bluuebunny/crossref_metadata_2025 huggingface dataset:
Seismic attribute inversion for velocity and attenuation structure using data from the GLIMPCE Lake Superior experiment
Authors: Matheney Michael P. , Nowack Robert L. , Tréhu Anne M.
Published: February 2004

A simultaneous inversion for velocity and attenuation structure using multiple seismic attributes has been applied to refraction data from the 1986 GLIMPCE Lake Superior experiment. The seismic attributes considered include envelope amplitude, instantaneous frequency, and travel time of first arrival data. Instantaneous frequency is converted to t* using a matching procedure which approximately removes the effects of the source spectra. The derived seismic attributes are then used in an iterative inversion procedure referred to as AFT inversion for amplitude, (instantaneous) frequency, and time. Uncertainties and resolution of the velocity and attenuation models are estimated using covariance calculations and checkerboard resolution maps. A simultaneous inversion of seismic attributes from the GLIMPCE data results in a velocity model similar to that of previous studies across Lake Superior. A central rift basin and a northern basin are the most prominent features with an increase in velocity near the Isle Royale fault. Although there is an indication of the central and northern basins in the attenuation model for depths greater than 4 km, the separation is not evident for shallower depths. This may result from microfractures masking compositional variations in the attenuation model for shallower depths. Attenuation Q values range from approximately 60 near the surface to near 500 at 10 km depth. A relationship between inverse Q and velocity of Q−1=0.0210–0.0028*v was found with a correlation coefficient of −0.96. This suggests a nearly linear, inverse relationship between Q−1 and velocity beneath Lake Superior which supports previous laboratory results. The inverted velocity and attenuation models provide important constraints on the lithology and physical properties of the Midcontinent rift beneath Lake Superior.
Electrical texture characterization of dipmeter microresistivity signals using multifractal analysis
Authors: Saucier A. , Huseby O. K. , Muller J.
Published: February 2004

We use multifractal analysis as a geostatistical tool for the characterization of microresistivity signals produced by dipmeter well‐logging tools. A sliding window of a fixed length is placed along the signal. For each window, several electrical texture indices characterizing the irregularity of the microresistivity signal are calculated. Plotted as a function of depths, these texture indices form what we call texture logs. The methods which are developed are tested on dipmeter data and compared to other log responses and core descriptions for the well 30/9–6 in the North Sea. We show that the texture logs can be used to identify geological lithofacies for this example.
Stress and time‐dependent failure of brittle rocks under compression: A theoretical prediction
Authors: Okui Yoshiaki , Horii Hideyuki
Published: February 2004

A micromechanics‐based continuum theory called interaction field theory (IFT) can reproduce localization phenomena such as shear faulting and axial splitting of rock under compression. The validity of IFT is examined by comparing its predictions of short‐term strength and creep behavior of hard rock with reported experimental data. The theory is formulated through an averaging scheme for an elastic solid containing many microcracks. The growth of the microcracks is assumed to occur through two mechanisms: stress‐induced crack growth and stress corrosion. Failure of rock under compression is known to be governed by the interactive behavior of microcracks. Yet in many earlier analyses the information on interaction is lost through conventional averaging schemes. IFT introduces a new field quantity which represents the interaction effect and the associated governing integral equation. IFT reproduces not only inelastic deformation of rock but also eventual macroscopic failure due to localization of microcracking. In the numerical analysis of short‐term behavior, macroscopic failure is reproduced as a bifurcation from a homogeneous state of deformation into a localized one. The strengths are obtained as a result of numerical analysis with material constants of an intact matrix and the quantities related to initial defects. The strengths of Westerly granite obtained as a function of the confining pressure are compared with published experimental data. For time‐dependent behavior of hard rock, creep tests of granite are analyzed. Comparisons are made between experimental failure times and numerical results for different confining pressures and environmental conditions.
A link between deformation history and the orientation of reflective structures in the 2.68–2.83 Ga Opatica belt of the Canadian Superior Province
Authors: Bellefleur G. , Calvert A. J. , Chouteau M. C.
Published: February 2004

True three‐dimensional reflector orientations can be derived from prestack seismic reflection data where a seismic profile is particularly crooked. This is accomplished by estimating a measure of coherency along travel time trajectories defined by the azimuth, dip, and depth of a reflector and the medium velocity. Results from Lithoprobe line 48, located in the Opatica belt of the Archean Superior Province, differentiate reflectors with two distinct orientations, which coincide with the attitudes of two deformational fabrics mapped at surface. Assuming a connection between reflectivity and strain induced by tectonic processes, the reflectors with NNE strikes and shallow dips toward the east are correlated with surface evidence for early west vergent thrusting in the Opatica belt. Other reflectors, which strike ENE‐WSW and dip shallowly to the north and to the south, indicate that most of the reflectors in the southern Opatica and beneath the Abitibi greenstone belt at middle and lower crustal levels formed during a later, approximately N‐S shortening event. Mantle reflections previously interpreted as a relict suture of an Archean subduction zone dip to the north at around 30°–45° and are also associated with this N‐S event. The distribution of reflector orientations estimated at the crooked parts of line 48 indicates that much of the Opatica crust was reworked during the N‐S shortening event, although a region of the middle and lower crust, characterized by the earlier D1 reflectors, is preserved in the central part of the belt.
The petrologic case for a dry lower crust
Authors: Yardley Bruce W. D. , Valley John W.
Published: February 2004

