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Munk, W, Gilbert F, Orcutt J, Zumberge M, Parker R.  2003.  The Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics (IGPP). Oceanography. 16:34-44.   10.5670/oceanog.2003.29   Abstract
Taesombut, N, Wu X, Chien AA, Nayak A, Smith B, Kilb D, Im T, Samilo D, Kent G, Orcutt J.  2006.  Collaborative data visualization for Earth Sciences with the OptIPuter. Future Generation Computer Systems. 22:955-963.   AbstractWebsite

Collaborative visualization of large-scale datasets across geographically distributed sites is becoming increasingly important for Earth Sciences. Not only does it enhance our understanding of the geological systems, but also enables near-real-time scientific data acquisition and exploration across distant locations. While such a collaborative environment is feasible with advanced optical networks and resource sharing in the form of Grid, many technical challenges remain: (1) on-demand discovery, selection and configuration of supporting end and network resources; (2) construction of applications on heterogeneous, distributed environments; and (3) use of novel exotic transport protocols to achieve high performance. To address these issues, we describe the multi-layered OptIPuter middleware technologies, including simple resource abstractions, dynamic network provisioning, and novel data transport services. In this paper, we present an evaluation of the first integrated prototype of the OptIPuter system software recently demonstrated at iGrid 2005, which successfully supports real-time collaborative visualizations of 3D multi-gigabyte earth science datasets.

Hedlin, MAH, Orcutt JA.  1989.  A Comparative-Study of Island, Seafloor and Subseafloor Ambient Noise-Levels. Bulletin of the Seismological Society of America. 79:172-179. AbstractWebsite

A study of seafloor and island stations shows that for the frequency band 0.1 to 10 Hz the seismic noise levels on islands are comparable to the levels on the seafloor. The microseism peak at the seafloor appears to be comparable to the highest levels observed on small islands. For this band, seafloor stations are realistic alternatives when island sites are not available.Seven year averages of the ambient noise levels recorded by Seismic Research Observatory (SRO) stations on three islands (Guam [GUMO], Taiwan [TATO], and New Zealand's north island [SNZO]) are compared with those recorded by the International Deployment of Accelerometers (IDA) station on Easter Island and on and beneath the ocean floor by Ocean Bottom Seismometers (OBSs) and the Marine Seismic System (MSS) deployed in a south Pacific DSDP drill hole at 23.8°S., 165.5°W (Adair et al., 1986). From 0.3 to 2 Hz the SRO displacement power levels fall in the range historically observed by the Scripps' OBSs (decreasing at 70 dB/decade from 1 by 10^6 nm^2/Hz at 0.3 Hz to 1 nm^2/Hz at 2 Hz) and are 10 to 15 dB above MSS levels. Above 2 Hz it appears that the same ratios hold (the SRO power levels decrease at 70 dB/decade to 1 by 10^−3 nm^2/Hz at a frequency of 10 Hz), although this correlation is based on very limited, high gain, short-period data. At frequencies below 0.3 Hz the SRO noise levels peak and decrease to approximately 2 by 10^3 nm^2/Hz at 40 mHz. The noise levels recorded at Easter Island are somewhat higher (decreasing at 70 dB/decade from 1 by 10^7 nm^2/Hz at 0.2 Hz to 1 nm^2/Hz at 10 Hz and to 1 by 10^5 nm^2/Hz at 50 mHz). At the microseism peak near 0.2 Hz the MSS levels are from 15 to 20 dB higher than observed by the SRO stations and equivalent to those recorded at Easter Island. There appears to be little dependence of the variance in noise level estimates on frequency. The upper 95 per cent confidence limit generally lies 10 dB above the average noise levels for all island stations.All island noise level curves are dominated by the broad double-frequency microseism peak centered between 0.15 to 0.2 Hz. The single-frequency peak ranges from absent (Easter Island) to discernable (Guam and New Zealand) to obvious (at Taiwan). The center frequency of this peak ranges from 0.07 Hz at Guam and New Zealand to 0.1 Hz at Taiwan. We speculate that the increased amplitude and frequency of the single-frequency microseism peak is due to the interaction between the shallow continental shelf and surface gravity waves and/or the presence of Taiwan in a region of limited fetch.

Kappus, ME, Harding AJ, Orcutt JA.  1990.  A Comparison of TAU-P Transform Methods. Geophysics. 55:1202-1215.   10.1190/1.1442936   AbstractWebsite

Many τ-p transform methods are available to seismic data analysts; selection of the appropriate method should depend on the nature of the source excitation, the intended use of the transformed data, limits imposed by sampling parameters, and computational cost. Using these criteria, we compare five methods on marine multichannel data and similar synthetic profiles. On fully sampled synthetic profiles, methods that handle the three‐dimensional (3-D) nature of the point source provide correct amplitude and phase information even at small slownesses, whereas 2-D and asymptotic approximate methods do not. When data from small ranges are not available, aliasing and truncation distort the amplitude of small slowness traces produced by all methods, but are most severe in the 3-D results. Transformation of data with increased input trace spacing or decreased depth to the reflector results in increased aliasing effects. When the intended use of the transformed data depends on correct arrival times only, and not on accurate amplitudes of small‐slowness traces, we recommend an asymptotic approximate method for its relative computational efficiency and comparative robustness with respect to aliasing and truncation. Uses that depend on correct amplitudes demand a τ-p transform method which honors the source geometry of the experiment.

