Ocean Bottom Seismographs Research Articles

T Waves Associated with Submarine Volcanic Eruptions in the Marianas Observed by Ocean Bottom Seismographs.

In September and December of 1989, swarms of T waves were observed by ocean bottom seismographs (BSOBS) off the Boso Peninsula of Japan, by a SOFAR hydrophone array near Wake Island, and by a station at Rangiroa. Using arrival times observed by these networks, the source location of the T waves was determined to be in the northern Marianas, where submarine eruptions had been observed frequently in recent years.The T waves were not accompanied by corresponding body waves. The T waves had shorter duration (from a few to ten seconds) and higher prominent frequency than those of tectonic earthquakes which also occurred in the northern Marianas. The amplitude variation of the T waves observed by BSOBS showed a different pattern from those from tectonic earthquakes. This is a manifestation of a difference in the spectral contents between them. The spectra of these T waves showed conspicuous harmonic peaks. These characteristics are similar to those of volcanic T waves as previously reported. Therefore we concluded that they are excited by submarine volcanic eruptions.From the detailed analysis of the T wave events, this volcanic activity is considered to proceed through three stages: the first which was not so explosive, the second which was composed of intermittent explosive eruptions, and the third which was small and similar to the first. The observed fundamental frequencies of the T waves varied with time. This could be caused by changes in the eruption site or in the size of the area where reverberations occurred.

Ocean Bottom Seismographs used in a crustal study of an area covered with flood‐basalt off Lofoten, N. Norway

ABSTRACTSeismic refraction measurements were made in August 1988 to study the crustal structure off Lofoten, Northern Norway. Twenty‐four 3‐component Ocean Bottom Seismographs (OBS) were used, of which seven were deployed in the area covered by landward‐flowing basalt deposited during the early Eocene break‐up between Norway and Greenland. The main purpose of the OBS survey was to investigate whether this method can be used to map structures below the basalt, which is not easy to penetrate with conventional seismic reflection techniques. The records obtained showed that the OBS data contain considerable information about structures below the flood‐basalt; preopening sediments up to 4.0 km thick is indicated below the 1.0–2.5 km‐thick landward‐flowing basalt. The success of the OBS survey indicates that such measurements can become an important tool in investigations on passive volcanic margins and, potentially, in other areas where highly reflective boundaries make the reflection technique difficult to apply.

Microearthquake hypocenter determination in the southern Aegean Sea from records of sea-floor and land-based seismographs

During the cruise of the Soviet research vessel RIFT in April 1987 in the southern Aegean Sea, an array of five ocean bottom seismographs was deployed. During its operation (19–27 April), more than 400 events with magnitudes, M L, varying between −0.5 and 4.4 were recorded. Their spatial distribution reconfirms the typical characteristics of a subducting slab (Mediterranean lithosphere) under the Aegean lithosphere. The thickness of the seismogenic layer in the southern Aegean sea is of the order of 30 km.

Spatial distribution of earthquakes associated with the Pacific plate subduction off northeastern Japan revealed by ocean bottom and land observation

Precise earthquake locations in the vicinity of a trench axis elucidate the characteristics of the shallower part of a subducting plate. We split the Japan Trench into two regions, off-Sanriku in the north and off-Fukushima in the south, on the basis of the seismic properties of the regions and the fine microearthquake structure investigated by ocean bottom seismographic surveys, as well as recent observation of major seismic activity. From ocean bottom seismograph data, the hypocentral distributions along a profile perpendicular to the trench axis, indicate that almost all earthquakes occur along the boundary separating the subducting Pacific plate and the landward plate. A difference of the hypocentral distribution between off-Fukushima and off-Sanriku is found at the shallow plate boundary level. The seismic thrust zone begins at more than 100 km landwards from the trench axis off-Fukushima, whereas it begins at 50 km off-Sanriku and is nearly flat between 50 and 100 km from the trench axis. This reflects regional variations of the interplate coupling. In spite of these variations, a common characteristic in recent observations indicates that in both regions almost all the hypocenters are deeper than 10 km. This suggests the plate boundary to be decoupled at least in the part shallower than 10 km.

