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Reichle, MS, Orcutt JA, Priestley K, Brune J.  1980.  The 1978 Oaxaca earthquake source mechanism: analysis from digital data. Geofisica Internacional. 17:295-301. Abstract
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Santelli, CM, Orcutt BN, Banning E, Bach W, Moyer CL, Sogin ML, Staudigel H, Edwards KJ.  2008.  Abundance and diversity of microbial life in ocean crust. Nature. 453:653-U7.   10.1038/nature06899   AbstractWebsite

Oceanic lithosphere exposed at the sea floor undergoes seawater rock alteration reactions involving the oxidation and hydration of glassy basalt. Basalt alteration reactions are theoretically capable of supplying sufficient energy for chemolithoautotrophic growth(1). Such reactions have been shown to generate microbial biomass in the laboratory(2), but field- based support for the existence of microbes that are supported by basalt alteration is lacking. Here, using quantitative polymerase chain reaction, in situ hybridization and microscopy, we demonstrate that prokaryotic cell abundances on seafloor- exposed basalts are 3 - 4 orders of magnitude greater than in overlying deep sea water. Phylogenetic analyses of basaltic lavas from the East Pacific Rise ( 9 degrees N) and around Hawaii reveal that the basalt- hosted biosphere harbours high bacterial community richness and that community membership is shared between these sites. We hypothesize that alteration reactions fuel chemolithoautotrophic microorganisms, which constitute a trophic base of the basalt habitat, with important implications for deep- sea carbon cycling and chemical exchange between basalt and sea water.

Rajasekar, A, Lu S, Moore R, Vernon F, Orcutt J, Lindquist K.  2005.  Accessing sensor data using meta data: a virtual object ring buffer framework. Proceedings of the 2nd international workshop on Data management for sensor networks. :35-42., Trondheim, Norway: ACM   10.1145/1080885.1080892   Abstract
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Albert, DG, Orcutt JA.  1990.  Acoustic Pulse-Propagation above Grassland and Snow - Comparison of Theoretical and Experimental Wave-Forms. Journal of the Acoustical Society of America. 87:93-100.   10.1121/1.398917   AbstractWebsite

Theoretical predictions are made of the effect of an absorbing ground surface on acoustic impulsive waveforms propagating in a homogeneous atmosphere for frequencies below 500 Hz. The lower frequencies of the pulse are enhanced as the effective flow resistivity of the ground surface decreases and as the propagation distance increases. The pulse waveforms and peak amplitude decay observed for propagation distances of 40 to 274 m over grassland were satisfactorily matched by calculations using an assumed effective flow resistivity of 200 kN s m^−4. Measurements over snow gave much greater amplitude decay rates, and the waveforms were radically changed in appearance, being dominated by the lower frequencies. These waveforms were satisfactorily matched only when a layered ground was incorporated into the calculations; then, an assumed surface effective flow resistivity of 20 kN s m^−4 gave good agreement with the observed waveforms and peak amplitude decay.

Orcutt, JA.  1998.  AGU's electronic publishing strategies. EOS Trans. AGU. 79:29-30. Abstract
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Adair, RG, Orcutt JA, Jordan TH.  1984.  Analysis of Ambient Seismic Noise Recorded by Downhole and Ocean-Bottom Seismometers on Deep-Sea Drilling Project Leg-78B. Initial Reports of the Deep Sea Drilling Project. 78:767-781.   10.2973/dsdp.proc.78b.112.1984   AbstractWebsite

