Abstract. The seismic structure of the crust and shallow mantle beneath the East Pacific Rise near 9ø30'N is imaged by inverting P wave travel time data. Our tomographic results constrain for the first time the three-dimensional structure of the lower crust in this region and allow us to compare it to shallow crustal and mantle structure. The seismic structure is characterized by a low-velocity volume (LVV) that extends from 1.2 km depth below the seafloor into the mantle. The cross-axis width of the LVV is narrow in the crust (5-7 km) and broad in the mantle (-18 km). Although the width of the top of the LVV is similar to previous estimates, its narrow shape at lower crustal depths and its significant widening in the mantle are previously unknown features of the rise velocity structure. In the rise-parallel direction the LVV varies in magnitude such that the lowest velocities are located between two minor rise axis discontinuities near 9ø28'N and 9ø35'N. From the seismic results we estimate the thermal structure and melt distribution beneath the rise. The thermal structure suggests that heat removal is relatively efficient throughout the crust yet inefficient at Moho and mantle depths. Estimates of the melt distribution indicate that magma accumulates at two levels in the magmatic system. One is at the top of the magmatic system and is capped by the shallow melt lens detected by seismic reflection surveys; the other is within the Moho transition zone and topmost portion of the mantle. The highest melt fractions occur within the upper reservoir, whereas the lower reservoir contains a lower melt fraction distributed over a broader area. By volume, however, there may be up to 40% more melt in the lower reservoir than in the upper reservoir. Along-axis variations in crustal melt content are similar to those in the mantle, supporting the hypothesis that the mantle, midway between the 9ø28'N and 9ø35'N devals, is presently delivering greater amounts of melt to the lower crust than to regions immediately to the north or south. We see no evidence (from seismic anisotropy) for diapiric mantle flow, suggesting that solid-state flow and melt migration are decoupled in the shallow mantle. Our results are not compatible with models that require a large, segment-scale redistribution of melt within the crust. Instead, our results imply that crustal magma chambers are replenished at closely spaced intervals along the rise.

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We have determined the three‐dimensional P wave velocity structure within the area of the Hengill‐Grensdalur central volcano complex, southwest Iceland, from the tomographic inversion of 2409 P wave arrival times recorded by a local earthquake experiment. The aperture of the 20‐element seismic network utilized in the inversion permitted imaging of a 5‐km‐thick crustal volume underlying a 15×14 km2 area. Within this localized volume are located the underpinnings of the active Hengill volcano and fissure swarm, the extinct Grensdalur volcano, and an active high‐temperature geothermal field. It was thus expected that the characteristic length scale of heterogeneity would be of the order of a kilometer. In order to image heterogeneous seismic velocity structure at this scale we paid particular attention to the fidelity of the assumed model parameterization, defined as the degree to which the parameterization can reproduce expected structural heterogeneity. We also discuss the trade‐off between the resolution of model parameters and image fidelity, compare results obtained from different parameterizations to illustrate this trade‐off, and present a synoptic means of assessing image resolution that utilizes the off‐diagonal information contained within the resolution matrix. The final tomographic image presented here was determined for a parameterization with fidelity that closely matches the geologic heterogeneity observed on the surface. For this parameterization, the resolution of individual parameters is generally low; however, a quantitative analysis of resolution provides an unambiguous assessment of well‐resolved volumes. Within the better resolved regions of the model the averaging volumes are 1–2 km and 2–4 km in vertical and horizontal extent, respectively. Results of tomographic inversion image three distinct bodies of anomalously high velocity, two of these extend from near the surface to a depth of about 3 km. These high‐velocity volumes are located directly beneath the surface expressions of the extinct Grensdalur volcano and the extinct Husmuli basalt shield. The third high‐velocity structure occurs in the depth range of 3–4 km but does not extend to the surface. These three high‐velocity bodies are interpreted to be solidified magmatic intrusions. Relatively low velocities underlay limited portions of the trace of the present accretionary axis and a low‐velocity body is imaged in the roots of the active Hengill volcano. The volume of lower velocities located beneath the surface expression of the Hengill volcano is interpreted to be a region of partial melt.

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 three‐dimensional seismic velocity structure of the upper 2.5 km of oceanic crust within a 20×18 km2 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 two‐dimensional. Three‐dimensional models reveal a seismic structure that correlates well with cross‐ and along‐axis variations in seafloor morphology, the location of the axial summit caldera, and the distribution of seafloor hydrothermal activity. The along‐axis segmentation of the seismic structure above the axial magma chamber is consistent with the hypothesis that mantle‐derived 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 500‐m‐thick thermal anomaly above a midcrustal melt lens; such an interpretation suggests that hydrothermal fluids may not have penetrated this region in the last 103 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 off‐axis magmatism; alternatively, the asymmetric thermal anomaly may be the consequence of differences in the depth extent of hydrothermal cooling.