bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of hydrogen fugacity and confining pressure on the interdiffusion rate of NaSi‐CaAl in plagioclase
Authors: Yund Richard A. , Snow Eleanour
Published: February 2008

Average
values for NaSi‐CaAl interdiffusion in the compositional interval from An0 to An26 have been determined at 1000°C by the method of lamellar homogenization. At 1500 MPa confining pressure (P),
increases 1 order of magnitude (5.0×10−21 to 4.0×10−20 m2/s) for 4 orders of magnitude increase in hydrogen fugacity (0.029, Mn3O4‐Mn2O3 buffer, to 197 MPa, FeO‐Fe3O4 buffer). At constant hydrogen fugacity (fH2),
increases rapidly at low pressure and becomes nearly independent of P above 1000 MPa. (For fH2 = 0.1 MPa,
= 2.8 × 10−22 m2/s at P = 0.1 MPa, 5.0 × 10−21 at P = 500, 1.3×10−20 at P = 1000, and 1.4×10−20 at P = 1500). The dependence of
on increasing pressure, when a hydrogen‐related species is present, is believed to be due to an increase in the concentration of the structural defect associated with increase in the hydrogen impurity. In most crustal igneous rocks, which are internally buffered near quartz‐fayalite‐magnetite, the dependence of
on fH2 is relatively minor compared to the effect of confining pressure.
Geodetic Predictions of a strike‐slip fault model: Implications for intermediate‐ and short‐term earthquake prediction
Authors: Lorenzetti Elizabeth , Tullis Terry E.
Published: February 2008

Mechanical models of faulting during an earthquake cycle now exist that are based on laboratory‐derived constitutive laws for rock friction. Such models allow calculation of geodetic changes as a function of depth, distance from the fault, and time and are useful for gaining insight on the magnitude and timing of geodetic changes that might occur prior to an earthquake. We have used the Tse and Rice (1986) model of a crustal strike‐slip fault to study the sensitivity of surface deformation to changes in the values of the friction constitutive parameters and the amount of dynamic overshoot during an earthquake. Models were examined with values of the characteristic decay distance Dc in the constitutive law ranging from 5 to 80 mm, with near‐surface values of the steady state velocity dependence a−b of −0.0015 and −0.003, and with either zero or 30% dynamic overshoot. The time variations of the predicted displacement, velocity, strain, and strain rate distributions are strongly affected by changes in Dc and a−b, and only slightly affected by the amount of dynamic overshoot. The larger values of Dc and the less negative value of a−b (i.e., large ratios of Dc to b−a) produce larger and earlier precursory signals. The most useful signals for predicting the model earthquakes are strain changes near the fault, because for nearly all model parameters studied, the predicted strain changes are considerably larger than the detectability limits of existing borehole strain meters in the presence of earth noise. Detectable changes occur a few minutes to a month prior to the earthquake, depending on parameter values. Premonitory increases in velocity also occur; a pattern of larger velocities at distances a few kilometers away from the fault trace is a diagnostic indication of accelerating slip at depth that would be useful for predictive purposes. Unfortunately, these velocity changes are detectable with creep meters and two‐color laser electronic distance measuring devices only for the largest ratios of Dc to b−a. In agreement with recent measurements for several earthquakes, the predicted amount of preseismic moment release is <0.5% of the earthquake moment for most simulations, and the postseismic moment release is a significant fraction of the earthquake moment. This two‐dimensional model suggests that earthquakes of magnitude 6 or greater on strike‐slip faults that cut the Earth's surface could be predicted using existing instrumentation, unless the ratio of Dc to b−a for natural faults is equal to or smaller than the smallest values investigated by Tse and Rice (1986). Study of three‐dimensional models is needed, since premonitory signals will be smaller if along‐strike variation is considered.
Centroid single force inversion of seismic waves generated by landslides
Authors: Kawakatsu Hitoshi
Published: February 2008

The centroid moment tensor (CMT) waveform inversion method of Dziewonski et al. (1981) is modified to analyze long period seismic waves generated by “single force” events such as the gigantic landslides associated with the 1980 eruption of Mount St. Helens. We refer to the method as centroid single force (CSF) inversion. As the result of the inversion we obtain the spatio‐temporal centroid and three components of a vector we call the CSF vector, which is the seismic point source representation of a landslide and whose direction is the opposite of the direction of sliding. The sealer magnitude of the CSF vector measures the overall size of the landslide and is given by MCSF = MD, where M is the mass of the sliding object and D is the sliding distance. We apply CSF inversion to the long period surface waves generated by the 1980 Mount St. Helens event, the 1975 Kalapana earthquake, and the 1974 Mantaro landslide, using seismograms from the GDSN, IDA and HGLP networks. The CSF solution for the St. Helens event is remarkably consistent with the actual geological observations and proves the efficacy of CSF inversion. The CSF solution for the Kalapana event does not fit the overall waveforms better than the CMT solution does. However, because the CMT solution does not explain the observed Love wave radiation pattern, it may be necessary to invoke a combination of both types of mechanism for this event. Although the geometry of the CSF solution (the direction of the force) for the Mantaro event is consistent with the actual landslide, MCSF is about 5 times smaller than that expected from the mass and travel distance of the landslide estimated by geological observations. This discrepancy may suggest either the relatively aseismic nature of this landslide or that the total volume of the slide was overestimated. By analogy to single station CMT inversion, single station CSF inversion also appears to be stable and useful.
Drainage pits in cohesionless materials: Implications for the surface of Phobos
Authors: Horstman K. C. , Melosh H. J.
Published: February 2008

