1999 earthquake: What Athens learned about its “earthquake-resistant foundations” and which areas have a natural shield

At 2:56 p.m. on Sept. 7, 1999, Athens was shaken by the most destructive earthquake the capital had experienced in decades. The magnitude 5.9 Parnitha earthquake killed 143 people, injured hundreds and destroyed or seriously damaged thousands of buildings.

It also left behind an important scientific lesson: a city’s seismic safety depends not only on the building, but on the ground beneath it. Twenty-seven years later, Greek experts have mapped more precisely a factor that often goes unnoticed. Attica’s subsurface is not uniform.

Some parts of the basin sit on solid bedrock. Others rest on younger, looser deposits formed in places that were once lakes, marine areas or riverbeds. That difference can become critical when seismic energy reaches the surface.

Areas with more favorable geological foundations include Hymettus, Vyronas, Kaisariani, Agia Paraskevi, Ilioupoli, Psychiko, Filothei, Dafni, Pnyx, the Acropolis, Plaka, Pangrati, Lycabettus, Attica Grove and parts of Old and New Penteli. Many contain rocks from the so-called Alpine geological basement. Marble, schist and other compact formations provide, according to geological studies, a more stable base for construction.

That does not mean a building on rock is automatically safe. Construction quality and age, the seismic code in force when it was built, the distance from the earthquake source, and the earthquake’s depth and mechanism all remain decisive.

The opposite conditions exist in areas dominated by younger, looser deposits. Moschato, Renti, Agioi Anargyroi, Kokkინოს Mylos, Metamorfosi, Kato Kifisia, Menidi, Ilion, Chalandri, Amfithea, Kallithea, Votanikos, Kolokynthou, Peristeri, Petroupoli and Nikaia are among the areas where ground behavior is a critical factor in seismic stress.

Attica did not always have its current shape. Areas such as Acharnes and Chalandri were once large lakes, while parts of the southern suburbs lay beneath the sea. Geological change and the successive accumulation of materials created the complex subsurface on which modern Athens developed. Under certain seismic conditions, looser soils can intensify ground motion. Human-made deposits also require particular attention.