bluuebunny/crossref_metadata_2025 huggingface dataset:
Crustal structure of the Ontong Java Plateau: Modeling of new gravity and existing seismic data
Authors: Gladczenko Tadeusz P. , Coffin Millard F. , Eldholm Olav
Published: February 2004

Seismic refraction and gravity‐based crustal thickness estimates of the Ontong Java oceanic plateau, the Earth's largest igneous province, differ by as much as 18 km. In an attempt to reconcile this difference we have evaluated available seismic velocity data and developed a layered crustal model which includes (1) a linear increase in velocity with depth in the Cenozoic sediments and the uppermost extrusive basement and (2) a reinterpretation of deep crustal and Moho arrivals in some deep refraction profiles. Previously, Moho had commonly been interpreted from later arrivals and in some cases constrained by precritical arrivals. However, if first arrivals at distal offsets are interpreted as Moho refractions, the maximum depth to Moho is reduced by about 10 km. Two‐dimensional gravity modeling along two transects from well‐determined oceanic crust in the Nauru Basin across the central On‐tong Java Plateau to the Lyra Basin, based on the reinterpreted crustal model, is regionally consistent with satellite altimetry derived and shipboard gravity fields yielding a 8.0 km/s Moho velocity at a depth of ∼32 km under the central plateau. The crust features a thick oceanic, three‐layer igneous crust comprising an extrusive upper crust, a 6.1 km/s middle crust and a ∼15 km thick 7.1 km/s lower crust. The total Ontong Java Plateau crustal volume is calculated at 44.4×106 km3 and 56.7×106 km3 for off‐ and on‐ridge emplacement settings, respectively. On the basis of velocities and densities we interpret the lower crust on the plateau to consist of ponded and fractionated primary picritic melts, which due to deformation and/or fluid invasion may have recrystallized to granulite facies mineral assemblages. The melts were emplaced during lithospheric breakthrough of a mantle plume in an oceanic, near‐ridge plate tectonic setting.
Three‐dimensional dislocation model for great earthquakes of the Cascadia Subduction Zone
Authors: Flück P. , Hyndman R. D. , Wang K.
Published: February 2004

There have been no historical Cascadia great subduction thrust earthquakes, but there is good recent evidence that very large earthquakes have occurred in the past and that strain is building up toward a future great event. Geodetic measurements in the coastal region from northern California to southern British Columbia show vertical and horizontal deformation as expected for the strain accumulation of a locked thrust fault. The segment of the subduction thrust that is locked and may rupture in future great events has previously been estimated through two‐dimensional (2‐D) elastic dislocation modeling of interseismic deformation geodetic data. In this study, a general 3‐D dislocation model for thrust faults has been developed that accommodates curved fault geometry and nonuniform interseismic locking or coseismic rupture. The model is based on die surface deformation due to shear faulting in an elastic half space. The 3‐D model of the Cascadia subduction zone calculates the surface deformation for a locked zone or a rupture zone of variable width along the margin. The bend in the margin trend and subducting slab end effects are included. There is a downdip transition zone between interseismic completely locked and free slip portions of the fault or between coseismic full rupture and no displacement. An initial 3‐D model based upon 2‐D dislocation models and upon thermal constraints was adjusted to optimize the fit of the predicted interseismic surface deformation to current deformation geodetic data. The best fit model has the thrust locked along the whole margin with an average locked zone width of 60 km and a transition zone width of 60 km. The two zones lie mainly offshore beneath the continental shelf and slope. The locked and transition zone widths vary smoothly along the margin, being greater off northern Washington where the thrust dip is shallow and narrower off central Oregon. Assuming that the locked plus transition zones approximate the maximum coseismic rupture area, these widths permit aMw=9 earthquake.
Cenozoic geodynamics of the Ross Sea region, Antarctica: Crustal extension, intraplate strike‐slip faulting, and tectonic inheritance
Authors: Salvini Francesco , Brancolini Giuliano , Busetti Martina , Storti Fabrizio , Mazzarini Francesco , Coren Franco
Published: February 2004

An integrated study of onshore and offshore geology of the Ross Sea region (namely, Victoria Land, north of Ross Island, and the Ross Sea, Antarctica) has revealed a complex, post‐Eocene tectonic framework. Regional NW‐SE right‐lateral, strike‐slip faults are the outstanding feature of this framework and overprint an older Mesozoic extensional event, responsible for formation of N‐S basins in the Ross Sea. The Cenozoic framework includes kinematic deformation and reactivation along the NW‐SE faults, including formation of pull‐apart basins, both positive and negative flower structures, and push‐up ridges. N‐S extensional faults are well developed between NW‐SE faults and indicate E‐W extension during the Cenozoic, produced by the NW‐SE right‐lateral strike‐slip motion together with regional crustal extension. NNW‐SSE compression, induced by the right‐lateral, strike‐slip kinematics, is indicated by locally inverted NE‐SW faults and basins. The evolution, geometry, and location of the Rennick Graben and the Lanterman Range fit well into this model. Variations in the deformational style across the region can be linked to corresponding variations in the bulk crustal rheology, from brittle behavior in the west, to ductile deformation (at subseismic‐scale resolution) near the Eastern Basin. A semibrittle region that favors N‐S clustering of Cenozoic magmatic activity lies in between. In this region, Cenozoic volcanoes develop at the intersections of the NW‐SE and the major N‐S faults. The NW‐SE faults cut almost continually from the Ross Sea to East Antarctica through lithospheric sectors with different rheology and thickness. At least two of the NW‐SE faults correspond to older Paleozoic terrane boundaries in northern Victoria Land. The NW‐SE faults link in the Southern Ocean with major transform faults related to the plate motions of Australia, New Zealand, and Antarctica.
A new analysis of the great 1970 Colombia earthquake and its isotropic component
Authors: Russakoff Daniel , Ekström Göran , Tromp Jeroen
Published: February 2004

