SEISMIC ATTENUATION STRUCTURE OF THE EAST PACIFIC RISE NEAR 9-DEGREES-30'N

We describe a spectral technique to measure the apparent attenuation o f compressional waveforms recorded during an active seismic tomography experiment centered at 9 degrees 32'N on the East Pacific Rise. Over 3500 estimates of t are obtained from 0.4- to 0.7-s-long windows alig ned with crustal P phases, including diffractions above and below a mi dcrustal magma chamber and Moho reflections which cross the rise axis at a range of lower crustal depths. We apply a smoothest model inversi on algorithm to the t measurements to derive images of apparent crust al Q(-1) both 20 km off axis and within a 16 x 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 l ower-crustal depths beneath the rise axis. Off axis, Q values in the u pper 1 km average 35-50, while at depths greater than 2-3 km Q is at l east 500-1000. The high levels of attenuation in the uppermost crust p robably result from the combined effect of frictional, fluid flow, and scattering mechanisms. Within 1-3 km of the rise axis, Q increase mar kedly in the uppermost 1 km to about 65. If the increase in attenuatio n off axis is entirely due to the similar to 300-m increase in the thi ckness of layer 2A extrusives required by seismic velocity measurement s, then Q in layer 2A must be about 10-20. No measurements of Q are ob tained in the immediate vicinity of the 1.6-km-deep axial magma lens b ecause no wave paths cross the rise axis through this region. The diff ractions beneath the magma chamber and the Moho reflections require a low-Q region, with minimum Q values of 20-50, which extends from no mo re than 2.5 km depth to the base of the crust. These values are simila r to laboratory measurements of Q obtained at solidus temperatures and constrain the low-e region to contain no more than a few percent melt . The axial magma chamber, which comprises a melt lens and an underlyi ng crystal mush zone, must be confined to a narrow, I-lan-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 a t 2-3 km depth north of 9 degrees 34'N but resolve no such trend at 4- 6 km depth. The along-axis variations may reflect the recent history o f volcanic eruptions and hydrothermal cooling and do not require syste matic along-axis variations in magma supply.