Earth

Aftershock analysis refutes claims of world’s deepest earthquake

Overview of the study area. Credit: The Seismic Record (2025). DOI: 10.1785/0320240035

The 7.9-magnitude Bonin Islands earthquake series that occurred in May 2015 caused ruptures deep underground near the base of the upper mantle, while record-breaking events in the lower mantle, according to research published in The Seismic Record. Aftershocks that spread to such depths were not included.

When Hao Jiang and colleagues at the University of Southern California reexamined the earthquake’s aftershock sequence, they found no evidence of aftershocks as deep as 751 kilometers (751 kilometers), as previous researchers had reported. This aftershock is said to be the deepest earthquake ever recorded.

Instead, their study found that the distribution of aftershocks matched a 12-kilometer strip of a mantle mineral called olivine, which could reveal at what depths earthquakes occur. There is sex.

The Ogasawara Islands earthquake, which occurred in a remote part of the Pacific Ocean 1,000 kilometers off the coast of Japan, was one of the deepest and largest earthquakes ever recorded. The earthquake occurred within the Izu-Ogasawara Trench, 680 kilometers from the earth’s surface.

The mechanisms behind deep earthquakes, or earthquakes that occur at depths of more than 500 kilometers, are something of a mystery to seismologists. The very high pressures and temperatures at these depths make the rocks more susceptible to bending and deforming plastically, rather than breaking in the brittle manner that causes seismic failure at shallow depths.

Zhang noted that these earthquakes also typically produce few aftershocks, which could provide useful data for understanding how these deep earthquakes occur in subduction zones. .

Plastic deformation “limits the formation of extensive fracture networks that typically cause aftershocks,” he said. “Furthermore, the high confining pressure facilitates the efficient redistribution of stresses after the main shock, further reducing the likelihood of subsequent seismic events.”

Previous studies of the Ogasawara Islands earthquake have reported foreshock sequences for this earthquake, and a second study detected potentially record-breaking deep aftershocks in the lower mantle.

“Both findings, if accurate, could significantly advance our understanding of deep earthquakes,” Zhang said. “However, these two catalogs are inconsistent and both have methodological limitations. Therefore, it is essential to reexamine the aftershock sequences using improved techniques.”

To get a closer look at distant, deep earthquakes, Zhang and his colleagues turned to data collected by Japan’s high-density seismic array, called Hi-Net, which precisely localizes the seismic signals coming from these earthquakes. We have combined the technologies for

Although their new analysis detected no foreshocks, they identified 14 aftershocks in the upper mantle within a 150-kilometre radius of the quake’s epicenter. One aftershock coincided with the rupture plane of the earthquake 1 week after the main shock, and the second aftershock was more widely distributed during the second week.

“Although it remains difficult to unequivocally rule out the existence of lower mantle-initiated seismicity and associated mechanisms, our results refute the most convincing case for lower mantle seismicity to date.” the researchers wrote in their paper.

The researchers suggested that the aftershock pattern was consistent with the presence of a metastable olivine wedge (MOW). In subducting slabs, olivine can retard the transition to other mineral states under high temperature and pressure.

“This delayed deformation generates stress, releases energy, and can trigger deep earthquakes,” Zhang said.

Because the MOW is a potential earthquake nucleation site, some researchers have proposed this metamorphic faulting mechanism as one of the main ways deep earthquakes occur, he added.

“Furthermore, MOW provides insight into the thermal structure and behavior of the subducting slab, with cooler slabs more likely to preserve metastable olivine at deeper depths,” Zhang added. . “Studying MOW can improve models of deep earthquake occurrence and improve our understanding of dynamic processes within the Earth.”

Further information: Hao Zhang et al., Planar rupture plane aftershocks of the deep focus Mw 7.9 Bonin Islands earthquake, seismic record (2025). DOI: 10.1785/0320240035

Provided by Seismological Society of America

Source: Aftershock analysis refutes claims of world’s deepest earthquake (January 22, 2025) from https://phys.org/news/2025-01-aftershock-analysis-world-deepest-earthquake.html Retrieved January 22, 2025

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