Kennett, BLN, Orcutt JA.  1976.  Comparison of Travel Time Inversions for Marine Refraction Profiles. Journal of Geophysical Research. 81:4061-4070.   10.1029/JB081i023p04061   AbstractWebsite

Systematic inversion techniques have been applied to travel time data from marine refraction profiles in the Pacific Ocean and are compared with the conventional uniform layer solutions for the same profiles. Extremal bounds are obtained on the possible velocity-depth distributions which fit the travel time data. Also a linearized inversion is used to construct suitable velocity-depth profiles together with a measure of their resolution. The velocity structures obtained indicate that layer 2 is a region of strong velocity gradients while layer 3 is relatively homogeneous, although it does show an increase in velocity with depth. The inverse schemes offer a useful alternative to fitting models containing uniform layers to the travel times from a seismic refraction profile.

Sandwell, DT, Renkin ML.  1988.  Compensation of Swells and Plateaus in the North Pacific - No Direct Evidence for Mantle Convection. Journal of Geophysical Research-Solid Earth and Planets. 93:2775-2783.   10.1029/JB093iB04p02775   AbstractWebsite

At intermediate and long wavelengths the ratio of geoid height to topography is sensitive to the depth and mode of compensation. A low geoid/topography ratio (<2 m/km) signifies shallow Airy compensation. A higher ratio (∼6 m/km) signifies thermal isostasy and/or dynamic uplift from a mantle plume. A very high geoid/topography ratio (>8 m/km) in conjunction with a poor correlation between geoid height and topography is evidence of mantle convection. After subtracting a reference geoid from the observed geoid, previous studies have found a regular pattern of geoid highs and lows with a characteristic wavelength of 3000–4000 km. Since these geoid highs and lows were poorly correlated with topography and resulted in very high geoid/topography ratios (10–20 m/km), they were believed to reflect the planform of mantle convection. We show that the regular pattern of geoid highs and lows is an artifact caused by truncating the reference geoid at spherical harmonic degree 10. Since the geoid spectrum is “red,” the residual geoid is dominated by degree 11. When the harmonics of the reference geoid are rolled off gradually, the regular pattern of geoid highs and lows disappears. In the Northeast Pacific, the new residual geoid reflects the lithosphere age offsets across the major fracture zones. In the Northwest Pacific, the residual geoid corresponds to isostatically compensated swells and plateaus. We have calculated the geoid/topography ratio for 10 swells and plateaus and have found a range of compensation depths. The highest geoidAopography ratio of 5.5 m/km occurs on the flanks of the Hawaiian Swell. Intermediate ratios occur in four areas, including the Midway Swell. These intermediate ratios reflect a linear combination of the decaying thermal swell and the increasing volume of Airy-compensated seamounts. Low geoid/topography ratios occur over the remaining five areas (e.g., Emperor Seamounts), reflecting the absence of a thermal swell. Our findings do not support the hypothesis that the planform of mantle convection is evident in the geoid. We see only indirect evidence of thermal plumes reheating the lower lithosphere.

Chapman, CH, Orcutt JA.  1985.  The Computation of Body Wave Synthetic Seismograms in Laterally Homogeneous Media. Reviews of Geophysics. 23:105-163.   10.1029/RG023i002p00105   AbstractWebsite

Synthetic seismograms, computed for realistic, horizontally stratified media, are now routinely used as an aid to seismic interpretation. This paper reviews the theoretical background to these methods and presents comparisons of the two popular algorithms, reflectivity and WKBJ seismograms, for a variety of earth models. The transformed wave equations are developed from the equations for a spherical, gravitating medium in a symmetric form suitable for body wave calculations. Four methods of solving these equations in general, inhomogeneous layers are described: the WKBJ and Langer asymptotic expansions and the WKBJ and Langer iterative solutions. Together with the earth-flattening transformation and the ray expansion, transformed solutions for body waves can then be obtained for realistic layered media. Four methods of inverting the frequency and wave number transformations are also described: the real and complex spectral and slowness methods. Although realistic seismic models are normally sufficiently complicated that numerical calculations are essential, before proceeding with numerical comparisons we have included a review of the canonical signals included in body wave seismograms. These analytic results for direct rays, partial and total reflections, turning rays on forward and reversed branches, head waves, interface waves, Airy caustics, and Fresnel and interface shadows are useful to anticipate and understand numerical problems and results. Finally, a comparison of Green's functions for crustal, mantle, and whole earth models, calculated using the WKBJ and reflectivity algorithms, is included.

