Slab temperature and thickness from seismic tomography 2. Izu-Bonin, Japan, and Kuril subduction zones

Delay times from teleseismic and local P wave arrivals are used to invert f or a high-resolution three-dimensional velocity model beneath the northwest Pacific. The model shows high-velocity slabs with average velocity anomali es of the order of 3-4%. Assuming the positive velocity deviations in the s ubducting lithosphere are to first order due to a temperature anomaly, the results of a theoretical slab temperature profile based on the diffusion eq uation are converted to a. synthetic slab velocity model. Temperature varia tions between the ambient mantle and the interior of the slab are converted to P wave velocity perturbations using dV(p)/dT approximate to 4.8 x 10(-4 ) km s(-1) degrees C-1. A nonlinear optimization scheme compares the tomogr ams obtained via tomography to the theoretically predicted models in order to determine the optimal values for slab thickness and mantle potential tem perature. Using 1180 +/- 100 degrees C, as the potential temperature, thick ness estimates of 88 +/- 8 km, 85 +/- 8 km, and 84 +/- 8 km are obtained fo r the Izu-Bonin, Japan, and Kuril slabs, respectively. A correlation exists between slab thickness and age, which is strong if mantle temperature vari ations along the slab strike can be ruled out. In the process of estimating slab thickness the predicted slab velocity model is used as a filter to en hance the initial minimum-noon tomographic result. The initial tomogram is modified to closely resemble the synthetic slab tomogram by using only null -space components. The use of the null-space components guarantees that the enhanced solution will satisfy the original seismic delay times. The enhan ced slab images show very continuous and narrow slabs compared to the initi al tomographic results.