Seismic refraction measurements of the compressional wave velocity of the upper mantle, obtained in the northeast Pacific Ocean between California and Hawaii, have shown a dependence on direction indicating anisotropy of velocity with a maximum of 8.6 km/sec in a roughly easterly direction and a minimum of 8.0 km/sec. In order to test whether or not these observations represent true anisotropy, rather than geographical variation of velocity, special anisotropy experiments were conducted at two locations centered roughly at 31°N, 121°W and 35°N, 126°W, respectively. At each of these locations significant anisotropy was observed, agreeing in direction with the apparent anisotropy indicated for the entire region. The magnitude of the velocity difference, 0.3 km/sec, on any single station is half the over‐all variation of 0.6 km/sec. These observations do not prove the hypothesis of anisotropy throughout the entire region from California to Hawaii. However, they are consistent with this hypothesis and give encouragement to similar observations elsewhere.

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Two major families of faults dominate the tectonics of slow spreading ridges: the transform faults paralleling the spreading direction and the 'GLORIA' faults, observed by large-scale side-scan sonar, normal to it. The observation of microearthquakes to 8 km depth beneath the Mid-Atlantic Ridge axis implies that not only the transform faults but the GLORIA faults as well penetrate into the mantle.Since large horizontal temperature gradients are often associated with them, both types of fault provide the means of circulating large volumes of sea water through the oceanic mantle. Under these conditions, serpentinization of upper mantle rocks will occur in a predictable way. Its influence on the tectonic development of slow spreading ridges is discussed.

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