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Mutter, JC, Barth GA, Buhl P, Detrick RS, Orcutt J, Harding A.  1988.  Magma Distribution Across Ridge-Axis Discontinuities on the East Pacific Rise from Multichannel Seismic Images. Nature. 336:156-158.   10.1038/336156a0   AbstractWebsite

Detailed studies of the morphology of the East Pacific Rise axis have shown that its linearity is disrupted by many small but distinct non-transform offsets, including overlapping spreading centres (OSCs) and deviations from axial linearity (devals), which display variable geochemical signals1–9. Using multichannel seismic reflection profiling, we have mapped the distribution of a bright, shallow reflector that Detrick et al. 10 have associated with an axial magma chamber. We have found that it is neither continuous across the 9°03' OSC6, nor separated into two parallel bodies2, and that its lateral offset does not conform to that of the topographic offset associated with the 9° 17' deval. These observations provide important insight into a causative relationship between morphological and petrological segmentation in this region of the East Pacific Rise, and we speculate that the discontinuities may be the morphological response to fluctuations in the spatial pattern of magma delivery.

Michael, PJ, Forsyth DW, Blackman DK, Fox PJ, Hanan BB, Harding AJ, Macdonald KC, Neumann GA, Orcutt JA, Tolstoy M, Weiland CM.  1994.  Mantle control of a dynamically evolving spreading center: Mid-Atlantic Ridge 31–34°S. Earth and Planetary Science Letters. 121:451-468.   10.1016/0012-821x(94)90083-3   AbstractWebsite

A segment of the slow-spreading Mid-Atlantic Ridge (MAR) at 33-degrees-S changes dramatically as its center is approached. Towards the center of the segment, the axis shoals from 3900 to 2400 m and a deep median valley nearly disappears. There is a prominent bullseye gravity low centered over the shallow summit, indicating thicker crust or lower density mantle or both. Incompatible element and radiogenic isotope ratios in MORB increase, creating a 'spike high' centered on the summit of the segment. The basalts' enrichment is confined to this robust ridge segment alone and is geochemically unlike the nearby hotspots at Tristan da Cunha, Gough and Discovery Islands. The average extent of mantle melting for the entire segment, as determined from mid-ocean ridge basalt (MORB) major element chemistry, is slightly greater than for adjacent segments. The segment has lengthened to 100 km by ridge propagation at both ends during the past 3.5 m.y., and is presently the longest and shallowest segment in the region. Although the ridge crest anomalies of this ridge segment strongly resemble those caused by the interaction of mid-ocean ridges with mantle hotspots, the geochemical and geophysical evidence suggests that they may instead be related to interaction of the ridge with a passively embedded chemical heterogeneity in the mantle.

Wolfe, CJ, Solomon SC, Laske G, Collins JA, Detrick RS, Orcutt JA, Bercovici D, Hauri EH.  2011.  Mantle P-wave velocity structure beneath the Hawaiian hotspot. Earth and Planetary Science Letters. 303:267-280.   10.1016/j.epsl.2011.01.004   AbstractWebsite

Three-dimensional images of P-wave velocity structure beneath the Hawaiian Islands, obtained from a network of seafloor and land seismometers, show an upper-mantle low-velocity anomaly that is elongated in the direction of the island chain and surrounded by a high-velocity anomaly in the shallow upper mantle that is parabolic in map view. Low velocities continue downward to the mantle tansition zone between 410 and 660 km depth and extend into the topmost lower mantle, although the resolution of lower mantle structure from this data set is limited. Comparisons of inversions with separate data sets at different frequencies suggest that contamination by water reverberations is not markedly biasing the P-wave imaging of mantle structure. Many aspects of the P-wave images are consistent with independent tomographic images of S-wave velocity in the region, but there are some differences in upper mantle structure between P-wave and S-wave velocities. Inversions without station terms show a southwestward shift in the location cif lowest P-wave velocities in the uppermost mantle relative to the pattern for shear waves, and inversions with station terms show differences between P-wave and S-wave velocity heterogeneity in the shallow upper mantle beneath and immediately east of the island of Hawaii. Nonetheless, the combined data sets are in general agreement with the hypothesis that the Hawaiian hotspot is the result of an upwelling, high-temperature plume. The broad upper-mantle low-velocity region beneath the Hawaiian Islands may reflect the diverging "pancake" at the top of the upwelling zone; the surrounding region of high velocities could represent a downwelling curtain; and the low-velocity anomalies southeast of Hawaii in the transition zone and topmost lower mantle are consistent with predictions of plume tilt. (C) 2011 Elsevier B.V. All rights reserved.

