We use the same 1D starting model for all time periods to help minimise
variations in the inversion process. This 1D model (P and Vp/Vs)
was selected because it provided the best fit (minimum RMS travel
time misfit) to a subset of the highest quality earthquake solutions.
This test did not assume fixed earthquake solutions. As in the 3D inversions,
hypocentral locations were allowed to vary in the inversions. The
model, given in Table 1, determines values of P-velocity at different
depth levels and a constant Vp/Vs ratio equal to 1.75.