bluuebunny/crossref_metadata_2025 huggingface dataset:
Pressure changes in the plasma sheet during substorm injections
Authors: Kistler L. M. , Möbius E. , Baumjohann W. , Paschmann G. , Hamilton D. C.
Published: February 2008

Using data from the CHEM instrument on AMPTE CCE, data from the three‐dimensional plasma instrument and the SULEICA instrument on AMPTE IRM, and magnetometer data from both spacecraft, we have determined the particle pressure and total pressure as a function of radial distance in the plasma sheet for periods before and after the onset of substorm‐associated ion enhancements over the radial range 7–19 RE. We have chosen events occurring during times of increasing magnetospheric activity, as determined by an increasing AE index, in which a sudden increase, or “injection”, of energetic particle flux is observed. During these events the particle energy of maximum contribution to the pressure increases from about 12 to about 27 keV. In addition, the particle pressure increases, and the magnetic pressure decreases, with the total pressure only changing slightly. For radial distances of less than 10 RE the total pressure tends to increase with the injection, while outside 10 RE it tends to decrease or remain the same. Because the fraction of the pressure due to particles has increased and higher energies are contributing to the pressure, a radial gradient is evident in the postinjection, but not preinjection, flux measurements. These observations show that the simultaneous appearance of energetic particles and changes in the magnetic field results naturally from pressure balance and does not necessarily indicate that the local changing field is accelerating the particles. The changes in the total pressure outside 10 RE are consistent with previous measurements of pressure changes at substorm onset and can be understood in terms of the unloading of energy in the magnetotail and the resulting change in the magnetic field configuration. The increase in pressure at R ≤ 10 RE may also be due to the changing magnetic field configuration if in the region close to the Earth where the field becomes more dipolar, the angle at which the solar wind interacts with the magnetopause increases, while farther down the tail it decreases. Finally, the radial pressure gradient observed in the 7–12 RE region agrees very well with that necessary to maintain stress balance for an isotropic plasma.
Spatial variations of the [H+]/[O+] ion concentration ratio in the topside F region: Observational evidence for H+ ion flows
Authors: Breig E. L. , Sanatani S. , Hanson W. B.
Published: February 2008

Ion concentration measurements during the initial elliptical phase of the Atmosphere Explorer C mission reveal an anomalous behavior as reflected in the height and longitudinal distributions of the [H+]/[O+] ion concentration ratio above 400 km. We emphasize a description and properties of this spatial variability with data acquired during August 1974 when the satellite sampled this height region during late afternoon at southern winter and northern summer mid‐latitudes. A well‐defined relative enhancement in the ion ratio near 500 km occurs in the winter hemisphere over a broad longitude interval about −90°; the general pattern persists throughout the month, but the magnitude of the relative variation decreases as the satellite orbit precessed southward. The observations at northern summer mid‐latitudes indicate a smaller relative depression of the ion ratio within the same general longitude sector. Available measurements and theoretical arguments preclude the presence of similar changes of the observed magnitude in the relative abundance of neutral hydrogen and oxygen at these altitudes. We must thus infer significant deviations of the ion ratio and the H+ concentration from charge exchange equilibrium to altitudes as low as 400 km in the longitude region of abnormal behavior. Height distributions of the observed H+ concentration are consistent with flows of this ion downward into the winter and outward from the summer afternoon F regions, but flows that occur preferentially at longitudes in the vicinity of −90° for the geophysical conditions being sampled. The corresponding horizontal patterns of this ion flow are in general accord with those indicated by Explorer 32 ion composition data at higher altitudes, with neutral winds as the probable driving source. Estimates further suggest that the wintertime H+ fluxes might be substantial, with unusually large vertical flow velocities possible within a localized geographic region. While quantitative aspects of this interpretation remain uncertain, the observed behavior in the [H+]/[O+] ratio provides strong evidence as to the presence and effects of ion flow in the topside ionosphere.
Transient events in the outer magnetosphere: Boundary waves or flux transfer events?
Authors: Sibeck D. G.
Published: February 2008

We present simultaneous solar wind and magnetospheric observations for a series of flux transfer events (FTEs) observed by the UKS and IRM spacecraft in the outer prenoon magnetosphere on October 28, 1984 (day 302). The CCE satellite, located near local noon, observed an enhancement in the magnetospheric magnetic field strength 4 min prior to each IRM/UKS event, suggesting the antisunward propagation of a compression in the magnetopause surface. We suggest that these compressions resulted from wavy magnetopause motion driven by variations in the solar wind dynamic pressure applied to the magnetosphere, rather than the motion of FTEs formed by magnetic merging. In support of this hypothesis we note that the events previously identified as FTEs occurred during a sequence of quasi‐periodic (5–6 min) oscillations in the magnetospheric plasma velocity and magnetic field. The oscillations exhibited all the characteristics expected for antisunward moving magnetopause boundary motion and those cycles identified as FTEs simply exhibited larger amplitudes. The magnetospheric compressions observed by the CCE and IRM/UKS corresponded to enhancements in the interplanetary magnetic field strength (IMF) observed by the ISEE 2 satellite, located just upstream of the subsolar bow shock. Some of the magnetospheric compressions occurred for a strongly northward IMF. There was no evidence for the type of accelerated flows commonly associated with magnetic merging at the magnetopause on this day, and the orientation of the magnetic field inside the UKS/IRM events was not consistent with that expected for FTEs.
The Martian bow shock: Wave observations in the upstream region
Authors: Skalsky A. , Grard R. , Klimov S. , Nairn C. M. C. , Trotignon J. G. , Schwingenschuh K.
Published: February 2008

