We present observations of anomalously broad and complex SH waveforms from two deep-focus earthquakes in the southern Kurils to Digital World-Wide Standard Seismograph Network and Regional Seismic Test Network Stations in North America. By making use of broadband, three-component seismograms and relying on a variety of phases, one can limit the class of possible explanations for these phases. In particular, it is shown that the anomalous arrivals are not due to source complexity, near-receiver structure, a triplication from a lower mantle velocity gradient, a low-Q zone in the mid-lower mantle, or shear wave splitting. We show that these arrivals correspond to relatively fast paths and have enhanced amplitudes, indicating that they are most probably caused by multipathing due to interaction with a high-velocity lower mantle feature below the source region. We examine the hypothesis that this feature may be a lower mantle extension of the Kuril slab. Ray-tracing experiments show that multipathing is indeed expected along the azimuths corresponding to the North American stations used in this study for a realistic slab model.

Observations of reversed-polarity arrivals are consistent with such an explanation, where it appears that two rays comprising the arrival emerge from different points on the focal sphere. One test of this explanation is that slab-induced multipathing should not be seen along back-of-slab (oceanic) azimuths. Examinations of seven events for which the stations are in this azimuth range do not exhibit the same broadening and complexity of the SH waveforms. Identification of slab-induced multipathing associated with deep-focus events has two important consequences. First, it appears to require slab penetration beyond the 650-km discontinuity, at least 300--400 km into the lower mantle under the Sea of Okhotsk. Second, these phases may act as a contaminant when attempting to study other seismic phenomena such as discontinuities, low-Q zones, near-receiver structure, or source complexity.