The seismicity and deep structure of Island arcs

Abstract

The hypocenters of approximately 1500 earthquakes in the Tonga-Fiji, Kermadec, Kuril-Kamchatka, and Caribbean regions were redetermined using a digital computer. Since these computations are more accurate than those used in most previous studies, the spatial distribution of the redetermined hypocenters can be used to resolve structural features with dimensions larger than about 20 km. In each of the regions investigated in this paper a zone of intense seismic activity was found beneath the inner (islandward) margin of the oceanic trench. Near its northern end the Tonga trench curves abruptly to the west; the belts of shallow and deep earthquakes and the chain of active volcanoes curve similarly. Thus the tectonic processes responsible for the earthquakes, the volcanoes, and the trench are intimately related for depths from 0 to 650 km. Theories that attempt to explain the curvature of the trench must account for a similar curvature in the belts of deep and shallow earthquakes. The distribution of earthquake hypocenters in the New Hebrides and Tonga-Fiji regions indicates that these arcs are part of a larger structural feature. Earthquake foci in the Tonga-Fiji region are confined to a zone about 50 to 100 km thick that dips under the arc. In at least some regions this focal surface can be regarded as a continuous zone of tectonic activity that extends from the surface to a depth of about 650 km. Within the accuracy of the computations the dip of this zone is independent of depth. Similar results were obtained for the Kermadec, Kuril-Kamchatka, and Lesser Antillean arcs. A discontinuity in seismicity between the Tonga and Kermadec arcs can be traced from the surface to a depth of about 650 km. Near the southern end of the Tonga arc the depth of maximum deep-focus activity decreases from about 625 km to about 475 km within a horizontal distance of about 500 km. It is unlikely that this pronounced peak in the deep-focus activity is associated either with a phase change or with a particular layer unless the physical or chemical properties of the mantle have large lateral variations.