The Santorini Seismic Swarm: A Geological Perspective

Introduction:

Santorini, a jewel of the Aegean, is no stranger to seismic and volcanic activity. However, recent events north of the island have drawn renewed attention to the dynamic geological forces shaping this region. Over the past several weeks, a seismic swarm of over 12,800 earthquakes has been recorded, with depths ranging from 0 to 22 km. This phenomenon, linked to movement along the Anydros Fault between Santorini and Amorgos, raises important questions about the geological processes at play.

Tectonic Setting: The Hellenic Volcanic Arc

Santorini is part of the Hellenic Volcanic Arc, a chain of volcanic islands formed by the subduction of the African Plate beneath the Eurasian Plate. As the African Plate moves northward, it sinks into the mantle, creating intense pressure and heat that drive both volcanic and seismic activity. This process has led to the formation of a complex structural environment with numerous faults, grabens, and volcanic systems.

The recent earthquake swarm is located in a back-arc extension basin, specifically in a graben structure north of Santorini. This area, known as the Kulumbo Active Field, is part of an extensive fault network that accommodates the stretching and thinning of the Earth's crust due to subduction-related extension.

Formation of Volcanic Arcs

Volcanic arcs, such as the Hellenic Volcanic Arc, form as a result of subduction zones, where one tectonic plate is forced beneath another. As the denser oceanic plate descends into the mantle, it experiences increasing pressure and temperature, leading to the release of volatiles such as water and carbon dioxide. These volatiles lower the melting point of the surrounding mantle, generating magma. The magma then rises through the overlying crust, often accumulating in magma chambers before erupting at the surface, forming volcanic islands. Over millions of years, this process builds chains of volcanoes, many of which remain active due to continued subduction and magma supply.

Back-Arc Basins: Definition and Formation

A back-arc basin is a geological feature that forms behind a volcanic arc, typically in a subduction zone setting. These basins develop due to the extension and thinning of the lithosphere as the overriding tectonic plate is pulled away from the subduction zone. The process is driven by slab rollback, where the subducting plate sinks into the mantle at an angle, creating tension in the overlying plate. This extension leads to the formation of rift structures, grabens, and fault systems, allowing magma to rise and contribute to volcanic and seismic activity in the region. The Hellenic Volcanic Arc exhibits back-arc extension, evident in the Kulumbo Active Field and the broader geological evolution of the Aegean region.

Understanding the Anydros Fault and Seismic Swarm

The Anydros Fault, running between Santorini and Amorgos, has been identified as the main driver of the recent seismic activity. Fault movement in this region is primarily tectonic rather than volcanic in nature. This means that while the shaking may be intense, it does not necessarily indicate an impending volcanic eruption. However, tectonic activity can still influence nearby volcanic systems by altering subsurface pressure and stress conditions.

The clustering of earthquakes suggests a localized zone of crustal deformation, where accumulated stress is being released in a series of small to moderate tremors rather than a single, large earthquake. The graben structure itself, formed by extensional forces, provides pathways for seismic energy to propagate through the region.

Volcanic Implications and the Kulumbo Chain

While Santorini's volcanic system remains closely monitored, the earthquake swarm is not directly linked to magma movement beneath the island. Instead, the tremors are occurring near the Kulumbo volcanic chain, an underwater field of volcanoes east of Santorini. Most of these volcanoes remain submerged, and while they are active, there are currently no indications of imminent eruptions.

If volcanic activity were to occur in the region, we would expect to see warning signs such as increased gas emissions, ground deformation, and harmonic tremors—seismic signals associated with magma movement. So far, no such signals have been detected. However, given Santorini’s volcanic history, geologists remain vigilant for any changes in the system.

Historical Context: Lessons from the Past

Santorini’s geological history is marked by catastrophic eruptions and significant seismic events. The most well-known event, the Minoan eruption (circa 1600 BCE), was one of the largest eruptions in human history. This cataclysmic event reshaped the island, creating the caldera we see today and sending tsunamis as far as Crete, potentially contributing to the decline of the Minoan civilization.

More recently, in 1956, the Amorgos Earthquake (M7.7) struck the region, triggering a devastating tsunami. This event underscores the potential for major seismic activity in the area and its associated risks.

Santorini’s Evacuation

Historical evidence suggests that the inhabitants of Santorini were likely evacuated before the catastrophic Minoan eruption. Archaeological excavations at Akrotiri, a Minoan settlement buried under volcanic ash, reveal well-preserved structures but a notable absence of human remains. This suggests that the population may have fled after experiencing precursor earthquakes and other warning signs, such as ground deformation or gas emissions. Unlike Pompeii, which was rapidly buried by ash, Santorini's Minoan inhabitants may have had enough time to escape to nearby islands. However, those who fled may have been affected by the massive tsunamis that followed the eruption, impacting coastal settlements across the Aegean, including Crete. The eruption's devastating effects likely disrupted trade, agriculture, and regional stability, contributing to the decline of Minoan civilization.

Future Monitoring and Geological Research

The ongoing seismic activity in the back-arc basin highlights the need for continued geophysical monitoring. Instruments such as GPS stations, seismometers, and satellite imaging provide real-time data on crustal movement and stress accumulation. Scientists are closely observing changes in the region to determine whether this swarm represents a temporary release of tectonic stress or signals broader geological shifts.

For now, the evidence suggests that the swarm is primarily a tectonic event rather than a precursor to volcanic activity. However, the interaction between seismic and volcanic processes in this highly dynamic region remains an active area of research.

Here's the video we made on the Santorini Seismic Swarm on the OzGeology Youtube Channel: