Institute of Volcanology and Seismology

Research Article| May 01, 1995 Magnesian andesite in the western Aleutian Komandorsky region: Implications for slab melting and processes in the mantle wedge G. M. Yogodzinski; G. M. Yogodzinski 1Institute for the Study of the Continents and Department of Geological Sciences, Cornell University, Ithaca, New York 14850 Search for other works by this author on: GSW Google Scholar R. W. Kay; R. W. Kay 1Institute for the Study of the Continents and Department of Geological Sciences, Cornell University, Ithaca, New York 14850 Search for other works by this author on: GSW Google Scholar O. N. Volynets; O. N. Volynets 2Institute of Volcanic Geology and Geochemistry, Petropavlovsk, Kamchatka 683006, Russia Search for other works by this author on: GSW Google Scholar A. V. Koloskov; A. V. Koloskov 2Institute of Volcanic Geology and Geochemistry, Petropavlovsk, Kamchatka 683006, Russia Search for other works by this author on: GSW Google Scholar S. M. Kay S. M. Kay 3Institute for the Study of the Continents and Department of Geological Sciences, Cornell University, Ithaca, New York 14850 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1995) 107 (5): 505–519. https://doi.org/10.1130/0016-7606(1995)107<0505:MAITWA>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation G. M. Yogodzinski, R. W. Kay, O. N. Volynets, A. V. Koloskov, S. M. Kay; Magnesian andesite in the western Aleutian Komandorsky region: Implications for slab melting and processes in the mantle wedge. GSA Bulletin 1995;; 107 (5): 505–519. doi: https://doi.org/10.1130/0016-7606(1995)107<0505:MAITWA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The role of the subducting lithospheric slab in the genesis of mantle-derived (primitive) magmas is investigated through a study of volcanic rocks formed in the tectonically strike-slip–dominated western Aleutian arc. Two types of chemically and petrologically distinctive primitive andesites have been found among the Miocene–late Pleistocene–age volcanic rocks in the western Aleutians. These are termed the "Adak-type" and "Piip-type" magnesian andesites. Trace element and isotopic characteristics indicate that Adak-type magnesian andesites (adakites) formed principally as small percentage melts of the basaltic portion of the subducting oceanic crust, leaving a clinopyroxene-garnet-rutile residual mineralogy. The resulting slab melt signature (high La/Yb, Sr) distinguishes Adak-type magnesian andesites from all other Aleutian volcanic rocks. Primitive characteristics (high Mg#, Cr, Ni) and intermediate compositions (∼59% SiO2) of Adak-type magnesian andesites were acquired by interaction with peridotite and/or basalt in the mantle wedge. The absence of olivine phenocrysts from Adak-type magnesian andesites indicates that they were not equilibrated with peridotite and so are unlike Piip-type magnesian andesites, which appear to have equilibrated under low pressure and hydrous conditions in the subarc mantle. Piip-type magnesian andesites also contain a slab melt component, but reaction-equilibration with peridotite has lowered La/Yb and Sr to levels like those of common Aleutian volcanic rocks.Miocene-age calc-alkaline rocks of the Komandorsky Islands have chemical characteristics transitional between those of Adak-type magnesian andesites and common Aleutian volcanic rocks from the central and eastern arc. In a source mixture of depleted mantle wedge, slab melt, and sediment, the Komandorsky rocks have a relatively large contribution from the slab melt endmember. The strong slab melt signature among western Aleutian rocks is attributed to highly oblique convergence that produced a slow subduction path into the subarc mantle. Geochemically, the slab melt provided a high Sr, La/Yb, La/Ta, and low Ti/Hf endmember to the western Aleutian source mixture. The enhanced role for slab melting in the western Aleutians may be like that predicted for Archean systems and for modern systems where the subduction zone is warm. In this regard, Adak-type magnesian andesites are probably the appropriate analog to sanukitoids and other primitive andesitic rocks of Archean age. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Journal Article Magnesian Andesites and the Subduction Component in a Strongly Calc-Alkaline Series at Piip Volcano, Far Western Aleutians Get access G. M. YOGODZINSKI, G. M. YOGODZINSKI 1Department of Geological Sciences & INSTOC, Cornell UniversityIthaca, New York 14850 Search for other works by this author on: Oxford Academic Google Scholar O. N. VOLYNETS, O. N. VOLYNETS 2The Institute of Volcanic Geology and GeochemistryPetropavlovsk, Kamchatka 683006, Russia Search for other works by this author on: Oxford Academic Google Scholar A. V. KOLOSKOV, A. V. KOLOSKOV 2The Institute of Volcanic Geology and GeochemistryPetropavlovsk, Kamchatka 683006, Russia Search for other works by this author on: Oxford Academic Google Scholar N. I. SELIVERSTOV, N. I. SELIVERSTOV 3The Institute of VolcanologyPetropavlovsk, Kamchatka, 683006, Russia Search for other works by this author on: Oxford Academic Google Scholar V. V. MATVENKOV V. V. MATVENKOV 4The Institute of OceanologyMoscow, 117218, Russia Search for other works by this author on: Oxford Academic Google Scholar Journal of Petrology, Volume 35, Issue 1, February 1994, Pages 163–204, https://doi.org/10.1093/petrology/35.1.163 Published: 01 February 1994 Article history Received: 06 July 1992 Accepted: 06 April 1993 Published: 01 February 1994