Fluid pressure in the crust may be controlled by different mechanisms according to depth, temperature, and the mineralogy of the host rocks. Where rocks are fluid‐saturated, fluid pressure may approach lithostatic or hydrostatic pressure depending on the ductility of the wall rocks and the connectivity of pores and fractures. However, if the host rocks contain minerals formed at temperatures higher than those currently prevailing, they will react with fluids to produce hydrated (or carbonated) retrograde minerals, and the fluid pressure will be limited by thermodynamic equilibrium between high‐grade reactant minerals and retrograde products. The thermodynamically constrained parameter, water fugacity, may have a value of tens to hundreds of bars in the lower crust. In practice, this means that for typical igneous or high‐grade metamorphic rocks now occurring in stable lower crust, notional fluid pressures are substantially (1 to 3 orders of magnitude) lower than lithostatic. No free, connected fluid phase can be present in deep stable crust, and alternative explanations must be sought for the relatively high electrical conductivity of such rocks. The proposal that high lower crustal conductivity is due to thin grain boundary films of graphite is also unlikely to be generally true because films of sufficient thickness would be readily visible on broken surfaces of hand specimens. An alternative explanation of the discrepancy between laboratory and field measurements of the conductivity of high‐grade rocks is that laboratory measurements are not normally made under appropriate conditions of rock‐buffered fluid pressure.
The October 30, 1994, seismic disturbance in South Africa: Earthquake or large rock burst?
Authors: Bowers David
Published: February 2004

The relative amplitude method is applied to body waves recorded at teleseismic distances from the October 30, 1994, mb 5.6 seismic disturbance, in South Africa, to determine the source depth and moment tensor. The results indicate that the source is shallow and that moment tensors with a positive volume change component are inconsistent with the observations. Observations of Rg in the frequency range 0.5–2.0 Hz at two regional distance stations also indicate that the source is shallow (less than about 3 km deep). Three‐component broadband and long‐period seismograms recorded at regional distances show strong Love wave excitation and a lobed Rayleigh radiation pattern, observations that are inconsistent with collapse mechanisms recently proposed for some large rock bursts (mine tremors). Modeling of teleseismic broadband P and regional distance surface waves shows that the source depth is shallow (2–3 km) and that the observed source radiation is consistent with that expected from a 45° normal dip‐slip fault. The results of the modeling are confirmed by reports from the mining region that the disturbance was a mine tremor caused by slip on the Stuirmanspan Fault in the President Brand mine at a depth of between 2 and 3 km. The close match between observed and synthetic P and Rayleigh wave amplitudes suggests that the explosion‐like mb: Ms of the mine tremor (mb 5.6, Ms 4.7) is attributable to the effect of the mechanism and the shallow depth of the source. The above results are at odds with those of Fan and Wallace [1995], who interpreted this disturbance as an earthquake at a depth of 12 km, based on a broadband inversion of three‐component waveforms from two regional distance stations. This study confirms the need to interpret both broadband waveforms (to match amplitude and polarity) and short‐period waveforms (to confirm (relative) onset times in the model structure) if inversion of regional broadband waveforms is to provide a reliable estimate of the moment tensor and source depth.
Self‐organization in the thermomechanical flow of ice sheets
Authors: Payne A. J. , Dongelmans P. W.
Published: February 2004

This paper uses a three‐dimensional, thermomechanical ice sheet model to investigate the initiation and evolution of ice streams within ice sheets. Ice streams are distinguished within general ice sheet flow by their relatively fast (of the order of kilometers per year) velocity and are thought to be crucial in determining the response of ice sheets to climatic change. We show that streaming can arise solely as a consequence of internal feedback between ice sheet flow and temperature. Inhomogeneities in topography and/or geology are not therefore necessary conditions for ice stream development. This spatial patterning replaces the large‐scale surging reported from previous modeling work in one horizontal dimension. We suggest that surging is restricted to situations where ice flow is channelized (e.g., valley glaciers) and that it is replaced by streaming where horizontal ice flow is unconstrained (ice sheets). The ice streams in some of our experiments show switching behavior reminiscent of that known from the Siple Coast ice streams, West Antarctica. The cause appears to be the competition for ice discharge and drainage basin area between adjacent ice streams. This may lead to the gradual reduction of a stream's discharge below the threshold necessary to generate enough frictional heat to maintain warm, viscous ice at the ice stream bed. In this case the stream will stagnate abruptly, which then prompts localized ice thickening and the initiation of a new stream in a different location.
Why blind thrust faults do not propagate to the Earth's surface: Numerical modeling of coseismic deformation associated with thrust‐related anticlines
Authors: Roering Joshua J. , Cooke Michele L. , Pollard David D.
Published: February 2004