Several methods of low‐frequency analysis are used to determine the source mechanism of the July 31, 1970, Great Colombia earthquake and to reexamine two controversial results previously obtained for this event. In a classic study of the earthquake, Dziewonski and Gilbert [1974] concluded (1) that low‐frequency (f<4 mHz) mode spectrum observations require an isotropic compression at the source of a magnitude similar to the deviatoric moment release and (2) that the isotropic component of moment release precedes the short‐period onset of the earthquake by 80 s. The original data set collected by Dziewonski and Gilbert is used in a reanalysis of the Great Colombia earthquake, taking advantage of the theoretical and computational advances that have been made during the past 20 years in predicting normal mode spectra. In particular, the splitting and coupling of modes induced by rotation, ellipticity, and three‐dimensional (3‐D) mantle structure are considered. When splitting and coupling are taken into account in the analysis of the observed spectra, the isotropic component is reduced to an insignificant size, and the data are well explained by a purely deviatoric source which does not require the initiation of moment release before the short‐period onset time of the earthquake. When the effects of splitting and coupling are ignored in the calculation, the results are instead similar to those of Dziewonski and Gilbert [1974]. Experiments with synthetic spectra generated for the deviatoric part of the moment tensor confirm that the distortion of the low‐frequency end of the normal mode spectrum caused by splitting and coupling is incorrectly interpreted as an isotropic component when these effects are ignored in the inverse problem. The detailed effects of modeling errors of this kind on the retrieved source parameters will, in general, depend both on the source mechanism and the distribution of observing stations.
Toward an experimental study of deep mantle rheology: A new multianvil sample assembly for deformation studies under high pressures and temperatures
Authors: Karato Shun‐ichiro , Rubie David C.
Published: February 2004

A new sample assembly for the multianvil high‐pressure apparatus has been developed that results in high‐strain plastic deformation at high pressures and temperatures with minimal deformation during the initial pressurization stage. In this assembly, the sample is a thin disk which is sandwiched between two pistons and oriented at 45° to their long axis. The sample and pistons are surrounded by a Pt tube and a polycrystalline MgO cylinder. Upon pressurization, a uniaxial stress develops because of the anisotropy of mechanical properties. Deformation during initial pressurization, which occurred in previous studies, is minimized by locating soft materials at the ends of the pistons and by the simple shear deformation geometry (as opposed to uniaxial compression) that allows sliding at the sample‐piston interfaces at low pressures. Large plastic strains, up to ∼100% shear strain, have been achieved in (Mg,Fe)2SiO4 phases at high pressures (up to 15 GPa) and high temperatures (up to 1900 K). A theoretical analysis has been made to evaluate the relative contributions to sample deformation from the relaxation of elastic strain in the sample column and from continuing advancement of the multianvil guide blocks. The observed dependence of strain on time, pressure and temperature suggests that deformation in the present experiments occurred mostly as a relaxation process rather than at a constant strain rate caused by continuous piston movement. A comparison of the creep strength of olivine inferred from the strain relaxation data at ∼15 GPa and ∼1900 K with low‐pressure data provides an estimate of the activation volume for creep of V* = 14 (±1) × 10−6 m3 mol−1. The theoretical analysis shows that constant strain rate deformation could result from the advancement of the guide blocks after complete stress relaxation, although the total strain will be much less than that attained in the relaxation process. Possible applications of this technique to studies of high‐pressure rheology and deformation microstructures in high‐pressure minerals are discussed, and strategies for future deformation experiments under high pressures and temperatures are proposed.
Three‐dimensional velocity structure and earthquake locations beneath the northern Tien Shan of Kyrgyzstan, central Asia
Authors: Ghose Sujoy , Hamburger Michael W. , Virieux Jean
Published: February 2004

