bluuebunny/crossref_metadata_2025 huggingface dataset:
Acoustic properties of a crack containing magmatic or hydrothermal fluids
Authors: Kumagai Hiroyuki , Chouet Bernard A.
Published: February 2004

We estimate the acoustic properties of a crack containing magmatic or hydrothermal fluids to quantify the source properties of long‐period (LP) events observed in volcanic areas assuming that a crack‐like structure is the source of LP events. The tails of synthetic waveforms obtained from a model of a fluid‐driven crack are analyzed by the Sompi method to determine the complex frequencies of one of the modes of crack resonance over a wide range of the model parameters α/a and ρf/ρs, where αis the P wave velocity of the rock matrix, a is the sound speed of the fluid, and ρf and ps are the densities of the fluid and rock matrix, respectively. The quality factor due to radiation loss (Qr) for the selected mode almost monotonically increases with increasing α/a, while the dimensionless frequency (v) of the mode decreases with increasing α/a and ρf/ρs. These results are used to estimate Q and v for a crack containing various types of fluids (gas‐gas mixtures, liquid‐gas mixtures, and dusty and misty gases) for values of a, ρf, and quality factor due to intrinsic losses (Qi) appropriate for these types of fluids, in which Q is given by Q−l = Qr−l + Qi−1. For a crack containing such fluids, we obtain Q ranging from almost unity to several hundred, which consistently explains the wide variety of quality factors measured in LP events observed at various volcanoes. We underscore the importance of dusty and misty gases containing small‐size particles with radii around 1 μm to explain long‐lasting oscillations with Q significantly larger than 100. Our results may provide a basis for the interpretation of spatial and temporal variations in the observed complex frequencies of LP events in terms of fluid compositions beneath volcanoes.
Viscoelastic‐coupling model for the earthquake cycle driven from below
Authors: Savage J. C.
Published: February 2004

In a linear system the earthquake cycle can be represented as the sum of a solution which reproduces the earthquake cycle itself (viscoelastic‐coupling model) and a solution that provides the driving force. We consider two cases, one in which the earthquake cycle is driven by stresses transmitted along the schizosphere and a second in which the cycle is driven from below by stresses transmitted along the upper mantle (i.e., the schizosphere and upper mantle, respectively, act as stress guides in the lithosphere). In both cases the driving stress is attributed to steady motion of the stress guide, and the upper crust is assumed to be elastic. The surface deformation that accumulates during the interseismic interval depends solely upon the earthquake‐cycle solution (viscoelastic‐coupling model) not upon the driving source solution. Thus geodetic observations of interseismic deformation are insensitive to the source of the driving forces in a linear system. In particular, the suggestion of Bourne et al. [1998] that the deformation that accumulates across a transform fault system in the interseismic interval is a replica of the deformation that accumulates in the upper mantle during the same interval does not appear to be correct for linear systems.
Exact and approximate formulas for P‐SV reflection and transmission coefficients for a nonwelded contact interface
Authors: Chaisri S. , Krebes E. S.
Published: February 2004

We present new and exact mathematical formulas for the P–SV particle displacement reflection and transmission coefficients for elastic plane waves incident upon a nonwelded contact (or linear slip) interface separating two solid half‐spaces. We represent the nonwelded contact at the interface with displacement discontinuity boundary conditions, that is, the traction is continuous across the interface, but the particle displacement is not: the discontinuity in the displacement is proportional to the traction. The formulas are derived by algebraically solving these boundary conditions. The formulas can be expressed in the form of those for the welded contact case, with some terms being modified due to nonwelded contact, plus additional terms due purely to nonwelded contact. Such formulas are useful because they provide insight into the nature of the coefficients and can be used to derive approximate formulas which have applications in the inversion and interpretation of seismic data. The exact formulas can also be applied to the case of a viscous nonwelded interface by simply replacing one of their parameters (the tangential specific compliance) with a modified parameter which includes the specific viscosity. We also apply the exact formulas to the specific case in which the incidence and transmission media are identical (e.g., a joint, fault, or fracture in a single homogeneous medium). In this case, reflected elastic waves are produced, unlike the case of welded contact, in which no reflections are produced because of the lack of an impedance contrast. This effect may partially explain occasional reports in the literature of anomalously large seismic amplitudes observed in areas where impedance contrasts are small. We present some numerical examples illustrating the effects of nonwelded contact on reflection and transmission coefficients. We show that energy is conserved at all incidence angles at a nonwelded interface between two different elastic solids. We also present some examples of applications of the exact formulas: (1) we derive approximate formulas for a weakly nonwelded interface between two identical solids, (2) we use them to study the sensitivity of the coefficients to the normal and parallel specific compliances (measures of the amount of nonweldedness), finding that they are generally more sensitive to the normal than the parallel compliance (except for the SS reflection), that the sensitivities vary considerably with incidence angle, and that the PP and SS transmission coefficients are basically insensitive to the compliances for a weakly nonwelded interface, and (3) we further approximate the formulas in example 1 to the case of small propagation angles, resulting in formulas that could be useful in amplitude versus offset studies.
Mantle plumes and flood basalts: Enhanced melting from plume ascent and an eclogite component
Authors: Leitch A. M. , Davies G. F.
Published: February 2004

