Ocean Bottom Seismographs Research Articles

Local site effects in the northern North Sea based on single‐station spectral ratios of OBS recordings

ABSTRACTIn recent years, there has been a growing awareness of the importance of local site effects in earthquake damage. A number of studies of recent destructive earthquakes have illustrated the relative contribution of enhanced ground shaking due to unconsolidated sediment layers.Among the different methods used to estimate local site response, the spectral ratio of shaking at a sedimentary site with respect to a bedrock reference site, has been successfuly applied in different geological environments. In this study, a technique recently proposed by Nakamura (1989) is used to evaluate site response using spectral ratios of horizontal vs. vertical components of earthquake recordings from a temporary ocean bottom seismograph (OBS) network in the northern North Sea and a permanent OBS at Oseberg oil field. Comparison with results obtained from the standard spectral ratios, indicate that the method is applicable also to subsea conditions, and the estimates obtained in this study indicate similar amplification factors to those obtained previously from analytical techniques. The ambient noise data on the other hand, gave unstable results, probably due to different noise characteristics in the marine environment. The results obtained on the earthquake data, however, provide an encouraging alternative to previously used analytical techniques for estimating local site response.

Aftershocks and Faults of the Hyogo-ken Nanbu Earthquake beneath Akashi Strait.

We carried out a seismic marine survey around Akashi Strait employing reflection and side-scan sonar imaging methods and an ocean bottom seismograph (OBS) array in order to closely examine the aftershocks in the immediate vicinity of the rupture initiation area of the 1995 Hyogo-ken Nanbu earthquake. A total of 71 aftershocks and some mechanisms were determined using OBS data for a period of 1 month with land temporary station data starting 12 days after the main shock. We identified a number of seismologically characteristic clusters to understand the heterogeneity that seem to exist around Akashi Strait. We determined that two major faults, the Nojima Fault and the Rokko fault system, are right-stepping within the strait cause en-echelon faultings in between. The Nojima Fault seems to terminate by deflecting westward in the strait. Focal mechanisms of these events are generally concordant with an E-W compression stress field, which is the tectonic trend. Events beneath the northern part of Awaji Island occur as a cluster dipping eastward, but mechanisms vary.

Generating interface waves using a freely falling, instrumented source

In recent years, interface waves such as the Scholte wave have become important tools in the study of the geoacoustic properties of near-bottom seafloor sediments. Traditionally, these waves have been generated by explosive or pneumatic sources deployed at or near the seafloor and monitored by ocean-bottom seismographs or geophone arrays. While these sources generate the requisite interface waves, they also produce higher frequency compressional waves in the water and sediment that tend to contaminate the surface wave and make inversion of the data difficult in the near field. In this paper, a new source consisting of a freely falling projectile instrumented with an accelerometer is described. When the projectile impacts the bottom, the exact time history of the vertical force applied to the sediment is known and therefore may be convolved with the transfer function of a sediment geoacoustic model to produce accurate synthetic seismograms. Moreover, the vertical force applied to the seafloor is very efficient in generating surface wave motion while producing very little compressional wave energy so that the near-field signals are much more easily analyzed. An example of the use of the new source is presented including inversion of the received signals to obtain shear-wave velocity and attenuation as a function of depth in the near bottom sediments at a shallow-water site.

Crustal structure related to the Philippine Sea plate subduction in the northeastern part of the Sagami Trough, Japan

A seismic refraction experiment was conducted with airguns and 17 pop-up type ocean bottom seismographs to delineate crustal structure related to the Philippine Sea plate subduction beneath Sagami Bay, at the northeastern end of the Sagami Trough, Japan. The P-wavespeed structure beneath Sagami Bay shows large variation in the range less than 10 km. As a whole, however, the structural model of the bay consists of four layers. The upper two layers are sediments, which are thick around the trough axis. The P wavespeed of the top of the fourth layer is consistent among profiles (5.6–6.1 km s −1), and the layer dips towards the northeast. This layer is also found as upper crust beneath the Izu-Bonin volcanic arc, to the south of Sagami Bay. This suggests that the island arc structure on the Philippine Sea plate subducts beneath the landward plate at the bay. The most significant lateral discontinuity in P-wavespeed structure across the trough axis is detected at the Sagami Tectonic Line. Faults along this structure are thought to be locus of the 1923 Kanto Earthquake ( M = 7.9).

