bluuebunny/crossref_metadata_2025 huggingface dataset:
Records of geomagnetic reversals from volcanic islands of French Polynesia: 2. Paleomagnetic study of a flow sequence (1.2–0.6 Ma) from the Island of Tahiti and discussion of reversal models
Authors: Chauvin Annick , Roperch Pierrick , Duncan Robert A.
Published: February 2008

A volcanic sequence almost 700 meters thick has been sampled in the Punaruu valley on the island of Tahiti, southern central Pacific Ocean. Detailed paleomagnetic results have been obtained from 123 sites. Three reversals are recorded in this sequence. Age determinations (K‐Ar) indicate that the youngest reversal corresponds to the Matuyama‐Brunhes transition while the two other transitions limit the Jaramillo normal polarity subchron. An apparent R‐T‐R excursion has been identified lower in the volcanic sequence and K‐Ar age determinations around 1.1 Ma suggest that it corresponds to the Cobb Mountain subchron, but no normal paleomagnetic directions were discovered. The Matuyama‐Brunhes and the lower Jaramillo transition are defined by only a few intermediate directions while many intermediate directions are observed for the upper Jaramillo transition and the Cobb Mountain excursion. We attribute these differences to variations in the rate of eruption of the volcanic rocks. The lower Jaramillo record is characterized by a steepening of the inclination at the beginning of the reversal suggesting a possible axisymmetric control of the field at this stage. However, the transition path for the most detailed record (upper Jaramillo) is characterized by large loops; this prevents simple modeling of the transition by low order zonal harmonics at all stages of a reversal. Paleointensity determinations were attempted on 48 samples with reliable results obtained for 26 of them. Paleointensities for the transitional field range from 3 to 8 μT. Such very low field strengths were first suggested by the low intensity of the natural remanent magnetization associated with intermediate directions. An analysis of the variation of the intensity of magnetization with the angular departure from the central axial field, including all other available data from Polynesia, indicates that (1) a paleomagnetic direction should be considered as intermediate when it is more than 30° from the expected axial dipole direction, and (2) the average transitional geomagnetic field intensity is about 1/5 of its value during stable polarity states. Comparison of these results observed in Polynesia with previous studies from Iceland suggests an increase of the average intensity of the intermediate field with latitude.
A stable chemical remanence in Holocene sediments
Authors: Snowball Ian , Thompson Roy
Published: February 2008

The ferrimagnetic iron sulphide, greigite, has been extracted from freshwater sediments from Loch Lomond and Llyn Geirionydd and identified by X‐ray diffraction and thermomagnetic analyses. The authigenic greigite is probably the dominant carrier of a stable natural remanent magnetization in the freshwater sediments underlying marine sediments in Loch Lomond and in the Lateglacial interstadial sediments from Llyn Geirionydd. Mineral magnetic analysis shows that greigite tends to have a higher SIRM/X ratio (approximately 70 kAm−1) than many natural magnetite assemblages. This magnetic characteristic coupled with greigite's ability to oxidize easily and to change to a less magnetic mineral upon exposure to air may be used to indicate its presence in downcore magnetic profiles. Alternating field demagnetization cannot be used to distinguish chemical from detrital remanences in Loch Lomond or Llyn Geirionydd on account of the similar magnetic stabilities of greigite and magnetite. The time lag between the acquisition of the primary detrital remanence and the secondary chemical remanence remains unknown and creates difficulties in understanding the record of paleomagnetic secular variation contained in these lake sediments.
The largest recorded earthquake swarm: Intraplate faulting near the Southwest Indian Ridge
Authors: Wiens Douglas A. , Petroy David E.
Published: February 2008

The most prolific source of moderate and large earthquakes along the mid‐ocean ridge system is located near the Atlantis II Transform along the Southwest Indian Ridge. A remarkable swarm of 48 earthquakes was recorded from 1925 to 1933, which included 29 events with M ≥ 6.0 and 4 events with M = 7.0. Examination of the global earthquake catalog suggests this is the largest known earthquake swarm. Relocated events cluster around 33.8°S, 58.0°E, approximately 100 km east of the southern intersection of the Atlantis II Transform and the spreading ridge, and epicentral uncertainties derived from both standard least squares and resampling techniques preclude locations along the plate boundary. Multiple event relocation shows that seven 1977–1983 events (mb 5.1–5.6) are distributed across a 40‐km‐diameter region at the location of the 1925–1933 swarm events. Mechanisms determined for three of the recent events using waveform inversion indicate normal faulting with variable strike‐slip components at depths of 12–17 km; neither the nodal planes nor the tensional axes show consistency between the three mechanisms. Grid search fits to body waveforms from the larger 1925–1933 swarm earthquakes suggest normal faulting mechanisms similar to the recent events, with depths of 11–20 km and seismic moments ranging up to 1.8×1027 dyn cm. The earthquake depths and tensional mechanisms are consistent with release of thermoelastic stress, which may be enhanced at this location by thermal bending moments associated with the transform fault. The total moment release of the 1925–1933 swarm (∼7×1027 dyn cm) is greater than the cumulative moment of all known oceanic intraplate seismicity since 1963 (∼3×1027 dyn cm). This suggests that intraplate stress release is dominated by infrequent but highly energetic episodes of seismicity and that estimates of intraplate moment release taken from observations over a limited time period underestimate intraplate seismicity levels.
Note concerning non‐double‐couple source components from slip along surfaces of revolution
Authors: Frohlich Cliff
Published: February 2008

