Effects of site geology on the attenuation of macroseismic intensity in Central Greece

Abstract

The contribution of different categories of local geological conditions to the observed macroseismic intensities in Central Greece is investigated. The method is developed using two large earthquakes in the Halmyros Basin, Central Greece. The main parameters of the earthquakes chosen are: 1975 March 8, 6.8 M_s and 1980 July 9, 6.2 M_s, both near Volos-Halmyros. They were both shallow earthquakes, with maximum observed intensities of IX+ and VIII+ respectively, while the available macroseismic and surface geological information comes from about 230 and 570 sites respectively. There are five main surface geological categories, in which the data can be classified. Differences between, and recent revisions of, intensity scales are considered in order to eliminate possible over- or under-estimates of intensity and to obtain a homogeneous intensity data set.

Attenuation laws are derived using both earthquakes and they are compared to determine the attenuation factors appropriate for different rock types. They are also tested for different azimuths from the macroseismic epicentre in order to assess the influence of the radiation pattern of the earthquake in relation to the degree of attenuation in a specific direction. The comparison of the detailed intensity results with strong motion data from the same area and in different geological formations, indicates that there is some relation between them.