Aftershock Monitoring of the Athens Earthquake of 7 September 1999

Abstract

On 7 September 1999 at 3:00 pm local time (11:56:50 GMT) an earthquake of Mw = 5.9 struck Athens, the capital of Greece. The intensity reached IX in the northwest out-skirts of the city (Figure 1), where 143 people were killed and more than 2,000 injured. More than 100,000 people were rendered homeless. Most of the damage occurred within 15 km of the epicenter. Structural damage decreased rapidly with the distance from the epicenter, where it was influenced by poor foundation conditions (building on top of artificial fill) and in certain cases by local topography. The maximum recorded peak ground acceleration downtown, at an epicentral distance of 15 km, was 0.30 g. Unfortunately, no accelerograph was installed either in the northern suburbs of Athens or at the industrial area close to the epicentral area. However, normalizing for distance one may predict a maximum peak ground acceleration in the near field of about 0.6 g. The duration of the strongest ground motion phase, however, was relatively short, and, judging from the damage distribution, intensities attenuated rapidly with distance from the epicenter. Had the strong motion lasted somewhere longer, the catastrophe would have been much greater (http://www.itsak.gr/report.html.). Figure 1. ▴ A typical example of partial collapse of a five-story reinforced-concrete factory from the epicentral region Historical monuments in the area of Athens are made of marble blocks placed one on the top of the other, without mortar. During strong ground shaking, the stones can slide or rock, usually accompanied by large displacements at the top of the structure. In spite of their apparent instability, however, such structures are impressively safe against collapse (Prof. Karidis, personal communication). Figure 2. ▴ Temporary seismic network (triangles) of Patras University, superimposed on the topographic relief image of the Athens region, Greece. The USGS focal mechanism of the M …