Eruption Loading: New Approaches to Earthquake Monitoring at Ontake Volcano, Japan
Understanding volcanic eruptions remains a critical challenge for earth scientists as communities worldwide face the devastating effects of sudden volcanic activity. Now, an innovative study from the University of Oxford, in collaboration with researchers from Japan and New Zealand, advances the frontier of eruption forecasting by harnessing subtle seismic clues embedded deep within the Earth’s crust. This pioneering research focuses on the phenomenon of shear-wave splitting in seismic waves—a subtle, yet revealing property of how seismic energy traverses fractured rock under stress. The results, presented in a recent publication in the journal Seismica, suggest that monitoring variations in shear-wave splitting can provide not only early warnings of an imminent volcanic eruption but also indications of its likely scale.
Volcanic eruptions release enormous energy and magma from the Earth’s interior, often preceded by complex movements of fluids and rock that generate seismic signals. Disentangling these signals to extract meaningful predictive parameters is notoriously difficult due to the intricate interactions of fractures, cracks, and fluids beneath the volcano. Shear-wave splitting emerges as a powerful candidate for this task. When shear-waves—seismic waves that oscillate perpendicularly to their direction of travel—pass through anisotropic media such as fractured and stressed rock, they become split into two polarized waves traveling at different speeds. This process is exquisitely sensitive to the orientation and state of cracks and fractures, offering a window into evolving stress patterns beneath the volcano.
Professor Mike Kendall, the leading author of this study from Oxford’s Department of Earth Sciences, explains: “Shear-wave splitting reflects the anisotropic nature of the volcanic edifice. As pressures within the magma chamber and conduits increase, the internal rock fabric undergoes notable changes. Our research has shown that these changes have a distinct seismic signature, potentially enabling us to delineate between minor and major eruption events.” By quantitatively tracking these seismic anisotropies over time, scientists gain access to a dynamic record of stress accumulation and release within the volcano’s structure.
Ontake Volcano in Honshū, Japan, served as the natural laboratory for this investigation. The team analyzed seismic data from two contrasting eruptions—one in 2007, a relatively small event with limited impact, and another in 2014, a much larger, catastrophic explosion that shook the region profoundly. By correlating shear-wave splitting parameters with eruption magnitude, the researchers discovered an insightful pattern: during the smaller eruption, the shear-wave splitting remained largely stable, whereas prior to and during the larger 2014 eruption, the shear-wave splitting ratio increased substantially, doubling just before the eruption climaxed.
This observation provides compelling evidence that seismic anisotropy measured by shear-wave splitting can serve as a proxy for eruption explosivity. The underlying physical mechanism relates to the stress-induced opening and closing of microcracks within the volcanic rocks. When magma pressure intensifies, it reorganizes the fracture network, aligning cracks and increasing anisotropy. This evolving crack system causes differential speeds in shear-wave propagation to become more pronounced, effectively serving as an early warning signal that the volcano is gearing toward a more violent rupture.
Co-author Professor Toshiko Terakawa from Nagoya University underscores the synergy of combining multiple seismic observables in eruption forecasting. “Seismic focal mechanisms, which describe earthquake source orientations, shifted dramatically around the 2014 eruption. Integrating these data with shear-wave splitting analyses enriches our understanding of the subsurface stress regime and its temporal evolution before eruptions.” Such multidisciplinary approaches are central to developing more robust and reliable monitoring frameworks, reducing false alarms while enhancing timely alerts.
From a hazard mitigation perspective, the implications of this work are profound. Existing volcano monitoring systems often rely on a suite of indicators, including changes in gas emissions, ground deformation, and seismicity rates. However, these measurements can sometimes produce ambiguous signals that hamper decision-making processes. Shear-wave splitting offers an additional, quantitative seismic parameter directly linked to the volcano’s internal stress state, improving the confidence and lead time of eruption forecasts.
