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  • 1
    Electronic Resource
    Electronic Resource
    S-wave anisotropy off Lofoten, Norway, indicative of fluids in the lower continental crust? (1995)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 120 (1995), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: P- and S-wave velocity models along two 145 and 175 km long perpendicular profiles on the continental shelf off Lofoten, northern Norway, have been obtained from a study of nine three-component ocean-bottom seismographs. The S-wave model has been achieved from a study of the high-quality, horizontal component data. On one OBS the S-wave reflection from the Moho can be followed almost continuously from vertical to wide-angle incidence. This unique observation indicates the presence of about 10 per cent S-wave anisotropy in the lower crust along the NE-SW profile. No such anisotropy is observed along the NW-SE profile, and it is suggested that the inferred anisotropy might be caused by liquid-filled microcracks or pores aligned vertically along this profile. The vertical NW-SE alignment of the microcracks/pores might be a result of the present-day stress-field (the maximum compressive stress trends NW-SE), or it might be influenced by recent ductile strain fabrics or by ductile strain fabrics inherited from earlier deformation episodes. Another possible explanation for the inferred anisotropy might be alignment of anisotropic minerals. Many highly anisotropic minerals like olivine, pyroxene and hornblende can be excluded in this case, since the inferred S-wave anisotropy is at least three times higher than the P-wave anisotropy. Alignment of the mineral kyanite. however, might possibly explain the observations, since this mineral has much higher S-wave anisotropy than P-wave anisotropy. The hypothesis involving fluids is preferred since the very high seismic velocities of kyanite do not seem to be compatible with the estimated lower crustal velocity in this area (VP= 6.8 km s-1).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1995.tb05912.x
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  • 2
    Electronic Resource
    Electronic Resource
    Seismic Refraction Crustal Study Along the Sognefjord, South-West Norway, Employing Ocean-Bottom Seismometers (1994)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 119 (1994), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: This paper presents a crustal structure model derived from an extensive ocean-bottom seismographic refraction experiment conducted on a 150km profile line along the Sognefjord, south-west Norway. the main part of the profile was located in the Western Gneiss Region (WGR) which is characterized by deformation and metamorphic overprinting during the Caledonian Orogeny. the western and eastern ends of the profile crossed the Solund Devonian Basin and the allochthonous unit of the Jotun Nappe, respectively. Within the WGR, the fjord bottom is covered with 200–250 m thick glacial sediments with a low P-wave velocity (1600m) and a high Poisson's ratio (0.48–0.49). the P-wave velocity and velocity gradient of the uppermost crystalline basement is 6.05 km s−1 and 0.03s−1, respectively. the Poisson's ratio within the upper 12 km crust has an almost constant value of 0.26 in spite of the westward increase in the Caledonian metamorphism. This may indicate that the uniform bulk composition of the WGR is the predominant factor on the seismic wave velocities rather than the ‘fossil’ Caledonization. the P-wave velocity and the Poisson's ratio in the lower crust (deeper than 19–20 km) are 6.6–7.0 km s−1 and 0.27, respectively, which are comparable to those for an amphibolitic-granulitic rock type. the Moho gently deepens eastward from 31 to 36km. the velocity contrast at the Moho is large (1.0–1.2 km s−1), as seen in the other areas of the WGR. At the eastern end of the profile (the Jotun Nappe), the P-wave velocity at the uppermost basement shows a higher value (6.20 km s−1) than that within the WGR (6.05 km s−1), representing relatively mafic rock components in the Jotun Nappe. Such a lateral velocity change becomes obscure at depths deeper than 6–7 km, which indicates that the detachment between the WGR and the Jotun Nappe is situated at a very shallow depth. This strengthens the hypothesis that the WGR is a westward extension of the Precambrian Baltic Shield buried beneath the Caledonian Nappes. A mid-crustal interface with a velocity jump of 0.2–0.3 km s−1 was observed at a depth of 19–20 km. This interface disappears under the western part of the profile, indicating the intense deformation of the WGR during the Caledonian Orogeny.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1994.tb04018.x
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  • 3
    Electronic Resource
    Electronic Resource
