George H. Sutton

Continental margins and geosynclines: The east coast of North America north of Cape Hatteras

Abstract Many geophysical measurements, including seismic refraction, gravity, magnetics, and echo soundings, have been made along the continental margin of eastern North America north of Cape Hatteras in the last twenty years. These have revealed the presence of two sedimentary troughs, one under the shelf, the other in deeper water under the continental slope and rise which are separated by a ridge in the basement near the edge of the shelf. The sediments in the shelf trough have been drilled to a depth of 10,000 ft and are of shallow water character. Cores of the upper part of the sediments of the outer trough have revealed features attributed to slumping, sliding, and turbidity current action, and, in part, sediments similar to the graywackes of Pettijohns (1949) classification. This sedimentary system is quite comparable to the Appalachian system as restored for early Paleozoic time. The sediments of the inner and outer troughs are similar to those of the Appalachian miogeosyncline and eugeosyncline ( Kay , 1951), respectively, and the basement ridge resembles the Pre-Cambrian axis which separates these two troughs in the Appalachians. While there is no active volcanism in the outer (eugeosynclinal) trough at the present time, evidence of past volcanism is present in the form of partially buried seamounts with large magnetic anomalies. Conditions in the Appalachian eugeosyncline appear to have been similar with but limited volcanism prior to the beginnings of Taconic activity. The gravity calculations reveal an abrupt change in depth of the Mohorovicic discontinuity near the edge of the shelf. There is some indication that the boundary between the crust and the mantle in this area is gradational rather than a sharp discontinuity. The major process necessary to convert the present continental margin into a mountain system is the one which thickens the crust under the outer, or eugeosynclinal trough. Since the miogeosyncline is already based on a crust of continental proportions, its deformation requires only a means of folding and thrusting the surficial sediments. This is but a minor part of the overall tectonic activity.

Passive seismic experiment.

Seismometer operation for 21 days at Tranquillity Base revealed, among strong signals produced by the Apollo 11 lunar module descent stage, a small proportion of probable natural seismic signals. The latter are long-duration, emergent oscillations which lack the discrete phases and coherence of earthquake signals. From similarity with the impact signal of the Apollo 12 ascent stage, they are thought to be produced by meteoroid impacts or shallow moonquakes. This signal character may imply transmission with high Q and intense wave scattering, conditions which are mutually exclusive on earth. Natural background noise is very much smaller than on earth, and lunar tectonism may be very low.

Seismic data from man-made impacts on the moon

Unusually long reverberations were recorded from two lunar impacts by a seismic station installed on the lunar surface by the Apollo 12 astronauts. Seismic data from these impacts suggest that the lunar mare in the region of the Apollo 12 landing site consists of material with very low seismic velocities near the surface, with velocity increasing with depth to 5 to 6 kilometers per second (for compressional waves) at a depth of 20 kilometers. Absorption of seismic waves in this structure is extremely low relative to typical continental crustal materials on earth. It is unlikely that a major boundary similar to the crustmantle interface on earth exists in the outer 20 kilometers of the moon. A combination of dispersion and scattering of surface waves probably explains the lunar seismic reverberation. Scattering of these waves implies the presence of heterogeneity within the outer zone of the mare on a scale of from several hundred meters (or less) to several kilometers. Seismic signals from 160 events of natural origin have been recorded during the first 7 months of operation of the Apollo 12 seismic station. At least 26 of the natural events are small moonquakes. Many of the natural events are thought to be meteoroid impacts.

Crustal Structure of the Hawaiian Archipelago, Northern Melanesia, and the Central Pacific Basin by Seismic Refraction Methods*

Abstract The crustal structure of the Hawaiian Archipelago, northern Melanesia, and parts of the Central Pacific Basin have been studied by seismic refraction methods. The systematic variation found in crustal thickness in the Hawaiian Islands is explainable by a hypothesis of differential subsidence. The crustal structure of northern Melanesia points to tensional forces in an east-west direction and compressional forces in a north-south direction. In the Central Pacific Basin, a 7.4 km/sec layer in the lower crust seems to be present over a wide area.

Correlation of deep ocean noise (0.4–30 Hz) with wind, and the Holu Spectrum—A worldwide constant

One year of ambient ocean noise data, 0.4 to 30 Hz, from the Wake Island hydrophone array in the northwestern Pacific are compared to surface wind speeds, 0–14 m/s (0–28 kn). Between 0.4 and 6 Hz, noise levels increase with wind speed at rates of up to 2 dB per m/s until a saturation is reached having a slope of about −23 dB/octave and a level of 75 dB relative to 1 μPa/√Hz at 4 Hz. This noise saturation, called the ‘‘Holu Spectrum,’’ likely corresponds to saturation of short‐wavelength ocean wind waves. It is probably a worldwide constant. Between 4 and 30 Hz, noise also increases with wind speed at rates of up to 2 dB per m/s, but no saturation level is observed and the slope increases to about 4 dB/octave. This may be acoustic noise from whitecaps. On a hydrophone less than 3 km from Wake, noise between 0.5 and 10 Hz increases with wind speed at a rate up to 2 dB per m/s, but absolute noise levels are significantly higher than levels on the other hydrophones more distant from Wake, and no saturation is...

