bluuebunny/crossref_metadata_2025 huggingface dataset:
Substorm dependence of chorus amplitudes: Implications for the acceleration of electrons to relativistic energies
Authors: Meredith Nigel P. , Horne Richard B. , Anderson Roger R.
Published: February 2004

Intense interest currently exists in determining the roles played by various wave‐particle interactions in the acceleration of electrons to relativistic energies during/following geomagnetic storms. Here we present a survey of wave data from the CRRES Plasma Wave Experiment for lower band (0.1–0.5fce) and upper band (0.5–1.0fce) chorus, fce being the electron gyrofrequency, to assess whether these waves could play an important role in the acceleration of a seed population of electrons to relativistic energies during and following geomagnetic storms. Outside of the plasmapause the chorus emissions are largely substorm‐dependent, and all chorus emissions are enhanced when substorm activity is enhanced. The equatorial chorus (|λm| < 15°) is strongest in the lower band during active conditions (AE > 300 nT) with average amplitudes typically >0.5 mV m−1 predominantly in the region 3 < L < 7, between 2300 and 1300 magnetic local time (MLT). This is consistent with electron injection near midnight and subsequent drift around dawn to the dayside. The high‐latitude chorus (|λm > 15°) is strongest in the lower band during active conditions, with average amplitudes typically >0.5 mV m−1 in the region 3 < L < 7 over a range of local times on the dayside, principally in the range 0600–1500 MLT, consistent with wave generation in the horns of the magnetosphere. An inner population of weak, substorm‐independent emissions with average amplitudes generally <0.2 mV m−1 are seen in both bands largely inside L = 4 on the nightside during quiet (AE < 100 nT) and moderate (100 nT < AE < 300 nT) conditions. These emissions lie inside the plasmapause and are attributed to signals from lightning and ground‐based VLF transmitters. We conclude that the significant increases in chorus amplitudes seen outside of the plasmapause during substorms support the theory of electron acceleration by whistler mode chorus in that region. The results suggest that electron acceleration by whistle mode chorus during/following geomagnetic storms can only be effective when there are periods of prolonged substorm activity following the main phase of the geomagnetic storm.
Dynamic evolution of relativistic electrons in the radiation belts
Authors: Réveillé T. , Bertrand P. , Ghizzo A. , Simonet F. , Baussart N.
Published: February 2004

Geomagnetic storms are responsible for the large increase of trapped electron fluxes in the magnetosphere resulting in the formation of radiation belts. Such belts can also be artificially produced by injection of electron beams. The dynamic evolution of the electron fluxes is very important for satellite protection purpose. Up to now most of the existing studies have been restricted to the stationary description of natural radiation belts. Recently, great effort has been made for their dynamic description [Bourdane et al., 1996]. We have developed a numerical code, which is able to follow the time behavior of the electron population under influence of collisions with tenuous atmosphere and resonant scattering by plasma waves. The code solves the phase‐averaged Fokker‐Planck equation of the electron momentum distribution function on magnetic shells within the Earth's inner magnetosphere. The physical arguments incorporated in the code include phase‐averaged, approximate collision operators and quasi‐linear diffusion due to resonant interaction with whistler plasma waves. The magnetic field is supposed to be tilted and decentered. Numerical experiments to simulate the decay and particles' loss in an artificial Van Allen belt are presented, and comparisons with available published data are discussed to validate the code [Abel and Thorne, 1998].
Strain accumulation and rotation in the Eastern California Shear Zone
Authors: Savage J. C. , Gan Weijun , Svarc J. L.
Published: February 2004

Although the Eastern California Shear Zone (ECSZ) (strike ∼N25°W) does not quite coincide with a small circle drawn about the Pacific‐North America pole of rotation, trilateration and GPS measurements demonstrate that the motion within the zone corresponds to right‐lateral simple shear across a vertical plane (strike N33°W±5°) roughly parallel to the tangent to that local small circle (strike ∼N40°W). If the simple shear is released by slip on faults subparallel to the shear zone, the accumulated rotation is also released, leaving no secular rotation. South of the Garlock fault the principal faults (e.g., Calico‐Blackwater fault) strike ∼N40°W, close enough to the strike of the vertical plane across which maximum right‐lateral shear accumulates to almost wholly accommodate that accumulation of both strain and rotation by right‐lateral slip. North of the Garlock fault dip slip as well as strike slip on the principal faults (strike ∼N20°W) is required to accommodate the simple shear accumulation. In both cases the accumulated rotation is released with the shear strain. The Garlock fault, which transects the ECSZ, is not offset by north‐northwest striking faults nor, despite geological evidence for long‐term left‐lateral slip, does it appear at the present time to be accumulating left‐lateral simple shear strain across the fault due to slip at depth. Rather the motion is explained by right‐lateral simple shear across the orthogonal ECSZ. Left‐lateral slip on the Garlock fault will release the shear strain accumulating there but would augment the accumulating rotation, resulting in a secular clockwise rotation rate ∼80 nrad yr−1 (4.6° Myr−1).
Reservoir‐induced deformation and continental rheology in vicinity of Lake Mead, Nevada
Authors: Kaufmann Georg , Amelung Falk
Published: February 2004