Viking orbiter images show grooves and chains of pits crossing the surface of Phobos, many of which converge toward the large crater Stickney or its antipode. Although it has been proposed that the pits and grooves are chains of secondary craters, their morphology and geometric relations suggest that they are the surface traces of fractures in the underlying solid body of Phobos. Several models have been proposed to explain the pits, of which the most plausible are gas venting and drainage of regolith into open fractures. The latter mechanism is best supported by the image data and is the mechanism studied in this investigation. Drainage pits and fissures are modeled experimentally by using two rigid substrate plates placed edge to edge and covered by uniform thicknesses of dry fragmental debris (simulated regolith). Fracture extension is simulated by drawing the plates apart, allowing drainage of regolith into the newly created void. A typical drainage experiment begins with a shallow depression on the surface of the regolith, above the open fissure. Increased drainage causes local drainage pits to form; continued drainage causes the pits to coalesce, forming a cuspate groove. The resulting experimental patterns of pits and grooves have pronounced similarities to those observed on Phobos. Characteristics such as lack of raised rims, linearity of grooves and chains of pits, uniform spacing of pits, and progression from discrete pits to cuspate grooves are the same in the experiments and on Phobos. In contrast, gas‐venting pits occur in irregular chains and have raised rims. These experiments thus indicate that the Phobos grooves and pits formed as drainage structures. The pit spacing in an experiment is measured at the time that the maximum number of pits forms, prior to groove development. The average pit spacing is compared to the regolith thickness for each material. Regression line fits indicate that the average spacing of drainage pits in unconsolidated, noncohesive regolith is nearly equal to the thickness of regolith and appears to be independent of the angle of repose, within the resolution of our experiments. This provides a simple means of estimating regolith thickness where drainage pits are present. On Phobos, two locations differing by 90° in longitude have average pit spacings that suggest regolith thicknesses of 290 and 300 m, suggesting that large areas of Phobos have a nearly uniform regolith thickness of approximately 300 m.
Mantle heterogeneities, geoid, and plate motion: A Monte Carlo inversion
Authors: Ricard Yanick , Vigny Christophe , Froidevaux Claude
Published: February 2008

Seismic tomography in both the upper and the lower mantle, as well as subducting oceanic slabs defined by seismicity, has been translated into density heterogeneities to generate models of mantle circulation. These models can predict both the surface velocities and the geoid, which can be compared with plate tectonics and gravity data. A given model is specified by 6 parameters related to the viscosities of 3 mantle layers and the absolute amplitudes of density variations in the upper and lower mantle as well as in the slabs. The values of these parameters are chosen at random within an acceptable range. Each model is submitted to an appropriate test comparing observations with predictions. The results of the most successful models selected by this Monte Carlo inversion are displayed. They yield preferred mantle viscosity structures exhibiting large variations at depth. With a physical interface between upper and lower mantle, i.e., with the possibility for the circulation to penetrate the 650 km discontinuity, two classes of viscosity profiles stand out. The first one implies a regular increase of the viscosity in the sublithospheric mantle, with reasonable values for the density parameters. The second one is unexpected in the sense that it predicts a very stiff bottom for the upper mantle. It also requires vanishingly small amplitudes for the upper mantle density heterogeneities defined by tomography, which would thus have to be of lithological rather than thermal origin. With a chemical interface at 650 km the outcome is very similar: the same two classes of viscosity structures do yield a satisfactory geoid prediction. However only the class of models with a stiff layer at midmantle depths predicts acceptable surface velocities. Altogether the best models out of some 60,000 which have been tested only explain one third of the geoid and two thirds of the surface divergence for spherical harmonic degrees 1 to 6. Nevertheless the main features of these two observed patterns are present in the computed maps, and 4 out of 6 correlation coefficients lie close to the 90% confidence level. This is true for the geoid as well as for the surface divergence of the displacement velocity. However, as the internal viscosity structure has been assumed to have spherical symmetry, the rotational component of the surface velocities cannot be predicted.
Spectral characteristics of chlorites and Mg‐serpentines using high‐resolution reflectance spectroscopy
Authors: King Trude V. V. , Clark Roger N.
Published: February 2008

The present laboratory study using high‐resolution reflectance spectroscopy (0.25–2.7 μm) focuses on two primary phyllosilicate groups, serpentines and chlorites. The results show that it is possible to spectrally distinguish between isochemical end‐members of the Mg‐rich serpentine group (chrysotile, antigorite, and lizardite) and to recognize spectral variations in chlorites as a function of Fe/Mg ratio (∼8–38 wt % Fe). The position and relative strength of the 1.4‐μm absorption feature in the trioctahedral chlorites appear to be correlated to the total iron content and/or the Mg/Si ratio and the loss on ignition values of the sample. Spectral differences in the 2.3‐μm wavelength region can be attributed to differences in lattice environments and are characteristic for specific trioctahedral chlorites. The 1.4‐μm feature in the isochemical Mg‐rich serpentines (total iron content ∼1.5–7.0 wt%) show marked spectral differences, apparently due to structural differences.
Rupture characteristics of the 1982 Tonga and 1986 Kermadec earthquakes
Authors: Lundgren Paul R. , Okal Emile A. , Wiens Douglas A.
Published: February 2008

