Subducted slabs stagnant above, penetrating through, and trapped below the 660 km discontinuity

Abstract

A new P wave tomographic model of the mantle was constructed using more than 10 million travel times. The finite‐frequency effect of seismic rays was taken into account by calculating banana‐donut kernels at 2 Hz for all first arrival time data, and at 0.1 Hz for broadband differential travel time data. Based on this model, a systematic survey for subducted slab images was developed for the circum‐Pacific; including the Kurile, Honshu, Izu‐Bonin, Mariana, Java, Tonga‐Kermadec, southern and northern South America, and Central America, arcs. This survey revealed a progressive lateral variation of the configuration of slabs along arc(s), which we interpret as an indication for successive stages of slab subduction through the Bullen's transition region with the 660 km discontinuity at the middle. We identified the four distinct stages: I ‐ slab stagnant above the 660 km discontinuity; II ‐ slab penetrating the 660 km discontinuity; III ‐ slab trapped in the uppermost lower mantle (at a depth of 660–1000 km); and IV ‐ slab descending well into the deep lower mantle. The majority of slab images are found to be either at Stage I or III, suggesting that Stages I and III are relatively stable or neutral and II and IV are relatively unstable or transient. There is a remarkable distinction for the deepest hypocentral distribution between slabs at Stage I and slabs at Stages II or III.