Earthquakes associated with diffuse zones of deformation in the oceanic lithosphere: some examples

Abstract

We review the results of source parameters of earthquakes associated with the eastern end of the Azores-Gibraltar plate boundary, the Davie ridge near Madagascar, and the Horizon bank in the north Fiji basin to characterize the nature of present-day tectonics in diffuse zones of deformation in the oceanic lithosphere. In comparison with typical plate boundaries, diffuse zones of deformation are in general characterized by complex morphological expressions and scattered seismicity of up to several hundred kilometers width. Although the average rate of displacement across these regions is not resolvable by current models of global plate reconstruction, the deformation is not truly intraplate in nature because strain is concentrated and often distinct tectonic boundaries can be identified. Earthquakes with seismic moments as large as 8 × 10 20 Nm have occurred in bands of scattered seismicity that are associated with diffuse zones of deformation in the oceanic lithosphere. In two cases, the zones of diffuse deformation continue into the continental lithosphere east of Gibraltar and north of the Davie ridge. However, the largest earthquakes occur in the oceanic part of these two zones. In contrast to previous reports, the depth of earthquakes is quite shallow (< 15 km) near the Horizon bank (also known as the Hazel-Holme fracture zone) where the predominant mode of deformation is strike-slip motion. This zone is an important tectonic feature in the southwestern Pacific because the rate of seismic strain release along it in recent years is several orders of magnitude higher than that of a proposed nascent plate boundary north of the north Fiji basin. The maximum focal depth reaches about 50 km in the zone of ocean-ocean collision on the eastern end of the Azores-Gibraltar plate boundary where most of the mechanical lithosphere must have been broken during large earthquakes. Focal depths of up to 35 km were observed beneath ocean floors of Mesozoic age at the zone of extension along the Davie ridge. The current deformation is apparently unrelated to the past motion along the Davie ridge and this region seems to be part of a wide zone of diffuse extension which marks the southern termination of the Nubian-Somalian plate boundary. In regions where a mixture of focal mechanisms occur, the consistent parameter is usually the orientation of the P-or the T- axes , instead of that of the nodal planes or slip vectors. Thus, as in the continental lithosphere, zones of diffuse deformation in the oceanic lithosphere seem to be better described by a regional compression or extension field rather than by displacement on discrete faults.