bluuebunny/crossref_metadata_2025 huggingface dataset:
Progressive inversion for hypocenters and P wave and S wave velocity structure: Application to the Geysers, California, Geothermal Field
Authors: O'Connell Daniel R. H. , Johnson Lane R.
Published: February 2008

Seismicity at The Geysers is induced by some aspect of steam production. Accurate earthquake locations, particularly hypocentral depth, are needed to determine the relationship between geothermal energy production and seismicity. Progressive P and S wave velocity‐hypocenter inversions were done using data from 39 microearthquakes at The Geysers to estimate microearthquake locations and determine if the geothermal field has a distinctive seismic signature. Comparable final Vp, Vs and Vp/Vs velocity models were obtained from four different starting velocity models. The elevation interval of maximum steam production coincides with minimum observed Vp/ Vs and Vp/ Vs increases below the primary production zone, suggesting that reservoir rock becomes more fluid saturated with increasing depth. A Vp/ Vs peak at the condensation zone‐production zone elevation delineates the top of the stream reservoir. Earthquake locations are confined to two depth intervals. A zone of shallow seismicity occurs immediately below the condensation zone and above maximum depths of steam production. A more arealy restricted zone of seismicity is located below maximum production depths and is abruptly terminated at an elevation of −3.7 km.
A flow band model of the Ross Ice Shelf, Antarctica: Response to CO2‐induced climatic warming
Authors: Lingle Craig S. , Schilling David H. , Fastook James L. , Paterson W. S. B. , Brown Timothy J.
Published: February 2008

A time‐dependent model is applied to the Ross Ice Shelf flow band discharging ice stream B, West Antarctica. The model includes the effects of temperature, depth‐dependent density, and backpressure from the coasts of the Ross embayment and Crary Ice Rise. Data from the Ross Ice Shelf Geophysical and Glaciological Survey and the Siple Coast Project are used as input data. Accuracy and stability are verified by reproducing the flow band for 10,000 model years with equilibrium distributions of accumulation, surface temperature, and basal balance. Sensitivity is tested by forcing the model with increasing accumulation rates, surface temperatures, and basal melt rates, respectively, while other factors are held constant. The response of the ice shelf to three climatic scenarios that may result from increasing carbon dioxide and trace greenhouse gases is simulated. The results range from slight thickening with moderately increased backpressure in the grounding zone to rapid thinning accompanied by rapidly decreasing backpressure during 175‐ to 600‐year simulations, depending primarily upon whether increasing surface temperatures and accumulation rates are accompanied by increased rates of basal melting. The central ice shelf, about 400 km upglacier from the calving front, thins by 22% in 600 years when basal melting is increased linearly to a maximum of 0.5 m/yr after 150 years, then held steady. The ice shelf thins by 40% in 175 years at the same location when basal melting is increased linearly to 2.0 m/yr after 150 years, then held steady. The present calculated equilibrium rate of basal melting, averaged over the bottom surface of the flow band, is 0.17 m/yr.
Petrology of the Lower Tertiary Clarno Formation in north central Oregon: The importance of magma mixing
Authors: Suayah Ismail B. , Rogers John J. W.
Published: February 2008

Predominantly Eocene rocks of the Clarno Formation in north central Oregon consist of lava flows, volcaniclastic rocks, and tuffaceous sedimentary rocks lying between Cretaceous shallow marine/deltaic sedimentary rocks and younger volcanic rocks. Whole rock Rb and Sr isotopic data yield a poor isochron with an age of 47±5 Ma. Clarno rocks are broadly calc‐alkaline, range from basalt to rhyolite, and show complex textural and compositional properties. Plagioclase and clinopyroxene phenocrysts are present in most lavas, with olivine in mafic rocks and orthopyroxene, quartz, and subordinate biotite and hornblende in intermediate rocks. Spongy (sieve‐textured) plagioclase and glass‐bearing plagioclase are both common. Many quartz grains are resorbed, and some pyroxene grains are spongy. Some samples contain quartz in which plagioclase inclusions have undergone partial melting along the plagioclase‐quartz boundaries; plagioclase inclusions in these rocks are more calcic (An54–66) than the cores of their companion xenocrysts (An41), suggesting digestion of earlier mafic rocks. Mafic cumulates have high MgO, Cr, and Ni values. Some oxide‐ and element‐SiO2 diagrams show linear trends, but scatter from the trends is large. Plots of SiO2 versus Al2O3, Sr, Y, and possibly other components show inflections in trends in the range of 55–60% SiO2. The abundance of disequilibrium textures shows that the dominant process in the evolution of the Clarno suite was mixing of magmas with liquid and (partly) solid sources, probably including earlier crystallized Clarno magmatic rocks. Cumulate textures and compositions, however, indicate that Clarno magmas were modified by crystal fractionation, which also may have contributed to the scatter from linear trends on composition diagrams. Initial 87Sr/86Sr ratios (corrected for a model age of 50 Ma) range from 0.70350 to 0.70397, consistent with mantle source(s) or with contamination by young mafic material but not with interaction with old upper continental crust.
Improvements in absolute gravity observations
Authors: Klopping Fred J. , Peter George , Robertson Douglas S. , Berstis Knute A. , Moose Robert E. , Carter William E.
Published: February 2008

