We describe a spectral technique to measure the apparent attenuation of compressional waveforms recorded during an active seismic tomography experiment centered at 9¿32'N on the East Pacific Rise. Over 3500 estimates of t* are obtained from 0.4- to 0.7-s-long windows aligned with crustal P phases, including diffractions above and below a midcrustal magma chamber and Moho reflections which cross the rise axis at a range of lower crustal depths. We apply a smoothest model inversion algorithm to the t* measurements to derive images of apparent crustal Q-1 both 20 km off axis and within a 16¿16 km area centered on the rise crest. The models resolve regions of high attenuation in the uppermost crust and in a low-Q zone which extends from midcrustal to lower-crustal depths beneath the rise axis. Off axis, Q values in the upper 1 km average 35--50, while at depths greater than 2--3 km Q is at least 500--1000.

The high levels of attenuation in the uppermost crust probably result from the combined effect of frictional, fluid flow, and scattering mechanisms. Within 1--3 km of the rise axis, Q increases markedly in the uppermost 1 km to about 65. If the increase in attenuation off axis is entirely due to the ~300-m increase in the thickness of layer 2A extrusives required by seismic velocity measurements, then Q in layer 2A must be about 10--20. No measurements of Q are obtained in the immediate vicinity of the 1.6-km-deep axial magma lens because no wave paths cross the rise axis through this region. The diffractions beneath the magma chamber and the Moho reflections require a low-Q region, with minimum Q values of 20--50, which extends from no more than 2.5 km depth to the base of the crust. These values are similar to laboratory measurements of Q obtained at solidus temperatures and constrain the low-Q region to contain no more than a few percent melt. The axial magma chamber, which comprises a melt lens and an underlying crystal mush zone, must be confined to a narrow, 1-km-thick region through which no rise-crossing paths pass. Inversions for along-axis structure in the low-Q anomaly show a 20--25% increase in attenuation at 2--3 km depth north of 9¿34'N but resolve no such trend at 4--6 km depth. The along-axis variations may reflect the recent history of volcanic eruptions and hydrothermal cooling and do not require systematic along-axis variations in magma supply. ¿ American Geophysical Union 1995