Lake Mead is a large reservoir in Nevada, formed by the construction of the 221‐m‐high Hoover Dam in the Black Canyon of the Colorado River. The lake encompasses an area of 635 km2, and the total volume of the reservoir is 35.5 km3. Filling started in February 1935. On the basis of a first‐order leveling in 1935, several levelings were carried out to measure the deformation induced by the load of the reservoir. Subsidence in the central parts of the lake relative to the first leveling was around 120 mm (1941), 218 mm (1950), and 200 mm (1963). The subsidence pattern clearly shows relaxation of the underlying basement due to the water load of the lake, which ceased after 1950. Modeling of the relaxation process by means of layered, viscoelastic, compressible flat Earth models with a detailed representation of the spatial and temporal distribution of the water load shows that the thickness of the elastic crust underneath Lake Mead is 30±3 km. The data are also consistent with a 10‐km‐thick elastic upper crust and a 20‐km‐thick viscoelastic lower crust, with 1020 Pa s as a lower bound for its viscosity. The subcrust has an average viscosity of 1018±0.2 Pa s, a surprisingly low value. The leveling data constrain the viscosity profile down to ∼200 km depth.
Experiments on conduit flow and eruption behavior of basaltic volcanic eruptions
Authors: Seyfried Ralf , Freundt Armin
Published: February 2004

Multiphase flow in basaltic volcanic conduits is investigated using analog experiments and theoretical approaches. Depending on gas supply, large gas bubbles (gas slugs) may rise through basaltic magma in regimes of distinct fluid‐dynamical behavior: ascent of single slugs, supplied slugs fed from the gas source during ascent, and periodic slug flow. An annular flow regime commences at the highest gas supply rates. A first set of experiments demonstrates that the growth of gas slugs due to hydrostatic decompression does not affect their ascent velocity and that excess pressure in the slugs remain negligible. The applicability of theoretical formulae describing slug ascent velocity as a function of liquid and conduit properties is evaluated in a second set of experiments. A third set of experiments with continuous gas supply into a cylindrical conduit are scaled to basaltic conditions over Morton, Eotvös, Reynolds, and Froude numbers. Gas flow rate and liquid viscosity are varied over the whole range of flow regimes to observe flow dynamics and to measure gas and liquid eruption rates. Foam generation by slug bursting at the surface and partial slug disruption by wake turbulence can modify the bubble content and size distribution of the magma. At the transition from slug to annular flow, when the liquid bridges between the gas slugs disappear, pressure at the conduit entrance drops by ∼60% from the hydrostatic value to the dynamic‐flow resistance of the annular flow, which may trigger further degassing in a stored magma to maintain the annular flow regime until the gas supply is exhausted and the eruption ends abruptly. Magma discharge may also terminate when magma ascent is hindered by wall friction in long volcanic conduits and the annular gas flow erodes all magma from the conduit. Supplied slugs are found to reach much higher rise velocities than unsupplied slugs and to collapse to turbulent annular flow upon bursting at the surface. A fourth set of experiments uses a conduit partially blocked by built‐in obstacles providing traps for gas pockets. Once gas pockets are filled, rising gas slugs deform but remain intact as they move around obstacles without coalescence or significant velocity changes. Bursting of bubbles coalescing with trapped gas pockets causes pressure signals at least 3 orders of magnitude more powerful than gas pocket oscillation induced by passing liquid. Our experiments suggest a refined classification of Strombolian and Hawaiian eruptions according to time‐dependant behavior into sporadically pulsating lava fountains (driven by stochastic rise of single slugs), periodically pulsating lava fountains (resulting from slug flow), and quasi‐steady lava fountains (oscillating at the frequency of annular‐flow turbulence).
Relationship of the Central Indian Ridge segmentation with the evolution of the Rodrigues Triple Junction for the past 8 Myr
Authors: Mendel Véronique , Sauter Daniel , Patriat Philippe , Munschy Marc
Published: February 2004

Located near 25°33′S, 70°00′E, the Rodrigues Triple Junction is the joining point of the intermediate‐spreading Southeast Indian and Central Indian Ridges with the ultraslow spreading Southwest Indian Ridge. Bathymetric data and magnetic anomalies are used to analyze the relationship between the evolution of the Central Indian Ridge segmentation and the evolution of the Rodrigues Triple Junction for the past 8 Myr. The Central Indian Ridge domain exhibits a complex morphotectonic pattern dominated by ridge‐normal and oblique bathymetric lows interpreted as the off‐axis traces of axial discontinuities. The short‐lived nontransform discontinuities as well as the segments that lengthen or shorten along the ridge axis reveal that the Central Indian Ridge segmentation is unstable near the Rodrigues Triple Junction. The combined study of the Central Indian Ridge and Southeast Indian Ridge domains shows that the triple junction evolves between two modes: a continuous mode where the Central Indian Ridge and Southeast Indian Ridge axes are joined and a discontinuous mode where the two ridge axes are offset. Owing to spreading asymmetry, and differences in axis direction or in lengthening rates of the Central Indian and Southeast Indian ridges, the continuous mode is unstable and evolves rapidly (<2 Myr) into a discontinuous mode. This last one is more stable and can evolve into a continuous mode only through the formation of a new Central Indian Ridge segment, which takes place facing the northern Southeast Indian Ridge segment. The evolution of the Rodrigues Triple Junction configuration and the evolution of the Central Indian Ridge segmentation are thus closely related.
Salt tectonics as a self‐organizing process: A reaction, transport, and mechanics model
Authors: Tuncay K. , Ortoleva P.
Published: February 2004