New numerical models of starting plumes reproduce the observed volumes and rates of flood basalt eruptions, even for a plume of moderate temperature arriving under thick lithosphere. These models follow the growth of a new plume from a thermal boundary layer and its subsequent rise through the mantle viscosity structure. They show that as a plume head rises into the lower‐viscosity upper mantle it narrows, and it is thus able to penetrate rapidly right to the base of lithosphere, where it spreads as a thin layer. This behavior also brings the hottest plume material to the shallowest depths. Both factors enhance melt production compared with previous plume models. The model plumes are also assumed to contain eclogite bodies, inferred from geochemistry to be recycled oceanic crust. Previous numerical models have shown that the presence of nonreacting eclogite bodies may greatly enhance melt production. It has been argued that the eclogite‐derived melt would react with surrounding peridotite and refreeze; however, recent experimental studies indicate that eclogite‐derived melts may have reached the Earth's surface with only moderate or even minor modification. Combined with an assumed 15 vol % component of eclogite, our models yield a sharp peak in melting of about 1–3 Myr duration and volumes of melt that encompass those observed in flood basalt provinces.
New constraints on the structure, thermochronology, and timing of the Ailao Shan‐Red River shear zone, SE Asia
Authors: Leloup P. H. , Arnaud N. , Lacassin R. , Kienast J. R. , Harrison T. M. , Trong T. T. Phan , Replumaz A. , Tapponnier P.
Published: February 2004

New structural, petrographic, and 40Ar/39Ar data constrain the kinematics of the ASRR (Ailao Shan‐Red River shear zone). In the XueLong Shan (XLS), geochronological data reveal Triassic, Early Tertiary, and Oligo‐Miocene thermal events. The latter event (33–26 Ma) corresponds to cooling during left‐lateral shear. In the FanSiPan (FSP) range, thrusting of the SaPa nappe, linked to left‐lateral deformation, and cooling of the FSP granite occurred at ≈35 Ma. Rapid cooling resumed at 25–29 Ma as a result of uplift within the transtensive ASRR. In the DayNuiConVoi (DNCV), foliation trends NW‐SE, but is deflected near large‐scale shear planes. Stretching lineation is nearly horizontal. On steep foliations, shear criteria indicate left‐lateral shear sense. Zones with flatter foliations show compatible shear senses. Petrographic data indicate decompression from ≈6.5 kbar during left‐lateral shear (temperatures >700°C). 40Ar/39Ar data imply rapid cooling from above 350°C to below 150°C between 25 and 22 Ma without diachronism along strike. Along the whole ASRR cooling histories show two main episodes: (1) rapid cooling from peak metamorphism during left‐lateral shear; (2) rapid cooling from greenschist conditions during right‐lateral reactivation of the ASRR. In the NW part of the ASRR (XLS, Diancang Shan), we link rapid cooling 1 to local denudations in a transpressive environment. In the SW part (Ailao Shan and DNCV), cooling 1 resulted from regional denudation by zipper‐like tectonics in a transtensive regime. The induced cooling diachronism observed in the Ailao Shan suggests left‐lateral rates of 4 to 5 cm/yr from 27 Ma until ≈17 Ma. DNCV rocks always stayed in a transtensive regime and do not show cooling diachronism. The similarities of deformation kinematics along the ASRR and in the South China Sea confirms the causal link between continental strike‐slip faulting and marginal basin opening.
First‐order reversal curve diagrams: A new tool for characterizing the magnetic properties of natural samples
Authors: Roberts Andrew P. , Pike Christopher R. , Verosub Kenneth L.
Published: February 2004

Paleomagnetic and environmental magnetic studies are commonly conducted on samples containing mixtures of magnetic minerals and/or grain sizes. Major hysteresis loops are routinely used to provide information about variations in magnetic mineralogy and grain size. Standard hysteresis parameters, however, provide a measure of the bulk magnetic properties, rather than enabling discrimination between the magnetic components that contribute to the magnetization of a sample. By contrast, first‐order reversal curve (FORC) diagrams, which we describe here, can be used to identify and discriminate between the different components in a mixed magnetic mineral assemblage. We use magnetization data from a class of partial hysteresis curves known as first‐order reversal curves (FORCs) and transform the data into contour plots (FORC diagrams) of a two‐dimensional distribution function. The FORC distribution provides information about particle switching fields and local interaction fields for the assemblage of magnetic particles within a sample. Superparamagnetic, single‐domain, and multidomain grains, as well as magnetostatic interactions, all produce characteristic and distinct manifestations on a FORC diagram. Our results indicate that FORC diagrams can be used to characterize a wide range of natural samples and that they provide more detailed information about the magnetic particles in a sample than standard interpretational schemes which employ hysteresis data. It will be necessary to further develop the technique to enable a more quantitative interpretation of magnetic assemblages; however, even qualitative interpretation of FORC diagrams removes many of the ambiguities that are inherent to hysteresis data.
Tomographic inversion of S–SKS times for shear velocity heterogeneity in D″: Degree 12 and hybrid models
Authors: Kuo Ban‐Yuan , Garnero Edward J. , Lay Thorne
Published: February 2004

