Major geological faults and fractures in the Earth's crust can both guide rising magma and help prevent volcanic eruptions, according to an international team of researchers.
Their study examined a "stealth" injection of magma, which travelled undetected through an existing fault system before stalling and creating a series of earthquakes in its wake.
This powerful volcano-tectonic unrest affected São Jorge Island in the Azores, an active volcanic island, which has erupted at least three times in the last 400 years and is home to 8,000 people.
In March 2022, the region experienced lots of earthquakes, prompting fears of a new eruption and posing a real challenge to civil authorities who were grappling with how to tackle the crisis.
Drawing on the support of a Natural Environment Research Council (NERC) Urgency Grant, a team led by UCL, the Spanish National Research Council and supported by Cardiff University, deployed land-based and ocean bottom seismometers to study the crisis and contribute to a better understanding of the volcanic system.
Their findings, published in Nature Communications , provide key insights for improving volcanic hazard forecasting, showing how large magma intrusions can occur rapidly and with limited warning, and that major geological faults can strongly influence whether magma erupts or stalls underground.
"This was a stealthy intrusion," said lead author Dr Stephen Hicks from UCL.
"Magma moved quickly through the crust, but much of its journey was silent, making it difficult to forecast whether an eruption would occur."
The team reconstructed the detailed underground movement of magma using high-resolution earthquake locations from land and ocean-bottom seismometers and ground deformation from satellite radar and GPS data.
They found that a vertical sheet of magma, known as a dike, rose rapidly from more than 20km below the surface before stalling just 1.6 km beneath the island.
Much of this ascent occurred with little seismic activity, with most earthquakes occurring after the magma stopped ascending.
We almost always think that magma injections are inevitably accompanied by significant earthquake activity as soon they enter the Earth's crust. Our data show many can be stealthy, posing a real challenge to volcano-monitoring agencies and civil authorities.
The team accessed satellite observations, which showed the volcano's surface rose by 6cm, confirming that magma had entered the shallow crust.
Intrusions of this kind – known as a failed eruption – help grow islands, according to the team, who demonstrate the process in their study's unprecedented sharp earthquake maps.
The magma rose through the Pico do Carvão Fault Zone – one of the island's main fault systems.
By studying geological traces left by ancient earthquakes, scientists had previously found that this fault system has produced large earthquakes.
But instead of a single large earthquake, the unrest from rising magma produced many small earthquakes clustered along this fault.
The team interpreted that the fault helped guide magma upward and may also have allowed gases and fluids to escape sideways, lowering pressure in the magma and helping halt its ascent.
"The fault acted like both a highway and a leak," said Dr Pablo J González, the study's co-lead author from the Spanish National Research Council (IPNA-CSIC) in Tenerife.
"It helped magma rise but may also have prevented an eruption."
Their research leads to a better understanding of volcanic systems and how they operate in complex tectonic settings, the team say.
"Our findings will support local authorities in assessing a potential volcanic threat, in the future" added Dr Ramalho.
The lessons learned here can be applied to many other places, such as Iceland, East Africa, the Caribbean and more, which really highlights the value of combining onshore and offshore geophysical data for accurate detection and localisation of seismic events and ground deformation.
Professor Ana Ferreira, co-author from UCL, reflected: "Securing urgent NERC funding to access equipment from its Geophysical Equipment Facility (GEF), alongside additional support from Portugal, was a tremendous collective effort and a clear example of transnational cooperation between academic and civil institutions in Portugal, the UK, and Spain."
Their paper, 'Fault-mediated magma propagation and triggered seismicity revealed by the 2022 São Jorge Azores unrest', is published in Nature Communications.