Abstract Major and trace element and Sr–Nd–Pb isotopic variations in mafic volcanic rocks hve been studied in a 220 km transect across the Kamchatka arc from the Eastern Volcanic Front, over the Central Kamchatka Depression to the Sredinny Ridge in the back-arc. Thirteen volcanoes and lava fields, from 110 to 400 km above the subducted slab, were sampled. This allows us to characterize spatial variations and the relative amount and composition of the slab fluid involved in magma genesis. Typical Kamchatka arc basalts, normalized for fractionation to 6% MgO, display a strong increase in large ion lithophile, light rare earth and high field strength elements from the arc front to the back-arc. Ba/Zr and Ce/Pb ratios, however, are nearly constant across the arc, which suggests a similar fluid input for Ba and Pb. La/Yb and Nb/Zr increase from the arc front to the back-arc. Rocks from the Central Kamchatka Depression range in 87Sr/86Sr from 0·70334 to 0·70366, but have almost constant Nd isotopic compositions (143Nd/144Nd 0·51307–0·51312). This correlates with the highest U/Th ratios in these rocks. Pb-isotopic ratios are mid-ocean ridge basalt (MORB)-like but decrease slightly from the volcanic front to the back-arc. The initial mantle source ranged from N-MORB-like in the volcanic front and Central Kamchatka Depression to more enriched in the back-arc. This enriched component is similar to an ocean-island basalt (OIB) source. Variations in (CaO)6·0–(Na2O)6·0 show that degree of melting decreases from the arc front to the Central Kamchatka Depression and remains constant from there to the Sredinny Ridge. Calculated fluid compositions have a similar trace element pattern across the arc, although minor differences are implied. A model is presented that quantifies the various mantle components (variably depleted N-MORB-mantle and enriched OIB-mantle) and the fluid compositions added to this mantle wedge. The amount of fluid added ranges from 0·7 to 2·1%. The degree of melting changes from ∼20% at the arc front to &amp;lt;10% below the back-arc region. The rocks from volcanoes of the northern part of the Central Kamchatka Depression—to the north of the transect considered in this study—are significantly different in their trace element compositions compared with the other rocks of the transect and their source appears to have been enriched by a component derived from melting of the edge of the ruptured slab.