Source time functions are obtained for the December 19, 1982, Tonga and October 20, 1986, Kermadec earthquakes from deconvolution of P waves, using a model including a significant dip of the ocean bottom. In such a geometry, the amplitudes of subsequent water multiples are large and irregular. As a result, we can model these two events as consisting of a single pulse of short duration (23 s in the case of the 1982 event, and 19 s for the 1986 one), despite complex teleseismic waveshapes. This is in contrast to inversions using flat‐layered models, which required very complex source time functions, which were mutually inconsistent at different stations. The anomalously large tsunami generated by the 1982 event is modeled as the result of propagation of the rupture into the sedimentary wedge at the trench, rather than due to a long‐lived complex source time function, the latter being also incompatible with the total lack of excitation of the Earth's normal modes in the low‐frequency limit. For the 1986 event, we derive a source duration (19 s) very short in relation to its moment (8×1027 dyn‐cm). This earthquake, a high stress drop rupture featuring a unique combination of strike‐slip and thrust motion on a plane dipping oceanwards of the trench, is interpreted as expressing internal deformation of the subducting Pacific slab at a depth of 40 km, under the compressional stresses resulting from the collision.
Pillow basalts of the Angayucham terrane: Oceanic plateau and island crust accreted to the Brooks Range
Authors: Pallister John S. , Budahn James R. , Murchey Benita L.
Published: February 2008

The Angayucham Mountains (north margin of the Yukon‐Koyukuk province) are made up of an imbricate stack of four to eight east‐west trending, steeply dipping, fault slabs composed of Paleozoic (Devonian to Mississippean), Middle to Late Triassic, and Early Jurassic oceanic upper crustal rocks (pillow basalt, subordinate diabase, basaltic tuff, and radiolarian chert). Field relations and geochemical characteristics of the basaltic rocks suggest that the fault slabs were derived from an oceanic plateau or island setting and were emplaced onto the Brooks Range continental margin. The basalts are variably metamorphosed to prehnite‐pumpellyite and low‐greenschist facies. Major element analyses suggest that many are hypersthene‐normative olivine tholeiites. Classification based on immobile trace elements confirms the tholeiitic character of most of the basalts but suggests that some had primary compositions transitional to alkali basalt. Although field and petrographic features of the basalts are similar, trace element characteristics allow definition of geographically distinct suites. A central outcrop belt along the crest of the mountains is made up of basalt with relatively flat rare earth element (REE) patterns. This belt is flanked to the north and south by LREE (light rare earth element)‐enriched basalts. Radiolarian and conodont ages from interpillow and interlayered chert and limestone indicate that the central belt of basalts is Triassic in age, the southern belt is Jurassic in age, and the northern belt contains a mixture of Paleozoic and Mesozoic ages. Data for most of the basalts cluster in the “within‐plate basalt” fields of trace element discriminant diagrams; none have trace‐element characteristics of island arc basalt. The Triassic and Jurassic basalts are geochemically most akin to modern oceanic plateau and island basalts. Field evidence also favors an oceanic plateau or island setting. The great composite thickness of pillow basalt probably resulted from obduction faulting, but the lack of fault slabs of gabbro or peridotite suggests that obduction faults did not penetrate below oceanic layer 2, a likely occurrence if layer 2 were anomalously thick, as in the vicinity of an oceanic island. The presence of basaltic tuff interbeds indicates proximity to an explosive basaltic eruptive center. The juxtaposition of submarine basalts of differing chemical affinity and age, adjacent to higher‐grade Paleozoic metamorphic rocks of the Brooks Range to the north, may be explained by obduction of internally complex (thickened) oceanic crust formed in an ocean plateau setting. Emplacement and rotation of thrust plates to steep attitudes occurred during accretion of the Brooks Range passive margin, probably beginning in the Late to Middle Jurassic.
Isotopic and trace element variations in the Ruby batholith, Alaska, and the nature of the deep crust beneath the Ruby and Angayucham terranes
Authors: Arth Joseph G. , Zmuda Clara C. , Foley Nora K. , Criss Robert E. , Patton W. W. , Miller T. P.
Published: February 2008