Differential travel times between S (or diffracted S) and SKS were measured to study the global distribution of shear wave velocity heterogeneity in the lowermost 250 km of the mantle (the D″ region). Commencing with ∼3000 S‐SKS times with variable qualities, we minimize uneven path coverage by thinning redundantly sampled regions (e.g., the Fiji‐Tonga to North America corridor) and collect additional data for sparsely sampled areas, especially in the Southern Hemisphere. About 1500 paths were retained, distributed to reconcile both high‐density and homogeneous sampling. We compare (1) spherical harmonic and (2) equal‐area block parameterizations of D″ shear velocity heterogeneity for identical minimum resolving lengths and mean model errors. We show that the two parameterizations result in indistinguishable patterns of heterogeneity and power distribution for resolution down to a block size of 4.5°×4.5° and maximum spherical harmonic degree of 40. We demonstrate synthetically that a high‐degree (L) inversion followed by a lower‐degree (Ls) spherical harmonic synthesis effectively circumvents model contamination from expansion truncation effects. Tomographic inversion of this data set yields a global distribution of D″ heterogeneity robust up to degree 12. In our preferred model (L = 40, Ls = 12, rms ∼ 1%), surface hotspots and estimated lower mantle plume roots are located in or at edges of low‐velocity regions (δVs< −2%), for both the Atlantic‐Indian low‐velocity corridor as well as the low velocities beneath the central Pacific. Although the above parameterizations entail no intrinsic difference in resolvability, higher resolution is regionally achieved by removing the derived degree 12 model from the data then inverting the corrected residuals using 4.5°×4.5° blocks. This hybrid method prevents power loss at intermediate to long wavelengths from otherwise severe damping and recovers smaller‐scale structure where constraints are better than the global average, such as beneath the Pacific and Eurasia. A synthetic recovery test yields resolvability maps that reflect both the path geometry and the quality of data, which help to identify robust features in both the degree 12 and the hybrid D″ models.
Distorted drainage basins as markers of crustal strain east of the Himalaya
Authors: Hallet B. , Molnar P.
Published: February 2004

Extensive lithospheric deformation on the eastern margin of the India/Eurasia collision has brought three major rivers into exceptionally close proximity. For over 300 km the Salween, Mekong, and Yangtze Rivers are only tens of kilometers apart, ∼10 times closer than rivers of comparable length elsewhere in the world. We view deeply incised river valleys as displacement markers that move with the underlying crust over time scales of at least tens of millions of years. Accordingly, the close spacing of these major rivers and their nearly parallel trends reflect the large crustal strains near the eastern Himalayan syntaxis. These river characteristics are consistent with two primary phases of deformation in this tectonic setting: NE‐SW shortening of ∼0.5 oriented radially from the eastern syntaxis, followed by dextral shear strain of 6±1 along a north trending zone. Pervasive shearing of this magnitude throughout the ∼150‐km‐wide region of anomalous drainage would account for ∼1000 km of northward motion of India relative to southern China. Directly east of this region, another ∼1000 km of northward motion is likely to have been accommodated by pervasive shearing through a zone hundreds of kilometers wide. Finally, we draw attention to the remarkable accord between the arcuate paths of each of the three great rivers in the broader southeastern Tibet region and small circles that curve around the eastern Himalayan syntaxis.
Experiments on vertical basement fault reactivation below volcanoes
Authors: Merle Olivier , Vidal Nathalie , van Wyk de Vries Benjamin
Published: February 2004

Scaled modeling has been conducted to study the role played by the reactivation of vertical basement faults in the destabilization process of overlying volcanoes. Results show that basement fault reactivation induces the formation of faults within the volcanic edifice. These faults delimite a central block which is extruded from between the two undeformed lateral parts of the edifice. Collapse of this central block can occur when heterogeneous cones are used in experiments, revealing that mechanical interfaces are of paramount importance in triggering rockslide avalanches. Collapse structures may display horseshoe shapes in map view and are then surprisingly similar to avalanche scars resulting from failure created by magmatic intrusions in natural volcanoes. Large‐scale destabilisation of half the surface of the cone may also occur in a single event depending on the position of the reactivated basement fault below the edifice. It is emphasized that the process under consideration may occur on a dormant volcano as well, provided that a fault is reactivated below it.
On the conditions for lower crustal convective instability
Authors: Jull M. , Kelemen P. B.
Published: February 2004