In the absolute gravity instruments developed by the Joint Institute for Laboratory Astrophysics (JILA) (Zumberge et al., 1982; Niebauer et al., 1986), the release of the dropped object induces systematic vibrations in the floor‐gravimeter system. These vibrations can cause significant errors in the observed time‐distance values from which the gravitational acceleration is computed. Detailed study of the vibrations affecting the gravity observations shows them to contain random noise and site dependent systematic components, which can be modeled by decaying sinusoids in the range of 10 to 120 Hz. This paper discusses (1) a mathematical filtering method to correct the observed time‐distance array by identifying and removing all non‐random signals from each individual drop, (2) upgrades of the gravimeter controller, which allow the collection of the data required to implement the mathematical filtering, and (3) mechanical filtering experiments using shock dampening pads and braces to minimize the vibrations. The maximum correction to observed gravity has been 23 μGal using the mathematical filter; typical corrections are in the 2–7 μGal range. The use of the shock dampening devices alone resulted in a three‐fold reduction in the amplitudes and decay times of the systematic vibrations.
Foreword [to Long‐Term Sea Level Changes]
Authors: Cloetingh Sierd
Published: February 2008

This special section contains a selection of papers presented at the AGU Chapman Conference on Long‐Term Sea Level held in Snowbird, Utah, April 17–20, 1989. The conference was convened by Dork Sahagian and Anthony Watts and was attended by a group of researchers active in a wide range of subjects dealing both with causes and expressions of sea level change. Sea level variations form one of those central topics in earth sciences that is beginning to bridge the gap between otherwise remote fields such as postglacial rebound dynamics, intraplate tectonics, sedimentary geology, and isotope analysis.

The conference was the site of intensive debate, and the papers presented in this special section demonstrate the need for an integrated approach to make further advances in our understanding of sea level change. Integrated research on fundamental questions, including the relative roles of tectonics and eustasy and the issue of global synchroneity of sea level change, has actually only recently begun. It is our hope that the present section will stimulate much further research on these topics.
Formation of magnetic anomalies antipodal to lunar impact basins: Two‐dimensional model calculations
Authors: Hood L. L. , Huang Z.
Published: February 2008

The production of large‐scale magnetic fields and associated crustal magnetization in lunar basin‐forming impacts is investigated theoretically. Two‐dimensional numerical models of the partially ionized vapor cloud produced in such impacts show that the low‐density periphery of the cloud expands thermally around the Moon and converges near the antipode in a time of the order of 400 to 500 s for silicate impactor velocities of 15 to 20 km/s. Fields external to the impact plasma cloud are produced by the magnetohydrodynamic interaction of the cloud with ambient magnetic fields and plasmas. For the most typical case in which the Moon is immersed in the solar wind plasma and its embedded magnetic field, an MHD shock wave forms upstream of the cloud periphery separating the shocked solar wind from the free‐stream solar wind. For impacts occurring on the downstream (antisunward) hemisphere, convergence of the impact plasma cloud and associated MHD shock waves occurs on the upstream side and results in a large antipodal field amplification. For impacts occurring on the upstream (sunward) hemisphere, some antipodal field amplification is still expected due to the finite electrical conductivity of the lunar interior (requiring an induced external magnetic field) and the likely presence of some residual plasma in the wake of the impact plasma cloud. During the period of compressed antipodal field amplification, seismic compressional waves from the impact converge at the antipode resulting in transient shock pressures that have been calculated to be as large as 2 GPa (20 kbar). This is near to the range of 50–250 kbar at which stable shock remanent magnetization has been found experimentally to occur in lunar soils. Significant crustal magnetization anomalies antipodal to lunar impact basins are therefore expected, consistent with orbital mapping results. Weaker magnetization observed peripheral to the Imbrium basin may also be explained by shock effects together with compressed ambient fields in a surface boundary layer. Although other processes such as cometary impacts and a former core dynamo may have contributed significantly to the observed paleomagnetism, meteoroid impact plasmas appear capable of explaining a major part of the large‐scale magnetization detected thus far from lunar orbit.
Inversion for the rupture details of the 1987 East Chiba Earthquake, Japan, using a fault model based on the distribution of relocated aftershocks
Authors: Fukuyama Eiichi
Published: February 2008

The rupture of the 1987 east Chiba earthquake was investigated using an empirical Green's function method and a nonlinear inversion technique. In order to discuss what happened on the fault plane in more detail, we relocated the aftershocks as well as the main shock. Relocations were done using a joint hypocenter method in order to increase the accuracy of the relative locations. Based on this aftershock distribution, we propose a double‐fault model. The main fault strikes N11°W with nearly vertical dip. A subsidiary fault is perpendicular to the main fault. The results of a source inversion using strong motion waveforms are as follows: (1) moment release was large both in the deep northern area of the main fault and on the subsidiary fault, where aftershock activity was weak; (2) rupture propagated on both main and subsidiary faults simultaneously, and rupture velocity decreased at the depth of around 35 km, near the upper boundary of the Philippine Sea plate; and (3) stress drop was high on the lower northern corner of the main fault where the rupture terminated and on the deep southern area of main fault where the rupture initiated. A negative correlation was found between moment release during the main shock and that during the aftershock sequence, but they did not compensate each other quantitatively. This suggests the existence of a heterogeneous strain distribution in the fault area either before the main shock or after its aftershock sequence.
Paleomagnetism of the Cambrian Moore Hollow Group, Texas: Evidence for a primary magnetization carried by detrital magnetite
Authors: Farr M. R. , Gose W. A.
Published: February 2008

