Ocean Bottom Seismometer Array Research Articles

An anomalous spatial pattern of shear-wave splitting observed in Ocean Bottom Seismic data above a subducting seamount in the Nankai Trough

In the summer of 2000, an ocean bottom seismometer (OBS) array was deployed along the western end of the Nankai Trough, off Cape Muroto. The array crosses an area where a buried subducting seamount was identified under the Nankai accretionary prism sediments during a multichannel seismic (MCS) reflection survey. In this paper we report the first results of shear-wave splitting (SWS) analysis of the three-component OBS data. One interesting result is a peculiar anomaly in the behaviour of shear-wave polarizations observed near the subducting seamount. We attempt to explain the observed spatial pattern in shear-wave polarizations by complex structure of stress field in accretionary prism sediments around seamount. We conclude, that the spatial pattern of SWS consistent with expected stress perturbations around seamounts, but there are several other reasons that can result in a similar type of seismic anisotropy. Finally, we have a somewhat broader look at the spatial pattern of shear-wave polarizations observed with another OBS array near the active front of Nankai Trough.

Imaging magmatic rocks on the Faroes Margin

Break-up of the North Atlantic in the Paleocene was accompanied by widespread magmatism, which provides a challenge both to imaging basin structure and to modelling the subsidence and development of the continental margins. The rationale for new methods of seismic acquisition and modelling are reported here, followed by an illustration of the techniques with data from the Faroes region, which lay close to the hottest part of the mantle plume at the time of continental break-up. Igneous rocks were added to the crust during continental break-up on rifted margins as extrusive lavas, as sills intruded into the subsurface, and as lower crustal intrusions or underplate. Each provide different, but tractable problems to seismic imaging. It is shown that many of these difficulties can be surmounted by using very long offsets (long streamers or two-ship methods) with a broad-band, low-frequency source, and by using dense arrays of ocean bottom seismometers to provide complementary P- and S-wave velocity control. These methods are illustrated using new results from the integrated Seismic Imaging and Modelling of Margins (iSIMM) project, which acquired regional data across the Faroes shelf and margin in summer 2002. The iSIMM profile used 85 four-component ocean bottom seismometers and a 104 litre (6340 in 3 ) airgun array for wide-angle data, with vertical arrays for far-field source signature recording. The wide-angle profiles were overshot using three single-sensor, Q-Marine streamers, one 12 km long, the other two 4 km long. A 48-gun, 167 litre (10 170 in 3 ) deep-towed, broad-band airgun array was used with the Q-marine streamers. The airgun arrays were designed and tuned to enhance the bubble pulses, with peak frequencies at 8–11 Hz. They performed well, with excellent arrivals visible to ranges beyond 120 km on the OBS, and penetration through the basalts and well into the upper mantle. Examples are shown of the new seismic data, which enable imaging of the basalt flows and underlying sediments, as well as lower-crustal igneous intrusions. This allows good constraints to be placed on the igneous distribution and volumes on the rifted margin and adjacent continental shelf.

An evaluation of peak and bubble tuning in sub-basalt seismology: modelling and results from OBS data

As part of the iSIMM project (White et al. 2002), a 6360 in3 airgun source array was used to acquire, in two passes, a deep seismic profile into a 480 km array of ocean bottom seismometers (OBS) east of the Faroe Islands. The first pass used peak tuning, and the second pass used bubble tuning, with other source parameters constant. The objective was to deliver low frequency energy for deep, long-offset, sub-basalt penetration. The results suggest that towing large guns deep is more important than the tuning method. However, for the gun configuration used, the bubble-tuned data are more compact, less reverberant and easier to pick.

Coupled seismic slip on adjacent oceanic transform faults

[1] In a four and a half hour period, more than 60 events in an earthquake swarm on the western boundary of the Easter microplate were detected by an array of ocean bottom seismometers. The larger events of the swarm were strike-slip earthquakes located on two transform faults separated by about 25 km. Slip on the faults was closely coupled, with activity alternating back and forth randomly between the two transforms. Coupled seismic activity is usually attributed to triggering by static stress changes or dynamic stresses in propagating shear waves generated by another earthquake, but these earthquakes are too small for either mechanism to be plausible. We suggest that the swarm may have been the seismic manifestation of a larger, primarily aseismic, slip event or slow earthquake involving both transforms, perhaps triggered by dike injection on the Easter-Pacific spreading center.

