Geochemistry of newly discovered quaternary Shiribeshi Volcano,Northeast Japan Sea

Summary Shiribeshi Seamount is located to the east of the Okushiri Ridge, in the northeast Japan Sea. Whole rock K-Ar age of olivine-augite andesite dredged from the Seamount was determined to be 0.9 ± 0.2 Ma (Tsuchiya et al., 1989), indicating that Shiribeshi Seamount is a Quaternary volcano in the back-arc region off the junction of the Northeast Japan and Kurile arcs. Shiribeshi volcano is composed of basalt to rhyolite, which show a typical island arc calc-alkaline nature on the basis of petrographical characteristics of 95 samples dredged from four sites. Abundances of incompatible elements including K, Rb, Sr, Nb, P, Ti, Y and Zr in 16 representative rocks are discussed, together with those in the Quaternary volcanic rocks from the NE Japan and Kurile arcs in terms of compositional variation across the arcs. The estimated composition of the primary magma of Shiribeshi volcano is characterized by higher incompatible element contents and a higher Zr/Y ratio than primary magmas in the volcanic front side. Based on HFS element concentrations the degree of partial melting for three primary magmas of Oshima-Oshima, Shiribeshi and Rishiri volcanoes in the northeast Japan Sea may decrease gradually with increasing distance from the volcanic front. However, LIL element contents, especially K and Rb are lower in the primary magma of Rishiri volcano located far from the volcanic front than in the remaining two primary magmas, which would imply that LIL/HFS ratios (or degree of contribution of LIL elements originating from the subducted oceanic crust) become minimal at Rishiri volcano. One basalt and three andesites from Shiribeshi volcano have the restricted range of low⁸⁷Sr/⁸⁶Sr ratios of 0.70297–0.70300, which indicates that the magma source for Shiribeshi volcano may be slightly more enriched in Sr isotopic compositions than theN-type MORB source.

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Geochemie des neuentdeckten quartären submarinen Vulkans Shiribeshi im Nordöstlichen Japanischen Meer

Zusammenfassung Der Shiribeshi Seamount liegt östlich des Okushiri Rückens im nordöstlichen Japanischen Meer. Gesamtgesteins K-Ar Alter von Olivin-Augit-Andesiten von diesem Seamount ergeben Werte von 0,9 ± 0,2 Ma (Tsuchiya et al., 1989), und weisen darauf hin, daß Shiribeshi ein quartärer Vulkan im back-arc Bereich nahe dem Kreuzungspunkt des nordostjapanischen und des Kurilen-Inselbogens ist. Er besteht aus Gesteinen, deren Zusammensetzung von Basalt bis Rhyolit reicht. Petrographische Daten von 25 Proben, die von vier submarinen Lokationen durch Dredging aufgesammelt wurden, weisen auf eine typische kalk-alkalische Inselbogenzusammensetzung hin. Die Verbreitung von inkompatiblen Elementen, die unter anderem K, Rb, Sr, Nb, P, Ti, Y und Zr umfassen, in 16 representativen Proben wird zusammen mit denen von quartären vulkanischen Gesteinen aus dem nordöstlichen Japanischen und den Kurilen-Inselbogen diskutiert; dabei wird Variationen der Zusammensetzung über die Bögen hinweg besondere Beachtung geschenkt. Die so ermittelte Zusammensetzung des primären Magmas des Shiribeshi Vulkans wird durch höhere inkompatible Elementgehalte und höhere Zr/Y Verhältnisse charakterisiert, wenn man sie mit primären Magmen an der vulkanischen Stirn des Inselbogens vergleicht. HFS Element-Konzentrationen lassen erkennen, daß der Grad teilweiser Aufschmelzung für drei primäre Magmen von Oshima-Oshima, Shiribeshi und Rishiri im nordöstlichen Japanischen Meer graduell mit zunehmender Entfernung von der vulkanischen Stirm abnimmt. Die Gehalte an LIL Elementen und besonders an K und Rb sind in dem primären Magma des Rishiri Vulkans, der weit von der vulkanischen Front entfernt liegt, höher als in den zwei anderen primären Magmen. Dies weist darauf hin, daß LIL/HFS Verhältnisse (oder der Beitrag von LIL Elementen, die aus subduzierter ozeanischer Kruste stammen) am Rishiri Vulkan ein Minimum erreichen. Ein Basalt und drei Andesite von Shiribeshi zeigen⁸⁷Sr/⁸⁶Sr Verhältnisse von 0,70297 bis 0,70300; dies läßt erkennen, daß die Quelle des Magmas für Shiribeshi etwas mehr an⁸⁷Sr angereichert war, als dieN-Typ Quelle.