One suggested explanation for the origin of deviatoric, non‐double‐couple earthquake sources is that there is slip along curved fault surfaces. Here we compute the moment tensor components for earthquakes along idealized faults which are surfaces of revolution. The calculations show that when the slip direction is around the axis of revolution, the earthquakes may possess purely double‐couple sources, or purely compensated linear vector dipole (GLVD) sources, depending on the geometry of the fault and the slip. However, a surprising result is that the CLVD component is always zero for unidirectional slip along surfaces having a suitable degree of symmetry. Thus slip along a large variety of simple and plausible curved fault geometries produces a purely double‐couple source.
Computation of synthetic seismograms for stratified azimuthally anisotropic media
Authors: Mallick Subhashis , Frazer L. Neil
Published: February 2008

We outline a method of computing synthetic seismograms for stratified, azimuthally anisotropic, viscoelastic earth models. This method is an extended form of the Kennett algorithm that is efficient for multioffset vertical seismic profiling. The model consists of a stack of homogeneous plane layers, and the response is computed iteratively by successive inclusion of deeper layers. In each layer, the 6×6 system matrix A is diagonalized numerically; this permits treatment of triclinic materials, i.e., those with the lowest possible symmetry. Jacobi iteration is an efficient way to diagonalize A because the entries of A change little from one wavenumber to the next. When the material properties are frequency dependent, the wavenumber loops are inside the frequency loop, and the computation is slow even on a supercomputer. When the material parameters are frequency independent, it is better to make frequency the deepest loop, with diagonalization of A outside the loop, in which case vectorization gives a relatively rapid computation. Temporal wraparound is avoided by making use of complex frequencies, and spatial aliasing is avoided by using a generalized Filon's method to evaluate both the wavenumber integrals. Various methods of generating anisotropic elastic constants from microlayers, cracks, and fractures and joints are discussed. Example computations are given for azimuthally isotropic and azimuthally anisotropic (AA) earth models. Comparison of computations using single and double wavenumber integrations for a realistic AA model shows that single wave‐number integration often gives incorrect answers especially at near offsets. Errors due to use of a single wavenumber integration are explained heuristically by use of wave front diagrams for point and line sources.
Heat flow, tectonics, and fluid circulation at the toe of the Barbados Ridge Accretionary Prism
Authors: Foucher J. P. , LePichon X. , Lallemant S. , Hobart M. A. , Henry P. , Benedetti M. , Westbrook G. K. , Langseth M. G.
Published: February 2008

Detailed patterns of near surface tectonics and heat flow were investigated at two locations at the toe of the Barbados Ridge accretionary prism. At the two locations, in the thickly sedimented southern portion of the prism near 12°20′N and in the northern area drilled during Ocean Drilling Program (ODP) leg 110 near 15°32′N, relative highs of heat flow were associated with zones of thrusting. We attribute them to upward advection of water rising from the décollement along faults through the sediments. In the ODP area, a significant heating plate effect from warm fluids flowing along the décollement and thrusts is required to account for the high regional heat flow. This effect implies high rates of fluid flow at the décollement, which can only be non‐steady state or would require massive lateral transport of fluids along strike.
Quantitative measurements of shear wave polarizations at the Anza Seismic Network, southern California: Implications for shear wave splitting and earthquake prediction
Authors: Aster Richard C. , Shearer Peter M. , Berger Jon
Published: February 2008

We analyze shear wave polarizations from local earthquakes recorded by the Anza network in southern California, using an automated method which provides unbiased and quantitative measurements of the polarization and the duration of linear motion following the shear wave arrival (the linearity interval). Initial shear wave particle motions are strongly aligned at four stations, a feature that is not predicted by focal mechanisms. The particle motion alignment is most likely caused by shear wave splitting due to anisotropy beneath these stations, a result supported by the clear shear wave splitting seen in a borehole recording near one of the Anza stations. These results are consistent with an earlier analysis of these data by Peacock et al. [1988]. However, our analysis does not support claims by Crampin et al. [1990] that shear wave splitting delay times at station KNW exhibit temporal variations which can be correlated with the occurrence of the North Palm Springs earthquake (ML=5.6) of July 8, 1986. Automatically determined linearity intervals scatter widely from 0.02 to 0.15 s and exhibit no clear temporal trends. We find a correlation between earthquake moment and the linearity interval, possibly a result of longer effective source time functions for the larger events. The inability to identify a distinct slow quasi‐shear wave pulse for the vast majority of these events indicates that scattering strongly affects the particle motion, even in the very early shear wave coda. Analysis of earthquake clusters with similar waveforms recorded at KNW shows that seismic Green functions are stable throughout the observational period and that most linearity interval variation is due to source and ray path differences between events. If shear wave splitting is causing the observed delay times between horizontal components, the waveform stability for events in these clusters restricts any temporal changes in shear wave splitting delay times to less than 5–10%.
Relationships between lava types, seafloor morphology, and the occurrence of hydrothermal venting in the ASHES Vent Field of Axial Volcano
Authors: Hammond Stephen R.
Published: February 2008

