bluuebunny/crossref_metadata_2025 huggingface dataset:
Three‐dimensional geometry of the Gorda Plate beneath northern California
Authors: Jachens Robert C. , Griscom Andrew
Published: February 2008

The isostatic residual gravity field over northern California displays a gravity gradient interpreted to reflect the south edge of the Gorda plate where it is subducted eastward beneath the North American plate. The locus of points of maximum slope defines a line trending S60°E from a coastal point approximately 20 km south of Cape Mendocino, a point where the buried plate boundary is inferred from magnetic and seismicity data. Southeast from the coast for a distance of 120 km the gravity anomaly parallels the strike of the Blanco fracture zone and the present direction of relative motion between the Pacific and northern Gorda plates. Calculations from the form of the anomaly yield depth estimates that fit an east–southeast plunge of approximately 9° for the top of the Gorda south edge. The sense of the anomaly (higher gravity to the south) supports the hypothesis that a window developed in the subducted slab east of the San Andreas fault and south of the Gorda plate. South of the Gorda boundary the base of the North American plate is thus in contact with hot material from the asthenosphere that invaded the window. Because the overlying North American plate has been moving relatively south across the Gorda boundary, the North American plate beneath the Coast Ranges east of the San Andreas fault in central California may be decoupled from the underlying material at a depth slightly deeper than the depth to the top of the boundary at the time the North American plate passed over it.
Normal mode study of the Earth's rigid body motions
Authors: Chao B. Fong
Published: February 2008

In this paper it is shown that the earth's rigid body (rb) motions can be represented by an analytical set of eigensolutions to the equation of motion for elastic‐gravitational free oscillations. Thus each degree of freedom in the rb motion is associated with a rb normal mode. We study both cases of nonrotating and rotating earth models and show that the rb modes do incorporate neatly into the earth's system of normal modes of free oscillation. The excitation formulae for the rb modes are also obtained, based on normal mode theory. Physical implications of the results are summarized and the fundamental differences between rb modes and seismic modes are emphasized. In particular, we ascertain that the Chandler wobble, being one of the rb modes belonging to the rotating earth, can be studied using the established theory of normal modes.
Temperature stratification and refraction errors in geodetic leveling
Authors: Shaw Roger H. , Smietana Peter J.
Published: February 2008

Refraction errors in geodetic leveling due to temperature stratification of the lower atmosphere are presented within the framework of Monin‐Obukhov similarity theory. Universal expressions for the nondimensional mean temperature gradient, accounting for both buoyant convection and mechanically induced mixing, were incorporated into the integration of ray path height with imposed refractive curvature to obtain estimates of the net error in elevation difference on sloping terrain. Exact solutions were found for the refraction error, but approximate versions had the advantage that a normalized refraction error was independent of shot length and terrain slope. Calculations demonstrated that refraction errors increase monotonically with increasing surface heat flux but that maximum errors are found at a finite value of the wind speed such that the ratio of the telescope height to the Obukhov length is independent of heat flux.A free convection approximation provided a close match to the maximum refraction error found from the more general solution for the same surface heat flux, indicating that a free convection approximation yields an upper bound to the refraction error. Calculations of refraction error based on environmental measurements taken during a 1981 survey between Saugus and Palmdale, California, showed that a free convection approximation overestimated the refraction error by a factor ranging from 1.09 to 1.53 depending on the magnitude of the surface roughness length.
Root mean square velocities of the continents with respect to the hot spots since the Early Jurassic
Authors: Schult Frederick R. , Gordon Richard G.
Published: February 2008

The root mean square velocities of the major continents with respect to the hot spots were estimated for the past 180 m.y. by combining previously published plate reconstructions. These velocities agree well with velocities relative to the spin axis inferred from paleomagnetic data. From ∼169 m.y. B.P. to ∼l44 m.y. B.P., the hot spot data indicate that Laurasia moved rapidly, ∼70 mm/yr, but the reconstructions permit a velocity as low as 50 mm/yr. We also estimate that from ∼63 m.y. B.P. to ∼48 m.y. B.P., India moved rapidly (∼140−20+30 mm/yr), faster than the present velocity of any plate and more than twice as fast as the present velocities of several subducting oceanic plates (those attached to downgoing slabs along a substantial fraction of their margin). The data require no other continent to have exceeded a velocity of 45 mm/yr during the last 180 m.y. In agreement with previous studies based on paleomagnetic data and in agreement with previous studies based on no‐nettorque analyses, we conclude that the continents have moved faster, occasionally much faster, than at present. From this we infer that continental lithosphere is unlikely to have much greater resistance to plate motion than oceanic lithosphere. Instead, oceanic plates tend to move more rapidly than continental plates because the subduction and rapid motion of a plate that includes no continents tends to persist longer than the subduction and rapid motion of a plate that includes one or more continents. On the basis of the very rapid early Tertiary motion of India, we conclude that the velocities of subducting plates can vary by a factor of 2. Therefore models of the driving forces of the plates that predict a uniform velocity of subducting slabs are invalid.
Spectral analysis of gapped data: Search for mode 1S1 at the South Pole
Authors: Rydelek P. A. , Knopoff L.
Published: February 2008