Toomey, DR, Wilcock WSD, Solomon SC, Hammond WC, Orcutt JA.  1998.  Mantle seismic structure beneath the MELT region of the East Pacific Rise from P and S wave tomography. Science. 280:1224-1227.   10.1126/science.280.5367.1224   AbstractWebsite

Relative travel time delays of teleseismic P and S waves, recorded during the Mantle Electromagnetic and Tomography (MELT) Experiment, have been inverted tomographically for upper-mantle structure beneath the southern East Pacific Rise. A broad zone of low seismic velocities extends beneath the rise to depths of about 200 kilometers and is centered to the west of the spreading center. The magnitudes of the P and S wave anomalies require the presence of retained mantle melt; the melt fraction near the rise exceeds the fraction 300 kilometers off axis by as little as 1%. Seismic anisotropy, induced by mantle flow, is evident in the P wave delays at near-vertical incidence and is consistent with a half-width of mantle upwelling of about 100 km.

Wolfe, CJ, Solomon SC, Laske G, Collins JA, Detrick RS, Orcutt JA, Bercovici D, Hauri EH.  2009.  Mantle Shear-Wave Velocity Structure Beneath the Hawaiian Hot Spot. Science. 326:1388-1390.   10.1126/science.1180165   AbstractWebsite

Defining the mantle structure that lies beneath hot spots is important for revealing their depth of origin. Three-dimensional images of shear-wave velocity beneath the Hawaiian Islands, obtained from a network of sea-floor and land seismometers, show an upper-mantle low-velocity anomaly that is elongated in the direction of the island chain and surrounded by a parabola-shaped high-velocity anomaly. Low velocities continue downward to the mantle transition zone between 410 and 660 kilometers depth, a result that is in agreement with prior observations of transition-zone thinning. The inclusion of SKS observations extends the resolution downward to a depth of 1500 kilometers and reveals a several-hundred-kilometer-wide region of low velocities beneath and southeast of Hawaii. These images suggest that the Hawaiian hot spot is the result of an upwelling high-temperature plume from the lower mantle.

Singh, SC, Kent GM, Collier JS, Harding AJ, Orcutt JA.  1998.  Melt to mush variations in crustal magma properties along the ridge crest at the southern East Pacific Rise. Nature. 394:874-878.   10.1038/29740   AbstractWebsite

The determination of along-axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle, eruption history along the ridge crest, and the process of crustal accretion. Seismic reflection images(1-4) have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths (250-4,500 m), but variations in its seismic properties and thickness have remained elusive, despite several attempts to constrain these properties(5-7). Here we report that the melt sill along the southern East Pacific Rise, which is about 50 m thick, undergoes abrupt changes in its internal properties, ranging from pure melt to mush. The 60-km-long ridge-crest segment near 14 degrees 00' S is characterized by three 2-4-km sections containing pure melt embedded within a magma chamber rich in mush. These small pure melt pockets may represent a fresh supply of magma from the mantle, capable of erupting and forming the upper crust. Conversely, the 80-90% of the magma chamber which is mushy is unlikely to erupt and may influence the lower-crustal accretion.

Orcutt, JA, Parker RL, Stark PB, Garmany JD.  1988.  A method of obtaining a velocity-depth envelope from wide-angle seismic data - Comment. Geophysical Journal-Oxford. 95:209-211. Abstract
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Riedesel, M, Orcutt JA, Macdonald KC, McClain JS.  1982.  Microearthquakes in the Black Smoker Hydrothermal Field, East Pacific Rise at 21°N. Journal of Geophysical Research. 87:613-623.   10.1029/JB087iB13p10613   AbstractWebsite

