Ocean Bottom Seismographs Research Articles

A lithospheric seismic refraction profile in the western North Atlantic Ocean

Summary. A study of the seismic velocity structure of the subcrustal oceanic lithosphere was conducted in the western North Atlantic north of the Lesser Antilles, where the age of the lithosphere is 80-95 Ma. A 1000km long north-south array of 18 ocean-bottom seismographs (OBS), spaced at 60 km intervals, recorded arrivals from explosions at ranges up to 800km and arrivals from earthquakes in the northern Caribbean region at ranges up to 1400 km. Analysis of the travel-time and amplitude data from the explosions provided the velocity structure down to about 65 km below sea-level. The earthquake data were intended to extend the travel times to ranges over 1100 km, and thus sample the deeper part of the lithosphere. However, the geometry and velocity structure of the downdipping slab beneath the Puerto Rico Trench deflected the ray paths between the Caribbean hypocentres and the OBS array such that no rays travelling through the lower lithosphere were detected by the array. Only rays through the upper lithosphere reached the OBS array. The results show that the subcrustal oceanic lithosphere consists of two main regions. The uppermost region starts just below the M discontinuity and exhibits velocities of 8.2-8.4 km s-1. This region is characterized by three alternating zones of low (8.2 km s-1), high (8.4 km s-1) and low (8.2 km s-1) velocity. The second region lies below 53 km and exhibits high velocities of 8.5-8.6 km s-1. A sharp boundary separates these two regions, suggesting that a major change in composition or phase occurs there. The velocity structure within the upper region shows broad agreement with petrological models for the generation of oceanic lithosphere starting from a pyrolite composition. The alternating zones of low and high velocity correspond to zones depleted of, and enriched in, the basaltic fraction of the primary mantle material, respectively. This is a consequence of the processes of migration and differentiation of the upwelling asthenosphere near the mid-ocean ridge axis. However, none of the petrological models based on a pyrolite or peridotite initial composition can easily explain the high velocities of over 8.5 km s-1 observed for the lower region of the seismic velocity model. We speculate that this region may consist instead primarily of eclogite which is acceted from the asthenosphere as the lithosphere cools and ages. The results do not permit definition of the lower boundary of the lithosphere which probably lies below the 65 km maximum depth resolved in this study. S-wave observations were less reliable than the P-wave data, but generally support the P-wave velocity model. No evidence was found for azimuthal anisotropy within the upper region of the subcrustal lithosphere. However, the existence in the western North Atlantic of a weakly anisotropic, or even isotropic, lower lithosphere with a significant component of eclogite requires verification by future experiments.

Crustal structure of the western Arafura Sea from ocean bottom seismograph data

Abstract Refraction data taken from ocean bottom seismograph recordings in the western Arafura Sea indicate a continental‐type structure for the region. This structure is characterised by a thin column (2 km) of sediments, with velocities ranging from about to 2 to 4 km s‐1, overlying an essentially two layer crust. The compressional wave velocities in the upper and lower crust are 5.97 and 6.52 km s‐1, respectively, with the boundary between the layers at a depth of 11 km. Very weak mantle‐refracted arrivals with a velocity of about 8.0 km s‐1 were recorded. Large‐amplitude, later arrivals, beginning at distances near 100 and 150 km, have been interpreted to be part of the retrograde branches from the 8.0 and 7.33 km s‐1 layers, respectively. Model studies indicate that a small positive velocity gradient is required between 17 and 30 km, and that the Moho is at a depth of 34 km. A third set of large amplitude, later arrivals starting at a distance near 250 km has been interpreted as most probably multipl...

Spatial distribution of microearthquakes beneath the Japan Trench from ocean bottom seismographic observations