Detection of shallowest submarine seismicity by acoustic coupled shear waves

An outstanding slow wave train similar to T wave in appearance has been observed on records of an ocean bottom seismometer array, off Boso, central Japan. These slow wave trains are not observed on land, are usually spindle‐shaped and dominant in vertical component, and have apparent velocities of ∼1.3 km/s along the array. In order to understand the observations, we calculate synthetic seismograms by a finite difference method for horizontally stratified layers of seawater, sediment, and basement, which show a well‐defined wave train consistent with the observations only if the source is placed above the basement layer. This result and normal mode calculations indicate that the slow wave train comprises essentially the sediment‐trapped shear waves coupled with ocean acoustic waves. The observed events of slow wave trains occur near the boundary between the outer arc high and the forearc basin, where vertical differential motion has been suggested to occur. We suggest that these small events represent the shallowest crustal activity unique to the triple junction off Boso.

Large aperture seismic imaging at a convergent margin: Techniques and results from the Costa Rica seismogenic zone

In March and April 1995 a cooperative German, Costa Rican, and United States research team recorded onshore-offshore seismic data sets along the Pacific margin of Costa Rica using the R/V Ewing. Off the Nicoya Peninsula we used a linear array of ocean bottom seismometers and hydrophones (OBS/H) with onshore seismometers extending across much of the isthmus. In the central area we deployed an OBS/H areal array consisting of 30 instruments over a 9 km by 35-km area and had land stations on the Nicoya Peninsula adjacent to this marine array and also extending northeast on the main Costa Rican landmass. Our goal in these experiments was to determine the crustal velocity structure along different portions of this convergent margin and to use the dense instrument deployments to create migrated reflection images of the plate boundary zone and the subducting Cocos Plate. Our specific goal in the central area was to determine whether a subducted seamount is present at the location of the 1990, M 7 earthquake off the Nicoya Peninsula and can thus be linked to its nucleation. Subsequently we have processed the data to improve reflection signals, used the data to calculate crustal velocity models, and developed several wide-aperture migration techniques, based on a Kirchhoff algorithm, to produce reflection images. Along the northern transect we used the ocean bottom data to construct a detailed crustal velocity model, but reflections from the plate boundary and top and bottom of the subducting Cocos plate are difficult to identify and have so far produced poor images. In contrast, the land stations along this same transect recorded clear reflections from the top of the subducting plate or plate boundary, within the seismogenic zone, and we have constructed a clear image from this reflector beneath the Nicoya shelf. Data from the 3-D seismic experiment suffer from high-amplitude, coherent noise (arrivals other than reflections), and we have tried many techniques to enhance the signal to noise ratio of reflected arrivals. Due to the noise, an apparent lack of strong reflections from the plate boundary zone, and probable structural complexity, the resulting 3-D images only poorly resolve the top of the subducting Cocos Plate. The images are not able to provide compelling evidence of whether there is a subducting seamount at the 1990 earthquake hypocenter. Our results do show that OBS surveys are capable of creating images of the plate boundary zone and the subducting plate well into the seismogenic zone if coherent reflections are recorded at 1.8 km instrument spacing (2-D) and 5 km inline by 1 km crossline spacing for 3-D acquisition. However, due to typical high amplitude coherent noise, imaging results may be poorer than expected, especially in unfavorable geologic settings such as our 3-D survey area. More effective noise reduction in acquisition, possibly with the use of vertical hydrophone arrays, and in processing, with advanced multiple removal and possibly depth filtering, is required to achieve the desired detailed images of the seismogenic plate boundary zone.

