Inversion of phase velocity alone results is greater scatter of the final models, particularly at shallow depths. This is because the phase velocities are only measured for periods greater than 25 s. Reliability aside, the models differ considerably. The resulting Lhasa model shows a pronounced low velocity zone in the lower crust ending at 70 km. The resolution at these depths appears reliable. The Qiangtang model shows no such velocity decrease. Both models show a significant increase in mantle velocities between 80 and 100 km.
The phase velocities are fit quite well in the Lhasa terrain and fairly well in
the Qiangtang terrain. Since group velocities were not constrained in the
inversion, they clearly do not fit. The scatter inthe group velocities below 30
s period is a result of the lack of phase velocity constraints at these
frequencies. Clearly, group velocities are needed to constrain shallow
structure.
| MODEL PLOT: range of initial models: gray final models: violet mean of final models: black VELOCITY PLOT: Initial group velocity: pale blue Initial phase velocity: pink/gray Final models: black Measured group velocity: blue Measured phase velocity: red |
