In the vast depths of our planet, where light never reaches and pressures rise to unimaginable levels, a geological mystery once unfolded that continues to puzzle scientists: the deepest earthquake ever recorded occurred nearly 750 kilometers (466 miles) beneath the Earth’s surface.
A Record-Breaking Quake
This unprecedented event took place in 2015 beneath the Bonin Islands, located off the southeastern coast of Japan. The quake registered a magnitude of 7.9 and originated at an astonishing depth of 751 kilometers. It holds the record for the deepest earthquake ever reliably measured by seismologists.
What makes this seismic event so extraordinary is not just its depth, but the fact that earthquakes at such depths should theoretically be impossible. Under extreme pressure and temperature conditions, rocks should deform plastically, not fracture and slip—the primary mechanism of most earthquakes. Yet, this quake was powerful, with seismic waves strong enough to be detected worldwide.
How Can Earthquakes Occur So Deep?
Earthquakes usually occur in the Earth’s crust or upper mantle, up to around 100 kilometers deep. These are regions where the brittle failure of rocks can easily occur. Deeper quakes, like those in subduction zones, are linked to the descent of tectonic plates into the mantle.
In the case of the Bonin quake, researchers believe the earthquake originated within the subducting Pacific Plate, which dives beneath the Philippine Sea Plate. As the plate descends, it remains colder and more rigid than the surrounding mantle for a significant depth. One possible explanation is a process known as “phase transformation faulting”, where mineral structures change under extreme pressure, creating internal stresses that can cause sudden failure—essentially, a deep-earth version of a brittle fracture.
Implications for Earth Science
The 751-kilometer-deep quake challenges our understanding of how stress behaves under such intense conditions. It suggests that tectonic plates may remain colder and more mechanically coherent much deeper into the Earth than previously thought.
Moreover, studying such deep quakes helps seismologists refine their models of Earth’s interior and understand the long-term dynamics of plate tectonics. It also has implications for hazard assessment in subduction zones, even though deep quakes usually don’t cause surface damage like their shallow counterparts.
A Window into the Planet’s Heart
While the 2015 Bonin earthquake remains the deepest known, it’s likely not the last surprise lurking in Earth’s depths. With advances in seismic monitoring and deeper understanding of geological processes, scientists continue to push the boundaries of what we know about the planet beneath our feet.
