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de Groot-Hedlin, C, Orcutt J.  2001.  T-phase observations in northern California: Acoustic to seismic coupling at a weakly elastic boundary. Pure and Applied Geophysics. 158:513-530.   10.1007/PL00001193   Abstract

Plans for a hydroacoustic network intended to monitor compliance with the CTBT call for the inclusion of five T-phase stations situated at optimal locations for the detection of seismic phases converted from ocean-borne T phases. We examine factors affecting the sensitivity of land-based stations to the seismic T phase. The acoustic to seismic coupling phenomenon is described by upslope propagation of an acoustic ray impinging at a sloping elastic wedge. We examine acoustic to seismic coupling characteristics for two cases; the first in which the shear velocity of the bottom is greater than the compressional velocity of the fluid (i.e., v(p) > v(s) > v(w)), the second is a weakly elastic solid in which vs much less than v(w) < v(p). The former is representative of velocities in solid rock, which might be encountered at volcanic islands; the latter is representative of marine sediments. For the case where v(s) > v(w), we show that acoustic energy couples primarily to shear wave energy, except at very high slope angles. We show that the weakly elastic solid (i.e., v(s) much less than v(w)) behaves nearly like a fluid bottom, with acoustic energy coupling to both P and S waves even at low slope angles. We examine converted T-wave arrivals at northern California seismic stations for two event clusters; one a series of earthquakes near the Hawaiian Islands, the other a series of nuclear tests conducted near the Tuamoto archipelago. Each cluster yielded characteristic arrivals at each station which were consistent from event to event within a cluster, but differed between clusters. The seismic T-phases consisted of both P- and S-wave arrivals, consistent with the conversion of acoustic to seismic energy at a gently sloping sediment-covered seafloor. In general, the amplitudes of the seismic T phases were highest for stations nearest the continental slope, where seafloor slopes are greatest, however noise levels decrease rapidly with increasing distance from the coastline, so that T-wave arrivals were observable at distances reaching several hundred kilometers from the coast. Signal-to-noise levels at the seismic stations are lower over the entire frequency spectrum than at the Pt. Sur hydrophone nearby, and decrease more rapidly with increasing frequency, particularly for stations furthest from the continental slope.

Blackman, DK, de Groot-Hedlin C, Harben P, Sauter A, Orcutt JA.  2004.  Testing low/very low frequency acoustic sources for basin-wide propagation in the Indian Ocean. Journal of the Acoustical Society of America. 116:2057-2066.   10.1121/1.1786711   AbstractWebsite

Low/very low frequency acoustic signals were transmitted to distant receivers in the Indian Ocean. The aim was to test methods for characterizing the hydroacoustic capability of the International Monitoring System (IMS) that discriminates for nuclear tests in the region. Several acoustic sources were deployed between Seychelles and Fremantle, Australia, and the IMS receivers comprised a network of hydrophones off Diego Garcia and Australia. Two of the three acoustic sources tested produced basin-scale propagation of impulsive signals. Single glass spheres imploded within the sound channel produced a clear signal at frequencies above similar to40 Hz, at ranges of hundreds to a thousand kilometers. Five-sphere glass implosions were recorded at ranges up to 4400 km. Near-sea surface shots from a large airgun array were recorded in several cases at ranges of hundreds to thousands of kilometers, the frequency of the highest signal-to-noise ratio arrivals varied within the 5-100 Hz band. High background noise level was a key factor at IMS stations that did not detect the airgun signals in the 5-15 Hz band. In a few cases, details of bathymetric features that are not well represented in the digital elevation model contributed to unexpected variation in relative signal levels between IMS stations. (C) 2004 Acoustical Society of America.