Distinct electric field oscillations in the frequency range from 4 to 40 kHz are detected by the plasma wave system in the upstream region of the bow shock during the operation of the Phobos 2 spacecraft around Mars. The upstream electron foreshock boundary approximately coincides with the magnetic field line tangential to the bow shock surface. A polarization analysis shows clearly that the electric field vector of the waves is aligned with the local magnetic field. The features of these high‐frequency oscillations appear to be very similar to those of the emissions seen near the electron plasma frequency in the electron foreshock regions upstream of the bow shocks of Venus and Earth.
Distributed extension at the Indian Ocean triple junction
Authors: Mitchell N. C.
Published: February 2008

The Indian Ocean triple junction is a ridge‐ridge‐ridge type joining two medium‐spreading ridges with one slow‐spreading ridge. GLORIA long‐range side scan sonar images show that, while the axial valleys of the two medium‐spreading ridges are almost colinear, apart from a small ∼5 km offset, the valley of the slow‐spreading third axis does not meet the other two in a simple fashion. The axis of this slow‐spreading Southwest Indian Ridge (SWIR), beyond the rift valley walls of the other two ridges, steadily deepens by over 1000 m away from the triple junction to reach 5000 m at 35 km. The GLORIA images show large normal faults around the deep SWIR rift valley, which increase in heave away from the triple junction and crosscut the abyssal hills formed at the faster‐spreading Central Indian Ridge, indicating that extension across the SWIR is distributed over a zone 10 km or more wide. This zone also shows no evidence for the formation of new oceanic crust, suggesting that the extension may be amagmatic near the triple junction. The high relief of the SWIR rift flanks, containing tilted seafloor of the other two ridges, may be an isostatic response of the lithosphere to the deep valley produced by this rifting. These observations, which suggest the progressive development of a propagating SWIR rift by the extension of preexisting seafloor, may have general implications for the dynamics of oceanic spreading centers. In particular, the deepening and widening of the valley away from the triple junction is attributed to the competing effects of tectonic thinning and lithospheric cooling. Thermal models of mid‐ocean ridges spreading at ∼30 mm/yr (e.g., Lin and Parmentier, 1989) predict that the depth to the 700°C isotherm increases with distance from the spreading axis by a proportion similar to the observed doubling in the total width of the deformation with distance from 10 to 35 km away from the triple junction. This rate of widening of the deformation zone may therefore support lithospheric necking models for oceanic rift valleys, if the width and depth of the deforming lithosphere are related in a simple manner.
Seismic coupling along the Chilean Subduction Zone
Authors: Tichelaar Bart W. , Ruff Larry J.
Published: February 2008

Underthrusting at subduction zones can cause large earthquakes at shallow depths but is accommodated by aseismic creep below a certain depth. The maximum depth of the seismically coupled zone (or seismogenic zone) is a transition from unstable to stable sliding. We have determined the maximum depth of the coupled zone and its variability along the Chilean subduction zone. The maximum depth of seismic coupling is defined by the depth of large (M > 6) underthrusting earthquakes that have occurred at the downdip edge of the coupled plate interface. Earthquake depth is determined with omnilinear waveform inversion of long‐period P waves. The statistical uncertainty in the depth is estimated using bootstrapping. Omnilinear inversion formally accounts for the scaling incompatibility between the P waveforms and decreases the uncertainty in the depth estimate. We have found the depths of 27 earthquakes in the time period from 1961 to 1987. Seismic coupling in Chile extends down to 48–53 km. There is a resolvable change in the maximum depth of coupling around 28°S. The region immediately north of this latitude has a coupled zone that extends no deeper than 36–41 km, while south of this latitude coupling extends down to 48–53 km. This transition is not simply related to the oceanic lithospheric age and plate convergence rate but coincides with a change in several geophysical and geological phenomena. With respect to shallower (north) and deeper (south) coupling there is a change from active volcanos to no volcanos, the dip of the deep slab changes from steep to shallow, the thickness of trench sediments changes from thin to thick, and the depth of the oceanic basement changes from deeper to shallower at 28°S. Due to the multiplicity of changes at 28°S there is no clear unique interpretation of what physical mechanism controls the variability in the maximum depth of the seismically coupled zone in Chile.
The phase screen method for vector elastic waves
Authors: Fisk M. D. , McCartor G. D.
Published: February 2008