Peridotite xenoliths from the Avacha volcano, Kamchatka, derived from the mantle beneath the volcanic front of the Kamchatka arc, are mainly highly depleted clinopyroxene-poor harzburgites with highly forsteritic olivine (Fo 90^92) and high Cr-number spinel (05^07). The Avacha peridotites have experienced metasomatism to various extents, with the formation of metasomatic orthopyroxene replacing primary olivine, by infiltration of SiO2-rich fluids. The metasomatic orthopyroxenes can be subdivided into two textural types; (1) radially aggregated prismatic grains (opx II-1); (2) stout grains associated with interstitial glass and metasomatic minerals (opx II-2).The Avacha peridotites exhibit light rare earth element (LREE) enrichment relative to heavy REE (HREE), even in primary lithologies with52 vol. % of metasomatic orthopyroxene. However, the metasomatism has not altered the chemical character-istics of the Avacha peridotites significantly, and the Fo contents

Detailed tephrochronological studies in Kamchatka Peninsula, Russia, permitted documentation of 24 Holocene key-marker tephra layers related to the largest explosive eruptions from 11 volcanic centers. Each layer was traced for tens to hundreds of kilometers away from the source volcano; its stratigraphic position, area of dispersal, age, characteristic features of grain-size distribution, and chemical and mineral composition confirmed its identification. The most important marker tephra horizons covering a large part of the peninsula are (from north to south; ages given in 14 C yr B.P.) SH 2 (≈1000 yr B.P.) and SH 3 (≈1400 yr B.P.) from Shiveluch volcano; KZ (≈7500 yr B.P.) from Kizimen volcano; KRM (≈7900 yr B.P.) from Karymsky caldera; KHG (≈7000 yr B.P.) from Khangar volcano; AV 1 (≈3500 yr B.P.), AV 2 (≈4000 yr B.P.), AV 4 (≈5500 yr B.P.), and AV 5 (≈5600 yr B.P.) from Avachinsky volcano; OP (≈1500 yr B.P.) from the Baraniy Amfiteatr crater at Opala volcano; KHD (≈2800 yr B.P.) from the “maar” at Khodutka volcano; KS 1 (≈1800 yr B.P.) and KS 2 (≈6000 yr B.P.) from the Ksudach calderas; KSht 3 (A.D. 1907) from Shtyubel cone in Ksudach volcanic massif; and KO (≈7700 yr B.P.) from the Kuril Lake-Iliinsky caldera. Tephra layers SH 5 (≈2600 yr B.P.) from Shiveluch volcano, AV 3 (≈4500 yr B.P.) from Avachinsky volcano, OP tr (≈4600 yr B.P.) from Opala volcano, KS 3 (≈6100 yr B.P.) and KS 4 (≈8800 yr B.P.) from Ksudach calderas, KSht 1 (≈1100 yr B.P.) from Shtyubel cone, and ZLT (≈4600 yr B.P.) from Iliinsky volcano cover smaller areas and have local stratigraphic value, as do the ash layers from the historically recorded eruptions of Shiveluch (SH 1964 ) and Bezymianny (B 1956 ) volcanoes. The dated tephra layers provide a record of the most voluminous explosive events in Kamchatka during the Holocene and form a tephrochronological timescale for dating and correlating various deposits.

Abstract. We present a general concept of mechanisms of preseismic phenomena in the atmosphere and ionosphere. After short review of observational results we conclude: 1. Upward migration of fluid substrate matter (bubble) can lead to ousting of the hot water/gas near the ground surface and cause an earthquake (EQ) itself in the strength-weakened area; 2. Thus, time and place of the bubble appearance could be random values, but EQ, geochemistry anomaly and foreshocks (seismic, SA and ULF electromagnetic ones) are casually connected; 3. Atmospheric perturbation of temperature and density could follow preseismic hot water/gas release resulting in generation of atmospheric gravity waves (AGW) with periods in a range of 6–60min; 4. Seismo-induced AGW could lead to modification of the ionospheric turbulence and to the change of over-horizon radio-wave propagation in the atmosphere, perturbation of LF waves in the lower ionosphere and ULF emission depression at the ground.