Fidelity of ocean bottom seismic observations

The often poor quality of ocean bottom seismic data, particularly that observed on horizontal seismometers, is shown to be the result of instruments responding to motions in ways not intended. Instruments designed to obtain the particle motion of the ocean bottom are found to also respond to motions of the water. The shear discontinuity across the ocean floor boundary results in torques that cause package rotation, rather than rectilinear motion, in response to horizontal ground or water motion. The problems are exacerbated by bottom currents and soft sediments. The theory and data presented in this paper suggest that the only reliable way of obtaining high fidelity particle motion data from the ocean floor is to bury the sensors below the bottom in a package with density close to that of the sediment. Long period signals couple well to ocean bottom seismometers, but torques generated by bottom currents can cause noise at both long and short periods. The predicted effects are illustrated using parameters appropriate for the operational OBS developed for the U. S. Office of Naval Research. Examples of data from ocean bottom and buried sensors are also presented.

Physical analysis of deep sea sediments

A sonic pulse system, similar to that used at Lamont Geological Observatory for seismic model experiments, was used aboard the Research Vessel VEMA during the summer of 1954 to determine high frequency seismic velocities in fresh deep sea sediment cores. Velocity profiles were obtained from 26 cores covering a wide range of lithologies and ages (Recent to Miocene). Density, porosity, median grain size, sorting, carbonate content, and salt content were also measured. The compressional wave velocity in the ocean‐bottom unconsolidated sediments studied is well represented by the equation: v′=2.093-(.0414±.0060)ϕ+(.00135±.00038)γ-(.44±.15)η where v′=compressional wave velocity in km/sec ϕ=median grain size in phi units γ=percentage of HCl soluble material η=porosity. Many measurements gave velocities less than the velocity of sound in sea water. Most of the low carbonate samples followed a velocity‐porosity relation given by the Wood (1941) equation. The regression coefficient, −.44η, agrees well with the ave...

Coupling of ocean bottom seismometers to soft bottom

Unlike response of seismometers resting on hard rock where the seismometer case moves with the rock to high frequencies, the response of ocean bottom seismometers (OBS) can be strongly affected by the low mechanical strength of ocean sediments. The motion as measured by the seismometer will not follow the expected relationships between pressure and particle motion for different wave types. Cross coupling between horizontal and vertical motions can occur, especially when there is differential motion between water and sediment. Resonant amplification and attenuation of higher frequencies also occur. Secondary seismic arrivals are especially subject to distortion. Overall response is strongly dependent upon the mass and configuration of the OBS and the rigidity and density of the bottom material. Tests at Lopez Island, Puget Sound using both directly applied mechanical transients and seismic signals with various instrument configurations demonstrate the above effects and provide some guidance for improved designs.

Optimum design of Ocean bottom seismometers

Ocean bottom seismometers (OBS) have been widely used during the past decade to collect seismic data for determination of the structure of the oceanic lithosphere, stress patterns in regions of earthquake activity, and geoacoustic parameters of the ocean floor. Data quality from these experiments has often been disappointing because of poor signal quality and high noise levels. Many of these problems result from motion of the OBS package that is decoupled from motion of the ocean floor. These coupling problems are more serious in the ocean than on land because of the low shear strengths of most ocean sediments. In this paper we continue to develop the theory of coupling of OBSs to soft sediments and arrive at results suggesting that OBS packages should be designed with: (1) the minimum mass possible, (2) radius of area in contact with the sediment proportional to the cube root of the mass, and the maximum radius less than 1/4 of the shear wavelength, (3) density of the OBS approximately that of the sediment, (4) a low profile and a small vertical cross section with water, and (5) low density gradients, and maximum symmetry about the vertical axis. Agreement of the theory with test data is good; most deviations are reasonable, given limitations of the theory and experiments. The theory also suggests that the coupling frequency, the frequency above which the OBS does not follow the motion of the sediment, is directly proportional to the sediment shear velocity.

Ocean bottom seismograph development at Hawaii Institute of Geophysics

Three distinct ocean bottom seismograph (OBS) systems have been developed at the Hawaii Institute of Geophysics to satisfy the different requirements for short-range refraction and anisotropy experiments, long-range refraction experiments, and short-term and semi-permanent monitoring for earthquakes. One system, originally designed for semi-permanent use in conjunction with a monster buoy of the IDOE North Pacific Experiment has been modified for emplacement off Oahu. It contains 3-component 1 Hz seismometers and a hydrophone and obtains power and transmits data via tow conductor cable. Two additional systems were designed for short-term use: a 2 Hz telemetering system (TOBS); and 4.5 Hz free-fall pop-up system (POBS). The TOBS contains 3-component seismometers and a hydrophone and transmits data to the ship via light-weight single-conductor electromechanical cable and an HF-VHF radio link from a surface buoy. The bottom package also includes a backup tape recorder. This system exhibits the advantages of real-time data acquisition (e.g. precise timing, rapid appraisal of data quality, optimum use of explosives, and common recording with other data) and the complexities and difficulties associated with a deep-sea mooring. However, use of cable with near neutral bouyancy permits the design of a deep-water system with low weights and stress levels. The POBS is a self-contained package containing a vertical and single horizontal seismometer, hydrophone, cassette tape recorder, and pre-set timed release. This system is relatively simple and inexpensive. Total weight of 150 kg in air (before launch) permits emplacement and retrieval from a ship with no special equipment by two (strong) persons. Experience to data suggests that the optimum deployment scheme for many studies is a combination of TOBSs and POBSs.