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With 7 Figures     

Space-time distribution patterns of destructive earthquakes in the inner belt of central Japan: Activity intervals and locations of earthquakes

Based on a block structure model of the inner belt of central Japan, an examination was conducted of the space-time distribution patterns of destructiv magnitudes M 6.4 or greater (M =Japan Meteorological Agency Scale). The distribution patterns revealed a periodicity in earthquake activit seismic gaps. Major NW—SE trending left-lateral active faults divide the inner belt of central Japan into four blocks, 20–80 km wide. The occurrenc A.D. with M ≥ 6.4, which have caused significant damage, were documented in the inner belt of central Japan. The epicenters of these earthquakes close to the block boundaries.

Using the relationship between the magnitude of earthquakes which occurred in the Japanese Islands and the active length of faults that generated them, movement is calculated for each historical earthquake. Space—time distributions of earthquakes were obtained from the calculated lengths, the latitud of generation. When an active period begins, a portion or segment of the block boundary creates an earthquake, which in turn appears on the ground surf active period ends when the block boundary generates earthquakes over the entire length of the block boundary without overlapping.

Five seismic gaps with fault lengths of 20 km or longer can be found in the inner belt of central Japan. It is predicted that the gaps will generate ea magnitudes of 7.0. These data are of significance for estimating a regional earthquake risk over central Japan in the design of large earthquake resist

The time sequences of earthquakes on the block boundaries reveal a similar tendency, with alternating active periods with seismic activity and quiet pe activity. The inner belt of central Japan is now in the last stage of an active period. The next active period is predicted to occur around 2500 A.D.     

Source models for the 1993 Hokkaido Nansei-Oki earthquake tsunami

A source model for the 1993 Hokkaido Nansei-Oki tsunami must satisfy certain conditions. Such conditions are presented in this paper, and two methods are used to determine the best source model for this event. A trial-and-error method selects DCRC-17a as the best among 24 different models. This model has three fault planes dipping westward. To reproduce well the tide gauge records at two locations, an inversion analysis is used to modify the dislocation of DCRC-17a.     

∼3,850 Ma tonalites in the Nuuk region,Greenland: geochemistry and their reworking within an Eoarchaean gneiss complex

The Eoarchaean (>3,600 Ma) Itsaq Gneiss Complex of southern West Greenland is dominated by polyphase orthogneisses with a complex Archaean tectonothermal history. Some of the orthogneisses have c. 3,850 Ma zircons, and they vary from rare single phase metatonalites to more common complexly banded migmatites. This is due to heterogeneous strain, in situ anatexis and granitic veining superimposed during younger tectonothermal events. In the single-phase tonalites with c. 3,850 Ma zircon, oscillatory-zoned prismatic zircon is all 3,850 Ma old, but shows patchy ancient loss of radiogenic Pb. SHRIMP spot analyses and laser ablation ICP-MS depth profiling show that thin (usually < 10 μm) younger (3,660–3,590 Ma and Neoarchaean) shells of lower Th/U metamorphic zircon are present on these 3,850 Ma zircons. Several samples with this simple zircon population occur on islands near Akilia. In contrast, migmatites usually contain more complex zircon populations, with often more than one generation of igneous zircon present. Additional zircon dating of banded gneisses across the Complex shows that samples with c. 3,850 Ma igneous zircon are not just a phenomenon restricted to Akilia and adjacent islands. For example, migmatites from Itilleq (c. 65 km from Akilia) contain variable amounts of oscillatory-zoned 3,850 Ma and 3,650 Ma zircon, interpreted, respectively, as the rock age and the time of crustal melting under Eoarchaean metamorphism. With only 110–140 ppm Zr in the tonalites and likely magmatic temperatures of >850°C, zircon solubility–melt composition relationships show that they were only one-third saturated in zircon. Any zircon entrained in the precursor magmas would thus have been highly soluble. Combined with the cathodoluminesence imaging, this demonstrates that the c. 3,850 Ma oscillatory zoned zircon crystallised out of the melt and hence gives a magmatic age. Thus the rare well-preserved tonalites and palaeosome in migmatites testify that c. 3,850 Ma quartzo–feldspathic rocks are a widespread (but probably minor) component in the Itsaq Gneiss Complex. C. 3,850 Ma zircon with negative Eu anomalies (showing growth in felsic systems) also occurs as detrital grains in rare c. 3,800 Ma metaquartzites and as inherited grains in some 3,660 Ma granites (sensu stricto). These demonstrate that still more c. 3,850 Ma rocks were present, but were recycled into Eoarchaean sediments and crustally derived granites. The major and trace element characteristics (e.g. LREE enrichment, HREE depletion, low MgO) of the best-preserved c. 3,850 Ma rocks are typical of Archaean TTG suites, and thus argue for crust formation processes involving important contributions from melting of hydrated mafic crust to the earliest Archaean. Five c. 3,850 Ma tonalites were selected as the best preserved on the basis of field criteria and zircon petrology. Four of these samples have overlapping initial ɛ_Nd (3,850 Ma) values from +2.9 to +3.6± 0.5, with the fourth having a slightly lower value of +0.6. These data provide additional evidence for a markedly LREE-depleted early terrestrial mantle reservoir. The role of c. 3,850 Ma crust should be considered in interpreting isotope signatures of the younger (3,800–3,600 Ma) rocks of the Itsaq Gneiss Complex. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Logic-tree Approach for Probabilistic Tsunami Hazard Analysis and its Applications to the Japanese Coasts