High-precision electron microprobe analyses were obtained on olivine grains from Klyuchevskoy, Shiveluch and Gorely volcanoes in the Kamchatka Arc; Irazú, Platanar and Barva volcanoes of the Central American Arc; and mid-ocean ridge basalt (MORB) from the Siqueiros Transform. Calcium contents of these subduction zone olivines are lower than those for olivines from modern MORB, Archean komatiite and Hawaii. A role for magmatic H2O is likely for subduction zone olivines, and we have explored the suggestion of earlier workers that it has affected the partitioning of CaO between olivine and silicate melt. We provide a provisional calibration of DCaOOl/L as a function of magmatic MgO and H2O, based on nominally anhydrous experiments and minimally degassed H2O contents of olivine-hosted melt inclusions. Application of our geohygrometer typically yields 3–4 wt % magmatic H2O at the Kamchatka and Central American arcs for olivines having ∼1000 ppm Ca, which agrees with H2O maxima from melt inclusion studies; Cerro Negro and Shiveluch volcanoes are exceptions, with about 6% H2O. High-precision electron microprobe analyses with 10–20 μm spatial resolution on some olivine grains from Klyuchevskoy and Shiveluch show a decrease in Ca content from the core centers to the rim contacts, and a sharp increase in Ca in olivine rims. We suggest that the zoning of Ca in olivine from subduction zone lavas may provide the first petrological record of temporal changes that occur during hydration of the mantle wedge and dehydration during ascent, and we predict olivine H2O contents that can be tested by secondary ionization mass spectrometry analysis.

The origin of the volcanic tremor is still under debate. Many theories have been proposed in the last years, but none has yet been completely accepted. In 1993, highly sensitive pressure sensors (2.175 Pa/Volt) used to monitor the explosive activity at Stromboli have revealed unexpected correlation between small spike‐shaped pressure signals (1–2 Pa) and volcanic tremor. These pressure pulses repeat regularly in time with a recurrent period of ca. 1 s. Video camera images allowed us to correlate the pressure pulses with small gas bursts occurring at one of the active vents. The striking correlation (0.971) between infrasonic and seismic energy fluctuations is particularly meaningful in the frequency domain. Infrasonic and seismic signal share the same spectral content (3 Hz) for every station within a range of 700 m around the craters. Correlations in time and frequency domain remained unaltered during the 1994 field experiments. Moreover, during 1994, the increased degassing activity has been followed by an increase in pressure release (7–8 Pa) and by a shift towards higher frequencies (8 Hz) both in the infrasonic and seismic records. Infrasonic waves and volcanic tremor show similar energy fluctuations and frequency contents, appearing therefore to be produced by the same dynamical process. On this basis, we claim that volcanic tremor at Stromboli originates by continuous outbursting of small gas bubbles in the upper part of the magmatic column.

Abstract. Active faults are those faults on which movement is possible in the future. This draws particular attention to active faults in geodynamic studies and seismic hazard assessment. Here, we present a high-detail continental-scale geodatabase: The Active Faults of Eurasia Database (AFEAD). It comprises 48 205 objects stored in shapefile format with spatial detail sufficient for a 1 : 1 000 000 map scale. The fault sense, a rank of confidence in activity, a rank of slip rate, and a reference to source publications are provided for each database entry. Where possible, this information is supplemented by a fault name, fault zone name, abbreviated fault parameters (e.g., slip rate, age of the last motion, and total offset), and text information from the sources. The database was collected from 612 published sources, including regional maps, databases, and research papers. AFEAD facilitates a spatial search for local studies. It provides sufficient detail for planning a study of a particular fault system and guides deeper bibliographical investigations. This scenario is particularly significant for vast central and northern Asian areas, where most studies are available only in Russian and hard copy. Moreover, the database model provides the basis for regional- and continental-scale integrative studies based on geographic information systems (GISs). The database is available at https://doi.org/10.13140/RG.2.2.25509.58084 (Bachmanov et al., 2022) and via web map at http://neotec.ginras.ru/index/mapbox/database_map.html (last access: 5 May 2022). Database representations and supplementary data are hosted at http://neotec.ginras.ru/index/english/database_eng.html (last access: 5 May 2022).

