bluuebunny/crossref_metadata_2025 huggingface dataset:
Paleomagnetism of Miocene East African Rift sediments and the calibration of the geomagnetic reversal time scale
Authors: Tauxe L. , Monaghan M. , Drake R. , Curtis G. , Staudigel H.
Published: April 2008

Paleomagnetic stratigraphy and K‐Ar age determinations are reported for the type section of the middle Miocene Ngorora Formation, found in the Kenya rift valley. The magnetostratigraphy is well correlated to the geomagnetic reversal time scale (GRTS) and spans from the lower part of Chron C5 (9) to Chron C5AB‐r (14). K‐Ar dates were determined for euhedral sanidines, handpicked from seven tuff horizons within the Ngorora Formation and the underlying Turn phonolite flow. These dates can therefore be tied directly to the GRTS. The eight dates fall into three discrete groups averaging 12.5±0.22 Ma (mean and standard deviation of results from four tuffs), 11.6±0.06 Ma (mean and standard deviation from three tuffs), and 10.16±0.38 (average of three analyses from one tuff). We interpret the age groups as resulting from three successive eruptive episodes, the stratigraphic positions of which are well constrained. In spite of episodic supply at the eruptive source, sediment accumulation is continuous at the resolution of the GRTS. This suggests that accumulation is controlled by basin subsidence rather than sediment supply. Sanidine dates support an age for the older boundary of marine magnetic anomaly 5 of about 10 Ma, as opposed to 11.12 Ma, suggested by the most recent results from Icelandic basaltic lava flows.
Heat flow in southern Mexico and the thermal effects of subduction
Authors: Ziagos John P. , Blackwell David D. , Mooser Federico
Published: April 2008

Twenty new heat flow measurements in southern Mexico are presented. These measurements document a very broad zone of low heat flow between the coastline and the Trans‐Mexico volcanic belt. At the Trans‐Mexico volcanic belt, heat flow increases to high values and remains high into central Mexico. Heat flow values in the Trans‐Mexico volcanic belt and northward are in excess of 80 mW m−2(1.9 μcal/cm2s) and typical of values observed in the high heat flow regions of the western United States. The average heat flow in the Sierra Madre del Sur is 26 mW m−2(0.6 μcal/cm2s), in the Sierra Madre Oriental is 89 mW m−2(2.1 μcal/cm2s), and in the Trans‐Mexico volcanic belt is 91 mW m−2(2.2 μcal/cm2s). A surface heat flow profile was constructed perpendicular to the trench, and the data were matched with a simple thermal subduction model. The resulting final model parameter set suggests that the angle of subduction is low and that convective heat transfer in the back arc region is probable. For data south of the Trans‐Mexico volcanic belt, reduced heat flow values (upper crustal radioactive effect removed) are less than 15 mW m−2(0.4 μcal/cm2s). Thus essentially the total mantle heat flow is being absorbed by the subducting block. The values are lower than the heat flow observed in the Sierra Nevada Mountains of California and support the hypothesis that the heat flow couple associated with the Sierra Nevada Basin and Range province is a relic subduction zone thermal pattern.
T waves with Long Inland Paths: Synthetic seismograms
Authors: Cansi Yves , Bethoux Nicole
Published: April 2008

In order to provide more insight into land propagation of T phases, signal analysis of these phases recorded inland has been carried out. We found that the wave train is composed mainly of Pg and Sg waves which reach the seismic station from different azimuths. With the hypothesis that the signal is the sum of discrete arrivals due to different rays crossing the continental slope at different locations, synthetic seismograms of T waves are computed taking into account the variation of the conversion point along the slope. The transmission of energy from the ocean to the continental crust is mainly due to diffraction effects. The duration arid the amplitude of the T phase, which are two criteria used to evaluate tsunami risk, are greatly influenced by the shape of the continental slope.
Heat flow in southernmost California and the origin of the Salton Trough
Authors: Lachenbruch Arthur H. , Sass J. H. , Galanis S. P.
Published: April 2008