Deep‐towed and submersible photographic surveys within the caldera of Axial Volcano have been integrated with high‐resolution bathymetry to produce a geological map of the most active vent field (Axial Seamount Hydrothermal Expeditions (ASHES)) in the caldera. The ASHES vent field encompasses an approximately 200 m×1200 m area of active venting which is located adjacent to the southwest caldera wall. Locations for over 2000 photographs in and near the vent field were determined using a seafloor transponder network. Then each photograph was described utilizing a classification system which provides detailed information concerning lava type, hydrothermal activity, sediment cover, geological structure, and biology. Resulting data were entered into a digital data base, and computer‐generated maps were created that portray spatial relationships between selected geological variables. In general, the entire ASHES field is characterized by pervasive low‐temperature venting. The most vigorous venting is concentrated in an approximately 80 m×80 m area where there are several high‐temperature vents including some which are producing high‐temperature vapor‐phase fluids derived from a boiling hydrothermal system. Lava types within the ASHES vent field are grouped into three distinct morphologies: (1) smooth (flat‐surfaced, ropy, and whorled) sheet flows, (2) lobate flows, and (3) jumbled‐sheet flows. The most intense hydrothermal venting is concentrated in the smooth sheet flows and the lobate flows. The location of the ASHES field is mainly attributable to faulting which defines the southwest caldera wall, but the concentration of intense venting appears to be related also to the spatial distribution of lava types in the vent field and their contrasting permeabilities. Other structural trends of faults and fissures within the field also influence the location of individual vents.
The shallow structure of the Martian lithosphere in the vicinity of the ridged plains
Authors: Zuber M. T. , Aist L. L.
Published: February 2008

Wrinkle ridges in the smooth plains of the Coprates and Lunae Planum regions of Mars form linear or concentric patterns with a regular spacing of 25–50 km. We test the hypothesis that the periodic development of deformation was a consequence of unstable horizontal compression, that occurred prior to probable ridge‐related faulting, of a strength‐stratified lithosphere which consists of a mechanically strong surface plains unit that successively overlies a weak megaregolith and a strong lithospheric basement. Results show that a range of models with both rigid and deformable megaregolith‐basement interface conditions yield solutions which can explain the ridge spacing within the constraint provided by the estimated thickness of the smooth plains materials. In models that incorporate viscous and plastic rheologies, uniform and exponentially varying vertical strength distributions, and the presence or absence of interfacial slip at the base of the surface plains unit, the ridge spacing is primarily controlled by the power law exponent of the lithosphere, the megaregolith/plains unit thickness ratio, and the plains unit/megaregolith strength contrast. Deformable basement models with a viscous rheology (1 ≤ n ≲ 3) can explain the ridge spacing if the megaregolith was thicker than and approximately 1–3 orders of magnitude weaker than the plains unit. Similar models with a perfectly plastic (n → ∞) rheology require plains unit thicknesses greater than 5 km, which exceeds most observational estimates. The dominant wavelength in the plastic models is not sensitive to internal strength contrasts and provides no constraint on the relative competence of the plains and megaregolith. Ridge spacing in the combined viscous and plastic models can be consistent with either a dry, water‐rich, or ice‐rich megaregolith at the time of ridge formation. If the ridge spacing was controlled primarily by the mechanical properties of lithospheric basement with little or no influence from the plains unit, then the minimum megaregolith thickness in the vicinity of the ridges must have been of the order of 0.4 km. The fact that ridges are most commonly found in the smooth plains but ridge spacing can be explained by models without a plains unit suggests that the unit may have facilitated the nucleation of reverse faults that are interpreted to characterize ridge structure but conceivably could have folded passively in response to unstable compression of the basement.
New models for the origin of Valles Marineris closed depressions
Authors: Spencer John R. , Fanale Fraser P.
Published: February 2008

Current models for the formation of the abundant large closed depressions of Valles Marineris have serious shortcomings. Purely tectonic mechanisms are inconsistent with the morphology of many depressions, and removal of large quantities of ground ice from the canyon walls is inconsistent with the observed strength of the walls. Accordingly, we offer some alternatives. One possibility involves decay of ice‐rich bodies occupying partially sediment‐choked ancient graben that predated the overlying cratered and ridged plains. Other possibilities involve the removal of massive equatorial carbonate deposits storing much of the planet's CO2 inventory, generated during greenhouse conditions on early Mars. Solution by carbonic acid derived from the atmosphere (analogous to terrestrial karst) requires extensive recycling of the available water supply. Solution by various groundwater acids, possibly derived from the Tharsis magmas, requires less water, especially if only the smaller closed depressions are due to carbonate decay. Alternatively, volume loss due to decarbonation of carbonate during early high heat flow or a later Tharsis‐related heat pulse can produce extensive collapse, especially if the carbonates have high silica content.
The Mars thermosphere: 2. General circulation with coupled dynamics and composition
Authors: Bougher S. W. , Roble R. G. , Ridley E. C. , Dickinson R. E.
Published: February 2008