The area of the Japan Trench, about 40°N off the Tôhoku District of north-eastern Honshu, Japan, was for the first time extensively covered by an ocean bottom seismograph (OBS) array. Seven OBSs were available for analyses of natural earthquakes and the observation period lasted two weeks during 1980 July and August. During this period, each OBS registered between 300 and 600 earthquakes. About 100 events were simultaneously recorded by at least three OBSs. Hypocentres were calculated by using a generalized inversion technique. Sixty-five events, mostly with magnitudes (M) from 1.5 to 3.0, which were not located by a land network, were determined to be within or near the area covered by the OBS array. The OBS array had nearly uniform detectability for earthquakes with M over 2.0 within the latitude range of 39.5° — 41.5°N and the longitude range of 143° — 146.5°E. Within this area there is a highly active region of microseismicity beneath the seaward wall of the Japan Trench. The seaward activity is concentrated within 100 km of the trench axis and is low beneath the Northwest Pacific Basin. A seismicity gap was found just westward of the trench axis, beneath the landward wall of the trench at a water depth of between 6000 and 4000 m. Seismic activity continues landward from the region west of the gap. The OBS array enabled us to determine the focal depth of microearthquake activity with better resolution than was previously possible. The results show that the microearthquake activity is concentrated within the upper part of the oceanic lithosphere. On the seaward side, most events occur at a depth of less than 30 km beneath the trench wall, while the activity on the landward side starts to deepen westward exactly at the trench axis. This paper reports the first direct observations to clarify the spatial distribution of microearthquakes associated with the bending and/or subducting of the oceanic plate just beneath the Japan Trench axis.

Longshot experiments to study velocity anisotropy in the oceanic lithosphere of the northwestern Pacific

Several long-range explosion seismology experiments have been conducted in the northwestern Pacific basin, where one of the oldest oceanic lithospheres is postulated to exist. The experiments were conducted from 1974 to 1980. Highly sensitive ocean-bottom seismographs which had been developed for longshot experiments were used. The lengths of the profiles ranged from 1000 to 1800 km, and the directions were chosen to provide wide azimuthal coverage. One of the aims of this series of experiments was to test the existence of velocity anisotropy on a large, regional scale. The results show that the oceanic lithosphere has anisotropy wherein the velocity changes by 4–7%. The anisotropy extends from a depth of at least 40 to 140 km beneath the sea bottom; however, the magnitude of the anisotropy may vary with depth. The azimuth of the maximum velocity is 150–160° clockwise from north, and coincides with the “fossil” direction of spreading of the Pacific plate, whereas it differs from the present direction of plate motion by ∼ 30°. The azimuth does not seem to depend on depth. In the direction of maximum velocity, the lithosphere is basically two-layered: 8.0–8.2 and 8.6 km s −1. The depth of the interface is 50–60 km beneath the sea floor.

Aftershock distribution of the 1982 Urakawa-Oki earthquake determined by ocean bottom seismographic and land observations.

The Urakawa-Oki earthquake (M=7.1) occurred offshore of Urakawa, Hokkaido, Japan, on March 21, 1982. In order to investigate the aftershock activity of this event, we deployed four ocean bottom seismographs (OBS's) off Urakawa. During an observation period from March 29 to April 6, the OBS network detected more than 4, 500 earthquakes. Magnitude range of these events is from -2.0 to 5.0. Aftershock distribution of the Urakawa-Oki earthquake was determined on the basis of the OBS and land observations. In the analysis, we selected about 250 aftershocks with M≥2.0, because the seismic signals from these events were recorded clearly both by the OBS's and the land stations around the aftershock area. Hypocenter determination was carried out using a method of inversion analysis. Most of the aftershocks were located in a region of 42°00'-42°20'N and 142°25'-142°40'E. These events were distributed in a depth range of 10-30km. The characteristic dimensions of the aftershock area were estimated as 35km×25km. The events in the southern part(south of 42°15'N) were distributed on a plane with an area of 20-25km×10-15km dipping 20-30° northward. In the northern part of the aftershock area, we found another trend of aftershocks which inclined southward with a high angle of 60-70°. The events in this group were located on a plane with a relatively narrow area of 10km×10-15km. The aftershock distribution obtained in the present analysis suggests that the fracture mechanism of the Urakawa-Oki earthquake was very complicated.

