Abstract:
Modern concept of seismic hazard assessment is based on the assumption that what was observed in the past could likely be expected in the future. It could be easily recognized by comparison of spatial distribution of maximum intensity of shaking ( I max ) from past earthquakes and actual seismic hazard maps. This makes crucially important the re-evaluation of parameters of earthquakes close to the maximum observed ones in a certain area: even a small change of the parameters can affect considerably the final hazard assessment. A case study of the earthquake on 2 June, 1930 in southern Catalonia, Spain, is presented. A comprehensive analysis of macroseismic and instrumental data leads to a good agreement in magnitude re-evaluation. The earthquake magnitude MS is within the interval 4.5-4.8, which is significantly larger than that reported earlier (3.9). Relatively accurate magnitude evaluation allows us also to assess the hypocenter depth to be at 20-30 km, deeper than it is commonly assumed for this region. Occurrence of large earthquakes in southern Catalonia at various depths (from 10 km to 30 km) reflects the presence of seismogenic structures which are able to produce earthquakes with magnitudes at least as large as 4.5-4.8. Orientation of isoseismals lets us also to suggest that these seismogenic structures are oriented almost perpendicular to strike of topographic elevations, which follow along the seashore. Modern topography does not inherit deep (mid and low crust) structures.