Structures resulting from lava and ice interaction are common at glaciated stratovolcanoes. During summit eruptions at stratovolcanoes, meltwater is produced and travels freely down steep slopes and thin permeable valley glaciers, eroding the ice and enlarging preexisting glacial drainages. As a result, eruptions in this environment have produced few catastrophic floods. Lava flowing into the open channels and voids in the glaciers becomes confined and grows thicker, filling the available space and producing steep‐sided bodies with smooth, bulbous contact surfaces. Quenching of lava against ice or by water forms small‐scale features such as tensional fractures and glass. As the amount of meltwater in contact with the lava increases, the type and abundance of smaller‐scale features become similar to those produced during subglacial eruptions into meltwater lakes. Identification of large‐ and small‐scale lava‐ice contact features in the field can be used to reconstruct paleoglacial extent and, combined with geochronology of lavas, to determine past paleoclimate. An understanding of lava‐ice interaction allows us to better assess the hazards posed by future eruptions at glaciated volcanoes.

Abstract The southern sector of Soufrière Hills Volcano failed on 26 December 1997 (Boxing Day), after a year of disturbance culminating in a devastating eruptive episode. Sector collapse produced a c. 50 x 10 6 m 3 volcanic debris avalanche, and depressurized the interior of the lava dome, which exploded to generate a violent pyroclastic density current. The south-directed growth of a lava lobe and build-up of lava-block talus, since early November 1997, brought the hydrothermally weakened sector to a condition of marginal stability. Limit-equilibrium stability analyses and finite-difference stress-deformation analyses, constrained by geomechanical testing of edifice and debris avalanche materials, suggest that the sector collapse was triggered by a pulse of co-seismic exogenous lava shear-lobe emplacement. Slip-surface localization was influenced by strain-weakening. The source region fragmented into avalanche megablocks, and further disruption generated a chaotic avalanche mixture that included variably indurated and coloured hydrothermally altered material, and much talus. The avalanche consisted of several flow pulses that reflected complexities of source disruption and channel topography. In the proximal zone, within 1.5 km from source, many megablocks preserve pre-collapse stratigraphy. At major bends the avalanche separated into channelled and overspill flows. In the distal region, &lt;2.5km from source, stacked sets of the main lithologies occur with a hummocky surface and abrupt flowage snouts, beyond which sparse hummocks occur in a thinly spread deposit. Textures suggest emplacement by laminar mass transport of partly saturated debris riding on a frictionally sheared base. Three-dimensional numerical simulations of emplacement governed by a Coulomb-type (Pouliquen) basal friction law imply low values of friction (&gt; 15°), consistent with geotechnical test data and the localized presence of pore-water pressures. The best-fit model suggests an emplacement time &gt;3 minutes and a typical maximum velocity of about 40ms _1 , which are consistent with field estimates.

Abstract Abundant peridotite xenoliths have been found in pyroclasitics of Avacha (Avachinsky) volcano, the south Kamchatka arc, Russia. They are mostly refractory harzburgite with or without clinopyroxene: the Fo of olivine and Cr/(Cr + Al) atomic ratio of spinel range from 91 to 92 and from 0.5 to 0.7, respectively. They are metasomatized to various extents, and the metasomatic orthopyroxene has been formed at the expense of olivine. The metasomatic orthopyroxene, free of deformation and exsolution, is characterized by low contents of CaO and Cr 2 O 3 . The complicated way of replacement possibly indicates low viscosity of the metasomatic agent, namely hydrous fluids released from the relatively cool slab beneath the south Kamchatka arc. This is a good contrast to the north Kamchatka arc, where the slab has been hot enough to provide slab‐derived melts. High content of total orthopyroxene, 40 vol% on average, in metasomatized harzburgite from Avacha suggests silica enrichment of the mantle wedge, and is equivalent to some subcratonic harzburgite. Some subcratonic harzburgites therefore could have been formed by transportation of subarc metasomatized peridotites to a deeper part of the upper mantle.

