We present a new seismic model of the 
Pacific lithosphere/asthenosphere to a depth of about 300 km. 
The data-set consists of broad-band Rayleigh
and Love wave group-velocity (CU-Boulder; 15 - 200 s) and
phase-velocity (Harvard, Utrecht; 40 - 150 s) dispersion curves.
We construct the group and phase velocity maps using
"diffraction tomography"  which accounts for path-length dependent
sensitivity, wave-form healing and
associated diffraction effects that are particularly important
for long ocean-crossing paths. Monte-Carlo inversion 
of the dispersion maps produces an ensemble of acceptable 
shear velocity models of the crust and uppermost mantle from
which average characteristics and uncertainties are extracted.

We convert the seismic model into temperature and density
in order to compare the predicted isotherms and
seafloor topography with physical models of lithospheric
formation and cooling. The conversion is based on laboratory
measured properties of mantle minerals and a compositional 
model of the oceanic upper mantle.

The dispersion velocities, the intrinsic shear velocities
of the lithosphere, the temperatures and the topography
predicted from the seismic model correlate strongly with age.
At young ages (< 70Ma), this correlation is well described by a
simple half-space cooling model, but at older ages, the lithospheric
temperatures predicted from the seismic model are higher than
those from the half-space cooling model.