A phase screen method for vector elastic waves is developed. The method allows rapid calculation of the propagation of elastic waves for problems where backscatter is small; only forward propagation is included in the analysis. The method can be used for propagation through either deterministic or stochastic media. The work described here generalizes existing phase screen methods for scalar waves, accounting for the difference in phase velocities of the transverse and longitudinal polarizations. The method is tested on a two‐dimensional problem, whose exact solution is also computed, for constant Poisson ratios. Synthetic seismograms for two‐dimensional random media using this method are compared with existing finite difference synthetics. An important use of the method is to estimate the rate of interconversion of S and P waves in terms of the energy flux to analyze the effectiveness of potential discrimination techniques.
Compositional changes in Trans‐Pecos Texas Magmatism Coincident with Cenozoic stress realignment
Authors: James Eric W. , Henry Christopher D.
Published: February 2008

The composition and inferred sources of Cenozoic magmas in Trans‐Pecos Texas changed abruptly between 32 and 28 Ma, coincident with a fundamental change in stress regime. Magmas emplaced between 48 and 32 Ma comprise extended differentiation suites beginning with relatively evolved basaltic magmas. These rocks have low Nb and Ta compared to Zr, Hf, La, Ba, and K, characteristics typical of continental volcanic arcs. However, the Texas rocks are more alkalic, have higher concentrations of incompatible elements, and have higher ratios of Nb and Ta to Zr, Hf, La, Ba, and K than is typical of rocks from arcs near trenches. These differences reflect relative position within the Cordilleran arc. Trace element models involving partial melting in the mantle wedge combined with fractional crystallization and assimilation of upper and lower crust can account for most of the observed trace element concentrations and ratios. Trans‐Pecos magmatism may have involved smaller degrees of partial melting than is typical of arcs located closer to the subduction zone. Magmas emplaced after 31 Ma were either bimodal, alkali basalt‐rhyolite, or exclusively basaltic. They have high absolute abundances of incompatible trace elements and high ratios of Nb and Ta to Zr, Hf, La, Ba, and K. The mafic rocks are typical of continental rift or ocean island basalts. Trace element models using Nb, Ta, Y, Yb, Hf, and Zr reproduce the range of concentrations and ratios observed in these rocks and require very small degrees of partial melting, variable amounts of fractional crystallization, but little or no assimilation. The change in magma compositions and sources is best illustrated by a drop in Zr/Nb, from between 18 and 6.25 in the earlier, arc rocks to between 6.25 and 2 in the later, rift‐related rocks. Stress regime appears to play an important role in the process of magma generation and evolution. The change in trace element compositions indicative of tectonic setting supports interpretations of paleostress data that activity up to 31 Ma occurred in a continental volcanic arc, whereas later activity occurred in a setting of intraplate extension. The magmatic change may have been simultaneous with the change in stress regime or may have lagged by as much as 3–4 m.y.
Periodic changes of the angular momentum budget due to the tides of the World Ocean
Authors: Seiler U.
Published: February 2008

The tides of the world ocean are investigated with respect to their instantaneous angular momentum budgets. The most important semidiurnal (M2, S2, N2), diurnal (K1, O1, P1), and long‐period tides (Mf, mf, Mm, Ssa) have been simulated with a numerical model of the classical, unconstrained type where the effects of loading and self‐attraction are considered in a parametrized form. On the whole, the calculated oscillation systems agree very well with observations and results from other tidal models. With increasing period, the long‐period tides approach the equilibrium state, to which the semiannual tide is very close. The angular momentum budgets are analyzed, where especially the characteristics of each tidal band are identified. A common feature of all partial tides is the dominance of the pressure torque in the angular momentum balances. This torque, representing topographic coupling in the ocean‐solid earth system, is thus the only relevant mechanism to exchange angular momentum between hydrosphere and lithosphere. The quantities that enter the excitation functions are given, and the resulting theoretical fluctuations in the Earth's rotation are briefly discussed, details being presented in future studies.
Normal modes and seismograms in an anelastic rotating Earth
Authors: Lognonné Philippe
Published: February 2008

In order to account for rotation and anelastic effects in the normal modes of the Earth, a structure of the space of normal modes different from those generally used in the elastic self‐adjoint case is necessary. This can be done with a duality relation between the eigenproblem and the one obtained simply by reversing the Earth's rotation. This leads to new biorthonormality relations between modes and dual modes. Seismograms can then be expressed in terms of a normal mode expansion. The normal modes of an anelastic rotating Earth can be computed with perturbation theory. In order to take into account the coupling terms between different dispersion curves, as well as between toroidal and spheroidal modes, the perturbations start from an anelastic, non‐rotating Earth rather than from an elastic one. The secular terms of the perturbation series, due to coupling effects between modes of the same multiplet, can then be removed. This ensures that higher order perturbation theory converges to the anelastic modes with sufficient accuracy, and gives, up to third order, expressions for the eigenmodes and eigenfrequencies. These expressions can be used to compute modes and seismograms of an anelastic realistic Earth model, neglecting neither rotation, anelasticity, anisotropy or lateral heterogeneities.
Enriched and depleted source components for tholeiitic and alkaline lavas from Zuni‐Bandera, New Mexico: Inferences about intraplate processes and stratified lithosphere
Authors: Menzies Martin A. , Kyle Philip R. , Jones Michael , Ingram Gerry
Published: February 2008