Thirty‐six samples from plutons of the Ruby batholith of central Alaska were collected and analyzed for 22 trace elements, and many were analyzed for the isotopic compositions of Sr, Nd, O, and Pb in order to delimit the processes that produced the diversity of granodioritic to granitic compositions, to deduce the nature of the source of magmas at about 110 Ma, and to characterize the deep crust beneath the Ruby and Angayucham terranes. Plutons of the batholith show a substantial range in initial 87Sr/86Sr (SIR) of 0.7055–0.7235 and a general decrease from southwest to northeast. Initial 143Nd/144Nd (NIR) have a range of 0.51150–0.51232 and generally increase from southwest to northeast. The δ18O values for most whole rocks have a range of +8.4 to +11.8 and an average of +10.3‰. Rb, Cs, U, and Th show large ranges of concentration, generally increase as SiO2 increases, and are higher in southwest than in northeast plutons. Sr, Ba, Zr, Hf, Ta, Sc, Cr, Co, and Zr show large ranges of concentration and generally decrease as SiO2 increases. Rare earth elements (REE) show fractionated patterns and negative Eu anomalies. REE concentrations and anomalies are larger in the southwest than in the northeast plutons. Uniformity of SIR and NIR in Sithylemenkat and Jim River plutons suggests a strong role for fractional crystallization or melting of uniform magma sources at depth. Isotopic variability in Melozitna, Ray Mountains, Hot Springs, and Kanuti plutons suggests complex magmatic processes such as magma mixing and assimilation, probably combined with fractional crystallization, or melting of a complex source at depth. The large variations in SIR and NIR in the batholith require a variation in source materials at depth. The southwestern plutons probably had dominantly siliceous sources composed of metamorphosed Proterozoic and Paleozoic upper crustal rocks. The northeastern plutons probably had Paleozoic sources that were mixtures of siliceous and intermediate to mafic crustal rocks. The inferred sources could well have been the higher‐metamorphic‐grade lithologic equivalents of the exposed Proterozoic(?) to Paleozoic schists, orthogneisses, and metavolcanic rocks of Ruby terrane, the silicic portions of which are quite radiogenic. The deeper crustal sources that gave rise to most of the batholithic magmas are inferred to be similar under both the Ruby metamorphic terrane and the Angayucham ophiolitic terrane.
Numerical study of the diffraction of elastic waves by fluid‐filled cracks
Authors: Coutant Olivier
Published: February 2008

The diffraction of P‐SV waves by one or several fluid‐filled cracks is simulated numerically. The method used is based on the integral representation of the diffracted field. We establish the integral representation for fluid‐filled cavities embedded in elastic media, and we compute the boundary integral equation by discretizing the diffracting surface and by evaluating Green's functions by the discrete wave number method. We compare the effects of the diffraction by a single crack versus a fracture system consisting of several shorter cracks, for low and high frequencies, with and without the single scattering (Born) approximation. At low frequencies the cracks interact individually; the energy diffracted is weak compared with the incident energy and exhibits a large dependence with respect to the crack length. For higher frequencies, interactions between cracks are predominant; the single crack and multiple cracks present similar radiation and amplitude patterns for diffracted waves. The effects are close to the ray theory results if the first Fresnel zone is of the same order as the crack extension.
Elasticity of a grossular‐pyrope‐almandine garnet
Authors: O'Neill Bridget , Bass Jay D. , Smyth Joe R. , Vaughan Michael T.
Published: February 2008

The single‐crystal elastic properties of a natural, grossular‐rich garnet solid solution have been measured at ambient conditions using Brillouin spectroscopy. This garnet sample is unusual for a grossular in that it contains appreciable pyrope and almandine components, with a composition of Gr48.4Py27.6Alm23.4Sp0.6. It is thus suitable for investigating the variation of elastic properties with composition in Ca‐rich garnet solid solutions that are stable only at high pressure. The values obtained for the bulk modulus, K = 170.4(6), and for the shear modulus, μ = 101.6(3), of the garnet examined in this study, vary by less than 1% from those obtained from a weighted sum of the end‐member moduli. This suggests that any composition in the grossular‐pyrope‐almandine system, which comprise the majority of upper mantle garnets, is accurately estimated with this simple approach. More complex elasticity‐composition variations observed for the grossular‐andradite garnets may be specific to garnet solid solutions where the cation substitutions occur at six‐coordinated crystallographic sites.
Footwall rocks to the Mid‐Tertiary Chemehuevi Detachment Fault: A window into the middle crust in the Southern Cordillera
Authors: John Barbara E. , Mukasa Samuel B.
Published: February 2008

Crystalline rocks unroofed from the middle crust during Tertiary crustal extension by the Chemehuevi detachment fault record a complex history of comprises both subhorizontal and subvertically foliated mylonitic gneisses sharing a common gently SW‐plunging stretching lineation, which are intruded by members of the voluminous laccolith‐shaped Chemehuevi Mountains plutonic suite. Proterozoic and Mesozoic gneisses, migmatites and early members of the Chemehuevi Mountains plutonic suite in the eastern Chemehuevi Mountains are deformed into a gently southwest‐dipping zone of protomylonite, mylonite and locally, ultramylonite in excess of 1.5 km thick. A belt of steeply foliated mylonitic gneiss up to 5 km across, traceable for 12 km along strike (NE‐SW) crops out in the northern part of the range. This belt passes abruptly downward into the subhorizontal shear zone exposed in the eastern Chemehuevi Mountains. A preliminary shear sense study indicates NE‐directed shear within the sub‐horizontal mylonitic rocks, and sinistral shear parallel to the steeply foliated mylonites. Deformation in these two zones took place under upper greenschist to middle amphibolite facies conditions. Mylonitic deformation associated with the two shear zones was spanned by intrusion of the Chemehuevi Mountains plutonic suite. Cross‐cutting contacts within the suite indicate that ductile deformation occurred after intrusion of hornblende‐biotite granodiorite. Four zircon fractions from this unit are discordant, but define a chord with a lower intercept of around 90 Ma and an upper intercept of ∼1900 Ma. Thirteen zircon fractions from two separate undeformed members of the suite (biotite granodiorite and porphyritic biotite granodiorite) are variably discordant, but fractions for each unit define chords that yield latest Cretaceous crystallization ages with upper intercepts of ∼1600 Ma. Together these data constrain the timing of plutonism and ductile deformation, with associated thick zones of fabric reorientation and mylonitic transposition, to the Late Cretaceous. Superimposed on this fabric and involving all rocks types in the footwall, are the localized brittle and ductile effects of the mid‐Tertiary extensional deformation. Movement direction and the timing of mylonitic deformation in the Chemehuevi Mountains suggest that the effects of the Cordilleran fold and thrust belt extends into the Chemehuevi Mountains region, where it in part overlapped with, and was heated by, plutonism along the eastern margin of the Mesozoic magmatic arc during late Cretaceous time.
A model for lava flows with two thermal components
Authors: Crisp Joy , Baloga Stephen
Published: February 2008