The exigencies of operating an ultralong‐period seismometer/gravimeter at the South Pole have obliged us to search for evidence for the pendulum mode of the earth's inner core as a signal with poor signal‐to‐noise ratio, recorded with lengthy interruptions in the time series. In our method for identifying a weak, almost harmonic time signal in the presence of record gaps, we assume the signal is phase coherent for the full time series across the gaps and perform Fourier analysis of the full data sequence. The signal‐to‐noise ratio is improved by 1/(M)1/2 over the case where individual gapped segments are summed, where M is the number of segments. Analysis of the recordings from the South Pole both before and after the Sumbawa earthquake shows no spectral line identifiable with the pendulum mode that is above the noise level.
Elasticity of single‐crystal orthoferrosilite
Authors: Bass Jay D. , Weidner Donald J.
Published: February 2008

The elastic moduli of a synthetic single crystal of pure FeSiO3, orthoferrosilite, have been measured at 1 bar and 20°C by Brillouin spectroscopy. The moduli are (in megabars) c11 = 1.98, c22 = 1.36, c33 = 1.75, c44 = 0.59, c55 = 0.58, c66 = 0.49, c12 = 0.84, c13 = 0.72, c23 = 0.55. A comparison of the pyroxene crystal structure with elasticity data shows that distortion of the tetrahedral chains parallel to c is related to c33 Straight or extended chains increase c33, and highly kinked chains weaken the structure. However, the ferrosilite data show that beyond a certain degree of distortion the tetrahedral chains are essentially passive. Previous models of pyroxene elasticity have indicated that c11 should be most sensitive to composition. This is supported by the current study and is reflected in the elasticity data via a positive linear correlation of c11 with the size of the M1 octahedron, as measured by the average M1‐0 distance. The average longitudinal modulus
11 ((c11 + c22 + c33)/3) varies linearly with composition in the system orthoenstatite‐orthoferrosilite, whereas the average shear moduli
¯44 and
¯12 are constant for Fe contents of at least 20%. By comparison, data for the solid solution forsterite‐fayalite are consistent with a linear variation of modulus versus composition between the end‐members for all three average moduli. We suggest that this difference between orthopyroxene and olivine is due to a strong preference of Fe2+ for the M2 octahedral site in pyroxene. The isotropic shear modulus μ is 30% lower for ferrosilite than enstatite. Despite a similar contrast in end‐member values of μ, the olivine data are consistent with a linear variation of μ between Fo100 and Fa100. At high temperatures, Fe and Mg in orthopyroxene will be randomly distributed, and a linear variation of μ with molar content of Fe is expected. A comparison of laboratory elasticity data with observed mantle properties shows that Vp, Vs, and ρ of the upper mantle are satisfied by pure olivine. However, the data strongly suggest that without the coexistence of a garnet phase, no more than 10% orthopyroxene may be present in a model mantle assemblage. Ca‐clinopyroxene increases the upper limit on mantle garnet content but does not greatly affect constraints on the quantity of orthopyroxene in the upper mantle.
Radio interferometric determination of intercontinental baselines and Earth orientation Utilizing deep space network antennas: 1971 to 1980
Authors: Sovers O. J. , Thomas J. B. , Fanselow J. L. , Cohen E. J. , Purcell G. H. , Rogstad D. H. , Skjerve L. J. , Spitzmesser D. J.
Published: February 2008

A series of experiments has been conducted during the last decade to develop a radio interferometry system capable of measuring crustal and rotational motions of the earth, as well as source positions for a reference frame based on compact extragalactic radio sources. With the exception of one session between Big Pine, California, and Westford, Massachusetts, the observing stations have been those of NASA's Deep Space Network in California, Spain, and Australia. Approximately 2400 observations of extragalactic radio sources were made between August 1971 and February 1980 during 48 separate sessions. A single multiparameter fit was applied to the observed values of delay and delay rate to extract astrometric and geophysical parameters from this decade‐long sequence. The fit produced estimates of 684 parameters, including station locations, radio source positions, polar motion, universal time, the precession constant, and solid earth tides. The a priori model included gravitational bending, the 1980 International Astronomical Union (IAU) nutation series, the 1976 IAU expressions for Greenwich mean sidereal time and precession, Bureau International de l'Heure estimates of universal time and polar motion, and monthly mean values for zenith troposphere delay. The rms residuals were 0.52 ns for delay and 0.30 ps/s for delay rate. Intercontinental baseline lengths have been determined with formal uncertainties of 4 to 10 cm. Universal time and polar motion were measured at 49 epochs, with formal uncertainties (for the more recent data) of 0.5 ms for UT1 and 6 and 2 milliseconds of arc (mas), respectively, for the X and Y components of polar motion. Our 1971–1980 data produced an estimate of the lunisolar precession constant that is smaller than the 1976 IAU value by 3.8 mas/yr with an approximate accuracy (1σ) of 2 mas/yr. The earth tide results agree with the commonly accepted values.
Deformation in the White Mountain Seismic Gap, California‐Nevada, 1972–1982
Authors: Savage J. C. , Lisowski M.
Published: February 2008

