[1] We analyze seismic waveforms from deep-focus earthquakes occurring in the subducting slab beneath Japan, recorded by broadband ocean bottom seismometers (BBOBSs) installed on the northwestern Pacific Ocean seafloor. The data reveal waveforms with a low-frequency direct P onset, followed by large-amplitude, high-frequency, long-duration Po and So waves. From the analysis of the BBOBS records and a numerical finite-difference method simulation of seismic wave propagation, we elucidate the generation and propagation processes of such guided waves. We demonstrate that the low-frequency direct P and S waves propagate in the asthenosphere and that the following high-frequency, long-duration Po and So waves are developed by multiple forward scattering of P and S waves. The scattering occurs due to laterally elongated heterogeneities in both the subducting and horizontal parts of the oceanic lithosphere, with the apparent velocities (V p = 8.1 km/s, V s = 4.6 km/s) being close to the velocities of oceanic lithosphere. Citation: Shito, A., D. Suetsugu, T. Furumura, H. Sugioka, and A. Ito (2013), Small-scale heterogeneities in the oceanic lithosphere inferred from guided waves, Geophys. Res.

[1] The frequency dependence of Q in Earth's upper mantle is investigated using the continuous P wave spectra over 0.08 < f < 8.0 Hz. Regional waveform data from 23 events in the Pacific slab recorded on broadband stations in western Japan were used to create spectral ratio estimates of P waves which sample a subducting slab and mantle wedge. We assumed the power-law model Q( f ) = Q 0 ( f/f 0 ) a and applied a grid search to determine the best fit a and Q 0 . The inferred frequency dependence (0.2 < a < 0.4) in most regions is consistent with laboratory data on solid olivine but not on partially molten olivine-rich rocks except those data for deep events. Close agreement between seismological and laboratory observations on frequency dependence of Q suggests that physical mechanisms of seismic wave attenuation are common and therefore laboratory data can be used to infer the physical state of Earth's upper mantle from seismic wave attenuation.

Po/So waves are characterized by their high-frequency content and long-duration travel over great distances (up to 3000km) through the oceanic lithosphere. Po/So waves are developed by the multiple forward scattering of P and S waves due to small-scale stochastic random heterogeneities. To study the nature of these heterogeneities, Po/So waves are analyzed in the Philippine Sea Plate, which consists of three regions with different lithospheric ages. In the Philippine Sea Plate, Po/So waves propagate in the youngest region (15 Ma) and propagate more effectively in older regions. We investigate the mechanism of this propagation efficiency using numerical finite difference method simulations of 2-D seismic wave propagation. The results of this study demonstrate that the increase in propagation efficiency of Po/So waves depends on the age of the oceanic lithosphere, and this relationship can be qualitatively explained by thickening of the oceanic lithosphere including small-scale heterogeneities and a reduction in the intrinsic attenuation. These small-scale heterogeneities may form continuously in oceanic lithosphere from the time of its formation at a spreading ridge, via the solidification of melts distributed in the asthenosphere.