We used the arrival times of local earthquakes and quarry blasts recorded by the Kyrgyzstan Broadband Network (KNET) to obtain three‐dimensional (3‐D) P and S wave velocity models of the upper crust beneath an actively deforming mountain front and its associated foreland in the Kyrgyz Tien Shan. The continuous velocity models, described by cubic B spline interpolation of the squared slowness over a regular 3‐D grid, were computed by simultaneous inversion of hypocenter and medium parameters. Exact ray tracing was done in the smooth 3‐D medium by shooting rays from the sources to the stations by an analytical perturbation method based on the paraxial ray theory. The deduced large, sparse, linear system was solved using the damped, iterative, least squares algorithm LSQR. The stability and resolution of the result was qualitatively tested by two synthetic tests: the spike test and the checkerboard resolution test. We found that the models are well resolved up to a depth of ∼27 km for most parts of our image domain. The P and S wave velocity models are consistent with each other and provide evidence for marked heterogeneity in the upper crustal structure beneath the northern Tien Shan. At shallower depths (<7 km) the sediment‐filled foreland is imaged as a relatively lower velocity feature compared to the mountains, which are cored by crystalline basement rocks. In contrast, at midcrustal depths the mountains are underlain by relatively lower velocity materials compared to the foreland. A distinct contrast in velocity structure is also observed between the eastern and western parts of the Kyrgyz Range at midcrustal depths, with the presence of relatively higher velocities toward the east. The seismicity is concentrated near the traces of major active faults and extends deeper beneath the foreland compared to the mountains. The regional compression in the Tien Shan is accommodated along a series of high‐angle reverse faults distributed throughout the orogenic system that extend from the surface down to midcrustal depths. The range‐bounding fault zone can be identified by a sharp lateral gradient in seismic velocities with a pronounced southward dip combined with a zone of seismicity that also deepens to the south and reverse fault source mechanisms from moderate‐sized events. A pronounced low‐velocity zone (LVZ) is imaged in the P wave field, at midcrustal depths, beneath the western part of the Kyrgyz Range. This LVZ is presumably correctable with reported high‐conductivity zones in this region that have been proposed to mark active fault zones along which fluid migration occurs. The location of the LVZ, which is closely coincident with the depth of maximum earthquake generation, might imply that it is a crustal decoupling zone at the brittle‐ductile transition.
On the volcanic waveguide
Authors: Garcés Milton A.
Published: February 2004

An analytical solution for the sound field in a resonant magma conduit with depth‐dependent magma properties provides a versatile interpretative tool for the analysis of seismoacoustic signals generated by fluid processes in active volcanoes. Expressions are given for the acoustic field in the atmosphere and the seismic field in the ground radiated through the magma conduit vent and walls, respectively. A new source model allows the resonance in the conduit to be excited by the vertical fluid velocity at the bottom of the conduit. This source model represents acoustic excitation induced by low‐frequency fluid oscillations or large volume fluctuations originating at depth. The magma conduit is modeled as a three‐section duct, with each section having a different density, sound speed, viscosity, and dimension. The modal structure of the coupled resonant system is retained as the sound field in the conduit propagates into the surrounding bedrock and the overlying atmosphere. The source‐time function permits estimates of the total mass injected or removed from the magmatic system, and the source spectrum acts as a band‐pass filter on the resonant modes of the conduit. Theoretical or empirical source functions can be used to drive the seismoacoustic wave field, and the amplitude of the source velocity may be constrained by the ballistics of pyroclasts and the height of ash plumes. The acoustic properties of the melt may be predicted from the temperature, composition, and flow conditions of the magma as a function of depth. Comparisons of synthetic waveforms and spectra with those of seismic and acoustic signals recorded on active volcanoes permit estimates of the conduit geometry, density, sound speed, and attenuation properties of the melt, and characterize the temporal and spectral features of the source function. The ability to extract this information accurately and efficiently from seismic and acoustic data streams may permit near real time modeling of fluid‐driven processes in active volcanoes.
Seismic reflection images beneath Puget Sound, western Washington State: The Puget Lowland thrust sheet hypothesis
Authors: Pratt Thomas L. , Johnson Samuel , Potter Christopher , Stephenson William , Finn Carol
Published: February 2004

Seismic reflection data show that the densely populated Puget Lowland of western Washington state is underlain by subhorizontal Paleogene and Neogene sedimentary rocks deformed by west and northwest trending faults and folds. From south to north beneath the Lowland, features seen on the seismic data include: the horizontally‐stratified, 3.5 km thick Tacoma sedimentary basin; the Seattle uplift with south dipping (∼20°) strata on its south flank and steeply (50° to 90°) north dipping strata and the west‐trending Seattle fault on its north flank; the 7.5 km thick, northward‐thinning Seattle sedimentary basin; the antiformal Kingston arch; and the northwest trending, transpressional Southern Whidbey Island fault zone (SWIF). Interpreting the uplifts as fault‐bend and fault‐propagation folds leads to the hypothesis that the Puget Lowland lies on a north directed thrust sheet. The base of the thrust sheet may lie at 14 to 20 km depth within or at the base of a thick block of basaltic Crescent Formation; its edges may be right‐lateral strike‐slip faults along the base of the Cascade Range on the east and the Olympic Mountains on the west. Our model suggests that the Seattle fault has a long‐term slip rate of about 0.25 mm/year and is large enough to generate aM7.6 to 7.7 earthquake.
Regional waveform calibration in the Pamir‐Hindu Kush region
Authors: Zhu Lupei , Helmberger Donald V. , Saikia Chandan K. , Woods Bradley B.
Published: February 2004