A 100×40 km trilateration network extending from Bishop, California, to near Hawthorne, Nevada, crosses the east end of the Long Valley caldera, site of renewed magma inflation in the 1979–1980 interval, and spans most of the White Mountain seismic gap. The network was surveyed in 1972, 1973, 1976, 1979, 1980, and 1982. The 1980 survey may be contaminated by a scale error. In addition, leveling surveys across the caldera have been run in 1932, 1957, 1975, 1980, 1982, and 1983. Interpretation of the deformation is complicated by the occurrence of the May 1980 Mammoth Lakes earthquake sequence (four earthquakes ML≥6) at the south edge of the caldera as well as other moderate earthquakes within the White Mountain seismic gap. The vertical deformation is largely accounted for by 0.10‐ to 0.15‐km3 expansion of a spherical magma chamber 8–10 km beneath the resurgent dome within the Long Valley caldera sometime between July 1979 and September 1980 with an additional expansion of perhaps 0.05 km3 between September 1980 and July 1982. Some additional sources of deformation within the aftershock zone of the Mammoth Lakes earthquakes seem to be required to explain the horizontal deformation. We show that right‐lateral slip on vertical faults extending WNW from each of the three largest earthquakes in the Mammoth Lakes sequence provides the required additional deformation, but this solution is by no means unique. There are simply too few data to define the rather complex deformation that apparently occurred within the aftershock zone. There is little doubt, however, that inflation of a magma chamber beneath the resurgent dome within the Long Valley caldera was involved in the deformation.
Rare gases in cyclosilicates and cogenetic minerals
Authors: Saito K. , Alexander E. C. , Dragon J. C. , Zashu S.
Published: February 2008

The component and isotopic composition of the rare gases in beryls, cogenetic pegmatite minerals, and metamorphic cordierites has been measured. Beryls contain large amounts of excess radiogenic gases and significant amounts of nonradiogenic gases. Beryls contain the most pristine samples of the rare gases in pegmatite volatile phases but have imposed significant elemental fractionations during crystallization. Both the excess radiogenic and nonradiogenic components are progressively depleted in the heavy gases relative to the light gases. Other pegmetite minerals contain rare gases with much smaller excess radiogenic components and apparently did not sample the same source as did the beryls. The other minerals exhibit varying but mineral‐specific fractionation patterns for the nonradiogenic rare gases. Cordierites also contain large amounts of excess radiogenic gases and significant amounts of nonradiogenic gases. In addition, cordierites show a strong, consistent fractionation of both gas components. The fractionation pattern is significantly different from that found in beryls and is characterized by an enrichment of Xe relative to the Ar, though both gases are depleted relative to He. The cyclosilicates can serve as probes of the rare gases present in the volatile phases when the cyclosilicates crystallized. The cyclosilicates contain a record of the rare gases present in geologically interesting environments over a wide range of time and physical conditions.
Segregation vesicles of ocean floor basalts: 1. Petrological study of the segregation products
Authors: Bideau D. , Hékinian R.
Published: February 2008

Thin section studies of about 100 rock samples recovered from the Mid‐Atlantic Ridge (FAMOUS area, near 37°N) show that the occurrence of segregation vesicles is restricted to particular textural zones corresponding to the inner portion of the pillows. A detailed study of an aphyric olivine basalt shows that the mineralogy of the segregated material is mainly clinopyroxene (salite or Ca‐augite) and plagio‐clase (An 50 to An 60) intergrowths associated with titanomagnetite grains. Olivine and plagioclase microlites (An 65 to An 75) crystallized in the groundmass prior to the formation of the segregation vesicles. Scanning microprobe analyses of the segregated product show an enrichment in Si, Fe, Ca, Na, Ti, P, and K and a depletion in Mg and Al with respect to the bulk rock composition. The compositional variation of the segregated material across the sample suggests that the degree of segregated melt differentiation increases toward the center of the pillow. The formation of segregation vesicles is a progressive phenomenon which occurs during a period of the cooling time when viscous deformation is still possible. This study provides criteria for recognizing portions or loose fragments of pillows which may be oriented by the method of Bideau et al. [1977]. This method is helpful for paleomagnetic studies of ocean floor basalt dredged samples.
Folded cauldrons of the Early Proterozoic Labine Group, northwestern Canadian Shield
Authors: Hildebrand R. S.
Published: February 2008

