Caltech
Email:
helm@gps.caltech.edu
poster/oral: poster
| Broadband record sections from arrays are compared with synthetic predictions generated from tomographic models. In general, such models require sharpening and enhancing to fit body waveforms. Models from the circum-Pacific fast belts can be modified by adding a temperature-dependent phase jump to explain the timing and strengths of the Scd phase occurring between S and ScS, and a global map of D" constructed. This model appears to explain the observed data from most fast (slabs) regions, although anisotropic features require more work. Beneath South Africa, the so-called African Plume appears to have the strongest slow anomalies. Delays of ScS relative to S of up to 10 sec at 90° are commonly observed. Such delays can be explained by long-period tomographic models with the addition of a slow D" structure (3 to 6% drops in shear velocities). However, these structures cannot explain the (SKS-S) differentials sampling the same region. To explain the (SKS-S) and (ScS-S) datasets simultaneously requires instead a large-scale ridge-like structure with a relatively uniform 3% reduction in shear velocity. The structure is about 800 km wide and extends at least 1000 km above D". It is orientated roughly NW-SE and leans towards the east at latitudes from 15° to about 30°. The walls of this structure appear to be sharp, in that, a fortuitous set of SKS phases traveling along this boundary display multipathing. To model such data, we need a sharp transition of less than 50 km, implying a chemical boundary. To model the above features, we employ a modification of the WKMJ method that is easily used in conjunction with the Cagniard-de Hoop method to generate 2D synthetics. As in tomography, we initiate the first stage by computing the paths reflecting off each layer of a 1D reference model (PREM), and noting the position of all ray segments. The velocity of each segment (layer crossing) is then adjusted to fit the tomographic model and a set of new paths generated to form a set of travel times (ti) and ray parameters (pi). These values can be used to generate a P-T curve and synthetic or used directly in a Cagniard-de Hoop code to handle the more difficult propagational problems, diffractions, etc. Synthetics for this procedure are compared with other methods with generally favorable results. |