Recording teleseismic earthquakes using ocean-bottom seismographs at mid-ocean ridges

Abstract Existing teleseismic data recorded by ocean-bottom seismographs (OBS) are sparse, but they are sufficient for analysis of earthquake detection thresholds under various background noise conditions. Long-period P, S , and surface waves are consistently recorded by OBS9s for magnitude 5.7 to 6+ events at ranges greater than 100°. Both Love and Rayleigh waves are recorded for very large events, with high coherence in the 15- to 70-sec period range; high coherence in the 15- to 35-sec range is typical for events of magnitude 5.5 to 6+. Short-period body-wave arrivals (1 Hz), on the other hand, have only been clearly recorded by OBS9s in the North Atlantic, during calm-weather periods or, by OBS9s in the Pacific, for very large events at ranges less than a few tens of degrees. Seismograms recorded at the East Pacific Rise (EPR) and at the Mid-Atlantic Ridge (MAR) illustrate the high signal coherence between instruments deployed in an array of OBS9s. Recordings of the pressure field as well as the vertical and horizontal displacement fields are used to assess the capabilities of OBS sensors and the frequency range of high signal-to-noise arrivals. Contamination of long-period, body-wave arrivals by secondary phases, due to reverberation in the water column, can significantly hinder investigation of relative travel-time anomalies across an OBS array on rough seafloor, particularly at low signal-to-noise ratios. The nature of the reverberations is illustrated in short-period data, and the basic physics behind the differences between the pressure and displacement signals is discussed. A bias of about 0.5 sec can be introduced to relative arrival times, with deeper stations appearing erroneously late, for an array where seafloor depths vary by about 1 km. Reflectivity synthetics provide the basis for designing optimum filters for removing the reverberation bias in long-period, P -wave data from the Mid-Atlantic Ridge, 34° S. The resulting relative travel-time anomaly is 0.4 to 0.6 sec with delays increasing with distance from the axis on the east flank of the spreading center.

Crustal structure of the Lofoten continental margin, off northern Norway, from ocean-bottom seismographic studies

SUMMARY In 1988, an extensive geophysical experiment was performed on the Lofoten passive continental margin, which was formed by the continental break-up between Greenland and northern Norway at 58 Ma. The geological units of the outer Lofoten margin are characterized by seaward-dipping reflectors (SDRs) and landward flood basalt, which extends up to 100 km landwards of SDRs. In this study, we obtain the P-wave velocity structure beneath the Lofoten Basin, the SDRs, and the landward flood basalt by use of ocean-bottom seismograph refraction profiling, and we also discuss the formation of the northern Norwegian passive continental margin. In the Lofoten Basin the crust is of oceanic type, consisting of sedimentary layers, oceanic laver 2 (4.9−5.5 km s−1), layer 3A (6.3−6.8 km s−1) and layer 3B (7.0−7.1 km s−1). Beneath the SDRs the crustal layers are identical to those of the Lofoten Basin, but the thickness of the lower crust, which represents the same velocity as layer 3B, increases to 5 km towards the continent side, and a high-velocity lower crustal layer (7.3 km s−1) is formed at the base of the crust. The ocean-continent transition zone is situated between the landward side of the SDRs and the northward continuation of the Voring Plateau Escarpment. In this region the velocity of the lower crust gradually decreases and approaches the lower crustal velocity beneath the Lofoten Islands (6.8 km s−1). The model also indicates that the high-velocity layer disappears in this region. Comparing our model with the crustal structure on the Voring margin, it is clear that the lower crustal body (≥7 km s−1) thickens southwards along the northern Norwegian continental margin. Recent results from petrological and geophysical studies of the generation of the oceanic crust have shown that increasing the temperature of the upwelling asthenospheric material increases the thickness of the oceanic crust. We interpret this as that the oceanic crust in the southern area in the Voring-Lofoten margin was generated by hotter material than that of the northern area.

