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McClain, JS, Orcutt JA, Burnett M.  1985.  The East Pacific Rise in Cross-Section - a Seismic Model. Journal of Geophysical Research-Solid Earth and Planets. 90:8627-8639.   10.1029/JB090iB10p08627   AbstractWebsite

In 1982 we undertook a seismic refraction experiment, known as the MAGMA expedition, to examine the detailed structure of the East Pacific Rise near 12°50′N. This segment of the rise, where the full spreading rate is about 110 mm/yr, is near the projected trace of the O'Gorman fracture zone and is the site of an overlapping spreading center as well as “black smoker” hydrothermal activity. In this paper we describe the analysis of a subset of the travel time data collected during the MAGMA expedition, namely the data from profiles which were oriented normal to the rise axis. These profiles provide a data set roughly equivalent to those collected on other experiments and sample the cross-sectional structure of the rise. We have modeled these data using a two-dimensional ray-tracing program. We have found that the seismic velocities in the young oceanic crust are rather high, with velocity gradients of 4.0–5.5 s−1 in the uppermost crust. The highest velocities at the seafloor occur beneath the rise axis itself and seem to decrease as the crust ages to 0.1 Ma. This decrease in velocity must result from an increase in porosity in the upper crust and may coincide with the development of abundant surface fissures as the crust spreads. The decrease in velocity does not appear to penetrate deeper than about 0.5 km and may reverse itself as hydrothermal alteration fills the pores and cracks. The fact that the highest velocities occur under the rise axis suggests that the hydrothermal circulation responsible for the black smokers is confined to seismically unresolved conduits, a result consistent with the high temperatures and discrete nature of the vents. Our best model includes a magma chamber some 4 km wide and extending from the Moho to within about 1.1 km of the seafloor. This magma chamber is far smaller than many models for the rise axis have predicted but larger than those inferred from seismic refraction experiments at other sites on the East Pacific Rise. These discrepancies probably arise because the magma chamber under the East Pacific Rise is not a steady state feature but changes with time because of hydrothermal cooling and perhaps because of an episodic supply of magma from the mantle or along the rise axis.

Spiess, FN, Macdonald KC, Atwater T, Ballard R, Carranza A, Cordoba D, Cox C, Diaz Garcia VM, Francheteau J, Guerrero J, Hawkins J, Haymon R, Hessler R, Juteau T, Kastner M, Larson R, Leyendyk B, Macdougall JD, Miller S, Normark W, Orcutt J, Rangin C.  1980.  East Pacific Rise: hot springs and geophysical experiments. Science. 207:1421-1433.   10.1126/science.207.4438.1421   AbstractWebsite

Hydrothermal vents jetting out water at 380 30 C have been discovered on the axis of the East Pacific Rise. The hottest waters issue from mineralized chimneys and are blackened by sulfide precipitates. These hydrothermal springs are the sites of actively forming massive sulfide mineral deposits. Cooler springs are clear to milky and support exotic benthic communities of giant tube worms, clams, and crabs similar to those found at the Galapagos spreading center. Four prototype geophysical experiments were successfully conducted in and near the vent area: seismic refraction measurements with both source (thumper) and receivers on the sea floor, on-bottom gravity measurements, in situ magnetic gradiometer measurements from the submersible Alvin over a sea-floor magnetic reversal boundary, and an active electrical sounding experiment. These high-resolution determinations of crustal properties along the spreading center were made to gain knowledge of the source of new oceanic crust and marine magnetic anomalies, the nature of the axial magma chamber, and the depth of hydrothermal circulation.

Orcutt, JA, Harding AJ, Levander A.  1993.  The effects of seafloor roughness on reverberation: Finite difference and Kirchoff simulations. Natural physical sources of underwater sound : sea surface sound (2). ( Kerman BR, Ed.).:221-226., Dordrecht; Boston: Kluwer Academic Publishers Abstract
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Tong, CH, White RS, Warner MR, Barton PJ, Bazin S, Harding AJ, Hobbs RW, Kent GM, Orcutt JA, Pye JW, Singh SC, Sinha MC.  2004.  Effects of tectonism and magmatism on crack structure in oceanic crust; a seismic anisotropy study. Geology. 32:25-28.   10.1130/g19962.1   AbstractWebsite

We analyzed 25,675 traveltime residuals from a three-dimensional seismic tomographic inversion to investigate crack-induced seismic anisotropy in the upper oceanic crust. The study covered two regions with contrasting levels of magmatic activity on the western limb of the 9 degrees N overlapping spreading center on the East Pacific Rise. The level of anisotropy gradually decreases with depth in the magmatically and hydrothermally active ridge region. In contrast, we observed a highly variable anisotropic structure in the magmatically and hydrothermally less active tip region at the end of the dying ridge segment: a weakly anisotropic layer beneath strongly anisotropic extrusive volcanic rocks is likely to be the result of relatively shallow cracks closed by hydrothermal precipitation. Strongly anisotropic dikes with inferred narrow and water-saturated cracks provide important along-axis pathways for the circulation of hydrothermal fluids beneath the shallow cracks in the less magmatically active regions. Furthermore, a significant clockwise rotation (20 degrees -30 degrees ) of fast directions occurs in both regions with increasing depth. Such a rotation provides evidence that the geometry of the underlying crack structure of the western limb is significantly different from that defined by the bathymetric ridge crest.