Continental crust is thought to be formed as a result of arc magmatism, but many of the lavas produced in these settings are basaltic, while those that are silicic are commonly evolved, with lower Mg #s than the continental crust. The bulk composition of continental crust can be produced by mixing of end‐member basaltic and silicic compositions, via magma mixing or in mechanical, tectonic juxtaposition, but some process is required to remove the cumulates and residues formed during generation of the silicic, “granitic” end‐member. We consider convective instability of dense mafic and ultramafic lower crust as a means to remove mafic residues of basalt differentiation in order to produce end‐member compositions that can mix to form the bulk composition of the continental crust. Using a range of lower crustal and mantle bulk compositions, ranging from mafic and ultramafic cumulates to primary liquid compositions, we calculated the subsolidus phase assemblage and resulting density. The results show that densities of likely lower crustal lithologies can exceed those of the mantle (by ∼50–250 kg m−3), but the density contrast is a strong function of composition, temperature, and pressure. For a “cold” geotherm with a Moho temperature of 300°C, relevant to cratonic settings, densities of all of the lower crustal compositions that we considered, except granulite, exceed the density of the underlying mantle at pressures as low as 0.8 GPa. For a “hot” geotherm with a Moho temperature in the range of 800–1000°C, the density of the lower crust is much more variable, with gabbroic and granulite compositions having lower densities than the mantle, while “arc gabbronorite” and ultramafic cumulate compositions having higher densities than the mantle at pressures similar to that for the cold geotherm. Instability times calculated for a two‐dimensional Rayleigh‐Taylor convective instability, where a dense lower crustal layer sinks into a lower‐density mantle, show that high temperatures (>700°C, or >500°C with a background strain rate) are required for this process to occur on a timescale of 10 Myr with rheological parameters expected for the crust and mantle. The high temperature required for dense lower crustal mafic‐ultramafic cumulates to sink into the mantle suggests that this process is restricted to arcs, volcanic rifted margins, and continental regions that are undergoing extension, are underlain by a mantle plume, or have had part of the conductive upper mantle removed.
Fluid pressures at hypocenters of moderate to large earthquakes
Authors: Streit Jürgen E. , Cox Stephen F.
Published: February 2004

Many active faults are expected to develop fluid pressures in excess of hydrostatic pressures below 3 to 7 km depth during interseismic periods. Suprahydrostatic fluid pressures are known to reduce the stresses required for brittle failure. Stress differences that trigger moderate to large earthquakes typically range from 40 to 160 MPa, as indicated by earthquake shear stress drops and paleostress estimates obtained from mylonites. For stresses in this range (40–160 MPa), seismic fault reactivation requires in most cases pore fluid factors (fluid pressure/overburden pressure) higher than for hydrostatic fluid pressures (i.e., >0.37) along misoriented faults with fault angles ≥45°, such as many segments of the San Andreas fault system. For example, at a stress difference of 100 MPa and fault angles of 45°–55°, fault reactivation at 7 to 20 km depth requires hypocentral pore fluid factors of ≈0.8–1 for reverse faults, 0.6–0.9 for strike‐slip faults, and <0.8 for normal faults. Pore fluid factors increase with increasing cohesive strength of faults, increasing coefficient of internal friction, and increasing fault angle. Seismic reactivation of cohesive faults at stress differences of 40–160 MPa requires near the base of the seismogenic zone (≈15 km depth) suprahydrostatic fluid pressures at all possible fault angles. Pore fluid factors are ≈0.4–0.9 for normal faults, ≈0.6–1 for strike‐slip faults, and ≈0.8–1.05 for thrust or reverse faults. These constraints are potentially useful for the modelling of seismic faulting and earthquake recurrence times.
A new, high‐resolution digital elevation model of Greenland fully validated with airborne laser altimeter data
Authors: Bamber Jonathan L. , Ekholm Simon , Krabill William B.
Published: February 2004

A new digital elevation model of the Greenland ice sheet and surrounding rock outcrops has been produced at 1‐km postings from a comprehensive suite of satellite remote sensing and cartographic data sets. Height data over the ice sheet were mainly from ERS‐1 and Geosat radar altimetry. These data were corrected for a slope‐dependent bias that had been identified in a previous study. The radar altimetry was supplemented with stereophotogrammetric data sets, synthetic aperture radar interferometry, and digitized cartographic maps over regions of bare rock and where gaps in the satellite altimeter coverage existed. The data were interpolated onto a regular grid with a spacing of ∼1 km. The accuracy of the resultant digital elevation model over the ice sheet was assessed using independent and spatially extensive measurements from an airborne laser altimeter that had an accuracy of between 10 and 12 cm. In a comparison with the laser altimetry the digital elevation model was found to have a slope‐dependent accuracy ranging from −1.04±1.98 m to −0.06±14.33 m over the ice sheet for a slope range of 0.0–1.0°. The mean accuracy over the whole ice sheet was −0.33±6.97 m. Over the bare rock areas the accuracy ranged from 20 to 200 m, dependent on the data source available. The new digital elevation model was used as an input data set for a positive degree day model of ablation. The new elevation model was found to reduce ablation by only 2% compared with using an older, 2.5‐km resolution model, which suggests that resolution‐induced errors in estimating ablation are no longer important.
Frictional response induced by time‐dependent fluctuations of the normal loading
Authors: Perfettini Hugo , Schmittbuhl Jean , Rice James R. , Cocco Massimo
Published: February 2004

