Analysis of soil effects and distribution of damage from the Pyrgos 1993 (Greece) earthquake

Summary On 26 March, 1993, a moderate magnitude earthquake (M_s=5.5) occurred at 3 km epicentral distance from the town of Pyrgos, in Southern Greece, causing extensive damage to masonry houses. To explain the variability of seismic intensity over the town and to propose measures against future seismic activity, a microzonation study was undertaken which combined geological, geophysical and geotechnical investigations, site specific analyses of seismic ground response and detailed recording of structural damage. The analytical predictions of ground response are correlated to soil conditions and then used to identify (micro-)zones of sites with similar seismic response. Furthermore, they are compared to quantitative estimates of damage distribution over the town. It is concluded that the peak ground acceleration, normalized against the input peak seismic acceleration, is a function of the local soil conditions as well as the seismic excitation characteristics. Hence, it cannot be defined uniquely at a site, without reference to the seismic excitation. However, the normalized peak ground velocity and the acceleration response spectra are mainly functions of the soil conditions and can be used as criteria for the practical definition of (micro-)zones. The distribution of damage in various parts of the town is at least partially attributable to local soil effects. The small epicentral distance of the earthquake, connected with the direction of the fault rupture, as well as the quality and techniques of construction, are additional factors that may have influenced the extent and distribution of damage.