NONLINEAR TELESEISMIC TOMOGRAPHY AT LONG VALLEY CALDERA, USING 3-DIMENSIONAL MINIMUM TRAVEL-TIME RAY-TRACING

We explore the impact of three-dimensional minimum travel time ray tra cing on nonlinear teleseismic inversion. This problem has particular s ignificance when trying to image strongly contrasting low-velocity bod ies, such as magma chambers, because strongly refracted and/or diffrac ted rays may precede the direct P wave arrival traditionally used in s traight-ray seismic tomography. We use a simplex-based ray tracer to c ompute the three-dimensional, minimum travel time ray paths and employ an iterative inversion technique to cope with nonlinearity. Results f rom synthetic data show that our algorithm results in better model rec onstructions compared with traditional straight-ray inversions, We ree xamine the teleseismic data collected at Long Valley caldera by the U. S. Geological Survey. The most prominent feature of our result is a 25 -30% low-velocity zone centered at 11.5 km depth beneath the northwest ern quadrant of the caldera. Beneath this at a depth of 24.5 km is a m ore diffuse 15% low-velocity zone. In general, the low velocities tend to deepen to the south and east, We interpret the shallow feature to be the residual Long Valley caldera magma chamber, while the deeper fe ature may represent basaltic magmas ponded in the midcrust. The deeper position of the prominent low-velocity region in comparison to earlie r tomographic images is a result of using three-dimensional rays rathe r than straight rays in our ray tracing. The magnitude of our low-velo city anomaly is a factor of similar to 3 times larger than earlier mod els from linear arrival time inversions and is consistent with models based on observations of ray bending at sites within the caldera. Our results imply the presence of anywhere from 7 to 100% partial melt ben eath the caldera.