Abstract
Abstract. Probabilistic seismic hazard assessment (PSHA), despite its deep criticism, is still the most common tool used to decide on the acceptable seismic risk and corresponding mitigation measures. We propose two consistency tests to address the variability of earthquake generation models found in PSHA studies: (i) one rule-of-thumb test where the seismic moment release from the model is converted to an average slip on a typical fault and compared with known plate kinematics or GNSS deformation field and (ii) a test where using a neotectonic model the computed deformation is converted into seismic moment release and to a synthetic earthquake catalogue. We apply these tests to W and SW Iberia slowly deforming region, where two earthquake source areas are investigated: (1) the Lower Tagus Valley, one of the largest seismic risk zones of Portugal; and (2) the offshore SW Iberia area, considered to be the source for the 1 November 1755 event (M ∼ 8.7). Results show that some of the earthquake source models should be considered suspicious, given their high/low moment release when compared to the expected values from GNSS observations or neotectonic modelling. This analysis allowed for a downgrade of the weight of poorly compliant models in the PSHA analysis, and thus for a more realistic hazard assessment, and can be integrated in other studies of similar settings.
Highlights
In earthquake-prone areas modern societies with limited available resources must decide on the level of acceptable risk to define the appropriate mitigation measures
We address the first factor of Eq (1), the earthquake generation process, and propose a methodology to evaluate and validate earthquake generation models that are used for Probabilistic seismic hazard assessment (PSHA) studies
In offshore SW Iberia, the source area for the large 1755 and 1969 magnitude earthquakes (Fig. 1a), seismicity relocated by ocean bottom seismometers (OBSs) has shown that most of the small-magnitude events are occurring at mantle depths, down to 50 km focal depths, again without a clear relationship with the known active tectonic features identified by geological and geophysical surveys (Geissler et al, 2010; Grevemeyer et al, 2016; Silva et al, 2017)
Summary
In earthquake-prone areas modern societies with limited available resources must decide on the level of acceptable risk to define the appropriate mitigation measures. As pointed out by several authors (e.g. Stein et al, 2012; Mulargia et al, 2017) recent large earthquakes have occurred in areas where the seismic hazard was considered low or they caused ground shaking much greater than the one expected by PSHA studies These events have demonstrated the weaknesses of PSHA and a deficit in the evaluation of its uncertainties. This brings confusion and uncertainty to the users of these studies, who use results for emergency management or land use planning and question the foundations for establishing or revising building codes This large diversity is usually ascribed to the large uncertainty on the second term of Eq (1), the uncertainty and differences in the ground motion prediction equations (GMPEs) used in the studies. It means that the model predictions are tested (validated) against observations and/or independent parameter estimates
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