bluuebunny/crossref_metadata_2025 huggingface dataset:
Megaregolith insulation, internal temperatures, and bulk uranium content of the moon
Authors: Warren Paul H. , Rasmussen Kaare L.
Published: February 2008

The moon is covered by a blanket of porous impact rabble (megaregolith) that probably averages about 2–3 km in thickness, and has a thermal conductivity of roughly 0.20 W m−1K−1, i.e., about 13 × lower than the mean conductivity of the lunar lithosphere, and 5–10 × lower than the mean conductivity of the subjacent anorthositic crust We employ finite difference models to study the effects of insulation by the megaregolith on lunar thermal evolution. Results indicate that the megaregolith has two important influences on heat flow: (1) Because the megaregolith is exceptionally thin in mare regions, heat passes more readily through them than through highland regions, and even flows laterally from the highland toward the mare. As a result, heat flow is exceptionally high along a boundary between highland and mare regions. The two lunar heat flow determinations were made along highlands/mare boundaries. Our calculations indicate that heat flow at the Apollo 17 site is enhanced over the global mean by a factor of roughly 1.15–1.20. The effect is similar at the Apollo 15 site, which is probably further unrepresentative due to a local enrichment of (regolith) U and Th contents to 3.3 × the global mean. Adjusting the Apollo 17 heat flow for the boundary effect, our best estimate for global mean heat flow is 12 mW m−2. (2) On a global scale, megaregolith insulation combined with lithosphere insulation causes slow cooling, which as a cumulative effect results in high present‐day mantle temperatures and heat flow. Assuming that the global mean megaregolith thickness is 2 km, a heat flow of 12 mW m−2is best matched by models with bulk moon U contents of 20–21 ng/g. Independent constraints on lunar internal temperatures derived from magnetic and tectonic data are best matched by models with ≈14 ng/g U. Thus our best estimate for the bulk moon U content is roughly 17 ng/g. These results imply that the bulk moon contents of U, and related refractory lithophile elements such as Th, Al, Ca, etc., must be considerably lower than commonly assumed.
Temporal change in coda associated with the Round Valley, California, Earthquake of November 23, 1984
Authors: Peng J. Y. , Aki K. , Chouet B. , Johnson P. , Lee W. H. K. , Marks S. , Newberry J. T. , Ryall A. S. , Stewart S. W. , Tottingham D. M.
Published: February 2008

Seven thousand seismograms of small earthquakes in the Mammoth Lakes‐Bishop area were used to measure values of Q from the decay of the earthquake coda. These measurements were compared between events that occurred before and after the Round Valley earthquake (M=5.7). We found that in regions near the main shock epicenter, measurements of coda Q−1 for earthquakes that occurred after the main shock were higher than for those earthquakes that occurred prior to the main shock. The opposite behavior was found for regions farther away from the main shock, namely, lower coda Q−1 after the main shock than before. Measurements of coda Q−1 in the Long Valley caldera, outside of the immediate source region of the earthquake, were higher than in surrounding areas before the main shock, but the difference disappeared after the occurrence of the main shock. This indicates that the temporal variation in coda Q−1 is comparable to its spatial variation. The doughnut model (seismicity quiescence surrounded by a zone of activity) which was invoked for explaining the precursory seismicity pattern appears to be similar with the observed coda Q−1 variation associated with the Round Valley earthquake. The observed spatial variations in coda Q−1 also help to reconcile conflicting results published in previous studies of the coda Q−1 precursor.
Plate tectonics and aspherical earth structure: The Importance of poloidal‐toroidal coupling
Authors: Forte A. M. , Peltier W. R.
Published: February 2008

We derive poloidal flow internal loading Green functions for incompressible fluid models of the mantle consisting either of a single constant viscosity spherical shell or of two adjacent spherical shells having different viscosities. From the Green function we obtain kernels connecting the surface divergence, the geoid, and the surface topography fields to the lateral density heterogeneity inferred on the basis of the application of seismic tomographic imaging techniques, and with these kernels we argue that both the surface divergence and geoid fields, consisting of harmonic degrees 2–5, may be reasonably fit with only a factor of 8 viscosity increase at a depth of 1200 km. We point out, however, that the coupling from poloidal to toroidal flow which is required to understand surface velocity spectra, may allow these observations to be understood in terms of a viscosity increase at depth which is smaller than required by nonhydrostatic geoid data in the context of pure poloidal models. Using the observed kinetic energy in the surface plate motions as a constraint allows us to infer a value for the steady state upper mantle viscosity of (2.0 ± 0.5) × 1021 Pa s. We comment on the implication of the difference between this number and the somewhat lower value (1 × 1021 Pa s) which has been derived on the basis of analyses of signatures of the glacial isostatic adjustment process.
Temporal change of Qp−1 in focal area of 1984 Western Nagano, Japan, Earthquake as derived from pulse width analysis
Authors: Ohtake Masakazu
Published: February 2008