We study the effect of time‐variable normal stress perturbations on a creeping fault which satisfies a velocity‐weakening rate‐ and state‐dependent friction law and is slipping at constant speed. We use the spring‐block model and include the effect of inertia. To account for the variable normal stress, we use the description introduced by Linker and Dieterich [1992], which links normal stress fluctuations to changes of the state variable. We consider periodic perturbations of the normal stress in time (as caused, for instance, by tides) and compare the behavior for two commonly used friction laws (the “slip” and the “ageing” laws). Their mechanical response is shown to be significantly different for normal stress fluctuations. It could be used to probe these two laws during laboratory friction experiments. We show that there is a resonance phenomenon, involving strong amplification of the shear and velocity response of the interface, when the spring stiffness is modestly above its critical value (or when, at a given stiffness, the normal stress is modestly below its critical value). We show that such an amplification is also observed when periodic fluctuations of the shear loading are considered, making the resonance phenomenon a general feature of the response of a near‐critical creeping surface to periodic fluctuations of the external loading. Analytical solutions are based on a linear expansion for low amplitude of normal or shear stress variations and are in very good agreement with numerical solutions. A method to find the evolution of friction in the case of an arbitrary perturbation of the normal stress is also presented. The results show that a creeping fault may be destabilized and enter a stick‐slip regime owing to small normal stress oscillations. This may also account for a mechanism for the generation of “creep bursts.” However, these phenomena require very specific parameter ranges to excite the resonance, which may not be met very generally in nature. This study illustrates the importance of the normal stress fluctuations on stable sliding and suggests further friction laboratory experiments.
Time constraints for modeling source dynamics of volcanic explosions at Stromboli
Authors: Ripepe Maurizio , Ciliberto Sergio , Della Schiava Massimo
Published: February 2004

The explosive source mechanism, responsible for the volcanic activity of Stromboli, is analyzed by seismic and infrasonic measurements in correlation with digitally processed images of the explosive events. Infrasonic recordings associated with short‐period seismic signals give evidence that the high‐frequency (>5 Hz) wave field is produced by a seismic source coupled to the atmosphere. The time delay between seismic and infrasonic onsets is not stable and the fluctuations could reflect a change in the magma‐gas physical properties and/or a migration of the source in the magma conduit. The gas jet velocity, calculated by videocamera images, and the time delay between seismic and infrasonic onset show a weak inverse correlation. The foam collapse model has been checked in laboratory experiments by recording acoustic signals produced during gas bubbles growth, flow, and burst in a liquid. The observed signals present strong similarities to short‐period (>1 s) as well as broadband (10–1 s) seismic signals recorded at Stromboli. Low‐frequency rarefaction is observed when the foam collapse occurs and the gas bubble starts to rise up along the pipe, then the bubble film breaking at the liquid free surface emits an audible sound. By analogy, we infer that low‐frequency seismic decompression is generated by the rapid expansion of gas in the magma conduit, while high‐frequency seismic signal and infrasound are generated by the explosion at the magma free‐surface. The time differences between seismic, infrasonic and visible onset of the explosion locate the explosive source at 600 m above sea level.
Geochronologic and thermobarometric constraints on the evolution of the Main Central Thrust, central Nepal Himalaya
Authors: Catlos E. J. , Harrison T. Mark , Kohn Matthew J. , Grove Marty , Ryerson F. J. , Manning Craig E. , Upreti B. N.
Published: February 2004

The Main Central Thrust (MCT) juxtaposes the high‐grade Greater Himalayan Crystallines over the lower‐grade Lesser Himalaya Formation; an apparent inverted metamorphic sequence characterizes the shear zone that underlies the thrust. Garnet‐bearing assemblages sampled along the Marysandi River and Darondi Khola in the Annapurna region of central Nepal show striking differences in garnet zoning of Mn, Ca, Mg, and Fe above and below the MCT. Thermobarometry of MCT footwall rocks yields apparent inverted temperature and pressure gradients of ∼18°C km−1 and ∼0.06 km MPa−1, respectively. Pressure‐temperature (P‐T) paths calculated for upper Lesser Himalaya samples that preserve prograde compositions show evidence of decompression during heating, whereas garnets from the structurally lower sequences grew during an increase in both pressure and temperature. In situ (i.e., analyzed in thin section) ion microprobe ages of monazites from rocks immediately beneath the Greater Himalayan Crystallines yield ages from 18 to 22 Ma, whereas late Miocene and Pliocene monazite ages characterize rocks within the apparent inverted metamorphic sequence. A Lesser Himalayan sample collected near the garnet isograd along the Marysandi River transect contains 3.3±0.1 Ma monazite ages (P ≈ 0.72 GPa, T ≈ 535°C). This remarkably young age suggests that this portion of the MCT shear zone accommodated a minimum of ∼30 km of slip over the last 3 Ma (i.e., a slip rate of >10 mm yr−1) and thus could account for nearly half of the convergence across the Himalaya in this period. The distribution of ages and P‐T histories reported here are consistent with a thermokinematic model in which the inverted metamorphic sequences underlying the MCT formed by the transposition of right‐way‐up metamorphic sequences during late Miocene‐Pliocene shearing.
Geophysical confirmation of low‐angle normal slip on the historically active Dixie Valley fault, Nevada
Authors: Abbott Robert E. , Louie John N. , Caskey S. John , Pullammanappallil Satish
Published: February 2004

