THE SHALLOW ATTENUATION STRUCTURE OF THE FAST-SPREADING EAST PACIFIC RISE NEAR 9-DEGREES-30'N

Direct measurements of Q values in the shallow crust are difficult to obtain due to the lack of energy turning in this region. We analyze a unique data set consisting of seven refraction experiments carried out with both the source and receiver near the seafloor. This experimental geometry allows us to resolve the shallow compressional wave attenuation structure of young crust at the East Pacific Rise. All models depict a low-Q layer 2A (apparent Q=11-22), and several models resolve a layer 2B with a much higher Q (apparent Q >70). This structure is consistent with a layer 2A composed of extrusives overlying a layer 2B composed of sheeted dikes.