Bazin, S, Harding AJ, Kent GM, Orcutt JA, Tong CH, Pye JW, Singh SC, Barton PJ, Sinha MC, White RS, Hobbs RW, Van Avendonk HJA.  2001.  Three-dimensional shallow crustal emplacement at the 9 degrees 03 ' N overlapping spreading center on the East Pacific Rise: Correlations between magnetization and tomographic images. Journal of Geophysical Research-Solid Earth. 106:16101-16117.   10.1029/2001jb000371   AbstractWebsite

We report a three-dimensional (3-D) seismic reflection and tomographic survey conducted at the 9 degrees 03'N overlapping spreading center (OSC) on the East Pacific Rise to understand crustal accretion at this feature. Inversions of travel time data from 19 ocean bottom hydrophones provide a 3-D image of the shallow velocity structure beneath the nontransform offset and associated discordance zone. Seismic analysis indicates that layer 2A thickness varies between 100 and 900 in and averages 430 in throughout the study area. The heterogeneous upper crustal structure at the OSC region contrasts with the simpler symmetric structure flanking the midsegments of the East Pacific Rise. The crust affected by the OSC migration carries evidence for the complex accretion at the axial discontinuity where the overlap basin may act as a lava pond. An area of thick layer 2A covers the southern half of the overlap basin and the propagating ridge tip and shows good correlation with a high magnetization region. Comparison of the magnetic field anomaly derived from the seismic structure model with the observed sea surface magnetic anomaly suggests that a significant portion of the high magnetization can be related to magnetic source thickness variation rather than solely to the geochemistry of the volcanic rocks.

Bazin, S, Harding AJ, Kent GM, Orcutt JA, Singh SC, Tong CH, Pye JW, Barton PJ, Sinha MC, White RS, Hobbs RW, Van Avendonk HJA.  2003.  A three-dimensional study of a crustal low velocity region beneath the 9 degrees 03 ' N overlapping spreading center. Geophysical Research Letters. 30   10.1029/2002gl015137   AbstractWebsite

[1] Overlapping spreading centers (OSCs) play a key role in models of magma distribution at fast spreading ridges. To investigate the relationship between ridge-axis discontinuities and magma supply, we conducted a three-dimensional seismic reflection and tomography experiment at the 9degrees03'N OSC along the East Pacific Rise. Tomographic analysis imaged a broad mid-crustal low velocity zone (LVZ) beneath parts of the overlapper and the associated overlap basin, demonstrating that it is magmatically robust. The complementary datasets reveal a complex storage and tapping of melt: the LVZ and melt sill at either end of the overlap basin are not simply centered beneath the rise crest but are skewed inwards. The subsequent focussing of the LVZ and sill beneath the axis of the eastern limb appears to be due to melt migration toward the tip. The OSC western limb is less magmatically robust and may be in the process of dying.

Hedlin, MAH, Minster JB, Orcutt JA.  1989.  The Time Frequency-Characteristics of Quarry Blasts and Calibration Explosions Recorded in Kazakhstan, USSR. Geophysical Journal International. 99:109-121.   10.1111/j.1365-246X.1989.tb02019.x   AbstractWebsite

In this paper we consider two fundamentally different processes that can be responsible for the organization of energy in seismic coda into discrete time-independent frequency bands. One process involves the resonance of energy in low velocity horizons and the other requires the interaction of time offset wavefields produced by subevents within multiple-event mine explosions (ripple-fired quarry blasts). We examined data collected by high frequency seismometers in Kazakhstan, USSR, and observed regular time-independent spectral modulations in coda resulting from events strongly suspected to be Soviet quarry blasts, but not in the coda from single event calibration explosions detonated at similar ranges. We conclude these modulations are a source effect and due to ripple-firing. This modulation is independent of the source-receiver azimuth and we infer that the spatial array of subshots in each event must be small. We demonstrate that simple linear superposition theory can be used to reproduce effectively the spectral modulation observed in real quarry blasts. On the basis of these observations we attempt to discriminate between the two types of events using a spectral pattern-based algorithm that seeks time-independent features. We consider the detrimental effect that resonant energy in low velocity horizons can have on the successful application of our algorithm.

