The results of an experiment designed to measure the shear-wave velocity structure of the shallow-water sediments at three locations in the East China Sea, in water depths ranging from 40 m to 150 m, are reported. Both vertically polarized and horizontally polarized shear-wave velocities were measured by placing both source and receiver on the seafloor. The recorded data consist of interface waves of Love wave and Stoneley/Scholte wave type, and both horizontally polarized (SH) and vertically polarized (SV) body waves. With the aid of synthetic seismogram analysis, the travel times and amplitudes of these phases were used to constrain the velocity structure of the seabed to depths of ∼50 m. These results show that velocities range from less than 50 m/s at the seafloor to greater than 400 m/s at subseafloor depths of only 50 m. Vertical velocity gradients of up to 200 and 15 s−1 are required in the upper 1 and 10 m of the seabed, respectively. Comparison of profiles carried out at the same approximate location shows that the velocity structure of the upper 10 m varies by up to 80% over lateral distances of less than a few kilometers. In the data bandwidth of 5–30 Hz, shear Q values range from 10–20 (attenuation factor of 0.07–0.14 m−1 at 20 Hz) at the seafloor to 40 (attenuation factor of 0.004 m−1 at 20 Hz) at depth. The shear velocities immediately below the seafloor differ by less than a factor of 2 from the values predicted by applying the Biot theory to the geotechnical parameters measured on cores taken at the experiment locations. In two of three cases the differences between the predicted and measured values are less than 8%.

The EDGE seismic experiment across the Virginia continental margin delineated a Paleozoic suture, buried Appalachian terranes, and Mesozoic rifting and magmatic events. The seismic grid revealed that the Mesozoic Norfolk rift basin exists only in the northern one-third of the previously mapped area. The north-striking listric border fault of the Norfolk basin half-graben parallels seismic laminations in the basement. The Jurassic volcanic wedge pinches out just landward of the Baltimore Canyon trough hinge zone and downlaps on the hummocky oceanic basement under the continental rise. Under the continental slope, the volcanic wedge reaches depths >9 s (20 km). Two distinct intracrustal reflections at 4.0-5.0 s and at 7.0 s TWTT (two-way traveltime) dip southeastward at low angles (approximately 15-degrees). The Moho reflection is disrupted where it is intersected by the 7.0 s reflection. Northwest of this point the Moho dips landward; seaward it is horizontal. Seaward of this point, the lower-crustal boundary laminations exist in a narrow interval (10.5-11.0 s) and are of strong amplitude. These changes in the Moho and lower crust represent the seaward edge of the Grenville-age North American crust and the landward edge of Jurassic magmatic underplating. A northwest-dipping reflection observed for the first time on the U.S. Atlantic margin may be the top of the Jurassic magmatic-underplating layer, the northwest-dipping reflection truncates the southeast-dipping 7.0 s TWTT reflection. Landward projection of the 7.0 s reflection yields a north-south trace on the postrift unconformity under the center of lower Chesapeake Bay. This trace is near a basement fault between low-grade metamorphic rocks (Carolina slate-Avalonia) on the east and high-grade rocks (Goochland terrane) on the west. This fault boundary and the southeast-dipping 7.0 s reflection probably represent the Taconic suture.

We present new ocean-bottom, wide-angle seismic data, collected during the 1990 EDGE Mid-Atlantic multichannel seismic experiment, which provide a measurement of the deep velocity structure of rifted Appalachian continental crust beneath the U.S. East Coast continental margin. Reflections from the entire crust and Moho are visible from offsets of zero to 100 km. One-dimensional inverse traveltime modeling reveals a 34-km-thick crust consisting of four layers beneath the post-rift sediments, with velocities of 5.9, 6.3, 6.4, and 6.85 km/s. This velocity structure is indistinguishable from that found beneath several Appalachian terranes in the northern Appalachians of New England. The average velocity of 6.85 km/s in the lower crust limits the landward extent of the high-velocity (7.2-7.4 km/s) lower crust found farther seaward, and supports the interpretation of that layer as the result of rift-related magmatic underplating, rather than as thinned, pre-existing lower continental crust.

Spectral studies of commercial marine multichannel reflection profiling data demonstrate their utility for providing estimates of the degree of spatial variability in low-frequency (<100-Hz) transmission characteristics that is present in contrasting shallow water environments. The excellent spatial sampling provided by these readily available datasets allow them to resolve ∼10-dB changes in returned energy levels occurring over distances of as little as a few hundred meters. At frequencies below the cutoff frequency, when the wave field is no longer dominated by water-borne energy, these small scale variabilities in transmission losses cannot be straightforwardly predicted from routinely collected mapping data (e.g., reflection profiles and velocity analyses of pre-critical reflections). Existing commercial marine multichannel reflection profiling datasets constitute an important resource for the mapping of changes in propagation characteristics in shallow water.