The National Center for Atmospheric Research thermospheric general circulation model (TGCM) for the Earth's thermosphere has been modified to examine the three‐dimensional structure and circulation of the upper mesosphere and thermosphere of Mars (MTGCM). The computational framework and major processes unique to a CO2 thermosphere are similar to those utilized in a recent Venus TGCM. Solar EUV, UV, and IR heating alone combine to drive the Martian winds above ∼100 km. An equinox version of the code is used to examine the Mars global dynamics and structure for two specific observational periods spanning a range of solar activity: Viking 1 (July 1976) and Mariner 6–7 (August–September 1969). The MTGCM is then modified to predict the state of the Mars thermosphere for various combinations of solar and orbital conditions. Calculations show that no nightside cryosphere of the type observed on Venus is obtained on the Mars nightside. Instead, planetary rotation significantly modifies the winds and the day‐to‐night contrast in densities and temperatures, giving a diurnal behavior similar to the Earth under quiet solar conditions. Maximum exospheric temperatures are calculated near 1500 LT (≤305 K), with minimum values at 0500 LT (≤175 K). The global temperature distribution is strongly modified by nightside adiabatic heating (subsidence) and day side cooling (upwelling). The global winds also affect vertical density distributions; vertical eddy diffusion much weaker than used in previous one‐dimensional models is required to maintain observed Viking profiles. A solar cycle variation in dayside exospheric temperatures of ∼195–305 K is simulated by our Viking and Mariner runs. The dayside heat budget responsible consists of a balance of EUV heating, adiabatic cooling, and molecular thermal conduction, with CO2 cooling having little influence. Solar heating is probably not the only driving mechanism of the Mars upper atmosphere; gravity wave and tidal influence from below will also likely be significant, especially during dust storm periods.
Calibration of a belt apparatus to 1800°C and 6 GPa
Authors: Brey G. P. , Weber R. , Nickel K. G.
Published: February 2008

A belt type solid media high‐pressure apparatus has been calibrated to the same accuracy as piston‐cylinder apparatuses. This was achieved by ensuring high reproducibility of pressures and temperatures and by calibrating the system with well‐established, internally consistent phase transitions. We have used the melting of silver (Mirwald and Kennedy, 1979), the quartz‐coesite transition (Mirwald and Masonne, 1980), and the graphite‐diamond boundary (Kennedy and Kennedy, 1976). High reproducibility of experimental conditions can be achieved by machining the various components of the furnace assembly very accurately and by using components which do not contaminate the thermocouple and have 99–100% theoretical density (in our case single‐crystal MgO and natural polycrystalline CaF2). It is essential that the gasket material has well‐defined mechanical properties: its friction behavior must be reproducible in each experiment so that the ram pressure always acts on the pressure vessel and the sample in the same way. Factory‐processed pyrophyllite fulfills this requirement. With all precautions taken and on the basis of the reactions cited above, we claim an accuracy of ±7°C and ±(1% nominal pressure+0.05 GPa). This corresponds to the accuracy given by Mirwald et al. (1975) for a low‐friction cell for a piston‐cylinder apparatus. Our belt apparatus is used routinely to achieve pressures up to 6 GPa and temperatures up to 1800°C.
Upper mantle structure of southern Africa from Pnl waves
Authors: Clouser Robert H. , Langston Charles A.
Published: February 2008

We modeled Pnl waveforms from two moderate‐sized normal‐fault earthquakes in Zambia to determine an upper mantle P wave velocity model for southern Africa. The event parameters are May 15, 1968, mb = 5.7, depth = 28 km; and December 12, 1968, mb = 5.9, depth = 6 km. Teleseismic body wave inversions by previous researchers constrain the focal parameters for these events. We compute synthetic seismograms for six upper mantle velocity models using a wave number integration algorithm until an acceptable fit to the data is obtained. These models have different mantle P wave velocity gradients and low‐velocity zone (LVZ) depths. Quality of fit is measured primarily by the Pn/PL amplitude ratio. Source‐station geometry allows for the independent sampling of the upper mantle beneath the Kaapvaal‐Zimbabwe craton and mobile belt structural provinces. Synthetics from a constant velocity mantle model do not contain prominent precursor arrivals seen in the data; these are interpreted as P waves turning in the upper mantle. These synthetics also produce a too low Pn/PL amplitude ratio. Synthetics for a model with a mantle P wave velocity gradient of 0.0033 s−1 and a LVZ no shallower than 170 km fit the cratonic path data adequately. A slightly lower gradient, with a LVZ no shallower than 150 km, is indicated for the mobile belt regions. All the data preclude a mantle lid velocity gradient as high as 0.0066 s−1. Different velocity gradients between the two provinces implies different thermal gradients, which supports the hypothesis that a deep, cool lithospheric root exists beneath the Kaapvaal‐Zimbabwe craton. Cratonic thermal gradients derived from our velocity gradients agree closely with those derived from thermal modeling by Ballard and Pollack [1987].
Multicomponent diffusion and creep in olivine
Authors: Jaoul Olivier
Published: February 2008