Seismic seafloor observations: a study of ‘anomalous’ intraplate seismicity in the northeastern Indian Ocean

Present-day intraplate tectonic activity in the northeast Indian Ocean, which manifests itself in high seismicity, is unusual for oceanic lithosphere plates. The contradictory points of the various tectonic hypotheses proposed to explain this ‘anomalous’ seismicity may be due to sparsity of ocean intraplate earthquakes of M > 4.5, which are recorded by means of networks of land-based permanent seismic stations. Seismological observations on the ocean floor by means of ocean bottom seismographs may provide additional and important data. Some results of such observations carried out in the Central Indian Basin simultaneously with deep seismic sounding surveys are analysed in this paper. About 118 seismic events were recorded at a site near 3°45′S and 80°E for a period of 7.3 days, and 24 events were recorded at a site near 0°50′S and 83°53′E for a period of 3.3 days. Magnitudes of these events range approximately from 2.8 to 4.0. Estimates of epicentre locations based on the Jeffreys-Bullen travel-time table show that over 90% of the earthquakes recorded at the first site were related to the Chagos Trough, during a period when no event was recorded there by land-based seismic stations. The most probable reason for these events is the Chagos Bank earthquake of 30 November 1983 ( M s = 7.6), which caused many aftershocks extending eastward from the main shock. Thus the high aftershock activity east of the Chagos Bank earthquake revealed by ocean bottom seismic observations complements the data on epicentres of larger earthquakes, which in turn provide the scope to extend the known seismicity of the Chagos Bank further east. In general, our results are in accordance with the most popular tectonic model of the Central Indian Ocean, in which a diffuse plate boundary divides the Australian plate from the Indo-Arabian or the Indian plate, initiating a new subduction zone.

A crustal study off Lofoten, N. Norway, by use of 3-component Ocean Bottom Seismographs

Twenty-four 3-component Ocean Bottom Seismographs (Obs) were used in August 1988 in a combined seismic refraction and reflection study off Lofoten, N. Norway. The purpose of the experiment was to map the crustal structure from the continental shelf to the oceanic crust off Lofoten. The very high data quality is demonstrated by the strong P-wave and shear-wave reflections, as well as converted waves from the Moho observed on the continental shelf. These arrivals are observed continuously from near vertical to wide angle incidence. Very high seismic sea-floor velocities in this area (3.1 km/s to 5.1 km/s) indicates absence or very thin sequences of Mesozoic sediments. The 5.1 km/s refractor coincides with the base Cretaceous reflection interpreted from the multichannel reflection data. The crystalline continental crust is here divided in layers with velocities of 6.0 km/s, 6.4 km/s and 6.8 km/s, respectively. On the seaward side of the escarpment Tertiary sediments varying in thickness from 1.0–1.5 km are situated on top of a 1.5-2.0-km-thick layer of flood-basalt containing seaward-dipping reflectors. A layer with velocity of 6.7 km/s is observed above the lower crust, which in this area is found to have a velocity as high as 7.3 km/s. These high velocities indicate that the crust in this area is of oceanic origin or, alternatively, the high-velocity layer in the lower crust might represent a magmatic body underplating highly thinned and intruded continental crust. Seven obss were deployed in the area that was covered by landward-flowing basalt deposited during the early Eocene breakup between Norway and Greenland. The survey was partly performed to investigate whether this method can be used to map structures below the basalt, which is impenetrable with conventional seismic reflection techniques. The obs data contain considerable information about structures below the flood-basalt; pre-opening sediments up to 4.0 km thick are indicated below the 1.0–2.5-km-thick landward-flowing basalt. The velocities of the crystalline portion of the crust are found to be similar to those observed under the continental shelf (6.0–6.8 km/s), which implies that the crust east of the escarpment is of continental origin. The crystalline crust is strongly thinned in this area, showing a minimum thickness of about 6 km. The depth to the Moho increases from about 15 km in the western part of the area to about 26 km on the continental shelf. The success of the obs survey indicates that such measurements can become an important tool in investigations on passive volcanic margins, and, potentially, in other areas where highly reflective surfaces make the reflection technique inefficient.