During the Pleistocene and Holocene, alkaline and tholeiitic magmas were erupted in the Zuni‐Bandera volcanic field (ZBVF) on the western flanks of the Rio Grande Rift, New Mexico. While most of the alkaline basalts are magnesian (i.e., >8.0% MgO), the tholeiitic basalts show evidence of fractionation of olivine and clinopyroxene. The ZBVF lavas have intraplate chemistry similar to that of alkaline and tholeiitic basalts in other continental and oceanic environments, and Sr and Nd isotopes range from depleted compositions similar to mid‐ocean ridge basalts to enriched compositions similar to ocean islands like Kerguelen. Slightly higher Th/Ta and Ba/Nb ratios in some of the ZBVF tholeiitic basalts correlate with isotopic ratios, and this may indicate minor involvement of a Th rich component (i.e., crust). Alternatively, such characteristics may merely reflect intraplate processes if one considers that the isotopic variation observed in the ZBVF is not that different from what is observed in Hawaiian basalts. Systematic changes in elemental and isotopic ratios with the degree of partial melting (Zr/Y) reveal that the geochemistry of the ZBVF magmas is dominated by two components: (1) a depleted mantle component which produced an enriched alkaline magma as a small degree melt (e.g., Ta/Yb=6.0; {La/Yb}N=60; Zr/Y=19; 87Sr/86Sr=0.703); and (2) an enriched mantle component which produced tholeiitic magmas as a larger degree melt (e.g., Ta/Yb <0.5;{La/Yb}N < 15.0; Zr/Y=3–4; 87Sr/86Sr = 0.706). While the depleted component is interpreted to be MORB asthenosphere the enriched component is more problematical and may reside in plume‐contaminated asthenosphere or stratified lithosphere. The lower lithosphere beneath the Proterozoic crust of the western USA appears to be chemically stratified and comprises a depleted peridotite protolith stabilized in the Proterozoic that may have been subsequently enriched due to interaction with fluids related to (1) suprasubduction processes during Proterozoic‐Phanerozoic accretion, (2) upwelling plumes (Rayton‐Clayton) in the Phanerozoic, and (3) asthenospheric upwelling in the late Phanerozoic. Alternatively plume‐contaminated asthenosphere provides a possible source for the enriched intraplate aspect of the ZBVF lavas.
Evidence for magma mixing and a heterogeneous mantle on the West Valley Segment of the Juan de Fuca Ridge
Authors: Van Wagoner N. A. , Leybourne M. I.
Published: February 2008

We studied basaltic rocks of the northern end of the Juan de Fuca Ridge to compare the volcanology, geochemistry, and petrogenesis of spatially related rocks of an active spreading center (West Valley and West Ridge), the Sovanco Fracture Zone, the Middle Ridge, and the adjacent Endeavour and Northern Symmetrical ridges. Rocks were analyzed for mineral chemistry and major, trace, and rare earth elements (REE). Crystallization temperatures and equilibrium phase compositions were calculated. The complex zoning and disequilibrium compositions of plagioclase and olivine megacrysts and phenocrysts from all areas except West Ridge, Sovanco Fracture Zone (7123‐7), and Endeavour Ridge indicate mixing in shallow level magma chambers. The rocks range from primitive (up to Mg # = 68) in the West Valley south and West Ridge areas to evolved ferrobasalts (Mg # as low as 52) at West Valley central, West Valley north, and Northern Symmetrical Ridge. The rocks from the West Valley axis span the range of Mg #. Rocks from West Valley are transitional to enriched (e.g., [La/Ce]n= 0.89–1.06), as are rocks from Endeavour Ridge (e.g., [La/Ce]n = 0.88–1.06). Rocks from West Valley wall, Middle Ridge, Sovanco Fracture Zone, and Northern Symmetrical Ridge are light REE depleted (e.g., [La/Ge]n = 0.76–0.88), and the most depleted rocks are from West Ridge (e.g., [La/Ce]n = 0.62). The chemically heterogeneous character of the lavas exists both within site and within a single dredge haul (West Valley north and Endeavour Ridge rocks) and between sites, indicating that the mantle source is heterogeneous on a variety of scales. The range in incompatible element ratios can be modelled as binary mixing between an enriched source and a depleted source. Further, trace element systematics require variations in the degree of partial melting. Lava chemistry and ridge morphology are consistent with southward propagation of spreading in West Valley to form an overlap with the Endeavour Segment. West Valley (West Valley north, central and south) is dominated by extension over volcanism and is thus thermally and magmatically depressed. Rocks from the West Valley Segment span a broader compositional range than ridge systems to the south, reflecting the heterogeneity of the source and the lack of robust volcanism to homogenize the chemistry. Zero age rocks from West Valley are similarly enriched in [La/Ce]n as zero age rocks from the Endeavour and Explorer ridges. Rocks from the West Valley wall, the Middle Ridge, and the older Sovanco Fracture Zone rocks are depleted similar to rocks of the Juan de Fuca Ridge south of the Cobb Offset. This difference suggests that enrichment of the mantle source north of the Cobb Offset has been a recent phenomena.
Geology, geochemistry, and isotopic character of the Colville Igneous Complex, northeastern Washington
Authors: Carlson Diane H. , Fleck Robert , Moye Falma J. , Fox Kenneth F.
Published: February 2008