This paper shows how to construct the Onsager tensor of “phenomenological” coefficients which is the key element in the Thermodynamics of Irreversible Processes. In a diffusion controlled deformation process, these coefficients couple the forces involved during diffusional creep to the fluxes of individual species, which are simultaneously diffusing from places in compression (sources of atoms) to places in relative extension (sinks). During such processes an electrical field is generated and couples the different moving species. Global stoichiometry must be preserved between sources and sinks to avoid local accumulation of some species and precipitation of extra phases. The diffusion coefficients Di of all the major constituents of forsterite and olivine (10%Fe), with general formula Me2SiO4, are now quite well known in the temperature (T ∼ 1373–1873 K) and oxygen partial pressure (pO2) ranges which are also accessible to experimental creep studies. It is found that DSi ≪DOx ≪DMg,DFe. The activation energies and pO2 dependences are equally well constrained, both for diffusion and for high temperature creep. The main result of this model is that the diffusional creep rate ε is naturally controlled by the the diffusion rate of the slowest species Dsi, but with an important enhancement due to the presence of the most abundant and mobile defects, the octahedral vacancies, VMe, with concentration [VMe], which are “pulling” the slow silicons. The model yields an activation energy for creep which is the sum of that of silicon diffusion and of the VMe formation. The exponents of the (pO2)n power law dependences are additive: n(έ) = n(Dsi) + n([VMe]). A similar relation is also deduced for the activation volumes. Comparisons between the available creep and diffusion data show that these relations are reasonably obeyed. Finally, an attempt to compute έ by using the above model for an experimentally deformed olivine specimen yields a value within 1 order of magnitude of the έ which has been measured, and for which the dislocation microstructure clearly shows that climb is the rate controlling mechanism.
Deep crustal structure of the Cascade Range and surrounding regions from seismic refraction and magnetotelluric data
Authors: Stanley William D. , Mooney Walter D. , Fuis Gary S.
Published: February 2008

Several regional seismic refraction and magnetotelluric (MT) profiles have been completed across the Cascade Range and surrounding geologic provinces in California, Oregon, and Washington. Joint geologic interpretation of the two data sets provides constraints on the nature of the crust not available from either data set alone. Analysis of three MT and two seismic refraction profiles in Oregon and a coincident MT and refraction profile in northern California show a high degree of correlation between resistivity and velocity models. The main feature that is evident in both data sets is a highly conductive (2–20 ohm m) zone that occurs at depths of 6–20 km and largely within a midcrustal velocity layer of 6.4–6.6 km/s, overlying a lower crust with velocities of 7.0–7.4 km/s. Although this conductor and the midcrustal zone of 6.4–6.6 km/s velocities are generally rather horizontal, important structures do occur. For instance, near the boundary of Western Cascades and High Cascades the MT midcrustal conductor rises to within 6 km of the surface. In addition, on the coincident MT‐refraction profile in northern California a significant westward downdip occurs on both the MT deep conductor and the 6.4‐km/s velocity layer, with both occurring at very similar depths. However, in the Columbia Plateau of Washington, no deep crustal conductors occur shallower than 25 km; also, the velocity structure is quite different, with a 6.8‐km/s midcrust and a 7.5‐km/s lower crust. Complex accretionary structures occur on MT models for the southern Washington Cascades, quite unlike the more horizontal structures of the Oregon and California Cascades. The accretionary structures in the southern Washington Cascades have been shown to be related to stress release in the area of Mount St. Helens. In order to explain the similar structures in the MT and refraction models for Oregon and California, we propose a model involving the effects of metamorphic zonation to produce the velocity structure, combined with metamorphically produced fluids and partial melt to produce the deep conductor. The higher midcrustal velocities and larger depth to deep MT conductors under the Columbia Plateau are explained by a more mafic crustal composition and lower heat flow.
Late Cenozoic volcanism, subduction, and extension in the Lassen Region of California, southern Cascade Range
Authors: Guffanti Marianne , Clynne Michael A. , Smith James G. , Muffler L. J. P. , Bullen Thomas D.
Published: February 2008

