Ocean Bottom Seismographs Research Articles

Deep crustal structure of the eastern Nankai Trough and Zenisu Ridge by dense airgun–OBS seismic profiling

An unprecedentedly extensive seismic refraction and wide-angle reflection survey using 65 ocean bottom seismographs revealed detailed crustal structure around the eastern Nankai Trough. A previously published crustal model shows an abrupt offset of the Moho at the south of the Zenisu Ridge, a prominent topographic high along the oceanward slope of the Nankai Trough. Our crustal model indicates that this offset of the Moho extends southwestward continuously to 138°E, decreasing its gap. The survey area experienced the last two great earthquakes in 1854 and 1944. However, the northeastern part of the survey area seems to have remained unruptured since the 1854 event. Factors controlling the size of the rupture area for great earthquakes are still a matter of debate. There are several candidates for these factors in the survey area: hypothetical tectonic boundaries that may or may not be oceanward prolongation of major on-land tectonic lines, estimated locations of slab disruption, and the extent of Moho offset along the strike of the Zenisu Ridge. The main purpose of this survey is to clarify the relation between the crustal structure and these geophysical and geological features bounding the rupture area. Our crustal model from the trough axis to the continental slope is characterized by a well-developed sedimentary wedge bounded by island arc crustal blocks, consisting of upper and lower crust, to the northwest. Furthermore, the subducting oceanic crust, which can be traced down to 25 km depth, shows that the down-dip angle steepens at 55 km landward from the trough axis. On the basis of compilation of our crustal model with previously published models around the eastern Nankai Trough, we derived an image of the entire subducting plate geometry for depths shallower than 20 km, which is still poorly constrained by the land observation of microearthquakes. Significant lateral variations of the crustal structure and the slab geometry are recognized along one prominent canyon, and the offset of the Moho at the south of the Zenisu Ridge disappears to the southwest of the canyon. Moreover, it seems that the slab disruption recognized at a depth greater than 20 km is connected to this canyon. Therefore, the lateral variation of the crustal structure along the canyon may be one of the causes to stop rupture propagation of great earthquakes. Furthermore, the crustal variation may also form a tectonic boundary that distinguishes the subduction pattern of the Philippine Sea plate, including the influence of the Izu–Ogasawara collision, in the eastern Nankai Trough from the simple subduction pattern of the western Nankai Trough.

Crustal structure of the southern part of the Vøring Basin, mid-Norway margin, from wide-angle seismic and gravity data

During the summer of 1996, five profiles with a total length of 892.5 km and 30 recovered Ocean Bottom Seismographs (OBSs) were acquired along the southern part of the Vøring Margin, mid-Norway. These profiles are oriented in both strike and dip direction, mapping the crustal structure along the western-most part of this area. By performing both forward and inverse velocity travel-time modelling of the vertical components of the OBSs (P-waves) regional models have been established. The thickness of the Cenozoic sediments ranges from 0.5 km at the structural highs (Møre Marginal High) to more than 3 km in the Vigrid Syncline. The Cretaceous layers vary considerably in thickness, from 1.7 km close to the Jan Mayen Fracture Zone, increasing to approximately 8 km below the Helland Hansen Arch. Several intrusions are found in the Cretaceous layers and they individually range in thickness from 0.1 to 0.6 km. These intrusions can be followed through crossing profiles, in both in the models presented in this paper, and in the models inferred from earlier OBS surveys along this margin. The pre-Cretaceous sediments are modelled as a layer with P-wave velocity of more than 5 km/s. The thickness of this layer is generally less than 5 km, but locally a thickness of 6.5 km is modelled. The crystalline basement is modelled as a 6+ km/s refractor with a general trend shallowing towards south- and northwest. Along a strike-profile, the transition from continental to oceanic crust is modelled by increased P-wave velocity from 6.2 to 7.3 km/s, as the profile crosses the Jan Mayen Lineament towards the Møre Marginal High. The thickness of the continental crystalline basement is largest landward towards the Helland Hansen Arch where it exceeds 10 km. Towards the SW and NW, where the crystalline basement is elevated, the thickness decreases to less than 5 km. In the lower crust, a high velocity 7+ km/s body, interpreted as magmatic underplating, is found along three of the profiles in the present work. The SE boundary of this magmatic underplated layer is located beneath the Helland Hansen Arch and towards the Møre Marginal High. The southwestward termination is less certain, but the Jan Mayen Fracture Zone seems to control the emplacement of intrusions in the southern Vøring Basin. The depth to Moho varies from 12 to 28 km, deepening landwards.

