Cm. Weiland et al., NONLINEAR TELESEISMIC TOMOGRAPHY AT LONG VALLEY CALDERA, USING 3-DIMENSIONAL MINIMUM TRAVEL-TIME RAY-TRACING, J GEO R-SOL, 100(B10), 1995, pp. 20379-20390
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.