Metastable Olivine Wedge Beneath the Japan Sea Imaged by Seismic Interferometry

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 debatable. 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 Hi‐net, 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 thinning to a depth of 580 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. Plain Language Summary Most earthquakes occur within the top tens of kilometers of the Earth, but some can be more than 300 km deep. These deep earthquakes are puzzling, because they are not supposed to happen given the high pressure and temperature there. Scientists hypothesize that they might be caused by sudden phase changes of the mineral olivine, which should have broken down at the depth but survived metastably by hiding in the cold core of subducted plates. However, detecting the “surviving” metastable olivine has been difficult because of its small size and remoteness. Here we apply a novel method that can turn some deep earthquakes into virtual sensors closer to our target. Seismic waveforms recorded by these virtual sensors not only provide clear evidence for the metastable olivine but also constrain it dimension much better than before: a wedge 30 km thick at 410‐km depth and extending down to 580‐km depth at least. This finding supports the phase change hypothesis for deep earthquakes and also means that the slab core must be extremely dry, because even a small amount of water can break the “surviving” olivine mineral. Key Points We turn deep earthquakes into virtual receivers by inter‐source interferometry, to detect metastable olivine wedge in subducted slab We confirm the existence of the wedge about 30 km thick at 410‐km depth and gradually thinning with depth to at least 580 km Our results suggest the slab core in transition zone is dry and deep‐focus earthquakes in