東海沖海底地震計 (TKOBS) による観測結果

東海沖海底地震計 (TKOBS) による観測結果

Microearthquakes in the Black Smoker Hydrothermal Field, East Pacific Rise at 21°N

In July and August 1980, an array of five ocean bottom seismographs was deployed within 3 km of the 350°C hydrothermal vents at the Rivera submersible experiment (RISE) site at 21°N, on the East Pacific Rise. Two of these instruments were placed within 600 m of the vents, using a transponder navigation network. The array detected four basic types of events. The first type consisted of local, very small microearthquakes. Locations obtained for 11 of these events place three within 1 km of the vents, with the others elsewhere along the rise crest. They appear to originate either from movement on the faults in the area or from the hydrothermal system beneath this area. A study of the S‐P times of this type indicates a maximum hypocentral depth of 2–3 km, implying a similar limit to the depth of hydrothermal circulation and brittle fracturing in the vicinity of the vents. The second type of event found consisted of emergent earthquakes that have many of the characteristics of volcanic harmonic tremor. The frequency of these events falls in the 1–5 Hz range and are similar in appearance to those seen at Mount St. Helens prior to and during its May 1980 eruption. They may be either hydrothermal or volcanic in origin. The third type of event produced a very monochromatic, high‐frequency seismogram, with the energy concentrated at 20 Hz. These events also appear to have a local origin. The particle motion of these earthquakes shows almost pure Stonely wave propagation, and they appear to originate from a regularly oscillating dilatational source. The nature of the source of event types two and three is clearly not the usual sort of motion on a fault and is still not understood. The fourth type of earthquake recordings were regional events, apparently from the nearby Rivera and Tamayo fracture zones The low‐frequency acoustic noise in the water was 16–64 times higher within 300 m of the vents than it was 2 km away. This implies that the black smoker vents may be generating a large acoustical disturbance as the 350°C water pours into the ocean. The observations are among the first of seismicity on moderate to fast spreading ridges, and many of the events appear to be related to the hydrothermal activity found there.

Rivera Ocean Seismic Experiment (ROSE) overview

The Rivera Ocean Seismic Experiment (ROSE) was designed as a combined sea and land seismic program to utilize both explosive sources and earthquakes to study a number of features of the structure and evolution of a mid‐ocean ridge, a major oceanic fracture zone, and the transition region between ocean and continent. The primary region selected for the experiment included the Rivera Fracture Zone, the crest and eastern flank of the East Pacific Rise north of the Rivera, and adjacent areas of Baja California and mainland Mexico. These areas were to be instrumented with land and ocean bottom seismographs in order to determine good source parameter and location data for natural events and to record these events along a large number of paths crossing various parts of the region. Explosive charges were to be detonated at sea to supplement the natural events. However, the necessary permission to conduct the experiment was not received from Mexican authorities; therefore an alternate plan was implemented whereby the marine program had to be moved southward outside of territorial waters. This had the effect of transforming this experiment into three, almost independent components: (1) an experiment to study the East Pacific Rise south of the Orozco Fracture Zone primarily using ocean bottom recording and explosive sources, (2) a seismicity program at the Orozco, and (3) a land‐based program of recording natural events along the coastal region of Mexico. A considerable amount of useful data was obtained in each of the three subprograms. In the marine parts of the experiment we were able to address a variety of problems including structure and evolution of young oceanic crust and mantle, structure and dynamics of the East Pacific Rise, seismicity of the Orozco Fracture Zone, and partitioning of energy transmission between the ocean volume and the crust/lithosphere. On land, the fortuitous occurrence of the Petatlan M7.6 earthquake of March 14, 1979, permitted the acquisition of an excellent data set of foreshocks and aftershocks of this large event, which provide new insight into the filling of a major seismic gap in the region. This overview describes the scientific rationale and the design of the experiments, along with some general results. Other articles in this volume give preliminary scientific results from certain components of the overall experiment or, in some cases, report on other data pertinent to the scientific goals of ROSE.

Seismicity and crustal structure in the Orozco Fracture Zone: Project Rose Phase II

A total of 301 earthquakes were recorded in the vicinity of the Orozco fracture zone by seven Texas ocean bottom seismograph stations during the 2‐week period of the Rivera Ocean Seismic Experiment (ROSE) (phase II). Using data from the entire ROSE array, hypocenters of 50 earthquakes were determined. These revealed two distinct zones of seismic activity within the fracture zone. In addition to these earthquake families, many very small events were detected by a station located very close to the spreading center of the East Pacific Rise. The magnitudes of these earthquakes, defined by their duration times, were so small that most of them were recorded only at this station (station 14) in continual or swarmlike occurrences. The slope of the frequency‐magnitude distribution of these events is significantly larger than those of other earthquake groups detected during the experiment, i.e., they appear to have an unusually high b value. These results suggest that this new population of earthquakes may be associated with volcanic activity or stress release within highly fractured crustal material. Refraction studies in the fracture zone reveal the presence of a rather high‐velocity crustal layer (6.9–7.0 km/s) beneath the experiment zone. The Moho velocity and the crustal thickness are estimated at 7.8 km/s and 6.2 km, respectively.