The Colville Igneous Complex in northeastern Washington comprises Paleocene orthogneisses and leucocratic granitic rocks and Eocene volcanic‐plutonic suites which were emplaced during early Tertiary extension. The Colville Igneous Complex is divided into three components: (1) 65 ± 4 Ma orthogneisses of the Okanogan dome or metamorphic complex and similar gneisses in the Kettle and Lincoln complexes; (2) 61–51 Ma leucocratic granitic rocks; and (3) 53–47 Ma intermediate to felsic volcanic‐plutonic rocks. The leucocratic granites and younger volcanic‐plutonic rocks are divided on the basis of age, geochemical and isotopic characteristics, and emplacement mechanism. The older leucocratic granites are silica rich, corundum‐normative, and Sr and light rare earth element (REE)‐enriched, and they form several large, discrete, compositionally zoned plutons and intrusive suites. The younger, Eocene volcanic‐plutonic rocks can be divided into two suites: an older compositionally restricted (68–72wt % SiO2), Sr‐enriched, medium‐K suite and a younger intermediate to felsic, high‐K, calcalkalic suite. Sr isotopic measurements of volcanic and plutonic rock in the Colville Igneous Complex yield variable initial 87Sr/86Sr ratios (Sri), ranging from about 0.704 to values greater than 0.710. In general, the volcanic rocks are especially variable with little or no correlation between Sri and geographic location. The leucocratic granites, however, show a systematic west to east increase across the area which probably reflects an increased involvement with Precambrian crust. The variable nature of Sri in the Colville Igneous Complex may also be controlled by crustal thickness and/or depth of melting. Within individual volcanic and plutonic suites, there is a crude correlation of mean Sri with age. There is a systematic decrease in Sri with increasing age which may reflect either a migration of the site(s) of melting or a decreased upper crustal residence time, possibly related to the onset of extension. The leucocratic granites may have formed by small degrees of partial melting in the lower crust, perhaps facilitated by thermal blanketing of an initially overthickened crust. The ascent of large bodies of leucocratic magmas may have increased the geothermal gradient which allowed for upper, more radiogenic, crust to be melted.
Age constraints for the present fault configuration in the Imperial Valley, California: Evidence for northwestward propagation of the Gulf of California Rift System
Authors: Larsen Shawn , Reilinger Robert
Published: February 2008

Releveling and other geophysical data for the Imperial Valley of southern California suggest the northern section of the Imperial‐Brawley fault system, which includes the Mesquite Basin and Brawley Seismic Zone, is much younger than the age of the valley itself. A minimum age of 3000 years is calculated for the northern segment of the Imperial fault from correlations between surface topography and geodetically observed seismic/interseismic vertical movements. Calculation of a maximum age of 100,000 years is based upon displacements in the crystalline basement along the Imperial fault, inferred from seismic refraction surveys. This young age supports recent interpretations of heat flow measurements and the evolution of geothermal systems, which also suggest that the current patterns of seismicity and faulting in the Imperial Valley are not long lived. The current fault geometry and basement morphology suggest a northwestward growth of the Imperial fault and a northwestward migration of the Brawley Seismic Zone. If this localized process is representative of more regional tectonic processes along the extent of the Salton Trough, we suggest that this migration is a manifestation of the propagation of the Gulf of California rift system into the North American continent.
Relocation of teleseismically recorded earthquakes near Tennant Creek, Australia: Implications for midplate seismogenesis
Authors: Bowman J. Roger , Dewey James W.
Published: February 2008

The three MS > 6 Tennant Creek, Australia, earthquakes of January 22, 1988, were preceded by small and moderate earthquakes in 1986 and 1987. We have used the method of joint hypocenter determination to estimate the positions of teleseismically recorded 1986–1987 earthquakes, the 1988 main shocks, and aftershocks with respect to a system of surface fault scarps. The 1986–1987 shocks and the 1988 main shocks nucleated near the center of the zone of surface scarps, where fault‐segment boundaries at depth are implied by complexities in the distribution of scarps at the surface. This suggests that the fault segments that ruptured in 1988 were already in existence in 1986–1987, which is consistent with the hypothesis that strong midplate earthquakes occur on preexisting faults. The redetermined 1986–1987 hypocenters are, however, also consistent with the hypothesis that midplate seismicity is localized by stress concentration due to bulk rheological heterogeneity of the crust, because they are situated on a regionally prominent gravity anomaly. The teleseismically recorded seismicity of the Tennant Creek region prior to the 1988 main shocks has the temporal pattern of a swarm followed by a lull. The concentration of swarm earthquakes between two scarps is consistent with models in which precursory‐swarm earthquakes correspond to faulting that is spatially distinct from the site of primary main‐shock faulting. The time interval between swarm and main shocks is similar to intervals between intermediate‐term precursory swarms and main shocks in regions that have much higher rates of tectonic loading; this similarity suggests that the time intervals between precursory swarms and subsequent main shocks are not strongly influenced by the rate of tectonic loading, but are determined primarily by time dependence of the failure process. The spatial distribution of teleseismically recorded aftershocks is in most respects like that of aftershocks recorded by a local network of portable stations in the half year following the main shocks. The set of teleseismically recorded aftershocks, like the set of locally recorded aftershocks, includes some events that occurred well away from the causative faults of the main shocks. At the length scale of the 1988 main‐shock rupture, the distribution of aftershocks occurring more than one year after the main shocks is not representative of the distribution of earlier aftershocks.
Velocity field and thickness of a laminar boundary layer at the core‐mantle boundary
Authors: Morrish John S. , Moon Wooil M.
Published: February 2008