The interleaving of parallel isotropic lamellae of contrasting mineralogical composition makes almost all marine sediments anisotropic, the form of anisotropy being transverse isotropy with a vertical axis of symmetry. Conventional marine seismic experiments, however, cannot quantify the anistropy because they do not record unconverted shear waves. In 1986, Rondout Associates, Inc. (RAI) and Woods Hole Oceanographic Institution (WHOI) recorded direct shear waves in shallow marine sediments by using a newly developed ocean-bottom shear source and a multicomponent on-bottom receiver. No single isotropic model could be adequately fit to the data, implying anisotropy. The seismic experiment was conducted in 21 m deep water about 10 km east of the New Jersey coast. In this paper, we describe the anisotropy in the top 50 m of marine sediments beneath two of the RAI/WHOI refraction profiles. We use an anisotropic reflectivity program to produce synthetic seismograms to estimate the five independent elastic stiffnesses necessary for describing the transverse isotropy. Our synthetics fit the vertical and two horizontal components of the data for both profiles. The two intersecting refraction profiles are 150 and 200 m long. These profiles are not long enough to constrain compressional wave velocities and anisotropy, but are quite adequate to find the shear wave anisotropy. A nearby drill hole showed that the sediments are interbedded silty clays, clays, and sands. The data require low shear velocities (< 400 m s-1) and low Qs (< 100) in about the top 30 m of the sediments. In the top 10 m of the sediments, silty clay exhibits approximately 12-15 per cent anisotropy for shear waves.

In June 1986 a seismic experiment emphasizing the observation of shear waves in the bottom sediments was conducted off the southern New Jersey coast in water depths between about 10 and 56 m. This paper emphasizes the generation and detection of horizontally polarized shear waves SH produced by a pair of air (mud) guns, mounted to apply horizontal forces, on a sled deployed from the ship and capable of being dragged along the bottom. Signals were digitally recorded from three-component geophones and a hydrophone. Shear waves and boundary waves were recorded in the frequency band 2 to 40–50 Hz having group velocities ranging from about 50 to 300 m/s. Both SH/Love wave and P/SV/Rayleigh-Stoneley-Scholte wave signals were dearly recorded and resolved by combining sums and differences of two properly oriented horizontal components and comparing the resolved horizontal data with vertical and pressure components. Full-waveform synthetic seismograms are being used to aid inversion for shear velocity and attenuation models that are required to be consistent with information available from test borings as well as the shear wave data. [Work supported by ONR.]

Multiple ship multichannel seismic measurements in the Baltimore Canyon Trough reveal a deep crustal layer with p-wave velocity of 7.2 km s−1. It apparently continues into oceanic layer 3 from beneath the inner shelf where it presumably underlies the continental basement. The layer may be plutonically solidified mantle melt. Its continuity from continental shelf to deep ocean basin may reflect a continuous progression between plutonic emplacement into the continental crust and plutonic construction of the lower oceanic crust. The deep magmatic expression of late stage continental rifting and early seafloor spreading may be very similar and blur the structural expression of the continent-ocean boundary.

The ability to resolve both the seismic velocity of the seabed in shallow water and the lateral variability of these measured velocities using four conventional seismic methods is compared. The four methods described here differ in the ease and efficiency in which the field data are collected and interpreted, and in the resolution of the velocity of the seabed they provide. Narrow-aperture refraction studies provide limited constraints due to the frequent absence of first-arriving sediment refractions in the narrow portion of the wave field that is sampled. Wide-aperture refraction experiments using ocean bottom seismometers provide high resolution of both the compressional- and shear-wave velocities of the seabed but are time-consuming to perform and to interpret. Narrow-aperture multichannel reflection data are easily collected, and are ideal for areal mapping of sedimentary sequences, but resolution of seismic velocities is low unless the aperture of the multichannel streamer is several water depths in length. Wide-aperture multichannel reflection profiling generally provides the highest resolution in velocity in all geoacoustic environments, including those which produce little or no precritical reflected energy and little dispersion of the acoustic normal modes. Moreover, difference seismic wave fields calculated by subtracting a reference seismic wave field from adjacent wave fields provide an objective means of rapidly mapping the lateral variability of the seabed.