Investigations of the East Greenland continental margin between 70° and 72° N by deep seismic sounding and gravity studies

Results are presented from a deep seismic sounding experiment with the research vessel POLARSTERN in the Scoresby Sund area, East Greenland. For this continental margin study 9 seismic recording landstations were placed in Scoresby Sund and at the southeast end of Kong Oscars Fjord, and ocean bottom seismographs (OBS) were deployed at 26 positions in and out of Scoresby Sund offshore East Greenland between 70° and 72° N and on the west flank of the Kolbeinsey Ridge. The landstations were established using helicopters from RV POLARSTERN. Explosives, a 321 airgun and 81 airguns were used as seismic sources in the open sea. Gravity data were recorded in addition to the seismic measurements. A free-air gravity map is presented. The sea operations — shooting and OBS recording — were strongly influenced by varying ice conditions. Crustal structure 2-D models have been calculated from the deep seismic sounding results. Free-air gravity anomalies have been calculated from these models and compared to the observed gravity. In the inner Scoresby Sund — the Caledonian fold belt region — the crustal thickness is about 35 km, and thins seaward to 10 km. Sediments more than 10 km thick on Jameson Land are of mainly Mesozoic age. In the outer shelf region and deep sea a ‘Moho’ cannot clearly be identified by our data. There are only weak indications for the existence of a ‘Moho’ west of the Kolbeinsey Ridge. Inside and offshore Scoresby Sund there is clear evidence for a lower crust refractor characterised byp-velocities of 6.8–7.3 km s−1 at depths between 6 and 10 km. We believe these velocities are related to magmatic processes of rifting and first drifting controlled by different scale mantle updoming during Paleocene to Eocene and Late Oligocene to Miocene times: the separation of Greenland/Norway and the separation of the Jan Mayen Ridge/Greenland, respectively. A thin igneous upper crust, interpreted to be of oceanic origin, begins about 50 km seaward of the Liverpool Land Escarpment and thickens oceanward. In the escarpment zone the crustal composition is not clear. Probably it is stretched and attenuated continental crust interspersed with basaltic intrusions. The great depth of the basement (about 5000 m) points to a high subsidence rate of about 0.25 mm yr−1 due to sediment loading and cooling of the crust and upper mantle, mainly since Miocene time. The igneous upper crust thickens eastward under the Kolbeinsey Ridge to about 2.5 km; the thickening is likely caused by higher production of extrusives. The basementp-velocity of 5.8–6.0 km s−1 is rather high. Such velocities are associated with young basalts and may also be caused by a higher percentage of dykes. Tertiary to recent sediments, about 5000 m thick, form most of the shelf east of Scoresby Sund, Liverpool Land and Kong Oscars Fjord. This points to a high sedimentation rate mainly since the Miocene. The deeper sediments have a rather high meanp-velocity of 4.5 km s−1, perhaps due to pre-Cambrian to Caledonian deposits of continental origin. The upper sediments offshore Scoresby Sund are thick and have a rather low velocity. They are interpreted as eroded material transported from inside the Sund into the shelf region. Offshore Kong Oscars Fjord the upper sediments, likely Jurassic to Devonian deposits, are thin in the shelf region but thicken to more than 3000 m in the slope area. The crust and upper mantle structure in the ocean-continent transition zone is interpreted to be the result of the superposition of the activities of three rifting phases related to mantle plumes of different dimensions: During period 2 and 3 only a few masses of extrusives were produced, but large volumes of intrusives were emplaced. So the margin between Scoresby Sund and Jan Mayen Fracture Zone is interpreted to be a stretched margin with low volcanic activity.

Crustal Transect across the Lofoten Volcanic Passive Continental Margin, N. Norway, Obtained by Use of Ocean Bottom Seismographs, and Implications for Its Evolution.

