Modeling the Tonga slab: Can travel time data resolve a metastable olivinewedge?

We present the results of detailed modeling of the Tonga slab with the goal s of determining whether high-resolution travel time data(1) can be fit by simple thermal and petrological slab models and (2) can resolve a metastabl e olivine wedge at depths greater than the equilibrium olivine-spinel phase boundary. We model arrival times recorded by a 1000 km line of 23 ocean bo ttom seismometers (OBS) and island broadband seismic stations extending fro m the Lau backarc basin, across the Tonga trench and onto the Pacific plate . The data consist of 388 local, P wave travel times from 17 deep and 3 int ermediate earthquakes recorded during the 3-month OBS deployment in late 19 94. We locate the events using both local and teleseismic arrival times, an d apply a relocation operator to the theoretical travel times to simulate t he biases introduced in the data by locating the events with a reference Ea rth model. The modeling consists of grid searches using a three-dimensional finite difference algorithm to compute local, first arriving travel times for equilibrium and metastable P wave velocity models constructed from ther mal, mineralogical, and morphological constraints. The travel time anomalie s are well fit by standard slab thermal models and P velocity temperature d erivatives of -0.4 to -0.3 ms(-1)degrees C-1. Forward calculations indicate that the presence of a metastable olivine wedge has a subtle effect on the travel times due to the tendency of first arriving waves to avoid the low- velocity region. Wedge velocity models provide a slightly better fit to the data than equilibrium models, but F tests indicate the improvement is not significant at the 95% level. Our results suggest that providing direct sei smological evidence of a wedge of metastable olivine in subduction zones wi ll require either waveform modeling or the observation of later arriving ph ases created by the depressed phase boundary.