On the basis of the pulse width broadening of initial P waves, temporal change of Qp−1 was investigated for the focal area of the 1984 western Nagano, central Japan earthquake of M = 6.8. Qp−1 in the focal area, ranging between 3.9 × 10−3 and 10.0 × 10−3, was significantly higher than that of the surrounding area, 2.8 × 10−3, throughout the 1.3‐year postseismic stage that we examined. The highest value, however, appeared not just after the main shock but for the period from 1 day to 2 months following the main shock. After the maximum attenuation was attained, Qp−1 in the focal area monotonically decreased, approaching the value of the surrounding area. This characteristic feature of Qp−1 change is successfully accounted for by the model in which newly opened cracks are gradually saturated by interstitial fluid. Qp−1 of the focal area was much higher than the surrounding area even for the period prior to the main shock. The high attenuation of the preseismic stage is probably a consequence of the earthquake swarm which preceded the M = 6.8 earthquake. However, there remains a possibility that attenuation intensity in the focal area increased as a precursor to the western Nagano earthquake.
Evidence for a shallow pluton beneath the Goat Rocks Wilderness, Washington, from gravity and magnetic data
Authors: Williams David L. , Finn Carol
Published: February 2008

A gravity and aeromagnetic study was conducted in and adjacent to the Goat Rocks Wilderness area, Washington. This work was done in conjunction with geologic and geochemical studies which were critically important to our interpretation. The Bouguer gravity anomaly map displays a large central positive gravity anomaly and three superimposed smaller positive anomalies. The main anomaly, about 26 by 11 km with an amplitude of about 16 mGal seems to delineate a dense large pluton or sheeted dike complex intruding less dense, older volcanic and sedimentary rocks lying directly below the volcano. The pluton or sheeted dike complex may have fed the Goat Rocks volcano throughout its 1–2 m.y. history. Three small feeders, mapped on the surface as andesite intrusions, may be the cause of three smaller gravity anomalies superimposed on the main anomaly. The magnetic anomaly maps also exhibit a large central anomaly whose source is the Goat Rocks pluton. The magnetic data suggest that the three shallow intrusive cupolas have undergone a significant amount of hydrothermal alteration. In addition, to some extent the magnetic data reflect the terrain of the area. Topographic highs produce magnetic highs, and topographic lows produce magnetic lows. The deviations from this pattern delineate subsurface geology. Magnetic lows may be of particular interest in locating zones of mineralization where magnetic minerals have been altered and have become significantly less magnetic.
Lateral heterogeneity and azimuthal anisotropy in the North Atlantic determined from SS‐S differential travel times
Authors: Kuo Ban‐Yuan , Forsyth Donald W. , Wysession Michael
Published: February 2008

To explore upper mantle heterogeneity beneath the North Atlantic we have measured 70 SS‐S differential travel times using the waveform cross‐correlation method. Both the two‐way preliminary reference earth model (PREM) and J‐B (Jeffreys‐Bullen) residuals exhibit a maximum variation of 17 s, ranging from −10 to 7 and −9 to 8 s, respectively. The lack of correlation between SS‐S and S residuals, along with the strong correlation between SS‐S and SS residuals, suggests that SS‐S differential travel times eliminate source and receiver effects and sample the upper mantle anomalies near the SS bounce points without being strongly affected by lower mantle heterogeneities. We find evidence for a strong local anomaly in the North Atlantic; residuals for bounce points immediately to the north of the Azores‐Gibraltar plate boundary average about 4 s faster than for the bounce points to the south of the boundary. We infer that this velocity contrast exists in the lithosphere and continues below a depth of 200 km. A series of linear multiple‐regression experiments demonstrate that variation in velocity with increasing age of the seafloor and azimuthal anisotropy are both significant contributors to the PREM and J‐B residuals. Assuming that travel time delays are a linear function of the square root of the seafloor age, we find the coefficient for (age)½ to be about −1.0 s/(m.y.)½. This is smaller than but not significantly different from the corresponding slope for the S delays of intraplate earthquakes in the Atlantic reported by J. D. Duschenes and S. C. Solomon (1977) and favors the presence of small amounts of water (e.g., 0.1%) in the upper mantle. In modeling anisotropy we carry out an experiment using synthetic seismograms to examine the shear wave splitting phenomena in the anisotropic medium and investigate three possible forms for SH wave delay as a function of azimuth (θ): sinusoidal variations as a function of 2θ, 4θ, and a combination of 4θ (predominant) and 2θ that mimics the delay expected in olivine crystals when the α axis is aligned horizontally. Our regression models strongly prefer the predominance of a 4θ to a 2θ variation, and the last type of anisotropy explains our data best. From the preferred anisotropy model the olivine α axis, or the upper mantle shearing flow, is inferred to be oriented N‐S. The magnitude of the azimuthal variation suggests that anisotropy extends over a depth range of several hundred kilometers in the upper mantle.
Western transverse ranges crustal structure
Authors: Keller Barry , Prothero William
Published: February 2008