Petrographic and paleomagnetic studies of the Cambrian Moore Hollow Group, Texas, establish the presence of three magnetic minerals and several components of magnetization. Hematite is the dominant carrier of the magnetic remanence in most samples and is primarily a diagenetic replacement of preexisting Fe‐bearing phases. Hematite‐dominant samples have characteristic directions which range from shallow easterly to shallow southerly. This distribution suggests multiple components of magnetization acquired at various times in the Paleozoic. The paleopole obtained for an unmineralized breccia (135.7°E, 15.4°N) represents either a chemical remanent magnetization of Middle Ordovician to Early Silurian age or a vector addition of two or more Cambrian through Silurian components. Mineralized breccias at the Silver Creek Mississippi Valley‐type deposit yield a Permian direction (117.0°E, 51.6°N). Because hematite in this deposit predates mineralization, the Permian age represents a maximum age for the mineralization at Silver Creek. Many samples contain goethite as a replacement of Fe‐dolomite, glauconite, and pyrite. The direction of magnetization carried by goethite is near the present geomagnetic field or, in a minority of samples, antipodal to it. Rounded Ti‐magnetite was identified by SEM in magnetic extracts from siliciclastic‐poor facies. IRM acquisition and thermal demagnetization confirms the presence of magnetite in these samples. Some samples contain an apparent modern viscous remanent magnetization. The dominant component (72 samples, 10 sites) has a site mean declination of 93.8° and inclination of 14.5° with an ∝95 = 8.5°, corresponding to a pole position at 163.0°E, 0.6°S. The presence of detrital magnetite, polarity changes, lack of evidence for prolonged burial, and the proximity of this pole position to other late Precambrian/Cambrian poles suggest that this component of magnetization indeed reflects the Late Cambrian paleopole for cratonic North America.
Numerical study on crack evolution in diagenetically consolidated sandstones
Authors: Zang Arno , Stöckl Horst
Published: February 2008

Using the finite element method the formation of microcracks by stress redistribution is numerically simulated for an idealized sedimentary rock. A porous quartz matrix is first consolidated by applying a stress field. An unstressed cement of quartz or calcite is then built into the pore space. Two scenarios for cementation are discussed. In the first case the cementation occurs in one step at constant stress. In the second case the cementation is modeled by filling the pore space in several stages with increasing stress beginning at the pore space surface. Finally, stress relaxation is examined by removing the primary stress field. When a sample is cut free from its host rock, tensile stresses arise in the cement, which are approximately equal in magnitude to the compressive stress in the consolidation phase. Therefore crack formation in the cement is modeled. In the case of a crack starting at the center of a cement grain, the stress intensity factor increases until the crack reaches the grain boundary, after which it drops abruptly to very low values. From this it is concluded that a crack, once initiated, will penetrate the cement grain completely but will probably arrest at a short distance behind the grain boundary. The effective Young's modulus, the effective Poisson's ratio, and the effective seismic P wave velocity of the fractured rock are discussed. Finally, the crack closure process under external hydrostatic pressure is modeled. The crack‐closing pressure determined for this idealized sedimentary rock is equal to the preexisting in situ pressure at cementation. These results are relevant to the conditions in drill cores removed from greater depth and applicable for estimates of the in situ stress field.
The double integral solution of seismic problems
Authors: Abramovici Flavian
Published: February 2008

The paper reviews the theoretical basis and the historical background of the Fourier‐Bessel representation of the solution to seismic problems in layered media. The paper shows how to incorporate point sources into the process based on integrating an ordinary differential system. The final form of the solution is given for the following cases: (1) an explosive source acting inside a liquid, solid, or mixed structure, (2) an internal torque pulse, and (3) a concentrated force acting on the surface. The paper ends with an example showing that “upgoing” and “downgoing” waves are present in an inhomogeneous structure, even when the integration of the differential system is performed in only one direction.
Crustal resistivity structure from magnetotelluric soundings in the Colorado Plateau‐basin and range provinces, central and western Arizona
Authors: Klein Douglas P.
Published: February 2008