Twelve moderate‐magnitude earthquakes (mb 4–5.5) in the Pamir‐Hindu Kush region are investigated to determine their focal mechanisms and to relocate them using their regional waveform records at two broadband arrays, the Kyrgyzstan Regional Network (KNET), and the 1992 Pakistan Himalayas seismic experiment array (PAKH) in northern Pakistan. We use the “cut‐and‐paste” source estimation technique to invert the whole broadband waveforms for mechanisms and depths, assuming a one‐dimensional velocity model developed for the adjacent Tibetan plateau. For several large events the source mechanisms obtained agree with those available from the Harvard centroid moment tensor (CMT) solutions. An advantage of using regional broadband waveforms is that focal depths can be better constrained either from amplitude ratios of Pnl to surface waves for crustal events or from time separation between the direct P and the shear‐coupled P wave (sPn + sPmP) for mantle events. All the crustal events are relocated at shallower depths compared with their International Seismological Centre bulletin or Harvard CMT depths. After the focal depths are established, the events are then relocated horizontally using their first‐arrival times. Only minor offsets in epicentral location are found for all mantle events and the bigger crustal events, while rather large offsets (up to 30 km) occur for the smaller crustal events. We also tested the performance of waveform inversion using only two broadband stations, one from the KNET array in the north of the region and one from the PAKH array in the south. We found that this geometry is adequate for determining focal depths and mechanisms of moderate size earthquakes in the Pamir‐Hindu Kush region.
Very long baseline interferometry antenna axis offset and intersection determination using GPS
Authors: Combrinck W. L. , Merry C. L.
Published: February 2004

A novel method to determine the axis intersection and offset of the Hartebeesthoek Radio Astronomy Observatory (HartRAO) radio telescope using GPS is described. This method allows the very long baseline interferometry (VLBI), satellite laser ranging (SLR) and Global Positioning System (GPS) reference frames to be tied to a single point.
Glaciation‐induced perturbations in the Earth's rotation: A new appraisal
Authors: Mitrovica Jerry X. , Milne Glenn A.
Published: February 2004

We revisit the theory and predictions of glaciation‐induced perturbations in both the rotation rate of the Earth (or
) and the wander of the rotation pole with respect to the surface geography. The two most common theoretical formulations asociated with polar wander driven by the surface leading of a viscoelastic Earth [e.g., Sabadini et al., 1982; Wu and Peltier, 1984] appear to disagree (by one) on the number of rotational normal modes associated with a surface gravitational response having N normal modes. We show, using a generalized theory, that the two formalisms yield essentially equivalent predictions. We next consider a benchmark comparison of recent predictions of present‐day polar wander speed due to glacial isostatic adjustment (henceforth GIA). These comparisons confirm the validity of certain previous analyses [e.g., Spada et al., 1992a]. They also reveal that predictions of polar wander speed can be sensitive to an assumption of incompressibility. Previous studies have noted that predictions of
/rotation rate, as a function of lower mantle viscosity, differ significantly in form from analogous predictions of polar wander speed. We investigate these differences by considering the contribution to the predicted anomalies from the individual normal modes of relaxation. We find that the norma/mode structure of the
/rotation rate predictions is fundamentally different from that which characterizes the polar wander response, and this difference reflects the distinct physical mechanisms which govern these anomalies. Finally, using a large suite of forward computations, as well as the calculation of a set of Frechet kernels, we investigate the sensitivity of our predictions to variations in the viscoelastic Earth model. We find, for example, that a previously noted sensitivity of polar wander speed predictions to variations in lithospheric thickness is only valid for Earth models with relatively weak lower mantle viscosities (< 5 × 1021 Pa s).
Modeling the formation of a half graben using realistic upper crustal rheology
Authors: Bott Martin H. P.
Published: February 2004

The evolution of a half graben in a strong upper crust with a Byerlee strength envelope, underlain by an inviscid substratum, has been modeled using elasto‐viscoplastic finite element analysis. The nonlithostatic stresses, regions of failure, and flexure profiles have been computed as the layer is extended by increments. The weaker upper part of the layer fails on stretching, and more extensive regions of tensional failure extend progressively downward beneath the downbending half graben and upward from the bottom of the layer beneath the footwall uplift. Compressional failure occurs locally due to bending or unbending. Plastic yielding progressively reduces the widths of the half graben and footwall uplift as extension proceeds, as the outer hinge line of the half graben migrates toward the fault. The plastic deformation reduces the effective elastic thickness of the layer by up to 67%, helping to explain the anomalously low observed values associated with normal faulting. Throughgoing failure in tension eventually occurs beneath the footwall uplift near the fault, when subsidence slows down and ceases. The strength of the upper crust thus limits the amount of subsidence that can occur. For an 8 km thick layer and sediment infill 400 kg/m3 less dense than the basement, the half graben width decreases from over 80 km at the onset of extension to 42 km when the subsidence has reached its limit of 2.3 km; the equivalent elastic layer thickness is then reduced to 3.3 km. Thicker layers have correspondingly greater widths and limiting depths. Less dense sediments result in a narrower half graben and smaller limiting depth.
Composition and temperature of Earth's inner core
Authors: Stixrude Lars , Wasserman Evgeny , Cohen Ronald E.
Published: February 2004