A mathematical model is presented for the cooling of an active flow with two separate thermal components. One component is a crust that cools by radiation and generally thickens with time. The other component is an inner core that is vertically isothermal and partially exposed at the top surface, where heat is lost by radiation. This model provides a more realistic description of active lava flows than the existing models that assume thermal homogeneity in each vertical cross section perpendicular to the direction of flow advance. Characteristic time scales for heat loss from the core are comparable to typical durations for the emplacement of single‐lobed basaltic flows. The two‐component model predicts fractional areas of exposed core between 0.001 and 0.1, consistent with several types of observations of lava flows. Predicted temperature changes for the crust and core, as a function of distance from the vent, also comparable favorably to the limited observations available for active flows. Depending on eruption duration, fraction of exposed core, flow thickness, and initial eruption temperature, the thermal interaction between the crust and the core can have a significant effect on the core temperature, the growth of the crust, and various averages of thermal losses by radiation. This analysis suggests that flow dimensions may be strongly influenced by thermal dynamics in the core if emplacement duration is long, the fraction of exposed core is maintained at a high level, or the initial eruption temperature is low.
Searching for slow and silent earthquakes using free oscillations
Authors: Beroza Gregory C. , Jordan Thomas H.
Published: February 2008

Our knowledge of Earth structure can be used to distinguish far‐field seismic signals from near‐field noise. In particular, earthquakes excite ground motion at the eigenfrequencies of the Earth's normal modes. By observing the spectral levels in small bands centered on the known frequencies of the normal mode resonance peaks and comparing them to the spectral levels of ambient noise, it is possible to detect earthquakes at low frequencies without knowing anything about them at high frequencies. We have used this fact to develop an earthquake detection algorithm that compares the signal level in the mode bands, as measured by the modulus of zeroth‐order moment of the Fourier spectrum, with that in the noise bands using an F test. The excitation of each mode over a global network of stations and of all the modes over the network is evaluated from a summed‐score statistic using a binomial test and a Markov test, respectively, which are robust with respect to deviations from the Gaussian noise model. We apply the technique to the Earth's fundamental spheroidal modes 0S8‐0S43 as recorded by the IDA network for the 2‐year interval 1978–1979. We calculate the level of excitation at 3‐hour intervals using a time domain integration technique designed to optimize the signal‐to‐noise ratio for each mode. The algorithm detects 50% of all teleseismic earthquakes having seismic moments M0 ≥ 1×1018 N m (Mw ≥ 6.0), and it detects all events having M0 ≥ 3×1018 N m (Mw ≥ 6.3). Slow earthquakes, i.e., those having anomalously large characteristic durations, are observed to be enriched in low‐frequency mode excitation relative to other events of comparable magnitude. Using this criterion, we have found a number of slow earthquakes that had not been previously identified as anomalous. Most of these newly identified slow earthquakes occurred on oceanic transform faults. In addition to the cataloged normal and slow earthquakes, we observe 27 episodes of substantial mode excitation that are reasonably well isolated from significant earthquakes. Such anomalies may represent “silent earthquakes,” events with propagation velocities sufficiently low that they do not generate globally detectable wave trains on high‐frequency seismometers.
High‐resolution inversion for South Atlantic Plate kinematics using joint altimeter and magnetic anomaly data
Authors: Shaw Peter R. , Cande Steven C.
Published: February 2008

We present an inversion for plate kinematics that solves for finite rotation parameters using fracture zone (FZ) and magnetic anomaly location data jointly. We define misfit functions that incorporate properties unique to each data type; in particular, the FZ misfit function does not depend upon alignment of conjugate FZ traces in the same way as magnetic lineations under the finite rotations. This property is useful for FZ locations, in which the signals on conjugate sides of the ridge may include systematic differences, or where data from one side of the ridge are sparse or missing. Formal error bounds estimated for the pole parameters show that the magnetic and FZ data are complementary in their information content. Error bounds computed for the joint inversion are substantially smaller than for either the FZ or magnetics data used separately, indicating that simultaneous use of the data in an inversion is crucial. We apply this method to Seasat altimeter data and magnetic anomaly picks in the South Atlantic. We solve for finite poles corresponding to magnetic anomalies 5, 6, 8, 13, 21, 22, 25, 30, 33, 33r, and 34; these define a smooth path over the past 84 m.y. indicating that the plate motions have not been as erratic as found previously.
Dynamics of degassing at Kilauea Volcano, Hawaii
Authors: Vergniolle Sylvie , Jaupart Claude
Published: February 2008