Resistivity structure to about 25 km depth is defined from two‐dimensional modeling of 29 magnetotelluric (MT) soundings (0.002–5 Hz) that traverse 280 km of the southwestern Colorado Plateau, transition zone, and Basin and Range provinces in Arizona. From the surface to 5 km depth, the MT model suggests structural relationships between low‐resistivity sedimentary and volcanic rocks (50–300 ohm m) and high‐resistivity granitic and gneissic basement (500–9000 ohm m). In the Basin and Range province, the MT model and a seismic reflection section show a generally consistent distribution of supracrustal rocks that have relatively low to moderate resistivity (MT) and relatively strong, locally coherent reflectivity. The supracrustal zone defined by these physical properties is inferred to be composed of upper plate rocks above a middle Tertiary detachment fault system which has been mapped in surrounding ranges. Some low‐angle fault zones inferred from seismic reflections to extend into high resistivity basement below the supracrustal rocks are not resolved by the MT model. A low‐resistivity zone with a conductance of 500 S or more is modeled in the crust at an average depth of about 15 km in the Basin and Range province and transition zone and may deepen below the southwestern part of the Colorado Plateau. In the Basin and Range province, the top of the low resistivity may correspond to a reflective layer with a 6‐s two‐way‐travel time. This deep low‐resistivity zone might be caused by a small fraction of connected hydrous solutions or silicic melts.
Tertiary extension‐related volcanism, Old Woman Mountains area eastern Mojave Desert, California
Authors: Miller Jonathan S. , Miller Calvin F.
Published: February 2008

Tertiary volcanism in the Old Woman Mountains area, eastern California overlapped temporally and spatially with lithospheric extension during the early Miocene. Field relations and restoration of movement along major faults indicate that the primary locus of magmatism probably lay along or flanked the axis of the Piute Mountains. The maximum age of volcanism is unknown but is probably about 20–21 Ma (after regional extension began). Most of the volcanism terminated after emplacement of the 18.5 Ma Peach Springs Tuff. Contrary to conventional models which suggest that extension‐related volcanism is basaltic or bimodal, the volcanic rocks of the Old Woman Mountains area represent a calc‐alkaline continuum dominated by mafic andesite; the most mafic rocks are mildly alkalic. Compositional variability within the suite is the result of mixing a mantle component with silicic crust, accompanied initially by minor olivine fractionation. Petrographic and field observations indicate that both magma mixing and assimilation of granitic crust have operated to produce the observed chemical variation. Trace elements and isotopes preclude binary mixing and suggest that mixing involved several crustal end members. This is consistent with the heterogeneous nature of both the exposed basement and a lower crustal xenolith suite sampled by a Tertiary dike. The mildly alkalic nature of the mafic lavas, the enrichment in large ion lithophiles and light rare earth elements, and the relatively radiogenic character of the most primitive uncontaminated basalt suggest a source similar to enriched mantle lithosphere. Alternatively, these geochemical and isotopic signatures could be attributed to contamination of asthenospheric basalt with mafic lower crust or low‐degree partial melts of this crust.
Introduction to Special Section on the California‐Arizona Crustal Transect: CACTIS, Part 3
Authors: Simpson Robert W. , Howard Keith A. , Haxel Gordon B.
Published: February 2008

The CACTIS (California‐Arizona Crustal Transect Interim Synthesis) workshop in May 1988 brought researchers together in Flagstaff, Arizona, to discuss the geologic evolution and crustal structure of the southern Cordillera between the San Andreas fault in southeastern California and the Colorado Plateau in Arizona [Sass et al., 1988]. The first set of papers resulting from the workshop appeared in the Journal of Geophysical Research (JGR) as the special CACTIS 1 section in January 1990 [Howard et al., 1990], and a second as CACTIS 2 in November 1990 [Haxel et al., 1990]. Twelve papers in this issue form part 3 of the CACTIS series. A final collection of papers will appear in the future as a joint special section with the California Consortium for Crustal Studies (CALCRUST).
A new approach to modeling of the dynamics of the lithosphere
Authors: Li Q. , Nyland E.
Published: February 2008

The properties of the lithosphere are different when measured at different scales. At a macroscale the detailed structures of the lithosphere are smeared, and it behaves as a continuum. At an intermediate scale called the mesoscopic scale, the material is composed of many small solid phase blocks whose interfaces play a significant role in the mechanical behavior of the macro phase. The smallest scale is the microscopic scale that associates with the atomic structure of the material. We introduce two state variables to describe the mesoscopic structure; entropy is a measure of the randomness of the organization of lithosphere blocks, and β describes the random strain energy stored in the system. Here the macroscopic stress and strain cannot uniquely determine one another, the relationship also depends on the internal structure of mesoscopic arrangement of the blocks. We can in principle derive an equation of state for the material that is composed of solid blocks of known geometry and mechanical properties, but calculations requiring realistic models are extensive. A one‐dimensional model is useful as well as simple, however.
Calculation of bulk modulus and its pressure derivatives from vibrational frequencies and mode Grüneisen Parameters: Solids with cubic symmetry or one nearest‐neighbor distance
Authors: Hofmeister Anne M.
Published: February 2008

