Sharp Velocity Discontinuity Research Articles

Seismic investigation of crust-mantle transition in continental rift systems - Jordan-Dead Sea rift and Rhinegraben

The recent acquisition of high-quality seismic refraction data in the Jordan—Dead Sea rift and adjacent areas has made possible the investigation of the dynamic properties of seismic P-waves refracted and reflected at the crust—upper mantle boundary. These waves cause high-amplitude arrivals near the outer cusp of the travel-time curve which are followed by an abrupt decrease in amplitudes at increasing distances beyond the cusp. It has been shown that such amplitude distributions can only be the result of a smooth rapid increase of velocity with depth. In the case of the Jordan—Dead Sea rift the amplitude distribution indicates the presence of a transition zone between the lower crust and upper mantle in which the velocity increases smoothly. The interpretation of seismic refraction data in the Rhinegraben indicates the existence of a similar transition zone. In both rifts the crust—mantle boundary outside the rift is represented by sharp velocity discontinuity. The comparison of the velocity structure of the crust—upper mantle boundary suggests that a smooth transition zone at the base of the lower crust is a characteristic property of continental rifts which could be interpreted in terms of crust—mantle interaction.

The upper mantle velocity distribution

In plate tectonic theory one thinks of rigid blocks of lithosphere moving relative to one another over a weak asthenosphere. In the oceans the base of the lithosphere is associated with the top of the low velocity layer at a depth of 70–100 km. Current models of the upper mantle velocity distribution beneath continental platforms on shield regions give little indication of the depth of the base of the lithosphere in these regions. Gerver and Markusevitch (1966, 1967) have shown that, providing travel times from earthquakes covering a range of depths are available in addition to surface focus travel times, it is possible to determine the upper mantle velocity distribution even if low velocity layers are present. This paper summarizes information on surface focus travel times from controlled sources in platform or shield regions and describes a complex model of the upper mantle velocity distribution based on observations of the travel times of seismic waves from Banda Sea events covering a wide range of depth. It is pointed out that surface focus travel times in continental platform or shield regions differ significantly from the Jeffreys-Bullen travel times. The deviations from the Jeffreys-Bullen times are much larger than the differences between the deviations for different continental platform or shield regions. One of the features of the model based on the travel times from the Banda Sea events is a velocity discontinuity at a depth of about 200 km. It is difficult to account for this discontinuity in terms of a phase transformation. It is pointed out that there is supporting evidence for the existence of relatively sharp velocity discontinuities at 200, 400 and 650 km from studies of converted phases by Vinnik and Jordan and Frazer. The principal uncertainty in the upper mantle velocity distribution arises from uncertainties in the depths of foci of the earthquakes. It is in fact possible that there may be a low velocity zone in the upper mantle beneath continental platform or shield regions between depths of 150 and 200 km.

Turbulent Layer in an Ideal Two-Dimensional Fluid

The problem of the turbulent layer arising in the two-dimensional ideal fluid is considered. The sharp initial velocity discontinuity was replaced by a row of 100 point vortices whose evolution in time was found by numerical methods. The average velocity profile and velocity fluctuations are given. The picture is very similar to the ordinary turbulent layer in spite of the potential character of the ideal fluid motion outside the discontinuity.

Ions in He II at 1 degrees K. I. Velocity discontinuities

In a study of the mobility of ions in liquid He II in the temperature range 0.89 degrees K to 1.0 degrees K, the existence of the sharp periodic velocity discontinuities, first detected by Careri, was confirmed. The discontinuities were found to occur at integral values of a critical drift velocity vc+=5.21+or-0.08 m s-1 for positive ions, and vc-=2.12+or-0.16 m s-1 for negative ions. Velocities were measured by a time-of-flight method, and at low values of the grid- source field, the previously undetected first discontinuity of the negative ions was found. Measurements were made of the magnitude Delta mu and shape of the first three discontinuities. The constant Delta mu values found for successive discontinuities and the observed lack of dependence of Delta mu on the grid-collector separation are is disagreement with the predictions of the vortex ring creation theory of Huang and Olinto.

A travel time and amplitude interpretation of a marine refraction profile: Primary waves

Oceanic refraction surveys are interpreted almost exclusively on a time basis yielding layered crustal models. This study utilizes more of the observed information by requiring the model to produce similar waveforms as well as travel times. Theoretical seismograms based on possible travel time models are compared with the first few seconds of observed recording. Closely spaced comparisons are made along a 120-km profile located north of the Hawaiian Islands. The final model indicates: (1) a high positive velocity gradient in the upper layers above the main oceanic layer. (2) a relatively homogeneous oceanic layer with a sharp velocity discontinuity at the crust-mantle interface. (3) a small positive velocity gradient in the upper mantle.