Seismic Survey Reveals Magma Reservoir Refilling Beneath Japan's Kikai Supervolcano 7,300 Years After Giant Eruption

Deep beneath the seafloor of southern Japan, a large body of magma appears to be refilling 7,300 years after one of the most powerful volcanic eruptions of the Holocene. Researchers from Kobe University and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) report that underwater seismic surveys have revealed a substantial region rich in magma beneath Kikai caldera, and that this magma looks newly supplied rather than left over from the ancient blast.

Kikai is a giant caldera, most of which lies beneath the ocean. Calderas form when huge volumes of magma escape during an eruption and the rock above the emptied reservoir collapses, leaving a broad, relatively shallow depression. According to the source material, Kikai's last giant eruption, about 7,300 years ago, was the largest known volcanic eruption of the Holocene, the epoch that began roughly 11,700 years ago and continues today. Eruptions on this scale can release enough magma to bury New York City's Central Park under a layer about 12 kilometers (7.5 miles) thick.

Scientists know that giant caldera volcanoes can erupt more than once, but they still do not fully understand how the enormous quantities of magma needed for such events accumulate. That gap makes it hard to anticipate when another major eruption might occur. "We must understand how such large quantities of magma can accumulate to understand how giant caldera eruptions occur," says Kobe University geophysicist Nobukazu Seama.

To look beneath Kikai, the team used powerful airgun arrays to send seismic pulses through the crust under the ocean floor. Instruments called ocean bottom seismometers recorded how those waves traveled through the rock. Because seismic waves move differently through solid rock than through regions containing molten material, the recordings allow researchers to infer structures that cannot be observed directly. Kikai's underwater setting turned out to be an advantage, Seama explains: "The underwater location allows us to implement systematic, large-scale surveys."

The findings, published in Communications Earth & Environment, revealed a substantial region with a high proportion of magma directly beneath the site of the massive eruption. From the seismic measurements, the researchers could determine the approximate size and shape of the reservoir. Its position and extent suggest it occupies the same magma storage system involved in the ancient eruption. "Due to its extent and location it is clear that this is in fact the same magma reservoir as in the previous eruption," Seama says.

The more striking conclusion concerns where that magma came from. Earlier investigations showed that a new lava dome has been growing near the center of the caldera for roughly the past 3,900 years. A lava dome forms when thick, viscous lava piles up around a vent instead of flowing freely across the landscape. Chemical analyses also found that material from this dome and other recent activity differs in composition from what was erupted in the giant event. That difference suggests the magma now stored beneath the volcano was supplied by new injections of molten rock from deeper in the crust or mantle. "This means that the magma that is now present in the magma reservoir under the lava dome is likely newly injected magma," Seama says.

The researchers stress that the replenishment does not mean another catastrophic eruption is imminent. Instead, it shows how a reservoir can be recharged over thousands of years, a process scientists need to understand before they can reliably assess the long-term behavior of supervolcanoes.

The work may also have implications beyond Japan. The researchers say the proposed re-injection model, in which new magma gradually enters and replenishes an existing reservoir after a giant eruption, is consistent with the large shallow magma reservoirs seen beneath other giant calderas such as Yellowstone in the United States and Toba in Indonesia. Understanding the process could reveal patterns in the long-term cycles of magma accumulation, volcanic activity and major eruptions, and might eventually help scientists recognize warning signs of future eruptions.

The findings come with caveats. The conclusion that the magma is newly injected rests on combining the new seismic imaging with earlier chemical analyses of the lava dome and recent volcanic products, and the seismic data reveal the reservoir's approximate size and shape rather than a detailed picture of how magma moves within it. The connection to Yellowstone and Toba is described as consistency with the model, not a demonstration at those sites.

Next, the team hopes to refine the seismic methods used at Kikai to investigate magma movement and storage in greater detail. "We want to refine the methods that have proved to be so useful in this study to more deeply understand the re-injection processes," Seama says. The ultimate goal, the researchers say, is to better monitor the key indicators of future giant eruptions.

The research was funded by Japan's Ministry of Education, Culture, Sports, Science and Technology (MEXT) and the Japan Society for the Promotion of Science.