Salt tectonics is placed within the theory of nonlinear dynamical systems. Features such as waves, diapirs, and tears are viewed as natural consequences of the symmetry breaking instabilities and related self‐organized dynamics of the deforming salt body coupled to the reaction, transport, and mechanics of the surrounding sediments. The fundamental nonlinearities are in the surrounding‐rock and salt rheology. Our findings are based on a coupled RTM model simulated using finite element techniques. The centerpiece of the rheology of both rocks and salt is a nonlinear incremental stress formulation that integrates poroelasticity, continuous irreversible mechanical deformation (with yield behavior), pressure solution, and fracturing. In contrast to previously presented studies, in our approach the descriptive variables of all solid and fluid phases (stress, velocity, concentrations, etc.) and porous media (texture, i.e., volume fractions, composition, etc.) are solved from RTM equations accounting for interactions and interdependencies between them. The role of the coupling between the spatial distribution of sediment input rate and diapir growth and stalling is examined as is the creation of an array of salt tectonic minibasins.
On the Modified Mercalli intensities and magnitudes of the 1811–1812 New Madrid earthquakes
Authors: Hough Susan E. , Armbruster John G. , Seeber Leonardo , Hough Jerry F.
Published: February 2004

We reexamine original felt reports from the 1811–1812 New Madrid earthquakes and determine revised isoseismal maps for the three principal mainshocks. In many cases we interpret lower values than those assigned by earlier studies. In some cases the revisions result from an interpretation of original felt reports with an appreciation for site response issues. Additionally, earlier studies had assigned modified Mercalli intensity (MMI) values of V‐VII to a substantial number of reports that we conclude do not describe damage commensurate with intensities this high. We investigate several approaches to contouring the MMI values using both analytical and subjective methods. For the first mainshock on 02∶15 LT December 16, 1811, our preferred contouring yieldsMω7.2–7.3 using the area‐moment regressions ofJohnston[1996]. For the 08∶00 LT on January 23, 1812, and 03∶45 LT on February 7, 1812, mainshocks, we obtainMω7.0 andMω7.4–7.5, respectively. Our magnitude for the February mainshock is consistent with the established geometry of the Reelfoot fault, which all evidence suggests to have been the causative structure for this event. We note that the inference of lower magnitudes for the New Madrid events implies that site response plays a significant role in controlling seismic hazard at alluvial sites in the central and eastern United States. We also note that our results suggest that thrusting may have been the dominant mechanism of faulting associated with the 1811–1812 sequence.
Neotectonics of the Sumatran fault, Indonesia
Authors: Sieh Kerry , Natawidjaja Danny
Published: February 2004

The 1900‐km‐long, trench‐parallel Sumatran fault accommodates a significant amount of the right‐lateral component of oblique convergence between the Eurasian and Indian/Australian plates from 10°N to 7°S. Our detailed map of the fault, compiled from topographic maps and stereographic aerial photographs, shows that unlike many other great strike‐slip faults, the Sumatran fault is highly segmented. Cross‐strike width of step overs between the 19 major subaerial segments is commonly many kilometers. The influence of these step overs on historical seismic source dimensions suggests that the dimensions of future events will also be influenced by fault geometry. Geomorphic offsets along the fault range as high as ∼20 km and may represent the total offset across the fault. If this is so, other structures must have accommodated much of the dextral component of oblique convergence during the past few million years. Our analysis of stretching of the forearc region, near the southern tip of Sumatra, constrains the combined dextral slip on the Sumatran and Mentawai faults to be no more than 100 km in the past few million years. The shape and location of the Sumatran fault and the active volcanic arc are highly correlated with the shape and character of the underlying subducting oceanic lithosphere. Nonetheless, active volcanic centers of the Sumatran volcanic arc have not influenced noticeably the geometry of the active Sumatran fault. On the basis of its geologic history and pattern of deformation, we divide the Sumatran plate margin into northern, central and southern domains. We support previous proposals that the geometry and character of the subducting Investigator fracture zone are affecting the shape and evolution of the Sumatran fault system within the central domain. The southern domain is the most regular. The Sumatran fault there comprises six right‐stepping segments. This pattern indicates that the overall trend of the fault deviates 4° clockwise from the slip vector between the two blocks it separates. The regularity of this section and its association with the portion of the subduction zone that generated the giant (Mw9) earthquake of 1833 suggest that a geometrically simple subducting slab results in both simple strike‐slip faulting and unusually large subduction earthquakes.
Low‐temperature magnetic properties of pelagic sediments (Ocean Drilling Program Site 805C): Tracers of maghemitization and magnetic mineral reduction
Authors: Smirnov Alexei V. , Tarduno John A.
Published: February 2004