We compare a theoretical prediction of the equation of state of iron at high pressures and temperatures to the properties of the Earth's inner core. The theoretical result is based on a first principles treatment of the static pressure and the pressure due to thermal excitation of electrons and an approximate ab initio (cell model) treatment of the Vibrational pressure. The density of iron is found to be greater than that of the inner core even for unrealistically high temperatures of 8000 K. The isentropic bulk modulus of iron is found to be consistent with that of the inner core over a wide range of temperatures (4000–8000 K). We conclude on the basis of these comparisons that the inner core contains a substantial fraction of elements lighter than iron. Assuming ideal solutions, we find the temperature and light component mass fraction required to simultaneously match the density and bulk modulus of the inner core. For a temperature of 7000 K, 1 wt % O as FeO, satisfies the inner core observations. The temperature and mass fraction of S required depend on whether S is included as pyrite (2 wt % S, 5500 K) or as Fe0.9S (>8 wt % S, <3500 K). On the basis of this result and empirical mixing rules for Fe‐O solutions, we argue that Fe‐light element solid solutions at inner core conditions may be significantly nonideal. We derive expressions for the properties of nonideal multicomponent solutions that are valid in the limit of small amounts of impurities. These lead to general results for the properties of the alloy fraction that are required by comparisons of our equation of state of iron with seismological models.
Local wave propagation near the San Jacinto Fault Zone, southern California: Observations from a three‐component seismic array
Authors: Wagner Gregory S.
Published: February 2004

We present results from a study conducted to characterize the P and S wave trains and codas for local earthquakes observed near the San Jacinto fault zone, southern California. The data were recorded by a small‐aperture, three‐component seismic array and analyzed using a statistical array processing algorithm that provides estimates of the propagation direction, apparent wavenumber, and polarization characteristics for plane wave arrivals observed at the array. The P codas for these local earthquake data are similar in that each is composed of predominantly P waves traveling in a direction subparallel to the path of the direct P arrivals. These P coda exhibit a character consistent with propagation in a waveguide as opposed to propagation in a medium with randomly distributed isotropic scattering sources. The S codas differ from the P codas in two dramatic ways: (1) the S codas are composed of predominantly S waves, and (2) the S coda waves arrive from more random/spatially uniform directions. The dramatic differences between the P and S codas provide compelling evidence that P and S coda are the product of distinctly different scattering mechanisms and therefore that the P and S codas provide information about different source characteristics and/or structural features. A third important observation concerning the S coda is that the direction‐of‐arrival spectra for the S codas differ from event to event considerably more so than do those of the P codas. This observation is at odds with the conventional wisdom and suggests that the character of the S coda reflects a potentially complex interaction among several factors including, but not limited to, the source depth, the source distance, and the source radiation pattern.
Plate tectonic reconstructions of the Juan Fernandez microplate: Transformation from internal shear to rigid rotation
Authors: Bird Robert T. , Naar David F. , Larson Roger L. , Searle Roger C. , Scotese Christopher R.
Published: February 2004

Side‐scan sonar, swath bathymetry and magnetic anomaly date define a detailed, three‐phase history of the Juan Fernandez microplate. The ∼6 m.y. history is presented in a series of discrete time steps to document the growth and reorganization of propagating spreading centers and structural feature, and microplate kinematic evolution. Prior to the microplate, the East Pacific Rise at the Pacific‐Antarctic‐Nazca triple junction was offset by a long transform fault zone, likely the fastest slipping transform on Earth at anomaly 3A time. The microplate originated from an intratransform setting between anomaly 3A (5.95 Ma) and anomaly 3 (5.24 Ma) time. Its early development resembled a large propagating rift system, and microplate core structures suggest the entire offset zone may have experienced deformation. Fast propagation of the East Ridge dominated microplate growth until ∼2.6–1.9 Ma when seafloor spreading became the dominant growth process. The microplate rotation rate increased threefold (from 9 to 29° m.y.−1 average) from phase 1 (4.2–2.6 Ma) to phase 2 (2.6–1.1 Ma) of the microplate's history, then reduced fourfold (29 to 7° m.y. −1 average; phase 3, 1.1 Ma to Present). Phases 2 and 3 of the microplate's rotational history support the edge‐driven model for microplate kinematics of Schouten and others to a good approximation. The Pacific‐Nazca shear couple drove microplate rotation during phase 2, but development of the southeastern boundary enabled a transfer to the Nazca‐Antarctic plate pair (phase 3). West Ridge propagation and reorganization of the south‐western boundary may have decoupled the Pacific plate from the microplate, thus facilitating the shear couple transfer. The recent continued deceleration in microplate rotation rate and westward migration of the Pacific‐Antarctic ridge axis relative to the microplate may indicate that the process of microplate “death” has begun. We speculate that the Juan Fernandez microplate will accrete to the Antarctic plate, perhaps within the next million years, like the extinct Friday microplate has done, thereby accomplishing another northward migration of the Pacific‐Antarctic‐Nazca triple junction. Our reconstructions illustrate that the Easter and Juan Fernandez microplates are more similar than previously thought in terms of their origin, growth, rift propagation, ridge segmentation and overall tectonic evolution.
Investigating discrepancies among measurements of traveling and standing wave attenuation
Authors: Durek Joseph J. , Ekström Göran
Published: February 2004

