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Abstract:
The purpose of this study is to refine the probabilistic seismic hazard assessment for Vanuatu. The analysis is complicated by inconsistencies of the global and local
data for the investigated region and systematic inadequacies of the attenuation
relation available. An additional aspect in the field of which more research would
be desirable is the de-clustering algorithm for the identification of dependent earthquakes.
In order to quantify the inevitable uncertainties associated with our results,
we used a Logic Tree approach.
The report is subdivided into seven chapters. Chapter 2 summarises the geology
and tectonic setting of the Vanuatu island arc. Prior scientific investigations will be sketched briefly and to the degree to which they are relevant for our further considerations. The consecutive Chapter 3 describes the different data sources
used for the calculations. Together, the local earthquake catalogue provided by the IRD (Institut de Recherche pour le D´eveloppement) and the global catalogues constitute
a comprehensive data base for the region. The homogenisation of different
catalogues required the conversion of magnitudes through maximum likelihood regressions.
Chapter 4 gives a brief overview over the methodological concept applied.
The implementation of the Cornell methodology involves two key steps: The first
step is to construct a seismicity model (Chapter 5) including the definition of source
zones and seismicity parameters characterising the level and type of activity in the
respective zone. The second step is to determine an appropriate attenuation relation
for the earthquake-generated ground motion in dependence of magnitude and
distance. Chapter 6 compares several attenuation relations from the recent geophysical
literature. We argue that the modeling of attenuation is the weakest link in
this analysis and the major source of uncertainty. The new seismic hazard maps are
presented and discussed in Chapter 7. The map that we consider to be the most
relevant is also shown on the inner title page of this report. Our findings indicate
that the seismic hazard in Vanuatu has been underestimated by prior assessments
such as the Global Seismic Hazard Assessment Programme GSHAP (cf. Fig. 1.1).