A seismic P wave velocity and bulk density model of the crust of the western Transverse Ranges of California along a north‐south profile near the city of Santa Barbara shows south to north crustal thickening and appreciable lateral inhomogeneity within the crust. The seismic model is developed by ray tracing based on data from a land‐sea refraction experiment, Pn arrivals from regional earthquakes, and near‐vertical arrivals from teleseisms. The density model is based on published gravity data (Bouguer on land and free air at sea). The model extends to the known oceanic lithospheric structure west of the southern California Borderland, providing a boundary condition for modeling crustal thickness. The Borderland crust south of the western Transverse Ranges is 23 km thick, and the Coast Ranges crust to the north is 31 km thick. A low‐density, low‐velocity feature associated with deep Neogene sediment in the Santa Barbara Channel extends to depths greater than 10 km. A local high‐density, high‐velocity feature is associated with exposures of Mesozoic mafic crystalline rock on Santa Cruz Island. Low‐density and velocity material is resolvable to 5‐km depth south of the East Santa Cruz Basin fault.
Migration of noisy crustal seismic data
Authors: Milkereit Bernd
Published: February 2008

A slowness‐weighted diffraction stack algorithm is used to migrate crustal seismic reflection data. Slowness information is extracted from the data using a slant stack (beam‐forming) process. The main purpose of beam forming is to improve the signal‐to‐noise ratio over a given range of slowness values. Localized slant stacks are used to design a coherency‐weighted slowness filter in the time distance domain. Through migration the subsurface structure and the observed seismic data are tied via slownessselective diffraction curves. A combination of nonlinear slowness filtering with migration gives excellent noise rejection and leads to an improved image in the depth domain. The method, which is particularly effective for migrating moderate to steep dipping events with low signal‐to‐noise ratio, is applied to various poststack and prestack synthetic data examples and to a deep crustal data set collected on Vancouver Island as part of the Canadian Lithoprobe project.
Improved gravitational recovery from a Geopotential Research Mission Satellite pair flying en echelon
Authors: Wagner Carl A.
Published: February 2008

In the standard mission the low‐low Geopotential Research Mission (GRM) pair fly close coplanar orbits in tandem at nearly constant distance and with a fixed mutual orientation for extended periods of time. Sensitivity to the gravity field is predominantly along‐track at a single altitude. Increased geopotential accuracy and discrimination can be obtained in other configurations for which radial and cross‐track information are also sensed strongly. One such configuration which is only a small change from the standard has the pair flying en echelon in slightly different orbit planes. The addition of cross‐track information in this approach is shown to yield considerable improvement in the geopotential, especially in near‐sectorial terms. Overall field accuracy is estimated to almost double with plane separations of only a few tenths of a degree (permitting fixed antenna operation). Many near‐sectorial terms show gains of up to an order of magnitude over the equivalent standard mission. These benefits are particularly important for low‐degree terms (l< 6) expected to show significant changes in the GRM lifetime due to past and present glacial activity.
Ice dynamics at the mouth of ice stream B, Antarctica
Authors: Bindschadler R. A. , Stephenson S. N. , MacAyeal D. R. , Shabtaie S.
Published: February 2008

Field data collected at the mouth of Ice Stream B show that the flow dynamics of this region are distinctly different than either the major portion of the ice stream upstream or the ice shelf downstream. Surface slopes in this region are as low as ice shelf surface slopes, yet with the exception of patches of ice which may be floating, the ice is grounded. Basal shear stress is negligible, the resistance to flow being partitioned between shear at the sides and longitudinal gradients of longitudinal and transverse stress. The surface is generally crevasse‐free. Features similar to ice rises are observed upstream of the grounding line. Their origin is uncertain, but they move at velocities comparable to the surrounding ice. The flow is laterally extensive and longitudinally compressive, but there are large local variations of the strain rate from the regional trends. The boundary between the two major tributaries to Ice Stream B, followed with the radar, is characterized by a band of strain rates much smaller than average. Detailed measurements at the downstream B network highlight this local variability of strain rates but confirm that there is a strong correlation between surface topography and strain rates. The strain rates indicate that the undulating topography is locally generated. The lower‐elevation ice is thicker and moves faster. A velocity profile across the crevassed northern margin shows that the decrease of velocity toward the edge is nearly linear. A calculation of ice stream discharge at this location agrees closely with two rather rough estimates of balance flux and is considerably larger than a third estimate. The discharge of Ice Stream B does not appear to be significantly out of balance with published estimates of total ice accumulation within the present catchment basin.
Polar basal melting on Mars
Authors: Clifford Stephen M.
Published: February 2008

If ice is present throughout the cryosphere anywhere on Mars, thermodynamic considerations suggest that it is most likely at the poles. Given this condition, the deposition of dust and H2O at the polar surface will ultimately result in a situation where the equilibrium depth to the melting isotherm has been exceeded, melting ice at the base of the cryosphere until thermal equilibrium is once again established. Should deposition persist, the polar deposits will ultimately reach a thickness where melting will occur at their actual base. At this point the cap may reach a state of equilibrium, where the deposition of any additional ice is balanced by geothermal melting. Thermal calculations yield basal melting thicknesses that are consistent with the inferred 4–6 km thickness of the present north polar cap; however, in the south the deposits appear sufficiently thin (1–2 km) that geothermal melting is likely to be relegated to a depth that lies well below the regolith‐polar cap interface. Similar conclusions are reached from consideration of the polar caps' theoretical equilibrium profiles. In the north a basal yield stress characteristic of ice at or near the melting point is indicated, while in the south it appears the cap has yet to achieve the necessary height for significant deformation to occur at its base. The potential importance of basal melting is illustrated through the discussion of four examples: (1) the origin of the major polar reentrants, (2) the removal and storage of an ancient Martian ice sheet, (3) the mass balance of the polar terrains, and (4) the possibility of basal melting at temperate latitudes. This analysis suggests that the process of basal melting may play a key role in understanding the evolution of the Martian polar terrains and the long‐term climatic behavior of water on Mars.
Petrogenesis of Mount St. HElens dacitic magmas
Authors: Smith Diane R. , Leeman William P.
Published: February 2008