The Kurile-Kamchatka Late Cenozoic volcanic rocks can be categorized as magmatic series of island arc and within-plate geochemical types, respectively. These types clearly can be identified on geochemical discriminant diagrams. Across-arc geochemical, mineralogical, and Sr-Nd-isotope zoning is characteristic of island-arc volcanics, but is not found for within-plate volcanics. Such zoning probably is the result of modification of fluid-phase composition, which originates in the underlying subducting plate (as a result of dehydration of water-bearing secondary minerals) and rises into the zone of island-arc magma generation in the mantle wedge. Along-arc Sr, Be, H., and 0 isotopic zoning is noted for island-arc volcanics. Probably it is associated with various levels of island-arc magma contamination. 10Be data for modern island-arc lavas attest to the fact that young pelagic sediments from the subducting plate participate in island-arc magma genesis. Within-plate lavas are found in the region north of Avacha Bay. They may precede island-arc volcanics (East Kamchatka), coexist with them during the late stage of volcanic activity (Central Range), or be completely unrelated to island-arc volcanics in the far rear arc (West Kamchatka). The appearance of within-plate lavas probably is connected with deep faults that accompanied the formation of the northern Kurile-Kamchatka trench and the associated new subduction zone in this part of the island-arc system, an event caused by the accretion of the Kronotskiy terrane to eastern Kamchatka in Middle Miocene time.

This paper describes the technique used to create and maintain the Active Faults of Eurasia Database (AFED) based on the uniform format that ensures integrating the materials accumulated by many researchers, inclu­ding the authors of the AFED. The AFED includes the data on more than 20 thousand objects: faults, fault zones and associated structural forms that show the signs of latest displacements in the Late Pleistocene and Holocene. The geographical coordinates are given for each object. The AFED scale is 1:500000; the demonstration scale is 1:1000000. For each object, the AFED shows two kinds of characteristics: justification attributes, and estimated attributes. The justification attributes inform the AFED user about an object: the object’s name; morphology; kinematics; the amplitudes of displacement for different periods of time; displacement rates estimated from the amplitudes; the age of the latest recorded signs of activity, seismicity and paleoseismicity; the relationship of the given objects with the parameters of crustal earthquakes; etc. The sources of information are listed in the AFED appendix. The estimated attributes are represented by the system of indices reflecting the fault kinematics according to the classification of the faults by types, as accepted in structural geology, and includes three ranks of the Late Quaternary movements and four degrees of reliability of identifying the structures as active ones. With reference to the indices, the objects can be compared with each other, considering any of the attributes, or with any other digitized information. The comparison can be performed by any GIS software. The AFED is an efficient tool for obtaining the information on the faults and solving general problems, such as thematic mapping, determining the parameters of modern geodynamic processes, estima­ting seismic and other geodynamic hazards, identifying the tectonic development trends in the Pliocene–Quaternary stage of the Earth's development, etc. The Active Faults of Eurasia Database is created in the format providing for inputs of new information, as well the database updating and revision.

