The seismic attenuation structure of a fast-spreading mid-ocean ridge.

The two-dimensional P-wave attenuation structure of the axial crust of the East Pacific Rise was obtained from an inversion of waveform spectra collected during an active-source seismic tomography experiment. The structure shows that attenuation near the surface is high everywhere but decreases markedly within 1 to 3 kilometers of the rise axis. The near-axis variation is attributed to the thickening of the surface basalt layer and possibly to in situ changes in porosity related to hydrothermal circulation. High attenuation is also observed beneath the rise axis at depths ranging from about 2 kilometers (less than 1 kilometer beneath the axial magma lens) to the base of the crust. The levels of attenuation in this deeper region require at most only a small fraction of partial melt.