Spudich, P, Orcutt J.  1982.  Estimation of earthquake ground motions relevant to the triggering of marine mass movements. Marine slides and other mass movements. ( Saxov S, Nieuwenhuis JK, Eds.).:219-231., New York: Plenum Press published in cooperation with NATO Scientific Affairs Division Abstract
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Kent, GM, Harding AJ, Orcutt JA.  1990.  Evidence for a Smaller Magma Chamber Beneath the East Pacific Rise at 9-Degrees-30' N. Nature. 344:650-653.   10.1038/344650a0   AbstractWebsite

The size and shape of magma chambers beneath mid-ocean ridges are fundamental features that control the availability of melt, composition of magmas and formation of oceanic crust. Models derived from the study of ophiolites1, and from thermal considerations2, include a very large magma chamber, which can exceed 10–20 km in width. Cross-axis seismic reflection profiles from the East Pacific Rise, however, constrain the width of the axial magma chamber to be < 3–4 km (ref. 3). Even this may be an over-estimate, arising from the under-migration of diffracted energy generated at the edges of a smaller magma chamber. Here we show that forward modelling of these diffraction hyperbolae yields a distance between the best-fitting point diffractors, and by inference a magma chamber width, of only 800–1,200 m. Reflectivity modelling also suggests that the available data are consistent with a magma chamber comprising only a thin layer of melt. A narrow and thin axial magma chamber would inhibit along-axis mixing4, and might thereby account for variations in magma composition along the East Pacific Rise5.

Kent, GM, Singh SC, Harding AJ, Sinha MC, Orcutt JA, Barton PJ, White RS, Bazin S, Hobbs RW, Tong CH, Pye JW.  2000.  Evidence from three-dimensional seismic reflectivity images for enhanced melt supply beneath mid-ocean-ridge discontinuities. Nature. 406:614-618.   10.1038/35020543   AbstractWebsite

Quantifying the melt distribution and crustal structure across ridge-axis discontinuities is essential for understanding the relationship between magmatic, tectonic and petrologic segmentation of mid-ocean-ridge spreading centres. The geometry and continuity of magma bodies beneath features such as overlapping spreading centres can strongly influence the composition of erupted lavas(1) and may give insight into the underlying pattern of mantle flow. Here we present three-dimensional images of seismic reflectivity beneath a mid-ocean ridge to investigate the nature of melt distribution across a ridge-axis discontinuity. Reflectivity slices through the 9 degrees 03' N overlapping spreading centre on East Pacific Rise suggest that it has a robust magma supply, with melt bodies underlying both limbs and ponding of melt beneath large areas of the overlap basin. The geometry of melt distribution beneath this offset is inconsistent with large-scale, crustal redistribution of melt away from centres of upwelling(2,3). The complex distribution of melt seems instead to be caused by a combination of vertical melt transport from the underlying mantle and subsequent focusing of melt beneath a magma freezing boundary in the mid-crust.

Orcutt, J, Kennett B, Dorman L, Prothero W.  1975.  Evidence of Low Velocity Zone Underlying a Fast-Spreading Rise Crest. Nature. 256:475-476.   10.1038/256475a0   AbstractWebsite

WE present the results of an unreversed seismic refraction profile on the East Pacific Rise near the Siqueiros Fracture Zone recorded using a digital ocean bottom seismograph (OBS)1. An analysis of P wave arrival times and amplitudes indicates a velocity gradient in the top 2 km, with the velocity reaching 6.7 km s−1. This is underlain by a low velocity channel some 1.4 km thick in which the velocity decreases to around 4.8 km s−1. Below this low velocity region there is a velocity gradient from 6.2 to 6.8 km s−1 and mantle velocities of 7.7 km s−1 are reached at a depth of 6 km below the sea bed.