Magnetic properties of pelagic sediments from the western equatorial Pacific (Ocean Drilling Program (ODP) Site 805C) have been measured at low temperatures to investigate changes in magnetic mineralogy and the distribution of bacterial magnetite with depth. A saturation isothermal remanent magnetization (Mrs) was given to the samples by applying a strong magnetic field (2.5 T) at 20 K after cooling both in the absence (zero field cooling, ZFC) and presence of a 2.5 T magnetic field (field cooling, FC). The thermal demagnetization of both MrsZFC and MrsFC was measured from 20 to 300 K. No pronounced Verwey transition was observed in the data from samples shallower than the iron redox boundary. Below the boundary the Verwey transition is visible for all samples studied. We interpret this behavior as reflecting low‐temperature oxidation (maghemitization) of primary magnetite and the subsequent dissolution of maghemite coatings below the iron redox boundary. The low‐temperature oxidation may occur both at the water‐sediment interface and deeper in the suboxic zone of sediments as a result of bacteria‐mediated processes, including the reduction of nitrate compounds. MrsZFC and MrsFC thermal demagnetization curves of all samples diverge for the entire temperature range measured. The divergence may be caused by the low‐temperature stabilization of spontaneous magnetization vectors along with exchange interaction in the magnetite maghemite system. Together, maghemitization and magnetic reduction diagenesis appear to play a dominant role in controlling the low‐temperature properties of the sediment studied. As a result, low‐temperature magnetic studies may provide more information on differences in reduction diagenesis than on the distribution of bacterial magnetite in natural sediments. The changes in effective oxidation state below the iron redox boundary detected by low‐temperature methods may be useful in the study of some paleoenvironmental processes.
Mode of Cenozoic east‐west extension in Tibet suggesting a common origin of rifts in Asia during the Indo‐Asian collision
Authors: Yin An
Published: February 2004

Cenozoic rifts in Tibet were traditionally interpreted as a result of topographic collapse of the Tibetan Plateau, reaching the maximum elevation that can be supported by its mechanical strength. Recent studies have emphasized possible similarities between rifting in Tibet and extension in the Basin and Range of the western United States. However, when examined in detail, one finds that spacing of long (>100 km) rifts in Tibet (∼100–300 km) is significantly greater than that in the Basin and Range (∼20–40 km). From south to north, rift spacing decreases systematically: from 191±67 km in the Himalaya south of the Indus‐Yalu suture to 146±34 km in south Tibet between the Indus‐Yalu and Bangong‐Nujiang sutures and farther north to 101±31 km in central Tibet between the Bangong‐Nujiang and Jinsha sutures. Instability analysis suggests that the mantle lithosphere must have been involved in east‐west Tibetan extension. Specifically, the widely spaced rifts in The Himalaya and Tibet may have been related to the presence of a relatively light crust (density <∼2.90 g cm−3) and a strong mantle lithosphere (∼40 km thick and a factor of 5 stronger than the upper crust). The observed systematic decrease in rift spacing can be explained by the known decrease in the crustal thickness in Tibet, from ∼70–80 km in The Himalaya in the south to ∼50–55 km in central Tibet in the north. A regional comparison of rifts in east Asia suggests that both the involvement of the mantle lithosphere and the age of rift initiation are similar for Tibetan rifts, the Baikal rift, and the Shanxi graben. This implies that topographic collapse or a convective event in the mantle cannot be the sole cause for the development of the Tibetan rifts. A regional boundary condition applied throughout east Asia must be required.
Impact of a complex overburden on analysis of bright reflections: A case study from the Mendocino Triple Junction
Authors: Henstock Timothy J. , Levander Alan
Published: February 2004

Determination of the physical properties and the geometry of features within the subsurface is a common application of active source seismic data. We use the example of unusually bright reflections identified in the lower crust of the Coast Ranges of northern California during the Mendocino Triple Junction Seismic Experiment to investigate the robustness of such determinations. The sources of these reflections are significant because they lie in a region of transition between subduction and strike‐slip tectonics, but they also have an overburden of heterogeneous Franciscan terrane rocks. We use finite difference synthetic seismograms including a stochastic overburden representative of Franciscan rocks and a realistic target geometry to examine how typical seismic diagonostics are affected under these conditions. The addition of a realistic overburden to the models replicates variations in the amplitude of the observed Pg arrival and leads to variations in measures of the amplitude of the reflected arrivals that are also similar to those in the data. Some of these variations are due to the measures of amplitude used, some represent real variations at the target, and others are due to distortion of the illumination field by the overburden. Overall reflection amplitudes are reduced due to transmission losses within the overburden, and the P wave seismic velocity within the reflectors may be as low as 2.5 km s−1 rather than the 3.5 km s−1 estimated when the overburden was not taken into account. Migrations of simulations, including combinations of overburden, target dip, and uneven spatial sampling, recover somewhat consistent but inaccurate approximations to the true target geometry: Discontinuous migrated images may result from a discontinuous wave field even where reflectors were continous. The simulations also imply that neither reversed reflection polarity nor converted shear waves are a reliable aspect of the reflected wave field if the overburden or target is complex. Comparison of migrated simulations to the data suggests that the reflections originate from two relatively continuous layers at the top and base of the lower crust, 300m and 100–150m in thickness, respectively. We believe that the reflections are from lenses of basaltic melt recently emplaced in the crust and generated by decompression melting in a “slab‐free” window.
Archeomagnetism of Ontario potsherds from the last 2000 years
Authors: Yu Yongjae , Dunlop David J. , Pavlish Larry , Cooper Martin
Published: February 2004

