Earth tremor near Agistri and the Pausanias volcano

An earth tremor measuring 4.9 on the Richter scale that was recorded early last week between the island of Agistri and Methana on the eastern coast of the Peloponnese was judged by seismologists to be an isolated incident. However, this western end of the volcanic arc carries particular seismological interest, both for its specific geotectonic state and the underwater volcano Pausanias, discovered just 17 years ago. The epicenter of the tremor that came at 11.44 p.m. on January 3 was located 50 kilometers south-southeast of Athens in the Saronic Gulf, south of Agistri. «The quake occurred at a depth of 123 kilometers so it was not felt by many people in the region itself, but as far away as northern Attica and the Argolid,» said Geodynamic Institute seismologist Moisis Kourouzidis. «Earthquakes in the Saronic Gulf are unusual in that the energy they produce is channeled in different directions, so the macro-seismic center (damage epicenter) is far from the true epicenter. For example, in 1965 there was a 6.1 Richter quake in the region that affected the prefectures of Arcadia and Messinia.» This is because of the tectonic nature of the entire volcanic arc. «The lithospheric plate has already fallen in this zone and earthquakes like the one this week occur at a great depth. Depending on the geotectonic structure of the crust and the mantle above the heart of the quake, the seismic energy is directed over great distances, although we cannot determine in which direction.» Although this most recent quake occurred above the volcanic arc, it was considered to be of tectonic and not volcanic origin. However, it occurred in a region which has attracted scientific interest in recent years, given the new data that has come to light. «In 1987, during research with the Aigaio oceanographic vessel, we discovered an underwater volcano, the existence of which had not been previously known,» said Professor Dimitris Papanikolaou of Athens University’s department of dynamic tectonic and applied geology. «The volcano is 1.5-2 kilometers northwest of the Methana peninsula, on the seabed in the western Saronic Gulf. Its ‘peak’ is at a depth of 140 meters and the volcanic cone begins at a depth of 400 meters. Based on the surrounding sediments, it appears to date from the Holocene period.» The volcano was named Pausanias after the famous traveller of antiquity who referred in his description of Corinth to a major volcanic explosion in about the third century BC, north of the Methana peninsula. He wrote that the sea boiled for several days and that the explosion altered the position of the hot springs that still exist today. Naturally the date cannot be confirmed, but historians’ descriptions coincide with the geological data and the position of the volcano, which is distinct from that above the Methana peninsula. According to Papanikolaou, this week’s quake does not appear to be linked to the region’s volcanos. «The volcanic arc has a special characteristic: Along its entire length, there are seismic phenomena caused by the movement of the lithospheric plates combined with the melting of the plate that creates the volcanos. However, volcanic activity in the western Saronic Gulf began about 4.4 million years ago in Aegina and was completed about 1 million years ago. Volcanic activity around Methana is more recent, from about 0.9 million years ago until about 2,200 years ago.»