At Kilauea volcano, Hawaii, the recent long‐lived eruptions of Mauna Ulu and Pu'u O'o have occurred in two major stages, defining a characteristic eruptive pattern. The first stage consists of cyclic changes of activity between episodes of “fire fountaining” and periods of quiescence or effusion of vesicular lava. The second stage consists only of continuous effusion of lava. We suggest that these features reflect the dynamics of magma degassing in a chamber which empties into a narrow conduit. In the volcano chamber, gas bubbles rise through magma and accumulate at the roof in a foam layer. The foam flows toward the conduit, and its shape is determined by a dynamic balance between the input of bubbles from below and the output into the conduit. The foam thickness is proportional to (μlQ/ε2 ρl g)1/4, where μ l and ρl are the viscosity and density of magma, ε is the gas volume fraction in the foam, g is the acceleration of gravity, and Q is the gas flux. The bubbles in the foam deform under the action of buoyancy, and the maximum permissible foam thickness is hc = 2σ/ερlgR, where σ is the coefficient of surface tension and R is the original bubble radius. If this critical thickness is reached, the foam collapses into a large gas pocket which erupts into the conduit. Foam accumulation then resumes, and a new cycle begins. The attainment of the foam collapse threshold requires a gas flux in excess of a critical value which depends on viscosity, surface tension, and bubble size. Hence two different eruption regimes are predicted: (1) alternating regimes of foam buildup and collapse leading to the periodic eruption of large gas volumes and (2) steady foam flow at the roof leading to continuous bubbly flow in the conduit. The essential result is that the continuous process of degassing can lead to discontinuous eruptive behavior. Data on eruption rates and repose times between fountaining phases from the 1969 Mauna UIu and the 1983–1986 Pu'u O'o eruptions yield constraints on three key variables. The area of the chamber roof must be a few tens of square kilometers, with a minimum value of about 8 km2. Magma reservoirs of similar dimensions are imaged by seismic attenuation tomography below the east rift zone. Close to the roof, the gas volume fraction is a few percent, and the gas bubbles have diameters lying between 0.1 and 0.6 mm. These estimates are close to the predictions of models for bubble nucleation and growth in basaltic melts, as well as to the observations on deep submarine basalts. The transition between cyclic and continuous activity occurs when the mass flux of gas becomes lower than a critical value of the order of 103 kg/s. In this model, changes of eruptive regime reflect changes in the amount and size of bubbles which reach the chamber roof.
Effects of clay mineralogy and the electrical conductivity of water on the acquisition of depositional remanent magnetization in sediments
Authors: Lu Ran , Banerjee Subir K. , Marvin James
Published: February 2008

New experiments on the interrelationship of clay minerals, solution properties, and postdepositional remanent magnetization (pDRM) provide interesting insights into pDRM acquisition. We have found that clay mineral content, composition, and solution conductivity affect pDRM strongly. With increasing clay concentration or electrolyte concentration, pDRM intensity decreases. Clay minerals may also be responsible for the inclination shallowing of pDRM. Different clay minerals behave differently: for example, montmorillonite diminishes pDRM intensity more rapidly than kaolin. Anson and Kodama (1987) have suggested an electrostatic model to explain their results of a similar study. However, we cannot explain all of our results by using their electrostatic model. To us, it seems that colloid coagulation is the key to explaining the results. Two kinds of forces exist in a colloidal solution: electric repulsion and van der Waals attraction. In low electrolyte concentration solutions, repulsion is larger than attraction. Strong Brownian motion is, therefore, required to enable the particles to cross over the repulsive energy barrier in order to stick to each other. But in high electrolyte concentration solutions, repulsion becomes relatively small, and particles stick together much more easily. Thus the effective mass of the grain becomes larger (clay + magnetite), but the magnetic moment remains the same, and hence the magnetic torque is relatively less effective. As a result, the magnetic carriers do not align effectively in the direction of the ambient field, making the pDRM intensity weaker.
Seismic evidence for continuity of the deep slab beneath central and eastern Peru
Authors: James David E. , Snoke J. Arthur
Published: February 2008

