A direct measurement of the distance between a hypocenter in a Benioff‐Wadati Zone and the Slab‐Asthenosphere contact

We made a direct measurement of the distance from a hypocenter in the Benioff‐Wadati zone to the boundary between the downgoing slab and overlying asthenosphere. This was accomplished by identifying a low amplitude P wave reflection off the slab‐asthenosphere contact which arrives at teleseismic stations several seconds after the initial P arrival. The measured delay time (after P) of the reflected phase shows a consistent azimuthal variation, thus eliminating possible source or receiver effects. The relative amplitude, polarity, and delay time of this observed phase are consistent with a model in which the distance between hypocenter and slab‐asthenosphere contact at this depth is about 38±5 km. The event studied was an intermediate depth (181 km) earthquake in the southern Kuriles which occurred in the lower layer of a double seismic zone. Our measured distance, when compared to the distance between the two seismic zones, indicates that the upper layer of seismicity is within 15 km of the slab‐asthenosphere contact. Appendix is available with entire article, on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, U.C. 20009. Document JB1‐010; $1.00. Payment must accompany order.