New analysis refutes the claim that the world's deepest earthquake happened in Japan

For almost ten years, an aftershock linked to the May 30, 2015 magnitude 7.9 Bonin Islands earthquake was touted as the deepest tremor ever measured, reported at 467 miles beneath the Pacific.

That label has now been stripped away by a fresh look at the data that places the aftershock, and our understanding of deep seismicity, on firmer ground.

The new investigation combed through recordings from Japan’s dense Hi‑Net array and came up empty for any signal that would support a quake in the lower mantle.

Instead, it found a modest cluster of events no deeper than the original mainshock at 422 miles, overturning the record‑depth claim.

Deepest earthquake ever recorded

“Our results do reject the most compelling lower mantle seismicity claim to date,” Hao Zhang, a geophysicist at the University of Southern California, writes.

He led the re‑analysis that identified only fourteen aftershocks within about 93 miles of the mainshock’s focus. 

Deep earthquakes behave differently from their shallow cousins, because pressures so intense tend to make rock flow rather than snap.

Decades of research show that quakes deeper than 310 miles rarely produce vigorous aftershock sequences. 

Earlier teams had claimed to spot a foreshock swarm and a single record‑breaking aftershock, but their techniques relied on template matching vulnerable to false alarms.

Zhang’s group used a cleaner beamforming approach that lines up full‑wave data without preselecting templates, reducing the chance of mistaking noise for a quake.

Mapping the aftershock footprint

By stacking thousands of seismograms with a beamforming technique, the study recreated the exact arrival times of faint aftershocks that earlier catalogs missed or misplaced.

Eight events popped up during the first week after the mainshock and traced a neat plane that aligns with the initial rupture surface.

During the second week, six more quakes appeared in a loose halo above the rupture, suggesting that stress migrated upward through the descending Pacific slab.

The deepest of the fourteen was only 16 miles below the mainshock, still safely within the upper mantle.

Aftershocks this sparse are not unusual at great depth. The Tonga earthquake of March 9, 1994, for instance, produced just a few dozen detectable aftershocks despite its comparable size.

All fourteen aftershocks released only about one‑ten‑thousandth of the energy of the main event, a ratio typical for very deep shocks. Such low productivity hints at rock that deforms smoothly once the initial rupture relaxes the stress.

A thin slice of stubborn olivine

Zhang’s team argues that the pattern of events fits a metastable olivine wedge, a sliver of the mineral olivine that lingers in a crystal state favored by colder temperatures.

Laboratory experiments indicate that olivine can keep this structure far outside its usual comfort zone, provided the surrounding slab stays cool.

When olivine finally flips to a denser form, it shrinks, building local stress that can trigger transformational faulting, one of the leading candidates for deep‑quake initiation.

In the Bonin slab, the wedge appears to be about 7.5 miles thick, narrower than similar structures in the Tonga or Bolivia slabs, a difference the authors tie to slower subduction and greater heating over time.

The narrow wedge also helps explain why the 2015 mainshock had few friends. Without a broad zone of brittle behavior, the slab loses its ability to generate the fracture networks that spawn rich aftershock clouds.

Experiments that squeeze olivine at high pressure show that it can keep its ambient structure down to roughly 1200°F lower than predicted if the temperature drops suddenly, mirroring conditions in a cold slab.

This delay stretches the mineral’s stability field and seeds the stress needed for transformational faulting.

Keeping the lower mantle quiet

The study adds weight to the view that the mantle’s 410 mile boundary, where minerals rearrange and the lower mantle begins, is a formidable barrier for earthquake rupture.

Seismic experiments show that this boundary can sag to 460 miles under cold slabs, yet even that depression leaves little room for quakes to form below it.

By knocking out the strongest evidence for a true lower‑mantle aftershock, Zhang and colleagues strip the record depth back to familiar territory.

The deepest confirmed events stay rooted in the transition zone, where mineral transformations, not classic rock breakage, appear to rule.

Seismic conversions recorded in the western Pacific reveal that the 410 mile discontinuity sags by almost 50 miles beneath the Bonin arc, yet it still separates the upper‑mantle quakes from any deeper disturbance.

That observation matches the picture from global tomography studies that even the coldest slabs rarely puncture the lower mantle while still brittle.

Why the deepest earthquake matters

Earthquake depth records may feel like bookkeeping, but they feed directly into models of plate recycling, mantle flow, and even volcanic plumbing.

A verified lower‑mantle quake would force geophysicists to rethink the strength and temperature of slabs far below our feet.

The updated catalog also sharpens the focus on transformational faulting as a viable engine for deep quakes, lending support to decades of lab and field evidence that point to phase transitions rather than friction as the spark.

For hazard planners, the findings dampen concerns that an ultra‑deep quake might directly unleash tsunamis or stress neighboring faults, because energy at these depths dissipates before reaching the surface.

The work underscores the value of dense seismic networks and careful signal tracking in a part of the planet where direct sampling is impossible. It also reminds us that science advances as much by pruning shaky claims as by adding new discoveries.

The more immediate impact is the academic one, refining boundary conditions in numerical models that aim to reproduce subduction over millions of years.

Armed with a cleaner catalog, those models can now test how small wedges of metastable crystal might seed the planet’s deepest shocks.

The study is published in The Seismic Record.

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