We explore the impact of three‐dimensional minimum travel time ray tracing on nonlinear teleseismic inversion. This problem has particular significance when trying to image strongly contrasting low‐velocity bodies, such as magma chambers, because strongly refracted and/or diffracted rays may precede the direct P wave arrival traditionally used in straight‐ray seismic tomography. We use a simplex‐based ray tracer to compute the three‐dimensional, minimum travel time ray paths and employ an iterative inversion technique to cope with nonlinearity. Results from synthetic data show that our algorithm results in better model reconstructions compared with traditional straight‐ray inversions. We reexamine 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 northwestern quadrant of the caldera. Beneath this at a depth of 24.5 km is a more 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 feature may represent basaltic magmas ponded in the midcrust. The deeper position of the prominent low‐velocity region in comparison to earlier tomographic images is a result of using three‐dimensional rays rather than straight rays in our ray tracing. The magnitude of our low‐velocity anomaly is a factor of ∼3 times larger than earlier models 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 beneath the caldera.
Preliminary results are presented from an expedition to the ultra-fast spreading segment on the East Pacific Rise. The combined multibeam and sidescan sonar worked extremely well, and provided the authors with surprising discoveries of abundant off-axis volcanism. Individuals from several institutions have contributed to a collection of public domain software for processing and displaying SeaBeam 2000 data, much of which is being used to visualize the East Pacific Rise.< >
Along‐axis profiles of three‐dimensional magnetic inversions for the Mid‐Atlantic Ridge (MAR) 31°–35°S show low magnetization near the middle of ridge segments and high magnetization at the segment tips for three adjacent spreading segments; thus there is an inverse relation between axial magnetization and axial topography. The ridge segment at 26°S on the MAR has the same inverse relationship between magnetization and topography. The common occurrence of this relationship suggests that it reflects a fundamental process of crustal accretion at the MAR. We analyze the rock magnetic properties from 42 locations within the four ridge segments in the South Atlantic to constrain the inherent trade‐off between source intensity and source thickness in the magnetization model. The natural remanent magnetization (NRM) intensities from the four ridge segments, averaged together, correlate with the magnetic inversion profiles. This finding implies that changes in the magnetization of the extrusives may account for much of the observed magnetic anomaly amplitude variation. A direct correlation of FeO content and magnetization suggests that magnetic anomaly amplitudes may be an indicator of FeTi‐rich basalts at the slow spreading MAR, even though the iron content of the basalts from high magnetization areas is not as high as observed at Pacific spreading centers. Despite the different magma plumbing systematics of the Pacific spreading centers and the MAR, it appears that the segment‐scale magma system of the MAR also results in segment‐scale crustal magnetization variations. Further evidence that the axial magnetic variations result from source intensity variations is that older isochrons have higher intensities near the ridge‐discontinuities, similar to the behavior on‐axis. Between 0 and 5 Ma the decay in magnetization is ∼50% independent of location within a spreading segment.
