Focal Mechanisms of Deep and Shallow Earthquakes in the Tonga-Kermadec Region and the Tectonics of Island Arcs

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Abstract
Well-determined focal mechanisms based on reliable first motions of both compressional and shear waves are presented for 18 shallow, 6 intermediate, and 15 deep-focus earthquakes in the Fiji-Tonga-Kermadec region of the Southwest Pacific. The double-couple model is an adequate representation for earthquake mechanisms at all depths; most of the mechanisms are characterized by a predominance of dip-slip motions. The orientations of the mechanisms of deep and shallow earthquakes appear to be systematically and fundamentally different in respect to the orientation of the seismic zone. Whereas the shallow mechanisms all appear to accommodate movements between the adjacent sides of the seismic zone, the slip planes of the deep earthquake mechanisms are systematically nonparallel to the deep seismic zone. Hence, the deep zone of activity does not appear to be a simple thrust fault. The P, B, and T axes of the double-couple solutions tend to parallel the dip, strike, and normal directions, respectively, of the portions of the Tonga seismic zone deeper than about 80 km. The P axes tend to be more stable in orientation than the B and T axes. Large variations in the orientations of some of the deep mechanisms may reflect contortions of the deep seismic zone in a simple geometrical fashion. The shallow mechanisms indicate that thrust faulting is occurring beneath the inner (islandward) margins of the Tonga and Kermadec Trenches and that transform faulting is occurring at the northern end of the Tonga Arc. The over-all interpretation of the shallow mechanisms also includes hinge faulting south of Samoa at the juncture of the thrust and transform faults. These results, which are also in agreement with mechanism data for other regions such as Japan where both numerous and reliable data are available, are most simply interpreted by a tectonic model of an island arc in which (a) shallow earthquakes occur between a segment of lithosphere that moves downward into the mantle and the segments of lithosphere on the surface, and (b) deep earthquakes occur within the downgoing slab in response to a compressional stress within it.

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