COMPLEX SCATTERING WITHIN D'' OBSERVED ON THE VERY DENSE LOS-ANGELES REGION SEISMIC EXPERIMENT PASSIVE ARRAY

Several seismic phases that scattered within a few hundred kilometers of the base of the mantle are observed in a very dense seismic section . The Los Angeles Region Seismic Experiment passive phase array was co mposed of 88 seismometers placed along a 175 km profile. Records from two deep earthquakes in Tonga and one earthquake near Honshu, Japan sh ow a secondary arrival between clear P and PcP arrivals. Modeling with layered structures shows that the Tonga and Honshu seismic sections a re consistent with an increase in seismic velocity 140 and 240 km abov e the core-mantle boundary, respectively, and a similar to 10-km thick low-velocity zone at the base of the mantle beneath a region in the m id Pacific. Several of these arrivals are not coherent enough to appea r in higher resolution stacks from the much larger Southern California Seismic Network. This experiment illustrates that fine-scale passive array data can reveal small-scale deep Earth structure invisible to la rger-scale seismic networks.