Unexpected shock rocks an “aseismic” area



A strong earthquake with surface wave magnitude 6.6 hit the small town of Grevena in northwest Greece on May 13, 1995. This unexpected earthquake surprised the seis-mological community: it damaged more than 5,000 houses and caused more than half a billion dollars damage, but no deaths.

This event was just one of the frequent, strong (M≥6.0) earthquakes that hit Greece but cause relatively few deaths. There are two main reasons for this phenomenon: houses in most parts of Greece are designed to withstand earthquakes, and epicenters of most shocks are in the sea. Seismic waves are therefore attentuated when they reach inhabited areas. This event was even more remarkable, however. Where in the past, most of the epicenters of the shocks were in the sea, and the seismic waves were attenuated when they reached inhabited areas, in the case of the recent Grevena earthquake, the epicenter of the shock was inland, close to inhabited areas, while most of the houses were old and of low strength. Yet, the earthquake occurred on a nice spring Saturday morning, and most people alerted by a small foreshock were luckily outside of the buildings that collapsed. This event proves that lack of fatalities does not necessarily suggest weak earthquake magnitude. This is an important consideration to keep in mind when evaluating the 2500-year record of seismicity in Greece and other areas.