Abstract
A low compressional‐wave velocity region in the midcrust below the San Francisco Mountain stratovolcano, Arizona, has been detected by the teleseismic P residual technique. This region is approximately 6 km wide, lies between elevations of 9 km and 34 km below sea level, and has a compressional velocity reduction of more than 6% with respect to the surrounding rocks. Several mechanisms are found to be quantitatively sufficient to produce such a feature. These include (1) a cool silicic pluton enclosed in a more mafic crust, (2) high temperature (near but below the solidus) in a quartz‐bearing rock in the low‐velocity region, (3) high density of water‐filled cracks having pore pressures nearly equal to lithostatic pressure, and (4) the presence of melt, either in intergranular pores or in crystal‐poor dikes.
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