Shallow Seismicity in the North Fiji Basin

Abstract

This paper presents a new compilation of shallow seismicity and focal mechanism data that help constrain the model of extension in the North Fiji Basin. Earthquakes are broadly distributed throughout the basin, in marked contrast to the narrow earthquake zones observed near mid-ocean ridge spreading centers. Areas of relatively high activity include the Fiji Fracture Zone, the Hazel Holme Ridge, the western Hunter Fracture Zone, and the proposed spreading center immediately west of Fiji. Areas of deep water in the northern and western North Fiji Basin are notably aseismic and may represent older, presently undeforming portions of the basin. Twenty-three focal mechanism solutions for the basin indicate that the region is dominated by strike-slip deformation. We observe no simple basin-wide system of stress distribution, but consistent stress orientations for groups of mechanisms provide evidence for (1) transcurrent faulting along the Fiji Fracture Zone; (2) hinge faulting of the Indo-Australian Plate near the Hunter Fracture Zone; (3) strike-slip faulting near the Hazel Holme Ridge; (4) strike-slip faulting within the central North Fiji Basin; and (5) normal faulting in the western North Fiji Basin. The orientations of the normal faulting events' tension axe, and the strike-slip events' fault planes are at odds with the configuration of the major spreading centers proposed by previous investigators. A model of "diffuse extension" in the North Fiji Basin provides an explanation for the broadly distributed shallow seismicity, the paucity of normal faulting mechanisms, the obliquity of earthquake fault planes to the strike of proposed spreading centers, and the lack of a uniform basin-wide pattern of stress orientation.