This study presents a 500-km-long crustal transect across the Lofoten volcanic passive continental margin, N. Norway, by compiling the results of two successive Ocean Bottom Seismographic (OBS) experiments performed in 1988. The OBS profiles were acquired from the Norwegian mainland, across the continental shelf, over an area covered with landward flood basalts, to the Lofoten basin. The land side end of the crustal model represents a thinned continental structure. The crust in this part has strong structural complexity, mainly due to faulting during pre-Tertiary continental thinning phases. Between the continental shelf and the seaward dipping reflectors (SDR), the model represents an extremely thinned continental crust and ocean/continent transition zone. This region is interpreted to be dominated by an early Tertiary continental rifting phase that progressed until early Eocene. The observed lower crustal reflectors, which are interpreted as intrusions in the lower crust, as well as the landward flood basalts indicates an extensive magmatic activity during the continental rifting phase. Between the SDR and magnetic anomaly 21, an oceanic crust with thick lower crust and a high velocity layer at the bottom of the crust (7.3km/s) are obtained. This high velocity layer is believed to be created by anomalously hot asthenospheric material rising around a hot spot. The comparison of the crustal structure across the Lofoten margin with the structure of the Voring-More margin shows significant differences in the volume of the lower crustal high velocity layer, which can be interpreted in terms of a NE-ward decrease of the influence of the hot spot.

S-wave anisotropy off Lofoten, Norway, indicative of fluids in the lower continental crust?

SUMMARY 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 (V, = 6.8 km s-').

Crustal structure of the Goban Spur rifted continental margin, Ne Atlantic

SUMMARY We present results that constrain the crustal structure across a typical ‘non-volcanic’ rifted continental margin, the Goban Spur in the NE Atlantic. Traveltime and amplitude modelling of wide-angle ocean-bottom seismograph profiles shows that the continental crust thins in a seaward direction to about 7 km before it breaks to allow a new oceanic spereading centre to develop. The crustal-velocity structure, the stretching factors and the density distribution inferred from gravity anomalies are consistent with bulk pure shear stretching for the development of the rifted continental margin. The seismic-velocity structure of the thinned continental crust suggests that thinning was accompanied by only limited igneous intrusion and extrusion in the upper crust. The adjacent oceanic crust is abnormally thin (5.4km). This is explained most readily by accretion at a very slow spreading centre (approx 11 mm yr-’ full rate), although additional cooling by conductive heat loss from the parent mantle as it rose slowly beneath the stretched and thinned continental lithosphere, together with minor igneous intrusion into, and surface flows onto, the adjacent continental crust may also have reduced the oceanic igneous thickness immediately adjacent to the continent-ocean transition.

Recording of ocean-bottom seismic noise and of a strong earthquake in the Himalayas by broadband digital OBS installed on the Mid-Atlantic Ridge

Abstract A broadband digital-analog ocean-bottom seismograph (OBS) with frequency pass band 0.003–30 Hz was installed in October 1991 on the Mid-Atlantic Ridge in the area of the Azores, where it operated for 3 days in programmed mode. The general structure of long-period ocean bottom seismic noise previously observed in several sea-floor experiments was confirmed, i.e. storm microseisms lie in the range 0.15–0.30 Hz and minimum noise is observed in the range 0.04–0.12 Hz (the band most appropriate for registering remote earthquakes). For still lower frequencies the level of noise increases. The level of background ocean-bottom seismic oscillations is time dependent. Strong ‘storms’ of microseisms were observed. In addition, a strong ( M = 7.2) earthquake occurred in the Himalayas on 19 October 1991 and was recorded in its entirety during one of the 2 h operational periods. The dispersive Love wave train was the most interesting one on the record, and dispersion curves of group velocity for two harmonics of the wave were derived. The dispersion curves are characteristic of a medium which is intermediate between transitional (from continental to oceanic) and continental models.