Heat flow in the Imperial Valley and adjacent crystalline rocks is very high (∼140 mW m−2). Gravity and seismic studies suggest the crust is about 23.5 km thick with the lower half composed of gabbro and the upper fourth composed of low‐density sediments. Conduction through such a crust resting directly on asthenosphere would give the observed heat flow if there were no extension or sedimentation. However, both processes must have been active, as the Imperial Valley is part of the Salton Trough, a pull‐apart sedimentary basin that evolved over the past 4 or 5 m.y. To investigate the interrelations of these factors, we consider a one‐dimensional model of basin formation in which the lower crustal gabbro and upper crustal sediments accumulated simultaneously as the crust extended and sedimentation kept pace with isostatic subsidence. For parameters appropriate for the Salton Trough, increasing the extension rate has little effect on surface heat flow because it increases effects of heating by intrusion and cooling by sedimentation in a compensating manner; it does, however, result in progressively increasing lower crustal temperatures. Analytical results suggest that the average extensional strain rate during formation of the trough was ∼20–50%/m.y. (∼1014s−1); slower rates are inadequate to account for the present composition of the crust, and faster rates would probably cause massive crustal melting. To achieve the differential velocities of the Pacific plate at one end of the trough and North American plate at the other with this strain rate, extension must have, on the average, been distributed (or shifted about) over a spreading region ∼150 km wide. This is about 10 times wider than the present zone of active seismicity, suggesting that the seismic pattern is ephemeral on the time scale for the trough's formation. Narrow spreading zones are typical where sustained spreading is compensated by basaltic intrusion to form the thin oceanic crust, but where such spreading occurs in thicker continental crust, broader zones of distributed extension (with smaller strain rates) may be required for heat balance. The Salton Trough model suggests that distributed extension can be associated with substantial magmatic additions to the crust; their effect on crustal buoyancy has important implications for the relation between crustal extension and subsidence.
Recurrence rates and probability estimates for the New Madrid Seismic Zone
Authors: Johnston Arch C. , Nava Susan J.
Published: April 2008

An updated frequency‐magnitude relation for the New Madrid seismic zone is used to derive conditional probabilities for future, large New Madrid earthquakes. We estimate that there is a 40–63% probability of an mb ≥ 6.0(Ms ≥ 6.3) event occurring by the year 2000 and an 86–97% probability by the year 2035. The estimates for a great 1812‐type event (Ms ≥ 8.3) are less than 1% probability by 2000 A.D. and less than 4% by 2035 A.D. These probabilities are contingent on many factors, a number of which remain assumptions because of the lack of a geological or paleoseismological chronology of past New Madrid activity. A conditional probability requires knowledge of a mean recurrence time, the type of distribution, and the standard deviation of actual repeat times about this mean. Four assumed distribution functions (Gaussian, lognormal, Weibull, and Poisson) were fit to recurrence estimates based on a combination of historical and instrumental seismicity data. Standard deviation was allowed to vary between one third and one half of the mean recurrence time, and a range of conditional probabilities was generated for time intervals of 15 and 50 years from the year 1985. The largest uncertainty in this procedure was the size of the seismic source area to use for recurrence estimation. Calculations were done for both a large and a small source zone which led to variation in estimated recurrence intervals by a factor of 2. The large source zone was favored for the final probability estimates because of the large crustal volume required to store elastically the strain energy for great New Madrid earthquakes.
A source extent analysis of the Imperial Valley earthquake of October 15, 1979, and the Victoria earthquake of June 9, 1980
Authors: Silver Paul , Masuda Tetsu
Published: April 2008

A new method for estimating source extent parameters has been applied to the Imperial Valley earthquake of October 15, 1979, and the Victoria earthquake of June 9, 1980. From the stations of the World‐Wide Standard Seismograph Network and Canadian Seismic Network, estimates of the duration of far‐field long‐periodSHwaves were made by computing the variance or second‐central moment τ2of instrument‐corrected, attenuation‐corrected pulses. Seventeen measurements for the Imperial Valley event and 25 for the Victoria event were made. We inverted for the source durationT, the fault lengthL, and the directivity parameterDby least squares. We find that for the Imperial Valley,T= 9.0 ± 0.5 s,D= 130 ± 43 km s, andL= 46 ± 13 km, and for the Victoria event,T= 10.9 ± 0.6 s, andD= 251 ± 71 km s.Lis not well constrained for this second event; the inversion gives 23 km, the lower bound obtained by constraining τ2to be positive everywhere on the focal sphere, although a value as high as 40 km would still be within the uncertainties. The positive values ofDfor both events indicate that rupture propagation was predominantly to the northwest. In order to estimate the degree of “unilateralness” we have computed the rupture mode index defined byR= |D|/LT, which is 1 for a unilateral rupture and zero for a bilateral rupture. We find a value ofR= 0.31 ± 0.14 for the Imperial Valley event, significantly less than unity, which indicates that a southern component of rupture is also present. Assuming uniform slip, we estimate the northern and southern fault segments to be 26 and 20 km, respectively. The presence of a bilateral component is supported by near‐field studies which have made use of stations along the southern half of the Imperial fault. For the Victoria event, due primarily to the uncertainty inL, the value ofRcan vary from 0.6 to 1 with the value of 1 being most probable. Assuming that the rupture is unilateral, we can obtain an estimate of fault length from the relationL= |D|/T, which yieldsL= 23 ± 6 km. Geodetic data and the distribution of aftershocks are consistent with a unilateral rupture mode and the fault length that we have obtained. Values of seismic moment from measurements of the zeroth moment of the displacement pulse are found to be 5.8±0.4 for Imperial Valley and 4.8±0.4 for Victoria, in units of 1018N m. The presence of a southern component of rupture for the Imperial Valley event would suggest a closer and perhaps causal relationship to the Victoria event, which occurred 8 months later and about 60 km to the south. In a tectonic framework we interpret these two earthquakes as the failure of adjacent transform faults and consider the possibility that a concurrent spreading event may have occurred in the vicinity of Cerro Prieto.
Measurement of the resistance of imperfectly elastic rock to the propagation of tensile cracks
Authors: Peck Lindamae , Nolen‐Hoeksema Richard C. , Barton Christopher C. , Gordon Robert B.
Published: April 2008