Caress, DW, Burnett MS, Orcutt JA.  1992.  Tomographic Image of the Axial Low-Velocity Zone at 12°50'N on the East Pacific Rise. Journal of Geophysical Research-Solid Earth. 97:9243-9263.   10.1029/92jb00287   AbstractWebsite

The 1982 MAGMA seismic refraction experiment yielded a large set of accurate P wave travel times corrected for bathymetry and anisotropy which sample the structure of the East Pacific Rise (EPR) at 12-degrees-50'N. The arrivals were recorded using ocean bottom seismographs at three sites: on axis, and at 7 km and 16 km east of the axis. We invert 2320 travel times for the two-dimensional crustal seismic velocity structure across the EPR axis, assuming that the velocity structure is invariant along strike. The travel times provide strong evidence for compressional velocity anisotropy in the upper crust corresponding to approximately 10% faster velocities for propagation parallel to the axis than perpendicular to the axis; the travel times used for the tomography are corrected for the effects of this azimuthal anisotropy. Our preferred model contains only the structure clearly required by the data (structure which is stable under excessive smoothing) and achieves a variance reduction of 81% relative to the laterally homogeneous starting model. We resolve a substantial zone beneath the rise axis in which the velocity is reduced by 0.4 to 0.7 km/s; this low-velocity zone (LVZ) is about 7 km wide and extends from a depth of about 1.5-2.0 km down to Moho at a depth of 5.5 km. The LVZ is slightly asymmetric, extending 1 km further to the east than to the west of the axis. In the shallow (<1.0 km depth) crust, a pattern of velocity variations is imaged in which velocities are high at the spreading axis, decrease between 3 km and 7 km east of the axis, and then increase again between 10 and 15 km east of the axis. We investigate the resolution of the inversion using an impulse response method; the LVZ and off-axis upper crustal variations are well resolved. In addition, the travel time data indicate that an axial high-velocity anomaly less than 2 km wide exists in the upper crust but is not resolved by the inversion. The small velocity reductions of the LVZ are consistent with hot rock containing only a small quantity of melt. These results, combined with the multichannel seismic reflection lines and expanded spread profiles from the northern EPR, suggest that the zone of high melt fraction under the spreading center is confined to a narrow, thin lens capping a broad zone of hot plutonic rocks. The upper crustal velocity reduction within 1 km of the axis reflects near-axial thickening of the extrusive layer and the later reduction probably reflects porosity increases due to near-axial tectonism; the upper crustal velocity increase beyond 15 km off axis is attributed to porosity decreases associated with hydrothermal alteration.

Burnett, MS, Caress DW, Orcutt JA.  1989.  Tomographic Image of the Magma Chamber at 12° 50'N on the East Pacific Rise. Nature. 339:206-208.   10.1038/339206a0   AbstractWebsite

THE East Pacific Rise at 12°50' N (Fig. 1) is a fast-spreading ridge with intense hydrothermal activity1, and ophiolite studies2 and thermal modelling3 indicate that this is a likely setting for a magma chamber. A recent seismic-reflection experiment4 imaged the top of the magma chamber at this site, at a depth of 1.4 km below the sea floor, and found that it is continuous for tens of kilometres along the rise axis. Here we examine a large set of accurate P-wave travel times from a detailed seismic refraction experiment at the same site5. The patterns observed in the travel times demonstrate that a zone of low seismic velocities exists beneath the rise axis throughout the region studied. The best-fitting two-dimensional structure, obtained from linear inversion of the travel times, includes an axial low-velocity zone (magma chamber) only ~6 km wide, in which velocities are depressed by more than 0.5 km s-1.

Wyatt, FK, Orcutt JA, Sasagawa G, Staudigel H, Zimmer P.  1996.  Toward In Situ Monitoring of Active Submarine Volcanoes: A Progress Report. Fifth Circum-Pacific Energy & Mineral Resources Conference transactions. ( Salisbury GP, Salisbury AC, Eds.)., Houston, Tex.: Gulf Pub. Co. Abstract
Garmany, J, Orcutt JA, Parker RL.  1979.  Travel time inversion: A geometrical approach. Journal of Geophysical Research. 84:3615-3622.   10.1029/JB084iB07p03615   AbstractWebsite

A geometric formulation of the seismic travel time problem is given based upon the use of slowness as an independent variable. Many of the difficulties in the conventional treatment (e.g., singular kernels) are thereby, avoided. Furthermore, it is shown that the inverse problem possesses an inherently linear formulation. In this formalism we are able to provide extremal solutions giving upper and lower depth bounds using linear programing. This approach has been compared with two well-known nonlinear extremal inversions. We find our technique to be easier to implement and find that it often generates superior results.