The bulk modulus KT can be related to structural parameters and to the sum of the squares of the vibrational frequencies. ∑vi2, for N‐atom crystals of cubic symmetry or for any symmetry that contains only one nearest‐neighbor distance. Required assumptions are the existence of (1) electrostatic forces exclusively between atoms at equivalent positions in the primitive unit cell, (2) pair‐wise central repulsive potentials between all other atoms, and (3) rigid ions. Including pressure in the derivation requires a more stringent version of point 2, namely, (4) that the structure scales isotropically upon compression. This leads to structurally independent formulas for the pressure derivatives of bulk modulus at 1 atm: the only variables are KT and sums involving vi2 and the first or second pressurderivative of vi, i.e., the mode Grüneisen parameters. Implicit in all equations is the independence of the sums on wave vector. Thus, knowledge of zone center phonons (mostly infrared and Raman bands) is sufficient to calculate elastic properties. Investigating these relationships for 45 minerals with 10 different structures shows that KT(0) is predicted within 7% of experiment for 21 solids, all of which should lack strong interactions between atoms at equivalent sites in the Bravais unit cell (i.e., rutile, corundum, ilmenite, and spinel structures). The accuracy of the model strongly depends on the accuracy of the sum ∑vi2. Agreement for B1, B2, fluorite, and wurtzite structures is moderate to poor, as expected, because these structures place like atoms as second nearest neighbors and can thus violate assumption 1. For rocksalt, agreement varies, depends on the size of the cation, not on the polarizability (i.e. the ionic rigidity) as inferred earlier. Calculated KT for the garnets is 1.367 time experimental KT, instead of unity, which is related to the structure not scaling perfectly with pressure. Agreement is variable (3–20%) for the perovskite structure with large discrepancies associated with ill‐constrained sums. However, for all structures, the relative size of KT is predicted, so that the theory can be used for systematics for all minerals with the proper symmetry, reguardless of whether the assumptions are satisified. For all of the above mentioned structures, K′o and K″o are predicted within the experimental uncertainties from the available spectroscopic measurements of 13 solids for K′o and 6 solids for K″o at pressure. Based on this, the good agreement for ilmenite, which does not meet the symmetry requirements, and the structural independence of the formulas, the relations involving K′o and K″o are inferred to hold for structures more complex than cubic or AXBY.
Garnet and pyroxenes in the mantle: A test of the majorite fractionation hypothesis
Authors: Herzberg Claude , Gasparik Tibor
Published: February 2008

Experimental data are reported for the compositions of garnets and pyroxenes that coexist with liquid in the system CaO‐MgO‐Al2O3‐SiO2 (CMAS) at 80–160 kbar. All experiments were run using the uniaxial split sphere apparatus at Stony Brook. Our data have permitted a refinement of Gasparik's (1990a) thermodynamic analysis of garnets and pyroxenes that occur at arid below the solidus; our results should provide precise estimates of the temperature‐pressure conditions at which peridotite and garnet xenoliths equilibrated in the mantle. An examination is also made of the composition of garnet that crystallizes as a liquidus phase; it is demonstrated to be a function of both composition and pressure. Pressure has the effect of changing the composition of garnet from pyropic at around 80 kilobars to pyroxene‐like (majorite) at pressures in excess of 140 kbar, changes that mirror those seen for garnets in the subsolidus. Failure to consider the effects of pressure on the composition of liquidus garnet can result in an erroneous test of the hypothesis that garnet fractionation occurred in early Earth history. It is shown that primitive mantle peridotite could have formed as a liquid by 35% majorite garnet fractionation from a CV chondrite like Allende at 200–260 kbar. These results support the hypothesis that extensive or possibly total melting occurred during the growth of the Earth by accretion of planetesimals (e.g., Davies, 1985; Abe and Matsui, 1985; Stevenson, 1987; Wetherill, 1990) and that majorite fractionation could have occurred during this time (Ohtani and Sawamoto, 1987). Removal of majorite to the lower mantle during the crystallization of a magma ocean is a successful way of explaining differences in MgO/SiO2 and CaO/Al2O3 between mantle peridotite and chondrite.
A test of Newton's inverse square law of gravitation using the 300‐m tower at Erie, Colorado
Authors: Cruz J. Y. , Harrison J. C. , Speake C. C. , Niebauer T. M. , McHugh M. P. , Keyser P. T. , Faller J. E. , Mäkinen Jaako , Beruff R. B.
Published: February 2008

Gravity was measured at eight different heights on a 300‐m meteorological tower, using electrostatically nulled LaCoste and Romberg gravimeters. The observed values were corrected for tides, temperature, and gravimeter screw errors, and tested for systematic effects due to (wind‐induced) tower motion (no such effects were found). These corrected results were compared with values predicted by means of Newton's inverse square law from surface gravity values. The differences exhibit no systematic trends, and their rms value is only 10 μGal, well within the errors of the experiment, as the estimated measurement errors increase from 9 μ Gal at the lowest platform to 14 μ Gal at the top and those of the predictions from 10 μ Gal to 23 μ Gal. These results set new constraints on the magnitude of any non‐Newtonian gravitational force; if such a force is derived from a Yukawa potential, the absolute value of α must be less than 0.001 for λ = 1000 m.
Eocene propagating rifts in the southwest Pacific and their conjugate features on the Nazca Plate
Authors: Cande Steven C. , Haxby William F.
Published: February 2008