Structure of the upper mantle under the EPR from waveform inversion of regional events

Waveform inversions of seismograms recorded at the Mantle Electromagnetic and Tomography (MELT) Experiment ocean bottom seismometer array from regional events with paths following the East Pacific Rise (EPR) require that low shear velocities (<3.7 km/s) extend to depths of more than 100 km below the rise axis. Velocities increase with average crustal age along ray paths. The reconciliation of Love and Rayleigh wave data requires that shear flow has aligned melt pockets or olivine crystals, creating an anisotropic uppermost mantle.

The relation between earthquakes, faulting, and submarine hydrothermal mineralization

Although the relationship between submarine hydrothermal activity and earthquakes was recognized over 20 years ago, it has still not been precisely defined. Faulting and permeability control fluid flow in the oceanic crust and therefore submarine hydrothermal activity at mid‐ocean ridges. Microearthquakes associated with submarine hydrothermal activity tend to be small in magnitude and occur in swarms. Swarms of microearthquakes associated with hydrothermal vents are thought to be related to cracking of the oceanic crust. The depths of the microearthquakes may mark the brittle‐ductile transition which denotes the downward limit of seawater circulation into the oceanic crust. The maximum depth of microearthquakes on the Mid‐Atlantic Ridge is 10 km, compared with 3 km on the East Pacific Rise. The brittle layer tends to be thinnest where mid‐ocean spreading rates are fastest. Seismic pumping is a mechanism for the transport of hydrothermal fluids within fractures and may account for stockwork mineralization within the ocean crust. Submarine hydrothermal mineralization is episodic on time scales ranging from 1 to 105 years and is controlled by volcanic and tectonic processes in a manner not yet completely understood. Emphasis in the study of submarine hydrothermal activity is changing from observation to long‐term monitoring. The U.S. Navy's Sound Surveillance System (SOSUS) has been very successful in detecting megaplume events caused by dike injection on the Juan de Fuca and Gorda Ridges. The long‐term deployment of large arrays of ocean bottom seismometers on the ocean floor will become routine in the future. Such a program will contribute to an understanding of the factors controlling the episodicity of submarine hydrothermal activity and mineralization on the sea floor. On the Izu‐Bonin Arc, hydrothermal mineralization has been observed in a number of volcanic Calderas. This region appears to have many advantages for the future monitoring and evaluation of microseismicity in hydrothermally active areas.

Broadband seismology and noise under the ocean

Most of our understanding of the Earth's interior has been derived from measurements from global seismic networks, although no network has ever been truly “global” because some 71% of the Earth's surface is underwater. The resulting gaps in coverage produce a biased and incomplete image of the Earth. Work has begun toward establishing permanent observatories on the deep seafloor, although the technical difficulties remain severe. Will these stations be useful, and where and how shall they be established? Data from seafloor observatories will be of poorer quality than continental site data because the sea surface is an important and local source of broadband noise. This noise is derived from wind and waves through direct forcing at long periods and by nonlinear coupling to elastic waves at short periods. Our understanding of the generation and propagation of seismic noise and of wind and wave climatology can be used to predict the temporal and geographical variability of the noise spectrum and to assess likely sites for permanent seafloor observatories. High noise levels near 1 Hz may raise detection limits for short‐period, teleseismic arrivals above mb = 7.5, limiting the usefulness of many seafloor sites. Noise levels in deep boreholes will be 10 dB quieter than those at the seafloor, but sensors buried short distances below the seafloor may also provide comparable noise levels and fidelity. The retrieval of data from permanent seafloor observatories remains an unsolved problem, but long‐term temporary arrays of ocean bottom seismometers are now being used in regional scale experiments using earthquakes as sources. Such experiments are likely to be less successful in the Pacific basin than in either the Indian Ocean or North Atlantic Ocean because of higher noise levels.