Hundreds of short‐lived, small‐ to moderate‐volume, mostly mafic volcanoes occur throughout the Lassen region of NE California and surround five longer‐lived, large‐volume, intermediate to silicic volcanic centers younger than 3 Ma. Volcanic rocks older than 7 Ma are scarce in the Lassen region. We identify 537 volcanic vents younger than 7 Ma, and we classify these into five age intervals and five compositional categories based on SiO2 content. Maps of vents by age and composition illustrate regionally representative volcanic trends. By 2 Ma, the eastern limit of voicanism had contracted westward toward the late Quaternary arc. Late Quaternary volcanism is concentrated around and north of the silicic Lassen volcanic center. The belt of most recent volcanism (25–0 ka) has been active since at least 2 Ma. Most mafic volcanism is cakalkaline basalt and basaltic andésite. However, lesser volume of low‐potassium olivine tholeiite (LKOT), a geochemically distinctive basalt type found in the northern Basin and Range province, also has erupted throughout the Lassen segment of the Cascade arc since the Pliocene. Thus models of the mantle source and tectonic control of LKOT magmatism should be applicable both within and behind the subduction‐related arc. Normal faults and linear groups of vents are evidence of widespread crustal extension throughout most of the Lassen region. NNW alignments of these features indicate NNW orientation of maximum horizontal stress (ENE extension), which is similar to the stress regime in the adjacent northwestern Basin and Range and northern Sierra Nevada provinces. The large, long‐lived volcanic centers developed just west of a zone of closely spaced NNW trending normal faults. Within that zone of faulting, pervasive ENE extension has precluded growth of large, long‐lived crustal magma systems. We interpret the western limit of the zone of NNW trending normal faults as the western boundary of the Basin and Range province where it overlaps the Lassen segment of the Cascade arc. In our view, the Lassen volcanic region occurs above the subducting Gorda North plate but also lies within a broad zone of distributed extension that occurs in the North American lithosphere east and southeast of the present Cascadia subduction zone. An episode of ENE extension that began in the late Miocene in the northwestern Basin and Range province appears to have triggered widespread late Miocene to Quaternary mafic volcanism in the Lassen region. The scarcity of volcanic rocks older than 7 Ma suggests that a more compressive lithospheric stress regime prior to the late Miocene extensional episode may have suppressed volcanism, even though subduction probably was occurring beneath the Lassen region.
Geophysical constraints on Washington Convergent Margin Structure
Authors: Finn Carol
Published: February 2008

Gravity and magnetic maps of western Washington reveal the lateral structure and fabric of the Washington Coast Range, Puget Basin, and southern Washington Cascade Range. The magnetic and gravity maps show large amplitude positive anomalies associated with the shallow but largely buried section of Washington Coast Range mafic rocks which are separated by negative anomalies over deep sedimentary basins. The positive anomalies indicate that the Coast Range mafic basement extends farther east than previously thought, at least as far east as the longitude of Seattle. Linear and steep gravity and magnetic gradients indicate many unmapped, often buried faults in the Washington Coast Range Province. Magnetic highs are also associated with mapped batholiths in the Cascade arc. Several magnetic highs observed east of the Coast Range rocks and west of these batholiths may be associated with buried Tertiary plutons or ophiolites. Two‐dimensional gravity and magnetic modeling constrained with geological and other geophysical data indicate that the Coast Range Province rocks are about 1 km thick at the coast, thickening to as much as 30 km near their postulated eastern edge. A maximum boundary on the average density of the upper 15–20 km of the rocks that compose the Coast Range Province of 2920 kg/m3 was established by the modeling, suggesting a composition largely of basalt and gabbro with little interbedded sediments. Under these rocks may be mantle or a subduction complex composed of dense mafic, ultramafic, and sedimentary rocks like that proposed to underlie Vancouver Island. Previous gravity models of the Washington margin include lower densities for the proposed subduction complex than for Vancouver Island, suggesting a lower component of mafic and ultramafic rocks than the rocks underlying Vancouver Island. However, my Washington model requires that the proposed subduction complex be more dense than the trench sediments and, therefore, that material denser than sediments be incorporated within it. The absence of continental mantle and the modeled wedge shape of the Coast Range Province upper crust suggest that erosion of the bottom of the overriding plate by subduction processes may have occurred.
Positive feedback fracture process induced by nonuniform high‐pressure water flow in dilatant granite
Authors: Masuda Koji , Nishizawa Osamu , Kusunose Kinichiro , Satoh Takashi , Takahashi Manabu , Kranz Robert L.
Published: February 2008

During forced water infiltration into a dry granite rock in the dilatant state, migration of a water front and a clear positive feedback between acoustic activity and water flow into microcracks were revealed by means of AE hypocenter locations and velocity tomography. Water was injected into a cylindrical granite sample subjected to 40 MPa confining pressure and constant axial stress equal to about 70% of short‐term dry strength. The injection water pressure was held constant at 17 MPa until failure occurred. Soon after water injection into the bottom end of the sample, AE hypocenters began near the bottom end. Subsequently, they moved up with a strong preference for the region where a weak concentration of AE events had been observed during a steady state creep stage before the water injection. The P wave velocity structure was determined in the section parallel to the loading axis and crossing the cluster of AE hypocenters. The velocity structure, reconstructed by seismic tomography, showed a nonsymmetric distribution of the velocity with regard to the loading axis during the injection. The strong concentration of AE hypocenters suggests that water flow channels are preferentially formed in a very limited region in the dilatant rock. The concentration of AE events and formation of these water channels marks a positive feedback process in which the local strength of the rock is reduced by the decrease of the effective confining pressure caused by increasing pore pressure which, in turn, triggers more microfracturing.
Northeastward drift in the Northern Mid‐Atlantic bight during late spring and summer 1976
Authors: Frey Henry R.
Published: February 2008

Air drops of surface drifter cards were made approximately monthly from December 1975 to September 1976 by helicopter south of Long Island. Drifters from three successive releases on April 13, May 25, and July 1 were recovered at Cape Cod, Martha's Vineyard, and Nantucket. The northeastward drift is attributed to anomalously persistent southerly winds.
Typhoon June (1975) viewed by a scanning microwave spectrometer
Authors: Rosenkranz P. W. , Staelin D. H. , Grody N. C.
Published: February 2008