Lower crustal seismic velocity-anomalies; magmatic underplating or serpentinized peridotite? Evidence from the Vøring Margin, NE Atlantic

On the Voring volcanic passive margin offshore mid-Norway, NE Atlantic, a lower crustal body with P-wave velocities in the range of 7.1–7.7 km/s has been mapped by twenty two-dimensional Ocean Bottom Seismograph (OBS) profiles. The main aim of the present paper is to evaluate to what extent the lower crust is consistent with magmatic intrusions or serpentinized peridotite. The relatively low Vp/Vs ratios of 1.75–1.78 modelled for the lower crust under the continental part of the Voring Plateau are consistent with mafic intrusions mixed with blocks of stretched continental crust, but not with the presence of partially serpentinized peridotites. The lower crustal high-velocity body is restricted to the area of the Late Cretaceous/Early Tertiary rift that lead to continental break-up in Early Eocene. The same model can explain the observations in the northern Voring Basin, but in the central and southern Voring Basin the seismic velocities do not preclude a model involving serpentinized peridotite in addition to intrusions and continental remnants. On the west Iberia non-volcanic margin a similar layer is interpreted as serpentinized peridotite. The existence of Moho reflections, the observation of S-wave anisotropy but absence of P-wave anisotropy, uncertainties regarding supply of water to allow for significant serpentinization and very low stretching factors compared with the west Iberia Margin, are among factors that argue against the presence of serpentinized peridotite in the Voring Basin.

Breathing of the seafloor: Tidal correlations of seismicity at Axial volcano

Tidal effects on seafloor microearthquakes have been postulated, but the search has been hindered by a lack of continuous long-term data sets. Making this observation is further complicated by the need to distinguish between Earth and ocean tidal influences on the seafloor. In the summer of 1994, a small ocean-bottom seismograph array located 402 microseismic events, over a period of two months, on the summit caldera of Axial volcano on the Juan de Fuca Ridge. Harmonic tremor was also observed on all instruments, and Earth and ocean tides were recorded on tiltmeters installed within the seismometer packages. Microearthquakes show a strong correlation with tidal lows, suggesting that faulting is occurring preferentially when ocean loading is at a minimum. The harmonic tremor, interpreted as the movement of superheated fluid in cracks, also has a tidal periodicity.

Microearthquake seismicity and focal mechanisms at the Rodriguez Triple Junction in the Indian Ocean using ocean bottom seismometers

Hypocenters and focal mechanisms of microearthquakes have been investigated at the Rodriguez Triple Junction in the Indian Ocean. Little was known on microearthquake activity in this region. We deployed 18 ocean bottom seismographs during the KH93–3 cruise of the R/V Hakuho‐Maru (Ocean Research Institute, University of Tokyo) from July 30 to August 20, 1993. We obtained 579 well‐constrained hypocenters and 13 focal mechanisms. Microearthquakes were found to be active along all of the three ridges: the Central Indian Ridge, the Southeastern Indian Ridge, and the Southwestern Indian Ridge. Especially at the triple junction there was an earthquake swarm within narrow area of approximately 15×5 km2. All of the 13 focal mechanisms showed normal or strike‐slip faultings, which means that the extensional stress field characterizes this region.

Crustal investigation of the Hellenic subduction zone using wide aperture seismic data