Equally important is the potential applicability of these findings beyond Ontake. As co-author Dr. Tom Kettlety of Oxford remarks, “We anticipate similar shear-wave splitting changes in other volcanic systems worldwide as their internal stresses fluctuate before eruptions. Deploying this method globally could revolutionize early-warning networks, especially for communities living close to hazardous volcanoes.” The universality of shear-wave physics and its sensitivity to rock anisotropy position this approach for broad implementation.
Furthermore, the study highlights the value of international scientific collaboration. Involving experts from the University of Oxford, Nagoya University, Victoria University of Wellington, University of Bristol, Kyoto University, and NORSAR, this research exemplifies how pooling diverse datasets and expertise can overcome complex geophysical challenges. Professor Martha Savage of Victoria University of Wellington emphasizes this point: “Our coordinated effort allowed us to unlock signals that single-site studies might miss. This global cooperation is vital for addressing volcanic risk on a planetary scale.”
Technically, the methodology hinges on detailed seismological analysis using dense seismic arrays deployed around Ontake. By measuring the polarization and velocity differences of incoming shear-waves during the critical eruption periods, the team extracted splitting parameters such as delay time and fast-axis orientation. These measurements were cross-validated with independent records of seismicity and eruption chronology to ensure robustness. Advances in computational seismology and signal processing played a key role in isolating these subtle effects from noisy datasets.
Interpreting time-dependent changes in shear-wave splitting also demands an understanding of fracture mechanics and rock physics. The study bridges the geophysical observations with theoretical models of stress-induced anisotropy, correlating observed seismic wave-speed variations with microstructural modifications in the volcanic edifice. This coupling of theory and observation paves the way for predictive models that can simulate expected seismic signatures under various eruptive scenarios.
In addition to enhancing eruption forecasting, this research contributes to the broader understanding of volcanic plumbing systems—the networks of magma pathways beneath volcanoes. By monitoring how stress redistributes spatially and temporally through shear-wave splitting observations, scientists can infer the geometry and dynamics of these otherwise inaccessible subterranean structures. Such insights are invaluable for hazard mapping and understanding eruption mechanisms at a fundamental level.
Looking forward, the researchers advocate for integrating shear-wave splitting analysis into standard volcano monitoring protocols globally. The approach’s sensitivity, low cost compared to some other geophysical instruments, and non-invasive nature make it an attractive addition. Coupled with real-time data transmission and automated signal processing, this method promises to deliver actionable intelligence to civil protection agencies and local populations facing volcanic hazards.
This groundbreaking study not only advances seismological monitoring but also exemplifies how fundamental research in earth sciences can directly contribute to public safety. As volcanic hazard mitigation remains a priority worldwide, approaches that bring earlier, clearer warnings empower communities and authorities to prepare and respond effectively, potentially saving lives and reducing economic damage.
The research highlights the evolving paradigm in volcanology where detailed wave physics intersects with practical disaster risk reduction. By revealing the “seismic fingerprint” of eruptive stress buildup through shear-wave splitting, scientists are unlocking a new dimension of Earth’s dynamic behavior, turning elusive signals into tangible alarms.
Subject of Research: Volcanic eruption forecasting using shear-wave splitting and seismic anisotropy at Ontake Volcano, Japan.
Article Title: Changes in seismic anisotropy at Ontake volcano: a tale of two eruptions
News Publication Date: Not explicitly stated; recent publication in Seismica.
Web References:
- DOI link to article
- University of Oxford Department of Earth Sciences: https://www.earth.ox.ac.uk/people/mike-kendall
References:
- Kendall, M., Terakawa, T., Savage, M., Kettlety, T., et al. (2024). Changes in seismic anisotropy at Ontake volcano: a tale of two eruptions. Seismica, vol. 4, issue 1. DOI: 10.26443/seismica.v4i1.1101
Image Credits: Dr. Koshun Yamaoka – Aerial view of Ontake Volcano, Honshū Island, Japan.
Keywords: Volcanoes, Physical geology, Volcanology, Volcanic processes, Volcanic eruptions, Seismology, Earth tremors, Earthquakes, Earthquake forecasting, Geophysics