High fault‐tip stress concentrations are associated with coseismic slip on blind thrust faults and suggest that these structures should readily propagate to the Earth's surface. Seismic profiles of blind‐thrust‐related earthquakes reveal diffuse zones of aftershocks surrounding the fault tip which are attributed to inelastic deformation, such as flexural‐slip or extensional fracturing. The complex interaction between blind thrust faults and secondary structures may control the evolution of blind thrust systems. The influence of bedding‐plane slip on fault propagation is simulated with numerical models using the boundary element method. We use two parameters to estimate the tendency for thrust fault propagation, (1) the mode II stress intensity factor and (2) the maximum Coulomb stress near the fault tip. Calculations from both analyses suggest that shallow thrust faults may exhibit an increased tendency to propagate as a result of interaction with the Earth's surface and slip along bedding planes above the fault tip and a decreased tendency to propagate due to slip along bedding planes at or below the fault tip. Our results demonstrate that the magnitude and style of inelastic deformation in active fault systems control fault propagation.
The elastic constants of San Carlos olivine to 17 GPa
Authors: Abramson E. H. , Brown J. M. , Slutsky L. J. , Zaug J.
Published: February 2004

All elastic constants, the average bulk and shear moduli, and the lattice parameters of San Carlos olivine (Fo90) (initial density 3.355 gm/cm3) have been determined to a pressure of 12 GPa at room temperature. Measurements of c11, c33, c13, and c55 have been extended to 17 GPa. The pressure dependence of the adiabatic, isotropic (Hashin‐Shtrikman bounds) bulk modulus, and shear modulus may be expressed as KHS = 129.4 + 4.29 P and by GHS = 78 + 1.71P ‐ 0.027 P2, where both the pressure and the moduli are in gigapascals. The isothermal compression of olivine is described by a bulk modulus given as KT = 126.3 + 4.28 P. Elastic constants other than c55 can be adequately represented by a linear relationship in pressure. In the order (c11, c12, c13, c22, c23, c33, c44, c55, c66) the 1 bar intercepts (gigapascal units) are (320.5, 68.1, 71.6, 196.5, 76.8, 233.5, 64.0, 77.0, 78.7). The first derivatives are (6.54, 3.86, 3.57, 5.38, 3.37, 5.51, 1.67, 1.81, 1.93). The second derivative for c55 is −0.070 GPa−1. Incompressibilities for the three axes may also be expressed as linear relationships with pressure. In the order of a, b, and c axes the intercepts in gigapascals are (547.8, 285.8, 381.8) and the first derivatives are (20.1, 12.3, 14.0).
A hierarchical stress release model for synthetic seismicity
Authors: Bebbington Mark
Published: February 2004

We construct a stochastic dynamic model for synthetic seismicity involving stochastic stress input, release, and transfer in an environment of heterogeneous strength and interacting segments. The model is not fault‐specific, having a number of adjustable parameters with physical interpretation, namely, stress relaxation, stress transfer, stress dissipation, segment structure, strength, and strength heterogeneity, which affect the seismicity in various ways. Local parameters are chosen to be consistent with large historical events, other parameters to reproduce bulk seismicity statistics for the fault as a whole. The one‐dimensional fault is divided into a number of segments, each comprising a varying number of nodes. Stress input occurs at each node in a simple random process, representing the slow buildup due to tectonic plate movements. Events are initiated, subject to a stochastic hazard function, when the stress on a node exceeds the local strength. An event begins with the transfer of excess stress to neighboring nodes, which may in turn transfer their excess stress to the next neighbor. If the event grows to include the entire segment, then most of the stress on the segment is transferred to neighboring segments (or dissipated) in a characteristic event. These large events may themselves spread to other segments. We use the Middle America Trench to demonstrate that this model, using simple stochastic stress input and triggering mechanisms, can produce behavior consistent with the historical record over five units of magnitude. We also investigate the effects of perturbing various parameters in order to show how the model might be tailored to a specific fault structure. The strength of the model lies in this ability to reproduce the behavior of a general linear fault system through the choice of a relatively small number of parameters. It remains to develop a procedure for estimating the internal state of the model from the historical observations in order to use the model for forward prediction.
Displacement‐length scaling for terrestrial and Martian faults: Implications for Valles Marineris and shallow planetary grabens
Authors: Schultz Richard A.
Published: February 2004

The displacement‐length ratio for the central composite graben in Valles Marineris, inferred from topography and likely stratigraphic offsets to beDmax/L≈ 4–6x10−3, is consistent with the scaling relations derived from terrestrial faults and grabens that also demonstrate a systematic variation in displacement with position along the structure. The large, 10‐km depth in central Valles Marineris is therefore not unusual, but rather, should be expected for a fault system 2240 km long. In contrast, existing measurements of throw along the numerous shallow radial grabens in Tharsis, while apparently inconsistent with the scaling relations, demonstrate the importance of recognizing mechanical‐segment length along linked or echelon graben sets. Faults on other planetary surfaces should scale with their terrestrial counterparts as long as they extend substantially deeper than the gravity‐dependent near‐surface rock‐mass zone (∼1 km for Earth, ∼2.5 km for Mars).
Detailed joint structure in a geothermal reservoir from studies of induced microearthquake clusters
Authors: Phillips W. Scott , House Leigh S. , Fehler Michael C.
Published: February 2004