Steady velocity estimates for the core fluid make possible new input data to the boundary layer equations that describe a flow field at the core‐mantle boundary (CMB). Low valued estimates of the core fluid viscosity are adopted to solve the equations with large dimensions of 3000–6000 km or, alternatively, smaller scale lengths of 10 m of CMB topography which are used to make boundary layer thickness estimates of 163.6 and 0.2988 m for the large and small models, respectively. The laminar velocity flow field within the layer thickness is calculated according to a numerical method developed for parabolic equations and is slower and thicker than boundary layers modelled in the same way for alternate atmospheric and benthic conditions. The model uses a zero pressure gradient and a flat rigid wall for the calculations to simplify this analysis. The surface topography at the CMB may alter the velocity field and produce turbulence.
Observations constraining near‐source ground motion estimated from locally recorded seismograms
Authors: McGarr A.
Published: February 2008

To estimate the seismic hazard to underground facilities or operations in the environs of a mining‐induced tremor or a natural earthquake, it is useful to be able to relate locally recorded seismic waveforms to peak ground velocity and slip at the causative fault. For this purpose, far‐field S wave pulses are analyzed to define the faulting slip D and near‐fault peak ground velocity D/2 that give rise to the most significant ground motion. This most intense region of faulting, an assumed circular asperity, has radius r within a broader source zone of radius r0, which is traditionally calculated from the corner frequency of the S wave spectrum. In developing relationships between peak far‐field velocity v and peak acceleration a, and the source processes of the asperity, D and D, as well as its radius r, the key model assumption is that r = kβ/ω, where ω is the angular frequency of the sinusoidal velocity pulse of maximum amplitude, β is the sheaf wave speed, and k is a constant. Observations in deep‐level gold mines of fault slip and slip velocity as well as laboratory observations of slip rate as a function of stress drop for stick‐slip failure support a choice of about k = 2.34, the value commonly used for estimating r0 using the Brune model. In particular, observations of fault slip up to 410 mm for mining‐induced tremors in the moment magnitude range 4–5 are consistent with D = 8.1 Rv/β, where R is hypocentral distance. Moreover, estimates based on underground damage of near‐fault ground velocities ranging up to 3.5 m/s are in accord with D/2 = 1.28(β/μ)ρRa, where μ is the modulus of rigidity and ρ is the density. Alternatively, the average slip velocity 〈D〉 can be expressed in terms of the stress drop Δσa of the asperity as 〈D〉 = 0.51 β Δσa/μ, and the agreement of this relationship with measurements made during stick‐slip failure in the laboratory is good. To the extent that seismic slip exterior to the asperity is a consequence of preevent suppression of slip due to the asperity, the broader‐scale(r0) slip can be related to that of the asperity. Just as the asperity radius r can be estimated from r = 2.34 βv/a, an alternative estimate for r0 is given by r0 = ρRaM0/[75.8ρμ(Rv)2], the results of which are generally in good agreement with estimates based on the spectral corner frequency method.
Late Tertiary tectonic evolution of the seafloor spreading system off the coast of California between the Mendocino and Murray Fracture Zones
Authors: Fernandez Laurie Skaer , Hey Richard N.
Published: February 2008

A forward modeling program of two‐plate spreading on a sphere was utilized to reconstruct the late Tertiary tectonic evolutionary history of the seafloor spreading system off the coast of California between the Mendocino and Murray fracture zones. This analytical technique was applied to a unique set of magnetic anomaly data consisting principally of a new digital data set: the original Raff‐Mason data from one of the most significant marine geophysical surveys: the 1955–1956 USCGSPioneersurvey. All the data, critical for the development of seafloor spreading and plate tectonic theory, were digitized (32 ° to 52°N); however, only the southern half was analyzed in this study. Investigation of the Raff‐Mason data, supplemented by additional magnetic anomaly data, indicates that the primary mechanism for reorganization of this part of the Pacific‐Farallon ridge, as the spreading center migrated toward and arrived at the North American trench, was rift propagation. The model presented here, containing a total of 21 propagation episodes from 36 Ma to 19.8 Ma (initiation time of the last propagator), shows generally excellent agreement with the isochrons identified from the magnetic anomaly pattern, although minor exceptions exist locally in complex areas. A step‐by‐step evolution of the model is presented in a series of reconstructions documenting the history of spreading between the Pacific plate and five smaller plates created from the breakup of the large ancestral Farallon plate. These plates include the Vancouver plate north of the Pioneer, the Farallon plate existing between the Murray and Pioneer transforms prior to the major reorganization at chron 10 (30.33 Ma), and the Arguello, Monterey, and Reyes microplates with northwest‐southeast spreading existing after chron 10.
Evolution of porosity and seismic structure of upper oceanic crust: Importance of aspect ratios
Authors: Wilkens Roy H. , Fryer Gerard J. , Karsten Jill
Published: February 2008