On March 24, 1978, a magnitude 6.0 intraplate earthquake occurred 380 km southwest of Bermuda near magnetic anomaly M4 (≈118 m.y.B.P.). A catalog of seismicity for the Bermuda rise indicates that this is an area of significant intraplate seismicity in the western North Atlantic Ocean. The fault plane solution for the 1978 event is of thrust type and strikes 340°, in an intermediate direction to the trends of major fracture zones (300°) and abyssal hill topography (035°) in the area. The P axis of this mechanism is nearly horizontal and trends 259°, subparallel to the absolute plate motion vector for North America. Aftershock activity was detected teleseismically for approximately 8 months after March 24, and the entire sequence is best described as a prolonged mainshock‐aftershock series. During June 18–28, 1978, we conducted a microaftershock survey of the area using ocean bottom hydrophones and recorded 250 events (0 < m b < 2). These smaller events are located in a tight cluster 5–10 km northeast of the NEIS mainshock location. SP‐P data from microaftershocks recorded during this survey indicate a relatively narrow band of activity (about 10 km wide), smaller than would be inferred based on NEIS aftershock locations alone. Many of the recorded microaftershocks exhibited prominent water wave phases, which have been used to constrain further the epicentral locations. The depths of these events, while not well deter mined, appear to be shallow, i.e., less than 5 km below the seafloor. Estimates of the b value, determined from two instruments for the 10‐day recording period, are 1.10 ±0.30 to 1.50 ±0.60. Overall, both the fault plane solution and subsequent aftershock locations of the 1978 event suggest that faulting did not occur along a major oceanic fracture zone but, rather, along a smaller‐scale feature within the M11‐M4 spreading discontinuity. The spatial clustering of seismicity near the perimeter of the rise suggests that variations of crustal thickness associated with the Bermuda rise may be responsible for triggering intraplate seismicity in this region.

We describe a method to constrain the seismic velocity structure of the upper 500–800 m of young oceanic crust. Conventional interpretations of seismic refraction profiles provide little or no information about this shallowmost crust. The method described here requires the assumption of a linear velocity gradient and utilizes measured values of range, travel time, and slowness of the observed refracted waves. Results are reported from several refraction profiles that sample crust between 0 and 4 m.y. in age on the flanks of the East Pacific Rise at 12°N. These data give an average gradient in the uppermost 800 m of the crust of 3.5 s−1 and a velocity at the seafloor of 2.5 km/s. Ray path and travel time modeling show that the refracted waves sampling the uppermost part of the crust are effectively masked by reflections from the seafloor.

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.

Seismic-reflection data collected using 0.66–1 (40-in3) airgun and a single hydrophone towed within a few hundred meters of the seafloor defines the basement morphology close to IPOD drill sites 417A and 417D in the western Central Atlantic Ocean. The high resolution provided by this technique, together with accurate navigation from acoustic transponder beacons, allows the basement hill into which these holes were drilled to be defined. Excellent agreement exists between the drilling results and these deep hydrophone reflection data.

Reflecting horizons which have anomalously high amplitude and which are conformable to the seafloor at about 500 to 600 m subbottom have been reported in two locations off the United States east coast--one along the crest of the Blake Outer Ridge, and another beneath the upper continental rise off New Jersey and Delaware. Detailed mapping of these horizons shows that: (1) the horizons cut across bedding planes in the sediment; (2) subbottom depth of the horizons increases with increasing seafloor depth and thus with decreasing seafloor (bottom water) temperature; and (3) the horizons are restricted to areas where sediment strata dip landward; such anomalous horizons are uncommon within the normal seaward-dipping continental-rise strata. Deep-sea drilling into or close to the anomalous horizon on the Blake Outer Ridge (Sites 102, 103, 104) recovered methane-rich sediment. Pressure/temperature conditions within the sediment column in both areas of anomalous horizons are appropriate for formation of gas hydrates to several hundred meters depth, thus suggesting that a zone of gas-hydrates overlies the anomalous horizons. A plot of the hydrate/gas phase transformation of the methane/seawater system in the sedimentary column, using geologically reasonable values for seafloor temperature and for thermal gradient and sound-velocity in the sediment, shows a good correlation between the depth of the phase change and the minimum depth of the anomalous reflecting horizons. The horizons therefore are thought to represent an imped nce contrast caused by the downward change from gas hydrate to gas in the sediment. Landward-dipping strata and the gas-hydrate layers in the areas exhibiting anomalous horizons appear to form traps for free gas, whereby the gas-hydrate layer blocks seaward gas migration and the dipping strata restrict landward migration.