Microearthquake clusters form distinct, planar patterns within five study regions of a geothermal reservoir undergoing hydraulic fracturing at Fenton Hill, New Mexico. The patterns define individual, slipping joint surfaces of dimension 40–120 m, containing 80–150 events each. Sharp, straight edges truncate the clusters; we interpret these as marking intersections with aseismic joints. Each edge orientation is consistent with an intersection between the active joint and a plane oriented parallel to one of the other clusters we identify. Therefore it appears that cluster shapes constrain the geometry of seismic and aseismic joints; both could be important components of the fluid‐flow network. The distribution of inferred slip plane orientations is consistent with but fails to provide sufficient constraint to differentiate conclusively between two, very different, stress field estimates, one measured using pressurization and wellbore breakouts, the other using focal mechanisms of the largest microearthquakes. An impermeable joint model, requiring pore pressure in excess of the normal stress on a joint before slip can occur, was inconsistent with many of the inferred slip plane orientations. The high‐quality locations were possible because events from the same cluster generated nearly similar waveforms, permitting the precise determination of relative arrival times. Standard deviations of arrival‐time residuals fall between 0.1 and 1.1 ms for these clusters. Major axes and aspect ratios of the 90% confidence ellipsoids range from 6 to 28 m and 1.5 to 8, respectively. Small events dominate the seismic energy release and thoroughly populate the identified, active joints, allowing the hypocenters to reflect details of the joint structure. To further investigate the reservoir structure, we applied a source‐array, slant‐stack technique to waveforms from the well‐located clusters, yielding directions that scattered energy left each cluster. By studying paths of scattered waves we expected to pinpoint impedance contrasts that might have indicated concentrations of fluid‐filled joints. However, results show that scattered energy in the S wave coda left the source region in the same direction as the direct S wave. Direct waves may have excited borehole tube waves that became trapped in the vicinity of the geophone tool, overwhelming any energy scattered from the reservoir.
Estimating crustal thickness in southern California by stacking PmP arrivals
Authors: Richards‐Dinger K. B. , Shearer P. M.
Published: February 2004

We use observations of the Moho‐reflected phase PmP to constrain crustal thicknesses and upper mantle velocities in southern California. We stack normalized absolute values of seismograms from local events in time and range bins after aligning on the initial P arrival and applying a range correction to adjust the various source depths to the surface. Although most individual seismograms do not allow accurate determination of a PmP arrival time, imaging the whole data set in this way shows clear PmP arrivals at ranges from about 90 km to over 250 km. PmP‐ Pg and PmP‐ Pn differential times can be measured from the image and used to estimate the Moho depth and upper mantle velocity. For southern California, we obtain an average crustal thickness of 28 km and an upper mantle velocity of 7.8 km/s. Next, we map lateral variations by repeating this procedure for stacks of subsets of the data in which the traces are grouped in caps by Moho reflection point. Estimates of Moho depth range from 18 km in the Salton Trough to 33 km beneath the eastern Transverse Ranges and 36 km beneath the southernmost Sierra Nevada. The upper mantle velocities generally increase from southwest to northeast across the region. We also map postcritical PmP/Pg amplitudes which vary by a factor of 4 with the highest amplitudes in the northwest Mojave Desert. Preliminary experiments with stacking SmS arrivals indicate strongly correlated SmS and PmP amplitude variations. These results provide a guide to source‐receiver paths that may produce anomalously strong Moho‐reflected phases during future earthquakes.
Permian deposition in the north central Brooks Range, Alaska: Constraints for tectonic reconstructions
Authors: Adams Karen E. , Mull Charles G. , Crowder R. Keith
Published: February 2004

Two opposing tectonic models have been offered to explain the regional structural relations in the north central Brooks Range fold‐thrust belt of northern Alaska. The first suggests that rocks of the northern Endicott Mountains were thrust from south to north over the area of the present Mount Doonerak high and are therefore highly allochthonous. The second implies that the rocks of the northern Endicott Mountains were deposited in a basin that lay north of the Mount Doonerak high and later were thrust a short distance southward onto the northern flank of the high and are thus parautochthonous. To provide stratigraphic constraints for these models, this study examines Permian facies of the north central Brooks Range. Permian rocks in the north central Brooks Range comprise a thin (40 to 160 m thick), fining‐upward succession of clastic, storm‐influenced shelf deposits. When the rocks of the northern Endicott Mountains are restored south of the Mount Doonerak area, a minimum distance of 80 km, the Permian deposits grade systematically from distal facies (Siksikpuk Formation) in the southwest to proximal facies (Echooka Formation) in the northeast. Facies trends in the reconstructed Permian basin include, from southwest to northeast, (1) an increase in carbonate content and corresponding decrease in silica content, (2) a general darkening and thickening of shaley intervals, (3) an increase in proximal features of storm beds, including coarser, thicker, more abundant, and more closely spaced beds, and (4) an increase in abundance and diversity of the faunal assemblage with a corresponding decrease in age. These stratigraphic relations imply that rocks of the northern Endicott Mountains are allochthonous and structurally overlie a proximal stratigraphie succession similar to that exposed in the Mount Doonerak area and northeastern Brooks Range.
Three‐dimensional flow and reaction in porous media: Implications for the Earth's mantle and sedimentary basins
Authors: Aharonov E. , Spiegelman M. , Kelemen P.
Published: February 2004