Shear waves from three-component ocean bottom seismographs off Lofoten, Norway, indicative of anisotropy in the lower crust

SUMMARY Data from four three-component ocean bottom seismographs (OBS) situated on a 145 km long profile on the Rost High on the continental shelf off Lofoten, northern Norway, have been modelled using 2-D seismic ray tracing, and a P- and S-wave velocity model along the profile is presented. The S-wave model has been obtained from a study of the high quality, horizontal component data. A Vp/Vs ratio of 1.95–2.15 is found in the sedimentary layers on the Rost High. These high values are consistent with shaly sediments, which are identified in dredged bedrock samples. South of the Rost High a considerably lower Vp/Vs ratio (ca. 1.6) is estimated for the sediments, which can be attributed in part to a higher degree of compaction in the deepest sediments and in part to a more sandy lithology. In the upper and middle crystalline crust a Vp/Vs ratio of 1.75 is obtained. Strong S reflections and P-to-S conversions at the Moho are observed on all four instruments, and the modelling of these arrivals indicates that the S-wave velocity in the lower crust varies with angle of incidence, from 3.5 km s−1 (Vp/Vs= 1.95) at vertical incidence to 4.0 km s−1 (Vp/Vs= 1.7) at 60–70d. This angle of incidence dependent S-wave anisotropy (the order of 10 per cent) is consistent with some horizontal or subhorizontal layering in the lower crust.

Tomographic image of the axial low‐velocity zone at 12°50′N on the East Pacific Rise

The 1982 MAGMA seismic refraction experiment yielded a large set of accurate P wave travel times corrected for bathymetry and anisotropy which sample the structure of the East Pacific Rise (EPR) at 12°50′N. The arrivals were recorded using ocean bottom seismographs at three sites: on axis, and at 7 km and 16 km east of the axis. We invert 2320 travel times for the twodimensional crustal seismic velocity structure across the EPR axis, assuming that the velocity structure is invariant along strike. The travel times provide strong evidence for compressional velocity anisotropy in the upper crust corresponding to ∼10% faster velocities for propagation parallel to the axis than perpendicular to the axis; the travel tunes used for the tomography are corrected for the effects of this azimuthal anisotropy. Our preferred model contains only the structure clearly required by the data (structure which is stable under excessive smoothing) and achieves a variance reduction of 81% relative to the laterally homogeneous starting model. We resolve a substantial zone beneath the rise axis in which the velocity is reduced by 0.4 to 0.7 km/s; this low‐velocity zone (LVZ) is about 7 km wide and extends from a depth of about 1.5–2.0 km down to Moho at a depth of 5.5 km. The LVZ is slightly asymmetric, extending 1 km further to the east than to the west of the axis. In the shallow (<1.0 km depth) crust, a pattern of velocity variations is imaged in which velocities are high at the spreading axis, decrease between 3 km and 7 km east of the axis, and then increase again between 10 and 15 km east of the axis. We investigate the resolution of the inversion using an impulse response method; the LVZ and off‐axis upper crustal variations are well resolved. In addition, the travel time data indicate that an axial high‐velocity anomaly less than 2 km wide exists in the upper crust but is not resolved by the inversion. The small velocity reductions of the LVZ are consistent with hot rock containing only a small quantity of melt. These results, combined with the multichannel seismic reflection lines and expanded spread profiles from the northern EPR, suggest that the zone of high melt fraction under the spreading center is confined to a narrow, thin lens capping a broad zone of hot plutonic rocks. The upper crustal velocity reduction within 1 km of the axis reflects near‐axial thickening of the extrusive layer and the later reduction probably reflects porosity increases due to near‐axial tectonism; the upper crustal velocity increase beyond 15 km off axis is attributed to porosity decreases associated with hydrothermal alteration.

Seafloor shear wave velocity variability.

