Ocean bottom seismograph measurements in the Central Aleutians

Abstract

Bathymetrie profiles of most oceanic trench regions show a well-defined outer rise seawards of the trench with numerous normal faults between the trench axis and the crest of the outer rise. Several investigators (see ref. 1 for review) have shown that the gross bathymetrie features can be modelled quite well if they result from a simple mechanical bending of the subducting lithospheric plate. These results have encouraged searches for seismic evidence of the bending stresses predicted by the models. However, the available teleseismic studies2 do not determine precisely the relative depths of events on the outer rise, or the precise locations of events relative to the trench. In 1979 we successfully deployed nine ocean-bottom seismometers3 (OBS) which operated for 6 weeks on the outer trench slope in the Central Aleutians between the trench axis and the crest of the outer rise (Fig. 1). The locations, depths and focal mechanisms of the earthquakes located by this network all strongly support the bending hypothesis. In particular, we observed a band of seismicity on the outer trench slope extending ∼60 km seaward of the trench axis (Fig. 1). The shallow events (Fig. 2) exhibit normal faulting (Fig. 3) and suggest tension in the upper part of the plate, while the deeper events (Fig. 2) exhibit thrusting mechanisms and suggest compression (Fig. 3). We suggest here that low magnitude seismic activity may be a common feature associated with the well-known normal faulting on the outer trench slope.