Great Bear Magmatic Zone (1.875–1.84 Ga) is a linear belt, 100 km wide by 800 km long, of little metamorphosed volcanic rocks and allied plutons of calc‐alkaline affinity. The entire zone was folded shortly after magmatism, and sections many kilometers thick through several cauldron complexes are exposed. Eruptions of rhyolite led to collapse of Black Bear Cauldron in which 1.5 km of tuff ponded. The cauldron then became the locus of fluvial and lacustrine sedimentation followed by augite‐plagioclase porphyritic andesite volcanism, both of which also accumulated within the cauldron. Ash flow eruptions of dacite caused collapse of Clut Cauldron which was accompanied by landsliding and avalanching of the cauldron walls. Abundant andesitic and intrusive debris is intercalated with the propylitized intracauldron facies tuff adjacent to the walls and some blocks are as large as 1 km across. Clut Cauldron also became the site for fluvio‐lacustrine sedimentation after collapse and was likely resurgent. Resurgence was probably related to the emplacement of a quartz monzonite pluton which occupies the core of the cauldron. Cornwall Cauldron, which is exposed entirely in cross section, contains 1–2 km of intracauldron facies tuff overlain by a complex of dacite lava flows. The cauldron was intruded by a granodiorite‐monzogranite pluton probably responsible for resurgence. Data from the area suggest the following: (1) the similarity of early Proterozoic cauldrons to Cenozoic examples indicates that cauldron collapse is a process that has occurred since 1.9 Ga; (2) batholiths beneath ash flow tuff fields are probably composite bodies made up of many individual plutons; (3) most of the plutons are sheetlike in cross section; and (4) cauldron collapse begins as soon as ash flow eruptions, and relief on the cauldron margin becomes large early in the subsidence history.
Explosive‐induced wave propagation in nugget sandstone
Authors: Larson Donald B.
Published: February 2008

Explosion‐induced particle velocity time histories have been recorded in Nugget sandstone at confining pressures between 0.1 and 34.5 MPa using explosive sources with energies between 0.6 and 300 kJ. The measured peak particle velocities and peak displacements are insensitive (within the experimental scatter of ±3%) to confining pressures. Smaller cavities are produced at higher confining pressure, but this reduction in cavity size can be attributed to increasing rock strength with confining pressure rather than to confining pressure per se. The recorded free field motion shows a strong departure from elastic behavior, giving average Q or attenuation factors between 8 and 20. Integration of the measured particle velocities to determine permanent displacements gives values an order of magnitude larger than expected from the measured cavity size. This observation suggests that the stress wave causes significant dilation of the rock near the explosion as it travels outward from the spherical energy source.
The distribution of hot spots
Authors: Stefanick Michael , Jurdy Donna M.
Published: February 2008

Hot spots have an irregular distribution over the earth's surface. Part of this irregularity is obviously due to the finite number (40–100) of hot spots, but the distribution does not appear to be completely random. In this paper, statistical analyses are compared for two published hot spot data sets, one minimal set of 42 and another larger catalog of 117. Three approaches are taken: (1) chi‐square tests of equal area boxes, (2) cumulative distributions about principal axes, and (3) construction of a density function. These methods all indicate that hot spots have a nonuniform distribution, even when statistical fluctuations are considered. To the first order, hot spots are concentrated on one half of the surface area of the earth; within that portion, the distribution is consistent with a uniform distribution. A prediction of hot spot density is made based on an empirical model relating number of hot spots to plate speed. Comparison of this predicted density function to the constructed hot spot density functions for the two data sets yields some similarities; however, the observed hot spot densities for neither data set are explained solely by plate speed.
Interpretation of gravity data in a complex volcano‐tectonic setting, southwestern Nevada
Authors: Snyder David B. , Carr W. J.
Published: February 2008

This regional gravity study, based on an irregular 2‐km data grid, was conducted during the past few years at Yucca Mountain, southern Nye County, Nevada, as part of a program to locate a suitable repository for high‐level nuclear waste. About 100 surface rock samples, three borehole gamma‐gamma logs, and one borehole gravity study provide excellent density control. A nearly linear increase in density of 0.26 g/cm3 per kilometer of depth is indicated in the thick tuff sequences that underlie the mountain. Isostatic and 2.0‐g/cm3 Bouguer corrections were applied to the observed gravity values to remove regional gradients and topographic effects, respectively. The Bare Mountain gravity high, with an isostatic anomaly maximum of 48 mGal, is connected with a greater gravity high over the Funeral Mountains, to the southwest; together, these highs result from a continuous block of dense, metamorphosed Precambrian and Paleozoic rocks that stretches across much of the Walker Lane from the east edge of Death Valley to Bare Mountain. The Calico Hills gravity high appears more likely to originate from a northeast trending buried ridge of Paleozoic rocks that extends southwestward beneath Busted Butte, 5 km southeast of the proposed repository, where two‐ and three‐dimensional modeling indicates that the pre‐Cenozoic rocks lie less than 1000 m beneath the surface. Tuff, at least 4000 m thick, fills a large steep‐sided depression in the pretuff rocks beneath Yucca Mountain and Crater Flat. The gravity low and the thick tuff section lie within a large collapse area that includes the Crater Flat‐Timber Mountain‐Silent Canyon caldera complexes. Gravity lows in Crater Flat itself are interpreted to coincide with the source areas of the Prow Pass Member, the Bullfrog Member, and the Tram Member of the Crater Flat Tuff; these source areas add nearly 350 km2 to the previously recognized extent of the local caldera complexes. Southward extension of the broad gravity low associated with Crater Flat into the Amargosa Desert is evidence for sector graben‐type collapse segments related to the formation of the Timber Mountain caldera and superimposed on the other volcanic and extensional structures within Crater Flat.
The thickness of the continental lithosphere
Authors: Peltier W. R.
Published: February 2008

