Buoyant zones, great earthquakes, and unstable boundaries of subduction

Abstract

The distribution of large shallow earthquakes along subduction boundaries does not agree with the distribution pattern that might be predicted from a simple model of plate tectonics. That is, along extensive sections of some island arcs, large shocks occurred infrequently or not at all during recorded history. Most of these zones of long-term quiescence are nearly coterminous with segments of the margin where zones of seamounts, aseismic ridges, or other bathymetric highs of the underthrust slab appear to be interacting with the subduction process. This spatial correlation suggests that at least some of the long-term absences of great shocks may result from a tectonic origin and not from temporary intervals of strain accumulation. The zones where rises interact with active trenches are also characterized in many instances by a near absence of low-angle thrust type mechanisms, by gaps in intermediate depth hypocenters, and by gaps and offsets in the line of active volcanoes. Thus major departures from classic subduction activity may develop where significant bathymetric features interact with a convergent margin. To explain these observations, we favor the hypothesis that aseismic ridges or other uplifted regions may delineate zones of oceanic lithosphere which are relatively buoyant and resist subduction upon collision with an active trench. In place of ‘typical’ oceanic lithosphere therefore there may exist a broad spectrum of average densities for oceanic lithosphere, and the relative buoyancy of both the underthrusting and the overthrusting slabs near the subducting margin may be a dominant influence in the development of subduction tectonics and in the locations and frequency of great earthquakes.