ALBERT

Description: During 1968 about 7500 km of new magnetic data were recorded by the USNS J. W. Gibbs between the Canary and Cape Verde islands, from the continental shelf to approximately 30øW. These data, together with an equal amount of data from other sources, reveal major magnetic features. The magnetic boundary between relatively undisturbed and disturbed magnetic zones is delineated near the middle of the northwest African continental rise. A sequence of linear north-south-trending anomalies immediately seaward of the magnetic boundary comprise a band about 300 km wide and can be correlated from about 15øN at the Cape Verde Islands to about 26øN just south of the Canary Islands. The band of magnetic anomalies appears to have a right lateral offset of about 100 km near 25øN where intersected by a west-northwest-trending fault near the eastward projection of the Atlantis fracture zone from the mid-Atlantic ridge. At least one prominent positive magnetic anomaly is associated with the northwest. African continental shelf. The magnetic disturbance boundary and the associated band of linear magnetic anomalies are nearly mirror images of similar anomalies associated with the continental margin off eastern North America. Major features of the magnetic-field-strength anomalies in the North Atlantic are highintensity anomalies associated with the continental terrace (shelf plus slope), a magnetic quiet zone, a magnetic boundary, and a sequence of characteristic anomalies associated with the

Description: Two new species of cranchiid cephalopod are described. These were both collected by opening-closing midwater trawls (RMTs) when vertical series were fished in the North Atlantic from R.R.S. ‘Discovery’.

Description: This report includes discussions of elastic and viscoelastic models for water‐saturated porous media, and measurements and computations of elastic constants including compressibility, incompressibility (bulk modulus), rigidity (shear modulus), Lamé's constant, Poisson's ratio, density, and compressional‐ and shear‐wave velocity. The sediments involved are from three major physiographic provinces in the North Pacific and adjacent areas: continental terrace (shelf and slope), abyssal plain (turbidite), and abyssal hill (pelagic). It is concluded that for small stresses (such as from a sound wave), water‐saturated sediments respond elastically, and that the elastic equations of the Hookean model can be used to compute unmeasured elastic constants. However, to account for wave attenuation, the favored model is ‘nearly elastic,’ or linear viscoelastic. In this model the rigidity modulus μ and Lamé's constant λ in the equations of elasticity, are replaced by complex Lamé constants (μ + iμ′) and (λ + iλ′), which are independent of frequency; μ and λ represent elastic response (as in the Hookean model), and iμ′ and iλ′ represent damping of wave energy. This model implies that wave velocities and the specific dissipation function 1/Q are independent of frequency, and attenuation in decibels per unit length varies linearly with frequency in the range from a few hertz to the megahertz range. The components of the water‐mineral system bulk modulus are porosity, the bulk modulus of pore water, an aggregate bulk modulus of mineral grains, and a bulk modulus of the structure, or frame, formed by the mineral grains. Good values of these components are available in the literature, except for the frame bulk modulus. A relationship between porosity and dynamic frame bulk modulus was established that allowed computation of a system bulk modulus that was used with measured values of density and compressional‐wave velocity to compute other elastic constants. Some average laboratory values for common sediment types are given. The underlying methods of computation should apply to any water‐saturated sediment. If this is so, values given in this paper predict elastic constants for the major sediment types.

Description: Geodetic data along the San Andreas fault between Parkfield and San Francisco, California (latitudes 36°N and 38°N, respectively), have been re-examined to estimate the current relative movement between the American and Pacific plates across the San Andreas fault system. The average relative right lateral motion is estimated to be 32 ± 5 mm/yr for the period 1907-1971. Between 36°N and 37°N it appears that most, if not all, of the plate motion is accommodated by fault creep. Although strain is presumably accumulating north of 37°N (San Francisco Bay area), the geodetic evidence for accumulation is not conclusive.

Description: An analysis of the reproducibility of Geodolite measurements at distances ranging from 1 to 35 km indicates a standard deviation for each length measurement of about σ = (a2 + b2L2)1/2, where a = 3 mm, b = 2 × 10−7, and L is the line length. Thus σ ranges from 3 to 8 mm for line lengths of 1 and 37 km, respectively. Corrections for atmospheric refractivity must be determined from temperature and humidity readings made with an aircraft flying along the line of sight at the time of the range measurements in order to attain this precision. Even at this level of precision, determination of the strain accumulation at sites along the San Andreas fault system will require annual observation of many line lengths over a period of at least 5 years.