Ground-Motion Models for Seismic-Hazard Assessment in Western Iberia: Constraints from Instrumental Data and Intensity Observations

The lack of constraints regarding ground-motion propagation characteristics in western Iberia remains one of the major hindrances to accurate seismic-hazard and seismic-risk assessment in the region. The fact that western Iberia is geographically located near the confluence of oceanic crust, active continental crust, and stable continental crust further complicates the selection of appropriate ground-motion models. To address this question we compare, through visual inspection and residual analysis, the regional ground-motion data for moderate magnitude earthquakes with a set of six ground-motion prediction equations, some of which were developed for active tectonic regions and the others for stable continental regions. Although the available data pertain mostly to offshore moderate magnitude earthquakes (M 4.8-6.0) at large distances (100-400 km), the few records from an onshore M 4.0 earthquake and from the 1969 M 7.8 earthquake display similar trends. The results show that the performance of the models is period-dependent, with shorter spectral periods (0.01 s, 0.1 s, 0.2 s, and 0.5 s) being strongly underpredicted by models developed for shallow active tectonic regions. More interestingly, the performance of the models for stable continental regions seems to be magnitude-dependent, with response spectra amplitudes for magnitudes of engineering interest being accurately estimated by the models. The inspection of the residual distributions of intensity observations leads to similar conclusions for the offshore data, while highlighting the extent of epistemic uncertainty in predicting the ground motions for moderate magnitude earthquakes at short to moderate distances.