In studies of long‐period (150 s to 300 s) seismic wave attenuation, the mean decay reported for surface waves is generally 10 to 15% greater (lower quality factor Q) than that measured from equivalent fundamental modes of oscillation. Towards reconciling this inconsistency, we first establish through forward modeling that observed waveforms are more compatible with the rapid rate of decay inferred from traveling wave analyses. Second, we apply traveling and standing wave measurement techniques to a common set of predicted and observed seismograms and compare the inferred levels of surface wave attenuation. We demonstrate that the discrepancy documented in the literature is a consequence of the measurement techniques applied and not an effect of the individual data sets used in previous studies. Experiments with synthetically generated seismograms illustrate that the presence of noise elevates the mean Q inferred for the standing wave methods considered. This bias is related to the nonrandom influence of noise on the phase spectrum. However, for realistic noise levels, the experiment fails to reproduce the magnitude of the discrepancy present in the literature or observed in this study. The synthetic experiments motivate a new measurement approach based on the decrease of spectral power with time that incorporates the behavior of noise into the parameterization. When this technique is applied to data, the mean observed Q decreases from values consistent with normal mode studies to values consistent with traveling surface wave analysis. We thus conclude that noise is the dominant cause of the discrepancy.
Regional variations of seismic attenuation ofLgwaves in southern Mexico
Authors: Domínguez Tonatiuh , Rebollar Cecilio J. , Castro Raúl R.
Published: February 2004

Attenuation ofLgwaves is estimated using 61 events located in the subduction zone of the Middle American trench and recorded by the seismic network of Laguna Verde, Veracruz, Mexico. We estimate the anelastic attenuation coefficient γ ofLgwaves or, equivalently,QLgby calculating the rate of acceleration spectra decay with distance. We consider paths from two regions: Guerrero‐Veracruz (NE‐SW direction) and Oaxaca‐Veracruz (North‐South direction). Assuming a frequency dependence of γ of the form γ(ƒ) = γ0ƒη, we find that η=0.175±0.05 for both regions and that γ0was lower for paths from Guerrero to Veracruz (γ0=0.0071±0.002) than for paths from Oaxaca to Veracruz (γ0=0.0161±0.003) in the frequency range from 2 to 7 Hz. In terms of the quality factorQLg, values ofQLg= 134±30ƒ0.83±0.2andQLg=59±10ƒ0.81±0.2were found for the Guerrero‐Veracruz and the Oaxaca‐Veracruz paths, respectively. This difference in attenuation may be due to the state of stresses that prevails in both regions including the density and fluid content of fractures, which are attenuation mechanisms also suggested for other regions [Mitchell,1995].
Formation of a metastable olivine wedge in a descending slab
Authors: Devaux Jean Philippe , Schubert Gerald , Anderson Charles
Published: February 2004

We present a thermal model of a descending slab in which the transformation of olivine to spinel is controlled by pressure‐ and temperature‐dependent reaction kinetics. Two different formulations of the kinetics are considered with the main discriminant being the temperature range over which olivine converts to spinel at pressures of about 15 GPa (about 500°–515°C and 560°–650°C). We use a finite element method to solve the coupled heat conduction (perpendicular and parallel to the dip of the slab) and kinetics equations, and we include the latent heat of the phase transformation. Latent heat release together with heat conduction parallel to the dip of the slab reduces significantly the length of the metastable olivine wedge and results in a very thin (≲5 km) two‐phase region. We employ the thermal parameter υτ sin δ (υ is the velocity of the descending slab, τ is the age of the slab, and δ is the dip of the slab to the horizontal) to interpret the results for the length of the metastable wedge. For values of the thermal parameter smaller than about 4000 and 7000 km, depending on the model of the kinetics, no metastable olivine wedge exists (the critical value of 7000 km for the thermal parameter corresponds to the kinetics model with the lowest transition temperature range). The length of the metastable olivine wedge is also found to be very sensitive to the model of the kinetics and to the effects of adiabatic heating. If the occurrence of deep earthquakes is related to the transformation of metastable olivine to spinel, then data on earthquake depth versus thermal parameter require that the onset of the reaction takes place at temperatures of about 550°–575°C. In this case the slab thermal parameter must be larger than 10,000 km for the metastable olivine wedge to extend down to 660 km depth. But deep earthquakes occur near 660 km depth in slabs with thermal parameter as small as about 5000 km (South America, for example). Either some deep earthquakes are unrelated to olivine metastability or our knowledge of olivine‐spinel reaction kinetics is incomplete.
Partial advection of equidensity surfaces: A solution for the dynamic topography problem?
Authors: Le Stunff Yves , Richard Yanick
Published: February 2004