We present a theoretical and numerical study of the reorganization of a porous matrix due to fluid flow coupled with dissolution or precipitation processes. We find that under certain conditions, flow of corrosive fluids results in unstable growth of the permeability and increasing disequilibrium in fluid chemistry with time. High‐permeability channels may form parallel to the direction of flow. In time, these channels cause the distribution of porosity to become increasingly correlated and anisotropic and cause flow rates to be increasingly variable. Flow coupled with crystallization has the opposite effect: With time, permeability reduction occurs at a decreasing rate. Mineral composition in the fluid approaches chemical equilibrium. Precipitation destroys existing preferred paths for flow and acts to homogenize and disperse the flow. Connectivity of the porous media is reduced. Implications of these results for two geological systems are discussed: (1) Modes of melt extraction from the Earth's mantle, where the expected different modes of flow and reaction may help explain different geochemical and geological observations at hot spots and mid‐ocean ridges, and (2) Precipitation and formation of abnormal pressure zones in sedimentary basins.
Introduction to Special Section: The Trans‐Alaska Crustal Transect (TACT) Across Arctic Alaska
Authors: Plafker George , Mooney Walter D.
Published: February 2004

This special section of the Journal of Geophysical Research addresses the composition and structural evolution of the lithosphere in northern Alaska. Investigations reported in this section were mainly undertaken as part of the Trans‐Alaska Crustal Transect (TACT), an integrated geological and geophysical transect of the entire Alaskan lithosphere along a north‐south corridor undertaken from 1984 to 1992 (Figure 1). The onshore segment of the transect approximately follows along the route of the trans‐Alaskan pipeline; the offshore segment extends across the continental margin in the Gulf of Alaska to the Pacific plate. The TACT line is unique in that it provides a coordinated onshore/offshore geological and geophysical traverse of the North American plate in Alaska from the active convergent Pacific margin to the passive Arctic margin of the continent.
Fluid flow and water‐rock interaction in the East Rift Zone of Kilauea Volcano, Hawaii
Authors: Conrad Mark E. , Thomas Donald M. , Flexser Steven , Vennemann Torsten W.
Published: February 2004

The East Rift Zone of Kilauea Volcano in Hawaii represents a major area of geothermal activity. Fluid inclusion and stable isotope analyses of secondary hydrothermal minerals in core samples from three scientific observation holes (SOH) drilled into the rift zone indicate that the geothermal system is dominated by meteoric waters to depths of as much as 1500 m below sea level. Calculated δ18O and δD values for fluids on the north side of the rift zone indicate that the deep meteoric fluids may be derived from precipitation on the upper slopes of Mauna Loa Volcano. In the interior of the rift zone, recharge is dominated by seawater mixed with local meteoric water. Water/rock ratios in the rift area are approximately 2, but strongly 18O‐enriched fluids in the deeper parts of the SOH‐2 and SOH‐4 drill holes (on the north side of the rift) indicate that the fluids underwent extensive interaction with rocks prior to reaching this part of the rift zone. Marine carbonates at the subaerial to submarine transition (between 1700 and 1780 m depth) in SOH‐4 have not fully equilibrated with the fluids, suggesting that the onset of hydrothermal activity in this area was relatively recent (<2000 years). This may represent increased volcanic activity along the rift after the end of the Ai La'au phase of eruptive activity at the Kilauea summit approximately 1000 years ago, or it may reflect progressive evolution of the hydrothermal system in response to southward migration of intrusive activity within the rift.
A simple method for migrating narrow aperture, noisy seismic reflection data and application to Project INDEPTH (International Deep Profiling of Tibet and the Himalaya) deep seismic profiles
Authors: Alsdorf Doug
Published: February 2004

Migration of deep seismic data is often hindered by a narrow recording aperture (line length by record length) and a low signal‐to‐noise ratio. The severity of typical migration artifacts (e.g., lateral smearing of discontinuous reflections into synforms, “smiles”) increases with travel time such that interpreters of deep seismic data have often substituted migrated line drawings for the actual sections. As part of Project INDEPTH (International Deep Profiling of Tibet and the Himalaya), a new migration method was developed to address both the noise and migration issues. The method works in the time‐space domain and uses the simple, constant velocity, straight ray path to perform the migration. First, only amplitudes within a given range are retained for migration, thus avoiding high‐amplitude noise bursts and low‐amplitude background noise. Then, the local dip of a reflection is found by automatically fitting a straight line to the highest amplitudes within a small window (several time samples by several traces) and calculating the dip of the line using a constant velocity. Finally, using this dip, the method migrates a selected amplitude value. The dips, lateral positions, and depths of the migrated events compare very well with output from more conventional algorithms (e.g.,fk‐Stolt, finite difference, etc.). The advantages of the new method include fewer artifacts, fast computer run times, low memory use and the ability to migrate long profiles and travel times (e.g., 50 s). The output of the method is a grid of migrated amplitudes (not wavelets) or dip values which is particularly effective for making small figures, such as those needed for publication. The principal disadvantage is the use of a constant migration velocity.
Influence of chemical buoyancy on the dynamics of slabs in the transition zone
Authors: Christensen Ulrich R.
Published: February 2004

