[1] Using reference source time functions, broadband velocity waveforms of PKIKP in the distance range 150-180°are inverted for a viscoelastic model of inner core attenuation. A mean Q a at 1 Hz of 307 ± 90 is determined from 345 available PKIKP ray paths. Both global and regional results find a depth-dependent attenuation in the deep inner core, with anisotropic attenuation resolved in regional results. Attenuation is much stronger in the upper 300 km of the inner core. The preferred model of viscoelastic attenuation is frequency dependent, with very weak velocity dispersion. Our data do not resolve any hemispherical differences of attenuation in the deep inner core. Citation: Li, X., and V. F. Cormier, Frequency-dependent seismic attenuation in the inner core, 1, A viscoelastic interpretation,

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[1] Broadband velocity waveforms of PKIKP in the distance range 150-180°are inverted for a model of inner core attenuation due to forward scattering by a threedimensional heterogeneous fabric. A mean velocity perturbation of 8.4 ± 1.8% and a scale length of heterogeneity of 9.8 ± 2.4 km are determined from 262 available PKIKP ray paths. The velocity perturbations are larger for polar than for equatorial paths, decrease with depth, and show anisotropy in both global and regional data. For paths beneath North America, the smallest scale lengths (1-5 km) tend to lie in either the upper 200 km of the inner core or along paths close to the rotational axis. The depth dependence of attenuation is roughly similar to that obtained assuming a viscoelastic origin, except a more abrupt transition is seen between higher attenuation in the upper inner core and lower attenuation in the lower inner core. This transition may be sharp enough to produce either a first-or second-order discontinuity with depth in the long-wavelength (composite) elastic moduli. A fabric that satisfies the observed depth dependence and anisotropy of attenuation requires solidification of iron crystals having high (>10%) intrinsic anisotropy, which are preferentially aligned in time and depth. Since weak velocity dispersion, elastic anisotropy, attenuation anisotropy, and their depth dependence agree with that predicted by such a fabric, we suggest that scattering attenuation is not a small fraction but, rather, the predominant mechanism of attenuation in the inner core in the 0.02-2-Hz frequency band. Citation: Cormier, V. F., and X. Li, Frequency-dependent seismic attenuation in the inner core, 2, A scattering and fabric interpretation,

[1] We systematically searched for seismograms of waves diffracted around the inner core (PKP Cdiff ) from all the temporary seismic arrays with data currently available at the IRIS DMC, as well as some permanent regional seismic arrays including F-NET in Japan and GRF in Germany, to assemble the largest high-quality PKP Cdiff database ever created. PKP Cdiff waves preferentially sample the base of the outer core and so contain important clues about Earth structure in this region. We measured PKP DF -PKP Cdiff differential traveltimes and PKP Cdiff /PKP DF amplitude ratios in the distance range of 154°-160°and modeled the observations using grid searches and full wave theory synthetic seismograms. The optimum model found by fitting the differential traveltimes has relatively low velocity at the base of the outer core as in AK135, which is consistent with many previous traveltime studies. However, the optimum model found by fitting the amplitude ratios (PKP Cdiff /PKP DF ) does not exhibit this feature, and instead is closer to PREM. The discrepancy may be explained by two likely causes. One is that small-scale topography or roughness on the ICB tends to scatter energy away from PKP Cdiff waves by generating trailing coda waves. The other is that there exists a thin layer with relatively low Q at the base of the outer core. This might be expected if there are suspended solid particles at the base of the outer core, as proposed decades ago. Both mechanisms could generate smaller PKP Cdiff amplitudes without significantly affecting PKP Cdiff traveltimes.Citation: Zou, Z., K. D. Koper, and V. F. Cormier (2008), The structure of the base of the outer core inferred from seismic waves diffracted around the inner core,

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Broadband displacement and velocity records of PKP phases are formed by the simultaneous deconvolution of the instrument responses of waveforms recorded by the short-and long-period channels of the Global Digital Seismograph Network. Revisions of standard earth models are inferred from the comparison of synthetic and observed waveforms. The synthetics incorporate a Q-model designed for the bandwidth of the data and a source model that accounts for the variations of waveform about the focal sphere due to the effects of finiteness and complexity of the rupture process.

Seismograms are synthesized by superposition of Gaussian beams for teleseismic $ waves radiated by deep focus earthquakes located within the zone of high seismic velocities that define a descending lithospheric slab. The $ wave synthetics demonstrate a phenomenon of slab diffraction that acts to lengthen the apparent pulse width of the $ wave. Low-frequency energy leaks out of the slab and travels to the receivers along paths in the lower-velocity region surrounding the slab. Seismograms synthesized in fully three-dimensional models of slab structure show that slab diffraction strongly depends on the azimuth of the receiver with respect to strike of the slab. Slab diffraction lengthens the pulse width of S waves in the entire azimuthal range on the dipping side of the slab. As the azimuth approaches the strike of the slab, the diffracted phase may evolve into a higher-frequency, impulsive phase, which arrives after the minimum time arrival. Some slab models predict the slab phase to become a superposition of multipathed, ray theoretical arrivals as the azimuth approaches the strike of the slab. As the azimuth varies through the strike of the slab toward the side opposite the direction of dip, the slab phase is rapidly extinguished. Rapid variations in the complexity of $ waveforms recorded along azimuths parallel to the strike of the slab are consistent with the predictions of synthetic seismograms. Evidence of slab diffraction in the $ waveforms of the deepest focus earthquakes supports the hypothesis of slab penetration beyond 650 km depth. Broadband $ waveforms from deep focus earthquakes in the Kuril-Kamchatl• slab suggest that this slab penetrates below 650 km and that it may suffer some distortion in shape and advective thickening b•low this depth. zones. Inversions of local and teleseismic P travel times suggest that aseismic extensions of slabs are a common feature beneath island arcs [Engdahl and Gubbins, 1987; Creager and Jordan, 1986]. The aseismic extensions associated with the deepest Wadati-Benioff zones have been found to penetrate to at least 1000 km depth [Jordan, 1977; Creager and Jordan, 1984, 1986]. This depth is greater than the depth of the deepest earthquakes and the last significant phase transition in the upper mantle, both near 650 km depth. Global tomographic inversions suggest the existence of a convective process associated with deep subduction along the rim of the Pacific Ocean. A ring of high velocities is found to surround the Pacific and extend down to the coremantle boundary [Dziewonski and Wocdhouse, 1987]. Isolated patches of high velocity in the midmantle and deep mantle may represent remnants of subducted slabs. One such patch is the Caribbean anomaly described by Jordan and Lynn [1974], Lay [1983], and Grand [1987]. This is a 1-2% high velocity perturbation of the surrounding mantle, lying between 1200-1600 km depth, extending northward from the Caribbean along the east coast of North America. Vidale and Garcia-Gonzalez [1988] have used $H waveforms to refine estimates of th...