Azimuthal Variation of Lithospheric Heterogeneity in the Northwest Pacific Inferred From <i>Po/So</i> Propagation Characteristics and Anomalously Large Ground Motion of Deep In‐Slab Earthquakes

<jats:title>Abstract</jats:title><jats:p>High‐frequency oceanic <jats:italic>Pn/Sn</jats:italic> (<jats:italic>Po/So</jats:italic>) phases (>2 Hz) recorded at ocean bottom seismometers in the northwest Pacific display strong azimuthal variations in propagation characteristics. In the direction parallel to former Pacific plate motion (N30°W), seismograms show a gentle rise at the onset of <jats:italic>Po/So</jats:italic> followed by large, long spindle‐shaped coda; <jats:italic>Po</jats:italic> has a low‐frequency (<0.25 Hz) precursor and much delayed high‐frequency signals, showing weak dispersion with frequency. For orthogonal propagation, the onset of <jats:italic>Po/So</jats:italic> rises sharply and bursts of <jats:italic>Po</jats:italic> reverberations in the seawater follow. These differences indicate a strong azimuthal dependence of the scattering waveguide effect of the oceanic lithosphere. Numerical simulations of seismic waves in three‐dimensional heterogeneous structures reveal that much of the observed <jats:italic>Po/So</jats:italic> propagation variability can be explained by laterally elongated fine‐scale heterogeneity in the oceanic lithosphere, with a correlation distance of 20 km in the direction parallel to the magnetic anomaly, and a much shorter correlation distance in the perpendicular and depth directions. The longer axis corresponds to the observed <jats:italic>Pn/Sn</jats:italic>‐wavespeed anisotropy in the northwest Pacific, so the heterogeneity pattern was also developed during the formation and growth of the Pacific plate; competing processes produce different styles of fine‐scale effects. The elongated heterogeneity distributions in the oceanic lithosphere are carried into the subducting Pacific slab allowing energy from deep‐focus earthquakes to propagate to large distances, producing observations of anomalously large ground motions in specific directions. The behavior can be matched with three‐dimensional simulation of high‐frequency wave propagation with a heterogeneous Pacific slab.</jats:p>