shallow earthquakes

Results of investigation of the lithosphere in the Kamchatka seismic focal zone from dynamic characteristics of earthquake records obtained at regional stations are presented. It is assumed that the specificity of the source zone can be estimated by the relation Cr = K P − bK S − c characterizing relative energies (energy classes, according to [Fedotov, 1972]) of short period transverse and longitudinal waves in the source. Azimuthal, spatial, and temporal variations in Cr and their relation to focal mechanisms are examined. Spatiotemporal variations in this parameter are shown to be caused by the influence of variations in the conditions in the source zone (its substance or process) on the radiation of P and S waves.

Velocity properties of the lithosphere in the ocean-continent transition zone in the Kamchatka region from seismic tomography data

Arrival times of P and S waves from local earthquakes in the Kamchatka area of the Kurile-Kamchatka Island Arc are used for calculating a spatial model of the elastic wave velocity distribution to a depth of 200 km. The lithosphere is shown to be strongly stratified in its velocity properties and laterally heterogeneous within the mantle wedge and seismic focal zone. A lower velocity layer (an asthenospheric wedge) is identified at depths of 70–130 km beneath the Eastern Kamchatka volcanic belt. The morphology of the Moho interface and the velocity properties of the crust are studied. The main tectonic structures of the region are shown to be closely interrelated with deep velocity heterogeneities. Regular patterns in the statistics of the earthquakes are analyzed in relation to variations in the elastic wave velocities in the focal layer. A mechanism of lithospheric block displacements along weakened zones of the lower crust and upper mantle is proposed.

Earthquake focal mechanisms and seismotectonic deformation in the Kamchatka-Commander region

The system of stresses in the lithosphere (0–59 km) and upper asthenosphere (60–150 km) was analysed on the basis of focal-mechanism data for events of M > 5.0 in 1964–1982. The zoning was made considering types of faulting and predominant stress systems. Four uniformly-stressed zones have been identified within the lithosphere and two zones within the upper asthenosphere. The deep distribution of types of faulting and stresses shows the presence of a double seismic focal zone under Kamchatka. Schemes of seismotectonic-deformation (STD) rate-tensor components were dran separately for the lithospheric and asthenospheric layers. To this end, data on individual and averaged focal-mechanism solutions and regional seismic moment-energy class relationships were used. The average STD rate for the lithosphere is 10 −9 yr −1, and for the upper asthenosphere 10 −10 yr −1. Vertical and horizontal components of STD are of the same order.

Geodynamics and seismicity of the Pamir-Himalayas region

Deep seismic sounding studies carried out in 1974–79 allowed an important peculiarity of the deep structure of the Pamir-Himalayas region to be established: the thickness of the Earth's crust is almost twice as large here as on the stable plates (65–75 and 35–37 km, respectively). The absence of any evidence for doubling of crustal thickness provides grounds for rejecting the hypothesis of subduction of the rigid Hindustan plate under the geosynclinal folded constructions of the Punjab syntaxis of the Himalayas. The steep inclination of all major faults, dissecting the Earth's crust and often dislocating the M surface, is also counter to this hypothesis. Several faults reflect the dynamics and conditions of formation of deep layers of the lithosphere. For example, the structural seam of the Indus, which has an almost sheer tilt and which penetrates to subcrustal depths, is a channel along which ophiolite associations of crystalline rocks were squeezed from the mantle. The Fore Himalayan and Major Himalayan faults are the boundaries between different structural facial zones. The band of greatest thickness of crust extends within the zone of greatest thickness of the asthenospheric layer; a deep minimum in the Bouguer anomalies (−550 mGal) corresponds to this zone, as does also a depression on the surface of the geoid. Seismicity of the lithosphere of the Pamir-Himalayas region is caused by geodynamic processes manifested in the higher lithospheric layers by block displacements of the Earth's crust (mostly uplifts), and in the lower parts by shifts of the steeply inclined mantle blocks (the Pamir-Hindukush seismic focal zone).

Investigation on earthquake prediction in Kamchatka

Investigations on earthquake prediction in Kamchatka are mainly carried out along three lines: 1. (1) The detailed investigations of Kamchatka-Komandorskiye Islands seismicity laws and long-term prediction (“seismic weather prediction”). The location of the Japanese earthquake on May 16, 1968, M = 7.9 was predicted well. 2. (2) The investigation of the connection between earthquakes and irregularities of electrotelluric field. The observed electric anomalies are found to arise some hours before the earthquakes and at the moment of the shock. The periods of such irregularities change from some minutes to half an hour. The amplitudes reach 50 mV/km. 3. (3) Investigations of velocities, amplitudes and other features of P-wave in the Pacific seismic focal zone near Kamchatka and their variations in time. Places of shot points and receivers at coast and in the ocean were constant. Distances were 40–200 km. Standard deviations of traveltime determinations were there ± 0.03 sec. In 1966–1968, when there were no earthquakes with M ≥ 6 in the investigated region, travel-time variations were in error limits. That means P velocities were constant here with an accuracy of 0.3–0.5%.