Ambient seismic noise at depth in the ocean crust is characterized using data from the Marine Seismic System (MSS), a vertical-component, digitally recording, short-period seismograph system which was part of the borehole instrumentation deployed on Deep Sea Drilling Project Leg 78B. The instrument package rested undamped in Hole395A, 516 m sub-basement. Reliable estimates of microseismic noise levels were obtained between 0.16 and 2.2 Hz; instrument noise dominated outside this band. The observed microseismic noise was quasi-stationary on a time scale of 1 hr., but not 10. Although spectral shapes were stable, noise amplitudes grew with time over the 26-hr, observation period by 3 to 5 dB. The borehole noise levels increased concurrently with local swell height, suggesting a causal relationship. An estimate of displacement power densities obtained early in the experiment had a peak value of 4 x 10^6 nm^2/Hz at 0.21 Hz, and decreased at 80 dB/decade from 1 × 10^6 nm^2/Hz at 0.33 Hz to 1 nm^2/Hz at 1.9 Hz. Noise levels observed at the seafloor near Hole 395A were greater than those observed in the borehole by a factor which increased with frequency from 10 dB at 0.2 Hz to 28 dB at 2 Hz. This is consistent with noise propagating as a fundamental-modeStoneley wave trapped near the sediment/seawater interface. If the relationship observed between noise at and below the seafloor during Leg 78B is a general one, ocean-bottom borehole noise levels could approach those at quiet continental sites.

Tong, CH, Pye JW, Barton PJ, White RS, Sinha MC, Singh SC, Hobbs RW, Bazin S, Harding AJ, Kent GM, Orcutt JA.  2002.  Asymmetric melt sills and upper crustal construction beneath overlapping ridge segments: Implications for the development of melt sills and ridge crests. Geology. 30:83-86.   10.1130/0091-7613(2002)030<0083:amsauc>2.0.co;2   AbstractWebsite

A new three-dimensional tomographic velocity model and depth-converted reflection images of the melt sills beneath the 9degrees03'N overlapping spreading center on the East Pacific Rise show that the upper crustal construction at this ridge discontinuity is highly asymmetric with reference to the bathymetric ridge crests of the overlapping limbs. Despite the similarly curved ridge crests, the asymmetries are markedly different under the two limbs and appear to be related to the contrasting evolutionary history of the limbs. The overlap basin is closely related to the propagating eastern limb in terms of its seismic structure. By contrast, the western limb forms a distinct morphologic region that displays little structural relationship to the adjacent overlap basin and other relict basins. As the overlapping spreading center is migrating southward, the differential development of melt sills and ridge crests may be inferred from the results of this study. Ridge propagation appears to involve two major processes: the advancement of the melt sill at the ridge tip and the development of ridge-crest morphology and the neovolcanic axis to the north of the overlap basin region near the existing propagating limb. The latter process may result in the abandonment of the current neovolcanic axis, leading to the self-decapitation of the propagating limb. By contrast, the self-decapitation of the western limb is related to the receding melt sill, which lags behind the anticlockwise rotational motion of the ridge crest.

Laske, G, Markee A, Orcutt JA, Wolfe CJ, Collins JA, Solomon SC, Detrick RS, Bercovici D, Hauri EH.  2011.  Asymmetric shallow mantle structure beneath the Hawaiian Swell-evidence from Rayleigh waves recorded by the PLUME network. Geophysical Journal International. 187:1725-1742.   10.1111/j.1365-246X.2011.05238.x   AbstractWebsite

We present models of the 3-D shear velocity structure of the lithosphere and asthenosphere beneath the Hawaiian hotspot and surrounding region. The models are derived from long-period Rayleigh-wave phase velocities that were obtained from the analysis of seismic recordings collected during two year-long deployments for the Hawaiian Plume-Lithosphere Undersea Mantle Experiment. For this experiment, broad-band seismic sensors were deployed at nearly 70 seafloor sites as well as 10 sites on the Hawaiian Islands. Our seismic images result from a two-step inversion of path-averaged dispersion curves using the two-station method. The images reveal an asymmetry in shear velocity structure with respect to the island chain, most notably in the lower lithosphere at depths of 60 km and greater, and in the asthenosphere. An elongated, 100-km-wide and 300-km-long low-velocity anomaly reaches to depths of at least 140 km. At depths of 60 km and shallower, the lowest velocities are found near the northern end of the island of Hawaii. No major velocity anomalies are found to the south or southeast of Hawaii, at any depth. The low-velocity anomaly in the asthenosphere is consistent with an excess temperature of 200-250 degrees C and partial melt at the level of a few percent by volume, if we assume that compositional variations as a result of melt extraction play a minor role. We also image small-scale low-velocity anomalies within the lithosphere that may be associated with the volcanic fields surrounding the Hawaiian Islands.