Mackenzie, K, McClain J, Orcutt J.  1982.  Constraints on Crustal Structure in Eastern Iceland Based on Extremal Inversions of Refraction Data. Journal of Geophysical Research. 87:6371-6382.   10.1029/JB087iB08p06371   AbstractWebsite

In the summer of 1978 the Iceland Research Drilling Project undertook the drilling of a deep crustal hole near Reydarfjörder in eastern Iceland. As a part of this project, the Scripps Institution of Oceanography and the University of Washington undertook a small-scale seismic refraction experiment near the drill site in an attempt to compare surface geophysical measurements with observations of samples from and logging in the hole. Using recent advances in the methods of extremal inversion of seismic data, we have determined an approximate one-dimensional velocity structure for the drill site. This structure indicates that the 1.9-km hole failed to penetrate the layer 2 layer -3 transition which was at some 3.0–4.5 km beneath the drill site. The transition appears to be rather abrupt, unlike that beneath the ocean, with velocity increasing from 5.2–5.5 km/s in the upper crust to about 6.7 km/s in layer 3. We observe a steep eastward dip and a shallow westward dip in the lower crust away from the nearby Thingmúli and Reydarfjördur volcanic centers, respectively, in agreement with previous work associating shallow depths to layer 3 with Tertiary volcanic centers as a result of increasing metamorphic grade and increased dyke swarm intensity.

Van Avendonk, HJA, Harding AJ, Orcutt JA, McClain JS.  2001.  Contrast in crustal structure across the Clipperton transform fault from travel time tomography. Journal of Geophysical Research-Solid Earth. 106:10961-10981.   10.1029/2000jb900459   AbstractWebsite

A three-dimensional (3-D) seismic refraction study of the Clipperton transform fault, northern East Pacific Rise, reveals anomalously low compressional velocities from the seafloor to the Moho, We attribute this low-velocity anomaly to intensive brittle deformation, caused by transpression across this active strike-slip plate boundary. The seismic velocity structure south of the Clipperton transform appears unaffected by these tectonic forces, but to the north, seismic velocities are reduced over 10 km outside the zone of sheared seafloor. This contrast in seismic velocity structure corresponds well with the differences in mid-ocean ridge morphology across the Clipperton transform. We conclude that the amount of fracturing of the upper crust, which largely controls seismic velocity variations, is strongly dependent on the shallow temperature structure at the ridge axis. Intermittent supply of magma to the shallow crust north of the Clipperton transform allows seawater to penetrate deeper, and the cooler crust is brittle to a greater depth than south of the transform, where a steady state magma lens is known to exist. The crustal thickness averages 5.7 km, only slightly thinner than normal for oceanic crust, and variations in Moho depth in excess of similar to0.3 km are not required by our data. The absence of large crustal thickness variations and the general similarity in seismic structure imply that a steady state magma lens is not required to form normal East Pacific Rise type crust. Perhaps a significant portion of the lower crust is accreted in situ from a patchwork of short-lived gabbro sills or from ductile flow from a basal magma chamber as has been postulated in some recent ophiolite studies.

Magde, LS, Detrick RS, Kent GM, Harding AJ, Orcutt JA, Mutter JC, Buhl P.  1995.  Crustal and Upper-Mantle Contribution to the Axial Gravity-Anomaly at the Southern East Pacific Rise. Journal of Geophysical Research-Solid Earth. 100:3747-3766.   10.1029/94jb02869   AbstractWebsite

This paper reassesses the crustal and upper mantle contribution to the axial gravity anomaly and isostatic topography observed at two segments (14 degrees S and 17 degrees S) of the southern East Pacific Rise (SEPR) in order to determine what constraints these data place on the amount of melt present in the underlying mantle. Gravity effects due to seafloor topography and relief on the Moho (assuming a constant crustal thickness and density) overpredict the amplitude of the gravity high at the EPR by 8-10 mGal. About 70% of this mantle Bouguer anomaly (MBA) low (6-7 mGal) can be explained by a region of partial melt and elevated temperatures in the mid-to-lower crust beneath the rise axis. Compositional density reductions in the mantle due to melt extraction are shown to make a negligible contribution to the amplitude of the observed MBA. Temperature-related mantle density variations predicted by a simple, plate-driven, passive flow model with no melt retention can adequately account for the mantle contribution to the observed MRA within the experimental uncertainty (+/- 1 mGal). However, the retention of a small amount of melt (less than or equal to 1-2% at 14 degrees S;less than or equal to 4% at 17 degrees S) in a broad region (tens of kilometers wide) of upwelling mantle is also consistent with the observed gravity data given the uncertainty in crustal thermal models. The anomalous height of the narrow, topographic high at the EPR provides the strongest evidence for the existence of significant melt fractions in the underlying mantle. It is consistent with the presence of a narrow (similar to 10 km wide) partial melt conduit that extends to depths of 50-70 km with melt concentrations up to 2% higher than the surrounding mantle. Along-axis variations in mantle melt fraction that might potentially indicate focused upwelling are only marginally resolvable in the gravity data due to uncertainties,in crustal thermal models. The good correlation between along-axis variations in depth, and changes in axial volume and gravity, argue against the mantle melt conduit as being the major source of this along-axis variation. Instead, this variability can be adequately explained by a combination of along-axis changes in crustal thermal structure and/or alone-axis crustal thickness changes of a few hundred meters.