The influence of compositional buoyancy on slab descent through the transition zone is studied in a two‐dimensional, semidynamical model of plate subduction. The viscosity is temperature‐ and depth‐dependent. Plate motion is imposed by a time‐dependent velocity boundary condition including retrograde trench migration. High rates of trench rollback favor stagnation of the slab in the transition zone, and low values favor deep penetration. Buoyancy forces arise from thermal expansion, from phase boundary deflection at 660 km depth, and from density differences between normal mantle and both basaltic and harzburgitic material in the slab. An anomalous density deficit makes basaltic rock buoyant between 680 km and 900 km. In each case, results with and without chemical buoyancy are compared. Its relative importance depends on the plate age. For a 100 m.y. old plate the compositional effect is found to be negligible. For younger plates it does not lead to qualitatively new modes of slab deformation but causes the slab to stagnate near 660 km already at lower trench velocity. However, penetration into the lower mantle is not inhibited even for a 25 m.y. old plate and for very strong chemical buoyancy when the trench is nearly stationary. A section of 24 km thick crust, intended to simulate an oceanic plateau, is found to pass into the deep mantle along with a penetrating slab. Slab break off occurs only for 36 km thick plateau crust. Although the influence of compositional buoyancy is not negligible, it does not seem to be the main controlling factor for the fate of slabs.
A comparison of methods for the modeling of thermochemical convection
Authors: van Keken P. E. , King S. D. , Schmeling H. , Christensen U. R. , Neumeister D. , Doin M.‐P.
Published: February 2004

We have compared several methods of studying thermochemical convection in a Boussinesq fluid at infinite Prandtl number. For the representation of chemical heterogeneity tracer, marker chain, and field methods are employed. In the case of an isothermal Rayleigh‐Taylor instability, good agreement is found for the initial rise of the unstable lower layer; however, the timing and location of the later smaller‐scale instabilities may differ between methods. For a simulation of entrainment by thermal convection of a dense layer at the bottom of the mantle we found good agreement for a few overturn times. After this, differences between the results can be large. We propose intrinsic differences between the methods and possibly chaotic mixing effects may be the cause of the lack of detailed agreement. The comparison shows that high resolution is necessary for a reasonable thermochemical study. This will pose severe restrictions on the applicability of these methods to three‐dimensional situations.
Depth dependence of earthquake frequency‐magnitude distributions in California: Implications for rupture initiation
Authors: Mori Jim , Abercrombie Rachel E.
Published: February 2004

Statistics of earthquakes in California show linear frequency‐magnitude relationships in the range of M2.0 to M5.5 for various data sets. Assuming Gutenberg‐Richter distributions, there is a systematic decrease in b value with increasing depth of earthquakes. We find consistent results for various data sets from northern and southern California that both include and exclude the larger aftershock sequences. We suggest that at shallow depth (∼0 to 6 km) conditions with more heterogeneous material properties and lower lithospheric stress prevail. Rupture initiations are more likely to stop before growing into large earthquakes, producing relatively more smaller earthquakes and consequently higher b values. These ideas help to explain the depth‐dependent observations of foreshocks in the western United States. The higher occurrence rate of foreshocks preceding shallow earthquakes can be interpreted in terms of rupture initiations that are stopped before growing into the mainshock. At greater depth (9–15 km), any rupture initiation is more likely to continue growing into a larger event, so there are fewer foreshocks. If one assumes that frequency‐magnitude statistics can be used to estimate probabilities of a small rupture initiation growing into a larger earthquake, then a small (M2) rupture initiation at 9 to 12 km depth is 18 times more likely to grow into a M5.5 or larger event, compared to the same small rupture initiation at 0 to 3 km.
Southern California permanent GPS geodetic array: Error analysis of daily position estimates and site velocities
Authors: Zhang Jie , Bock Yehuda , Johnson Hadley , Fang Peng , Williams Simon , Genrich Joachim , Wdowinski Shimon , Behr Jeff
Published: February 2004

We analyze time series of daily positions estimated from data collected by 10 continuously monitoring Global Positioning System (GPS) sites in southern California during the 19‐month period between the June 28, 1992 (Mw=7.3), Landers and January 17, 1994 (Mw=6.7), Northridge earthquakes. Each time series exhibits a linear tectonic signal and significant colored noise. Spectral power at frequencies in the range 5 yr−1to 0.5 d−1is dominated by white noise or possibly fractal white noise and is several orders of magnitude higher than what would be expected from random walk noise (in this short‐period range) attributed by others to geodetic monument motions. Estimating a single slope for the time series' power spectra suggests fractal white noise processes with spectral indices of about 0.4. Site velocity uncertainties assuming this fractal white noise model are 2–4 times larger than uncertainties obtained assuming a purely white noise model. A combination white noise plus flicker noise (spectral index of 1) model also fits the data and suggests that the velocity uncertainties should be 3–6 times larger than for the white noise model. We cannot adequately distinguish between these two noise models, nor can we rule out the possibility of a random walk signal at the lowest frequencies; these questions await the analysis of longer time series. In any case, reducing the magnitude of low‐frequency colored noise is critical and appears to be best accomplished by building sites with deeply anchored and braced monuments. Otherwise, rate uncertainties estimated from continuous GPS measurements may not be improved significantly compared to those estimated from infrequent campaign‐mode measurements.
Isotopic and trace element compositions of upper mantle and lower crustal xenoliths, Cima volcanic field, California: Implications for evolution of the subcontinental lithospheric mantle
Authors: Mukasa Samuel B. , Wilshire Howard G.
Published: February 2004