The most frequent and voluminous eruptive products at Mount St. Helens are dacitic in composition, although a wide variety of magma types (basalt to rhyodacite) is represented. To address the petrogenesis of the dacites, we present major and trace element analyses of samples of pumice clasts and dome or flow lavas erupted during the past ∼40,000 years. The dacites have similar (in some cases even lower) contents of many incompatible elements (e.g., Zr, Hf, REE, U, Be, Ta, Nb) compared with those in associated basalts and andesites, whereas Ba, Rb, K, Cs, and Sr are relatively enriched. The unusual depleted nature of the dacites and generally low bulk distribution coefficients (estimated from glass/whole‐rock pairs) for numerous trace elements preclude an origin of these magmas principally by crystal fractionation of associated mafic magmas. A more plausible model for their origin involves melting of metabasaltic crustal rocks that have been enriched in Ba, Rb, Cs, and Sr by either intercalation of sediments with depleted basalt or selective metasomatic enrichment of the source region. Melting at crustal levels presumably is related to intrusion of mantle‐derived basaltic magmas. Compositional diversity among the erupted dacites can be attributed to spatial or temporal heterogeneity of the magma sources or, in some specific cases, to such processes as crystal fractionation, assimilation, and magma mixing.
Effect of crustal layering upon dislocation modeling
Authors: Savage J. C.
Published: February 2008

Slip distribution at depth on a fault may be inferred from the deformation observed on the surface. In inverting the surface deformation data to obtain the slip distribution, the Earth is generally approximated by an elastic half‐space. Slip distributions inferred from a half‐space model may contain artifacts, including zones of reversed slip, due solely to effects of layering in the real Earth. This effect is demonstrated for a vertical strike‐slip fault in an Earth consisting of an elastic layer overlying an elastic half‐space. Slip on the fault is taken to be independent of the along‐strike coordinate (i.e., antiplane strain is assumed). For a given slip distribution in this model the slip distribution on a similar fault in an elastic half‐space is found that produces the identical surface deformation. Comparison of the two slip distributions reveals structure introduced into the half‐space equivalent slip profile by crustal layering. The comparisons suggest that low‐resolution inversion schemes (e.g., single screw dislocation models) are not drastically affected by Earth structure, but attempts at detailed inversion are likely to produce profiles contaminated by artifacts of Earth structure.
Constitutive parameter measurements of fluids and soil between 500 kHz and 5 MHz using a transmission line technique
Authors: Kraft Clifford
Published: February 2008

Electromagnetic properties of soil in the 500 kHz to 5 MHz range are determined experimentally to predict plane wave propagation. The ratio of the characteristic impedance of a coaxial probe filled with fluid or soil to the same probe filled with air is used to determine the material's effective permittivity and conductivity. Open‐ and short‐circuit impedances are measured, corrected, and used to predict wave parameters. Measurements made on dry desert alluvium demonstrated the method. Wave attenuation increased linearly with frequency between 0.5 × 10−2 Np/m at 1 MHz to 2.5 × 10−2 Np/m at 5 MHz because of increasing soil conductivity; the ratio of soil wavelength to free‐space wavelength remained approximately constant at 0.4 because of the almost constant permittivity in this frequency range.
The downward continuation of magsat crustal anomaly field over Southeast Asia
Authors: Achache José , Abtout Abdeslam , Le Mouël Jean‐Louis
Published: February 2008

A general method for the local downward continuation at ground level of geopotentials measured by low‐orbiting satellites at different altitudes is proposed. Such a continuation is the inverse solution of the external Dirichlet problem. The stability of the solution with respect to the altitude and the distribution of data points is discussed. Principal component analysis is shown to perform well and leads to a stable solution. The method proposed has the advantage of being directly applicable to measured data points which are not evenly spaced and at different altitude. As an illustration, it is applied to Magsat data over southeast Asia. Two independent maps of the vertical component of the anomaly field at a constant altitude near ground level are derived for dawn (descending orbits) and dusk (ascending orbits) data separately. The two maps reveal similar anomalies and confirm the correlation of the Magsat‐derived anomaly field with the mosaic structure of the continental crust in southeast Asia. Stable continental blocks are associated with positive anomalies, and intervening collision belts correspond to well‐defined negative anomalies. A very prominent low is observed above the Himalaya and extends over the Tibet plateau, confirming an anomalous crustal and thermal structure of the plateau.
Simultaneous stochastic inversion for geomagnetic main field and secular variation: 1. A large‐scale inverse problem
Authors: Bloxham Jeremy
Published: February 2008

