An earthquake jolted Athens on Sunday although there were no immediate reports of damage or injuries.

According to early reports from the Greek Geodynamic Institute, the 4.2-magnitude quake's epicenter was 23 kilometers (14 miles) northwest of Athens.

It is believed to have been an aftershock of the strong tremor that hit the city on July 19, knocking out phone and cellphone service.

According to the institute, that tremor was a 5.1-magnitude quake with an epicenter 23 kilometers (14 miles) northwest of the Greek capital.

There is "no reason for concern. It is a normal aftershock of the 5.1 quake," the institute's director Akis Tselentis told Athens News Agency.

Scientists had said they were expecting an 4.0-magnitude aftershock.

Greece lies on major fault lines and is regularly hit by earthquakes, but they rarely cause casualties.

In 2017, a 6.7-magnitude earthquake killed two people on the island of Kos in the Aegean sea, causing significant damage.

In 1999, a 5.9-magnitude quake left 143 people dead in Athens and the region northwest of the capital.