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Researchers have built on past studies and introduced new methods to explore the nature and role of subsurface fluids, including water, in the instances and behaviors of earthquakes and volcanoes. Their study suggests that water, even heavy rainfall, can play a role in or even trigger seismic events. This could potentially lead to better early warning systems.

The study, appearing in Communications Earth & Environment, improves models of seismic activity and can even help identify optimal sites for drilling to tap sources of supercritical geothermal energy.

As far as is currently known, earthquakes and volcanic eruptions cannot be predicted, certainly not on the timescales we expect from typical weather reports. But as physical theories improve, so does the accuracy of statistical models, which could be useful for planning, and potentially also early warning systems, which can save lives when disaster does strike. Another benefit of improving such models is that they could help locate areas suitable for tapping into geothermal energy.

Geologists and other researchers strive for the improvement of theories based on good observations. A recent development in this field has added another factor into the mix, which may be more significant than previously expected.

"Our latest paper using advanced seismic imaging shows for the first time, how deep volcanic fluids such as water, in their high-pressure supercritical state, can become trapped, migrate and undergo phase changes that influence earthquakes," said Professor Takeshi Tsuji from the Graduate School of Engineering at the University of Tokyo.

"Applying machine learning to our seismometer data allowed us to map earthquake distribution and mechanism in detail, and to investigate the brittle-ductile transition zone, where rocks change from seismically active to largely inactive. This is a prime area for fluids to accumulate. Unlike earlier low-resolution electromagnetic surveys, our seismic approach revealed these systems in unprecedented three-dimensional detail."

Supercritical fluids are the key here. They are special because they act like both a liquid and a gas. Due to high pressures and temperatures, they flow easily like gas, but with the ability to store and transfer huge amounts of heat like a liquid. This means that as they pass through different mediums or a medium with varying conditions—for example, from a tightly sealed area to one that's fractured—supercritical fluids can rapidly heat an area, changing how it and the magma beneath it behave. This supercritical fluid is not isolated from the world; it can even be affected by rain.

"When heavy rain falls, the groundwater level rises, increasing pressure in cracks and faults deep below. If those faults are already close to breaking, this added pressure can trigger earthquakes," said Tsuji. "In volcanic areas, where the crust is weakened by high-pressure fluids, this effect can be especially strong. Our study clearly showed a correlation between rainfall and seismicity. By imaging magma and understanding how pressure builds up underground, we may improve how we look for early warning signs of eruptions."

Beyond predicting disasters, though, there is a potentially great benefit: the chance to tap into nearly limitless clean geothermal energy, something a country like Japan is ideally suited to do but has not yet begun. The project began as a means to find reliable drilling targets to more easily reach the supercritical water reserves necessary for realizing geothermal power use. With their new method, Tsuji and his team were able to identify fluid pathways, reservoirs beneath sealed layers, and the fractures that let fluids escape.

"Underground supercritical water contains vast thermal energy, making it an incredibly promising renewable resource in the future. Importantly, because it is drawn from deep reservoirs, it does not interfere with surface hot spring systems, a major concern for geothermal power in Japan," said Tsuji.

"The main obstacle to widespread use of supercritical geothermal energy is drilling. These fluids exist at great depths under extreme pressure and temperature, so both drilling technology and equipment must be adapted. Even though we can now locate supercritical fluids and their reservoirs, we still need to develop safe and efficient designs for wells to make this energy resource practical."