The method of stochastic inversion, previously applied to modeling the secular variation and to modeling the main field, is extended to the simultaneous inversion for both main field and secular variation. A priori information is specified in a form analogous to that used previously in studies of the main field and of the secular variation; the time dependency is represented by an expansion in Legendre polynomials, resulting in a simple diagonal form for the a priori covariance matrix. With the field expanded spatially to spherical harmonic degree 14 and temporally to degree 8 the resultant inverse problem has over 2000 parameters: solution of such a large system by conventional means requiring the formation of normal equations is prohibitively expensive on all but a few machines. Alternative means of solution are required: an efficient algorithm, the preconditioned memoryless Broyden‐Fletcher‐Goldfarb‐Shanno algorithm, which exploits the partially known eigenvalue structure of the problem, is adopted. The method is applied to the analysis of observatory data spanning the period 1900–1980. The resulting model, MFSV/1900/1980/OBS, fits the data over this long time span to better than 30 nT, while providing temporally and spatially smoothly varying models of the magnetic field at the core‐mantle boundary. The model compares very favorably with other models of the main field and secular variation.
Helium isotopes: Lower Geyser Basin, Yellowstone National Park
Authors: Kennedy B. M. , Reynolds J. H. , Smith S. P. , Truesdell A. H.
Published: February 2008

High 3He/4He ratios associated with the Yellowstone caldera reflect the presence of a magmatic helium component. This component is ultimately derived from a mantle plume capped by a cooling batholith underlying the caldera. In surface hot springs, fumaroles, etc., the 3He/4He ratio varies from ∼1 to 16 times the air ratio. The variations are produced by varying degrees of dilution of the magmatic component with radiogenic helium. The radiogenic helium is crustal‐derived and is thought to be scavenged from aquifers in which the hydrothermal fluids circulate. We determined the helium isotopic composition in 12 different springs from the Lower Geyser Basin, a large hydrothermal basin within the caldera. The 3He/4He ratio was found to vary from ∼2.7 to 7.7 times the air ratio. The variations correlate with variations in water chemistry. Specifically, the 3He/4He ratio increased with total bicarbonate concentration. The dissolved bicarbonate is from gas‐water‐rock interactions involving CO2 and Na silicates. The concentration of bicarbonate is a function of the availability of dissolved CO2, which, in turn, is a function of deep boiling with phase separation prior to CO2‐bicarbonate conversion. The correlation of high 3He/4He ratios with high bicarbonate is interpreted as the result of deep dilution of a single thermal fluid with cooler water during ascent to the surface. The dilution and cooling deters deep boiling, and therefore both CO2 and 3He are retained in the rising fluid. Fluids that are not diluted will boil to a greater extent, losing a large proportion of 3He, as well as CO2, leaving a helium‐poor residual fluid in which the isotopic composition of helium will be strongly affected by the addition of radiogenic helium. The helium isotopic compositions are also affected by the presence or absence of reactive volcanic glass in shallow reservoirs, the extent of these shallow reservoirs, and by the absorption of steam, derived from the early stages of deep boiling, into near‐surface cold waters.
High 3He emanation and seismic swarms observed in a nonvolcanic, forearc region
Authors: Wakita Hiroshi , Sano Yuji , Mizoue Megumi
Published: February 2008

The 3He/4He and 4He/20Ne ratios of gases emergent in the southwestern part of Japan have been measured by a mass spectrometer. The geographical distribution pattern of the 3He/4He ratios in the Kinki district, a non volcanic, forearc region where microseismic activity at shallow depths has continued over a long period of time, is strikingly different from those found in the rest of the Japanese Islands, including the Shikoku district which lies next to Kinki. Anomalously high 3He/4He ratios, up to 9.69 × 10−6, are found in the Kinki district. All the observed phenomena (high emanation of 3He of magmatic origin, high‐temperature hot springs, high terrestrial heat flow, hypocentral distribution of microearthquakes, a local irregularity in the dip direction of the subducting Philippine Sea plate, and the presence of molten materials inferred from the characteristic phases of SxP and SxS reflected waves in microseismograms) strongly suggest the presence of a shallow magma body beneath the area.
Lg wave propagation in a laterally varying crust and the distribution of the apparent quality factor in central France
Authors: Campillo Michel
Published: February 2008

The aim of this study is to evaluate the sensitivity of Lg waves to lateral changes of the earth's structure. Considering a simple model of uplift of the Moho, numerical simulations show that the geometrical attenuation of Lg is not much affected by a smooth anomaly of the Moho depth. On the other hand, the passing of the Lg wave through the region of the Moho uplift results in a clear deterioration of the wave shapes, which confirms the occurrence of mode conversions. The presence of an overlying sedimentary basin causes a local amplification of Lg above the basin itself and the appearance, behind the basin, of a secondary surface wave guided in the sediments. The effect of the basin on the Lg wave is found to be reasonably taken into account by applying a local amplification function to the data. We use a data set consisting of records of Lg phases in France to test the conclusions of our numerical study. The mapping of the apparent quality factor, computed from Lg at different frequencies, confirms the interpretation of the strong attenuation of S waves around 1 Hz in terms of scattering and shows the weak sensitivity of the amplitude of Lg to smooth changes in the depth of the Moho.
Determination of relative site motions in the western United States using Mark III Very Long Baseline Interferometry
Authors: Clark T. A. , Gordon D. , Himwich W. E. , Ma C. , Mallama A. , Ryan J. W.
Published: February 2008