Detection and location of earthquakes in the Central Aleutian Subduction Zone using island and ocean bottom seismograph stations

A network of eight University of Texas ocean bottom seismographs (OBS) operated for 6 weeks in 1978 about 50 km offshore of Adak Island, Alaska, and nearby islands. In 1979 a similar network of nine instruments was deployed for 7 weeks farther offshore within and up to 100 km seaward of the Aleutian trench. For shallow earthquakes on the outer trench slope, for shallow earthquakes in the thrust zone, and for intermediate‐depth events we have analyzed the OBS and island‐based network data and evaluated the island network's capabilities for earthquake detection and location and for focal mechanism determination. Our three major conclusions are presented. The first concerns shallow earthquakes on the outer trench slope. In 1979 about 30 earthquakes occurred within the Aleutian trench and up to 60 km seaward of the trench axis. The island network located none of these events and detected P phases for only three of them. Ray tracing shows that the islands lie in a geometric shadow zone for events on the outer trench slope. The best located events are shallower than 20 km and exhibit first motions consistent with normal faulting. Several authors have suggested that these events are caused by bending of the oceanic lithosphere at the outer rise prior to subduction. If so, then the event locations reported here show that the bending stresses exceed the strength of lithosphere only in a narrow zone extending about 10 km landward and 60 km seaward of the trench axis. The second conclusion concerns shallow earthquakes in the thrust zone. Epicenters determined by island stations alone are virtually identical to epicenters determined using data from both island and OBS stations. The locations are similar whether they are determined using flat‐layered velocity models or a more realistic model and a ray‐tracing scheme. Nearly all the earthquakes detected by the OBS network are also recorded by island stations. Three composite focal mechanism solutions that use data from both island and OBS stations have P axes parallel to the direction of plate convergence. The thrust zone events are separated from events on the outer trench slope by a gap in seismicity about 50 km wide situated on the inner trench slope. The third conclusion concerns earthquakes deeper than 70 km. Epicenters determined using island network stations alone lie 10 to 80 km south of those determined using OBS and island stations, with the differences between epicenters depending both on event depth and on the velocity model used. Ray tracing with a realistic velocity model shows that for events at 200‐km depth locations are about 80 km north of locations determined from island data alone. These results do not support the observed increase in the dip of the Benioff zone beneath 100‐km depth that is suggested by locations determined from island network data alone using a flat‐layered velocity model.

A seismicity study in northern Baffin Bay

A temporary seismograph network was operated in a seismically active region of northern Baffin Bay during part of September–October 1978. The array comprised three portable smoked-paper seismographs on Baffin Island and three ocean-bottom seismographs in Baffin Bay. During the experiment a number of earthquakes were recorded sufficiently well to be located by an iterative procedure. Two seismicity regimes are distinguished. The first is a narrow onshore zone of shallow (<10 km) earthquakes near and parallel to the coast. Most of this activity was concentrated in the Cape Jameson region of Baffin Island, with one earthquake of magnitude 3. A second, more diffuse group of events was offshore in Baffin Bay. These were deeper [Formula: see text] and indicated a possible trend normal to the coast. They may be associated with the fault plane of the 1933 magnitude 7.3 Baffin Bay earthquake. The seismicity results and the Baffin Island – Baffin Bay margin structure deduced from earthquake and explosion travel times are in agreement with previous studies.