Seismic properties of the uppermost igneous crust of the oceans are dominated by porosity effects, that is, the size, concentration, and shape of void spaces. Porosity is initially determined by the physics of extrusion (does an eruption form breccia, pillows, or massive flows?) but is very rapidly modified by alteration and hydrothermal deposition. Laboratory data provide insight into compressional wave velocity‐porosity behavior of basalts at a hand sample scale, while well logs provide data at outcrop scale. Relating observations at all scales to porosity structure and extrapolating to seismic scale requires application of rock physics theory. Using information from ophiolites and deep ocean cores, we have defined rock physics parameters for two simple models of upper oceanic crust. The models approximate different levels of void filling by alteration products by differing in the amount of crack (low aspect ratio) porosity they contain. From the models we compute theoretical compressional wave velocity and porosity profiles. Calculated profiles agree well with both well logs and seismic data and illustrate that the increase in seismic velocities measured seismically in the upper crust need not be accompanied by large changes in total porosity.
Crustal structure of the active margin, south central Alaska: An interpretation of seismic refraction data from the Trans‐Alaska crustal transect
Authors: Wolf Lorraine W. , Stone David B. , Davies John N.
Published: February 2008

Seismic refraction and wide‐angle reflection data from the U.S. Geological Survey's Trans‐Alaska Crustal Transect are used to investigate the upper crustal structure of southcentral Alaska and to develop a model using two‐dimensional asymptotic ray theory. The data considered here are from the N‐S 126‐km Cordova Peak refraction profile, which was positioned to cross the Chugach and Prince William terranes. These two tectonostratigraphic terranes form part of a large accretionary complex related to the Alaska subduction zone. Interpretation of data from the Cordova Peak profile indicates systematically higher velocities in Chugach terrane rocks as compared with Prince William terrane rocks at comparable depths. Unconsolidated sediment and glacial ice overlie Chugach terrane rocks Of unusually high compressional velocities (4.7–7.0 km/s) in the upper 10 km of crust. A northward thickening metasedimentary and metavolcanic flysch sequence, having an average velocity of 6.0 km/s, is underlain by metavolcanic mafic basement rocks having an average velocity of 6.6 km/s. The upper 10 km of the Prince William terrane have compressional velocities ranging from 3.7 to 6.2 km/s and are correlated with clastic sedimentary and volcanic rocks which are overlain by younger terrigenous sedimentary rocks and underlain by mafic to ultramafic Prince William terrane basement rocks. The upper crustal velocity structures of the Chugach and Prince William terranes are distinct to at least 10 km depth, which suggests that the Contact fault zone is a boundary which extends to at least 10 km. Midcrustal layers beneath the Chugach terrane contain two velocity reversals which may indicate the presence of off‐scraped oceanic sediment, overpressurized zones, shear zones, or some combination of these possibilities. Data from energy sampling midcrustal layers beneath the Prince William terrane do not provide similar evidence for low‐high velocity zones in that area.
Thermoelastic parameters for six minerals at high temperature
Authors: Anderson Orson L. , Isaak Donald L. , Oda Hitoshi
Published: February 2008

The complete high‐temperature data for six minerals measured in the University of California, Los Angeles, Mineral Physics Laboratory are analyzed and presented as a group. These minerals include CaO and a calcium rich garnet, grossular. We find no significant differences in the thermoelastic parameters for the calcium rich minerals when compared with the calcium deficient minerals. Evidence of anharmonicity is shown in the specific heat, CV, of corundum, forsterite, pyrope, and grossular garnet, but no evidence is found for the case of MgO and CaO. The patterns for the thermoelastic parameter γ show that in general γ decreases with temperature T and that γ at constant volume always decreases rather markedly with T. The thermoelastic parameters, δT, δS, and αKT tend to remain independent of T at high T (above Θ), whereas they are not independent of T near room temperature. The thermal pressure at P = 0 is quite linear with T at temperatures above Θ for all six minerals. We find no evidence of anharmonic behavoir for the thermoelastic parameters of forsterite, correcting an earlier report by Anderson and Suzuki (1983) that anharmonicity appeared in γ, δT;, δS, and αKT at high T for this mineral.
Patterns of seismic activity preceding large earthquakes
Authors: Shaw Bruce E. , Carlson J. M. , Langer J. S.
Published: February 2008

We analyze the patterns of seismic activity which precede large events in a mechanical model of a fault. The model generates a statistical distribution of events similar to that observed for a single fault, with a scaling region consistent with the Gutenberg‐Richter law at small and moderate magnitudes, and an excess of events at large magnitudes. We find only slight variation in the scaling behavior during a loading cycle. However, we do observe systematic variations in space and time of the overall rate of activity. In the model, the activity accelerates dramatically preceding a large event and is usually a maximum hi the neighborhood of the future epicenter. These results are compared to California seismicity data, where we find that activity patterns vary regionally. Looking at patterns of activity in the San Francisco Bay Area since 1948, we find an increase of activity on the Calaveras fault near San Jose beginning in the 1980s which, if our model is relevant, would forecast a large earthquake in that region. The 1989 Loma Prieta earthquake occurred on the San Andreas fault within 30 km of the section of the Calaveras fault showing increased activity.
Least squares inversion of borehole temperature measurements in functional space
Authors: Shen P. Y. , Beck A. E.
Published: February 2008