In November 1975, typhoon June formed in the Philippine Sea and was observed by the scanning microwave spectrometer on the Nimbus 6 satellite. This instrument measures thermal radiation emitted by the earth in the microwave band of oxygen, a water vapor line, and a transmittance window in between. The oxygen band measurements provide temperature maps, which show a single peak at the eye of the typhoon that changes in amplitude with time. The water line and window measurements are transformed into a coordinate system in which the atmospheric variables of column water vapor content and cloud liquid water content are mutually orthogonal. The vapor showed a broad maximum around the typhoon while it was intensifying. More structure developed in the vapor as the typhoon weakened. Inside the typhoon, surface roughness and white water induced by the wind are evident in the measurements. By assuming that the troposphere was saturated with respect to water vapor inside the typhoon, surface wind speed and cloud liquid water content are inferred. The temperature and the wind inferred from the microwave measurements show time and space correlations that are expected in light of the thermal wind equation. Comparisons are made with infrared cloud imagery and aircraft reconnaissance flight data. In the microwave‐aircraft data comparison the corresponding parameters show somewhat different time variation, which we attribute to the fact that the eye of the typhoon was not resolved in the microwave images.
On the tensor form of the bulk dispersion coefficient in a bounded skewed shear flow
Authors: Fischer Hugo B.
Published: February 2008

In 1953, G. I. Taylor showed that longitudinal dispersion of matter in a pipe can be represented by a one‐dimensional dispersion equation and derived the magnitude of a coefficient to represent the effect of the shear flow. Taylor's analysis is extended to a shear flow in which the velocity vector rotates with depth but is horizontally homogeneous. We show that horizontal dispersion can be represented by a two‐dimensional equation in which a tensor coefficient represents the effect of the vertical shear. The terms in the tensor are derived for an approximation to the mean flow on the Middle Atlantic Bight of the U.S. Atlantic continental shelf.
Electromagnetic fields radiated from a lightning return stroke: Application of an exact solution to Maxwell's equations
Authors: Le Vine D. M. , Meneghini R.
Published: February 2008

A solution is presented for the electromagnetic fields radiated by an arbitrarily oriented current filament over a conducting ground plane in the case where the current propagates along the filament at the speed of light, and this solution is interpreted in terms of radiation from lightning return strokes. The solution is exact in the fullest sense: no mathematical approximations are made, and the governing differential equations and boundary conditions are satisfied. The solution has the additional attribute of being specified in closed form in terms of elementary functions. This solution is discussed from the point of view of deducing lightning current wave forms from measurements of the electromagnetic fields and understanding the effects of channel tortuosity on the radiated fields. In addition, it is compared with two approximate solutions, the traditional moment approximation and the Fraunhofer approximation, and a set of criteria describing their applicability are presented and interpreted.
The anomalous winter thunderstorms of the Hokuriku Coast
Authors: Takeuti T. , Nakano M. , Brook M. , Raymond D. J. , Krehbiel P.
Published: February 2008

Multistation electric field change measurements have confirmed earlier findings that lightning ground strokes in the Hokuriku winter storms generally lower positive charge to earth. Only one out of eight flashes involved the ‘normal’ negative strokes commonly found in summer storms. The mode of discharge appears to favor the continuing current type: eight out of twelve strokes to ground were followed by a continuing current. Despite the ‘abnormal’ stroke polarity, the Hokuriku storms appear to exhibit a normal dipolar structure (positive charge uppermost), although the dipole is apparently severely tilted in the direction of the wind shear. A crude estimate of charge height based upon one negative cloud‐to‐ground flash and one positive flash locates the negative charge below the positive charge in a cloud system whose bases are at 400 m and radar tops are at 7.5 km. The reason for the occurrence of dominantly positive return stroke flashes in these winter storms remains unclear.
Direct measurements of the atmospheric conduction current
Authors: Burke H. K. , Few A. A.
Published: February 2008

A new method of measuring the atmospheric conduction current above the ground has been employed to obtain data for 12 weeks during the first half of 1974. The instrument consists of a split aluminum sphere suspended by insulated wires to a wooden frame. The measuring electronics and the transmitter are enclosed within the spherical structure. The interaction of the instrument with its atmospheric electrical environment is analyzed, and it is shown that in steady state conditions, predictable differences in the instrumentally measured currents and the atmospheric conduction current will be less than 5% and in the nonsteady state situations the difference is less than 20%. Diurnal variations, a probable winter‐summer variation, sunrise, and fog effects were observed for the data obtained during fair‐weather conditions. Disturbed weather data are interpreted for the effects of low clouds on the atmospheric current. The charge concentrations within overcast clouds sufficient to produce the observed reversed atmospheric currents are estimated to be small (around ±5 ×10 −12 C/m3) in relation to values in thunderclouds (around ± 10−10 C/m3).
Thermal microwave radiances from horizontally finite clouds of hydrometeors
Authors: Weinman J. A. , Davies R.
Published: February 2008