An archeomagnetic study was carried out on potsherds samples from sites in Ontario with ages ranging from A.D. 90 to A.D. 1640 as determined by 14C dating. Thellier double‐heating paleointensity experiments were performed in air on 65 specimens of 52 samples from seven sample sets. Reliable paleointensity estimates were obtained for 49 specimens. Alternating field and thermal demagnetization, temperature dependence of weak‐field susceptibility, and hysteresis measurements indicate that magnetite of pseudo‐single‐domain grain size is the carrier of natural remanent magnetization. The paleointensity results follow a half‐cycle sine curve, with a steady decrease from 54.0±5.9 μT to 37.6±5.7 μT between A.D. 90 and A.D. 885 and a monotonic increase from 52.0±6.1 μT to 59.4±1.7 μT between A.D. 1200 and A.D. 1900. The paleointensities determined yield virtual axial dipole moments (VADMs) of the Earth's magnetic field that agree well with those from other parts of North America, except between A.D. 900 and A.D. 1400, when they are systematically lower. This discrepancy is probably caused by a substantial non‐dipole field in southwestern North America from the tenth to the fifteenth century, since secular variation studies using potsherds from Arizona and lake sediments from Minnesota show different inclination variations during that period.
Continuous deflation of the Askja caldera, Iceland, during the 1983–1998 noneruptive period
Authors: Sturkell Erik , Sigmundsson Freysteinn
Published: February 2004

The Askja volcano at the spreading plate boundary in north Iceland consists of nested calderas, the latest formed in an eruption in 1875. Several eruptions have occurred since in Askja, the most recent in 1961. Precise leveling has been conducted yearly at Askja since 1983. In 1993, a dense GPS network was measured in and around the Askja caldera consisting of more than 20 points, and we remeasured this complete network for the first time in 1998. Askja subsided during the period from 1983 to 1998. Observed deformation fits broadly with a “Mogi” point source model with best fitting location near the center of the main Askja caldera (65.0448°N, 16.7805°W) at a depth of 2.8 km. From 1983 to 1991 the yearly subsidence rate between the end points of a leveling profile decayed gradually from ∼10 mm/yr to an average of ∼7 mm/yr in the 1991–1998 period. Total subsidence at the Askja center in the 1983–1998 period is at least 75 cm, and the integrated volume of surface subsidence is ∼0.037 km3. This period has been a “quiet” period at Askja with no eruptions, large earthquakes or known dike injections. Such a high rate of “background” deformation has not been observed at other volcanoes in Iceland. The eruption of a neighboring volcano in 1996 has not resulted in a modified deformation pattern at Askja, indicating no pressure connection at depth between the two systems. Solidification may account for part of the observed contraction and subsidence in Askja.
The stability and P–V–T equation of state of CaSiO3 perovskite in the Earth's lower mantle
Authors: Shim Sang‐Heon , Duffy Thomas S. , Shen Guoyin
Published: February 2004

Energy dispersive X‐ray diffraction measurements for polycrystalline CaSiO3 perovskite were carried out at in situ transition zone and lower mantle P‐T conditions (P = 18–96 GPa, T= 1238–2419 K) using the diamond anvil cell and double‐sided laser heating at the GeoSoilEnviro Consortium for Advanced Radiation Sources (GSECARS) sector of the Advanced Photon Source. An analysis of the temperature error sources in laser heating reveals that the axial and radial thermal gradients are the greatest error source. We have used measurements where the combined temperature error (1σ) from all sources is <150 K. By obtaining X‐ray diffraction patterns at 8–22 GPa and 300–2200 K range, the high‐temperature phase boundary between CaSi2O5+Ca2SiO4 and CaSiO3 perovskite was determined to be 14–16 GPa, in contrast to the results of previous large‐volume press (LVP) measurements (9–11 GPa). The stability of cubic CaSiO3 perovskite was confirmed to 2300 km depth in the Earth's interior. No evidence of phase transformation or break down to oxides was observed. The proposed tetragonal distortion, and hence the phase transformation from distorted phase to cubic, was not observed. The combined data set of this study and earlier LVP measurements was fit to a Birch‐Murnaghan‐Debye equation. By fixing V0 = 27.45 cm3/mol, KT0 = 236 GPa, and K′T0 = 3.9 from recent static compression data and θ0 = 1000 K, we obtain γ0 = 1.92±0.5 and q = 0.6±0.3. Although data to 69 GPa and 2380 K were used in the fitting, this result is also consistent with measurements to 96 GPa. This result yields not only density and bulk modulus but also higher‐order thermoelastic parameters, such as thermal expansivity and temperature dependence of bulk modulus, at lower mantle P‐T condition in an internally consistent way. This direct measurement of the equation of state at lower mantle condition verifies that the density and bulk modulus of CaSiO3 perovskite at lower mantle P‐T conditions are very close to seismic values (within 1.5 and 3.0%, respectively). These differences are sufficiently small that the abundance of CaSiO3 perovskite will have negligible effects on density and bulk modulus profiles for the mantle.
Permeability enhancement due to microcrack dilatancy in the damage regime
Authors: Simpson Guy , Guéguen Yves , Schneider Frédéric
Published: February 2004