Ultramafic xenoliths of spinel dunite, harzburgite, lherzolite, amphibole/phlogopite‐bearing pyroxenite, and clinopyroxenite occur in andesitic pyroclastic debris from the 1964 eruption of Shiveluch volcano, Kamchatka. Peridotites have coarse/protogranular, porphyroclastic, and granuloblastic textures, abundant kink‐banded olivine, and refractory mineral compositions with forsteritic olivine (Fo 88–94 ) and Cr‐rich spinel (100*Cr/Cr + Al = 47–83). Orthopyroxene (opx) is also Mg‐rich but occurs only as a fibrous mineral present along olivine grain boundaries, in monomineralic veins that crosscut coarse olivine, and in veins with amphibole and phlogopite that crosscut coarse‐grained peridotites. Textural evidence and mineral compositions indicate that the peridotites and hydrous pyroxenites were replacive dunites that formed by melt‐rock reactions involving the dissolution of pyroxene and precipitation of olivine. The fibrous opx and millimeter‐scale veins of phlogopite, amphibole, and opx are interpreted as the autometasomatic products of hydrous magmas that were trapped in the uppermost mantle (&lt;45 km). In this interpretation, opx was produced by reaction between late‐stage, silica‐rich, hydrous fluids/melts and olivine in the dunite protolith, and the millimeter‐scale veins of phlogopite, amphibole, and opx are the volatile‐enriched, deuteric products that were liberated during the final stages of magma crystallization. The absence of textural equilibrium suggests that the late‐stage replacement process which produced the fibrous opx occurred shortly prior to the eruption that carried the xenoliths to the surface. On the basis of two‐pyroxene thermometry and Ca‐in‐olivine barometry, the xenoliths equilibrated between 800 and 1000°C and 1.03 and 2.21 GPa. This implies that the xenoliths were carried from sub‐Moho depths, a result consistent with geophysical estimates of crustal thickness. Olivine‐opx‐spinel equilibria indicate that the xenoliths are strongly oxidized with fO 2 from +1.4–2.6 log units above the fayalite‐magnetite‐quartz (ΔFMQ) buffer in peridotites, +1.7–2.3 ΔFMQ in hydrous pyroxenites, and +2.4–3.3 ΔFMQ in cumulate clinopyroxenites. High fO 2 in the peridotites is attributed to the melt‐rock reactions that formed the dunite protolith. These results therefore suggest that interaction between oxidized melts and peridotite wall rock at shallow mantle depths plays a significant role in creating and modifying the uppermost mantle and deepest crust in some subduction settings.

The 15 November 2006 Kuril earthquake (Mw 8.1-8.4) and tsunami enabled us to collect a compelling data set of coastal geomorphic change in the Kuril Islands from ~3 months before to 9 (and 21) months after the tsunami. Our pre-tsunami and post-tsunami surveys of the islands, including four topographic profi les measured in 2006 and reoccupied in 2007, allow us the confi dence to attribute many changes to the tsunami, in spite of an absence of eyewitness accounts in the central islands. Areas with low runup, <8 m, underwent limited geomorphic change, primarily confi ned to beach or stream channels. Regions with high runup, >15 m, underwent massive erosion that dramatically altered the coastline. Tsunami deposits roughly corresponded with the extent of tsunami runup and inundation. The amount of sediment eroded by the tsunami far outweighed the amount deposited on land in all cases studied. The tsunami was dominantly erosive in the Kuril Islands because the high-relief topography of the coastline accelerated tsunami outfl ow.

Envelopes of scalar waves are simulated at various distances from an instant point source embedded in a random uniformly scattering medium by means of direct Monte-Carlo modelling of wave-energy transport. Three types of scattering radiation pattern ('indicatrix') are studied, for media specified by (1) a Gaussian autocorrelation function of inhomogeneities, (2) a power-law ('fractal', k−α) inhomogeneity spectrum and (3) the mix of case (1) and the isotropic indicatrix (very small + large inhomogeneities). We look for a model that can qualitatively reproduce the two most characteristic features of real S-wave envelopes of near earthquakes, namely (1) the broadening of the ‘direct’ wave group with distance and (2) the monotonously decaying shape of the coda envelope that does not deviate strongly from that expected in the isotropic scattering case. Both properties are observed for any band over a wide frequency range (1–40 Hz). The well-studied isotropic scattering model realistically predicts the appearance of codas but fails to predict pulse broadening. The model of large-scale inhomogeneity realistically predicts the mode of pulse broadening but fails to predict codas. We have found that, for a particular frequency band, within each class of inhomogeneity studied, both requirements can be qualitatively satisfied by a certain choice of parameters. In the Gaussian-ACF case, however, this match can be obtained only for a narrow frequency range. In contrast, the fractal case (with a value of exponent a of about 3.5–4) reproduces qualitatively the observed wide-band behaviour, and we consider it a reasonable representation of the gross properties of the earth medium.

