V p/V s-ratios from the central Kolbeinsey Ridge to the Jan Mayen Basin,North Atlantic; implications for lithology,porosity and present-day stress field

The horizontal components from twenty Ocean Bottom Seismometers deployed along three profiles near the Kolbeinsey Ridge, North Atlantic, have been modelled with regard to S-waves, based on P-wave models obtained earlier. Two profiles were acquired parallel to the ridge, and the third profile extended eastwards across the continental Jan Mayen Basin. The modelling requires a thin (few 100 m) layer with very high V _p/V _s-ratio (3.5–9.5) at the sea-floor in the area lacking sedimentary cover. The obtained V _p/V _s-ratios for the remaining part of layer 2A, 2B, 3 and upper mantle, correspond to the following lithologies: pillow lavas, sheeted dykes, gabbro and peridotite, respectively. All crustal layers exhibit a decreasing trend in V _p/V _s-ratio away-from-the-axis, interpreted as decreasing porosity and/or crack density in that direction. A significant S-wave azimuthal anisotropy is observed within the thin uppermost layer of basalt near the ridge. The anisotropy is interpreted as being caused by fluid-filled microcracks aligned along the direction of present-day maximum compressive stress, and indicates crustal extension at the ridge itself and perpendicular-to-the-ridge compression 12 km off axis. Spreading along the Kolbeinsey Ridge has most likely been continuous since its initiation ca. 25 Ma: The data do not suggest the presence of an extinct spreading axis between the Kolbeinsey Ridge and the Aegir Ridge as has been proposed earlier. The V _p/V _s-ratios found in the Jan Mayen Basin are compatible with continental crust, overlain by a sedimentary section dominated by shale.     

Refinement of the Micro‐Distillation Technique for Isotopic Analysis of Geological Samples with pg‐Level Osmium Contents

In recent years, the ¹⁸⁷Re–¹⁸⁷Os isotope system has been increasingly used to study samples containing very small quantities of Os. For such samples, optimisation of measurement procedures is essential to minimise the loss of Os before mass spectrometric measurements. Micro‐distillation is a necessary purification step that is applied after the main Os chemical separation procedure, prior to Os isotope ratio measurements by negative‐thermal ionisation mass spectrometry (N‐TIMS). However, unlike the other separation steps, this procedure has not yet been optimised for small samples. In this study, we present a refined micro‐distillation method that achieved higher yields and allowed high‐precision R(¹⁸⁷Os/¹⁸⁸Os) expressed as ¹⁸⁷Os/¹⁸⁸Os measurements for small‐sized geological samples that contain only a few pg Os. The Os recovery in the micro‐distillation step was tested by changing the operating conditions including heating time and temperature, and amounts of oxidant and reductant. Recoveries were measured by the isotope dilution ICP‐MS method after the addition of ¹⁹⁰Os‐enriched spike solution. We found that the most critical factor controlling the chemical yield of Os during micro‐distillation is the extent of dilution of the reductant (HBr) by H₂O evaporated from the oxidant. A refined micro‐distillation method, in which the amount of oxidant solution is reduced from the conventional method, achieved an improved chemical yield of Os (~ 90%). This refined method was applied to the measurement of ¹⁸⁷Os/¹⁸⁸Os by N‐TIMS of varying test portions of the geological reference material BIR‐1a. The resulting ¹⁸⁷Os/¹⁸⁸Os ratios of BIR‐1a matched the literature data, with propagated uncertainties of 0.2, 1.1 and 11% digested sample quantities containing 150, 10 and 1 pg of Os, respectively.     

Anomalous Rayleigh-Wave Propagation Along Oceanic Trench

A clear later phase of amplitude larger than the direct surface wave packet was observed at stations in Hokkaido, Japan, for several events of the December 1991 off-Urup earthquake swarm in the Kuril Islands region. From its particle motion, this phase is likely to be a fundamental Rayleigh wave packet that arrived with an azimuth largely deviated from each great-circle direction. As its origin, Nakanishi (1992) proposed that the sea-trench topography in this area as deep as 10 km may produce a narrow zone of low velocity for Rayleigh waves of periods around 15 sec. Following this idea, we compute ray paths and estimate how Rayleigh waves would propagate if we assume that lateral velocity variations are caused only by seafloor topography. We confirm that thick sea water in the trench indeed produces the phase velocity of Rayleigh waves to be smaller than in a surrounding area by the degree over 100%. Such a low-velocity zone appears only in a period range from 12 to 20 sec. Although this strong low-velocity zone disturbs the direction of Rayleigh wave propagation from its great circle, the overall ray paths are not so affected as far as an epicentre is outside this low-velocity zone, that is, off the trench axis. In contrast, the majority of rays are severely distorted for an event within the low-velocity zone or, in other words, in the neighborhood of the trench axis. For such an event, a part of wave energy appears to be trapped in this zone and eventually propagates outwards due to the curvature or bend of trench geometry, resulting in very late arriving waves of large amplitude with an incident direction clearly different from great circles. This phenomenon is observed only at a very limited period range around 16 sec. These theoretical results are consistent with the above mentioned observation of Nakanishi (1992).     