The French Pilot Experiment OFM-SISMOBS: first scientific results on noise level and event detection

Abstract The seismic data recorded during the French Pilot Experiment OFM-SISMOBS (Observatoire Fond de Mer) were carefully analyzed. This experiment was successfully conducted between 28 April and 11 May 1992. Notwithstanding the technical goal of the experiment, which was to show the feasibility of installing and recovering two sets of three-component broadband seismometers (one inside an Ocean Drilling Program (ODP) borehole and another inside an ocean-bottom seismograph (OBS) sphere in the vicinity of the hole), the second goal of the experiment was to obtain for the first time the seismic noise level in the broadband range 0.5–3600 s, to conduct a comparative study of broadband noise on the sea-floor, downhole and on a continent, and to determine the detection threshold of seismic events. After the installation of both sets of seismometers, seismic signals were recorded continuously for 10 days. The analysis of these signals shows that the seismic noise is smaller in the period range 4–30 s for both sea-floor seismometers (OFM; Observatoire Fond de Mer) and downhole seismometers (OFP; Observatoire Fond de Puits) than at a typical broadband continental station such as GEOSCOPE Station SSB. The noise is smaller than that at SSB up to 600 s for OFM. The noise on vertical components is much smaller than on the horizontal ones. This difference might be explained by instrument settling. It was also observed that the noise level tends to decrease with time for both OFM and OFP, which means that the equilibrium stage was not attained by the end of the experiment. The patterns of microseismic noise in oceanic and continental areas are completely different. The background microseismic noise is shifted towards shorter periods for OFM and OFP compared with a continental station. This might be related to the difference in the crustal structure between oceans and continents. The low level of seismic noise implies that the detection threshold of earthquakes is very low, and it has been possible to record correctly teleseismic earthquakes of magnitude as small as 5.3. It was also possible to extract the Earth tide oceanic signal. Therefore, the experiment was a complete technical and scientific success, and demonstrated that the installation of a permanent broadband seismic and geophysical observatory on the sea-floor is now possible and can provide the scientific community with high-quality seismic data.

The ocean/continent transition along a profile through the Lofoten basin, Northern Norway

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 Voring 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.

Comparisons of ridge crest earthquake waveforms recorded on SOSUS hydrophones with ocean bottom seismometers in the near field

The U.S. Navy’s SOSUS network of hydrophone arrays is allowing an unprecedented continuous look at oceanic seismicity. Hydrophones in the SOFAR channel are sensitive detectors of acoustic T-waves resulting from earthquakes in the ocean crust. Since Fall of 1991 a program at the NOAA Pacific Marine Environmental Laboratory has catalogued over 500 seismic events originating on the boundaries of the Pacific, Gorda, and Juan de Fuca tectonic plates. Though extremely sensitive, observations through SOFAR hydrophones have the drawback that the earthquake signal travels long distances though the ocean sound channel. The recorded signal is thus a highly distorted representation of the earthquake. There is as yet no reliably determined relationship between T-wave strength and source magnitude, mainly because many local factors can affect the transfer of energy from the ocean floor into the SOFAR waveguide. We have compared earthquake and explosive events recorded in 1991 simultaneously by the SOSUS network and three small-scale arrays of ocean bottom seismographs placed near the intersection of the Gorda spreading ridge and the Blanco Transform Fault. By this effort, we are attempting to gather enough statistics on long range T-wave strength compared to local measurements to provide a rough magnitude scale.