A three dimensional microscopically based permeability model incorporating inelastic deformation has been developed to account for the modification of transport properties due to fracturing. The basic hypothesis investigated is that permeability enhancement during brittle deformation is caused by the formation of dilatant microcracks which are associated with friction sliding on a preexisting random population of shear cracks. Additional dilatancy, produced as a result of frictional sliding over asperities, is also accounted for by the introduction of a crack roughness parameter. Linear elastic fracture mechanics is used to calculate the evolution of crack length and crack area as a function of applied stress and fluid pressure. After making a geometrical simplification the microcrack parameters derived from the deformation model can be used to calculate the permeability tensor assuming that fluid transport results from Poiseuille flow through a connected distribution of cracks. The model enables investigation of the macroscopic permeability variation as a function of two loading parameters, three constant material parameters, and the crack roughness parameter. Results demonstrate that permeability is a smooth but strongly increasing (near power law) function of the Terzaghi effective stress ratio and is strongly dependent on the initial crack density, Young's modulus, the friction coefficient, and the effective confining pressure. Numerical results are in quantitative agreement with published experimental measurements and display behavior similar to results obtained with a relatively simple analytical model. The model enables calculation of the degree of stress‐induced anisotropy, which is shown is be relatively small (< × 10) for resonable effective stress ratios. The modeling presented provides a quantitative tool with which the effects of microcrack‐induced permeability enhancement can be investigated within the broader context of coupled fluid flow and brittle deformation in the crust.
Scaling of time‐dependent stagnant lid convection: Application to small‐scale convection on Earth and other terrestrial planets
Authors: Solomatov V. S. , Moresi L.‐N.
Published: February 2004

Small‐scale convection associated with instabilities at the bottom of the lithospheric plates on the Earth and other terrestrial planets occurs in the stagnant lid regime of temperature‐dependent viscosity convection. Systematic numerical simulations of time‐dependent, internally heated stagnant lid convection suggest simple scaling relationships for a variety of convective parameters and in a broad range of power law viscosities. Application of these scaling relationships to the Earth's oceanic lithosphere shows that for either diffusion or dislocation viscosity of olivine, convective instabilities occur in the lower part of the lithosphere between 85 and 100 km depth (the rheological sublayer). “Wet” olivine satisfies constraints on the heat flux and mantle temperature better than “dry” olivine, supporting the view that the upper mantle of the Earth is wet. This is also consistent with the fact that the rheological sublayer is located below the Gutenberg discontinuity which was proposed to represent a sharp change in water content. The viscosity of asthenosphere is (3–6)×1018 Pa s, consistent with previous estimates. The velocities of cold plumes are relatively high reaching several meters per year in the dislocation creep regime. A low value of the heat flux in old continental cratons suggests that continental lithosphere might be convectively stable unless it is perturbed by processes associated with plate tectonics and hot plumes. The absence of plate tectonics on other terrestrial planets and the low heat transport efficiency of stagnant lid convection can lead to widespread melting during the thermal evolution of the terrestrial planets. If the terrestrial planets are dry, small‐scale convection cannot occur at subsolidus temperatures.
Evidence for temporal variation of seismic velocity within the upper continental crust
Authors: Bokelmann Götz H. R. , Harjes Hans‐Peter
Published: February 2004

Observations of systematic temporal variations of seismic anisotropy are presented for an induced‐seismicity experiment at 9 km depth. These observations were made under particularly well‐controlled conditions in the German Continental Deep Drilling Program (KTB) borehole, using shear wave splitting from similar events recorded at a three‐component instrument located at 4 km depth from a hydraulic fracturing experiment at 9 km depth. In a large set of seismic events recorded during the experiment, many can be associated with multiplets exhibiting essentially identical waveforms. Since they must have approximately the same source location and source radiation pattern, these events are particularly useful for testing the hypothesis of time‐dependent anisotropy. Anisotropy itself is clearly a very prominent feature in the data. A simple approach for waveform matching of split shear waves allows unprecedented resolution of variations in shear wave splitting. Importantly, the variation of shear wave splitting with time is a relative measurement, which can be performed with higher accuracy than the associated absolute measurement. In particular, the relative measurement is not affected by timing errors nor by event distance variations. During the experiment the difference between shear wave velocities decreases by ∼2% within ∼12 hours. After that, the medium apparently approaches a state which is stable for at least 5 hours. We suggest that the temporal variation is due to the tectonic stress release from seismic events caused by the fluid injection. This model requires the presence of fluid‐filled cracks at depths larger than 4 km.
Deep structure of the Baringo Rift Basin (central Kenya) from three‐dimensional magnetotelluric imaging: Implications for rift evolution
Authors: Hautot Sophie , Tarits Pascal , Whaler Kathy , Le Gall Bernard , Tiercelin Jean‐Jacques , Le Turdu Caroline
Published: February 2004