Four years of very long baseline interferometry (VLBI) measurements involving fixed and mobile stations in the western United States are analyzed using weighted least squares. Baseline solution time series are used to estimate rates of change of the vector baselines between pairs of VLBI stations. Horizontal vector motions are determined for 18 sites with respect to a site in the Mojave desert. Six sites west of and two sites just to the east of the San Andreas fault show northwestward motion closely parallel to the San Andreas fault. Their motions range from 18 to 43 mm/yr and are significant at confidence levels greater than 99%. Motions in California relative to interior North American plate sites are used to estimate an integrated contribution from Basin and Range spreading to the total relative Pacific plate motion as 9–10 mm/yr. A maximum relative velocity of 50–51 mm/yr is found between the interior North American sites and the western California sites, in close agreement with the NUVEL‐I plate motion model.
Microfracture and crack healing in experimentally deformed peridotite
Authors: Rovetta Mark R. , Blacic James D. , Delaney John R.
Published: February 2008

Secondary CO2 fluid inclusion arrays in peridotite xenoliths from basalt are formed by the healing of CO2‐filled microcracks. We have produced healed microcracks and deformation textures comparable to those in xenoliths by deforming peridotite ifi the presence of excess CO2 fluid at a constant strain rate of 10−5 s−1, a confirming pressure of 1.0–1.5 GPa, and a temperature of 800°–1050°C. Plastic and micro‐clastic deformation textures coexist, and healed microcracks extending out of (100) and (011) olivine deformation lamellae have the crystallographic orientation of microcracks nucleated on pileups of (100)[010] and (011)[100] edge dislocations. Healed microcracks in olivine grains without deformation lamellae tend to be in these orientations or in the orientation of microcracks nucleated on pileups of (110)[001] and (010)[001] edge dislocations. The same crystallographic orientation of healed microcracks is present in peridotite xenoliths from basalt. The steady flow stress data of the experiments without added CO2 fluid fit a power law with a stress exponent n of 3 and a thermal activation coefficient of 410 ± 50 kJ mol−1. The maximum stress that can be supported by samples with added CO2 fluid is 100–300 MPa less than the flow stress of samples without added CO2 fluid. The reduction in niechanical strength correlates with an increase in the concentration of intragranular healed microcracks and an increase in the proportion of healed microcracks inclined to the maximum compressive stress direction. Strength and microcrack characteristics of experiments with CO2 performed at 1.5 GPa are similar to experiments without CO2 at 1.0 GPa. The dependence of strength and microcrack concentration on confining pressure suggests that the principal effect of the CO2 fluid is to reduce the effective confining pressure and to promote microclastic deformation mechanisms. The similarity of experimental textures to xenoliths suggests that CO2 fluid may produce semibrittle mechanical behavior in the regions of the lower lithosphere where xenoliths originate.
Deep earthquakes in the southwest Pacific: A tectonic interpretation
Authors: Hamburger Michael W. , Isacks Bryan L.
Published: February 2008

This paper examines the spatial distribution of deep earthquakes in the Tonga‐Fiji‐New Hebrides region of the southwest Pacific. Our interpretation emphasizes the complex Cenozoic tectonics of the Pacific/Indo‐Australian plate boundary as a primary control on the distribution and deformation of subducted lithosphere. Most deep earthquakes in the interarc region are associated with the contorted Pacific plate lithosphere subducted at the Tonga Trench. However, anomalous groups of deep earthquakes located west of the Tonga zone are unrelated to the present plate configuration. Tectonic reconstructions of the region to 8 m.y. B.P. provide circumstantial evidence that (1) the anomalous events west of the Tonga zone occur in two pieces of detached lithosphere, subducted at the Vitiaz and proto‐New Hebrides trenches during the late Miocene, (2) the flattening of the inclined seismic zone in northernmost Tonga is related to the rapid opening of the Lau Basin since 4 m.y. B.P., and (3) the sharp westward curvature of the Tonga seismic zone in this area coincides with a preexisting bend in the late Miocene Vitiaz arc. The sharpness of its present curvature appears to be a secondary effect of shear flow in the lower mantle and compression between detached (Vitiaz) and attached (Tonga) lithosphere. Thus much of the contortion of the subducted lithosphere beneath Tonga‐Fiji may be produced by local tectonic interactions, rather than collision of the slab with an impenetrable boundary in the midmantle.
Tidal triggering of intermediate and deep focus earthquakes
Authors: Curchin John M. , Pennington Wayne D.
Published: February 2008

