High Resolution Global Delay Time Tomography

In the field of seismic tomography, a large difference in resolving power exists between high resolution regional studies and long wavelength global studies. Regional studies tend to resolve features on a scale of 100 km or so (e.g. Spakman et al., 1993), whereas global studies usually image structure on a scale of 1000 km (e.g. Su et al., 1994). Hence, global studies have been unable to map important small scale lateral heterogeneity such as subduction zones, rift zones and hotspots.
To improve on the global modeling done so far, we concentrate on two aspects of the tomographic modeling: the data set and the model parameterization. The applied data have been derived from relocated hypocenters of the ISC and NEIC data sets (Engdahl et al., in press). This resulted in a data set of nearly 11 million arrivals. We perform simultaneous inversions for slownesses, event locations and station corrections using depth phases to increase global coverage and constrain the hypocenters. The earth is parameterized with a 'flexible cell model' in which the chosen cell size depends on the hitcount, thus allowing for more detailed solutions in high hitcount regions.
Inversions are performed with single as well as composite rays (Spakman and Nolet, 1988) to test the validity of the 'average' ray approach. Several synthetic tests are used to visualize the model's resolving power. In general, small scale structure is resolved well in densely sampled regions, notably near Tonga, Japan, Indonesia and the Mediterranean. In these areas a qualitative comparison shows good agreement with regional models (e.g. Widiyantoro and Van der Hilst, 1996 and Spakman et al., 1993). In the low hitcount parts of the model the solution resembles existing global models such as the IASP3D model (Vasco et al., 1994).