Three‐dimensional modeling of data from 31 vertical electrical and 24 magnetotelluric soundings collected in the Baringo‐Bogoria Basin (central Kenya Rift Valley) shows a thick succession of well‐defined tectonostratigraphic units beneath the Recent deposits of the Marigat‐Loboi Plain. They include from top to bottom, a sedimentary basin, ∼1.5 km thick, controlled by N‐S and N140° structural trends, and a thick homogeneous resistive layer related to the bottom of the basin, overlying a conductive structure, which cannot be clearly correlated with the Proterozoic basement. It is suggested that the resistive layer correlates with the mid‐Miocene plateau‐type flood phonolites which flowed over the early Kenya Rift during a major volcanic activity period. The conductive structure overlain by these lava flows could be a sedimentary basin developed during the initial phase of rifting, during the Oligocene‐Miocene. The absence of a significant gravity low associated with this deep basin suggests a zone of dense intrusion deeper than 5–10 km, not discernible with the magnetotelluric data but required to explain the gravity anomalies. The recognition of a deeply buried sedimentary succession lying between 4 and 8 km beneath the lower Miocene volcanic series of the Baringo valley would provide new insights into the regional volcano‐sedimentary stratigraphie succession and the rift development of the Kerio and Baringo Basins.
Three‐dimensional magnetic imaging of the Chicxulub Crater
Authors: Pilkington Mark , Hildebrand Alan R.
Published: February 2004

Although few magnetization measurements are available for the structural elements of the Chicxulub impact crater, the magnetization intensities of the melt sheet, upper breccia unit, and central uplift are 3–4 orders of magnitude greater than the 3‐ to 4‐km‐thick carbonate and evaporite stratigraphy covering the Yucatan block. This allows three‐dimensional modeling of the crater's structure by inversion using a two‐layer model. Two layers are separately inverted by dividing the crater's magnetic field expression into <40‐ and >40‐km wavelength components. The upper layer (average depth 2 km) models the distribution of highly‐magnetized zones in the crater's melt sheet. The lower layer (average depth 5 km) represents relief on the Yucatan block's basement surface and effectively maps the crater's ∼50‐km‐diameter central uplift and possibly the expression of the surrounding collapsed disruption cavity fill. The shallower magnetized zones consist of two generally concentric distributions, at radii of ∼20 and ∼45 km. These highly magnetized zones are thought to result from hydrothermal systems, localized at the edge of the central uplift and the collapsed disruption cavity, having produced magnetic phases during alteration of the melt sheet.
Fault rupture between dissimilar materials: Ill‐posedness, regularization, and slip‐pulse response
Authors: Cochard A. , Rice J. R.
Published: February 2004

Faults often separate materials with different elastic properties. Nonuniform slip on such faults induces a change in normal stress. That suggests the possibility of self‐sustained slip pulses [Weertman, 1980] propagating at the generalized Rayleigh wave speed even with a Coulomb constitutive law (i.e., with a constant coefficient of friction) and a remote driving shear stress that is arbitrarily less than the corresponding frictional strength. Following Andrews and Ben‐Zion [1997] (ABZ), we study numerically, with a two‐dimensional (2‐D) plane strain geometry, the propagation of ruptures along such a dissimilar material interface. However, this problem has been shown to be ill‐posed for a wide range of elastic material contrasts [Renardy, 1992; Martins and Simões, 1995; Adams, 1995]. Ranjith and Rice [2000] (RR) showed that when the generalized Rayleigh speed exists, as is the case for the material contrast studied by ABZ, the problem is ill‐posed for all values of the coefficient of friction, f, whereas when it does not exist, the problem is ill‐posed only for f greater than a critical value. We illustrate the ill‐posedness by showing that in the unstable range the numerical solutions do not converge through grid size reduction. By contrast, convergence is achieved in the stable range but, not unexpectedly, only dying pulses are then observed. RR showed that among other regularization procedures, use of an experimentally based law [Prakash and Clifton, 1993; Prakash, 1998], in which the shear strength in response to an abrupt change in normal stress evolves continuously with time or slip toward the corresponding Coulomb strength, provides a regularization. (Classical slip weakening or rate‐ and state‐dependent constitutive laws having the same kind of abrupt response as Coulomb friction also do not regularize the problem.) Convergence through grid size reduction is then achieved in the otherwise ill‐posed range. For sufficiently rapid shear strength evolution, self‐sustained pulses are observed. When the generalized Ray‐leigh wave speed exists, they propagate essentially at that velocity and, in consistence with Weertman's [1980] analysis, the propagation occurs only in one direction, which is that of slip in the more compliant medium. When the generalized Rayleigh wave speed does not exist, similar self‐sustained pulses propagate at about the slower S wave speed and in the same direction. RR also suggested that for sufficiently high coefficient of friction, another kind of (less unstable) self‐sustained pulses, propagating at a velocity close to the slower P wave speed and in the opposite direction, could also exist. We numerically verify that prediction.
Fracture process zone in granite
Authors: Zang Arno , Wagner F. Christian , Stanchits Sergei , Janssen Christoph , Dresen Georg
Published: February 2004