Interval velocities, refraction velocities, and layer depths from 38 airgun-sonobuoy profiles made along a west-east traverse of the Colombia and Venezuela basins at about 15°N are presented in a schematic structure section, which includes details from single-channel reflection-profiler records converted to depths. The seismic data are correlated with earlier reflection and refraction surveys and with the results of JOIDES drilling. Two prominent subbottom reflectors, Horizon A^Prime and Horizon B^Prime, are traced across each basin. Interval and refraction velocities in the material beneath B^Prime in the Venezuela basin range between 3.5 and 5.7 km/sec, but most are in the range 4.5 to 5.3 km/sec. These values are in close agreement with published laboratory measurements of sonic velocities made on DSDP cores of basalts from Horizon B^Prime. It is therefore unlikely that the material below B^Prime can contain large amounts of sediment unless the sediment has a velocity near that of basalt. The discontinuous reflectors below B^Prime are thought to be thin layers of interbedded ash or sediment. It is argued that the Venezuela basin has normal oceanic that has been depressed about 3 km by the addition of flood basalts, and that the velocity structure of typical Caribbean crust (6.2 on 7.3 km/sec material) does not exist in the Colombia and Venezuela basins, except as arbitrary divisions in a scattering of crustal and basement velocities that range between 4.0 and 7.5 km/sec, as they do in other ocean basins. It is suggested that the upper interface of primordial basement coincides with the deepest reflector below B^Prime. Therefore, it is likely that the Late Cretaceous basalts and dolerites drilled at the depth of Horizon B^Prime do not represent primordial crust, but reflect the last major igneous event in the history of development of the Caribbean Sea.

Research Article| November 01, 1974 Bathymetry and Sediment Geometry of the Greater Antilles Outer Ridge and Vicinity BRIAN E. TUCHOLKE; BRIAN E. TUCHOLKE 1Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543 Search for other works by this author on: GSW Google Scholar JOHN I. EWING JOHN I. EWING 2Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 Search for other works by this author on: GSW Google Scholar Author and Article Information BRIAN E. TUCHOLKE 1Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543 JOHN I. EWING 2Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1974) 85 (11): 1789–1802. https://doi.org/10.1130/0016-7606(1974)85<1789:BASGOT>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation BRIAN E. TUCHOLKE, JOHN I. EWING; Bathymetry and Sediment Geometry of the Greater Antilles Outer Ridge and Vicinity. GSA Bulletin 1974;; 85 (11): 1789–1802. doi: https://doi.org/10.1130/0016-7606(1974)85<1789:BASGOT>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Detailed examination of 20,000 km of seismic reflection profiles and data on cores and abyssal currents suggests that the Greater Antilles Outer Ridge has been constructed by differential deposition of sediment from the Western Boundary Undercurrent above the seismic reflector, Horizon A. Deposition of acoustically stratified sediments under the eastern sector of the outer ridge was apparently terminated in the late middle Eocene when the incipient formation of the present Puerto Rico Trench cut off downslope sediment movement from the northeastern Antilles. At about the same time, the newly formed Western Boundary Undercurrent, possibly interacting with Antarctic Bottom Water entering from the South Atlantic, began to preferentially deposit acoustically transparent sediment to form the eastern outer ridge. Stratified sediments were deposited in the region of the present western outer ridge until the middle Miocene, when increased Antarctic Bottom Water flow apparently diverted the sediment-laden Western Boundary Undercurrent and produced a flow pattern which initiated deposition of the acoustically transparent sediment now forming the western ridge sector. Since that event, the resultant morphology of the Greater Antilles Outer Ridge has diverted the Western Boundary Undercurrent into a contour-following flow, causing further depositional construction of the entire outer ridge. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

ABSTRACTAll core evidence supports the conclusion that abyssal plains are sites of graded‐bed sequences deposited from turbidity flows. The deposits appear to be present‐day equivalents of thin‐ to medium‐bedded, graded, flysch‐type sands and silts of many ancient geosynclines and sedimentary basins.

The results of 57 seismic-refraction profiles recorded in the South Atlantic Ocean south of 47°S are used to describe the gross structure of the Scotia Sea basins, the Falkland plateau, and, to a lesser extent, the North Scotia and South Sandwich ridges. The Drake passage and Scotia Sea basins have normal or nearly normal oceanic crust. Their floors are covered by 300–1000 meters of sediment, except in a band in the western part, where the sediment cover is thin and nonuniform. There is direct correspondence between the position of the band of thin sediments and the position of a low oceanic ridge, as defined by morphology, seismic refractions, magnetic lineations, and earthquake epicenters; this correspondence suggests that the band may represent young oceanic crust produced by sea-floor spreading. Scouring or nondeposition due to strong bottom currents may also have reduced accumulation in this band. The Falkland plateau is a southward-tilted continental block capped by a thick wedge-shaped body of sediments, the top-most layers of which continue farther south to form the floor of the adjacent Falkland trough. The section beneath the Falkland trough is similar to a depressed or subsided oceanic crust overlain by 4 km of low-velocity sediments. A complete set of travel-time graphs can be ordered from the American Geophysical Union, Suite 435, 2100 Pennsylvania Ave., N.W., Washington, D.C. 20037. Document J71-001; $1.00 per microfiche card.