In gross classification, sediments of the deep seafloor may be of terrigenous or marine origin. Those of marine origin are classified as oozes (generally calcareous) or clays (generally siliceous) depending largely on whether their CaCO3 content is greater or less than 30%. The carbonate contents is, in turn, controlled both by the biological productivity of the surface waters and the circulation patterns of the deep waters of the oceans. Older waters have more dissolved CO2 and attack carbonates more aggressively. There are now a few deep-sea in-situ shear velocity measurements in siliceous and calcareous sediments. The siliceous sediments show velocities that are the lowest, 20–30 m/s at the surface, while calcareous sediments have slightly higher velocities. Both of these have velocities lower than those found in shallow silts and sands. Measurements of the shallow shear velocities show high variability, approaching 50%, over a few km distance in the siliceous sediment (Dorman etal., 1991). These measurements are made using interface waves from seafloor explosions observed using ocean bottom seismographs. These may be correlated with the channeling of sediments but the mechanism is far from clear. [Work supported by ONR.]

Microseismicity of the frontal Hellenic arc according to OBS observations

Five ocean bottom seismographs (OBS) of the pop-up and tethered types were installed southeast of the island of Crete at depth ranging from 1700 to 4000 m, and worked continuously for one week (July 2–9, 1988). Some one thousand microearthquakes with local magnitude M L from approximately 0 to 1.9 were registered. A total of 105 quakes were recorded by three or more instruments. Foci of these microearthquakes in the earth's crust and upper mantle are concentrated off the eastern flank of Crete along the contact between two blocks of the Hellenic arc. The en-echelon arrangement of the Pliny and Strabo deep-water trenches to the south of Crete is accompanied by a twofold Benioff zone. The seismic zones dip under the Hellenic arc with the same angle of 60° and are also en-echelon. The Strabo zone is probably the younger with a maximum focal depth of 70 km against 120 km for Pliny zone. Seismic activity in the crust beneath the island is revealed to extend along the entire slope of the Pliny trench, but there is a principal maximum of activity at ±5 km, a minimum at 15–20 km, and a second maximum at 20–30 km. These findings are in agreement with results from OBS observations made to north of Crete in 1987.

Deep seismic crustal structure beneath the Bonin Trough

The Bonin Trough and the Bonin Ridge are located in the fore-arc region of the southern Bonin Arc System. Such a conspicuous trough-ridge formation along an arc cannot be recognized in other trench-island arc systems. The southern Bonin Arc is characterized by other several unique geological and geophysical features. The tectonics controlling their formation and evolution has attracted interest of many earth scientists but has not been understood well. In 1987, we made a seismic refraction experiment across the Bonin Trough to clarify the structure of the crust and the upper mantle of this area which has not been known well to date. High quality seismic data of airgun shooting and of explosive sources were acquired by ocean bottom seismograph and hydrophone (OBSH) systems. A detailed seismic velocity structure model was obtained by modeling of the observed travel times and amplitudes using a two-dimensional ray tracing technique. The structure is considerably variable across the Bonin Trough, especially in the sedimentary layers, in spite of its flat sea-floor topography. Thinning of the upper crust is recognized in the trough suggesting that the Bonin Trough was formed by rifting of the island arc crust. Uplifting of the crust and the uppermost mantle beneath the Bonin Ridge is required in our structure model. The depth to the Moho boundary decreases from 18 km beneath the Shichito Ridge to 14 km beneath the Bonin Ridge; this is in good agreement with the crustal structure expected from large positive gravity anomaly observed over the Bonin Ridge.

Attenuation of S-waves in the lithosphere of the Sea of Crete according to OBS observations

Abstract Five OBS were installed in the Sea of Crete in 1987. During a period of nine days 430 local earthquakes were recorded. Hypocentres were determined for 85 microearthquakes. A correlation between magnitude ML and duration of seismic events on the records of the ocean bottom seismographs (OBS) was found from data of 14 shocks recorded simultaneously by OBS and some land-based Greek stations. A magnitude-frequency relationship for earthquakes recorded by OBS was obtained in the magnitude range ML = 2–4. Amplitude curves describing the attenuation of body waves in the lithosphere of the Sea of Crete were compiled. Assessment of quality factor Qs was made by comparison of empirical and theoretical amplitude curves. The values of Qs for the lithosphere of the Sea of Crete were calculated and found equal to 200–300. Low values of Qs and consequently strong attenuation of S-waves in the lithosphere of the Sea of Crete could be explained by extension of the lithosphere accompanied by its partial contamination with melted magma.

