Greece’s Aegean islands are typically associated with relaxing vibes and tranquil scenery. But some of them, including Amorgos, Anafi, and Santorini, are currently besieged by a seemingly never-ending swarm of intense earthquakes.
Over the last few days, residents have been startled by tremors as high as magnitude 5.3. More than two-thirds of those living on Santorini have voluntarily chosen to leave the island, and authorities have ordered the closure of schools and dissuaded large indoor gatherings. People have been told to stay away from the coast and any of the island’s steep slopes.
A state of emergency has been declared, but the Prime Minister of Greece, Kyriakos Mitsotakis, has urged calm. In a recent crisis meeting, he said: “The government has trust in science,” but noted that scientists are dealing with an “extremely perplexing phenomenon.”
The drumbeat-like cacophony of unusually potent quakes, overlayed atop the region’s baroque, messy geologic jigsaw, has researchers foxed. “It’s not clear to me why we have this repetitive sequence of earthquakes,” says David Pyle, a volcanologist at the University of Oxford. “It’s a real puzzle.”
Although Santorini is itself a partially submerged, and still active, volcanic island, the current consensus is that the cause of these quakes is anything but volcanic. Their source seems to be related to a strange, difficult-to-forecast tectonic process—one that could quickly wind down or dangerously escalate.
The situation is constantly evolving, and things could either get worse or (hopefully) improve. For now, “there’s no reason for panic,” says Jonas Preine, a geophysicist at the Woods Hole Oceanographic Institution. “But there is for sure reason for concern.”
The Aegean Sea is no stranger to earthquakes and eruptions. To the south and west of Greece is the deepwater Hellenic Trench. There, an ancient piece of dense oceanic crust is being swallowed by the underlying mantle. This descent isn’t calm, but chaotic, and the ongoing stress on the crust above is pulling Greece apart in multiple directions. That tectonic pandemonium has led to the formation of spiderweb-like fault zones and myriad volcanoes, both below and above water. Scientists, aware of these hazards, keep a close eye on it all.
Santorini – a giant magmatic cauldron with two small volcanic isles nested in its submerged center – has a dark and dangerous volcanic history stretching back 650,000 years. It features a massive blast in 1560 B.C. that helped end a civilization, a significant explosion in A.D. 726, and even a modest outburst in 1950. It’s still an active volcanic system—and it’s not alone. A nearby submarine volcano called Kolumbo also exploded in 1650, creating a series of tall tsunamis and a lethal cloud of toxic gases.
The current seismic tempest, which only recently made headlines, has been going on for some time now. “It started toward the end of last year, there started to be slightly more seismicity, but nobody really noticed,” says Isobel Yeo, a submarine volcanologist at the National Oceanography Centre in Southampton, England.
At the end of January, there was a profound uptick in the power and frequency of these quakes; there have been almost a thousand per day over the last few days. “It must be an awful thing to experience,” says Pyle.
Why the current quakes are odd
The earthquakes are not just happening in one place. The unrest started within Santorini itself, but the seismic focus quickly shifted offshore.
The pattern of quakes is also unlike a classic earthquake sequence. Often, a fault ruptures and produces its strongest quake—the mainshock—which is then (usually) followed by a series of increasingly weaker aftershocks. But in this case, there is no clear mainshock.
Instead, the region is being rattled by a cornucopia of quakes of somewhat similar magnitudes—and, for several days, the quakes appeared to be getting stronger. “This is super unusual,” says Preine.
This atypical seismic pattern is known as an earthquake swarm. They happen in various places around the world, but each is idiosyncratic. For example, swarms in Iceland’s Reykjanes Peninsula are associated with the migration of magma, and several have terminated in dramatic eruptions. But swarms beneath America’s Yellowstone National Park have not culminated in any modern volcanic activity.
Compared to classic quakes, swarms “don’t play by the rules,” says Hubbard. And that makes forecasting their futures difficult.
What’s causing these earthquakes?
To speculate on what may happen in the coming days and weeks, we first need to answer a key question. “Are these earthquakes related to volcanic activity or tectonic activity?” says Preine.
In the early days of this swarm, several quakes shook Santorini’s northern caldera basin, the bowl-shaped, sunken edifice at the island’s heart. There were briefly concerns that this could indicate the start of a fresh injection of magma into the crust, which could generate additional, damaging quakes or potentially trigger an eruption.
Fortunately, the caldera-focused quakes were only temporary. “Right now, the earthquake activity is focusing to the northeast. It’s not clustering underneath any of the volcanoes that we know,” says Preine. Now, most quakes are coming from the Santorini-Amorgos fault zone, a patch of fragmented seafloor between the two islands.
These offshore quakes also show none of the hallmarks of ascending magma, nor are they accompanied by typical pre-eruption signs, like the ground swelling up and deforming on a volcano’s slopes. “Right now, this is tectonic activity,” says Yeo—meaning it’s something happening on faults.
Swarms are often associated with the movement of fluids, like water or carbon dioxide, through faults. Fluids escaping from one fault and invading another can essentially force that fault open and cause it to rupture. “They can continue to trigger earthquakes simply by moving,” says Judith Hubbard, an earthquake scientist at Cornell University.
Forecasting future hazards is tricky
For now, the major threat is an especially strong earthquake, and any resulting landslides or localized tsunamis it may cause. “Santorini may not be about to blow up,” says Yeo. But, she adds, that doesn’t mean that a major earthquake can be ruled out.
Indeed, that is why schools are closed on Santorini, and why people are being urged to avoid shorelines—not because of any volcanic activity, but because of the potential quake and tsunami hazards.
This swarm could suddenly die out. “But there is still an anxiety that there could be an acceleration to a much larger earthquake,” says Pyle. On July 9, 1956, the magnitude 7.8 Amorgos earthquake—the largest to strike Greece in the 20th century—and resulting tsunami caused infrastructural damage to multiple islands and killed dozens of people.
“Living with uncertainty as the situation evolves… is very difficult for people to do, especially when buildings are crumbling and the earth is shaking,” says Amy Donovan, a volcanologist and natural hazards researcher at the University of Cambridge.
Although a catastrophic quake is of concern, it’s not the only thing causing trepidation. “At the moment, I think the biggest worry is if the earthquakes start to focus more toward the volcanic chain,” says Preine, including the nearby submarine volcano Kolumbo. “There is always a risk of some kind of coupling.”
In other words, “if you shake your magma chamber up, you can trigger a volcanic eruption,” says Yeo. Presently, there is no indication that this is happening, but it’s something scientists are watching out for closely.
Rather than relying on a preexisting network of volcanic and tectonic probes, a galvanized international team of scientists—led by Greek researchers—are deploying new instruments, including ocean-bottom seismometers and self-piloting robotic submersibles, all of which are scanning the depths for any suspicious geologic changes.
The Greek authorities have communicated the danger to the public carefully, quickly, and transparently and “are managing the crisis very well,” says Pyle. But it is disquieting that the near future is unclear. “I’m not sure if there’s any obvious way that geologists could say, ‘This is the most likely scenario for how it’ll pan out.’”
Sending our prayers to the families and hoping they are safe. Your trusted reporter will keep you all informed.