Under the assumption that the Earth's thermal field is one‐dimensional and purely conductive, the temperature w is related to the Earth model m through a partial differential equation (PDE), where m is the set of model parameters consisting of the ground surface temperature, the background heat flow density, the thermal conductivity, the specific heat capacity, and the rate of heat protection; by setting the time origin sufficiently far in the past, the initial temperature field may be taken as the steady state temperature field. Given data (d0, Cd) on w and a priori information ( m0, Cm) on m, where Cd and Cm are covariance operators describing uncertainties in d0 on m0 respectively, the aim of the least squares inversion is to determine the most probable model that minimizes the misfit function
. We formulate this problem in the functional space as opposed to the conventional discrete formulation and solve it using iterative gradient methods. The formulation reduces the computation in each iteration to essentially two forward solutions of the PDE, the first for the primal problem: given m, solve for the actual field w, and the second for the dual problem: using the weighted data residuals as heat source, solve for the residual temperature field in the same medium, but with homogeneous boundary conditions and with time reversed. The correlation of the residual and the actual fields, then, gives the gradient and also the Hessian of S, the latter of which evaluated at the most probable model is the approximate a posteriori covariance operator. Because discretization is required only when solving the forward problems, we avoid the computing and storing of partial derivatives of d with respect to discretized m, which can be a prohibitive task when the number of data and the number of discretized m are large.
Fractal features of the earthquake phenomenon and a simple mechanical model
Authors: Matsuzaki Mitsuhiro , Takayasu Hideki
Published: February 2008

A simplified two‐dimensional stick‐slip model is introduced. The system shows a kind of dynamical order‐disorder phase transition with a control parameter proportional to the strength of interaction. At the critical point, this model is consistent with the empirical laws of earthquakes such as Gutenberg‐Richter's law, the spatial distribution of hypocenters, and the correlation function of the occurrence of earthquakes as results of a dynamical critical phenomenon. This model can naturally explain the locality that local magnitude distributions often deviate from Gutenberg‐Richter's law.
An empirical method for calculating melt compositions produced beneath mid‐ocean ridges: Application for axis and off‐axis (seamounts) melting
Authors: Niu Yaoling , Batiza Rodey
Published: February 2008

We present a new method for calculating the major element compositions of primary melts parental to mid‐ocean ridge basalt (MORB). This model is based on the experimental data of Jaques and Green (1980), Falloon et al. (1988), and Falloon and Green (1987, 1988) which are ideal for this purpose. Our method is empirical and employs solid‐liquid partition coefficients (Di) from the experiments. We empirically determine Di = ƒ(P,F) and use this to calculate melt compositions produced by decompression‐induced melting along an adiabat (column melting). Results indicate that most MORBs can be generated by 10–20% partial melting at initial pressures (P0) of 12–21 kbar. Our primary MORB melts have MgO = 10–12 wt %. We fractionate these at low pressure to an MgO content of 8.0 wt % in order to interpret natural MORB liquids. This model allows us to calculate Po, Pƒ, To, Tƒ, and F for natural MORB melts. We apply the model to interpret MORB compositions and mantle upwelling patterns beneath a fast ridge (East Pacific Rise (EPR)8°N to 14°N), a slow ridge (mid‐Atlantic Ridge (MAR) at 26°S), and seamounts near the EPR (Lament seamount chain). We find mantle temperature differences of up to 50°–60°C over distances of 30–50 km both across axis and along axis at the EPR. We propose that these are due to upward mantle flow in a weakly conductive (versus adiabatic) temperature gradient. We suggest that the EPR is fed by a wide (−100 km) zone of upwelling due to plate separation but has a central core of faster buoyant flow. An along‐axis thermal dome between the Siqueiros transform and the 11°45′ Overlapping Spreading center (OSC) may represent such an upwelling; however, in general there is a poor correlation between mantle temperature, topography, and the segmentation pattern at the EPR. For the Lament seamounts we find regular across‐axis changes in Po and F suggesting that the melt zone pinches out off axis. This observation supports the idea that the EPR is fed by a broad upwelling which diminishes in vigor off axis. In contrast with the EPR axis, mantle temperature correlates well with topography at the MAR, and there is less melting under offsets. The data are consistent with weaker upwelling under offsets and an adiabatic temperature gradient in the sub axial mantle away from offsets. The MAR at 26°S exhibits the so‐called local trend of Klein and Langmuir (1989). Our model indicates that the local trend cannot be due solely to intracolumn melting processes. The local trend seems to be genetically associated with slow‐spreading ridges, and we suggest it is due to melting of multiple individual domains that differ in initial and final melting pressure within segments fed by buoyant focused mantle flow.