Two independent sets of geophysical data are shown to be particularly sensitive to the thickness of the continental lithosphere. Both are connected with the response of the planet to the last deglaciation event of the current ice age. The first set of data consists of relative sea level histories in the age range 0–10 kyr B.P. from sites along the eastern seaboard of North America. Such data appear to require a continental lithospheric thickness in excess of 200 km. The second set of data consists of the ILS record of the motion of the rotation pole over the time range 0–75 years B.P., which reveals a secular drift at the rate of 0.95 (±0.15) deg/106 years toward Hudson Bay. When the viscosity profile of the mantle is fixed to that required to explain free air gravity and relative sea level data from within the ice margin and the observed nontidal acceleration of planetary rotation, then these observations also suggest a continental lithospheric thickness of the same order.
Mineralogical comparison and cooling history of lunar and chondritic vesicular melt breccias
Authors: Miyamoto Masamichi , Takeda Hiroshi , Ishii Teruaki
Published: February 2008

Lunar vesicular melt breccia (77135) was mineralogically compared with LL chondritic analog Yamato 790964 to elucidate impact melting processes. Microprobe analyses were made of pyroxenes, plagioclases, and olivines and of bulk compositions of the glasses which appear to be solidified from impact melts. Notable similarities between the lunar and chondritic melt breccias are (1) abundant vesicles, (2) similar pyroxene chemical zoning trends, (3) presence of variable amounts of clastic material, and (4) similar chemical compositions except for K and Na contents of the glass and mesostasis. Some constraints on the cooling history are estimated from Mg‐Fe diffusion profiles in olivine and pyroxene. The burial depth of lunar sample 77135 during cooling was not large (0.2–100 m). For the chondrite the burial depth during cooling is probably smaller than that of the lunar analog. Even if the size of the chondrite parent body is small, it is probable that impact melts were produced and a layer of ejecta debris was produced which was sufficiently thick to homogenize Mg‐Fe chemical zoning of olivines.
Formation of Apollo 15 green glass beads
Authors: Arndt J. , Engelhardt W. V. , Gonzalez‐Cabeza I. , Meier B.
Published: February 2008

Size frequency distributions of the cross‐sections of green glass beads in a thin section of regolith breccia 15427 have been determined. Cross‐section medians of vitrophyric and glassy beads are 0.22 mm and 0.094 mm, respectively. Vitrophyric beads contain exclusively olivine crystals of three crystallographically different morphologies. With a synthetic melt of green glass composition, free flight cooling rates have been determined. Spherules of 0.22 mm and 0.094 mm in diameter cool at rates of 1500°C/s and 4200°C/s, respectively, in the temperature range 1050°C–1000°C. The critical cooling rate for green glass formation, measured under controlled conditions, is about 1°C/s, indicating that lunar green glass beads have not been cooled in free flight but in a hot gaseous medium. By controlled cooling of synthetic green glass melt droplets from above the liquidus temperature, olivine morphologies have been produced that are identical to those in lunar vitrophyric beads. On the other hand, after heating synthetic glass spherules, textures have been observed that do not occur in lunar green glass. It follows that the lunar beads have been continuously cooled without later reheating. It is inferred that green glass melt droplets have been erupted from the lunar interior together with a large mass of gas, the cooling history of which is recorded by the internal textures and the size distributions of the green glass beads.
Geoid lows over deep‐sea trenches
Authors: Cazenave A. , Rosemberg‐Borot C. , Rabinowicz M.
Published: February 2008

In many instances, negative gravity anomalies over trenches are slightly displaced toward the inner slope of the subduction complex indicating an accumulation of low density material. This effect, known for many years in gravity data, is also observed in geoid data. We have analyzed regularly spaced geoid height profiles from the Seasat satellite across the Aleutian, Kuril, Mariana and Tonga‐Kermadec trenches. Regional variations of the offset between topography and geoid minima have been recorded. Along the Aleutian trench, the offset increases from a few kilometers in the eastern part to a few tens of kilometers (up to 40 km) in the central part (between 170° W and 180° W), and then decreases again to less than 10 km in the western Aleutians. Along the Kuril trench, the offset is more or less constant (∼20 km) except for a larger offset (up to ∼30 km) at the southern end of the arc. Along the Mariana and Tonga trenches, the geoid low is in near coincidence with the trench axis, while in the southern end of the Kermadec trench, a small offset is detected. The displacement of the geoid low away from the trench axis is related to the width of accreted material beneath the inner trench slope: no offset is observed in regions of very narrow accretionary prism whereas a ∼30‐km offset may result from an accretionary prism of ∼60 km in width supported without isostatic compensation by the subducting elastic lithosphere; geoid lows centered over the axis of trenches with narrow accretionary prism (e.g., Mariana and Tonga) are almost completely explained by the seawater trench topography alone, unlike displaced geoid lows which require an additional negative density contrast. On the other hand in areas of very extensive accretionary prism (as in the eastern Aleutians) the situation is complicated by sediment loading and additional flexure of the descending plate which insures partial compensation of the prism.
Radiocarbon dating of open systems with bomb effect
Authors: McKay Christopher P. , Long Austin , Friedmann E. Imre
Published: February 2008