In uniaxial compression tests performed on Aue granite cores (diameter 50 mm, length 100 mm), a steel loading plate was used to induce the formation of a discrete shear fracture. A zone of distributed microcracks surrounds the tip of the propagating fracture. This process zone is imaged by locating acoustic emission events using 12 piezoceramic sensors attached to the samples. Propagation velocity of the process zone is varied by using the rate of acoustic emissions to control the applied axial force. The resulting velocities range from 2 mm/s in displacement‐controlled tests to 2 μm/s in tests controlled by acoustic emission rate. Wave velocities and amplitudes are monitored during fault formation. P waves transmitted through the approaching process zone show a drop in amplitude of 26 dB, and ultrasonic velocities are reduced by 10%. The width of the process zone is ∼9 times the grain diameter inferred from acoustic data but is only 2 times the grain size from optical crack inspection. The process zone of fast propagating fractures is wider than for slow ones. The density of microcracks and acoustic emissions increases approaching the main fracture. Shear displacement scales linearly with fracture length. Fault plane solutions from acoustic events show similar orientation of nodal planes on both sides of the shear fracture. The ratio of the process zone width to the fault length in Aue granite ranges from 0.01 to 0.1 inferred from crack data and acoustic emissions, respectively. The fracture surface energy is estimated from microstructure analysis to be ∼2 J. A lower bound estimate for the energy dissipated by acoustic events is 0.1 J.
Instantaneous geodetic positioning at medium distances with the Global Positioning System
Authors: Bock Yehuda , Nikolaidis Rosanne M. , de Jonge Paul J. , Bevis Michael
Published: February 2004

We evaluate a new method of Global Positioning System (GPS) data analysis, called instantaneous positioning, at spatial scale lengths typical of interstation spacings in a modern crustal motion network. This method is more precise and versatile than traditional GPS static and kinematic processing of multi‐epoch batches of data. The key to instantaneous positioning is the ability to resolve integer‐cycle phase ambiguities with only a single epoch of dual‐frequency phase and pseudorange data, rendering receiver cycle slips irrelevant. We estimate three‐dimensional relative coordinates and atmospheric zenith delay parameters independently every 30 s over a 12‐week period for baseline distances of 50 m, 14 km, and 37 km. Horizontal precision of a single‐epoch coordinate solution is about 15 mm and vertical precision is about 7–8 times worse. Removing that component of each time series which repeats with a period of exactly 1 sidereal day, and thus manifests signal multipath, reduces the scatter by about 50% in all components. Solution averaging of the high‐frequency time series can be performed using any number of measurement epochs to further improve coordinate precision. We demonstrate that the daily coordinates estimated with instantaneous positioning are more precise (by 20–50% per coordinate component) than those estimated with 24‐hour batch processing. Spectral analysis of the single‐epoch solutions indicates that the flicker noise characteristic of GPS time series observed in lower‐frequency bands also affects GPS solutions in the frequency band 0.01 mHz to 10 mHz. We argue that the flicker noise is induced by tropospheric effects. Since modern GPS receivers are capable of observing at frequencies as high as 10 Hz, our technique significantly overlaps and complements the frequency band of broadband seismology and benefits other research areas such as earthquake geodesy, volcanology, and GPS meteorology.
Thermochemical convection in and beneath intracratonic basins: Onset and effects
Authors: Schoofs Stan , Trompert Ron A. , Hansen Ulrich
Published: February 2004

Observations of episodic subsidence and high organic maturity in the intracratonic Michigan basin cannot be explained by simple conductive models. Moreover, the sylvinite deposits within the basin indicate an evaporite evolution other than a simple evapoconcentration of standard seawater. By means of numerical simulations we have investigated whether these deviations can be attributed to periods of free thermochemical convection (TCC) of aqueous fluids within a fractured igneous rift body underlying the sediments. Since crustal rocks are known to be heterogeneous and anisotropic, we have first investigated the onset of TCC in an idealized domain, for temperature and salinity linearly increasing with depth. The sensitivity study considers several heterogeneous permeability fields, by varying the stochastic correlation lengths of the medium and by introducing anisotropy on the subgrid scale. Due to destabilization of the fluid in pockets of relatively high porosity, the onset of convection in the heterogeneous media occurs at ratios of chemical to thermal buoyancy 1 order higher than in homogeneous media. Next we have studied the thermal, chemical, and subsidence effects of the convective flow in and beneath the Michigan basin, for the situations before and after closure from the open sea. TCC explains the elevated level of organic maturity in the sediments and up to a quarter of the observed fast subsidence rates. Finally, we argue that basement involvement could have changed the seawater composition, and so leads to the direct deposition of sylvinite.