This figure pulls together my summer research nicely, but needs quite a bit of explaining. This summer I was comparing magnetotelluric (MT) data and p-wave seismic tomography residuals to get a better understanding of the Rio Grande Rift. I was helping out with installing MT sites across the rift. These sites were arranged in 3 lines that transect the rift -- one northern line near Denver, Colorado, one middle line near Taos, New Mexico, and one southern line near Las Cruces, New Mexico. I modeled some MT data from 9 sites on the northern line creating models of resistivity versus depth. I also have produced p-wave travel time residuals over the rift with 22 events, and averaged all the residuals at each site to produce average travel time residuals. Each MT site was matched with the closest (in location) average residual time. This figure depicts the last resistivity layer of the MT models ("at infinity") and the corresponding p wave travel time residuals versus longitude of the northern line. The bottom plot shows the slow area below the rift (around the center of the line) while the top plot shows higher resistivity (lower conductivity) under the rift. While its accepted that the rift's upper mantle is slow, it is expected that areas of slow velocity have high conductivity. This is because higher temperature, more hydration, and more partial melt all cause slower velocities and higher conductivities. Changes in composition, however, can either produce slow or fast velocities and low or high condictivities. This figure raises some questions and definitely requires further investigation, but sums up my summer nicely by combining MT and P-wave tomography.
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