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

Abstract

High-frequency oceanic Pn/Sn (Po/So) 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 Po/So followed by large, long spindle-shaped coda; Po 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 Po/So rises sharply and bursts of Po 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 Po/So 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 Pn/Sn-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.