We present the results of a wide-aperture seismic onshore–offshore study (Crete Seismic Experiment) in the Cretan region as part of the Hellenic arc compressional system. Three seismic lines were carried out on and around the island of Crete in order to investigate the crustal structure of the region. Up to 119 three-component recording stations were deployed on each profile that observed seismic energy generated by a 48-l airgun array and eight 20-kg landshots. A total of 6208 shots were fired. Upon completing the fieldwork, the vertical components of all stations were evaluated; 300 Common-Receiver-Gather (CRG) sections of the ocean bottom seismographs (OBS) and land stations as well as 100 Common-Source-Gather (CSG) sections of the land shots and selected airgun shots were compiled and modeled in order to generate a 2D P-wave velocity–depth model for each profile. The accuracy of the model depends on the depth and position along the profiles and does not exceed 5% for both depth and P-wave velocity. We identified strong lateral variations in crustal and sedimentary thickness mainly in a north–south direction but also along strike (east–west). The crust is continental and has a maximum thickness of 32.5 km below northern central Crete. Its subdivision in an upper ( v p=5.8–6.3 km/s, locally up to 6.5 km/s) and a lower ( v p=6.4–6.9 km/s) part is justified by a first-order discontinuity with v p-velocity a contrast of up to 0.6 km/s. The eastern part of Crete shows a significantly thinner crust of 24 to 26 km. To the North, the crustal thickness decreases to 15 km below the central Cretan Sea. The prominent decrease of the Moho depth north of central Crete is interpreted to represent the northern end of a microcontinent that was subducted in Oligocene times and later surfaced by ‘buoyant escape’ (Stöckhert et al., 1999; Thomson et al., 1999). The P– T– t– D history of the high-pressure rocks of Crete, Greece: denudation by buoyant escape. In: Exhumation Processes: Normal Faulting, Ductile Flow and Erosion. Ring, U., Lister, G., Willet, S., Brandon, M. (Eds.), Spec. Publ. of the Geol. Soc. of London, p. 154]. To the south and southwest of the island, the continental crust gradually thins to a minimum of 17 km and at approximately 100 km off the southern coast of Crete, it is in contact with oceanic crust below the Mediterranean Ridge. Upper mantle velocities were determined to be 7.7 km/s below the Cretan Sea and 8.0 km/s south of Crete. Below the continental Cretan crust, a 6- to 7-km-thick layer with v p-velocities between 6.6 and 7.1 km/s was identified on each line and could be followed by reflections to a depth of 42 km. It is decoupled from the overlying continental crust at central Crete and is interpreted as oceanic crust presently under subduction towards the NNE below the Aegean Sea.

Deep crustal structure of the Chicxulub impact crater

We present the results of a wide‐angle seismic survey conducted over the Chicxulub impact crater. Profile Chicx‐A/A1 is a chord across the offshore portion of the crater, and Chicx‐B/F is an onshore‐offshore profile through the center of the crater. We use travel times recorded by 32 ocean bottom seismograph and 19 land‐based receivers to model the crustal structure of the crater. The primary feature in the shallow velocity structure is a low‐velocity Tertiary basin, the edge of which correlates with a steep gravity gradient in some regions of our survey. Directly beneath the Tertiary basin at the center of the crater is a region of relatively low velocities and densities that is interpreted as a unit of melt rocks (either suevite breccias with isolated melt pods or suevite breccias overlying a coherent melt sheet). This unit has a thickness of ∼1 km and a diameter of ∼100 km. Central uplift with a diameter of 40–60 km is observed on Chicx‐B/F. A lower limit on the vertical extent of uplift is 9 km. Shallow basement is also observed along the northwest portion of Chicx‐B/F, associated with a prominent northwest trending high in the gravity field. Superimposed on a regional trend is Moho uplift of ∼1 km near the center of the Chicx‐A/A1 profile, with adjacent Moho deepening of ∼1.25–1.5 km. The Moho topography may be related to deformation processes associated with the formation of the outer ring or to the excavation and collapse of the transient cavity.

Thinned continental crust below northern Evoikos Gulf, central Greece, detected from deep seismic soundings