We have mapped a 1200 km long NW‐SE trending lineament in Seasat and Geosat radar altimeter data crossing a remote portion of the Southwest Pacific Basin. This lineament runs obliquely both to the fracture zones (FZs) and magnetic lineations between the Austral and Agassiz FZs. By examining shipboard magnetics, gravity, and bathymetric profiles in this region and in the conjugate regions of the Nazca plate, we identify this feature as the site of a series of small ridge jumps, highly asymmetrical spreading and small propagating rifts, during the mid‐to‐late Eocene (chron C21 to chron C13). Between the Austral and Agassiz FZs there is roughly 250 to 350 km of missing Pacific plate crust, which must have been transferred to the Nazca plate by the propagating rifts and small ridge jumps. On the Nazca plate, the conjugate area to the gravity lineament is now being subducted beneath the Chile Trench. North of the Challenger FZ this area corresponds to a region of undecipherable magnetic anomalies. South of the Challenger FZ, there is evidence for a small 50 km ridge jump offshore of the Chile Trench. This small ridge jump by itself is not sufficient to account for the missing crust on the Pacific plate, and we conclude that there must have been repeated small ridge jumps at the same site. One consequence of these ridge jumps is that crust on the Nazca plate entering the trench is not as old as would be predicted by simply mirror imaging the anomaly pattern on the Pacific plate.
The Yellowstone‐Hebgen Lake Geoid obtained through the integrated geodesy approach
Authors: Milbert D. G. , Dewhurst W. T.
Published: February 2008

The 1987 Crustal Deformation Survey of the Yellowstone‐Hebgen Lake region produced accurate geodetic measurements in an area unique for its rugged topography and volcanic processes. Using the integrated geodesy approach, Global Positioning System (GPS) baseline vectors, geodetic leveling, and gravity measurements were combined in a simultaneous least squares adjustment. Using 30″ × 30″ digital terrain data, and an Airy model, isostatically corrected complete Bouguer gravity disturbances highlight both the Yellowstone Caldera and a second gravity low near Hebgen Lake. A linear relationship between these disturbances and the departures of geoid height from the Airy isostatic model have been found (r = 0.84). Adjusting the covariance function while incorporating the GPS contribution yields a function whose correlation length is significantly longer than that predicted by autocorrelation statistics of the gravity disturbances alone. The integrated adjustment produced a geoid whose low‐frequency components are in good agreement with OSU86F. Formal statistics indicate precision of 2–5 cm. GPS residuals for 35 leveled points in the adjustment have a root‐mean‐square value of 2.9 cm and a standard error estimate of 4.6 cm. In comparison, a free adjustment of 152 GPS vectors yields a differential height standard error of 3.8 cm.
Correlation of P wave amplitudes and travel time residuals for teleseisms recorded on the Southern California Seismic Network
Authors: Mori Jim , Frankel Arthur
Published: February 2008

P waves from 13 teleseismic events, well distributed in azimuth and recorded on a large short‐period network in southern California, show a positive correlation between travel time residuals and relative amplitudes. Slower arrivals are consistently larger than faster arrivals. Although there is a large amount of scatter, the data show an amplitude increase of about a factor of 2 for a 1‐s increase in travel time. The correlation can be seen in various subsets of the data: individual earthquakes, single stations recording 10 or more earthquakes and data averaged over all recorded events at each station. Because the variations in amplitude and travel time can be associated with surface geology, much of the data recorded at non bedrock sites can be explained in terms of a near‐surface layer of varying velocity. The travel time residual would then be directly proportional to the velocity in the layer and the amplitude proportional to the square root of the velocity contrast at the base of the layer. However, some stations show azimuthally varying travel time residuals that are attributed to upper mantle velocity structures. We use a two‐dimensional finite difference calculation to show that velocity anomalies in the upper mantle can also contribute to the observed correlation of travel time and amplitude.
A long‐term geomagnetic excursion from Plio‐Pleistocene sediments in Java
Authors: Hyodo Masayuki , Sunata Wahyu , Susanto Eko E.
Published: February 2008

A paleomagnetic record from the Plio‐Pleistocene sediments in Sangiran, central Java represents a long‐period change of the geomagnetic field and reveals an existence of large‐scale swing of declination between the Olduvai and Jaramillo events. The field directions in reverse polarity show a gradual clockwise swing to almost west‐pointing directions, followed by a rapid return to south‐pointing directions. The geomagnetic behavior has been well duplicated from the Plio‐Pleistocene sediments in Mojokerto, 200 km west of Sangiran. At both sites, the declinations have maximum deflection more than 70° from the axial dipole field direction and the inclinations remain low during the swing. The full duration of the swing, recorded over sections about 40 m thick at both sites, was estimated about 130,000 years and a zone of most deviated fields 50,000 years, assuming a constant sedimentation rate. The phenomenon was regarded as an excursion. Neither geomagnetic excursion nor geomagnetic event corresponding to the entire duration of the excursion has been reported at present, although only the final stage, a duration of most deviated fields, may be associated with the geomagnetic events reported from North America and Europe. An upward‐pointing eccentric radial dipole model rather than a downward‐pointing one seems plausible to explain the geomagnetic behavior during the excursion. The former becomes one possible source for the inclination anomalies in the time‐averaged fields of the Plio‐Pleistocene times in the equatorial areas. A short sequence of the Upper Olduvai transitional fields obtained from Mojokerto exhibits a VGP path confined to a band of longitude 150°W ± 20°, inconsistent with those from other sites on the globe. This suggests that a dipolar component was not dominant globally in the Upper Olduvai transitional fields.
Shear at the surface of a lake in light winds
Authors: Kenney Bernard C.
Published: February 2008

