Lateral variations of attenuation in the upper mantle and discontinuities in the lithosphere

Abstract

This study demonstrates the existence and determines the pattern of lateral variations of attenuation in the uppermost mantle on a worldwide scale. The evidence comes mostly from a comparison of the gross characteristics of the seismic phase Sn for over 1500 paths, which taken together cross as many regions of the earth as possible with the current configuration of the World-Wide Standardized Seismograph Network. Sn is a seismic shear wave that propagates in the uppermost mantle and that does not penetrate the low-velocity channel. It propagates very efficiently across the stable regions of the earth, the continental shields, and deep-ocean basins, but propagation is very inefficient when paths cross the crests of the mid-ocean ridge system or the concave sides of most island arcs. These observations suggest that attenuation is more pronounced in the uppermost mantle near the ridge crests and the islands arcs than in the more stable regions. If low attenuation, or high Q, correlates with high strength, the data imply that the uppermost mantle is considerably weaker under the ridge crests and the concave sides of the island arcs than it is elsewhere. Thus, the part of the strong outer shell, or lithosphere, in the mantle is discontinuous with gaps in it at the ridges and island arcs. The low attenuation for Sn for paths crossing the transform faults that connect ridge crests suggests that any gap at the transform faults is very narrow. In addition, S waves recorded at stations in and near the Mariana, New Britain, Solomon, and South American arcs from local deep shocks were found to follow the pattern previously observed in the Tongan and Japanese arcs. As was suggested for the Tongan arc, the observations imply that one piece of lithosphere has underthrust another to great depth at these arcs. Hence, data presented in this paper are in accord with recent ideas of sea-floor spreading in which large plates of lithosphere move with respect to each other as rigid bodies, spreading apart at the ocean ridges, sliding past one another at the transform faults, and underthrusting at the island arcs.