    VpIVs ratio structure of the Lofoten continental margin, northern Norway, and its geological implications (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 124 (1996), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: In 1988, an extensive seismic refraction and wide-angle reflection study was performed at the Lofoten passive continental margin, off northern Norway, by using three-component ocean bottom seismographs. The Lofoten margin was formed by continental rifting and sea-floor spreading with extensive volcanism, which is responsible for landward flood basalts and seaward-dipping reflectors (SDR). The P-wave structure beneath the seaward side of the Voting Plateau escarpment at the Lofoten margin is characterized by a thick oceanic lower crust and the existence of an additional high-velocity layer at the bottom of the crust.In this study, the distribution of the P-wave velocity (Vp) to S-wave velocity (Vs) ratio down to the uppermost mantle is obtained by studying P-to-S converted shear-wave data. Vp/Vs ratios of 3.0–5.5 are obtained in the sedimentary layers. These high ratios are attributed to unconsolidated sediments. In the upper crust, a significant difference in Vp/Vs ratios is found between oceanic-side profiles and land-side profiles, i.e. 1.86 oceanward of the SDRs and 1.76 landward of the SDRs. This variation may have been caused by a difference in porosity, due to the difference in extrusive processes: the upper crust of the oceanic side of the SDRs was created by submarine eruptions, while the land side of the SDRs was formed by submarian eruptions. The Vp/Vs ratios in the middle crust and lower crust are estimated as 1.76–1.78 and 1.80, respectively. Comparing Vp and VsV, data from the lower crust to those in previous studies clearly indicates the presence of mafic gneisses. This result represents new seismic evidence for the hotspot hypothesis concerning the formation of passive volcanic continental margins. Clear S-wave upper-mantle refractions can be observed at offsets of up to 100 km, even where P-wave arrivals cannot be identified. In order to model these S-wave arrivals, a VS=4.8 km s-1 layer is necessary 2 km below the Moho.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1996.tb05634.x
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  • 4
    Electronic Resource
    Electronic Resource
    Earthquake distribution in the subduction zone off eastern Hokkaido, Japan, deduced from ocean-bottom seismographic and land observations (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 105 (1991), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: In 1986, observations of microearthquakes were conducted with the use of five ocean-bottom seismometers (OBSs) on the continental slope east of the Kushiro Submarine Canyon (KSC) off Hokkaido Island, Japan. The observation area is located about 50-100 km landward of the southern Kuril Trench beneath which the Pacific Plate is subducted toward Hokkaido Island. The OBS data were analysed together with land data on Hokkaido Island to determine the precise earthquake distribution and the geometry of the subducted plate.The OBS data revealed for the first time the seaward seismicity east of KSC. High and shallow seismicity is concentrated in an area between the coast line of Hokkaido Island and the 2000 m isobath. A region between the 2000 and 4000 m isobaths corresponds to the source area of an interplate earthquake occurring in 1973 (the Nemuro Hanto-Oki earthquake, M= 7.4). The OBS array detected more than 10 events in this region. Most of them, however, are classified as earthquakes occurring within the subducted plate, and the remarkable aftershock activity of the 1973 event is not recognized. The seismic activity is extremely low between the 4000 and 6000 m isobaths as is commonly seen in subduction zones. The seismicity is also very low beneath the trench, in contrast with the adjacent trench areas.In a vertical section taken perpendicular to the trench axis, most of the located events form a seismic plane dipping landward. The dip-angle of the plane is 20d̀-25d̀ beneath the continental slope, while 35d̀-40d̀ under Hokkaido Island. The change in dip angle occurs 150 km landward of the trench axis. This seismic plane is located 30-40 km below the plate boundary defined from the aftershock distribution of the Nemuro Hanto-Oki event. The seismicity on the plate boundary is correlated with the geometry of the subducted plate. The high activity occurs in a restricted region where the subducted plate shows a remarkable bend.The seismic activity obtained is different from those in the adjacent regions along the southern Kuril Trench. In particular, the regional difference in seismicity is remarkable between the western and eastern sides of KSC. Our result suggests that the subduction zone along the Kuril Trench is divided into several tectonic blocks and KSC is one of their boundaries.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb00806.x
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  • 5
    Electronic Resource