Microearthquake activity in the Suakin Deep, Red Sea and tectonic implications

Abstract During a 10-day microearthquake survey in February 1988 in the Suakin Deep, Red Sea, twelve 3-component Ocean Bottom Seismographs (OBS) recorded nearly 1000 events. Precise localizations were made in 221 cases and indicated the existence of two different types of microearthquakes: very shallow ones with a focal depth of less than 1000 m, associated with hydrothermal circulation and those that are tectonically induced and occur in the oceanic crust and lithosphere (maximum hypocentre depths of 18 km). Magnitudes were determined using the signal duration and range between 0.3 and 2.8 ML. The distribution of the hydrothermally induced seismicity shows intense clustering in the central and eastern parts of the deep trough (water depths ranging from 2500 m to 2800 m) in association with brine occurrences. Crustal and lithospheric events are scattered along the escarpments of the axial trough and blockfaulted elements within it. The eastern graben flank, however, tends to be more active than the western one. The deformation is controlled to a great extent by microfracturing, expressed by the many very small events (ML

P wave velocity anomalies between the Tonga subducting slab and the overlying mantle beneath the Lau Basin

During a 3‐week period in November‐December 1984, nine ocean bottom seismograph (OBS) stations were operating in the Lau basin concurrently with a land‐based station in the Tongan island of Vava'u (VAV). Data from two intermediate‐depth (200 km < h < 300 km) and 10 deep (.h > 400 km) earthquakes in the Tonga Benioff zone allowed the P wave velocity contrast between the subducting slab and the overlying mantle beneath the Lau basin to be determined. Based on relative residuals OBS‐VAV and on three‐dimensional ray tracing, a P wave velocity contrast of 7% is inferred. This is an average contrast for depths above 400 km. The standard error of the estimates is of the order of 2%. The data are consistent with a hot upper mantle beneath the Lau basin where temperatures are close to the solidus temperature, but they do not necessarily require the presence of partial melt in the asthenosphere beneath the Lau basin.

Ocean bottom tiltmeter developed for submarine volcano monitoring

Most of the oceanic crust is formed by intrusive processes, yet these processes at mid‐ocean ridges are poorly understood when compared to on‐land volcanos that are well monitored with respect to their surface deformation and seismic activity. This limited knowledge of tectonic processes at mid‐ocean ridges stems from an absence of direct observations and inhibits our understanding of their magmatic segmentation, and many first‐order magmatic processes, including fractionation and mixing. As a means to improve this situation, we have installed tiltmeters in the Scripps Ocean Bottom Seismographs (OBS) and begun initial performance testing in sediment and hard rock environments. These tests indicate that the TILT‐OBS is now ready for realistic, designated testing on submarine volcanos. Such tests may be in the form of a rapid response monitoring of recently discovered submarine volcanic activity, or as the first trial measurements of strain in more stable, mid‐ocean ridge volcanos.

Introduction to Special Section on EDGE and Related Seismic Projects, Onshore‐Offshore California

The continental margins hold a special interest to Earth scientists because they mark the complex transition from oceanic to continental crust. This transition often involves a number of interacting processes, including subduction, accretion, and deformation of the oceanic lithosphere and growth and modification of the continental lithosphere, particularly the crust. The past decade has seen a great deal of progress in geophysical studies of continental margins, in large part due to the successful application of marine multi channel seismic (MCS) surveys that have provided unprecedented details on the deep structure.An opportunity arose in 1986 to augment, at a relatively modest cost, an MCS survey commissioned by the Pacific Gas and Electric Company (PG) across the central California margin. At the initiative of the Houston Area Research Center (HARC) and Rice University, funds were provided to the M CS contractor to record several additional MCS profries, and land seismographs were deployed to record the marine air gun shots and land shots prepared by the U.S. Geological Survey. Ocean bottom seismographs were deployed on two of the profiles by Oregon State University. This study constitutes the first of many profiles of opportunity completed or planned by EDGE, a National Science Foundation and industry funded consortium of institutions committed to geophysical studies of continental margins.