The application of radiocarbon dating is extended to include systems that are slowly exchanging carbon with the atmosphere. Simple formulae are derived that relate the true age and the exchange rate of carbon to the apparent radiocarbon age. A radiocarbon age determination does not give a unique true age and exchange rate but determines a locus of values bounded by a minimum age and a minimum exchange rate. It is found that for radiocarbon ages as large as 10,000 years it is necessary to correct for the anthropogenic radiocarbon produced in the atmosphere by nuclear weapons testing. A one‐term exponential approximation, with an e‐folding time of 14.43 years, is used to model this effect and is shown to be accurate to within 3% for exchange time constants of 100 years and greater. The approach developed here is not specific to radiocarbon and can be applied to other radioisotopes in open systems.
Earthquakes near Albuquerque, New Mexico, 1976–1981
Authors: Jaksha Lawrence H. , Sanford Allan R.
Published: February 2008

The U.S. Geological Survey conducted a 66‐month‐long study of earthquake activity near Albuquerque, New Mexico, during 1976–1981. Analysis of over 1000 hypocenters with magnitudes as large as 3.2 revealed that most of the seismicity fell into three regions: (1) the Socorro and Albuquerque areas of the Rio Grande rift, (2) the Mount Taylor area of the Jemez lineament, and (3) the Estancia basin. In the rift the hypocenters are shallow and largely confined to regions beneath the basins. Composite fault plane solutions suggest both normal and strike‐slip faulting with an average T axis oriented WNW‐ESE. Most of the young faults of the rift are seemingly inactive, and the rift as a whole cannot be delineated on the basis of seismicity.
Elasticity of uvarovite and andradite garnets
Authors: Bass Jay D.
Published: February 2008

The adiabatic single‐crystal elastic properties of synthetic end‐member uvarovite garnet (Ca3Cr2Si3O12) and a natural sample of 96% andradite garnet (Ca3Fe2Si3O12) have been measured at ambient conditions by Brillouin spectroscopy. The bulk modulus (Ks) and shear modulus (μ) of andradite (And) are Ks = 158 ± 2 GPa, μ = 90 ± 1 GPa, and those of uvarovite (Uv) are Ks = 165 ± 2 GPa and μ = 94 ± 1 GPa. Observed trends of the single‐crystal elastic moduli (Cij) as a function of composition, and specifically the deviation from the Cauchy relation c12 = C44 are analogous between the Ca‐bearing garnets grossular (Gr = Ca3Al2Si3O12)‐And‐Uv and other chemical systems such as the rock salt and corundum structure oxides, whereas other silicate garnets exhibit distinctly different behavior. Ks of andradite is substantially higher than previously estimated from data on garnet solid solutions. Garnets on the Gr‐And join show a pronounced minimum in the Ks‐composition relationship at a composition of approximately Gr24And76, indicating that ideal linear mixing of end‐member elastic properties is inadequate to explain the elasticity of garnet solid solutions. The elastic properties of Gr‐And and Al2O3‐Cr2O3 solid solutions can be explained by incorporating a composition‐dependent volume of mixing term into the expression for solid‐solution elasticity. An evaluation of elasticity systematics relating the bulk sound speed and mean atomic weight, or Ks and molar volume, indicates that these relationships yield closer estimates of garnet elastic properties than does the seismic equation of state of Anderson (1967).
Temporal variations in the gravitational field of the Mississippi Embayment
Authors: Ervin C. Patrick , McGinnis L. D.
Published: February 2008

A network of gravity base stations in the Mississippi Embayment was repeatedly occupied, beginning in November 1975, although the majority of the occupations were between September 1976 and September 1978. Variations in the gravitational difference between most pairs of stations had a standard deviation of 15 μGal and may be attributed to random error. However, the gravitational difference between three station pairs exhibited a temporal change of approximately 45 μGal between the July and November 1977 surveys, while the difference between three other pairs changed by smaller and less statistically significant amounts. Neither observed changes in groundwater elevation nor regional tectonic activity, as indicated by seismicity, can explain the observed variations. The most probable source is a localized change in elevation caused by the surface loading associated with changes in stage of the Mississippi River.
Introduction to Special Section on Partial Melting Phenomena in Earth and Planetary Evolution
Authors: Waff Harve S.
Published: February 2008

Partial melting has been a major element in the dynamic evolution of the moon, earth and other terrestrial planets since the earliest stages of their existence. The entire outer crust of these bodies has, at one time, passed through the melting process; partial melting is probably of major importance in formation of the zonal structure of the terrestrial planets including separation of their cores. Throughout the history of the earth and probably Venus, partial melting has been an active process in separation and redistribution of materials. Chemical fractionation of the crust and mantle is the integrated result of ongoing magmatic transport via partial melting through geologic history.
Tectonic motions in California inferred from Very Long Baseline Interferometry observations, 1980–1984
Authors: Lyzenga Gregory A. , Wallace Karen S. , Fanselow John L. , Raefsky Arthur , Groth Polly M.
Published: February 2008

