The epicentre of the tremor was located 4 kilometres west southwest of Marathon.
A 2.6-magnitude earthquake occurred at 03:45 on Monday morning in Attica.
Initially, the tremor's epicentre was located 4 kilometres east-southeast of Agios Stefanos, and the focal depth was estimated at 5 kilometres, according to the Geodynamic Institute.
A second report was later issued by the citizen-seismograph network of RaspberryShake, which also listed it as a magnitude 2.6 earthquake.
Towns or cities near the epicentre where the quake might have been felt as very weak shaking include Stamata (pop. 2,900), located 2 km from the epicentre, Kifissia (pop. 47,300) 10 km away, Marousi (pop. 72,300) 13 km away, Iraklion (pop. 49,600) 15 km away, and Agia Paraskevi (pop. 59,700) 15 km away. In Acharnes (pop. 99,300, 15 km away), Chalandri (pop. 74,200, 15 km away), Nea Ionia (pop. 67,100, 17 km away), Athens (pop. 664,000, 23 km away), and Piraeus (pop. 163,700, 31 km away), the quake was probably not felt.