Although one‐layer dynamic models of the Earth's mantle have successfully explained the geoid, they generate a surface dynamic topography that seems too large relative to geological observations. In this study, we hypothesize the possibility of partial advection of mantle equidensity surfaces by vertical motion induced by “driving” loads. These large‐scale “flow‐dependent” loads would greatly reduce the dynamic topography amplitude, while preserving a good fit to the observed geoid. Various physical processes related to nonequilibrium phase changes or to the existence of chemical heterogeneity in the mantle could justify a partial advection of the mean density. In this paper, we simply consider the flow‐dependent loads as proportional to the vertical flow velocity. Two density mantle models are considered, one from subduction reconstruction [Ricard et al., 1993] and one from seismic tomography [Li and Romanowicz, 1995]. We show that a very moderate entrainment (a few kilometers) of the equidensity surfaces in the transition zone is sufficient to reduce dynamic topography amplitude by a factor of 2 or 3. The seismic velocity signal associated with this entrainment would be hidden by the signal of thermal origin. Using this new hypothesis, we compute sea level changes associated with epeirogeny for the Cretaceous, Paleocene, and Oligocene periods. The amplitude and phase of these changes are in fairly good agreement with geological hypsometric curves. Our results suggest that not only the thermodynamics, but also the kinetics of mineralogical phase changes in the transition zone are of crucial importance.
Lithospheric and upper mantle structure of southern Tibet from a seismological passive source experiment
Authors: Yuan Xiaohui , Ni James , Kind Rainer , Mechie James , Sandvol Eric
Published: February 2004

Fifteen seismic stations were operated with about 20‐km spacing in southern Tibet across the Zangbo suture (the collision zone between India and Asia) between May and October 1994 as part of the International Deep Profiling of Tibet and the Himalaya project (INDEPTH II) for wide‐angle recording of the controlled source experiment and for passive earthquake recording. In addition, a dense deployment (4‐km spacing) of stations within the German Depth Profiling of Tibet and the Himalayas (GEDEPTH) project also recorded a number of teleseismic earthquakes. The third data source used in this study is the records of the permanent broadband station at Lhasa. Teleseismic records have been obtained in sufficient quantity and quality to derive an image of the structure of the lithosphere and upper mantle from P‐to‐S converted phases. Important results are as follows. The Moho at 70–80 km and a second discontinuity at 50–60 km depth are observed over the entire profile south and north of the Zangbo suture. The data from the GEDEPTH dense array enable the detection of inclined structures penetrating the crust at the Zangbo suture. A pronounced low‐velocity zone exists north of the Zangbo suture at about 10–20 km depth. The locations of the upper mantle discontinuities at 410 and 660 km depth are in agreement with the global reference model IASP91 [Kennett, 1991] over a large region of the Himalaya and southern Tibet.
Crustal deformation measurements using repeat‐pass JERS 1 synthetic aperture radar interferometry near the Izu Peninsula, Japan
Authors: Fujiwara Satoshi , Rosen Paul A. , Tobita Mikio , Murakami Makoto
Published: February 2004

We have examined the precision of interferometric synthetic aperture radar (SAR) measurements of surface deformation of the Earth using 24‐cm wavelength data acquired by the Japanese Earth Resources Satellite 1 (JERS 1) spacecraft, over the Izu Peninsula, Japan. Radar images acquired from 1993 to 1994 were made into 11 independent interferograms. Apparent water vapor signatures with equivalent path delay up to 16 cm preclude reliable estimates of small deformation from only one interferogram. Averaging of interferograms improves the measurement, such that residuals between precise leveling data and corresponding radar observations are less than 1 cm RMS. There is a clear correlation between cloud cover extent on a given day and the spectral power of the water vapor signature in an interferogram formed from the corresponding day's SAR image. Also, several water vapor features associated with local topography are consistent with accumulation of moisture on the windward side of an on‐shore flow. Analysis shows that decorrelation in mixed conifer and deciduous forest is a weak function of the time between observations, imposing a roughly constant level of additional decorrelation relative to scatter‐stable urban areas. Furthermore, in high‐relief areas, correlation is high when the baseline is tens of meters but drops rapidly with increasing baseline length. With strict control of the interferometric baseline length, adequate correlation in interferograms over very long time periods is possible even in forested, high relief regions. Observed inflation over Ito city is consistent with a localized magma source at depth, suggesting a different mechanism from the previous episode in 1989.
Electrical conductivity of magnesiowüstite/perovskite produced by laser heating of synthetic olivine in the diamond anvil cell
Authors: Duba Al , Peyronneau Jean , Visocekas Fabrice , Poirier Jean‐Paul
Published: February 2004