In March 1996, a wide aperture reflection/refraction profiling (WARRP) seismic survey was carried out across the Maliakos–Sporades and northern Evoikos basins, central Greece. Two onshore/offshore seismic lines were recorded, using 14 ocean bottom seismographs (OBS) and 10 land stations. As an energy source, we used one sleevegun of 60 l volume, operating at 120-bar pressure. The results acquired by kinematic, two-point raytracing modelling of the time sections provided evidence of a thinned, stretched continental crust, of only 20 km thickness, below the central part of the northern Evoikos Basin. This basin was developed by the separation of the island of Evia—with continental crust of approximately 30 km thickness—from the Greek mainland, by transtension and stretching of the crust, forming the north Evia and the Sperchios valleys. Along the Maliakos–north Sporades Basin, the crust thins from 34 km at the mainland to 22 km below the Sporades Basin. The crust at the northern part of the island of Evia and the Trikeri Straits is 30–32 km thick and of normal continental structure; it is separated by an intercrustal discontinuity between an upper and lower crust. The sediments thicken significantly towards the Sporades Basin, where they obtain maximum thicknesses of the order of 8–10 km. The igneous and metamorphic crust thins significantly in the basin maintaining however its continental character along the entire section. The transition from the Maliakos–Trikeri Straits to the Sporades Basin is controlled by a steep listric fault that downthrows the basin crust by more than 6–7 km. The crustal transition of the thinned Sporades Basin to the 30-km thick crust of the northern Evia and the Maliakos Straits is poorly understood. Simple or pure shear stretching cannot have developed this margin, and we favour the assumption that it denotes the limit of two different crustal domains; these were merged together during a compressive tectonic episode, prior to the present-day extension and stretching of the Aegean Sea that was activated 5 Ma ago.

Micro‐seismicity around the seaward updip limit of the 1946 Nankai Earthquake dislocation area

We observed micro‐seismicity around the seaward updip limit of the 1946 Nankai earthquake dislocation area by using pop‐up type ocean bottom seismographs deployed for three months in 1998. At the subduction zone, we must consider strong lateral heterogeneity of the seismic velocity structure to locate earthquakes precisely. We constructed 3‐D P‐ and S‐wave velocity structure models based on the results of airgun‐OBS seismic surveys along the Nankai Trough. We located 83 earthquakes using these 3‐D models. These hypocenters show two groups of seismicity. One seismicity group locates in oceanic layer 3 within a subducting seamount. These earthquakes may reflect the deformation of the seamount caused by its subduction. The other seismicity group locates near the updip limit of the thermally modeled locked zone. These hypocenters are determined to be near the boundary between the accretionary prism and the subducting oceanic crust.

Crustal structure of the outer Vøring Plateau, offshore Norway, from ocean bottom seismic and gravity data

Four ocean bottom seismograph (OBS) profiles acquired across the continent‐ocean transition zone of the outer Vøring Plateau have been modeled by inversion and forward modeling of the OBS vertical component traces (P waves). The thickness of the sedimentary layers deposited since the early Eocene continental breakup varies from up to 2.2 km above the basaltic inner flows (IF) to a minimum of 0.2 km about 20 km seaward of the Vøring Escarpment (VE), with a velocity of 1.6–2.55 km/s. The thickness of the uppermost layer of basalt varies from 0.5 to 4.5 km, and its velocity ranges from 3.4 to 5 km/s. The modeling of the two dip profiles suggests that strongly intruded preopening sedimentary rocks extend ∼40 km seaward of the VE. Farther seaward, another layer of extrusives/intrusives is found beneath the uppermost layer of basalt. The lateral variations in velocity in the main crustal layer along the dip profiles indicate that the continent/ocean transition extends over a 30–50 km wide zone in between (strike) profile 3 (oceanic; 6.6–7.35 km/s) and (strike) profile 4 (continental; 6.1–7.0 km/s). An up to 7 km thick lower crustal high‐velocity body (7.1–7.4 km/s) is interpreted in terms of intrusions in the lower crust beneath and landward of the continent/ocean transition. The total thickness of the normal oceanic crust is estimated to be 7–9 km, the anomalously thick oceanic crust reaches a maximum thickness of about 25 km, and the crustal thickness is estimated to be 17–22 km beneath the IF. Gravity modeling of the profiles confirmed all main aspects of the velocity models and provided constraints on the crustal structures toward the ends of the profiles where the ray coverage was low.

Three-component OBS-data processing for lithology and fluid prediction in the mid-Norway margin, NE Atlantic