    Electronic Resource
    Observations of microseismicity in the southern Kuril Trench area by arrays of ocean bottom seismometers (1989)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 98 (1989), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Two ocean bottom seismometer (OBS) arrays were deployed to study microearthquake activity in the southernmost Kuril Trench area; at a junction of the Kuril and Japan Trenches. The first OBS observation was carried out from 1982 June to 1982 July and the second was from 1983 July to 1983 August. Each array consisted of 10 OBSs covering adjacent areas with an overlap. During the observation periods each OBS recorded about 50 earthquakes a day in average, a tenth of which were located in and near the arrays.A detailed picture of seismic activity associated with subduction and/or bending of the Pacific plate was obtained at the very point where it started to subduct. Spatial distribution of microseismicity in the southern Kuril Trench area is, in general, similar to that found in the Japan Trench area off Sanriku. A prominent microseismicity was found both landward and seaward of the axis of the Kuril Trench. The seaward activity had a sharp seaward boundary; along the edge of the seaward trench wall, in the Pacific Basin beyond which no detectable event with magnitude of greater than 2 was observed. Although seismicity was high beneath a continental slope between the Kuril Trench and Hokkaido, there was a seismicity gap beneath the inner trench wall. The gap coincided with an area of between 3 and 5 km in sea depth. The depth of the events beneath the seaward trench wall ranged from 0 to 30 km and just beneath the trench axis it was slightly deeper (0–50 km).A detailed comparison of seismicity between the Kuril and Japan Trenches showed a variation in spatial distribution along the trench axis: microseismicity near the trench axis is not uniform. The activity beneath the continental slope formed a seismic zone dipping landward with a low dip angle of less than 10°, which seemed to be a shallower extension of the lower plane of the double structured seismic zone in the Kuril Trench area. It shows a contrast to the Japan Trench area where the seismicity corresponds to the upper plane.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1989.tb05513.x
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  • 6
    Electronic Resource
    Electronic Resource
    Ocean bottom tiltmeter with acoustic data retrieval system implanted by a submersible (1988)
    Springer
    Marine geophysical researches 9 (1988), S. 237-254 
    Details
    ISSN: 1573-0581
    Keywords: ocean bottom tiltmeter ; subduction rate measurement ; crustal deformation observation on seafloor ; acoustic data retrieval ; acoustic control of bottom instruments ; marine geodesy ; deep sea submersible
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Sensitive Ocean Bottom Implanted Tiltmeters (OBITs) with a sophisticated data retrieval system have been developed in order to observe directly the subduction of oceanic lithosphere. The OBIT is the first long-term geophysical instrument which was designed to be deployed by a manned deep-sea submersible. When the OBIT is put on oceanic lithosphere which is bending and is about to subduct under a deep sea trench, the OBIT records the subduction by observing the tilt of the surface of the lithosphere. The OBIT system has a sensitivity of 10-8 radian, which is enough to detect the ongoing subduction in months or years. The OBIT may give an answer to the question whether there are fluctuations in the subduction rate. Two OBITs were successfully deployed on a seaward slope of the Kuril Trench by the newly built French deep submersible, Nautile. The OBITs were installed on the northwest shoulder of Erimo seamount, at a depth of 3930 m, in the Kuril Trench. In order to attain stable long-term observations of crustal deformations, the sensing unit was cemented onto bare rock by mortar. We have not yet had an opportunity to recover the data. The life of the instruments is expected to be more than five years. An acoustic data transmission system has been developed for the OBIT data recovery. The stored data can be retrieved at any moment during the observation period, with no need to retrieve the instruments nor to interrupt the observation, by use of the acoustic system. The acoustic system has a high data transmission rate as well as extremely low power consumption. This will be the first long-term crustal deformation measurement on the sea floor.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00309975
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  • 7
    Electronic Resource
    Electronic Resource
    Precise Positioning of Ocean Bottom Seismometer by Using Acoustic Transponder and CTD (1997)
    Springer
    Marine geophysical researches 19 (1997), S. 199-209 
    Details
    ISSN: 1573-0581
    Keywords: Ocean bottom seismometer ; acoustic transponder ; positioning