東海沖でのフィリピン海プレートの詳細な沈み込み形態の解明

In 1992, the eastern part of the Nankai Trough off Tokai area, Japan, is seismically surveyed by the dense refraction profiling using an ocean bottom seismograph (OBS) array and airguns. In this area, the Philippine Sea plate is subducting under the Japan Arc along the Nankai Trough, and a remarkable topographic high, the Zenisu Ridge, exists along the southern side of the trough. The aim of this experiment was to obtain detailed seismic velocity structures of the crust at the plate boundary and to reveal the subducting pattern of the Philippine Sea plate at its north margin. We could obtain detailed velocity structure models because of the dense profiling, the good data quality and precise analysis by a ray-tracing method using both the travel-time data and the amplitude data. Results of this study are summarized as follows:1) The eastern Nankai Trough, similar to the southwestern Nankai Trough off Shikoku, has a typical oceanic crust with the thickness of 8-10km, which varies along this profile. Igneous layers are composed of the oceanic layer 2A (P-wave velocity of 3.6-4.3km/s), 2B (4.7-5.6km/s), 3A (6.3-6.4km/s) and 3B (6.7-6.9km/s). And the velocity of the uppermost mantle is 7.8-8.0km/s.2) The Zenisu Ridge has a similar velocity structure to that of the Nankai Trough. It shows a difference with the crustal structure of the Izu-Ogasawara Arc which is characterized by existence of the 6km/s layer. This is clear evidence that the Zenisu Ridge is a topographic high originated from the oceanic crust. Beneath the ridge, the sediment thickness is thin (0.5-2km) and the igneous basement, the oceanic layer 2, shows severe undulations as compared with the structure of the Nankai Trough.3) Existence of a subducting oceanic crust beneath the continental slope was imaged by clearly observed wide-angle reflection phases. Our data support that the subducting oceanic crust lacks the layer 2A and the upper part of the layer 2B. It indicates a remarkable contrast to crustal structure models obtained at the southernmost Kuril Trench and the Ryukyu Trench where the oceanic crust subducts with the whole of the oceanic layer 2.4) The velocity of the upper crust on the subducting oceanic crust was determined as 5.3-5.8km/s, which is relatively low as compared with that of the Japan Arc. The continental slope is characterized by an accretional prism composed of a thick sediment layer of 3.0-3.5km/s and a wedge shaped layer of 3.8-4.2km/s. The wedge seems to contain material of the upper part of the oceanic layer 2 which was scraped off during the subduction because of the similarity of velocities between them.5) Beneath the south margin of the Zenisu Ridge, the oceanic layer 3 was found to have a discontinuity in layering. Furthermore, the layer and the Moho show a sudden dip to the north in the south side of the discontinuity. Although the feature of the structure indicates a possible subduction of the oceanic crust beneath the ridge, the extent of the dipping layer just reaches beneath the south margin of the ridge. This result may suggest that a new subduction of the Philippine Sea plate has started there.

Seismic anisotropy in the mantle beneath an oceanic spreading centre

BENEATH an active mid-ocean ridge, the mantle upwells in response to the divergence of the newly formed plates, leading to high temperatures and pressure-release melting below the ridge axis. The width of the upwelling region and the amount of melting depend on mantle rheology1–5, but all models predict a maximum decrease in seismic velocity at the ridge axis. It has also been suggested, however, that the alignment of anisotropic minerals by shear in the upwelling mantle will increase seismic velocity for rays travelling subvertically through the upwelling zone6,7. Here we report the observation of a consistent pattern of anomalously early P-wave arrival times at an array of ocean-bottom seismographs deployed across the axis of the southern Mid-Atlantic Ridge: P-waves from distant earthquakes arrive earlier at stations near the axis than at those further away. Our results are consistent with a model of anisotropy in which the degree of mineral alignment is greatest directly beneath the ridge axis, and significant anisotropy extends tens of kilometres from the axis.

Possible shallow crustal shear wave anisotropy off Lofoten, Norway, inferred from three-component ocean-bottom seismographs

Summary P- and S-wave velocity models along two perpendicular profiles 145 and 175 km long on the undrilled continental shelf off Lofoten, northern Norway, have been obtained from the study of nine three-component ocean bottom seismographs. A VP/VS ratio of 1.96–2.15 is found in the sedimentary layers along a NE-SW profile. These high values are consistent with shaly sediments, which are identified in dredged bedrock samples. Along a NW-SE profile the S-wave velocities in the sedimentary layers are 5-10 per cent higher than along a NE-SW profile. The most likely explanation for this velocity difference is seismic anisotropy, which might be caused by liquid-filled microfractures alligned along the direction of the present-day maximum horizontal compressive stress. The Vp/Vs ratio in the upper crystalline crust is found to be 1.75 along both profiles. If the inferred anisotropy described above is caused by aligned microcracks, the lack of anisotropy in the crystalline basement indicates that these cracks are restricted to the sedimentary layers. It must be emphasized, however, that the observed azimuthal difference in S-wave velocities could be caused by lateral velocity variations in the sediments, although the anisotropy hypothesis is considered more likely.