Madsen, JA, Detrick RS, Mutter JC, Buhl P, Orcutt JA.  1990.  A two- and three-dimensional Analysis of Gravity-Anomalies Associated with the East Pacific Rise at 9°N and 13°N. Journal of Geophysical Research-Solid Earth and Planets. 95:4967-4987.   10.1029/JB095iB04p04967   AbstractWebsite

We have used a unique data set collected during the 1985 East Pacific Rise multichannel seismic experiment to reevaluate the constraints that gravity data can place on the crustal structure of the East Pacific Rise (EPR). The close spacing of track lines within the two main survey areas (8°45′N-9°55′N and 12°20′-13°30′N) allowed us to perform three-dimensional analyses of high-quality gravity data (+/−1 mGal uncertainity) obtained with the Bell Aerospace BGM-3 gravity meter. our gravity modeling was enhanced by the availability of high-resolution Sea Beam bathymetric data and by independent structural constraints provided by seismic reflection and refraction data. To model the crustal and upper mantle density structure at the ridge axis, we first calculated the gravity anomalies due to the density contrasts at the water/crust and crust/mantle boundaries and the changes in density caused by the cooling of the lithosphere with age and subtracted these predictable signals from the observed free-air anomaly. The residual anomalies were then used to place constraints on the magnitude and distribution of anomalous mass at the EPR. Our results show that over 90% of the power in the observed free-air anomaly can be modeled by these predictable components of the gravity signal. The amplitude and wavelength of the small residual anomaly can be modeled by a broad region (∼20 km wide), centered on the rise axis, of slightly lower-than-normal crustal and/or upper mantle densities (−0.03 Mg m^−3). This region of anomalous mass and density variations in the underlying mantle provide the principle isostatic support for the axial topographic high. The anomalies are consistent with, but do not require, the presence of a low-density, crustal magma chamber at the rise axis. If a largely molten magma chamber exists, then the gravity data require that it be a narrow, volumetrieally small body. There are first-order differences in the distribution of anomalous mass between the two survey areas with the 9°N region generally being characterized by lower densities in the axial region. Along-axis variations in anomalous mass within each of the survey areas correlate with first-order changes in the depth and cross-sectional shape of the EPR axial high. We attribute these differences in axial morphology and the amplitude of the associated gravity anomalies to along-strike changes in the size of the crustal magma chamber, the width of the surrounding zone of cooling crustal rocks, and densities in the uppermost mantle beneath the rise axis.

Van Avendonk, HJA, Harding AJ, Orcutt JA, McClain JS.  1998.  A two-dimensional tomographic study of the Clipperton transform fault. Journal of Geophysical Research-Solid Earth. 103:17885-17899.   10.1029/98jb00904   AbstractWebsite

From the marine refraction data recorded on five instruments during the Clipperton Area Seismic Survey to investigate Compensation (CLASSIC) experiment in 1994 we construct a compressional velocity model for a 108 km long profile across the Clipperton transform. We apply a new seismic tomography code that alternates between ray tracing and linearized inversions to find a smooth seismic velocity model that fits the observed refraction travel times. The solution to the forward ray-tracing problem is a hybrid of the graph (or shortest path) method and a ray-bending method. The inversion is performed with least squares penalties on the data misfit and first derivatives of the seismic structure. Starting with a one-dimensional compressional velocity model for oceanic crust, the misfit in the normalized travel time residuals is reduced by 96%, decreasing the median travel time residual from 110 to 25 ms. The compressional velocity structure of the Clipperton transform is characterized by anomalously low velocities, about 1.0 km/s lower than average, beneath the median ridge and parallel troughs of the transform domain. The low compressional velocities can be explained by an increased porosity due to fracturing of the oceanic crust. We found crustal thicknesses of 5.6-5.9 km under the transform fault to produce the best fit of the PmP phase arrivals and Pg/Pn crossovers. Since the crust is not thin beneath the transform parallel troughs and the velocity anomaly is not confined to the median ridge, we find uplift by serpentinite diapirs unlikely as an explanation for the relief of the median ridge. A median ridge that is the result of brittle deformation due to compression across the transform domain is, however, compatible with our results. The upper crust is thicker to the north of the transform than to the south, which is likely a consequence of the contrast in temperature structure of these two spreading segments.