Where the land shaked: The Pisia fault, Corinth Greece

Greece is restless. Africa pushing northwards has created a tectonic mess of the northeastern Mediterranean. It is a long history of one continent piece of after another clashing into Europe, pushing up the Alps here, the mountains on Balkan there, subduction creating the volcanism of Santorini, of Etna and Vesuvio – and the mountains of Greece itself.

Within this mess, there are also areas of stretching. It can happen because squeezed land masses try to escape perpendicular to the compression, and because mountains collapse under their own weight, like a heap of snow piled up in front of a shovel. The Corinth Basin, which we visited on the Peloponnese peninsula, may have developed because of such a collapse, followed by what geologists call back-arc extension:

The Corinth Basin sits on the line going westwards from the arc of volcanoes in the Aegean sea, and one explanation for the rift that creates the basin is that the crust is lifted up by hot, light rocks from the subducted slab of Africa –not enough to make volcanoes (yet), but enough for the crust to slide down on each side of the ascending hot rocks.

Such back-arc basins are common behind volcanic arcs, the line of volcanoes in front of subduction zones. We do not know for sure how and why they develop, but a likely explanation is that the ascending magma also pushes up and this the crust in front of the volcano chain.

So, counterintuitive as it sounds, this part of Greece stretches. The stretching makes earthquakes, they are as common as raki and souvlaki, although most are small. But sometimes, big quakes hit.

On February 25th 1981, two quakes hit the Perachora peninsula on Corinth, with maximum magnitude of 6,7. The Pisia fault goes across the Perachora peninsula, from west bending towards northeast, and the quake slipped along a 25 km long part of the fault. The epicenter was near the village Pisia (red dot in the map below), and here we can still see the movement on the fault itself.

Going up to the Pisia fault, muggles might just note that a part of the hillside is more planar than usual. Geologists will recognise it as a fault plane, the last plane to develop in this fault zone. The plane is not entirely planar, more like a wavy, corrugated roof plate, with their longitudinal direction plunging downwards. It looks like some giant had run a cheese grater on it.

The plane is also weathered, partly covered by moss and lichens, with bushes growing in cracks. Originally limestone, the rock is etched by rain. But the lowermost half meter of the surface is not weathered, only a few lichens have settled. This part of the surface was laid bare by the 1981 quake, and it has not had time to be much weathered.

Looking at the fault plane like this gives some impression of the enormous force involved in rubbing the blocks against each other. And these three earthquakes were just a tiny few of the many quakes on many faults through millions of years, that it took to build up the mountains of Greece – and then tear them apart and open the Corinth Gulf. Seeing the small movement on the fault for one quake, we appreciate the deep time it takes to accumulate all the displacement on the fault plane.

This fault plane is in turn only the last zone of deformation on the Pisia fault. The fault plane cuts through a zone of fault breccia, a limestone crushed and fractured by fault movements, then partly dissolved by acid water flowing through, to large grooves – or cemented and filled in again by dissolved calcite and… lather, rinse and repeat.

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