The December 16, 1954, Dixie Valley earthquake (Ms = 6.8) followed the nearby Fairview Peak earthquake (Ms = 7.2) by 4 min, 20 s. Waveforms from the Fairview Peak event contaminate those from the Dixie Valley event, making accurate fault plane solutions impossible. A recent geologic study of surface rupture characteristics in southern Dixie Valley suggests that the Dixie Valley fault is low angle (<30°) along a significant portion of the 1954 rupture. To extend these observations into the subsurface, we conducted a seismic reflection and gravity experiment. Our results show that a portion of the Dixie Valley ruptures occurred along a fault dipping 25° to 30°. As such, the Dixie Valley event may represent the first large, low‐angle normal earthquake on land recorded historically. Our high‐resolution seismic reflection profile images the rupture plane from 5 to 50 m depth. Medium‐resolution reflections, as well as refraction velocities, show a smoothly dipping fault plane from 50 to 500 m depth. Stratigraphic truncations and rollovers in the hanging wall show a slightly listric fault to 2 km depth. Gravity profiles conservatively constrain maximum basin depth and define overall geometry. Extension along the low‐angle section may have occurred in two phases during the Cenozoic. Current fault motion postdates a 13 to 15 Ma basalt, imaged in the hanging wall, and inherits from a fault formed during an earlier extensional pulse, concentrated at 24.2 to 24.4 Ma. The earlier extension suggests extraordinary slip rates as high as 18 mm/yr, resulting in the formation of the low‐angle fault break. Sections of the Dixie Valley fault where there is no evidence for current low‐angle slip correlate well with areas where no pre‐15 Ma slip has been documented.
High‐frequency variations in Earth rotation from Global Positioning System data
Authors: Rothacher M. , Beutler G. , Weber R. , Hefty J.
Published: February 2004

Using the data of the global, dense Global Positioning System (GPS) network established by the International GPS Service a continuous, uninterrupted series of subdaily Earth rotation parameters (ERPs) with a time resolution of 2 hours has been generated at the Center for Orbit Determination in Europe. The series starts in January 1995 and has a length of more than 3 years. Starting from the 2‐hour ERP values of this, to our knowledge, unique time series, the high‐frequency variations in Universal Time (UT1) and polar motion (PM) due to ocean tides are studied and a set of sine and cosine coefficients is estimated for all the major tidal terms at nearly diurnal and semidiurnal frequencies. The GPS series is not very homogeneous (various processing changes during the 3 years) and still short compared to the length of very long baseline interferometry (VLBI) and satellite laser ranging (SLR) data sets. However, the results derived from this series are already of the same quality as the results from VLBI and SLR. A comparison of the tidal coefficients stemming from all three space‐geodetic techniques shows an agreement on the 1 μs level for UT1 and 10 microarc seconds (μas) for PM, respectively. The RMS difference between the ocean tide amplitudes estimated from GPS data and from TOPEX/Poseidon altimeter data amounts to 0.7–0.9 μs in UT1 and 9–13 μas in PM. The residual spectrum that remains after the removal of all tidal terms has a noise level of ∼5–10 μas in PM and 0.5–1 μs in UT1 and contains nontidal signals (up to 55 μas in PM and 3 μs in UT1) that might be due to the impact of the satellite orbit modeling (12‐hour revolution period of the satellites) or, alternatively, due to atmospheric or oceanic normal modes.
Gravity anomalies and crustal structure at the southeast Greenland margin
Authors: Korenaga Jun , Holbrook W. Steven , Detrick Robert S. , Kelemen Peter B.
Published: February 2004

Free‐air gravity anomalies across the southeast Greenland margin are investigated in conjunction with a well‐constrained seismic velocity model to provide a constraint on subsurface density structure. This volcanic rifted margin is characterized by the presence of ∼30‐km‐thick igneous crust, which correlates with a positive gravity high of ∼60 mGal. A new systematic approach is adopted for gravity modeling, which consists of (1) full error propagation from the velocity model to predicted gravity anomalies through a posteriori model covariance represented by Monte Carlo ensembles, (2) the inversion of residual gravity anomalies for density variations within geological subdomains, and (3) the joint inversion of seismic travel times and gravity anomalies. A density model derived from the velocity model, using conventional conversion laws for the continental and oceanic crust, substantially underpredicts the observed gravity by ∼70 mGal over the continental shelf. Neither errors in the velocity model nor the uncertainty of the chosen conversion laws are shown to be sufficient for such a large gravity misfit. A possible range of mantle contribution is first investigated by modeling various thermal evolution and depletion scenarios, which suggests that the maximum contribution is only ∼20 mGal, assuming constant source mantle composition throughout continental rifting and subsequent seafloor spreading. If most of the residual gravity anomaly has a crustal origin, applying a conversion law with a denser upper crust in the continent‐ocean transition zone seems to be the only plausible option to resolve this difficulty. Contrasting eruption environments for the transition zone crust (subaerial) and the oceanic crust (submarine) probably result in different porosity structures, to which a velocity‐density relationship is highly sensitive. The wire log and laboratory measurements of plateau basalts recovered from recent drilling legs on North Atlantic margins seem to support this explanation. An alternative explanation, which invokes a strong degree of source mantle heterogeneity, is also plausible on the basis of a recent geochemical study of the North Atlantic igneous province.
A two‐phase model for compaction and damage: 3. Applications to shear localization and plate boundary formation
Authors: Bercovici David , Ricard Yanick , Schubert Gerald
Published: February 2004

