Patterns of seismicity in the Vanuatu (New Hebrides) arc: regional variations and systematic evolution
Several sequences of major shallow earthquakes occurred in the Vanuatu island arc from 1961 through 1981. The space-time evolution of these sequences is described and analyzed in part I of this thesis in relation to the regional and arcwide development of the shallow and intermediate-depth seismicity that occurred during the 21 year period. Contrasting patterns of shallow seismicity characterize distinct segments of the arc. In particular, major events are preceded by donut patterns in the central part of the arc and by intense precursor activity in the northern part. In contrast to these regional variations, remarkable regularities appear in the development in time of the seismicity in relation to the major episodes of interplate slippage. Part II of the thesis is a detailed study of the relationship of the subducting d'Entrecasteaux Fracture Zone (DFZ) to the spatial distribution of intermediate-depth seismicity in the central part of the arc. The extrapolation to depth of the subducting DFZ is spatially related to a remarkable gap of intermediate-depth seismicity and to a zone of anomalous attenuation of high-frequency shear waves in the subducted lithosphere. The ocean bottom bathymetry south of the DFZ is also anomalous. The absence of bathymetric expression of a trench in this area may be an effect of the flanking slope of the already subducted DFZ. In this model the downblending effect of subduction is offset by the landward shoaling of the subducted plate due to the subducting DFZ.
- OSTI ID:
- 5862583
- Country of Publication:
- United States
- Language:
- English
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