Tomographic imaging of the shallow crustal structure of the East Pacific Rise at 9°30′N
Abstract
Compressional wave travel times from a seismic tomography experiment at 9°30'N on the East Pacific Rise are analyzed by a new tomographic method to determine the threedimensional seismic velocity structure of the upper 2.5 km of oceanic crust within a 20×18 km^{2} area centered on the rise axis. The data comprise the travel times and associated uncertainties of 1459 compressional waves that have propagated above the axial magma chamber. A careful analysis of source and receiver parameters, in conjunction with an automated method of picking P wave onsets and assigning uncertainties, constrains the prior uncertainty in the data to 5 to 20 ms. The new tomographic method employs graph theory to estimate ray paths and travel times through strongly heterogeneous and densely parameterized seismic velocity models. The nonlinear inverse method uses a jumping strategy to minimize a functional that includes the penalty function, horizontal and vertical smoothing constraints, and prior model assumptions; all constraints applied to model perturbations are normalized to remove bias. We use the tomographic method to reject the null hypothesis that the axial seismic structure is twodimensional. Threedimensional models reveal a seismic structure that correlates well with cross and alongaxis variations in seafloor morphology, the location of the axial summit caldera, and the distribution of seafloor hydrothermal activity. The alongaxis segmentation of the seismic structure above the axial magma chamber is consistent with the hypothesis that mantlederived melt is preferentially injected midway along a locally linear segment of the rise and that the architecture of the crustal section is characterized by an en echelon series of elongate axial volcanoes approximately 10 km in length. The seismic data are compatible with a 300 to 500mthick thermal anomaly above a midcrustal melt lens; such an interpretation suggests that hydrothermal fluids may not have penetrated this region in the last 10^{3} years. Asymmetries in the seismic structure across the rise support the inferences that the thickness of seismic layer 2 and the average midcrustal temperature increase to the west of the rise axis. These anomalies may be the result of offaxis magmatism; alternatively, the asymmetric thermal anomaly may be the consequence of differences in the depth extent of hydrothermal cooling.
 Publication:

Journal of Geophysical Research
 Pub Date:
 December 1994
 DOI:
 10.1029/94JB01942
 Bibcode:
 1994JGR....9924135T
 Keywords:

 Earth Crust;
 MidOcean Ridges;
 P Waves;
 Pacific Ocean;
 Seismic Waves;
 Three Dimensional Models;
 Tomography;
 Bathymeters;
 Diffraction Propagation;
 Explosive Devices;
 Fermat Principle;
 Inversions;
 Seismographs;
 Velocity Distribution;
 Geophysics;
 Marine Geology and Geophysics: Midocean ridge processes;
 Seismology: Structure of the crust;
 Tectonophysics: Physics of magma and magma bodies