Laboratory tests confirm the accuracy of the compliance equations for wedge‐loaded, linearly elastic, double cantilever beam test specimens used for the measurement of fracture energy GI but show that there are significant discrepancies with theory in tests on rock specimens of the same design. The dependence of the compliance on the length of the crack in the test specimen is not correctly predicted by theory for the experiments done on rock. The axial load applied to the arms of the double cantilever beam as a result of wedge loading reduces Young's modulus by as much as 44% and decreases the measured elastic anisotropy of specimens of granite. The experiments show that useful measurements of GI can be made on rock provided that the Young's modulus used in the determination of GI is measured on the same specimen under the same conditions of loading as are used in the fracture experiments. Values of GI for Sioux quartzite determined this way are within 12% of GI determined from the total work done on the specimens in the fracture experiments. Measurements on nine different types of rock show that a tensile crack must advance as much as 50 grain diameters at a constant speed before a steady state value of GI is attained. Hence methods of measuring GI based on crack initiation give results that are both variable and different from the steady state value of GI. The range of GI among the nine different types of rock tested with the wedge‐loaded double cantilever beam is from 40 to 260 J/m2, which is large compared to any error in the reduction of the data due to the assumption of linear elasticity.
Station coordinates, baselines, and Earth rotation from LAGEOS laser ranging: 1976–1984
Authors: Tapley B. D. , Schutz B. E. , Eanes R. J.
Published: April 2008

Satellite laser range (SLR) measurements to LAGEOS from May 1976 to January 1984 have been used to compute earth rotation parameters and the geocentric positions of 57 laser tracking station sites occupied during this interval. The station position determinations were part of a simultaneous solution which includes LAGEOS orbit elements in 15‐day arcs and earth rotation parameters in 3‐day subarcs using residuals from a 7.7‐year LAGEOS long arc (LLA 8402). Systematic differences between the terrestrial reference frames implied by the University of Texas station coordinate solution (LSC 8402) and by previous solutions computed at the University of Texas (UT) and at Goddard Space Flight Center (GSFC) are reviewed. The new solution resolves a 0.72 m z axis difference between the terrestrial reference system origins inferred from the GSFC solution (SL5) and the previous UT solution (LSC 8112). The implied origins of the LSC 8402 solution and the SL5 solution are consistent to less than 60 mm in all components, and after removing orientation and scale differences, the coordinates for 26 common tracking stations have an rms difference of less than 80 mm. The rms difference in baseline length for 22 station pairs used in the recent Crustal Dynamics Project very long baseline interferometry (VLBI)/SLR intercomparison experiment is 50 mm between the GSFC and UT laser solutions and 40 mm between the UT laser solution and the VLBI solution. The earth rotation parameters computed in 3‐day intervals have an internal precision of 2 marc sec (62 mm at the earth's surface) for xp and yp and 0.15 ms (70 mm at the equator) for UT1. The estimated accuracy of the series, based on dual‐baseline VLBI intercomparisons, is approximately 2 marc sec for xp and yp and 0.2 ms for UT1. The effects of plate motion on the determination of the secular drift in polar motion using the 7.7‐year series is shown to be at the level of 1 marc sec/yr. Finally, baseline changes have been investigated by repeated station coordinate determinations using 60‐day subsets of the 7.7‐year interval of the LAGEOS data set. In each 60‐day solution interval, orbit elements were estimated at 15‐day intervals, and earth rotation parameters were determined at 5‐day intervals. For the 4‐year period beginning in January 1980, baseline length changes inferred by the successive 60‐day solutions were found to be in general agreement with the motions predicted by the AM1‐2 plate motion model of Minster and Jordan.
Observing tectonic plate motions and deformations from satellite laser ranging
Authors: Christodoulidis D. C. , Smith D. E. , Kolenkiewicz R. , Klosko S. M. , Torrence M. H. , Dunn P. J.
Published: April 2008