Very long baseline interferometry (VLBI) has been used to measure three baselines of 150–300 km length within California and three baselines of 1500 km length between California and Texas. These measurements, which span nearly 4 years beginning in 1980 and have typical individual uncertainties of approximately 2 cm, have been fitted to a steady plane strain/rotation model. After solving for internal network strains and velocities with respect to the North American plate interior, the Jet Propulsion Laboratory site (35 km SW of the San Andreas fault trace) is found to move toward the northwest with respect to points > 100 km NE of the fault, at an average rate of 25 ± 4 mm yr−1, along an azimuth of N40°W ± 7°. No reliably determined motion is registered between the other sites, all of which are > 100 km from the inferred Pacific‐North American plate boundary. The observed rate of displacement across the San Andreas fault agrees with independent determinations of contemporary slip rates but appears to conflict with the rigid global plate tectonic rate. Differences between this rate determination and those from satellite laser ranging observations provide potentially important constraints on the distribution of tectonic strain in the region. These results are taken as evidence for a regional distribution of elastic strain that is offset in approximately the same sense and amount as the Big Bend of the southern San Andreas fault. Finite element simulations support the conjecture that tractions on the base of the crust that accompany mantle downwelling beneath the Transverse Ranges, could contribute to these strain inhomogeneities.
Strain accumulation in the Yakataga Seismic Gap, southern Alaska
Authors: Savage J. C. , Lisowski M.
Published: February 2008

Strain accumulation in the Yakataga seismic gap has been estimated from the deformation of a 60 km × 40 km trilateration network surveyed in 1979–1980, 1982, and 1984. The contraction in the approximate direction (N19°W) of plate convergence is 0.19 ± 0.04 μstrain/yr, and there is a minor (0.07 ± 0.04 μstrain/yr) orthogonal extension. A significant right‐lateral shear (0.09 ± 0.02 μstrain/yr) occurs across a vertical plane striking N71°E. A simple dislocation representation of the plate interaction model proposed by Lahr and Plafker for the Yakataga seismic gap quantitatively accounts for the observed N19°W contraction and qualitatively accounts for the right‐lateral shear, the two strains being associated with the normal and transverse components, respectively, of the oblique convergence between the Yakutat block and the North American plate. The measurements are consistent with strain accumulation building up to a great thrust earthquake in the Yakataga gap. In addition, there is a suggestion of right‐lateral shear strain accumulation across the Contact fault, but the interval over which data are available is probably too short to prove such accumulation.
Diffracted Sp generated under the Australian Shield
Authors: Baag Chang‐Eob , Langston Charles A.
Published: February 2008

Diffracted Sp is found to be generated under the Australian shield by deep‐seated Tonga, Honshu, and Bonin events. This seismic phase is set up at the Moho as a P head wave by a critically incident upgoing SV wave at an epicentral distance of around 50°. The simple source time history inferred from P or SH particle motion and the absence of source structure effects on SV waveforms from a deep source suggest a diffracted Sp wave for the explanation of distorted SV waves recorded at the Mundaring station. The polarity of this phase arriving immediately after direct SV indicates a P type of wave, excluding the possibility of other type of S waves like ScS. Using a WKBJ spectral method for SV wave propagation, synthetic seismograms for the direct SV and diffracted Sp are computed. With some modification of the P velocity model by Mathur and the S velocity model presented by Goncz and Cleary for the Australian shield, synthetics produce the distinctive interference between direct SV and diffracted Sp seen in the data. The arrival time difference between direct S and diffracted Sp is sensitive to the P velocity of the upper mantle. Results of the analysis reveal higher P velocities in the upper mantle along the east‐west path than along the north‐south path from Mundaring station. The analysis of teleseismic SV waveforms and the diffracted Sp phase is a new and useful technique for determining regional variations in upper mantle P velocity structure.
Magnetotelluric investigations at Mount Hood, Oregon
Authors: Mozley E. C. , Goldstein N. E. , Morrison H. F.
Published: February 2008

Magnetotelluric data, with both electric and magnetic field references for noise cancellation, were collected at accessible locations around and as close as possible to the Mount Hood andesite‐dacite volcano. The purpose of the study was to identify and map conductive features and to relate them to the thermal regime of the region. Several conductors could be discerned. The shallowest, at a depth of around 500 m below the surface, was identified as a flow of heated water moving away from the summit; the deepest (∼50 km) might be a melt zone in the upper mantle. Of particular interest is an elongate conductor that strikes Nl0° W and extends from a depth of 12 km down to 22 km. Because the conductor strike is close to the trend of the chain of Cascade volcanoes and because of the high conductive thermal gradients reported for the area, this feature was initially believed to be a zone of partial melt following the volcanic axis. However, because no teleseismic P wave velocity anomaly has been found, the cause of the conductor is more problematic. While the existence of small zones of melt cannot be ruled out, it is possible that the conductor is caused by a large volume of intensely deformed rocks with brine‐filled microfractures.
Focal depths and mechanisms of large earthquakes on the Arctic Mid‐Ocean Ridge System
Authors: Jemsek John P. , Bergman Eric A. , Nabelek John L. , Solomon Sean C.
Published: February 2008