Microwave brightness temperatures emerging from horizontally finite clouds of hydrometeors have been computed by means of an analytical model. The analytical model is sufficiently fast that simulations of multifarious aspects of this problem can be easily evaluated. The results of these computations were compared with the results of Monte Carlo simulations, and the agreement was found to be satisfactory. The results of the present computations show that radiation emerging from the sides of isolated hydrometeor clouds causes the brightness temperature of such clouds to differ markedly from the brightness temperature of plane parallel clouds. If, however, the effect of the underlying surface is included in the computations of upwelling radiances, then the microwave brightness temperatures of finite clouds depart somewhat less from the plane parallel cloud results. Radiances emerging from hydrometeor clouds over land are less sensitive to the effects of horizontally finite geometry than are radiances emerging from comparable clouds over water. The effect of the finite field of view of satellite‐borne microwave radiometers on measured brightness temperatures is considered; it is shown that ambiguities of the order of a factor of 2 in the inferred mean rainfall rate can occur if a rain cloud does not fill the radiometer field of view.
Laboratory studies of wind‐driven Langmuir circulations
Authors: Faller Alan J. , Caponi Enrique A.
Published: February 2008

Langmuir circulations in a wind wave tank have been observed and measured by a variety of methods including dye patterns and floating surface tracers. On the basis of measurements of cellular wavelength λc as a function of surface wavelength λw and depth H a tentative functional relation for the dependence of λc/H upon λw/H is proposed. The experiments clearly imply that the waves play an important role in the mechanism of formation of the Langmuir circulations but that the depth of the layer has a significant modifying influence. On the basis of certain ‘rake’ experiments and other indirect observations it is proposed that when λw/H < 1, the primary scale of Langmuir circulations, related to λw, transfers energy to a larger scale that is commensurate with the depth such that λc/H ≃ 2. Under these circumstances, two distinct scales of Langmuir circulations may exist simultaneously.
Instabilities of zonal equatorial currents, 2
Authors: Philander S. G. H.
Published: February 2008

A stability analysis of a realistic meridional profile of the surface currents in the equatorial Atlantic and Pacific oceans reveals that the most unstable waves are westward propagating and have a period of about 1 month, a wavelength of approximately 1100 km, and an e folding time near 2 weeks. This is proposed as an explanation for waves with these scales that have recently been observed in the Atlantic and Pacific oceans.
Ice growth in Duluth‐Superior Harbor
Authors: Sydor M.
Published: February 2008

The heat budget for ice in the Duluth‐Superior Harbor was obtained from measurements of temperatures and net radiation and from weather data. It was found that ice growth in the harbor is nearly uniform and depends mainly on the net radiation term. Some localized variations in ice thickness can be attributed to the spatial distribution in albedo and the variation of the turbulent heat flux in water. For localized areas with high turbulence and thermal pollution where the heat transport in water becomes significant, a 20–50% decrease in ice thickness is observed, depending on flow velocities and the proximity of the areas to the heated effluent sources. Outside of these localized areas there are smaller but persistent variations in ice thickness over broad regions of the harbor adjacent to the industrialized sections of Duluth and Superior. These variations can be attributed to the relative changes in the albedo due to particulate fallout. Landsat data were used for determination of the relative distribution in albedo, while the turbulent heat flux in water was obtained from temperature distribution measurements and from flow rates which were calculated from a numerical model of the harbor.
Low‐frequency fluxes of momentum, heat, salt, and nutrients at the edge of the Scotian Shelf
Authors: Smith Peter C.
Published: February 2008

Measurements of current, temperature, and salinity from a single mooring at the edge of the Scotian Shelf are analyzed for the period from December 1975 to April 1976. Major contributions to the subtidal Reynolds stresses consist of the wind‐driven flows (2‐ to 8‐day periods) and offshore forcing effects (10–30 days). These may be distinguished by a difference in sign of the onshore momentum flux in each band. Uniform divergence of these stresses across the shelf would not be significant to the general circulation. The shoreward eddy transport of salt and heat are maximum at 50‐m depth and are concentrated in the lowest frequency bands. Considered uniform at the open boundary, the 50‐m fluxes would support temperature and salinity gradients of 1.5°C and 0.5‰ per 100 km along the Scotian Shelf. Also, a method is presented by which estimates of nutrient fluxes at the shelf break are made by establishing a relationship between nutrients and TS properties through another variable, the concentration of dissolved oxygen. The subtidal shoreward transport of nitrate is found to be 12 ± 4 μg atom N m−2 s−1, which meets Fournier's (1977) estimate (15.5 μg atom N m−2 s−1) of the spring and summer phytoplankton requirements on the shelf. Finally, current meter and hydrographic data and satellite imagery are used to estimate the net salt and heat transports produced by the entrainment of surface shelf water by a Gulf Stream eddy. On an annual basis (6 events/yr) the volumetric exchange (∼104 km3 yr−1) is greater than the estimate for a similar event in the Mid‐Atlantic Bight (Morgan and Bishop, 1977) but consistent with figures for the transport in shelf water filaments along the Gulf Stream (Kupferman and Garfield, 1977). The resulting contributions of salt and heat to the shelf waters are 3 × 1013 kg yr−1 and 6 × 1019 cal yr−1, which are equivalent to the measured subtidal transports at the shelf break. These estimates exceed by a factor of 6 those of Voorhis et al. (1976) and Horne (1978) for small‐scale frontal mixing.