Abstract
Stochastic tomography, made possible by dense deployments of seismic
sensors, is used to identify previously undetected and poorly detected
changes in the composition and mineral structure of Earth’s mantle. This
technique inverts the spatial coherence of amplitudes and travel times
of body waves to determine the depth and lateral dependence of the
spatial spectrum of seismic velocity. The inverted spectrum is
interpreted using predictions from the thermodynamic stability of
different compositions and mineral phases as a function of temperature
and pressure, in which the metamorphic temperature derivative of seismic
velocities can be used as a proxy for the effects of heterogeneity
induced in a region undergoing a phase change. Peaks in the metamorphic
derivative of seismic velocity are found to closely match those found
from applying stochastic tomography to elements of Earthscope and FLEX
arrays. Within ± 12 km, peaks in the fluctuation of P velocity at 425,
500, and 600 km depth beneath the western US agree those predicted by a
mechanical mixture of harzburgite and basalt in a cooler mantle
transition zone. A smaller peak at 250 km depth may be associated with
chemical heterogeneity induced by dehydration of subducted oceanic
sediments, and a peak at 775 km depth with a phase change in subducted
basalt. Non-detection of a predicted endothermic phase change near 660
km is consistent with its width being much less than 10 km. These
interpretations of the heterogeneity spectrum are consistent with the
known history of plate subduction beneath North America.