Deep-Focus Earthquakes Under Northeast China-An Imprint of the Complex Tectonic History of Pacific Plate Subduction

Abstract

Deep-focus earthquakes and their association with metastable olivine wedges (MOWs) remain enigmatic. Here we perform a seismic-geodynamic analysis of the Pacific slab stagnant at the 660 km deep bottom of the mantle transition zone. All investigated deep earthquakes exhibit only minor (mostly implosive) isotropic components, yet exhibit strongly varying compensated-linear-vector-dipole components. For the largest studied earthquake (M W 6.9), we demonstrate a significant stress-drop heterogeneity on a subhorizontal fault and a spatial change in radiation efficiency. We interpret the earthquakes with an evolutionary numerical subduction model with realistic mineralogy and rheology, including non-uniform plate aging and subduction disruption due to the Izanagi-Pacific ridge sinking in the early Cenozoic. This process resulted in a present-day tomography-supported bent slab, preserving low temperatures (900-1000 K), which permits the metastable olivine presence. Further, we demonstrate that the potential MOW is also bent. The accompanying internal deformation controls the deep seismicity in the slab tip with apparent changes in seismic radiation efficiency and rupture speed across the modeled temperature gradients. From a broader perspective, the MOW contortion may contribute to deformational anisotropy in the shallow lower mantle. Our results underscore the importance of joint interpretations of the evolutionary subduction models and seismic source inversions.