The 7.9 magnitude earthquake sequence that struck the Bonin Islands, located deep within the Earth near the upper mantle, did not produce an aftershock that reached record depths into the lower mantle, as stated in a recent study.
The 7.9 magnitude earthquake sequence that occurred in the Bonin Islands, positioned deep within the Earth near the upper mantle’s base, did not result in an aftershock that penetrated record depths into the lower mantle, according to a study published in The Seismic Record.
In a review of the May 2015 earthquake’s aftershocks, Hao Zhang from the University of Southern California and his team found no evidence for a previously reported aftershock that was said to occur at a depth of 751 kilometers—an event labeled the deepest earthquake ever documented.
Rather, their analysis revealed a pattern of aftershocks consistent with a thin section of a mantle mineral known as olivine, which could provide insights into the mechanisms of deep earthquakes.
This Bonin Islands earthquake, which originated 1000 kilometers off Japan in a secluded area of the Pacific Ocean, stands out as one of the most profound and significant earthquakes recorded. It happened within the Izu-Bonin subduction zone at a depth of 680 kilometers below the Earth’s surface.
Understanding the processes that lead to deep earthquakes—those originating 500 kilometers or more below the surface—remains a challenge for seismologists. The extreme pressures and temperatures at such depths typically cause rocks to deform plastically, as opposed to shattering in the brittle manner that characterizes shallower earthquakes.
These deep earthquakes usually result in fewer aftershocks, as per Zhang’s observations, which could provide valuable information to comprehend how deep seismic events are generated in subduction zones.
Plastic deformation “limits the creation of widespread fracture networks that would usually generate aftershocks,” he explained. “Moreover, the high confining pressures assist in the efficient reassignment of stress after the mainshock, further lessening the chances of subsequent seismic activity.”
One previous analysis of the Bonin Islands earthquake suggested a foreshock series occurred, while another study claimed to have detected a deep aftershock in the lower mantle that could set a new record.
If accurate, “Both findings could greatly enhance our understanding of deep earthquakes,” remarked Zhang. “However, these two catalogs contradict each other and both have methodological shortcomings. Thus, it is crucial to reassess the aftershock sequence using enhanced techniques.”
To investigate this elusive deep earthquake in greater detail, Zhang and his colleagues utilized data gathered by Japan’s dense seismic network known as Hi-Net, employing a variety of techniques to pinpoint the seismic signals from the event accurately.
Their latest analysis found no foreshocks but identified 14 aftershocks within the upper mantle, all occurring within a 150-kilometer radius of the earthquake’s focal point. One cluster of aftershocks corresponded with the earthquake’s rupture plane one week after the main event, while another cluster spread over a broader area during the following week.
“While it’s still challenging to completely dismiss the possibility of seismicity originating in the lower mantle and its related mechanisms, our findings refute the most substantial claim regarding lower mantle seismicity to date,” the researchers stated in their paper.
The pattern observed in the aftershocks aligns with the hypothesis of a metastable olivine wedge (MOW), as proposed by the researchers. In a subducting slab, olivine can postpone its transition into different mineral forms when subjected to high temperatures and pressures. “This delayed transformation might create stress and release energy, potentially instigating deep earthquakes,” Zhang noted.
Several researchers have suggested that MOWs could serve as critical sites for earthquake initiation, proposing the mechanism of transformational faulting as a primary cause of deep seismic events, he added.
“Moreover, MOWs provide valuable information about the thermal characteristics and behavior of subducting slabs, with colder slabs more likely to retain metastable olivine at greater depths,” Zhang continued. “By researching MOWs, we can refine deep earthquake generation models and enhance our comprehension of the dynamic processes within Earth’s interior.”
