Slab diffraction of S waves

Abstract

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...

Waveform response to the morphology of 2-D subducted slabs

“…Using P-wave 2-D finite-difference modelling in a plane perpendicular to the strike of the slab, Vidale (1987) predicted that slab diffraction and defocusing will broaden the waveforms at take-off angles subparallel with the subducting direction. Such a waveform broadening, which is characterized by an increase in pulse-width and a decrease in amplitude, was confirmed by Cormier (1989) using Gaussian beam asymptotic ray theory. Cormier also demonstrated with 3-D slab models that slab diffraction strongly depends on the azimuth of the receiver with respect to strike of the slab.…”

Subduction zones-their influence on traveltimes and amplitudes of P-waves

“…See also Cormier (1989, fig. 2) for a demonstration of the fact that short-period energy remains mostly concentrated in the slab.…”

Resolution experiments for NW Pacific subduction zone tomography

“…Body wave tomography has been used to illustrate the nature of mantle heterogeneity [Dzicwonski, 1984;Grand, 1987]. It is especially successful in delineating steeply dipping anomalous velocity regions such as subducting slabs [Hirahara, 1981; Kamiya et al 1988;Spakrnan et al, 1988; Van der Hi 1st, 1989;Zhou, 1988;Zhou and Clayton, preprint, 1988], due to the thermal contrast between the cold downgoing slab and wanner ambient mantle, long exploited in travel time studies [Toksoz et al, 1971;Sleep, 1973;Engdahl et al, 1977;Jordan, 1977;Creager and Jordan, 1984; and recently incorporated in waveform studies [Silver and Chan, 1986;Vida le, 1987;Cormier, 1989].…”