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)

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 similar to 670 km was found beneath the 300-400-km event zone in the northern Bonin region near 33 degrees 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) degrees C cold anomaly estimated using a post-spinel transformation Clapeyron slope of (-2.0 +/- 1.0) MPa/K. In southern Bonin near 28 degrees 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 similar to 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 similar to 680 km on May 30, 2015, the discontinuity is depressed to similar 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.