Seismograms of Peru‐Brazil deep focus earthquakes recorded on the nearby Carnegie broadband station at Cuzco, Peru (CUS) are characterized by an anomalous large‐amplitude P wave arrival that follows about 1.5 s after direct P. The anomalous arrival is interpreted to be an underside wide‐angle reflection from the upper surface of the descending Nazca plate somewhere in the depth range 150–400 km and in a region where the slab is wholly aseismic. Particle motion analysis of the CUS seismograms shows that the direct P wave has arrival angles as predicted from simple ray tracing. The large, impulsive anomalous phase (termed Pr) also arrives as a P wave but is of opposite polarity and about 1.5 times the amplitude of the direct P arrival. The back azimuth of Pr is about 20° clockwise from the path of direct P, and its angle of emergence is significantly larger. Forward modeling calculations indicate that the seismic observations are consistent with a model in which Pr originates as a P wave that travels up (inside) the slab from the source and is then reflected at wide angle from the steeply dipping upper plate boundary. Results from modeling lead to the following conclusions: (1) the descending Nazca plate exists in and is probably continuous through the aseismic region between 150 and 500 km depth; (2) the plate is very steeply dipping (∼70°) below depths of about 150 or 200 km; and (3) the velocity discontinuity at the reflection point must be relatively sharp, probably less than about 10 km in thickness.
Buoyancy‐driven magma fracture: A mechanism for ascent through the lithosphere and the emplacement of diamonds
Authors: Spence D. A. , Turcotte D. L.
Published: February 2008

Magma‐driven fractures must be an important mechanism for the transport of magma through the crust and lithosphere. Dikes are pervasive throughout the crust and clearly play an important role in active volcanoes. A magma fracture is the only mechanism that allows sufficiently high magma velocities to prevent complete solidification during transport through cold country rock. The analysis of magma‐driven fractures is complex because it involves flow in the fracture (often turbulent), the elastic deformation of the country rock surrounding the fracture, and the propagation of the fracture. For vertically propagating magma fractures, buoyancy forces may be the dominant driving mechanism. In order to simplify the analysis of this problem we neglect the elastic distortion of the country rock and the fracturing process. We consider a buoyant fluid initially in an elliptical two‐dimensional cavity. The shape of the upwardly deforming cavity is shown to satisfy the nonlinear kinematic wave equation. For laminar flow the cavity initially deforms but does not move upward. At a critical time the solution near the top of the crack becomes multivalued indicating the presence of an upwardly propagating shock wave. The tail of the upwardly propagating crack closes but does not move upwards. Solutions have also been obtained for turbulent flow. Velocities of upward propagation for appropriate parameter values indicate this mechanism can explain the observed characteristics of kimberlite eruptions. Inclusion of elastic and fracture properties will determine the structure of the shock wave, that is, the crack tip, but would not effect the applicability of our solution.
Behavior of the southernmost San Andreas Fault during the past 300 years
Authors: Sieh Kerry E. , Williams Patrick L.
Published: February 2008

Surficial creep occurs at low rates along the Coachella Valley segment of the San Andreas fault, which has not produced a large earthquake during the period of historical record. Geodetic data indicate, however, that the crust adjacent to this segment of the San Andreas fault is accumulating strain at a high rate. Furthermore, neotectonic and paleoseismic data indicate that the fault does produce very large earthquakes every two to three centuries. In view of its long‐term behavior, the occurrence of creep along the surficial trace of the fault in the Coachella Valley is of particular interest. Along two short reaches of the San Andreas fault in the Coachella Valley, measurements of offset geological deposits and man‐made structures and from alignment arrays and creep meters show that slip rates of 2–4 mm/yr near Indio and near the Salton Sea have persisted for the past three centuries. This slow aseismic surficial creep is not a transient precursor to seismic failure of this segment of the fault. We suggest that the Coachella Valley segment of the San Andreas fault creeps in its upper few kilometers. This behavior may be due to tectonically induced high pore pressures in the coarse sediments that abut the fault.
Calculated elastic and thermal properties of MGO at high pressures and temperatures
Authors: Isaak Donald G. , Cohen Ronald E. , Mehl Michael J.
Published: February 2008

Using the potential‐induced breathing model, we calculate the pressure and temperature dependence of the thermoelastic properties of MgO. These calculations represent the first attempt to obtain a consistent set of thermodynamic elastic moduli for an oxide from an ab initio model over a wide range of pressure and temperature. By assuming the quasi‐harmonic approximation for the free energies, we find excellent agreement between the temperature dependence of calculated elastic moduli and those obtained from experiments. Comparison of the calculated athermal and isothermal elastic moduli shows approximations using athermal values to be unreliable at high temperature. The elastic moduli for MgO are presented for pressures and temperatures appropriate for the lower mantle, a regime in which elastic moduli cannot be obtained by direct measurement.
Microcoercivity, bulk coercivity and saturation remanence in multidomain materials
Authors: Xu Song , Merrill Ronald T.
Published: February 2008

Microcoercivity is associated with the pinning force of a domain wall and has typically, but erroneously, been taken to be identical to bulk coercivity. Indeed, excluding certain limiting cases, it is shown that bulk coercivity is nonlinearly related to microcoercivity. Unfortunately, the precise functional relation between microcoercivity and bulk coercivity requires making certain assumptions about the magnetic material of interest. This is illustrated by considering materials with either low or high defect concentration for which a rigid domain wall constraint is satisfied. Simplified theory for bulk coercivity versus temperature appears to compare very favorably with measured values for hydrothermally grown magnetites.
Experimental study on propagation of liquid‐filled crack in gelatin: Shape and velocity in hydrostatic stress condition
Authors: Takada Akira
Published: February 2008