Seafloor shear wave velocity structure determined from interface wave dispersion

The propagation of interface waves in the seafloor waveguide is primarily controlled by the sediment shear velocity, which increases rapidly with subbottom depth. This gradient causes the propagation velocity to be strongly frequency dependent and this dispersion allows one to infer the shear velocity structure from propagation velocity measurements. These waves were excited with small explosions on the seafloor and the dispersion was measured over distances of a few hundred meters, observing seafloor motion on ocean bottom seismographs (OBSs). It is common to observe these waves in the 0.5–5 Hz frequency range, even from explosions with bubble frequencies in the 100‐ to 400‐Hz range. Typically several modes are seen and shear velocity in the 1‐ to 30‐m depth range can be recovered. The attenuation is high (Q = 25) in the very near surface, consequently, modes with large energy just below the surface suffer severe attenuation, while modes with little energy in the highly absorbing regions (higher modes and all modes whose phase velocity is near that of water) show Q up to 500. [Work supported by ONR.]

日本列島周辺の海底地殻構造 地殻構造から見た海溝‐島弧‐背弧系

The results of the recent seismic refraction surveys using an Ocean Bottom Seismograph (OBS) array have clarified fine crustal structures of the trench=island-arc= back-arc systems around the Japan Islands, namely the Kurile, the Northeastern Japan, the Southwestern Japan, the Izu-Ogasawara and the Ryukyu. The present paper tries to look over theseismic velocity structures of the crust and of the uppermost mantle beneath the trenches, the island arcs and the back-arc basins revealed by the OBS studies in the last decade.New OBS data have shown regional variations in the crustal structures of the subduction zones. The crustal structures around trenches vary not only from a trench to another but also along one trench. The dip angle of a subducting oceanic lithosphere, the spatial extent of an accretionary prism and the shallow structure of a continental slope characterize each subduction zone and are deeply related to its tectonic evolution. Interesting similarity and dissimilarity are found between the structures of the Bonin Trough, a intra-arc rift, and of the Okinawa Trough, a back-arc rift, providing a clue to understanding of the mechanism of the rifting in the island-arc crust. We also note the crustal structure of the Yamato Basin, a part of the Sea of Japan (an inactive back-arc basin), a curious structure in which the crust has a composition similar to the normal oceanic crust but has a twice thickness.

海洋地殻深部地震探査

This paper reviews recent development of a study of oceanic crust. An old image of crustal structures is replaced by a modern picture with a large amount of new observations. Basically the classical three-layered crustal model is acceptable if we regard it as an averaged structure with depth. The detailed modern model provides a clue to understand crustal dynamics and an origin of the oceanic lithosphere. We are now able to image a magma chamber beneath the mid oceanic ridge, subducting oceanic crust at trench areas, and plutonically solidified mantle melt on passive margins.A new technology of controlled source seismology includes waveform observation with well-calibrated amplitudes and phases, a two-ship method with multichannel expanding seismic profiling and an ocean bottom seismograph. These methods enable us to use spatially dense seismic data, which make it possible to investigate a detailed structure of the oceanic crust varying vertically and laterally.

1980年代の海底地震観測

A review of seafloor observations of seismic waves is presented, highlighting progress in the 80's. Observations made by ocean bottom seismographs have proved superiority in the acquisition of better quality data than conventional surface hydrophone observations and in determining more accurate seismicity and seismic structure in ocean areas. The decade has seen developments of highly reliable instruments which can be deployed in tens of quantity. Efforts are continuing to broaden the observational window in the frequency range and dynamic range, which require to overcome noise problems linked to observational site and instrument coupling parameters. Long-term observation is also another important goal for the next generation of seafloor seismic instruments to construct a truly global seismic network.