A strong earthquake with a preliminary magnitude of at least 6.1 hit the western Greek island of Lefkada on Tuesday, killing two people and causing extensive damage to infrastructure and buildings on Lefkada and nearby islands.

The temblor was felt across western Greece, which is very earthquake-prone, with people on Lefkada and the nearby Ionian Sea island of Kefalonia rushing out onto the streets.

The Athens Geodynamic Institute said the undersea quake had a preliminary magnitude of 6.1 and occurred at 9:10 a.m. off Greece's western mainland, about 186 miles west of Athens. The U.S. Geological Survey put the preliminary magnitude at 6.5. Different agencies often have varying preliminary magnitudes in the hours and sometimes days after a quake.

Greek authorities were struggling to assess the damage, their efforts hampered by landslides that blocked Lefkada's network of narrow, mountainous roads.

The fire department said an 82-year-old woman was killed in the village of Athani when a wall collapsed on her. And a 69-year-old woman died in her home in another village when a boulder loosened by the earthquake tumbled down onto her house. Four people were hospitalized with non-life-threatening injuries.

State ERT TV said part of the harbor of Vassiliki in the worst-hit southwestern part of the island was submerged in the sea. Schools were shut on both Lefkada and Kefalonia as a precaution.

Authorities said several houses were badly damaged on Kefalonia and another nearby island, Ithaca.

Aftershocks were also hitting the area — including one with a 5.2 magnitude more than an hour after the main quake — and Ionian Islands regional governor Theodoros Galiatsatos called on residents to avoid any structures that appeared damaged until authorities could assess their safety.

Earthquakes are common in Greece, which is one of the world's most seismically active areas, though serious injuries and deaths are rare. A severe quake near Athens in 1999 killed 143 people, and caused extensive damage through the Greek capital.

The Ionian is particularly seismically active, and new buildings on the area's islands are constructed to strict anti-seismic standards. Kefalonia was struck by a series of strong earthquakes, two of them with magnitudes of around 6, in January 2014, causing damage and minor injuries but no fatalities.

Those temblors awakened memories of catastrophic 1953 quakes that flattened nearly all the island's structures, killing hundreds of people.