Earthquake activity at the Kodiak continental shelf, Alaska, determined by land and ocean bottom seismograph networks

abstractThe spatial pattern of earthquakes determined by a combined land and ocean bottom seismometer (OBS) network in the Kodiak Island shelf region differs systematically from the pattern determined by a land network and from locations determined by the International Seismological Centre (ISC). As a part of a larger study of seismic risk on the continental shelf near Kodiak Island, we augmented the University of Alaska land network by deploying 11 recoverable OBS units south of Kodiak Island for 2 months in the summer of 1979. Despite a relatively short operation time and various instrument malfunctions, the combined network detected 19 locatable earthquakes in the shelf region. Because of the structural heterogeneity of this area, the earthquakes were located with a scheme which allowed different velocity models to be used for travel-time calculations of phases traveling to different stations in the network. The locations of earthquakes determined using data from both land and OBS networks were displaced about 12 km from the hypocenters of the same earthquakes determined using only land network data. For these events on the continental shelf, azimuthal control of the joint land-OBS network is excellent, and thus the joint land-OBS network locations are considerably more reliable than locations determined with the land network data alone. When the locations of the combined land-OBS network are compared to 15 yr of teleseismic locations reported by the ISC, the center of teleseismic activity appears to be about 20 to 30 km north of the center of activity determined in our study. This difference between locally determined and teleseismically determined location is similar to that observed in other studies of earthquakes and nuclear explosions in the Aleutian arc.

A three-component ocean bottom seismograph for controlled source and earthquake seismology

A three-component pop-up ocean-bottom seismograph was built at the Institute of Oceanographic Sciences in 1978. It is constructed around a buoyant 71 cm diam aluminium alloy forged sphere which contains three 4.5 Hz orthogonal geophones and an external hydrophone. The instrument will record continuously in analogue mode for over eight days using a modified reel-to-reel tape-recorder running at 1.5 mm s-1. The geophones have a bandwidth of 2–25 Hz and the hydrophone bandwidth is 5–40 Hz. Ballast release is by pre-set clock or by acoustic command. Fifty-four deployments have been carried out in five cruises for the loss of only one instrument. Good recordings of dropped weights, airguns, explosions and earthquakes have been obtained.

A microearthquake survey at the junction of the East Pacific Rise and the Wilkes (9 S) fracture zone

Summary. An ocean bottom seismograph survey of the junction of the East Pacific Rise and the Wilkes fracture zone detected only three microearthquakes beneath the rise crest during seven days of recording. In contrast, during the same period 41 events were detected on the fracture zone, all at distances greater than 10 km from the junction. These results suggest that near the rise crest the thin crust can support sufficient stress only to generate infrequent small earthquakes and that most faulting may take place by aseismic slip. At 10 km from the rise axis part of the crest has become competent enough to support stress, resulting in earthquakes probably at depths of up to 5 km below the sea-bed. Gear 5-waves on the seismometer records indicate that a magma chamber, if it exists near the junction, is less than 10 km across.

Evidence that biological activity affects Ocean Bottom Seismograph recordings

Brief and impulsive signals of uncertain origin appear regularly on records from Ocean Bottom Seismographs (OBS) of several institutions. These signals have been recorded on nearly all deployments of the Texas OBS, including sites at depths greater than 7000 m. At some sites, they account for over 90% of the events recorded. They are of short duration (usually 0.5–4.0 s) and have a characteristic frequency (usually in the range of 4–18 Hz) that differs from site to site. When networks of OBS instruments are deployed, the signals are not recorded simultaneously by different instruments. Neither the frequency content nor the distribution of durations of these signals is similar to what is observed for known earthquake events. We present evidence suggesting that the signals are of biological origin, perhaps caused by animals touching the OBS units. (1) The distribution of these signals on instruments deployed at depths shallower than 1000 m shows a 24 h periodicity, while there is a 24 h periodic pattern on instruments deployed at sites deeper than 1000 m (where there is no visible light). (2) The frequency of occurrence of signals is similar to the vertical distribution of biomass in the oceans, i.e., they appear most frequently on OBS instruments deployed at very shallow depths. (3) Biological material has been found attached to several OBS units upon recovery.

Seismicity surveys with ocean bottom seismographs off western Canada

Three arrays of ocean bottom seismographs have been deployed to study the seismicity at the northern end of the Juan de Fuca ridge system off western Canada. Nearly 100 events were located with estimated accuracies generally better than ±10 km, all lying on or near the en echalon ridge‐transform fault plate boundaries as defined in this area by the magnetic anomalies, the seafloor morphology and by other geophysical data. The depths of 12 events were determined to lie between 2 and 6 km below the top of the crust. The seismograms exhibit clear P and S wave arrivals along with phases that involve P to S and sometimes S to P conversion probably at the base of the sediments beneath the instruments. The event magnitudes have been estimated from signal duration using four calibration events that were well recorded by a land station. The magnitude estimates permit the determination of rough magnitude‐frequency of occurrence relations over the magnitude range of 1 to 3 that are in surprisingly good agreement with the recurrence relations for the area at larger magnitudes from 75 years of land station data. The mean P wave velocity in the uppermost mantle from the earthquake data recorded by the sea floor arrays is 7.6 km s−1 and the mean Vp/Vs ratio is 1.71 or a Poisson's ratio of 0.24.