Holmes, RC, Tolstoy M, Harding AJ, Orcutt JA, Morgan JP.  2010.  Australian Antarctic Discordance as a simple mantle boundary. Geophysical Research Letters. 37   10.1029/2010gl042621   AbstractWebsite

Several complex models require unique mantle conditions to explain the Australian Antarctic Discordance (AAD), an unusually deep and rugged section of the Southeast Indian Ridge (SEIR) between similar to 120 degrees-128 degrees E. Seismic evidence suggests the AAD is instead the manifestation of two contrasting mantle domains converging along its eastern edge. Variations in axial morphology and flanking topographic relief along the SEIR arise as ridge segments to the west (Indian mantle) grade into a cooler melting regime while those to the east (Pacific mantle) are more magmatically robust. Seismic refraction data show crustal thickness decreases from the west into the AAD at a rate of 0.1 km/100 km, then rapidly increases from 4.8 +/- 0.4 km to 7.3 +/- 0.2 km across the eastern border. The AAD thus appears to be the terminal end of a long-wavelength reduction in melt supply at what may be the simplest global example of a mantle boundary. Citation: Holmes, R. C., M. Tolstoy, A. J. Harding, J. A. Orcutt, and J. P. Morgan (2010), Australian Antarctic Discordance as a simple mantle boundary, Geophys. Res. Lett., 37, L09309, doi: 10.1029/2010GL042621.

Hedlin, MAH, Minster BJ, Orcutt JA.  1990.  An automatic means to discriminate between earthquakes and quarry blasts. Bulletin of the Seismological Society of America. 80:2143-2160. AbstractWebsite

In this article we discuss our efforts to use the NORESS array to discriminate between regional earthquakes and ripple-fired quarry blasts (events that involve a number of subexplosions closely grouped in space and time). The method we describe is an extension of the time versus frequency “pattern-based” discriminant proposed by Hedlin et al. (1989b). At the heart of the discriminant is the observation that ripple-fired events tend to give rise to coda dominated by prominent spectral features that are independent of time and periodic in frequency. This spectral character is generally absent from the coda produced by earthquakes and “single-event” explosions. The discriminant originally proposed by Hedlin et al. (1989b) used data collected at 250 sec−1 by single sensors in the 1987 NRDC network in Kazakhstan, U.S.S.R. We have found that despite the relatively low digitization rate provide by the NORESS array (40 sec−1) we have had good success in our efforts to discriminate between earthquakes and quarry blasts by stacking all vertical array channels to improve signal-to-noise ratios.We describe our efforts to automate the method, so that visual pattern recognition is not required, and to make it less susceptible to spurious time-independent spectral features not originating at the source. In essence, we compute a Fourier transform of the time-frequency matrix and examine the power levels representing energy that is periodic in frequency and independent of time. Since a double Fourier transform is involved, our method can be considered as an extension of “cepstral” analysis (Tribolet, 1979). We have found, however, that our approach is superior since it is cognizant of the time independence of the spectral features of interest. We use earthquakes to define what cepstral power is to be expected in the absence of ripple firing and search for events that violate this limit. The assessment of the likelihood that ripple firing occurred at the source is made automatically by the computer and is based on the extent to which the limit is violated.

Shearer, PM, Toy KM, Orcutt JA.  1988.  Axi-Symmetric Earth Models and Inner-Core Anisotropy. Nature. 333:228-232.   10.1038/333228a0   AbstractWebsite

Seismic waves passing through the middle of the Earth travel slightly faster in a N – S direction than in an E – W direction. A comparison of travel times for different P-wave phases shows that most of this anomaly is within the inner core, and that uniform anisotropy of the inner core is the most likely cause.

