Journal of Conference Abstracts

P-SV Hybrid Method and Ultra-low Velocity Zones at the CMB

Don Helmberger (helm@gps.caltech.edu) & Lianxing Wen

Caltech,Seismo Lab, Pasadena,Ca, 91125, U.S.A.

We develop a P-SV hybrid method for calculating synthetic seismograms involving 2D heterogeneous structures in the deep mantle. The finite-difference technique (FD) is applied in a heterogeneous region and the generalized ray theory (GRT) solutions from a seismic source are used in the finite-difference initiation process. The seismic motions after interacting with the target structure are propagated back to the surface of the Earth analytically with the aid of the Kirchhoff method. Two strategies are adopted to explore fine structures at the core-mantle boundary region. We first model anomalous SKS-SPdKS observations sampling regions near the core-mantle boundary (CMB) beneath the central Pacific, central Africa, and Iceland with this procedure. Localized structures just above the core-mantle boundary with lateral dimensions of 250 to 400 km can explain even the most extreme data observed to date, if we allow S-velocity drops of up to 30% (predicted by partial melt). We can also use broadband PKP precursors to study these ULVZ's. We present evidence and detailed modeling of strong broadband PKP precursors observed from events in the Fiji region. Hybrid method modeling indicates that strong seismic heterogeneities with length scales of several hundred kilometers to several kilometers near the core-mantle boundary in the western Pacific are required to explain the observed broadband PKP precursors. The sampling regions of those strong PKP precursors correlate quite well with the existence of ultra low velocity zones discovered from the long period SKS-SPdKS data. We will discuss geographic significance and variation of length scale and magnitude of seismic structures revealed from these precursors.