Seismic attenuation structure of the East Pacific Rise near 9°30′N

Abstract

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 factional, 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.