Ultramafic and mafic xenoliths from the Cima volcanic field, southern California, provide evidence of episodic modification of the upper mantle and underplating of the crust beneath a portion of the southern Basin and Range province. The upper mantle xenoliths include spinel peridótite and anhydrous and hydrous pyroxenite, some cut by igneous‐textured pyroxenite‐gabbro veins and dikes and some by veins of amphibole ± plagioclase. Igneous‐textured pyroxenites and gabbros like the dike rocks also occur abundantly as isolated xenoliths inferred to represent underplated crust. Mineral and whole rock trace element compositions among and within the different groups of xenoliths are highly variable, reflecting multiple processes that include magma‐mantle wall rock reactions, episodic intrusion and infiltration of basaltic melts of varied sources into the mantle wall rock, and fractionation. Nd, Sr, and Pb isotopic compositions mostly of clinopyroxene and plagioclase mineral separates show distinct differences between mantle xenoliths (εNd = −5.7 to +3.4; 87Sr/86Sr = 0.7051 – 0.7073; 206Pb/204Pb = 19.045 – 19.195) and the igneous‐textured xenoliths (εNd = +7.7 to +11.7; 87Sr/86Sr = 0.7027 – 0.7036 with one carbonate‐affected outlier at 0.7054; and 206Pb/204Pb = 18.751 – 19.068), so that they cannot be related. The igneous‐textured pyroxenites and gabbros are similar in their isotopic compositions to the host basaltic rocks, which have εNd of +5.1 to +9.3; 87Sr/86Sr of 0.7028 – 0.7050, and 206Pb/204Pb of 18.685 – 21.050. The igneous‐textured pyroxenites and gabbros are therefore inferred to be related to the host rocks as earlier cogenetic intrusions in the mantle and in the lower crust. Two samples of peridotite, one modally metasomatized by amphibole and the other by plagioclase, have isotopic compositions intermediate between the igneous‐textured xenoliths and the mantle rock, suggesting mixing, but also derivation of the metasomatizing magmas from two separate and distinct sources. Sm‐Nd two‐mineral “isochrons” yield apparent ages for petrographically identical rocks believed to be coeval ranging from ∼0 to 113±26 Ma, indicating the unreliability of dating these rocks with this method. Amphibole and plagioclase megacrysts are isotopically like the host basalts and probably originate by mechanical breakup of veins comagmatic with the host basaltic rocks. Unlike other Basin and Range localities, Cima Cr‐diopside group isotopic compositions do not overlap with those of the host basalts.
A seafloor long‐baseline tiltmeter
Authors: Anderson Gregory , Constable Steven , Staudigel Hubert , Wyatt Frank K.
Published: February 2004

Long‐term monitoring of seismicity and deformation has provided constraints on the eruptive behavior and internal structure and dynamics of subaerial volcanoes, but until recently, such monitoring of submarine volcanoes has not been feasible. Little is known about the formation of oceanic crust or seamounts, and we have therefore developed a stand‐alone long‐baseline tiltmeter to record deformation on active seafloor volcanoes. The instrument is a differential pressure, two‐fluid sensor adapted for use on the seafloor, combined with an autonomous data logger and acoustic navigation/release system. The tiltmeter can be installed without use of remotely operated vehicles or manned submersibles and, to first order, is insensitive to noise driven by temperature or pressure gradients. We recorded 65 days of continuous data from one of these tiltmeters on Axial Seamount on the Juan de Fuca Ridge during a multidisciplinary experiment that included ocean bottom seismographs, magnetotelluric instruments, and short‐baseline tiltmeters. After instrument equilibration the 100‐m‐long tiltmeter provided a record with long‐term drift rates of 0.5–5 μrad day−1 and higher frequency variations of the order of 5–10 μrad. Comparison with records of subaerial volcanic tilt shows that this instrument can discriminate volcanic deflation events, though none occurred during our deployment, a conclusion supported by nearby short‐baseline tilt and bottom pressure recordings. The short‐ and long‐baseline data constrain volcanic inflation of Axial Seamount to be below 0.5–1 μrad day−1 during mid‐1994. Analysis of the long‐baseline tilt data in conjunction with electric field, temperature, and short‐baseline tiltmeter data shows that high‐frequency signals are largely driven by ocean currents. Improved coupling between the tiltmeter and seafloor should reduce this noise, improve stability and drift, and further enhance our ability to record tilt related to active submarine volcanism.
On the orientation and patterns of wing cracks and solution surfaces at the tips of a sliding flaw or fault
Authors: Willemse Emanuel J. M. , Pollard David D.
Published: February 2004