Samples prepared from synthetic single crystals of olivine have electrical conductivity that differs significantly from samples prepared, under similar conditions, from San Carlos olivine. In particular, the oxygen fugacity with which the sample was treated before being loaded into the laser‐heated diamond anvil cell (LHDAC) has a large effect on the electrical conductivity of magnesiowüstite/perovskite assemblages synthesized from San Carlos olivine and no significant effect on those synthesized from the synthetic olivines. This effect is likely due to the presence of nickel in San Carlos olivine. However, because the main effect of nickel in olivine is to diminish the extent of the redox stability field of olivine, this points to a possible effect of the internal oxygen fugacity of the LHDAC on the assemblages produced during transformation. Electron microscopy examination of samples transformed to magnesiowüstite/perovskite assemblages in the LHDAC under identical conditions detected substantially more metallic iron in assemblages produced from San Carlos olivine. Analysis of the experimental procedure employed for transformation of silicates in the LHDAC indicates that oxygen from air trapped in the porous powder during pressurization can provide a condition of constant oxygen fugacity in the material. In the temperature gradient that exists during laser heating, a condition of constant oxygen fugacity can lead to very complicated phase assemblages, varying from highly oxidized to highly reduced, at least for materials containing transition metals.
Least squares restoration of Tertiary thrust sheets in map view, Tajik depression, central Asia
Authors: Bourgeois Olivier , Cobbold Peter Robert , Rouby Delphine , Thomas Jean‐Charles , Shein Vassili
Published: February 2004

The Tajik depression, located west of the Pamirs and south of the Tien Shan, is a compressional intermontane basin, bounded by basement overthrusts and filled with Mesozoic and Cenozoic sediments. The internal structure is typical of a thin‐skinned fold and thrust belt. Kinematic data available in the literature suggest that indentation of the Pamirs into Asia during the Cenozoic collision of India and Asia has been accommodated in various ways within the depression, including westward extrusion, thickening combined with wrenching along N‐S folds and thrusts, and counterclockwise rotations. These various deformation processes can be analyzed and quantified by reconstruction of the predeformed state of the depression. However, the combination of thrusting, wrenching and block rotations implies a nonplane deformation, which cannot be restored properly using balanced cross sections alone. We have therefore developed a numerical method for restoration of stratigraphic surfaces, designed for regions of nonplane compressional tectonics. The deformed region is represented in map view as a mosaic of fault‐bounded blocks, overlapping each other along the faults. Blocks are separately unfolded and then numerically packed together by least squares minimization of overlaps, yielding fields of finite horizontal translations and rotations about vertical axes. To analyze the deformation postdating the collision of India and Asia, we have restored a stratigraphic surface at the base of the Cenozoic. First, in order to test the numerical method, we restored a map that had previously been restored by a purely manual method. Restoration of a second map, drawn from newly available subsurface data, leads to geometrical inconsistencies: overlaps and gaps, which cannot be reduced, remain in the restored state. After correction of these inconsistencies, restoration yields a complex mode of deformation for the depression. Individual thrust slices have undergone counterclockwise rotations about vertical axes, the magnitudes increasing from west to east, up to a maximum of 40° near the Pamirs. Horizontal shortening is 150 km (35%) in the center of the depression and 240 km (85%) in the northeastern part, between the Pamirs and the Tien Shan. Strike slip motions, commonly not revealed by balancing cross sections, are associated with thrusting on faults striking N‐S. Independent paleomagnetic data and slip directions on small‐scale faults provide positive checks on the results of our restoration.
The shallow ice approximation for anisotropic ice: Formulation and limits
Authors: Mangeney Anne , Califano Francesco
Published: February 2004

Ice sheet flow modeling is generally based on the shallow ice approximation (SIA) developed for isotropic ice. We extend this approximation to anisotropic ice and check its validity for both the isotropic and anisotropic cases by comparing the results of up to second‐order SIA with those obtained from a model solving the full set of mechanical equations. The theory is developed for a constitutive relation describing Newtonian and non‐Newtonian behavior, but numerical results are shown only for a Newtonian behavior. The results are compared for plane flow of isothermal ice under steady state conditions. SIA gives an excellent representation of the Newtonian flow of an isotropic ice sheet and a good description for the anisotropic case. The zero‐order approximation is sufficient to describe ice flow precisely over flat bedrock, for the anisotropic and isotropic cases, even close to an ice divide. For uneven bedrock, the second‐order SIA gives excellent results for isotropic ice and acceptable results for anisotropic ice. Finally, we quantify the error introduced by using an enhancement factor to represent the anisotropy when longitudinal stresses are taken into account.