Crustal Structure Beneath Lofoten, N. Norway, From Vertical Incidence and Wide-Angle Seismic Data

Summary A high-quality multichannel seismic reflection line was acquired in 1987 along a 175 km long profile across the continental shelf off Lofoten, northern Norway. A seismic wide-angle experiment was performed in 1988 along the same profile, using seven three-component Ocean Bottom Seismographs (OBS) with 20-25 km spacing and shotpoint intervals of 240 m. The study of the data has shown that the combination of the multichannel reflection and the wide-angle (OBS) technique provides information about the crustal structure beneath the Lofoten shelf that could not have been achieved using only one of the techniques. the multichannel reflection data provide a detailed image of the shallow (Cretaceous) structures, which represents an important basis for inversion of the OBS data. the lower crust and the Moho are also well mapped in some parts of the area with the multichannel reflection technique. The OBS data reveal that significant amounts of pre-Cretaceous sediments exist along almost the entire profile, with a maximum thickness of about 5 km in the Vestfjorden Basin. From the OBS data the thickness of the lower crust is inferred to decrease from about 11.5 km under the Rost High to about 2 km below the Lofoten Ridge. the OBS data indicate further that the Moho position under the Vestfjorden Basin is considerably deeper than can be inferred from the reflection data. About 10km below Moho a strong dipping event is observed in the OBS data. This upper mantle reflection might be related to a possible seaward dipping master fault, and/or presence of layers of partially hydrated peridotite.

Seismic anisotropy inferred from wide-angle reflections off Lofoten, Norway, indicative of shear-aligned minerals in the upper mantle

Data from six three-component Ocean Bottom Seismographs situated on a 175 km long profile on the continental shelf off Lofoten, northern Norway, have been modelled using 2-D seismic ray-tracing, and P- and S-wave velocity models for the upper mantle are presented. A dipping structure about 10 km below the Moho causes strong P and S reflections as well as P-to-S conversions. This upper mantle reflection might be related to a possible seaward dipping master fault, and/or the presence of partially hydrated peridotite. The modelling of these arrivals reveals significant seismic anisotropy in the upper mantle. The maximum P-wave anisotropy is inferred to be in excess of 15%, the maximum S-wave anisotropy in the order of 5–10%, and the axis of the fast velocity is inferred to be inclined at least 45° from the horizontal. The strong anisotropy for P-waves indicates that its cause is related to alignment of minerals. We postulate that the inferred steep inclination of anisotropic minerals might be a result of shearing along faults and fractures during post-Caledonian extensional episodes. It must be emphasized, however, that it cannot be excluded that more complicated anisotropic models with other geological implications can also explain the observations satisfactorily. The study demonstrates the importance of incorporating S-waves in analysis of seismic data, because the strong P-wave anisotropy would not have been discovered without the constraints given by the S-waves.

Real-time detection of anomalous geoelectric changes

Aiming at an accurate and quick detection of anomalous changes of the electric field so that we can detect tectonic signals, we developed observation methods for use both on land and on the ocean floor. With these observation methods, that is, observation on land using electrodes and the underground cables of the Nippon Telegraph and Telephone Company, and on the ocean floor using the power-feeding arrangement of the ocean-bottom seismograph of the Japan Meteorological Agency, we can obtain more reliable data than those from ordinary observation methods. Using these data, we further developed two techniques of analysis in real-time which are based on a statistical model, eliminating components induced by the variations of the geomagnetic field and/or the movement of the seawater. For both cases, the geomagnetic field at the Kakioka Magnetic Observatory and/or the seawater level data observed on the ocean floor were used as associated data. The predominant induced component in the variation of the geoelectric field is almost completely separated in the cases of the geoelectric field in the Mito region and on the ocean floor. Owing to the characteristic periodicity of the monopolar variation of noise in the Numazu region, in spite of the large amplitude of the noise (mainly due to the leak-current of the d.c. electric trains) in the original data from the Numazu region, the noises were considerably separated, with these methods, into the induced component part or tidal component part. With these methods, consequently, the detectability of anomalous changes was improved about ten times and about five times in the cases of the geoelectric field in the Mito region and on the ocean floor, and in the case of the geoelectric field in the Numazu region, respectively.