Sliding along a preexisting flaw can result in the formation of tensile cracks where stresses concentrate near the flaw tips. These tensile cracks are referred to as wing cracks and are generally oriented oblique to the preexisting flaw. Previous studies based on linear elastic fracture mechanics (LEFM) showed that the kink angle depends on the ratio of normal to shear loading on the flaw. We present analytical solutions for cohesive end zone (CEZ) flaw models and find that the relationship between kink angle and load differs significantly from that for LEFM flaws. Furthermore, the remote flaw‐parallel normal stress may significantly reduce or increase the kink angle, especially for CEZ flaws with large end zones. These results suggest that multiple interpretations are possible for some measured kink angles. In some materials, solution surfaces may form at the tip of the sliding flaw. By considering the angle between wing cracks and solution surfaces it is possible to determine whether the LEFM or CEZ model is more appropriate and thus to provide a better constrained interpretation of the boundary conditions that accompanied sliding. For some CEZ flaws the stress state in the cohesive end zone is nearly homogeneous, possibly promoting formation of arrays of opening mode cracks and solution surfaces that together form a shear zone. The CEZ flaw model can explain some orientations and patterns of cracks and solution surfaces commonly observed along natural faults that cannot be accounted for with the LEFM model.
Geodetic strain of Greece in the interval 1892–1992
Authors: Davies R. , England P. , Parsons B. , Billiris H. , Paradissis D. , Veis G.
Published: February 2004

A first‐order triangulation of Greece was carried out in the 1890s. Reoccupation, using Global Positioning System receivers, of 46 of the 93 original markers yielded estimates of the deformation of the region over the intervening interval. Broad regions have similar geodetic strain over the 100‐year time span. Strain north of the Gulf of Korinthos is predominantly north‐south extension, though with a significant east‐west component. The central Peloponnisos is relatively stable, whereas the gulfs of the southern Peloponnisos are all characterized by uniaxial east‐west extension. The seismic expression of strain for the entire region, calculated from the seismic moment tensors of earthquakes of MS≥5.8 during the past 100 years, accounts for only 20–50% of the geodetically determined strain. At a scale of 50–100 km, the fraction of the strain that is expressed seismically varies much more than this range. In particular, whereas seismic strain in the eastern Gulf of Korinthos over the past 100 years is commensurate with the geodetic strain, there is rapid extension across the western Gulf of Korinthos (∼0.3 μstrain yr−1), with negligible seismic strain for the 100 year period prior to 1992. The Egion earthquake of June 1995 in the western Gulf of Korinthos released only a small proportion (≤20%) of the elastic strain that had accumulated in that region. The observed distribution of displacements can be explained by the relative rotation of two plates with a broad accommodation zone between them, but it is equally consistent with the deformation that would be expected of a sheet of fluid moving toward a low‐pressure boundary at the Hellenic Trench. A simple calculation implies that if the region does behave as a fluid, then its effective viscosity is ∼1022–1023 Pa s. Such viscosities are consistent with the deformation of a lithosphere obeying a rheological law similar to that obtained for olivine in the laboratory.
The physical basis of glacier volume‐area scaling
Authors: Bahr David B. , Meier Mark F. , Peckham Scott D.
Published: February 2004

Ice volumes are known for only a few of the roughly 160,000 glaciers worldwide but are important components of many climate and sea level studies which require water flux estimates. A scaling analysis of the mass and momentum conservation equations shows that glacier volumes can be related by a power law to more easily observed glacier surface areas. The relationship requires four closure choices for the scaling behavior of glacier widths, slopes, side drag and mass balance. Reasonable closures predict a volume‐area scaling exponent which is consistent with observations, giving a physical and practical basis for estimating ice volumes. Glacier volume is insensitive to perturbations in the mass balance scaling, but changes in average accumulation area ratios reflect significant changes in the scaling of both mass balance and ice volume.
A numerical study of anisotropic, low Reynolds number, free surface flow for ice sheet modeling
Authors: Mangeney Anne , Califano Francesco , Hutter Kolumban
Published: February 2004

Few ice sheet flow models have been developed that solve the complete set of mechanical equations. Until now, these models were limited to isotropic conditions. We present here a two‐dimensional, finite difference method capable of solving the equations for the steady flow of a viscous, incompressible, anisotropic fluid with a free surface under isothermal conditions. It is not a standard method, especially with respect to the time discretization of the numerical scheme, and converges for very low Reynolds numbers. This method is applied here to the planar flow of anisotropic ice over flat or irregular bedrock, with no‐slip boundary conditions at the ice‐bedrock interface. The results are presented here for Newtonian behavior in the vicinity of an ice divide. The ice is assumed to be isotropic at the ice sheet surface, with continuous and prescribed development of anisotropy with increasing depth. Going from isotropic to anisotropic situations, our results indicate that the free surface becomes flatter and the shear strain rates larger and more concentrated near the bedrock. The flow is less sensitive to variations of the bedrock topography in the anisotropic case than in the isotropic case. Furthermore, a new phenomenon appears in the anisotropic case: the partial stagnation of ice in the holes of the bedrock. These effects have significant consequences when dating the ice. The isochrones obtained in the anisotropic case are flatter and the anisotropic ice is more than 10% younger above the bumps and more than 100% older within the holes than for the isotropic ice.