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Topography of the 660-km discontinuity within the Izu-Bonin subduction zone and evidence of slab penetration near the Bonin super Deep Earthquake(~680 km)

引用

Earthquake science 2023年第6期36卷 458-476页

作者： Gang Hao Graduate school of science Faculty of ScienceKyushu UniversityMotookaNishi-kuFukuokaJapan

The Izu-Bonin subduction zone in the Northwest pacific is an ideal location for understanding mantle dynamics such as cold lithosphere subduction. The slab produces a lateral thermal anomaly, inducing local topographic changes at the boundary of a post-spinel phase transformation, considered to be the origin of the ‘660-km discontinuity.’ In this study, the short-period(1–2 Hz) s-to-p conversion phase s660p was used to obtain the fine-scale structure of the discontinuity. More than 100 earthquakes that occurred from the 1980s to the 2020s and were recorded by high-quality seismic arrays in the United states and Europe were analyzed. A discontinuity in the ambient mantle with an average depth of ~670 km was found beneath the 300–400-km event zone in the northern Bonin region near 33°N. Meanwhile, the ‘660-km discontinuity’ has been pushed upward, away from the slab, possibly because of a hot upwelling mantle plume. In the central part of the subduction zone, the 660-km discontinuity is depressed to an average depth of(690 ± 5) km within the slab at approximately 150 km below the coldest slab core, indicating a(300 ± 100) ℃ cold anomaly estimated using a post-spinel transformation Clapeyron slope of(-2.0 ± 1.0) Mpa/K. In southern Bonin near 28°N, the discontinuity was found to be further depressed at an average depth of(695 ± 5) km below the deepest event and with a focal depth of ~550 km. The discontinuity is located where the slab bends abruptly to become sub-horizontal toward the west-southwest. Near the zone of the isolated Bonin super Deep Earthquake, which occurred at ~680 km on May 30,2015, the discontinuity is depressed to ~700 km, suggesting a near-vertical penetrating slab and an s-to-p conversion in the coldest slab core, where a large low-temperature anomaly should exist.

关键词： Izu-Bonin subduction zone discontinuity post-spinel transformation s-to-p wave conversion vertical subduction cold slab

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