Tidal stress changes within the Earth due to the gravitational attraction of the Sun and Moon can exceed 20 mbar/h, whereas tectonic stress rates are of the order of 0.2 mbar/h. Although the absolute magnitude of tidal stresses is less than 1% of the typical stress drops observed in earthquakes, a correlation between the tidal stress history on a fault plane and the time of failure should exist if (1) tidal and tectonic stresses are combined in a simple manner, (2) the tectonic stress buildup over time is smooth, and (3) failure occurs immediately upon attainment of some critical stress. To search for possible tidal triggering of intermediate and deep focus earthquakes, 7359 events were selected from a global catalog and subdivided into 30 regions. Each event was assigned a tidal phase relative to both the semidiurnal and biweekly tidal stress curves at the time of occurrence. All tidal phase distributions were checked for a preferred phase orientation using the Rayleigh test and for nonrandomness using the chi‐square test. Results of this analysis for the semidiurnal Earth tide show that three regions have a preferred phase direction and four regions display nonrandomness significant at the 90% confidence level, which would be expected for this number of random data sets. The biweekly tide, when normalized for the unequal distribution of biweekly phase over time after correction for clustering of events in some of the data sets, shows only three non‐random data sets at the 95% level as defined by the chi‐square test. Thus intermediate and deep focus earthquakes in most subduction zones occur independently of both the semidiurnal and biweekly tidal stress cycles, and we may conclude that failure below 70 km does not occur at the instant a stress curve, the simple sum of tectonic and tidal stresses, reaches the yield strength of subcrustal rocks. This implies that either tectonic stress rates prior to failure are not smooth and are unaffected by predictable sources of stress or that failure does not necessarily occur when a certain critical stress is reached. These may in turn be due to some time delay characteristic of deep failure, to the stress history of the material, to the diffusion of fluids, or to the mode of failure itself, at present a unique and unexplained phenomenon at such high temperatures and pressures.
Ascent of a granitoid diapir in a temperature varying medium
Authors: Mahon Keith I. , Harrison T. Mark , Drew Donald A.
Published: February 2008

A relationship for the ascent velocity of a granitoid diapir is derived and used in conjunction with a crustal heat flow model which incorporates a depth‐dependent temperature to predict emplacement histories. Ascent rate is strongly dependent on the width of a deformation layer of variable viscosity that is formed by the transfer of heat to the country rock and on the buoyancy force which propels the diapir through it. Ascent velocity is described by the expressionU∞= 2Δρgd3Z/μ0R, where Δρ is the density contrast between diapir and country rock, g is gravitational acceleration,dis the deformation zone width,Zis a parameter which describes the increase in viscosity across the zone, μ0is the country rock viscosity at the contact, andRis the diapir radius. The width of the deformation zone, as dictated by the temperature distribution and rheological function, is the point at which the viscosity increases by a factorFfrom the contact value. Although ascent rates vary with choice of this parameter, ascent distances are not strongly dependent onF.Results of simulations which incorporate a spectrum of input parameters reveal that ascent distance is strongly dependent on both the rheology and ambient temperature of the encompassing rock, but only weakly dependent on the initial diapir temperature. Ascent velocity varies with the diapir size, with larger bodies moving more slowly and ascending for longer periods relative to smaller plutons. As a result of the proportionately larger amount of thermal energy available for ascent, larger bodies will rise greater distances than smaller diapirs. However, the additional ascent appears nominal relative to the contrast in available energy. Modeling diapirs using a no‐slip boundary condition decreases ascent velocity by a factor of about 3 relative to results incorporating a stress‐free boundary condition. The time scale of granitoid diapirism over crustal distances cannot be less than 104years, due to the rate limiting effect of thermal conduction above the body, and not greater than ∼105years, reflecting solidification times of kilometer‐sized plutons. A consequence of ascent by a deformation zone mechanism is the generation, following emplacement, of a mechanically weakened crust which extends from beneath the diapir to near the source, perhaps explaining the relatively common occurrence of synmagmatic to postmagmatic mafic dikes within granites believed to be derived from interaction of basalt with the lower crust.
Flexural isostasy and uplift of the Sierra Nevada of California
Authors: Chase Clement G. , Wallace Terry C.
Published: February 2008

The Sierra Nevada of California has experienced several kilometers of uplift in the last 10 m.y., but its present elevation is isostatically compensated by a crustal root that has probably existed since Late Cretaceous. Flexural isostasy can reconcile these observations: Basin and Range faulting and extension starting 10 m.y. ago allowed rapid uplift of an eroded and regionally compensated magmatic arc underlain by an excess crustal root. A quantitative model of an unbroken elastic plate buoyed up by the excess root provides an explanation for the estimated 10‐m.y. paleotopography of the range. The same model fits the present topography when broken (shear and fiber stresses not transmitted) at the location of the Owens Valley fault zone. The broken‐plate model predicts the appropriate amount of westward tilt of the Sierra Nevada and can explain the pronounced eastward tilt of the White and Inyo mountains. The position where paleo and present drainages in the Sierra cross over in elevation is the most sensitive indicator of the elastic thickness required of the lithosphere. An effective elastic thickness of 50 km best fits the observed crossover distances and is consistent with the amount of post‐10 m.y. uplift. Calculated fiber and shear stresses suggest that actual extension of the crust is more important in permitting the uplift than the mere presence of normal faults. Part of the residual isostatic gravity anomaly pattern (high gravity over the eastern Great Valley and western Sierra Nevada, low residual gravity over the high Sierra) may be explained by the continuing flexural support of departure from local isostatic equilibrium. The model implies that before 10 m.y. ago the Sierra should have been marked by a negative isostatic gravity anomaly of more than −100 m Gal, flanked by isostatic gravity highs. Such large isostatic anomalies have not been observed to date over eroded arcs, but the Sierra has uniquely low heat flow and, presumably therefore, high lithospheric strength, and the Sierran root is at the upper limit of size that could be isostatically suppressed.
Uncertainties in reconstructions of the Pacific, Farallon, Vancouver, and Kula Plates and constraints on the rigidity of the Pacific and Farallon (and Vancouver) Plates between 72 and 35 Ma
Authors: Rosa José Wilson Corrêa , Molnar Peter
Published: February 2008

