Claims of world's deepest earthquake challenge new analysis

The 7.9 magnitude Bonin Islands earthquake sequence, which ruptured deep within the Earth near the base of the upper mantle, did not include an aftershock that extended to record depths in the lower mantle, according to a study published in The Seismic Record.

When Hao Zhang of the University of Southern California and his colleagues re-examined the aftershock sequence of the May 2015 earthquake, they found no evidence of a 751-kilometre-deep aftershock as previous researchers had reported. This aftershock has been called the deepest earthquake ever recorded.

Instead, their study found a distribution of aftershocks consistent with a 12-kilometre-long piece of mantle mineral called olivine that could shed light on how deep earthquakes can occur.

The Bonin Islands earthquake, which occurred 1,000 kilometers off the coast of Japan in a remote area of ​​the Pacific Ocean, is one of the deepest and largest earthquakes ever recorded. The earthquake occurred within the Iso-Bonin subduction zone, 680 kilometers below the Earth's surface.

The mechanisms behind deep earthquakes—those that occur at a depth of 500 kilometers or deeper—are a mystery to seismologists. The extremely high pressures and temperatures at these depths make rocks more likely to bend or deform ductively, rather than breaking in a brittle manner that causes seismic rupture at shallow depths.

Zhang noted that these earthquakes typically produce only a few aftershocks, which can provide useful data for understanding how these deep events are generated in subduction zones.

Plastic deformation “limits the formation of large-scale fracture networks that typically generate aftershocks,” he said. “In addition, high confining pressures promote effective stress redistribution after the mainshock, reducing the likelihood of subsequent seismic events.”

One previous study of the Bonin Islands earthquake reported a foreshock sequence for the event, while a second study revealed a potentially record-breaking deep aftershock in the lower mantle.

“Both results could greatly enhance our understanding of deep earthquakes, if they are accurate,” Zhang said. “However, these two catalogs are inconsistent, and both have methodological limitations. Therefore, it is necessary to re-examine the aftershock sequences using improved techniques.”

To get a better look at the deep, distant quake, Zhang and his colleagues turned to data collected by a dense seismic array in Japan called Hi-Net, using a combination of techniques to pinpoint the location of seismic signals coming from the event.

Their new analysis did not detect any aftershocks, but identified 14 aftershocks in the upper mantle within a 150-kilometre radius of the epicenter. One group of aftershocks aligns with the rupture level of the earthquake one week after the mainshock, with the second group spreading over a wider area during the second week.

“Although it remains difficult to conclusively reject the existence of earthquakes initiated in the lower mantle and associated mechanisms, our results reject the most compelling claim to date about earthquakes in the lower mantle,” the researchers wrote in their paper.

The researchers suggested that the pattern of aftershocks is consistent with the presence of a metastable olivine wedge, or MOW. In a subducted slab, olivine can delay its transition to other mineral states under high temperature and pressure. “This late metamorphism may generate stress and release energy, which could trigger deep earthquakes,” Zhang said.

He added that with MOW sites being potential sites for earthquake genesis, some researchers have proposed this transform faulting mechanism as one of the main ways deep earthquakes occur.

“Moreover, MOW devices provide insight into the thermal structure and behavior of subducting slabs, as cold slabs are more likely to preserve semi-stable olivine at greater depths,” Zhang added. “By studying MOWs, we can improve models of deep earthquake generation and improve our understanding of dynamic processes in the Earth's interior.”

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