The depth dependence of seismic background noise within the sea floor

Further results are reported of a study of seafloor noise using an array of ocean-bottom seismometers in a sediment pond in deep water off the California coast. Earlier (Schreiner and Dorman, 1990) measurements were presented of coherence between elements of the array that contained interelement spacings in the 8–156 m range and it was shown that they represented the fundamental and first three overtones of seismic surface and interface waves. Here, the ratios of power spectra (horizontal to vertical component) in the 0.500–10 Hz frequency band were used to refine the estimates of the interface wave mode excitation that were made earlier using the interelement coherences. There is a simple frequency dependence in the mode excitation functions. The fundamental mode dominated below 0.65 Hz and is succeeded by the first three overtones in order as the frequency increases. The other transition frequencies are 0.95 and 6.5 Hz. From these excitation functions, the depth dependence of background noise was synthesized, using the vertical wave functions from the known shear velocity structure. For some depths in the 0.8–10 Hz frequency range, the amplitude of vertical component noise is up to 29 dB greater than at the surface, due to the prominence of higher mode interface waves in this band. Varying degrees of noise suppression are obtained outside this band. The noise suppression (or enhancement) is strongly dependent on the relative mode excitation. At frequencies where the fundamental mode dominates there will be noise reduction downhole while at frequencies where the overtones are prominent, there will be noise enhancement at some depths. For horizontal-component noise there is noise suppression with depth at all frequencies. [Work supported by ONR and ONT.]

VLF/ULF noise on basalt and sedimented seafloor.

During project NOBS (7 Sept. 1991 to 10 Oct. 1991) four arrays of ocean bottom seismometers were deployed to record seafloor noise in the band 0.01 to 10 Hz. The arrays had 0-, 10-, 100-, and 1000-m sediment, and were all within a few hundred km of FLIP, which was located at 43° 42.63 N, 125° 59.19 W. Examination of two-dimensional wave-number spectra of waves from an earthquake and from noise shows that for frequencies from 0.1 to 0.5 Hz the wavelength is of order tens of kilometers, whereas above 0.5 Hz the wavelength of noise and signal-generated noise is of order tens of meters. Four conclusions can be drawn from the data at this time; from 0.1 to 0.5 Hz the noise is propagating at a relatively high velocity, probably as Rayleigh waves with speeds of order 3 km/s; from 0.5 to 10 Hz the noise is propagating at low speeds, probably as interface (Stonely) waves with speeds of order 0.1 km/s; shear energy undergoes a large amount of scattering from 0.5 to 10 Hz; noise from 0.5 and 10 Hz is probably produced by scattering at the seafloor and is largest where the sediment is thinnest.

Earthquake distribution in the subduction zone off eastern Hokkaido, Japan, deduced from ocean-bottom seismographic and land observations

SUMMARY 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 20d-25d beneath the continental slope, while 35d-40d 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.

Ambient noise and elastic wave propagation in the deep seafloor

In August/September 1989 a borehole seismic array was deployed at DSDP Site 534 in the Blake Bahama Basin. The purpose of the array was to study the physics of acoustic and elastic wave propagation and ambient noise near the seafloor. The array was inserted into the borehole using a wireline re‐entry capability and was recovered by grappling. Each node was individually clamped into the hole. The borehole array consisted of four three‐component seismometers (with a passband from 2–50 Hz) at depths of 10, 40, 70, and 100 m below the seafloor. A borehole hydrophone was also recorded at the top node. Over I Gbyte of data was acquired over a 2‐week period from the array. A shooting program of air‐guns and explosives was carried out. An array of ocean bottom seismometers and a vertical hydrophone array were also deployed. Preliminary observations are (1) the time spread as a result of late‐arriving energy is much less at 10‐m depth than at the seafloor; (2) the borehole hydrophone is susceptible to tube waves for transient signals and ambient noise; (3) ambient noise levels vary by at least 10 dB over time; (4) ambient noise levels decrease by 10 dB between 10‐ and 100‐m depth; and (5) ambient noise levels on a vertical seismometer are 10 dB less than colocated horizontal seismometers.