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Kappus, ME, Harding AJ, Orcutt JA.  1995.  A Base-Line for Upper Crustal Velocity Variations Along the East Pacific Rise at 13-Degrees-N. Journal of Geophysical Research-Solid Earth. 100:6143-6161.   10.1029/94jb02474   AbstractWebsite

A wide aperture profile of the East Pacific Rise at 13 degrees N provides data necessary to make a high-resolution seismic velocity profile of the uppermost crust along a 52-km segment of ridge crest. Automated and objective processing steps, including tau - p analysis and waveform inversion, allow the construction of models in a consistent way so that comparisons are meaningful. A continuous profile is synthesized from 70 independent one-dimensional models spaced at 750-m intervals along the ridge. The resulting seismic velocity structure of the top 500 m of crust is remarkable in its lack of variability. The main features are a thin low-velocity layer 2A at the top with a steep gradient to layer 2B. The seafloor velocity is nearly constant at 2.45 km/s +/- 3% along the entire ridge. The velocity at the top of layer 2B is 5.0 km/s +/- 10%. The depth to the 4 km/s isovelocity contour within layer 2A is 130+/-20 m from 13 degrees to 13 degrees 20'N, north of which it increases to 180 m. The increase in thickness is coincident with a deviation from axial linearity (DEVAL) rioted by both a slight change in axis depth and orientation and in geochemistry. The waveform inversion, providing more details plus velocity gradient information, shows a layer 2A with about 80 m of constant-velocity material underlain by 150 m of high velocity gradient material, putting the base of layer 2A at approximately 230 m depth south of 13 degrees 20'N and about 50 m thicker north of the DEVAL. The overall lack of variability, combined with other recent measurements of layer 2A thickness along and near the axis, indicates that the thickness of volcanic extrusives is controlled not by levels of volcanic productivity, but the dynamics of emplacement. The homogeneity along axis also provides a baseline of inherent variability in crustal structure of about 10% against which other observed variations in similar regimes can be compared.

Sandwell, DT, Gille S, Orcutt J.  2003.  Bathymetry from space is now possible. EOS Trans. AGU. 84:37,44. Abstract
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Hedlin, MAH, Minster JB, Orcutt JA.  1991.  Beam-Stack Imaging Using a Small Aperture Array. Geophysical Research Letters. 18:1771-1774.   10.1029/91gl02160   AbstractWebsite

We seek to gain a fuller understanding of seismic coda generation in the continental crust, by identifying secondary (scattering) sources illuminated by a distant primary source. We have developed a migration technique to scan seismic coda recorded by a small-aperture seismic array for phases generated locally by scattering from large heterogeneities, or topographic undulations. We use a widely distributed suite of seismic events to illuminate the local crustal volume from different directions and produce an image of the local crust. Stable apparent secondary seismic sources are observed, and interpreted as scatterers excited by the primary events.

Tolstoy, M, Vernon FL, Orcutt JA, Wyatt FK.  2002.  Breathing of the seafloor: Tidal correlations of seismicity at Axial volcano. Geology. 30:503-506.   10.1130/0091-7613(2002)030<0503:botstc>2.0.co;2   AbstractWebsite

Tidal effects on seafloor microearthquakes have been postulated, but the search has been hindered by a lack of continuous long-term data sets. Making this observation is further complicated by the need to distinguish between Earth and ocean tidal influences on the seafloor. In the summer of 1994, a small ocean-bottom seismograph array located 402 microseismic events, over a period of two months, on the summit caldera of Axial volcano on the Juan de Fuca Ridge. Harmonic tremor was also observed on all instruments, and Earth and ocean tides were recorded on tiltmeters installed within the seismometer packages. Microearthquakes show a strong correlation with tidal lows, suggesting that faulting is occurring preferentially when ocean loading is at a minimum. The harmonic tremor, interpreted as the movement of superheated fluid in cracks, also has a tidal periodicity.

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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
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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.   http://dx.doi.org/10.1016/j.future.2006.03.023   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.