How can we use seismic tomography to understand volcanic degassing?

• The islands of Vulcano and Lipari are part of a volcanic archipelago, named Aeolian Islands, in Southern Italy.
• A major degassing crisis recently occurred, causing the temporary evacuation of some sectors of Vulcano Island.
• Geophysicists can use the seismic waves from crustal local earthquakes to create a picture of the underground volcanic system.
  
• Dr Cristina Totaro and the geophysics team of the University of Messina identified a gas anomaly beneath Vulcano that is driving degassing activity.
  

Volcanoes are among the most fascinating natural hazards on Earth. Volcanic activity can range from small hydrothermal vents releasing hot water on the ocean floor to major explosive eruptions of lava and gases. There are long-lasting effusive eruptions on the volcano Kīlauea in Hawaii, where lava has flowed continuously over many years, and no less dangerous are the bubbling mud pools and shooting geysers that hint at magma chambers below the ground. These activities on the Earth’s surface are just a part of the complex volcanic systems beneath the Earth’s crust.

Volcanic systems include magma chambers deep underground and a network of fissures and channels along which it can travel, along with areas of gas buildup. However, scientists are unable to see directly into the depths of a volcanic system to know exactly what features there are and how they interact. While volcanologists can analyse erupted materials to understand the chemical composition of magma – which can tell us about the type of eruption that could occur – we need to be able to monitor what an active volcanic system is doing in the present day so that local communities can be prepared for a future eruption. For this, we also need to understand how gases and fluids distribution influence volcanic activity.

Clues from local earthquakes

The movement of magma or gasses through a volcanic system causes vibrations and earthquakes.

The vibrations from earthquakes spread across the Earth’s surface in elastic waves, known as seismic waves, and scientists can study the volcanic system by intercepting these waves and recording their arrival times at a seismometer. This technique, known as seismic tomography, helps geophysicists to picture the volcanic structure within the crust by analysing the variations in the seismic wave velocity below the surface. Regions with high concentrations of gases will furnish different signatures in seismic wave velocity with respect to areas rich in magma.

Seismic tomography helps geophysicists to picture the volcanic structure within the crust by identifying velocity anomalies.

Dr Cristina Totaro at Messina University is a specialist in the tomographic investigation in volcanic and tectonic frameworks. She and her colleagues obtain 3D seismic velocity models of the Lipari-Vulcano complex in Italy by using data collected from earthquakes. Their model, improving and refining the knowledge on crustal structure, has shed new light on this complex volcanic system, helping to better understand its behaviour.

Southern Italy’s Aeolian Archipelago

Lipari and Vulcano are two of seven islands that form the Aeolian Archipelago in southern Italy. Located in the southern Tyrrhenian Sea, part of the area is crossed by a volcanic band which includes Lipari and Vulcano, an active volcanic system.

Over the last decade, the area has experienced large hydrothermal activity (fluids circulation within Earth’s crust, transferring heat energy and chemicals towards the surface) and active degassing (where underground magma releases gasses such as carbon dioxide). Vulcano Island, the root of the word ‘volcano’ in English, has experienced a recent increase in volcanic activity and is home to one of the four above-ground active volcanoes in Italy. There are smoking fumaroles across the island (openings that release volcanic gasses and steam), and, from 2021 to 2023, a strong degassing crisis occurred which caused the temporary evacuation of the island and the closure of some stretches of the coast. As a popular tourist destination, with a winter population of approximately 450 residents, increasing to several thousand during the summer, it is crucial to understand the volcanic behaviour in this system.

Creating a model

Totaro and her team gathered seismic wave data from a vast seismic network, including information from around 4,400 earthquakes that have occurred over the last 30 years. By exploring the velocity (speed) distribution of the seismic waves generated by the earthquakes, the team can observe the volcanic system.

The new seismic imaging reveals gas-rich volumes that influence volcanic activity, degassing phases, and hydrothermal processes.

There are two specific types of seismic waves analysed in the investigation, P-waves and S-waves, and they travel at different velocities through the Earth’s crust away from the earthquake’s origin. P-waves travel faster and arrive at a seismometer first, closely followed by S-waves. Geophysicists have long used seismic waves to calculate the epicentre of an earthquake. In their research, the team uses the ratio of P-wave and S-wave velocity to create 3D seismic velocity patterns that help to discriminate between the presence of gases and fluids under the Earth’s surface.

The Lipari-Vulcano complex

The tomographic investigation revealed a clear presence of gas at very shallow depths, around 8km below the surface, under the island of Vulcano and off the west coast of Lipari. The anomaly beneath Vulcano is close to a caldera area (a hollow caused by the collapse of a volcano), fumaroles, and other degassing activity. The new model helps to define how gas-rich volumes of magma are spread across the area feeding the degassing activity and their impact on this system.

The gas anomaly the team identified off the west coast of Lipari has very similar characteristics to the one under Vulcano, but there is no evidence of degassing in this location. This is because the area is under the sea, so the pressure of the water column would promote gas dissolution (disintegration) into marine water and therefore inhibit the gas bubbling observation.

These seismic tomographic images have provided evidence of gas in the crust in this volcanic system and help to explain how gas-rich volumes feed the Aeolian volcanic arc degassing system in South Italy. By identifying the location of the gas-rich volumes, the team have provided a valuable contribution to improving modelling of volcanic systems and how they may change over time.