Zhichao Shen, Zhongwen Zhan

The metastable olivine wedge (MOW) within subducted slabs has long been hypothesized to host deep-focus earthquakes (>300 km). Its presence would also rule out hydrous slabs being subducted into the mantle transition zone. However, the existence and dimensions of MOW remain controversial. Here, we apply inter-source interferometry, which converts deep earthquakes into virtual seismometers, to detect the seismic signature of MOW without influence from shallow heterogeneities. With data from the Hinet, we confirm the existence of MOW beneath the Japan Sea and constrain its geometry to be ~30 km thick at 410-km depth and gradually diminishing to a depth of 610 km at least. Our result supports transformational faulting of metastable olivine as the initiation mechanism of deep earthquakes, although large events (M7.0+) probably rupture beyond the wedge. Furthermore, the slab core must be dehydrated at shallower depth and only transports negligible amount of water into the transition zone.