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  • 1
    Electronic Resource
    Electronic Resource
    Groundwater microtemperature in earthquake regions (1984)
    Springer
    Pure and applied geophysics 122 (1984), S. 933-946 
    Details
    ISSN: 1420-9136
    Keywords: Groundwater temperature ; Crustal strain ; Coseismic signal ; Earthquake precursor ; Atmospheric pressure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Microtemperature measurements of groundwater with a relative precision better than 1/1000°C have been made in several seismically active areas in Japan. The measured temperatures show clear coseismic signals as well as a correlation with atmospheric pressure. Simultaneous observations at various depths have shown that these temperature changes were not induced by simple groundwater level changes. Also, distinctive signals occurred before several earthquakes and seem to be caused by a different mechanism than the coseismic signals. The microtemperature at some observation sites shows excellent correlation with records of nearby sensitive borehole strainmeters. Simultaneous recording of microtemperature and strain has been initiated in some boreholes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00876394
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  • 2
    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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  • 3
    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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  • 4
    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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  • 5
    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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  • 6
    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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  • 7
    Electronic Resource
    Electronic Resource
    Coseismic changes in groundwater temperature of the Usu volcanic region (1981)
    [s.l.] : Nature Publishing Group
    Nature 291 (1981), S. 137-138 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Usu volcano, 70km south-west of Sapporo, erupted on 7 August 1977, after lying dormant for 30yr. A series of eruptions followed the first one, continuing until October 1978. Many earthquakes have been observed in the area, beginning just before the first eruption and continuing to the present. The ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/291137a0
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  • 8
    Electronic Resource
    Electronic Resource
    High shear velocity layer in the upper mantle of the Western Pacific (1977)
    [s.l.] : Nature Publishing Group
    Nature 269 (1977), S. 680-682 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] A linear array of high sensitive ocean bottom seismographs (OBSs) was positioned on the Western Pacific Basin, where the Pacific lithosphere is thought to be very old and about to subside (Fig. 1) The water depth is between 5,500 and 5,600m. The linear dimension of the array is about 400km, so that ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/269680a0
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  • 9
    Electronic Resource
    Electronic Resource
    Seismographic observation at the bottom of the Central Basin Fault of the Philippine Sea (1975)
    [s.l.] : Nature Publishing Group
    Nature 253 (1975), S. 177-179 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Fig. 1 Locations of observation sites of ocean-bottom seismographs. Bathymetric map (depths in fathoms) is reproduced from HO 1301 publication. Intersection lower left is a record of ship-borne depth recorder of RV Hakuho-maru. Position of A: 1506'N (15.10N), 12954'E (129.91). Position of A': ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/253177a0
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  • 10
    Electronic Resource
    Electronic Resource
    Application of the Single-Bubble Airgun Technique for OBS-Data Acquisition Across the Jan Mayen Ridge, North Atlantic (1997)
    Springer
    Marine geophysical researches 19 (1997), S. 81-96 
    Details
    ISSN: 1573-0581
    Keywords: Seismic sources ; wide-angle seismics ; Jan Mayen Ridge
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A 140 km long wide-angle seismic profile has been acquired by use of 6 Ocean Bottom Seismographs across the Jan Mayen Ridge, North Atlantic. The profile was acquired twice; once with a traditionally tuned standard source and secondly with a somewhat smaller source tuned on the first bubble pulse. Analysis of the frequency content of the data reveals that the single-bubble source within the 10-15 Hz frequency range generates a signal with a level about 5 db above that of the standard source. These differences can partly be related to differences in airgun depth. The higher output level for these frequencies enables the single-bubble source to resolve intra-crustal structures with a higher degree of certainty, when compared to the data acquired by use of the standard source array. The standard source seems to generate slightly more energy for frequencies around 6 Hz, probably due to the use of a large 1200 in/sup3 gun in this array. These low frequencies a re of importance for mapping of lower crustal and upper mantle structures, and it is recommended that this is taken into account when seismic sources for mapping of deep crustal and upper mantle structures are designed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1004275600265
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