TOMOGRAPHIC IMAGING OF THE SHALLOW CRUSTAL STRUCTURE OF THE EAST PACIFIC RISE AT 9-DEGREES-30'N

Compressional wave travel times from a seismic tomography experiment a t 9 degrees 30'N on the East Pacific Rise are analyzed by a new tomogr aphic method to determine the three-dimensional seismic velocity struc ture of the upper 2.5 km of oceanic crust within a 20 x 18 km(2) area centered on the rise axis. The data comprise the travel times and asso ciated uncertainties of 1459 compressional waves that have propagated above the axial magma chamber. A careful analysis of source and receiv er parameters, in conjunction with an automated method of picking P wa ve onsets and assigning uncertainties, constrains the prior uncertaint y in the data to 5 to 20 ms. The new tomographic method employs graph theory to estimate ray paths and travel times through strongly heterog eneous and densely parameterized seismic velocity models. The nonlinea r inverse method uses a jumping strategy to minimize a functional that includes the penalty function, horizontal and vertical smoothing cons traints, and prior model assumptions; all constraints applied to model perturbations are normalized to remove bias. We use the tomographic m ethod to reject the null hypothesis that the axial seismic structure i s two-dimensional. Three-dimensional models reveal a seismic structure that correlates well with cross- and along-axis variations in seafloo r morphology, the location of the axial summit caldera, and the distri bution of seafloor hydrothermal activity. The along-axis segmentation of the seismic structure above the axial magma chamber is consistent w ith the hypothesis that mantle-derived melt is preferentially injected midway along a locally linear segment of the rise and that the archit ecture of the crustal section is characterized by an en echelon series of elongate axial volcanoes approximately 10 km in length. The seismi c data ate compatible with a 300- to 500-m-thick thermal anomaly above a midcrustal melt lens; such an interpretation suggests that hydrothe rmal fluids may not have penetrated this region in the last 10(3) year s. Asymmetries in the seismic structure across the rise support the in ferences that the thickness of seismic layer 2 and the average midcrus tal temperature increase to the west of the rise axis. These anomalies may be the result of off-axis magmatism; alternatively, the asymmetri c thermal anomaly may be the consequence of differences in the depth e xtent of hydrothermal cooling.