Subslab ultra low velocity anomaly uncovered by and facilitating the largest deep earthquake

Abstract

Abstract It is enigmatic that M8+ earthquakes can take place at depth greater than 600 km inside the slab, where the P-T conditions generally do not favor seismic slip rate (~m/s) on faults. Here we provide new insights to the initial rupture and mechanism of the Mw 8.3 Sea of Okhotsk earthquake by analyzing high-frequency teleseismic array data. We determine the relative location and timing of two early subevents and the geometry and velocity perturbation of a nearby structure anomaly. We found a small-scale (~30x60x60 km) ultralow (-18±2%) P-wave velocity anomaly located beneath the Pacific slab around the 660km discontinuity. The highly melted nature of the anomaly provides significant buoyancy, inducing stress in the slab dramatically closer to the critical condition for thermal runaway weakening that allows the rupture to propagate beyond the metastable olivine wedge, forming M8+ events. Extremely large velocity reduction urges for further mineral physics and geodynamic investigations.