Group, EPRS.  1981.  Crustal Processes of the Mid-Ocean Ridge. Science. 213:31-40.: American Association for the Advancement of Science   10.2307/1687002   AbstractWebsite

Independent geological and geophysical investigations of the Mid-Ocean Ridge system have begun to focus on the nature of the magma chamber system underlying its central axis. Thermal models predict the existence of a steady-state chamber beneath a thin crustal lid ranging in thickness from 2 to 13 kilometers. The only aspect of the system that these models fail to account for is the extremely slow spreading rates. Seismological studies reveal the existence of a low-velocity zone beneath segments of the East Pacific Rise, which is thought to correspond to a chamber system having a half-width of approximately 5 to 10 kilometers. These estimates compare favorably with those derived separately through petrological investigations of deep-sea drilling results, various sampling programs, and field and laboratory studies of ophiolites. The chamber is thought to be wing-shaped and to remain continuously open; it is thought to be fed from the center while simultaneously solidifying at the sides as spreading carries the two halves apart. Progressive fractionation occurs by crystal settling coupled with repeated replenishment and magma mixing in an open steady-state system. Near-bottom studies reveal that the zone of extrusion above the chamber is narrow, but its eruptive history is cyclic in nature, in conflict with the predictions of a steady-state model. On-bottom gravity data at 21 ° N on the East Pacific Rise reveal a negative gravity anomaly that may be related to the uppermost part of the chamber. The anomaly is only 2 kilometers wide and 1 kilometer below the sea floor. This feature may be associated with a short-term upper magma reservoir. The cyclic volcanic activity is directly related to the active phase of hydrothermal circulation responsible for the observed negative thermal anomaly. The volume of water associated with this circulation is equal to the entire ocean volume passing through the accretion zone approximately every 8 million years. This is about 0.5 percent of the world's rivers, but the effective transport rates of elements are comparable to those of rivers in that anomalies for individual elements are frequently between 100 and 1000 times the average river composition. The degree of subsurface dilution determines the final exit temperature and composition of the hydrothermal fluids, ranging from manganese domination at extreme dilution to iron at intermediate levels to sulfide deposition when low dilution occurs. The discovery of massive sulfide deposits on the East Pacific Rise is destined to have a profound impact on our understanding of ore-forming processes. Whether it will have any economic significance remains to be seen.

Bazin, S, van Avendonk H, Harding AJ, Orcutt JA, Canales JP, Detrick RS, Grp M.  1998.  Crustal structure of the flanks of the East Pacific Rise: Implications for overlapping spreading centers. Geophysical Research Letters. 25:2213-2216.   10.1029/98gl51590   AbstractWebsite

Tomographic inversion of seismic refraction data from the flanks of the East Pacific Rise (EPR), 17 degrees 15'S, shows that the thickness of layer 2 varies by as much as 500 meters off axis. A thick layer 2 is found in crust affected by migration paths of overlapping spreading centers (OSC). However, no significant variation in crustal thickness is detected throughout the study area. The crustal structure differences documented in this paper are primarily related to this paleo-tectonic setting rather than the east-west asymmetries characteristic of this region of the southern EPR.

Tolstoy, M, Harding AJ, Orcutt JA.  1993.  Crustal Thickness on the Mid-Atlantic Ridge - Bulls-Eye Gravity-Anomalies and Focused Accretion. Science. 262:726-729.   10.1126/science.262.5134.726   AbstractWebsite

Spreading segments of the Mid-Atlantic Ridge show negative bull's-eye anomalies in the mantle Bouguer gravity field. Seismic refraction results from 33-degrees-S indicate that these anomalies can be accounted for by variations in crustal thickness along a segment. The crust is thicker in the center and thinner at the end of the spreading segment, and these changes are attributable to variations in the thickness of layer 3. The results show that accretion is focused at a slow-spreading ridge, that axial valley depth reflects the thickness of the underlying crust, and that along-axis density variations should be considered in the interpretation of gravity data.