In 1992, acomprehensive three-component ocean bottom seismic survey was performed in the central and northern area of the Voring Basin, offshore mid-Norway, NE Atlantic. An important part of the data acquisition program consisted of a local survey with 20 Ocean Bottom Seismographs (OBS) dropped at approximately 200 m interval in 1300 m water depth. The main purpose of the local survey was to acquire densely sampled P- and S-wave reflection data above a seismic flatspot anomaly observed earlier, in order to more accurately predict if hydrocarbons could be related to it. The conventional reflection data processing methods applied to the vertical components included predictive deconvolution in order to attenuate low frequency ringing, near offset mute and a series of constant velocity stacks in order to obtain the optimal velocity function. The final result is a “trouser” shaped, high resolution VZ stacked section with minor influence of water multiples. The inline (Vx ) component contains no strong multiples, and extensive near trace muting was hence not necessary to apply for this component. Velocity analysis together with ray-tracing modelling indicate that P-S-converted shear waves (reflections) represent the dominant mode. The results of the interpretation and modelling indicated a Vp/Vs-ratio of approximately 2.6 in the overburden, which suggests domination of partly unconsolidated shale, while the Vp/Vs-ratio in the assumed reservoir was approximately 1.8, which indicates a more sand dominated facies. Outside the flatspot area a higher Vp/Vs-ratio ratio (approximately 2.0) was estimated, indicating that hydrocarbons could be present in the assumed reservoir.

Controls on the tectono-magmatic evolution of a volcanic transform margin: the Vøring Transform Margin, NE Atlantic

Ocean bottom seismograph (OBS), multichannel seismic and potential field data reveal the structure of the Voring Transform Margin (VTM). This transform margin is located at the landward extension of the Jan Mayen Fracture Zone along the southern edge of the Voring Plateau. The margin consists of two distinctive segments. The northwestern segment is characterized by large amounts of volcanic material. The new OBS data reveal a 30–40 km wide and 17 km thick high-velocity body between underplated continental crust to the northeast and normal oceanic crust in the southwest. The southeastern segment of the mar is similar to transform margins elsewhere. It is characterized by a 20–30 km wide transform margin high and a narrow continent-ocean transition. The volcanic sequences along this margin segment are less than 1 km thick. We conclude from the spatial correspondence of decreased volcanism and the location of the fracture zone, that the amount of volcanism was influenced by the tectonic setting. We propose that (1) lateral heat transport from the oceanic lithosphere to the adjacent continental lithosphere decreased the ambient mantle temperature and melt production along the entire transform margin and (2) that right-stepping of the left-lateral shear zone at the northwestern margin segment caused lithospheric thinning and increased volcanism. The investigated data show no evidence that the breakup volcanism influenced the tectonic development of the southeastern VTM.

海底地震計と制御震源を用いた北部大和海盆, 秋田沖日本海東縁部海陸境界域の地震波速度構造

海底地震計と制御震源を用いた北部大和海盆, 秋田沖日本海東縁部海陸境界域の地震波速度構造

A crustal transect between Precambrian Australia and the Timor Trough across the Vulcan Sub-basin

Wide-angle seismic data from ocean bottom seismographs, together with gravity and deep marine reflection profiling data along the Vulcan transect in northern Australia, define the crustal-scale features between the Precambrian Australian craton and the Timor Trough. The transect provides an outline of crustal and upper mantle architecture across the major boundary between the Australian and SE Asian plates when linked with earlier deep marine seismic profiling. Near the Australian coast, relatively unaltered Precambrian Kimberley Basin rocks are inferred to extend to the edge of a shallow-water shelf area (Yampi Shelf) with a crustal thickness of 35 km. The crust then thins to 26 km under the outer shelf near the Timor Trough. Over the same distance Palaeozoic/Mesozoic basin sequences are interpreted to thicken to 12–13 km, inferring an attenuation of Precambrian basement rocks from 35 to 13–14 km across the margin ( β=2.6). On the outer shelf, the Vulcan Sub-Basin is a trans-tensional rift within Permo-Triassic platform areas (Ashmore Platform, Londonderry High). Within the lower crust under major bounding faults at the sub-basin/platform margins, there are elevated P-wave velocities to 7 km/s, suggesting emplacement of intrusive, more mafic rocks at depth during basin-forming processes. At mid-crustal levels, near the top of the inferred attenuated Precambrian crustal rocks, there are strong near-vertical-incidence reflections at about 13 km depth that are interpreted to be a detachment or further evidence of intrusive rocks. Additionally, seismic energy reflected at wide angles from within the upper mantle at 38–45 km depth indicates that compositional boundaries/heterogeneities continue at depth. Detailed information within the basin sediments includes the interpretation of crosscutting velocity patterns within the Mesozoic sequences as an indication of decreasing porosity with depth of burial, and the presence of a low-velocity sequence at 1–2 km depth across the Londonderry High.