Orcutt, JA, McClain JS, Burnett M.  1984.  Evolution of the ocean crust: results from recent seismic experiments. Ophiolites and oceanic lithosphere. ( Gass IG, Lippard SJ, Shelton AW, Eds.).:7-16., Oxford; Boston; St. Louis, Mo.: Published for the Geological Society by Blackwell Scientific Publications ; Blackwell-Mosby Book Distributors Abstract
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Babcock, JM, Harding AJ, Kent GM, Orcutt JA.  1998.  An examination of along-axis variation of magma chamber width and crustal structure on the East Pacific Rise between 13 degrees 30 ' N and 12 degrees 20 ' N. Journal of Geophysical Research-Solid Earth. 103:30451-30467.   10.1029/98jb01979   AbstractWebsite

We investigate the along-axis variations of magma chamber width and crustal structure along the East Pacific Rise (EPR) from 13 degrees 30'N to 12 degrees 20'N through reprocessed common depth point (CDP) reflection profiles. The magma lens is, predominantly, a continuous feature in the study area with an average width of similar to 500 m as determined from migrated cross-axis CDP profiles. This value is similar to widths estimated elsewhere along the EPR, suggesting that the axial magma chamber (AMC) width is not spreading rate dependent once the threshold for a steady state magma chamber is reached. The axial morphology of the 13 degrees N area is generally not a good predictor of magma lens width or continuity. A fairly continuous melt lens is imaged where the triangular axial topography might suggest waning magma supply. In fact, between 13 degrees 05'N and 13 degrees 01'N a shallow melt lens has been imaged which may be indicative of recent or impending eruptive activity. This shoaling is similar to that observed near the 17 degrees 26'S region of the EPR where the rise axis summit is domed and highly inflated. Generally, the thickness of seismic layer 2A beneath the ridge crest is uniform and comparable to that estimated for 9 degrees N, 14 degrees S, and 17 degrees S on the EPR, suggesting that the axial extrusive layer is invariant along fast spreading ridges. Uniformity of layer 2A thickness along-axis implies that variations in magma chamber depth are directly attributed to changes in thickness of the sheeted dike complex (seismic layer 2B). Contrary to expectations of decreasing melt sill depth with increasing spreading rate, the average thickness of seismic layer 2B is slightly less (similar to 165 m) at 13 degrees N than at the faster spreading, more robust 9 degrees N area. Finally, geochemical/petrologic boundaries, which may delineate separate melt supply regions, occurring at the 13 degrees 20'N and 12 degrees 46'N devals (deviation in axial linearity) are observed to coincide with subtle changes in AMC and layer 2A reflection characteristics.

de Groot-Hedlin, C, Orcutt JA.  2001.  Excitation of T-phases by seafloor scattering. Journal of the Acoustical Society of America. 109:1944-1954.   10.1121/1.1361057   AbstractWebsite

T-phases excited by suboceanic earthquakes are classified into two types: abyssal phases which are excited near the earthquake epicenter at seafloor depths far below the SOFAR velocity channel, and slope T-phases which are excited at continental, or ocean island slopes and ridges at distances up to several hundreds of kilometers from the epicenter. In this article, it is demonstrated that approximate time-frequency characteristics of both classes of T-phase can be synthesized under the assumption that T-phases are excited by scattering from a rough seafloor. Seafloor scattering at shallow depths preferentially excites low order acoustic modes that propagate efficiently within the ocean sound channel minimum. At greater depths, scattering excites higher order modes which interact weakly with the seafloor along much of the propagation path. Using known variations in near-source bathymetry, T-phase envelopes are synthesized at several frequencies for several events south of the Fox Islands that excited both types of T-phase. The synthesized T-phases reproduce the main time vs frequency features of each type of arrival; a higher frequency, nearly symmetric arrival excited near the epicenter and a longer duration, lower frequency arrival excited near the continental shelf, with a peak amplitude at about 5 Hz. (C) 2001 Acoustical Society of America.

Priestley, K, Orcutt J.  1982.  Extremal Travel Time Inversion of Explosion Seismology Data from the Eastern Snake River Plain, Idaho. Journal of Geophysical Research. 87:2634-2642.   10.1029/JB087iB04p02634   AbstractWebsite

We have inverted travel time data from seismic refraction profiles within the Snake River Plain, a volcanic-tectonic depression in southern Idaho, for crustal and uppermost mantle compressional velocity structure. The data in the vicinity of the youngest, northeastern volcanics in Yellowstone require the presence of a significant low velocity zone in the lower crust at depths between 20 and 40 km in order to explain the presence of a caustic at short ranges resulting from the crust-mantle transition. Data to the west are compatible with a model in which the velocity increases monotonically with depth. These observations are consistent with other data including the progressively lower crustal resistivity toward the northeast, the dated progression of volcanics from the southwest to the northeast at a rate of approximately 3.5 cm yr−1 and surface wave dispersion studies. The velocity reversals required by the data presented here and surface wave studies in the area are almost certainly related to elevated temperatures in the lower crust which approach or exceed the solidus of the deep crustal composition. The entire crustal section in the eastern Snake River Plain is characterized by very high velocities and can be represented by models with no first-order discontinuities with the possible exception of the Moho or crust-mantle transition.