In July and August 1980, an array of five ocean bottom seismographs was deployed within 3 km of the 350°C hydrothermal vents at the Rivera submersible experiment (RISE) site at 21°N, on the East Pacific Rise. Two of these instruments were placed within 600 m of the vents, using a transponder navigation network. The array detected four basic types of events. The first type consisted of local, very small microearthquakes. Locations obtained for 11 of these events place three within 1 km of the vents, with the others elsewhere along the rise crest. They appear to originate either from movement on the faults in the area or from the hydrothermal system beneath this area. A study of the S-P times of this type indicates a maximum hypocentral depth of 2–3 km, implying a similar limit to the depth of hydrothermal circulation and brittle fracturing in the vicinity of the vents. The second type of event found consisted of emergent earthquakes that have many of the characteristics of volcanic harmonic tremor. The frequency of these events falls in the 1–5 Hz range and are similar in appearance to those seen at Mount St. Helens prior to and during its May 1980 eruption. They may be either hydrothermal or volcanic in origin. The third type of event produced a very monochromatic, high-frequency seismogram, with the energy concentrated at 20 Hz. These events also appear to have a local origin. The particle motion of these earthquakes shows almost pure Stonely wave propagation, and they appear to originate from a regularly oscillating dilatational source. The nature of the source of event types two and three is clearly not the usual sort of motion on a fault and is still not understood. The fourth type of earthquake recordings were regional events, apparently from the nearby Rivera and Tamayo fracture zones The low-frequency acoustic noise in the water was 16–64 times higher within 300 m of the vents than it was 2 km away. This implies that the black smoker vents may be generating a large acoustical disturbance as the 350°C water pours into the ocean. The observations are among the first of seismicity on moderate to fast spreading ridges, and many of the events appear to be related to the hydrothermal activity found there.

Willoughby, DF, Orcutt JA, Horwitt D.  1993.  A Microprocessor-Based Ocean-Bottom Seismometer. Bulletin of the Seismological Society of America. 83:190-217. AbstractWebsite

For over 12 years, the Scripps Institution of Oceanography has operated a fleet of microprocessor-based ocean-bottom seismometers. These instruments free-fall to the seafloor and release their anchors and rise to the surface either at preset times or on receipt of an acoustic command, The instruments are contained in a single spherical pressure case and include geophones with a 1 -Hz natural period, and differential pressure gauges responsive to acoustic signals between 0.003 and 30 Hz. Recent improvements described in detail here include the implementation of a C-44 bus 80C88 microprocessor and cassette recorders capable of storing up to 10 days of data digitized at 128 samples/sec, or 40 days at 32 samples / sec. In addition, tiltmeters have been installed in the instruments. Serial links to the processor and release timers provide for instrument checkout and the setting of time and data parameters from outside the pressure case. A portable laboratory also described here is used to prepare the instruments for deployment at sea.

Harding, AJ, Hedlin MAH, Orcutt JA.  1998.  Migration of backscatter data from the Mid-Atlantic Ridge. Journal of the Acoustical Society of America. 103:1787-1803.   10.1121/1.421332   AbstractWebsite

In studies of low-frequency reverberation within the marine environment, a central concern is the relationship between reverberation events and morphological features of the seafloor. A time-domain migration algorithm for the reverberation intensity field is developed that produces scattering coefficient maps coregistered with a bathymetry database. The algorithm is tailored to broadband transient sources with good range resolution, and was developed to analyze an extensive set of reverberation records from a 200-255 Hz source collected on the flanks of the Mid-Atlantic ridge. The precise, sample-by-sample, tracking of wavefronts across elements of the bathymetry database that forms the foundation of the algorithms implementation results in reverberation maps that show a clear and detailed correlation between scattering and morphology with narrow scarp slopes consistently highlighted. Environmentally induced asymmetries in transmission loss and incidence angle are exploited to break the inherent left-right ambiguity of the receiver array. Iterative migration, assuming a dominant dependence of backscatter on grazing angle, produces images, even from individual records, that show good ambiguity resolution. Results from multiple records corroborate the effectiveness of the ambiguity resolution and demonstrate the stability of the scattering coefficient estimates and the acoustic system. (C) 1998 Acoustical Society of America.

