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 approx 6km wide, lies between elevations of 9km and 34km below sea level, and has a compressional velocity reduction of 65 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.-Author
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Two-dimensional compressional wave velocity structure under San Francisco volcanic field, Arizona, from teleseismic P residual measurements.