Rayleigh Wave Phase Velocities in the Atlantic Ocean

Abstract

The phase velocities of Rayleigh waves are found for many paths in the north Atlantic. The single station technique is used with earthquakes in the Mid-Atlantic Ridge. The major contributor to the error in phase velocity is the uncertainty in epicentral location and origin time. Focal depth and mechanism are sufficiently well controlled for these events that they do not significantly add to the phase velocity error. The resulting uncertainty is +O-02 km s-' for the period range 20-100 s.The ocean is regionalized into ridge and basin. Both ray theory and Rytov's method are used to extract the dispersion characteristics of each region from the observations. Large lateral variations of the basin dispersion suggest a further subdivision. These variations can be explained entirely by variations in sediment thickness.The deduced upper mantle model of the ocean basin is characterized by a shear velocity reversal at 40km depth with a lid shear velocity of 4.7kms-'.The lowest shear velocities are centered at a depth of 125krn. The lid shear velocity can be reduced to 4.6kms-' only if both the thickness of the lid increases and the density increases. Phase velocities for the Mid-Atlantic Ridge imply that the upper mantle shear velocity under the ridge is significantly lower than that under the ocean basin at depths greater than 20 km.We shall find strong evidence that the first shear velocity reversal occurs at 40 km in the ocean basins. While there is a great deal of non-uniqueness in surface wave inversions, it is difficult to thicken the lid if the shear velocity is 4.7 as suggested from the S, velocities of Hart & Press (1973). We shall also find evidences for a very shallow velocity reversal underlain by low shear velocities under the Mid-Atlantic Ridge.The ocean basins can be divided into two regions with distinct dispersion relations. At longer periods (100 s) the phase velocities differ by 0.05 kms-', while at the shorter periods (20s) they differ by as much as 0.5kms-'. These differences can be entirely explained in terms of sediment thickness. Observed Rayleigh wave phase velocitiesDescription of method observed phase, $obs, through the relation The Rayleigh wave phase velocity for a frequency, o, can be determined from the

Variations in structure and tectonics along the Mid-Atlantic Ridge, 23 degress N and 26 degrees N

“…The ratio is consistent with laboratory results (Vp!Vs = 1.81-1.91) for crustal layers 2 and 3 , and is within the range of values (1.73-2.08) from previous MAR microearthquake studies [Francis eta/., 1977;Lilwall et al, 1977Lilwall et al, , 1978, and the range (1.74-2.12) determined by combining sub-Moho P and/or S-wave velocities from refraction experiments and surface wave studies [Hart and Press, 1973;Weidner, 1974;Steinmetz et al, 1977;.…”

“…We fmd that the P delay for 26°N to be on average much smaller, by perhaps 1 s or more relative to the Herrin [1968] Earth model, than reported by other investigators [e.g., Girardin, 1980;Dorbath and Dorbath, 1981;Woodward and Masters, 1990]. Although the positiveS wave travel time delay indicates that lower S wave velocities than appropriate for the average Earth may be present, the small magnitude of the delay suggests that the ridge segment near 25°50'N may not be underlain by as large a low velocity anomaly as earlier studies have indicated for other regions of the MAR [e.g., Girardin and Poupinet, 1974;Weidner, 1974;Duschenes and Solomon, 1977;Steinmetz et al, 1977;Rowlett and Forsyth, 1979;Dorbath and Dorbath, 1981;Grand, 1989;Sheehan and Solomon, 1989].…”

“…Investigations to date have found in general the slowspreading Mid-Atlantic Ridge (MAR) to be characterized by anomalously low P and S wave velocities. Shear and surface wave studies in the vicinity of mid-ocean ridges have shown there to be zones of high-attenuation and/or low shear wave velocity beneath parts of the MAR [Girardin and Poupinet, 1974;Weidner, 1974;Duschenes and Solomon, 1977;Forsyth, 1977;Kuo et al, 1987;Solomon, 1988, 1989;Grand, 1989].…”

“…Estimates of the extent of the anomalous velocities within the upper mantle range from a few tens of kilometers to 100 km in width at depths down to 250 km beneath the ridge crest [e.g., Solomon, 1973;Solomon and Julian, 1974;Weidner, 1974;Grand, 1989].…”

The change of electrical structure between an orogenic and an ancient tectonic area (Carpathians and Russian platform).

“…That means that the asthenosphere is edging out in the transitional zone between the Carpathian system and the platform areas. The induction effect of the crustal anomaly in the deep fracture between Inner and Outer Carpathians (Pieniny pacific (LEEDS et al, 1974;LEEDS, 1975); bar with slash, East pacific (FORSYTH, 1975); bar with dot, Atlantic (WEIDNER, 1973).…”