Chapter 7 A Scattering Waveguide in the Heterogeneous Subducting Plate

The subducting plate is an efficient waveguide for high-frequency seismic waves. Such effects are often observed in Japan as anomalously large ground acceleration and distorted pattern of seismic intensity extending along the eastern seaboard of the Pacific Ocean from deep earthquakes in the Pacific plate, and Kyushu to Shikoku region from deep events in the Philippine Sea plate. Seismograms in these high intensity zones show low-frequency (f < 0.25 Hz) onset for both P and S waves, followed by large, high-frequency (f > 2 Hz) later arrivals with a very long coda. Such observations are not explained by a traditional plate model comprising just high wave speed and low attenuation material in the slab. A new plate model that can produce such guided high-frequency waves is characterized by multiple forward scattering of seismic waves due to small-scale heterogeneities within the plate. The preferred model requires anisotropic heterogeneity of elongated properties in the subduction slab with longer correlation distance (10 km) in the plate downdip direction and much shorter correlation distance (0.5 km) across the plate thickness. The standard deviation of P- and S-wave velocities and density from average is 3%. Such a quasi-laminated structure in the plate, which is equivalent to random distribution of anisotropic heterogeneities of elongated properties in parallel to the plate surface, can guide high-frequency signals with wavelengths shorter than the correlation distances along the plate. In contrasts, low-frequency signals with longer wavelength are not affected by the small scale heterogeneities and travel through the heterogeneous plate as a forerunner of the scattering signals. The high wave speed property of the plate and a strong velocity gradient from the center to the outer part of the plate due to the thermal regime allows low-frequency (f = 0.3-0.5 Hz) seismic waves to escape into the surrounding, low wave-speed mantle by refraction of seismic waves. The net result is a frequency-dependent waveguide in the subducting plate with efficient guiding of high-frequency (f > 2 Hz) signals by multiple forward scattering and loss of intermediate frequency (f = 0.3-0.5 Hz) signals due to internal velocity gradients. Very low frequency signals (f < 0.15 Hz) with wavelength larger than the plate thickness are not significantly affected by the presence of the plate. We demonstrate the presence of the frequency selective wave propagation effect from comparisons of observations from deep earthquakes that occurred recently in the Philippine Sea plate and in the Pacific plate. A good representation of the behavior of scattering waveguide is provided by 2D finite-difference calculations for seismic waves using heterogeneous slab models. The results of the simulations demonstrate that the frequency dependency of the models is quite sensitive to the thickness of the plate, and also depends on the scale lengths of heterogeneity distribution in the plate.