For Probabilistic Tsunami Hazard Analysis (PTHA), we propose a logic-tree approach to construct tsunami hazard curves (relationship between tsunami height and probability of exceedance) and present some examples for Japan for the purpose of quantitative assessments of tsunami risk for important coastal facilities. A hazard curve is obtained by integration over the aleatory uncertainties, and numerous hazard curves are obtained for different branches of logic-tree representing epistemic uncertainty. A PTHA consists of a tsunami source model and coastal tsunami height estimation. We developed the logic-tree models for local tsunami sources around Japan and for distant tsunami sources along the South American subduction zones. Logic-trees were made for tsunami source zones, size and frequency of tsunamigenic earthquakes, fault models, and standard error of estimated tsunami heights. Numerical simulation rather than empirical relation was used for estimating the median tsunami heights. Weights of discrete branches that represent alternative hypotheses and interpretations were determined by the questionnaire survey for tsunami and earthquake experts, whereas those representing the error of estimated value were determined on the basis of historical data. Examples of tsunami hazard curves were illustrated for the coastal sites, and uncertainty in the tsunami hazard was displayed by 5-, 16-, 50-, 84- and 95-percentile and mean hazard curves.     

Long-term climato-limnological cycles found in a 3.5-million-year continental record

Analysis of physical properties in long sediment cores (BDP96) from Academician Ridge in Lake Baikal indicates that major climato-limnological changes during the past 3.5 Myr occurred at about 2.5–2.8, 1.7–1.9, and 0.9–1.2 Ma, which were close to times of major geomagnetic polarity reversals (Matuyama/Gauss, Olduvai, Jaramillo + Matuyama/Brunhes). The principal climato-limnological oscillation has a long-term period of nearly 1,000 kyr, which corresponds to the periodicity of fluctuation in solar insolation. It also seems to be related to geomagnetic field intensity. Other long-term period of 400 kyr corresponds to Milankovitch parameters of eccentricity. These results suggest that changes in solar insolation were closely related to long-term environmental variations in the deep continental interior.     

Climato-limnological signals during the past 260 000 years in physical properties of bottom sediments from Lake Baikal

The St.16 core obtained from the Academician Ridge of Lake Baikal in eastern Siberia may span about 260 000 years, and some physical properties of the core samples are closely related to aquatic paleoproductivity and climatic change. The median of grain size, grain density, and water content fluctuate synchronously. They also are connected with change in the abundance of biogenic silica (diatoms). The physical parameters indicate that there were high aquatic productivity periods around interglacial periods (MIS 5 and 7; 70 000-125 000 yr B.P. and 180 000-250 000 yr B.P.). Comparatively large clastics were transported from outside of the lake through various routes (ice rafting, etc.) in addition to fluvial routes during the glacials or 'stadials. There are ca. 20 000 yr, 40 000 yr and 100 000 yr periods in the variations of physical properties. These are related to the three Milankovitch parameters of solar insolation.     

Groundwater-level Changes Due to Pressure Gradient Induced by Nearby Earthquakes off Izu Peninsula, 1997

Anomalous water level changes were observed at two wells associated with seismic swarm activity off Izu Peninsula on March, 1997. These are coseismic water level drops followed by gradual postseismic water level rise at the time of large earthquakes during the swarm activity. The post-seismic water level rises, which can be fitted by an exponential function with a time constant of about six hours, are explained in terms of the horizontal pressure diffusion due to the pressure gradient in the aquifer induced by the coseismic static strain.     

Diffuse Emission of CO2 from Showa-Shinzan, Hokkaido, Japan: A Sign of Volcanic Dome Degassing

Two soil CO₂ efflux surveys were carried out in September 1999 and June 2002 to study the spatial distribution of diffuse CO₂ degassing and estimate the total CO₂ output from Showa-Shinzan volcanic dome, Japan. Seventy-six and 81 measurements of CO₂ efflux were performed in 1999 and 2002, respectively, covering most of Showa-Shinzan volcano. Soil CO₂ efflux data showed a wide range of values up to 552 g m^-2 d^-1. Carbon isotope signatures of the soil CO₂ ranged from -0.9‰ to -30.9‰, suggesting a mixing between different carbon reservoirs. Most of the study area showed CO₂ efflux background values during the 1999 and 2002 surveys (B = 8.2 and 4.4 g m^-2 d^-1, respectively). The spatial distribution of CO₂ efflux anomalies for both surveys showed a good correlation with the soil temperature, indicating a similar origin for the extensive soil degassing generated by condensation processes and fluids discharged by the fumarolic system of Showa-Shinzan. The total diffuse CO₂ output of Showa-Shinzan was estimated to be about 14.0–15.6 t d^-1 of CO₂ for an area of 0.53 km².