A new two‐phase theory employing a nonequilibrium relation between interfacial surface energy, pressure, and viscous deformation [Bercovici et al., this issue] provides a model for damage (void generation and microcracking) and thus a continuum description of weakening, failure, and shear localization. Here we demonstrate applications of the theory to shear localization with simple shear flow calculations in which one phase (the matrix, representing, for example, silicate) is much stronger (more viscous) than the other phase (the fluid). This calculation is motivated as a simple model of plate boundary formation in a shear zone. Even without shear the two phases eventually separate due to gradients in surface tension. However, the influence of shear on phase separation is manifest in several ways. As shear velocity increases, the separation rate of the phases increases, demonstrating a basic feedback mechanism: Accumulation of the fluid phase causes focused weak zones on which shear concentrates, causing more damage and void generation and thus greater accumulation of fluid. Beyond a critical shear velocity, phase separation undergoes intense acceleration and focusing, leading to a “tear localization” in which the porosity becomes nearly singular in space and grows rapidly like a tear or crack. At an even higher value of shear velocity, phase separation is inhibited such that shear localization gives way to defocusing of weak zones suggestive of uniform microcracking and failure throughout the layer. Our two‐phase damage theory thus predicts a wide variety of shear localization and failure behavior with a continuum model. Applications of the theory to various fields, such as granular dynamics, metallurgy, and tectonic plate boundary formation are numerous.
Thermal thickness and evolution of Precambrian lithosphere: A global study
Authors: Artemieva Irina M. , Mooney Walter D.
Published: February 2004

The thermal thickness of Precambrian lithosphere is modeled and compared with estimates from seismic tomography and xenolith data. We use the steady state thermal conductivity equation with the same geothermal constraints for all of the Precambrian cratons (except Antarctica) to calculate the temperature distribution in the stable continental lithosphere. The modeling is based on the global compilation of heat flow data by Pollack et al. [1993] and more recent data. The depth distribution of heat‐producing elements is estimated using regional models for ∼300 blocks with sizes varying from 1°×1° to about 5°×5° in latitude and longitude and is constrained by laboratory, seismic and petrologic data and, where applicable, empirical heat flow/heat production relationships. Maps of the lateral temperature distribution at depths 50, 100, and 150 km are presented for all continents except Antarctica. The thermal thickness of the lithosphere is calculated assuming a conductive layer overlying the mantle with an adiabat of 1300°C. The Archean and early Proterozoic lithosphere is found to have two typical thicknesses, 200–220 km and 300–350 km. In general, thin (∼220 km) roots are found for Archean and early Proterozoic cratons in the Southern Hemisphere (South Africa, Western Australia, South America, and India) and thicker (>300 km) roots are found in the Northern Hemisphere (Baltic Shield, Siberian Platform, West Africa, and possibly the Canadian Shield). We find that the thickness of continental lithosphere generally decreases with age from >200 km beneath Archean cratons to intermediate values of 200±50 km in early Proterozoic lithosphere, to about 140±50 km in middle and late Proterozoic cratons. Using known crustal thickness, our calculated geotherms, and assuming that isostatic balance is achieved at the base of the lithosphere, we find that Archean and early Proterozoic mantle lithosphere is 1.5% less dense (chemically depleted) than the underlying asthenosphere, while middle and late Proterozoic subcrustal lithosphere should be depleted by ∼0.6–0.7%. Our results suggest three contrasting stages of lithosphere formation at the following ages: >2.5 Ga, 2.5–1.8 Ga, and <1.8 Ga. Ages of komatiites, greenstone belts, and giant dike swarms broadly define similar stages and apparently reflect secular changes in mantle temperature and, possibly, convection patterns.
Canonical approach to the free nutations of a three‐layer Earth model
Authors: Escapa Alberto , Getino Juan , Ferrándiz José M.
Published: February 2004

The Hamiltonian formalism is applied to a three‐layer Earth model composed of a rigid mantle, a fluid outer core and a rigid inner core, with no dissipation of frictional or electromagnetic origin. Analytical expressions for the free nutations are derived in a set of nonsingular canonical variables that allow a much simpler mathematical treatment. They are checked versus numerical solutions and compared with previous results by Mathews et al. [1991a] and Getino and Ferrándiz [1996, 1998a] with complete agreement up to the first order in the ellipticities.
Statistical analysis of synthetic earthquake catalogs generated by models with various levels of fault zone disorder
Authors: Lu Chunsheng , Vere‐Jones David
Published: February 2004