Numerical experiment for strontium-90 and cesium-137 in the Japan Sea

A numerical experiment is carried out to reproduce distribution of concentration of ⁹⁰Sr and ¹³⁷Cs, estimate their total amount and verify their source in the Japan Sea. Model results are in good agreement with observational findings in the Japan Sea expeditions between 1997 and 2002 by the Japan Atomic Energy Agency. Vertical profiles of the concentration of ⁹⁰Sr and ¹³⁷Cs show exponential decreases with depth from the sea surface to the sea bottom. From the model and observational results, it is suggested that the concentration of ⁹⁰Sr and ¹³⁷Cs in the surface layer is approximately in the range of 1.0–1.5 Bq/m³ and 2.0–2.5 Bq/m3, respectively. On the other hand, it is found that the concentration in the intermediate and deep layer is higher than that observed in the northwestern Pacific Ocean, suggesting active winter convection in the Japan Sea. The total amount of ⁹⁰Sr and ¹³⁷Cs in the seawater is evaluated to be 1.34 × 10¹⁵ Bq and 2.02 × 10¹⁵ Bq, respectively, in the numerical experiment, which demonstrates an estimation by observational data obtained in the Japan Sea expeditions. The total amount of ⁹⁰Sr and ¹³⁷Cs changed during the second half of 20th century corresponding to deposition at the sea surface with the maximums of 4.86 × 10¹⁵ Bq for ⁹⁰Sr and 7.33 × 10¹⁵ Bq for ¹³⁷Cs, respectively, in the mid-1960s. The numerical experiment suggests that the main source of ⁹⁰Sr and ¹³⁷Cs has been global fallout, although there have been some potential sources in the Japan Sea.     

An extended theory of equi-risk line for a general release rule

The expression of equi-risk line derived by the authors represents the relationship between discharge capacityy ₀ ^u and storage capacityz ₀ ^u to keep flood frequency under a certain risk level represented by the return periodT, i.e.,z ₀/z ₀ ^u ={(y ₀ ^u –y ₀ ^u )/y ₀ ^u } ^S , wherey ₀ ^u andz ₀ ^u areT-year probability peak discharge and total volume of a hydrograph. The shape parametersS is evaluated in this paper for various release rules of the storage facilities and correlations of durations and peaks of hydrographs. The expression forS is: , whereS ₀ andS are the values ofS forp=0 and , andp is the exponent of a general storage-release relation,q=az' ^p, wherea is the storage constant, andz' andq are the volume of stored water and the corresponding release. The values ofS ₀ andS are expressed in terms of the correlation coefficient of durations and peaks of inflow hydrographs.     

Direct ascent to the surface of asthenospheric magma in a region of convex lithospheric flexure

The stress field of oceanic lithosphere controls the distribution of submarine petit-spot volcanoes. However, the eruption sites of these petit-spot volcanoes are considered to be limited to concavely flexed regions of lithosphere off the outer rise. Here, we present new data for a recently identified petit-spot lava field on a convexly flexed section of the lithosphere adjacent to the subduction zone offshore of northeast Japan in an area containing more than 80 volcanoes. This area is marked by strongly alkaline lavas that were erupted on the convexly flexed region. As for the concavely flexed region where the petit-spots previously reported, the base of the lithosphere beneath the eruption sites is under extension, whereas the upper part of the lithosphere is under compression. This change in the stress field, from the lower to upper lithosphere, causes ascending dikes to stall in the mid-lithosphere, leading to metasomatic interaction with the surrounding peridotite. The new geochemical data of rocks and xenocrysts presented in this study indicate that strongly alkaline magmas erupted on the convexly flexed region would have ascended more rapidly through the mid-depth of lithosphere because of the extensional regime of the upper lithosphere and decreasing the degree of metasomatic reaction with the surrounding mantle peridotite. The results indicate that the degree of metasomatism and the compositional variations of petit-spot magmas are controlled mainly by the stress field of the lithosphere.