Enhanced waveguide effect for deep-focus earthquakes in the subducting Pacific slab produced by a metastable olivine wedge

摘要

Deep-focus (>400km) earthquakes in the subducting Pacific slab produce areas of anomalously large and lengthy shaking along the fore-arc (Pacific ocean) side of northern Japan. This effect arises because the slab acts as an efficient waveguide for seismic waves from earthquakes at depth within the slab. The mechanism of such slab-induced waveguide effect can be explained by efficient propagation of seismic waves in a cold and low-attenuation (high Q) slab, with multiple forward scattering of high-frequency (>1Hz) waves in a quasi-laminar stochastic waveguide. In this study, we demonstrate an additional mechanism to enhance the waveguide effect for deeper events, which is caused by reduction of the seismic wave speed in a metastable olivine wedge (MOW) in the core of the slab for depths between 400 and 600km. We present evidence for the presence of the low-wave speed MOW from analysis of broadband waveforms of deep-focus earthquakes in the Pacific slab descending beneath the Sea of Japan. For events below 400km depth, low-frequency and slightly dispersed P and S phases arrive ahead of the most energetic high-frequency direct phases. Numerical simulations of high-frequency seismic wave propagation for both 2-D and 3-D heterogeneous subduction zone models demonstrate a strong waveguide effect from the MOW due to focusing of high-frequency seismic waves toward the up-dip direction in the slab. The presence of the metastable olivine in slabs allows the development of very large and lengthy ground motion for stations on the fore-arc side of northern Japan in good agreement with the observations.