The results of deep seismic investigations on geotraverse in the Barents Sea from Kola peninsula to Franz-Joseph Land

In 1995 geophysical investigations in the southern part of the AP-1 geotraverse across the whole Barents Sea Plate were completed. The entire profile links the super-deep SG-3 well on the Kola peninsular (town of Zapolarny) and the base Hayes-1 well on Franz-Joseph Land. The extent of the investigated part of the profile was 700 km. The integrated seismic experiment included offshore and onshore wide-angle reflection/refraction observations (referred hereinafter as Deep Seismic Sounding — DSS) with 3-component ocean bottom seismographs (OBS) and air guns and multi-channel seismic reflection study (MCS) at different offsets ranges. Sufficiently dense deployment of OBS (5–20 km) and a 250 m shooting interval provided reliable phase correlation and wave identification. Intensive reflections from the Moho have been recorded practically continuously along the profile. The DSS data processing built a mid point dynamic reflection Moho image and produced a general depth section. MCS data defined boundaries in the sedimentary layer and the basement surface in the southern part of the profile. The lowermost observed boundary, the Moho, occurs at depths between 35 and 40 km. The thickness of sedimentary cover varies from zero in the southern part of the profile to 14–16 km in its northern part. Results of geological interpretation confirm the continental crust in the studied part of the Barents Sea Shelf Plate.

Aftershock distribution of the 1994 Sanriku‐oki earthquake (Mw 7.7) revealed by ocean bottom seismographic observation

The 1994 Sanriku‐oki earthquake (Mw 7.7) ruptured the entire seismogenic zone of the Pacific plate subduction beneath northeastern Japan arc. We deployed 18 ocean bottom seismographs and recorded its aftershocks, which span the whole seismogenic zone, with unprecedented accuracy. We calculate hypocenters and mechanisms to improve our understanding of the plate subduction geometry and plate coupling. The rupture of the mainshock initiated at the updip end of the seismogenic zone defined as the trenchward limit of the aftershock and background seismicity. The rupture continued to slip along a shallow dipping (<10°) interface for ∼60 km, and then slipped across an asperity where the dip angle starts to increase at ∼143°E. Here aftershocks are few, suggesting complete release of strain. The rupture continued another 100 km or so to the downdip end beneath the coastline, where the plate dip angle reaches 30° to smoothly connect to the dip angle of the Wadati‐Benioff zone. Coseismic slips are larger westward of 143°E, where the mantle wedge comes into contact with the subducting plate. The stress redistribution due to the mainshock seems to have caused aftershocks with normal faultings to occur offset above and below the plate boundary.

Acoustic Structure and Seismic Velocities in the Carnarvon Basin, Australian North West Shelf: Towards an Integrated Study

High quality refraction and wide-angle reflection seismic data recorded by ocean-bottom seismographs (OBSs) deployed by the Australian Geological Survey Organisation along the 700 km long transect in the Carnarvon Basin effectively supplement results obtained by means of the conventional reflection technology. Velocity information can now be derived from both CDP (nearvertical reflection) and OBS (refraction/wide-angle reflection) data. Generally, CDP-derived average velocities are lower than OBS-derived velocities and this deviation increases with depth: from ~0.1 km/s at 8 s two way time (TWT) to 0.8-1.6 km/s at 16 s TWT. If the CDP-derived velocities are used to depth convert reflection data, then depth to these TWTs would be underestimated by 0.4 to 6.4-12.8 km respectively. Some local anomalies (at ~6s TWT CDP-derived velocities may be more than 0.1 km/s higher than the OBS-derived velocities) distort this general trend. These would result in ~0.3 km local overestimates of the depth equivalent of 6s TWT. Co-analysis of the interval velocity field reconstructed from the travel time-based interpretation of the OBS data and the conventional reflection image of the crust in some cases shows their poor correlation. Prominent reflectivity seen in the conventional reflection data at depth 13-17 km does not correspond to any significant velocity increase imaged by refraction/wide-angle techniques. These reflections were taken in our pre-OBS interpretation to mark the top of the underplated layer in the crust. Velocity below that level estimated from the OBS data is in the range 5.7-6.4 km/s. This velocity range is considerably lower than anticipated for mafic rocks under appropriate PTconditions. Quite significant local velocity increase in the depth range 15-23 km does not produce high-amplitude near-vertical reflections. Only a combination of reflection and refraction/wideangle techniques provides a clue to a consistent geological interpretation of seismic data. Improved velocity estimation in the crust is crucial for accurate depth conversion of near-vertical reflection data.