Directional spectra observations of seafloor microseisms from an ocean-bottom seismometer array

Observations of the directional spectra of seabed motion in shallow water were conducted off the New Jersey coast during the summer of 1987. Using a six-point ocean-bottom seismometer array, each instrument supporting a pressure transducer, and two horizontal and vertical accelerometers, measurements of gravity and seismic waves across the ULF/VLF band were collected in 12.5 m of water. Array dimensions were tuned particularly for directional spectra observations of short-period seafloor microseisms. Directional spectra analysis indicates that in the short-period microseismic band, 1.5–2.5 s, motion of the seafloor is primarily a result of slow seismic waves traveling at apparent velocities near 200 m/s. These propagation velocities for the microseismic band in shallow water are an order of magnitude less than microseismic velocities from similar studies on land. Contemporaneous measurements of the directional spectra of long-period ocean gravity waves, 15–85 s, show an eastern direction of origin; short-period ocean gravity waves, 5–9 s, measured using particle motion analysis, are from the south. The direction of propagation of the microseisms, found from the maximum response of the array, is shown to be approximately N150E—a direction midway between long- and short-period ocean-wave propagation directions. Correlation of particle motion and directional spectra analysis indicates that microseisms have retrograde motion. These results suggest that the microseisms are most likely Scholte interface waves.

Observations of microseismicity in the southern Kuril Trench area by arrays of ocean bottom seismometers

Summary 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.

Directional spectral and particle motion measurements of underwater seismic noise

Observations of underwater seismic noise were measured using arrays of ocean bottom seismometers (OBS) deployed in shallow waters on the continental shelf off New Jersey. To reduce uncorrelated noise on the seafloor, instruments were buried to 0.5-m depth with the aid of an underwater hydraulic pump. Array geometry and dimensions were tuned to measure microseismic noise in the period band of 2–6 s. Directional spectrums measured by maximizing cross spectrums between array elements for a given wavenumber indicate that slow microseismic waves having an apparent velocity of 200–300 m/s were traveling out of the east. Phase velocities determined from normal mode analysis agree closely with measured phase velocities. From particle motion analysis for single OBS elements, microseismic noise is shown to exhibit retrograde motion at the seafloor. These experiment results indicate that microseismic noise is due to Scholte-type interface waves. [Work supported by ONR, code OA.]

The P-wave structure of the lithosphere-asthenosphere in the Western Pacific: a comparison of ocean versus continent

In this paper, we present the lithosphere-asthenosphere P-wave velocity structures in the Western Pacific as determined by ocean-bottom seismometer array observations to illustrate the contrasts and similarities between continental and oceanic regions. These structures are compared with representative velocity structures beneath continental shield regions and the tectonically active regions. The subcrustal lithosphere in the western Northwest Pacific Basin possesses a layered P-wave structure (apparent surface velocities are 8.0, 8.4 and 8.6 km/s) and extends to a depth of about 149 km. The lithosphere is underlain by a low-velocity layer which appears to be the major transition zone from the lithosphere to asthenosphere. This velocity structure is similar to that of the continental shield regions. In the East Mariana Basin, the uppermost mantle layer below the Moho is about 30 km thick with a thin high-velocity lid (8.3 km/s) at its base. These layers form the subcrustal lithosphere, below which several alternating low- and high-velocity layers together form a thick asthenosphere. The upper mantle structure beneath the East Mariana Basin seems to show an intermediate feature between continental shield regions and tectonically active regions. The differences between the upper mantle structure of the continent and ocean could be regarded as aspects of an evolution of upper mantle. The western Northwest Pacific Basin could be regarded as being almost matured, while the East Mariana Basin could be regarded as being rejuvenated.

