Waveform Effects of a High-velocity, Subducted Slab

Creager and Jordan [1986] propose that penetration of subducting slabs under the Kurile Islands and other subduction zones to depths of at least 1000 km is necessary to explain travel time anomalies of deep earthquakes. Such penetration would also be expected to affect the amplitudes and waveforms of the body waves from earthquakes. Synthetic seismograms appropriate for a record section in a plane perpendicular to the strike of the slab are presented using a coupled finite-difference and Kirchhoff method. Our shear-wave version of the compressional-wave velocity structure of Creager and Jordan [1986] produces an amplitude decrease up to a factor of four and waveform broadening up to 20 seconds for SH arrivals with a take-off angle pointing straight down the slab. Slabs that extend only 300 km below the earthquake but are half as thick and twice as anomalously fast as Creager and Jordan's [1986] velocity model will roughly preserve the travel time variation pattern, and show less waveform broadening, but produce first arrivals that are emergent. Slabs that become thicker with depth show less waveform broadening. Reconciliation of the amplitude, waveform distortion, and timing of body waves from deep events is necessary to understand the geometry of slabs in the upper mantle and near and below the 650 km discontinuity.