The stress release model, a stochastic version of the elastic rebound theory, is applied to the large events from four synthetic earthquake catalogs generated by models with various levels of disorder in distribution of fault zone strength (Ben‐Zion, 1996). They include models with uniform properties (U), a Parkfield‐type asperity (A), fractal brittle properties (F), and multi‐size‐scale heterogeneities (M). The results show that the degree of regularity or predictability in the assumed fault properties, based on both the Akaike information criterion and simulations, follows the order U, F, A, and M, which is in good agreement with that obtained by pattern recognition techniques applied to the full set of synthetic data. Data simulated from the best fitting stress release models reproduce, both visually and in distributional terms, the main features of the original catalogs. The differences in character and the quality of prediction between the four cases are shown to be dependent on two main aspects: the parameter controlling the sensitivity to departures from the mean stress level and the frequency‐magnitude distribution, which differs substantially between the four cases. In particular, it is shown that the predictability of the data is strongly affected by the form of frequency‐magnitude distribution, being greatly reduced if a pure Gutenburg‐Richter form is assumed to hold out to high magnitudes.
A constraint on the shear stress at the Pacific‐Australian plate boundary from heat flow and seismicity at the Kermadec forearc
Authors: Von Herzen R. , Ruppel C. , Molnar P. , Nettles M. , Nagihara S. , Ekström G.
Published: February 2004

New heat flow measurements and relocated hypocenters that constrain the subduction geometry of the Pacific plate at the Kermadec trench yield an estimate of shear stress on the thrust fault. With the exception of a few relatively high values (>60 mW m−2) on the upper forearc region near the active volcanic ridge, most of the 64 heat flow values along two profiles across the forearc range from 20 to 40 mW m−2. Corrections for bottom water temperature variations that caused nonuniform temperature gradients were required for most measurements. The means of the most reliable values for the northern and southern profiles are 28.9±5.8 mW m−2 (n = 33) and 28.9±7.2 mW m−2 (n = 19), respectively, and the measurements show no apparent systematic variation along the profiles over the range ∼50 to 150 km distance from the trench. The means of the 10 values at each of two sites on the Pacific plate seaward of the profiles are 57.2±6.3 and 60.2±6.6 mW m−2. Redeterminations of focal depths and fault plane solutions of earthquakes in the vicinity of the heat flow profiles indicate thrust faulting on a plane dipping 17°±2°. Calculated values of heat flow for a two‐dimensional analytical approximation to conduction through the upper plate, diffusion into the downgoing slab, and advection by that slab are consistent with either a uniform stress of ∼40±17 MPa along the thrust fault or stress increasing linearly at ∼0.5±0.2 MPa km−1 with distance from the trench axis. The comparable scatter in the heat flow measurements about those calculated from these simple stress distributions does not show one to be a better approximation than the other.
Reestablishing the circumpolar wave in sea ice around Antarctica from one winter to the next
Authors: Gloersen Per , White Warren B.
Published: February 2004

Remarkable correlation exists between warm water, poleward winds over the ocean, and low sea ice concentrations and extents over the winter sea ice pack around Antarctica, even to the point of continuing paterns across the ice‐sea boundary. Since the wind stress associated with poleward wind is expected to compact the ice during sea ice edge retraction, we conclude that the memory of the Antarctic circumpolar wave in the sea ice pack is carried from one austral winter to the next by the neighboring water temperatures since the sea ice pack retracts nearly to Antarctica in austral summer.
Surface‐to‐subsurface velocity projection for shallow water currents
Authors: Shen Colin Y. , Evans Thomas E.
Published: February 2004

Sea surface currents in coastal oceans are accessible to continuous direct observations by shore‐based high‐frequency Doppler radar systems. Inferring current structure in shallow water from such surface current observations is attempted. The approach assumes frictionally dominated flow and vertically varying current velocity on the scale of the Ekman boundary layer. The approximation of the velocity variation with depth is consequently derivable in terms of orthogonal basis functions from the sea surface kinematic and dynamic boundary conditions; specifically, the viscous momentum and shear equations evaluated at the sea surface. The inference procedure developed is demonstrated with sea surface data obtained in the coastal High‐Resolution Remote Sensing Experiment on the continental shelf off Cape Hatteras. Despite uncertainties in the surface measurements, qualitative agreement is obtained between the inferred subsurface current and the current measured in situ. The sensitivity of the inference to the measurement uncertainties as well as to the model assumptions is investigated, and the inferred result is found to be generally robust.
Absolute geostrophic velocity within the Subantarctic Front in the Pacific Ocean
Authors: Donohue Kathleen A. , Firing Eric , Chen Shuiming
Published: February 2004

Velocity measurements from a shipboard acoustic Doppler current profiler (ADCP) are used as a reference for geostrophic current calculations on six sections across the Subantarctic Front (SAF) in the Pacific Ocean. The resulting cross‐track velocity estimates near the bottom range from 4 to 10 cm s−1 to the east in the eastward jet at the SAF; in adjacent regions of westward surface flow, the near‐bottom velocity is usually to the west. On one section where simultaneous lowered ADCP velocity profiles are available, they confirm the results from the shipboard ADCP. Annual mean velocity sections from the Parallel Ocean Program numerical model also show near‐bottom velocities exceeding 5 cm s−1, with the same tendency for the zonal velocity component near the bottom to match the direction of the surface jets. Transport across the entire Antarctic Circumpolar Current (ACC) cannot be estimated accurately from ADCP‐referenced geostrophic sections because even a very small cross‐track bias integrates to a large error. A preliminary look at the 1992 model transport stream function shows that the effect of bottom‐referencing varies from section to section; it can cause 40‐Sv recirculations to be missed, and can cause net transport to be underestimated or overestimated by O (30 Sv).