Standard computer cards released at 1‐min intervals from the same point beneath a hydrometeorological tower were observed to segregate by color according to their depth of integration of the current. Green cards floated flat at the lake surface. Orange cards, on the other hand, curved downward when placed on the lake surface and averaged the currents in the top 1 cm. The separation of the cards into two distinct plumes resulted from wind‐directed shear in the first centimeter below the surface that was superimposed upon a barotropic current flowing crosswind. Using time‐lapse aerial photography, the magnitude and direction of the shear was quantified. The mean shear in the top cm was 3.5 s−1 and was aligned with the mean wind direction. The wind‐directed shear was similar to that expected for a viscous sublayer in light winds (1.3–1.8 m s−1).
An optimal XBT sampling network for the eastern Pacific Ocean
Authors: Sprintall Janet , Meyers Gary
Published: February 2008

An objective analysis technique is used to design an optimal oceanographic network for the eastern Pacific Ocean from 160°W–80°W, 20°S–20°N. Expendable bathythermograph observations from a sequence of transequatorial transects (1979–1984) were used to estimate temporal and meridional mean and correlation statistics, while bathy messages in the global telecommunications system (1979–1982) were used to calculate zonal statistics. The variables sea‐surface temperature and depth of 20°C isotherm are presented here as representing the thermal structure of both the surface and subsurface layers. In the eastern Pacific, the thermal temporal and spatial signals fluctuate according to the presence and strength of El Niño. During ENSO it is not uncommon for these scales to double in value. To resolve characteristically shorter signals during the typical years, data associated with the larger scales of ENSO events must be excluded. Scales of variability in the thermal field were also determined to be very different in five latitudinally delineated regions corresponding to the major equatorial current system of the eastern Pacific region. In general, space and time scales representative of the subsurface structure are smaller than scales of sea‐surface temperature and are consequently the limiting factor in network design. In the design of an XBT network for the entire tropical eastern Pacific, decorrelation scales of 3° latitude, 15° longitude and 2 months are determined as being most representative and capable of resolving the subsurface thermal signal. Sub‐regional inhomogeneity of signal‐to‐noise ratio for the eastern Pacific (ranging from 0.5–1.5) implies that a uniform scale is not appropriate especially for quantitative studies of mapping error. Structure of interpolation errors suggest that optimal sampling is achieved with two to three samples per decorrelation scale i.e. one XBT station every 1°–1.5° latitude and 5°–7.5° longitude.
Comparison of aircraft synthetic aperture radar and buoy spectra during the Norwegian Continental Shelf Experiment of 1988
Authors: Rufenach C. L. , Olsen R. B. , Shuchman R. A. , Russel C. A.
Published: February 2008

Aircraft synthetic aperture radar (SAR) measurements from seven flight paths at two different altitudes were acquired over a trimodal ocean wave system off the coast of Norway on March 11, 1988. These three wave systems traveling in different directions were also measured by a directional buoy with the buoy spectra rotated into the radar coordinates for comparison with SAR spectra. Fifteen subspectra were averaged to obtain the measured SAR spectra, and sixteen spectral bands were averaged to obtain the measured buoy spectra. The seven flight paths in conjunction with the three wave systems resulted in a nearly uniform distribution of azimuth peak directions varying from 0° to 83° and R/V varying from 28 to 110. The comparison of the SAR‐ and buoy‐derived peak wavelength and direction shows some scatter. We consider the scatter to be due primarily to the statistical uncertainty in the spectral estimates, the wave number resolution, and differences in the temporal and spatial averaging. Scanning distortion is shown to cause significant bias in the SAR and buoy comparison for slow‐flying aircraft imaging ocean wave swell. Correction of the SAR spectra for this special radar‐ocean condition gives significant improvement in the SAR‐buoy comparison. We investigate the relative importance of the motion‐related mechanisms (velocity bunching, velocity smearing, and scanning distortion), and the tilt contributions using four nondimensional imaging parameters.
On the nonlinear mapping of an ocean wave spectrum into a synthetic aperture radar image spectrum and its inversion
Authors: Hasselmann Klaus , Hasselmann Susanne
Published: February 2008