The three‐dimensional shape and velocity of propagating cracks in the hydrostatic stress condition were studied by using gelatin, the physical properties of which were controlled to be constant. Various liquids (with various densities, viscosities, and volumes as the governed parameters) were injected in gelatin to form liquid‐filled cracks. The directions of the crack growth and the propagation of an isolated crack are governed by the density difference between injected liquid and gelatin (Δρ), that is, a buoyancy. The propagation of a crack has two critical values: the first is the transition value to brittle fracture; the second is the value where segmentation begins to occur. The condition of a stable isolated crack formation is discussed. The crack shape of an isolated crack in the direction perpendicular to the crack plane is different from that of a growing crack with a fat tear drop form: the former has an elliptical top and a nearly flat bottom. The upper termination of an isolated crack in the vertical cross section has an elliptical shape, and the lower termination has a cusped shape. The lower part of the crack occupies the preexiting fracture which has formed by fracturing at the crack top. The crack thickness (w)/crack height (h) ratio is proportional to Δρ A, if the elastic moduli are constant. The crack length l/h ratio increase with h in the primary fracture, while the l/h ratio decreases with h in the preexisting fracture except for air‐filled cracks. The ascending velocity of an isolated crack is proportional to Δρ3 h4, that is, Δρ w2, if the other physical properties are constant. The height and length of a growing penny‐shaped crack are approximately proportional to A 3d1/3t4/9, so that the growth rate of height is in proportion to A3d3t−5/9 (A3d is constant injection rale). Some comparisons with the two‐dimensional crack theory and applications for magma‐filled cracks are discussed on the basis of these results.
Slip partitioning of the Calaveras Fault, California, and prospects for future earthquakes
Authors: Oppenheimer David H. , Bakun William H. , Lindh Allan G.
Published: February 2008

Examination of main shock and microearthquake data from the Calaveras fault during the last 20 years reveals that main shock hypocenters occur at depths of 8–9 km near the base of the zone of microearthquakes. The spatial pattern of pre‐main shock microseismicity surrounding the Coyote Lake and Morgan Hill hypocenters is similar to the pattern of the post‐main shock microseismicity. Microseismicity extends between depths of 4 and 10 km and defines zones of concentrated microseismicity and aseismic zones. Estimates of the fault regions which slipped during the Coyote Lake and Morgan Hill earthquakes as derived from seismic radiation coincide with zones which are otherwise aseismic. We propose that these persistent aseismic zones represent stuck patches which slip only during moderate earthquakes. From the pattern of microearthquake locations we recognize six aseismic zones where we expect future main shocks will rupture the Calaveras fault. From an analysis of historic seismic data we establish the main shock rupture history for each aseismic zone and identify two zones that are the most likely sites for the next M > 5 earthquakes. The first zone is located near Gilroy and was last ruptured by a M5.2 earthquake in 1949. The second zone is located south of Calaveras Reservoir and north of the 1988 M5.1 Alum Rock earthquake. It has not slipped seismically since at least 1903, and the size of the aseismic region is sufficiently large to sustain a M5.5 earthquake.
Macroscopic laws for immiscible two‐phase flow in porous media: Results From numerical experiments
Authors: Rothman Daniel H.
Published: February 2008

Flow through porous media may be described at either of two length scales. At the scale of a single pore, fluids flow according to the Navier‐Stokes equations and the appropriate boundary conditions. At a larger, volume‐averaged scale, the flow is usually thought to obey a linear Darcy law relating flow rates to pressure gradients and body forces via phenomenological permeability coefficients. Aside from the value of the permeability coefficient, the slow flow of a single fluid in a porous medium is well‐understood within this framework. The situation is considerably different, however, for the simultaneous flow of two or more fluids: not only are the phenomenological coefficients poorly understood, but the form of the macroscopic laws themselves is subject to question. I describe a numerical study of immiscible two‐phase flow in an idealized two‐dimensional porous medium constructed at the pore scale. Results show that the macroscopic flow is a nonlinear function of the applied forces for sufficiently low levels of forcing, but linear thereafter. The crossover, which is not predicted by conventional models, occurs when viscous forces begin to dominate capillary forces; i.e., at a sufficiently high capillary number. In the linear regime, the flow may be described by the linear phenomenological law ui = ΣjLijfj, where the flow rate ui of the ith fluid is related to the force fj applied to the jth fluid by the matrix of phenomenological coefficients Lij which depends on the relative concentrations of the two fluids. The diagonal terms are proportional to quantities commonly referred to as “relative permeabilities.” The cross terms represent viscous coupling between the two fluids; they are conventionally assumed to be negligible and require special experimental procedures to observe in a laboratory. In contrast, in this numerical study the cross terms are straightforward to measure and are found to be of significant size. The cross terms are additionally observed to be approximately equal, which is the behavior predicted by Onsager's reciprocity theorem. However, persistent transient effects can render the reciprocity unobservable. The numerical study is performed with a discrete numerical model of the molecular dynamics of immiscible mixtures called the immiscible lattice gas. The immiscible lattice gas models both the Navier‐Stokes equations and surface tension. Numerical tests presented here additionally provide quantitative validation of the method's ability to simulate wetting phenomena and the effects of capillary pressure. Whereas the numerical study of the linear phenomenological laws utilizes a highly simplified porous medium with one pore and two throats, numerical examples of wetting and nonwetting invasion experiments in a geometrically complex 2‐D porous medium are also provided.