Queen Charlotte fault zone: microearthquakes from a temporary array of land stations and ocean bottom seismographs

A temporary array of land and ocean bottom seismograph stations was used to accurately locate microearthquakes on the Queen Charlotte fault zone, which occurs along the continental margin of western Canada. The continental slope has two steep linear sections separated by a 25 km wide irregular terrace at a depth of 2 km. Eleven events were located with magnitudes from 0.5 to 2.0, 10 of them beneath the landward one of the two steep slopes, some 5 km off the coast of the southern Queen Charlotte Islands. No events were located beneath the seaward and deeper steep slope. The depths of seven of these events were constrained by the data to between 9 and 21 km with most near 20 km. The earthquake and other geophysical data are consistent with a near vertical fault zone having mainly strike-slip motion. A model including a small component of underthrusting in addition to strike-slip faulting is suggested to account for the some 15° difference between the relative motion of the North America and Pacific plates from plate tectonic models and the strike of the margin. One event was located about 50 km inland of the main active zone and probably occurred on the Sandspit fault. The rate of seismicity on the Queen Charlotte fault zone during the period of the survey was similar to that predicted by the recurrence relation for the region from the long-term earthquake record.

Sub-Moho seismic profile in the Mariana Basin — Ocean bottom seismograph long-range explosion experiment

Ocean bottom seismograph (OBS) long-range explosion experiments were carried out in the Mariana Basin in 1973 and 1976. Seven large shots (8.5–1.5 ton) as well as several tens of small shots were fired. The maximum range of observation was about 1900 km. As many as 25 OBS stations were deployed in an array of about 800 km. It is found that the sub-Moho P-wave velocity structure is of stratified nature, being composed of alternating high- and low-velocity layers. High-velocity layers with apparent velocities of 8.1, 8.2, 8.4, 8.6 and 8.7 km/s are identified. Low-velocity layers, sandwiched between the high-velocity layers of 8.4, 8.6 and 8.7 km/s, are very prominent. The sub-Moho high-velocity lid with an apparent velocity of 8.4 km/s is very thin. Thinning of this lid, thickening of the low-velocity layer, and the presence under it of another high-velocity layer (8.6 km/s) appear to characterize the uppermost mantle structure beneath the Mariana Basin.

An earthquake sequence studied with ocean-bottom seismographs

Summary. A tripartite ocean-bottom seismograph array at the junction of the East Pacific Rise and Rivera Fracture Zone recorded an eathquake sequence, consisting of three main shocks (mB= 4.3, 4.3 and 4.8) and numerous aftershocks from the fracture zone, in the distance range 35–50 km. Delineation of the rupture zones by aftershocks indicates that the first two main shocks took place on overlapping fault areas, while the third occurred over a fault area separated from the first by several kilometres. Both rupture zones were about 4 km long. Surface wave spectra indicate a shallow (about 3 km below the sea floor) source, as does OBS array phase velocity data. The seismic moments, obtained from teleseismic surface wave data, of 1.3, 2.1 and 2.8 × 1023 dyn cm, with the fault areas as delineated by aftershocks, imply a stress drop of about 8 bars for the main shocks. Aftershock sequences of each of the main shocks are similar, with a b-value of about 0.65. Teleseismic P travel times are similar to those from near-surface sources in Nevada.

Queen Charlotte [western Canada] fault zone; microearthquakes from a temporary array of land stations and ocean bottom seismographs : Hyndman, R.D. and R.M. Ellis, 1981 Can. J. Earth Sci., 18(4):776–788

Queen Charlotte [western Canada] fault zone; microearthquakes from a temporary array of land stations and ocean bottom seismographs : Hyndman, R.D. and R.M. Ellis, 1981 Can. J. Earth Sci., 18(4):776–788