A new, closed nonlinear integral transformation relation is derived describing the mapping of a two‐dimensional ocean wave spectrum into a synthetic aperture radar (SAR) image spectrum. The general integral relation is expanded in a power series with respect to orders of nonlinearity and velocity bunching. The individual terms of the series can be readily computed using fast Fourier transforms. The convergence of the series is rapid. The series expansion is also useful in identifying the different contributions to the net imaging process, consisting of the real aperture radar (RAR) cross‐section modulation, the nonlinear motion (velocity bunching) effects, and their various interaction products. The lowest term of the expansion with respect to nonlinearity order yields a simple quasi‐linear approximate mapping relation consisting of the standard linear SAR modulation expression multiplied by an additional nonlinear Gaussian azimuthal cutoff factor. The cutoff scale is given by the rms azimuthal (velocity bunching) displacement. The same cutoff factor applies to all terms of the power series expansion. The nonlinear mapping relation is inverted using a standard first‐guess wave spectrum as regularization term. This is needed to overcome the basic 180° mapping ambiguity and the loss of information beyond the azimuthal cutoff. The inversion is solved numerically using an iteration technique based on the successive application of the explicit solution for the quasi‐linear mapping approximation, with interposed corrections invoking the full nonlinear mapping expression. A straightforward application of this technique, however, generally yields unrealistic discontinuities of the best fit wave spectrum in the transition region separating the low azimuthal wave number domain, in which useful SAR information is available and the wave spectrum is modified, from the high azimuthal wave number region beyond the azimuthal cutoff, where the first‐guess wave spectrum is retained. This difficulty is overcome by applying a two‐step inversion procedure. In the first step the energy level of the wave spectrum is adjusted, and the wave number plane rotated and rescaled, without altering the shape of the spectrum. Using the resulting globally fitted spectrum as the new first‐guess input spectrum, the original inversion method is then applied without further constraints in a second step to obtain a final fine‐scale optimized spectrum. The forward mapping relation and inversion algorithms are illustrated for three Seasat cases representing different wave conditions corresponding to weakly, moderately, and strongly nonlinear imaging conditions.
Wave variance partitioning in the trough of a barred beach
Authors: Howd Peter A. , Oltman‐Shay Joan , Holman Robert A.
Published: February 2008

The wave‐induced velocity field in the nearshore is composed of contributions from incident wind waves (ƒ > 0.05 Hz), surface infragravity waves (ƒ < 0.05 Hz, |κ| < (σ2/gβ) and shear waves (ƒ < 0.05 Hz, |κ| > σ2/gβ), where ƒ is the frequency, σ = 2πƒ, κ is the radial alongshore wavenumber (2π/L, L being the alongshore wavelength), β is the beach slope, and g is the acceleration due to gravity. Using an alongshore array of current meters located in the trough of a nearshore bar (mean depth ≈ 1.5 m), we investigate the bulk statistical behaviors of these wave bands over a wide range of incident wave conditions. The behavior of each contributing wave type is parameterized in terms of commonly measured or easily predicted variables describing the beach profile, wind waves, and current field. Over the 10‐day period, the mean contributions (to the total variance) of the incident, infragravity, and shear wave bands were 71.5%, 14.3% and 13.6% for the alongshore component of flow (mean rms oscillations of 44, 20, and 19 cm s−1, respectively), and 81.9%, 10.9%, and 6.6% for the cross‐shore component (mean rms oscillations of 92, 32, and 25 cm s−1, respectively). However, the values varied considerably. The contribution to the alongshore (cross‐shore) component of flow ranged from 44.8–88.4% (58.5–95.8%) for the incident band, to 6.2–26.6% (2.5–32.4%) for the infragravity band, and 3.4–33.1% (0.6–14.3%) for the shear wave band. Incident wave oscillations were limited by depth‐dependent saturation over the adjacent bar crest and varied only with the tide. The infragravity wave rms oscillations on this barred beach are best parameterized by the offshore wave height, consistent with previous studies on planar beaches. Comparison with data from four other beaches of widely differing geometries shows the shoreline infragravity amplitude to be a near‐constant ratio of the offshore wave height. The magnitude of the ratio is found to be dependent on the Iribarren number, ξ0 = β(H/L0)−½. Shear waves are, as previous observation and theory suggest (Oltman‐Shay et al., 1989; Bowen and Holman, 1989), significantly correlated with a prediction of the seaward facing shear of the longshore current.
Variability of the sea surface temperature in the eastern equatorial Pacific during 1986–1988
Authors: Hayes S. P. , Chang Ping , McPhaden M. J.
Published: February 2008

Time series of surface wind and upper ocean temperature and velocity, obtained from equatorial moorings along 110°W, are used to assess the importance of various oceanic and atmospheric processes in the variation of the mixed layer temperature for the period January 1986 to June 1988. This period coincides with the onset and development of the 1986–1987 El Niño‐Southern Oscillation warm event and a subsequent cold event in 1988. Results of the temperature equation analyses indicate that seasonal and interannual variability of sea surface temperature (SST) in the eastern Pacific cannot be accounted for by observed surface heat flux; oceanic processes play an important role in the heating of the surface water. Although no single process dominated SST change, the most important processes in the mean balance were the net incoming surface heat flux, the penetrative solar radiation, and the vertical turbulent flux out the bottom of the mixed layer. The mean vertical entrainment could not be estimated with the available data. On the seasonal time scales, both the vertical turbulent heat flux and the vertical entrainment variations were well correlated with SST change. Zonal advection was a significant contribution to the heat flux variability, but its fluctuations were poorly correlated with the mixed layer heating. In particular, it was found that zonal advective heat flux tended to be out of phase with the spring warming. At higher frequencies, little zonal advective heat was found to be associated with the passage of a Kelvin event in January 1987. Surprisingly, meridional heat advection appeared more important than the zonal heat advection in modifying the local SST as this event passed the mooring location.