As part of a global study of the source characteristics and tectonic implications of large earthquakes on mid‐ocean ridges, we report on the focal depths and mechanisms of the six largest earthquakes that have occurred in the last 20 years on the Arctic mid‐ocean ridge system. For each earthquake we invert the long‐period P and SH waveforms to estimate the parameters of the best fitting point source, including seismic moment, centroid depth, double‐couple source orientation, and source time function. Three of the earthquakes occurred on the oceanic spreading center in the Eurasian Basin, along ridge segments spreading at half rates of 4–6 mm/yr. These events have mechanisms very similar to those of ridge crest earthquakes on the Mid‐Atlantic Ridge: almost pure normal faulting on planes that dip at approximately 45° and strike parallel to the rift axis, moments of 4–5 × 1024 dyn cm, centroid depths of 1–2 km beneath the seafloor, and water depths (inferred from the predominant period of water column reverberations) appropriate to epicentral locations within the median valley. The remaining three earthquakes, also characterized by normal faulting, are associated with the continuation of the divergent plate boundary (2–3 mm/yr half rate) onto the continental shelf of the Laptev Sea, where the crust becomes transitional in nature. One of the largest known spreading center earthquakes (August 25, 1964, M0 = 1 × 1026 dyn cm) occurred where the oceanic ridge intersects the outer edge of the continental slope. Waveform inversion for this event can resolve unilateral rupture from north to south (landward) along a fault at least 30 km in length. The preferred centroid depth is 5 km beneath the seafloor in crustal material with an unusually low shear velocity, but a centroid depth as great as 15 km cannot be ruled out. Two earthquakes beneath the continental shelf have significantly greater centroid depths (10–20 km) than mid‐ocean ridge earthquakes, indicating a thicker brittle regime and a cooler thermal structure than are typical of oceanic spreading centers. The tectonic environment of these events is more representative of rifted continental lithosphere than of a mid‐ocean ridge.
Sediment sound‐velocity provinces in the Cape Basin: New results using multichannel seismic reflection profiles
Authors: Austin James A. , Divins David L.
Published: February 2008

As part of a broad synthesis of geophysical data from passive continental margins bordering the South Atlantic, sediment sound‐velocity functions have been derived for the Cape Basin off southwest Africa using velocity/travel time information gathered from multichannel seismic reflection profiles and sonobuoys. These functions allow more accurate sediment thickness calculations than similar functions previously derived for this region solely from sonobuoy results. However, a single basin‐wide function is insufficient to predict thickness variations everywhere in the depocenter. Furthermore, sediment‐velocity equations developed for the southern Cape Basin do not precisely define sediment thicknesses in the north, and vice versa. Equations appropriate to both regional and previously published crustal subdivisions of the basin produce the most consistent results relative to available well control, although possible associations between underlying crustal type and overlying sediment distribution on this passive margin remain unclear. Instead, the location of the Orange Cone deltaic accumulation south of the present course of the Orange River may be the principal factor responsible for separate southern and northern sediment‐velocity provinces in the Cape Basin.
Vertical crustal deformation associated with the 1979 M = 6.6 Imperial Valley, California Earthquake: Implications for fault behavior
Authors: Reilinger Robert , Larsen Shawn
Published: February 2008

Elevation changes in the Imperial Valley, California, derived from repeated leveling surveys for the time period including the 1979 M = 6.6 earthquake, provide some constraints on fault geometry and slip distribution associated with this event. Many of the first‐order features of the observed vertical movements are well matched by simple models consisting of variable slip on planar faults in an elastic half‐space using fault offsets inferred from strong ground motion observations (Archuleta, 1984) and measured afterslip. The geodetic and seismic observations suggest that significant slip is confined to depths above 13 km with maximum right‐lateral offset reaching about 1.5 m on the Imperial fault. Dip slip occurs predominantly in the sediments on the upper 5 km of the Imperial fault and on the Brawley fault. Right‐lateral afterslip is confined to the upper 5 km of the Imperial fault and reaches about 30 cm for the period 1979–1981. In contrast, observed elevation changes near the 1979 hypocenter (on southern end of Imperial fault) and in the Brawley Seismic Zone show significant deviations from those predicted by models of fault slip inferred from strong ground motion measurements. Specifically, the geodetic data suggest that slip on the Imperial fault is significantly lower in the vicinity of the earthquake hypocenter than along the central and northern sections of the fault. In addition, there is marginal evidence that the dip of the Imperial fault changes along strike from approximately vertical just north of the U.S.‐Mexico border to between 70° and 80° near the northernmost extent of the 1979 surface break. This change in dip may be related to a change in local strike along the fault. Large elevation changes (>15 cm) also occur within the Brawley Seismic Zone well north of the primary surface faulting. While these movements are consistent with a number of possible fault models, our prefered interpretation based on geodetic and seismic observations (aftershock locations and focal mechanisms) involves right‐lateral, aseismic slip on a northwest striking fault along the east side of the Brawley Seismic Zone and conjugate left‐lateral faulting on a northeast striking fault (possibly associated with an M = 5.8 aftershock). Buried creep on this same right‐lateral fault in the Brawley Seismic Zone can also account for vertical deformation during the postseismic period of the 1940, M = 7.1 Imperial Valley earthquake as well as deformation during the interseismic period between the 1940 and 1979 events. Substantial right‐lateral slip in the Brawley Seismic Zone suggests that a significant part of the shear strain released during and following the 1940 and 1979 earthquakes on the Imperial fault is transferred through the Brawley Seismic Zone to the southern end of the San Andreas fault.