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

Oceanic Pn/Sn waves, which are often referred to as Po/So, are characterized by high-frequency (>2 Hz) arrivals with a long coda that travel for more than 2,000 km across the Pacific (Walker & Sutton, 1971). Fine-scale heterogeneity in the oceanic lithosphere with much longer horizontal than vertical correlation length produces multiple forward scattering of high-frequency waves within the lamella that guide high-frequency signals for large distances. The scattering waveguide effect of the heterogeneous lithosphere is further reinforced by seawater trapped P waves that interacts with Po and So at the seafloor .The propagation and attenuation properties of Po/So depends strongly on the thickness of the lithospheric waveguide. Propagation is efficient in the thick, old (120 Ma) northwest part of the Pacific Plate. However, attenuation is significant in propagation across the thin, younger (30 Ma) Philippine-Sea Plate Shito et al., 2013). The Po/So propagation intensity has a positive relationship with the age of the Pacific plate, that is the thickness of oceanic lithosphere (Kennett et al., 2014) and is also the case with the Philippine-Sea plate (Shito et al., 2015). Kennett and Furumura (2013) demonstrated that the general properties of oceanic Po and So traveling through the scattering waveguide can be captured by a stochastic heterogeneity model for the lithospheric mantle with much longer horizontal correlation distance (a h = 10 km) than in the vertical direction (a z = 0.5 km). A suitable description is provided by, for example, the von Kármán stochastic random heterogeneity model (e.g., see Frankel & Clayton, 1986) with 2% standard deviation of the root-mean-square

Small‐Scale Intraslab Heterogeneity Weakens Into the Mantle Transition Zone

“…Therefore, our 2D slab model can represent the general feature of intraslab scatterers within the updip dimension. Still, without comprehensive 3D simulations, we cannot fully understand the character of intraslab scatterers, such as the presence of strong azimuthal‐dependent scatterers (Furumura & Kennett, 2021). Note that the frequency contents of our interferometric observations are narrow‐banded due to the small magnitudes of deep earthquakes.…”

Earth Structure Across Many Scales

“…Yet, the stochastic waveguide model can reproduce the waveforms from deep events recorded at ocean bottom seismometers well away from the subduction zone (Shito et al., 2013). With the recent improvements in computation power, 3‐D simulations can be made to high frequencies incorporating 3‐D heterogeneity structure (Furumura & Kennett, 2021) that come closer to matching the full character of the guided waves.…”