SEISMICITY OF THE GALAPAGOS 95.5°W PROPAGATING RIFT

An array of ocean bottom seismometers was deployed for 21 days about the 95.5°W Galapagos propagating rift tip to obtain seismic evidence for the propagating rift hypothesis and to investigate the dynamic effects of rift propagation. Seismicity was fairly constant (10–17 events per day) with no indication of swarm activity. Hypocenters were determined for 304 earthquakes, the largest of which is estimated to be mb = 2.3, with a seismic moment of 2.9 × 1020 dyn cm. The cumulative seismic moment released by all local earthquakes is estimated to be 6.6 × 1020 dyn cm. A b value of 0.77 ± 0.08 was estimated for the region, implying a stress regime dominated by a broad tectonic stress field. The distribution of earthquakes is consistent with the propagating rift hypothesis. Most earthquakes occurred south of the propagating rift within the De Steiguer Deep. Seismicity within the transform zone is clustered in such a way as to suggest that two or three discrete fault segments may be active at the same time. The seismicity observed along the propagating rift is diffuse. Earthquakes were observed at the propagating rift tip, but not in front of it. Almost all earthquakes occur south of the northern pseudofault and its extension west of the rift tip; that is, earthquakes occur almost exclusively in lithosphere that has been or is being overtaken by the propagating rift. Six composite focal mechanisms are consistent with the propagating rift hypothesis. First motions from earthquakes located on the propagating rift indicate extension, whereas motions associated with the failing rift area indicate compression. A compressional regime is also indicated west of the transform zone. Twelve earthquakes from the transform zone produce a poorly constrained focal mechanism consistent with right‐lateral strike‐slip motion along a fault plane that is roughly parallel to the isochrons of the transferred lithosphere.

A microseismicity survey of Axial Seamount, Juan de Fuca Ridge

Abstract An array of ocean bottom seismometers and hydrophones were deployed within the caldera of Axial Seamount, located at the intersection of the Cobb-Eickelberg Seamount Chain and the Juan de Fuca Ridge. Recent manned submersible dives have discovered the presence of two distinct hydrothermal vent fields. The objective of the microseismicity survey was to determine baseline information necessary for designing an optimal experiment to monitor seismicity associated with submarine volcanic and hydrothermal systems. With an instrument deployment period of eight days, over 1500 events were recorded and examined. Most events occurred within three periods of swarm activity. The location of the swarm events is believed to be within the low temperature vent field in the northwest portion of Axial's caldera. These impulsive, high-frequency events are interpreted to be tectonic in origin. No harmonic tremor-like events were observed on any of the instruments, indicating a lack of magmatic movement. Only nine local events were recorded on more than one instrument, and none of these could be located unambiguously. Monochromatic 20-Hz signals were also recorded and are interpreted to originate from whales observed in the immediate vicinity.

Structure and seismotectonics of the Vema Fracture Zone, Atlantic Ocean

The structure and seismo-tectonics of the Vema Fracture Zone and the adjacent regions of its transverse ridge to the south and the Mid-Atlantic Ridge spreading centre to the north is discussed, based on an analysis of seismic refraction profiles, micro-earthquake activity and gravity anomalies. Crustal structures were determined by analysis of wide angle seismic data from explosive and airgun sources recorded on arrays of ocean bottom seismometers. Micro-earthquakes were recorded by six instruments for a period of 14 days in an array to the north of the ridge-transform intersection. Immature crust, similar to that reported by Fowler (1976) from the FAMOUS area, lies beneath the spreading centre. The lower crust thins and normal mantle appears as the fracture zone is approached. The thinnest crust lies beneath the northern end of the topographic deep at the fracture zone intersection. This is also the locus of current microearthquake activity at sub-bottom depths of 3–4 km, which a composite fault-plane solution suggests may originate on normal faults that parallel the trend of the local topography on the eastern wall of the median valley. Within the central region of the fracture zone valley, the crust thickens but still retains an anomalous velocity structure. Seismic layer 3 is absent and crustal velocities are low. This is probably the consequence of attenuated crustal accretion, of faulting and cracking, and of hydrothermal alteration. The pattern of residual gravity anomalies, after correction for topography, for sediment thickness and for mantle temperature variations, is consistent with the crustal structure defined by the seismic experiments. The gravity measurements indicate that there is a broad thinning of the crust over 20–40 km in the vicinity of the fracture zone, on which are superimposed localized regions of low and high densities. To the south of the fracture zone, the crust thickens and assumes a normal velocity structure including a mature seismic layer 3. The seismic Moho is warped upwards under the transverse ridge, suggesting a tectonic origin from buckling rather than uplift by serpentinization.