To study the rigidity of the Pacific and Farallon plates between 72 and 35 Ma, we analyzed the magnetic anomalies and fracture zones in the Pacific Ocean between longitudes 125°W–180° and latitudes 43°S–60°N. A set of consistent rotations indicates that the Pacific and Farallon plates between the Pioneer and Agassiz fracture zones were rigid plates between 72 and 35 Ma. Thus the location of a missing Tertiary boundary, within the Pacific or Antarctic plates, is restricted to be south of latitude 43°S. The Farallon plate broke into two plates at the time of the change in the Pacific‐Farallon spreading direction north of the Pioneer and Mendocino fracture zones, sometime between 59 and 49 Ma. The northern fragment, termed the Vancouver plate, was subducted beneath most of North America. We present rotations and uncertainties for the histories of motion of the Pacific and Kula plates from anomaly 32 to anomaly 25, of the Pacific and Vancouver plates from anomaly 21 to anomaly 13, and of the Pacific and Farallon plates from anomaly 32 to anomaly 13.
A statistical theory of seamount magnetism
Authors: Parker Robert L.
Published: February 2008

The standard least squares estimation of the average magnetization of a seamount is generally regarded as unsatisfactory. The reason for its poor performance is, I assert, due to the strong correlation and uneven magnitudes of the unfitted noise component of the magnetic anomaly. Least squares estimation depends for its efficiency and freedom from bias upon the statistical independence and common distribution of the random contamination. The primary contributor to the unfitted field is the part due to nonuniformity of the magnetic material within the seamount. If the magnetic heterogeneities are modeled as a random stationary vector field of known functional form, it is possible to calculate the covariance matrix of the magnetic field noise. Then, to the extent that the statistical description of the magnetic sources is valid, the correlation can be properly accounted for in the least squares procedure by weighting the observations to form an equilibrated set with random components that are independent and identically distributed. The main use of the average magnetization vector is in the determination of the associated paleopole position: I show how the least squares estimates of the magnetization vector and its uncertainty are mapped into a confidence region for the pole position. Application of this theory to a young seamount in the South Pacific is highly satisfactory. The model stationary vector field chosen for the magnetization is one with isotropic directional behavior and a very short length scale of correlation. It is found that the equilibrated residuals are far less correlated than those of the conventional procedure and the misfits are uniformly distributed throughout the equilibrated data. The estimated paleopole position is within a few degrees of the north geographic pole, which is the expected location because the seamount is geologically young. Traditional least squares fitting gave a pole at latitude 60°N. The 95% confidence zone associated with the paleopole position has semiaxes measuring 16° by 13°, and thus the uncertainty is not much larger than those of standard paleomagnetic work.
Yield strength of MgO to 40 GPa
Authors: Meade Charles , Jeanloz Raymond
Published: February 2008

Using the diamond cell, we have measured the quasi‐static yield strength of polycrystalline and single‐crystal MgO to 40 GPa at room temperature. In contrast to previous studies we find that the yield strength is pressure dependent, increasing from 1.1 GPa at 10 GPa to 4.0 GPa at 40 GPa for polycrystalline samples subjected to small strains. This increase is an order of magnitude larger than the change in the shear modulus over the same pressure range. At 20 GPa the critical resolved shear stress for slip on the
system is less than 0.2 GPa, while for
slip it is greater than 1.9 GPa. We infer that the

critical shear stress is relatively small and independent of pressure. We observe a large variability in the strength of polycrystalline samples depending on the amount of strain achieved in the diamond cell. Samples subjected to large strains are weakest, and they show evidence of a preferred orientation that allows slip on the weak
system.
Yield strength of the B1 and B2 phases of NaCl
Authors: Meade Charles , Jeanloz Raymond
Published: February 2008

We have measured the quasi‐static yield strength of NaCl to 41 GPa at room temperature. Between 0 and 28 GPa, the strength of the B1 phase increases eightfold, from 0.08 to 0.65 GPa. Across the B1‐B2 transition, the strength of NaCl decreases about 50%. We show that this result is consistent with the measurements of Bridgman [1937] on the shear strength of the B1 and B2 potassium halides to 5 GPa. We propose that the simultaneous increases in coordination and nearest‐neighbor distance have opposite effects on the change in shear strength across B1‐B2 transitions. Increasing the atomic coordination makes the B2 structure stronger than the B1 while increasing the nearest‐neighbor distance decreases the strength of the high pressure phase. In NaCl the large increase in nearest‐neighbor distance more than offsets the effect of increased coordination, and thus the B2 phase is weaker than the B1 phase.