When Greek tourist hotpot Santorini was rattled by an earthquake at the end of January, it wasn't a one-off.
Thousands of tremors have since shaken the south-eastern Aegean Sea region in what scientists call an "earthquake swarm".
More than a fortnight later, the majority of Santorini's nearly 16,000 residents have left the island as the mid-level quakes continued.
Now authorities have declared a state emergency on the nearby island of Amorgos too.
Where are Santorini and Amorgos?
Santorini and Amorgos are part of Greece's Cyclades island group, which is located in the Aegean Sea.
They're about 200 kilometres south-east of mainland Greece.
[datawrapper — greece map]The islands are extremely popular tourist destinations, with about 3.4 million visitors flocking to Santorini in 2023.
What is an earthquake swarm?
A bunch of earthquakes happening with no dominant main shock, New Zealand's Geonet website explains.
"Swarms typically involve earthquakes of lower magnitudes, they can last for days to weeks, or persist for months," earthquake scientists Dee Ninis and Konstantinos Michailos wrote in a piece for the Conversation.
"They can even slow down, and then intensify again, unsettling locals with intermittent ground shaking."
Greece is in a highly seismically active part of the world and earthquakes are common.
However, it's rare for there to be so many intense quakes over such a long period.
How many earthquakes have there been?
Athens University has recorded more than 14,000 quakes between January 26 and February 9.
But there's been more since then, with 109 quakes recorded on Monday alone.
And the university Siesmological Laboratory's website recorded dozens of further tremors on Wednesday.
Will there be more?
It's difficult to say.
Scientists say it's unclear whether frequent lower-magnitude quakes will continue for several weeks or months.
"The most likely scenario is for the seismic activity to continue for certain days or weeks at the same intensity," the head of Greece's earthquake planning and protection authority, Efthymios Lekkas, told Proto Programma radio.
What's the magnitude of the earthquakes?
The quakes have been ranging between a magnitude 3 to 5.
So far, the largest has had a magnitude of 5.2 — that one was Monday night.
Thirteen of Monday's quakes registered magnitudes greater than 4, and there were several more of a similar magnitude on Tuesday.
One area was hit by another barrage of up to magnitude-5 tremors on Wednesday morning, with a few 4.0 and 4.7 termors recorded at about midday.
Depending on the topography and local ground conditions, the effects of earthquakes can vary.
Earthquakes with magnitudes of less than 3.5 rarely cause damage in Australia, but Geosciences Australia points out that magnitude-4 earthquakes occasionally topple chimneys.
In Santorini, authorities cordoned off some areas on the island's renowned cliff top towns that were considered vulnerable to potential rock slides.
Amorgos, which has fewer than 2,000 permanent inhabitants, is considered less at risk owing to its rocky terrain and less dense construction than Santorini.
Has there been much damage?
So far there hasn't been any major damage reported, however the quakes have caused cracks in some older buildings and there have been been some minor rock slides.
Inspections at Amorgos have found no major damage to key buildings, local authorities said, and no injuries have been reported.
But authorities have been warning people to avoid indoor gatherings and stay well away from cliffs.
Locals and hotels have also been told to drain their swimming pools to avoid potential structural damage to buildings in the event of a large earthquake.
What is causing the earthquakes?
The Greek Civil Protection Ministry confirmed the tremors are not linked to volcanic activity.
The islands are near an undersea fault line, which runs for 120lm.
But, so far, there's only been quake activity between Santorini and Amorgos.
"Earthquake swarms are often associated with fluid movement in the earth's crust and the resulting seismic activity is usually less dramatic than the sudden movement of a strong main shock," Dr Ninis and Dr Michailos wrote.
What do we know about the area?
It's where Greece's biggest earthquake in the last century struck.
A 7.7-magnitude quake hit in 1956.
It triggered a 20m tsunami which killed more than 50 people and caused significant damage in Amorgos and Santorini.
[datawrapper — amorgos]The islands are in a zone known as the Hellenic Volcanic Arc, with two volcanoes in the area.
One is Kolumbo, an underwater volcano about 8km north-east of Santorini.
The other is Nea Kameni, which an islet within Santorini's ring-shape land mass.
Santorini used to be more rounded in shape, but back in 1600 BC there was a massive volcanic eruption which destroyed much of the land mass.
[datawrapper — santorini]Known as the Minoan eruption, it was one of the biggest volcanic eruptions in human history and is thought to have fuelled the downfall of the ancient Minoan civilisation.
Technically the volcano is still active, but its last notable eruption was way back in 1950.
"What we must realise is that the Santorini volcano produces very large explosions every 20,000 years," Mr Lekkas said last week.
"It's been 3,000 years since the last explosion, so we have a very long time ahead of us before we face a big explosion."
ABC with Wires