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We have obtained precise estimates of the position of Ocean Bottom Seismometers (OBS) on the sea bottom. Such estimates are usually uncertain due to their free falling deployment. This uncertainty is small enough, or is correctable, with OBS spacing of more than 10 km usually employed in crustal studies. But, for example, if the spacing is only 200 m for OBS reflection studies, estimates of the position with an accuracy of the order of 10 m or more is required. The determination was carried out with the slant range data, ship position data and a 1D acoustic velocity structure calculated from Conductivity–Temperature–Depth (CTD) data, if they are available. The slant range data were obtained by an acoustic transponder system designed for the sinker releasing of the OBS or travel time data of direct water wave arrivals by airgun shooting. The ship position data was obtained by a single GPS or DGPS. The method of calculation was similar to those used for earthquake hypocenter determination. The results indicate that the accuracy of determined OBS positions is enough for present OBS experiments, which becomes order of 1 m by using the DGPS and of less than 10 m by using the single GPS, if we measure the distance from several positions at the sea surface by using a transponder system which is not designed for the precise ranging. The geometry of calling positions is most important to determine the OBS position, even if we use the data with larger error, such as the direct water wave arrival data. The 1D acoustic velocity structure should be required for the correct depth of the OBS. Although it is rare that we use a CTD, even an empirical velocity structure works well.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1004246012551
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  • 8
    Electronic Resource
    Electronic Resource
    The ocean/continent transition along a profile through the Lofoten basin, Northern Norway (1994)
    Springer
    Marine geophysical researches 16 (1994), S. 201-224 
    Details
    ISSN: 1573-0581
    Keywords: Ocean/continent transition ; Lofoten Margin ; volcanic passive margin ; refraction seismics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The Cenozoic margins of the Norwegian-Greenland Sea offer ideal conditions for passive margin studies. A series of structural elements, first observed on these margins, led to the concept of volcanic passive margins. Questions still remain about the development of such features and the location of the boundary between oceanic and continental crust. Despite the thin sediment cover of the margins, seismic reflection data are not able to image the deeper structures due to the occurrence of igneous rocks at shallow depth. This paper presents a 320-km long profile perpendicular to the strike of the main structural units of the Lofoten Margin in Northern Norway. A geological model is proposed, based on observations made with ocean bottom seismographs, which recorded seismic refraction data and wide angle reflections, along with a seismic reflection profile covering the same area. Ray-tracing was used to calculate a geophysical model from the shelf area into the Lofoten basin. The structures typical of a volcanic passive margin were found, showing that the Lofoten Margin was influenced by increased volcanic activity during its evolution. The ocean/continent transition is located in a 30-km wide zone landwards of the Vøring Plateau escarpment. The whole margin is underlain by a possibly underplated, high velocity layer. Evidence for a pre-rift sediment basin landwards of the escarpment, overlain by basalt flows, was seen. These structural features, related to extensive volcanism on the Lofoten Margin, are not as distinct as further south along the Norwegian Margin. Viewed in the light of the hot-spot theory of White and McKenzie (1989) the Lofoten Margin can be interpreted as a transitional type between volcanic and non-volcanic passive margin.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01237514
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  • 9
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    Observations of microseismicity in the southern Kuril Trench area by arrays of ocean bottom seismometers (1989)
    Oxford University Press
    Details
    Publication Date: 1989-07-01
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Royal Astronomical Society.
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  • 10
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    Practical application of a sea-water battery in deep-sea basin and its performance (2009)
    Elsevier
    Details
    Publication Date: 2009-02-01
    Print ISSN: 0378-7753
    Electronic ISSN: 1873-2755
    Topics: Electrical Engineering, Measurement and Control Technology
    Published by Elsevier
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