Lithospheric structure north of Scothland--I. P-wave modelling deep reflection profiles and gravity

The W-reflector and the Flannan reflector are two enigmatic planar structures in the lithospheric mantle north of Scotland, mapped in three dimensions using deep reflection profiling. The Flannan reflector dips eastwards from the Moho to a depth of 80 km at an angle of ∼30°, whilst the W-reflector is subhorizontal, lying 10–20 km beneath the Moho and terminating westwards against the Flannan. Both reflectors have a strong impedance contrast and are laterally coherent over tens of kilometres. Wide-angle seismic reflection and refraction data are used here to constrain the P-wave velocity structure of the lithosphere in this region and to investigate the physical properties and geometry of these reflective structures. We report the results from an integrated seismic survey in which a series of explosive shots fired at sea were recorded by ocean bottom seismographs, land stations and a streamer towed by a second ship in an expanding spread configuration. A well resolved P-wave velocity structure for the lithosphere above the mantle reflectors is derived by inverting the seismic traveltimes to optimize the fit of the model to the data, and by generating synthetic seismograms from the model to match the relative amplitudes of the observed seismic phases. A high-amplitude post-critical reflection indicates the existence of a discrete reflecting interface dipping gently westwards between 40 and 50 km depth in the lithospheric mantle. The P-wave velocity model is converted to two-way reflection time and correlated with normal-incidence reflection data: the Moho structure agrees well, and the reflecting interface defined by the wide-angle data is coincident with the subhorizontal W-reflector. Less distinct later arrivals observed in the data are consistent with modelled reflections from the Flannan reflector in the mantle and from the extrapolated position of the Outer Isles Fault in the lower crust. Modelling of the relative amplitudes and critical distances of the wide-angle mantle reflection shows that the reflecting layer must be at least 3 km thick and that it has considerably higher velocity (8.5 km s−1) and density (3.5 g cm−3) than normal mantle. Further modelling tests the vertical structure within the layer, and constrains the sharpness of the upper interface and whether the reflection amplitude could have been enhanced by internal layering. Gravity data measured over the region are consistent with the velocity model and with the high-density layer in the mantle, which is constrained by the gravity modelling to be less than 10 km thick. Published laboratory measurements of the physical properties of mantle rocks indicate that mafic eclogite with high velocity and density satisfies all the observations from the mantle layer. Our preferred hypothesis for the origin for such a layer is that it is a fragment of oceanic crust that has been subducted and metamorphosed to eclogite facies.

Subducted seamount imaged in the rupture zone of the 1946 nankaido earthquake

The Nankai Trough is a vigorous subduction zone where large earthquakes have been recorded since the seventh century, with a recurrence time of 100 to 200 years. The 1946 Nankaido earthquake was unusual, with a rupture zone estimated from long-period geodetic data that was more than twice as large as that derived from shorter period seismic data. In the center of this earthquake rupture zone, we used densely deployed ocean bottom seismographs to detect a subducted seamount 13 kilometers thick by 50 kilometers wide at a depth of 10 kilometers. We propose that this seamount might work as a barrier inhibiting brittle seismogenic rupture.

Detailed plate boundary structure off northeast Japan coast

In 1997, a seismic experiment using an airgun array and ocean bottom seismographs (OBSs) was performed in the forearc region of the northeast Japan (NEJ) arc. The objectives of this experiment were to clarify whole of the velocity structure around the forearc region of NEJ arc including a detailed plate boundary structure and the heterogeneous structure. In this paper, we estimated the heterogeneous velocity structure around the forearc region of northeast Japan by applying 2‐D ray tracing and travel time inversion to the airgun‐OBS data. The depth where the island arc Moho comes into contact with the subducting oceanic crust is about 20 km. We suggest the existence of an oceanic layerl overlying the oceanic crust subducting down to at least near 18 km by comparing observed waveforms with these calculated using the reflectivity method.