Orcutt, J, Jordan TH.  1985.  MSS and OBS data from the NGENDEI experiment in the southwest Pacific. The VELA Program : a twenty-five year review of basic research. ( Kerr AU, Ed.).:758-770., [United States]: Executive Graphic Services for the Defense Advanced Research Projects Agency Abstract
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Detrick, RS, Buhl P, Vera E, Mutter J, Orcutt J, Madsen J, Brocher T.  1987.  Multichannel Seismic Imaging of a Crustal Magma Chamber Along the East Pacific Rise. Nature. 326:35-41.   10.1038/326035a0   AbstractWebsite

A reflection observed on multi-channel seismic profiles along and across the East Pacific Rise between 8°50′ N and 13°30′ N is interpreted to arise from the top of a crustal magma chamber located 1.2–2.4 km below the sea floor. The magma chamber is quite narrow (<4 – 6 km wide), but can be traced as a nearly continuous feature for tens of kilometres along the rise axis.

Harding, AJ, Kent GM, Orcutt JA.  1993.  A Multichannel Seismic Investigation of Upper Crustal Structure at 9-Degrees-N on the East Pacific Rise - Implications for Crustal Accretion. Journal of Geophysical Research-Solid Earth. 98:13925-13944.   10.1029/93jb00886   AbstractWebsite

Reprocessed multichannel seismic profiles from the 9-degrees-N segment of the East Pacific Rise reveal prominent shallow subbasement events. These events are identified as wide-angle reflections from the base of seismic layer 2A, based upon modeling of expanding spread profile data and velocity functions. The layer 2A reflections typically increase from 0.15 s after the seafloor reflection at the rise axis to 0.3-0.45 s within 1-2 km of the axis, corresponding to an increase in layer thickness of 200-600 m. No further systematic increase in layer thickness is observed, although lateral variability of the order of a few hundred meters in thickness is observed at greater offsets from the rise axis. However, the intermittent character of the imaged layer 2A reflection is attributed to focusing and defocusing of energy by the seafloor bathymetry rather than necessarily to intrinsic lateral variability at the base of the layer. The base of layer 2A is interpreted as corresponding to the transition between the extrusive section, pillow basalts and sheet flows, and a sheeted dike complex. The rapid thickening of the layer near the rise axis is attributed to successive lava flows burying the initially shallow top of the sheeted dike complex as the layer passes through the neovolcanic zone. Lateral variability of layer 2A can significantly affect the imaging of the underlying axial magma chambers as average velocities within layer 2A are approximately half that of layer 2B. For an along-axis profile, apparent along-axis variability in the depth of the axial magnma chamber is traced to variability in the thickness of layer 2A caused by wandering of the profile relative to axis. Within the resolution of the data, the time delay of the magma chamber reflection relative to the base of layer 2A is constant.

Mikhalevsky, PN, Sagen H, Worcester PF, Baggeroer AB, Orcutt J, Moore SE, Lee CM, Vigness-Raposa KJ, Freitag L, Arrott M, Atakan K, Beszczynska-Moeller A, Duda TF, Dushaw BD, Gascard JC, Gavrilov AN, Keers H, Morozov AK, Munk WH, Rixen M, Sandven S, Skarsoulis E, Stafford KM, Vernon F, Yuen MY.  2015.  Multipurpose Acoustic Networks in the Integrated Arctic Ocean Observing System. Arctic. 68:11-27. AbstractWebsite

The dramatic reduction of sea ice in the Arctic Ocean will increase human activities in the coming years. This activity will be driven by increased demand for energy and the marine resources of an Arctic Ocean accessible to ships. Oil and gas exploration, fisheries, mineral extraction, marine transportation, research and development, tourism, and search and rescue will increase the pressure on the vulnerable Arctic environment. Technologies that allow synoptic in situ observations year-round are needed to monitor and forecast changes in the Arctic atmosphere-ice-ocean system at daily, seasonal, annual, and decadal scales. These data can inform and enable both sustainable development and enforcement of international Arctic agreements and treaties, while protecting this critical environment. In this paper, we discuss multipurpose acoustic networks, including subsea cable components, in the Arctic. These networks provide communication, power, underwater, and under-ice navigation, passive monitoring of ambient sound (ice, seismic, biologic, and anthropogenic), and acoustic remote sensing (tomography and thermometry), supporting and complementing data collection from platforms, moorings, and vehicles. We support the development and implementation of regional to basin-wide acoustic networks as an integral component of a multidisciplinary in situ Arctic Ocean observatory.