A total of 5270 shallow and intermediate-depth earthquakes recorded by the 32 stations of the regional seismic network of the Geophysical Service of Russia are used to assess the P-wave velocity structure beneath the Kamchatka peninsula in the Western Pacific. The tomographic inversion is carried out in three steps. First, a 1-D tomographic problem is solved in order to obtain an initial velocity model. Based on the 1-D velocity model, 3-D tomographic inversions with homogeneous and heterogeneous starting models are obtained. The Conrad (15 km depth) and Moho (35 km depth) discontinuities determined from the 1-D tomographic inversion, and the upper boundary of the subducting slab are taken into account in the heterogeneous starting model for the traveltimes and ray-path determinations. Both velocity structure and hypocentral locations are determined simultaneously in the inversion. The spacing of the grid nodes is a half-degree in the horizontal direction and 20–50 km in the vertical direction. A detailed P-wave tomographic image is determined down to a depth of 200 km. The resulting tomographic image has a prominent low-velocity anomaly that shows a maximum decrease in P-wave velocity of approximately 6 per cent at 30 km depth beneath a chain of active volcanoes. At depth, low-velocity anomalies are also observed in the mantle wedge extending down to a depth of approximately 150 km. These anomalies are apparently associated with the volcanic activity. The sedimentary basin of the Central Kamchatsky graben, to the west of the volcanic front, and the accretionary prism at the trench correlate with shallow low-velocity anomalies. High-velocity anomalies observed at a depth of 10 km may be associated with the location of metamorphic basements in the Ganalsky–Valaginskoe uplift and upper crust of Shipunsky cape. The results also suggest that the subducted Pacific plate has P-wave velocities approximately 2–7 per cent higher than those of the surrounding mantle and a thickness of approximately 70 km.

Spatial geochemical variations of Quaternary lavas erupted along the northern segment of the Kamchatka arc are used to trace changes in magma generation across the subducting Pacific slab edge. The late Pleistocene–Holocene lavas of the northern end of the Central Kamchatka depression north of the Pacific slab edge show strong enrichment in high field strength elements and light rare earth elements, relatively unradiogenic strontium and lead but radiogenic neodymium isotope ratios, and oxygen isotope compositions similar to those of mid-oceanic-ridge basalts. These geochemical characteristics are distinct from the southern Central Kamchatka depression volcanoes located above the subducting Pacific slab. Extensive fluid-triggered mantle melting dominates magma genesis beneath the largest Kamchatka volcanoes in the south, whereas low-degree decompression melting of the Pacific asthenospheric mantle is the major magma generation process north of the Pacific slab edge. Quaternary detachment of the subducted Pacific plate fragment resulted in the influx of fertile mantle beneath Kamchatka. We propose that upwelling and southward flow of this hotter, more fertile mantle is the main reason for recent magmatism in northern Kamchatka and for the exceptional productivity of the Central Kamchatka depression volcanoes (Klyuchevskoy and Sheveluch), the most active arc volcanoes on Earth.

We analyse daily cross-correlation computed from continuous records by permanent stations operating in vicinity of the Klyuchevskoy group of volcanoes (Kamchatka). Seismic waves generated by volcanic tremors are clearly seen on the cross-correlations between some pairs of stations as strong signals at frequencies between 0.2 and 2 Hz and with traveltimes typically shorter than those corresponding to interstation propagation. First, we develop a 2-D source-scanning algorithm based on summation of the envelops of cross-correlations to detect seismic tremors and to determine locations from which the strong seismic energy is continuously emitted. In an alternative approach, we explore the distinctive character of the cross-correlation waveforms corresponding to tremors emitted by different volcanoes and develop a phase-matching method for detecting volcanic tremors. Application of these methods allows us to detect and to distinguish tremors generated by the Klyuchevskoy and the Tolbachik, volcanoes and to monitor evolution of their intensity in time.