Compressional wave velocity structure is determined for the upper crust beneath Newberry Volcano, central Oregon, using a high‐resolution active‐source seismic‐tomography method. Newberry Volcano is a bimodal shield volcano east of the axis of the Cascade Range. It is associated both with the Cascade Range and with northwest migrating silicic volcanism in southeast Oregon. High‐frequency (∼7 Hz) crustal phases, nominally Pg and a midcrustal reflected phase, travel upward through a target volume beneath Newberry Volcano to a dense array of 120 seismographs. This arrangement is limited by station spacing to 1‐ to 2‐km resolution in the upper 5 to 6 km of the crust beneath the volcano's summit caldera. The experiment tests the hypothesis that Cascade Range volcanoes are underlain only by small magma chambers. A small low‐velocity anomaly delineated about 3 km below the summit caldera supports this hypothesis for Newberry Volcano and is interpreted as a possible magma chamber of a few to a few tens of km3 in volume. A ring shaped high‐velocity anomaly nearer the surface coincides with the inner mapped ring fractures of the caldera. It also coincides with a circular gravity high, and we interpret it as largely subsolidus silicic cone sheets. The presence of this anomaly and of silicic vents along the ring fractures suggests that the fractures are a likely eruption path between the small magma chamber and the surface.

Utilizing teleseismic P residuals, we have detected a column of high P velocity material extending from within 10 km of the surface below the summit of Newberry Volcano, Oregon, to midcrustal depths near 25 km. We interpret this column to be the expression of a swarm of predominantly subsolidus gabbroic sills and dikes which were intruded as the volcano was built. The high P velocities observed below the volcano severely limit the size of magma chambers which could presently exist in the crust below Newberry Volcano. Those possible include a few percent of partial melt distributed through large volumes of a mafic intrusion zone in the midcrust; a few, smaller, higher melt fraction zones in the midcrust with dimensions less than 6 km and whose aggregate volume is only a few percent of enclosing volumes of 200 km3; small magma bodies with dimensions of a few kilometers located within the upper 10 km of the crust; or a mafic, crystal‐rich magma of arbitrary dimensions located in the upper few km. The low P velocities detected in the upper 4 km beneath the center of the summit caldera may be partially caused by a magma chamber in the second of these catagories.

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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.