For the first time satellite laser ranging (SLR) measurements from LAGEOS are revealing changes in geodetic station positions suggestive of predicted global tectonic plate motions. A detailed discussion of the latest SLR results is given herein. Since the accuracy of tectonic plate velocities increases with the repetition rate and accuracy in the measurement of interplate distances, activities are also described for increasing the temporal resolution and precision of baseline determinations. The LAGEOS laser data for the period January 1979 to the end of 1982 have been reanalyzed. The results of this analysis are now approaching the useful accuracies required by geophysics. For the better annually determined stations they include consistencies of 2–3 cm in station heights and 2 cm in annual baselines. The SLR results show overall agreement with the geologic tectonic rate model of Minster and Jordan with a linear cross correlation of 0.61 for 34 observed interstation rates. Improved temporal resolution was obtained in the measurement of specific baselines of moderate length through the large number of simultaneously observed passes.
Orbit determination and European Station Positioning from satellite laser ranging
Authors: Wakker K. F. , Ambrosius B. A. C. , Aardoom L.
Published: April 2008

Within the 1979–1982 time frame, at Delft University's Department of Aerospace Engineering some numerical experiments were performed by processing a limited number of multiday global tracking data arcs. Each arc consisted of laser ranging data acquired in the 1978–1980 period. These arcs were selected especially for their relative abundance of European observations of the satellites LAGEOS, Starlette, and GEOS 3. An evaluation of the orbit errors due to errors in the gravity models was performed both in terms of apparent range and timing biases of each pass over a tracking station and of orbit differences between solutions in which different gravity models were applied. In addition, from LAGEOS and Starlette tracking data, separate solutions were obtained for the positions of the European laser ranging stations at Kootwijk, Wettzell, Grasse, and Metsähovi. In spite of the use of the most accurate tailored gravity models available at the time, the results, both in terms of absolute coordinates and in baselines, often differ considerably from more recent solutions by other authors. It is hypothesized that intermittent data‐taking problems at some of the European stations are responsible for these differences. Their distorting effects are relatively strong because of the limited number of passes on which our solutions are based.
On the accuracy of recent Goddard gravity models
Authors: Lerch Francis J. , Klosko Steven M. , Wagner Carl A. , Patel Girish B.
Published: April 2008

Extensive evaluations of recent Goddard earth models against independent data generally confirm previous GEM accuracy estimates as conservative. Tests with new Seasat altimetric and surface gravimetric data as well as previous unused satellite lumped harmonics and accelerations and independent optical/laser solutions all suggest that the geoid errors in the satellite only GEM‐L2 (1982) may be as low as 20 cm (rms worldwide) through degree and order 6 and only 110 cm through degree and order 20. The GEM 10B (1981) geoid appears to be accurate to better than 150‐cm rms through degree 36. Recent independently developed European models from comprehensive surface and satellite data agree well with Goddard models at low and high degree compared to estimated GEM errors. Discrepancies at intermediate degree may be due to more extensive use of satellite orbits in Goddard fields. These conclusions (particularly on low‐degree terms) conflict markedly with those made in a recent paper by Lambeck and Coleman (1983), who used an unreliable error estimator.
Structure and petrology of the Reykjanes Ridge between 62°55′N and 63°48′N
Authors: Johnson G. L. , Jakobsson S. P.
Published: April 2008

Geophysical and petrologic data are presented on the northern extent of the Reykjanes Ridge. As it nears Iceland the crest of the Reykjanes Ridge is a small belt of rough topography superimposed on an elevated plateau. The rough topography consists of a series of en echelon ridges. Immediately to the southwest of the Reykjanes Peninsula the ridges strike about N35°E and apparently represent a rotational adjustment toward the N40°E trends on Iceland. To the south the en echelon ridges strike about N25°E. Magnetic profiles place the peak of the magnetic anomaly either directly below or along the eastern side of the topographic highs. Eight distinctive en echelon volcanic systems (swarms) are identified and are structurally and petrologically indistinguishable from those of the western Reykjanes Ridge. Petrologically, three rock types (picrite basalts, olivine tholeiites, and tholeiites) were recovered. These form distinctive groups which match those described from the western Reykjanes Peninsula.
On the mechanism of magma injection and plate divergence during the Krafla Rifting Episode in NE Iceland
Authors: Marquart Gabriele , Jacoby Wolfgang
Published: April 2008

Geodetic observations of horizontal and vertical point displacements in a ∼100‐km‐wide network across the Krafla fissure swarm of NE Iceland are analyzed with simple elastic models. The observed displacements accumulated from 1975 to 1980 in several events of magma injection into fissures that were widened. The fissure swarm was extended, and it subsided while the adjacent lithosphere contracted or was compressed accompanied by surface uplift. We applied two models to fit the far‐field deformation: an infinite horizontal pressure line source or a pipe and an infinite, vertically limited, vertical dyke source loaded by normal stress at the walls, both embedded in an elastic half‐space. In the far‐field inversion the two models cannot be distinguished, but geology and the observed axial graben subsidence clearly discriminate against the pipe and for the dyke. The inversion suggests that the widened dykes cut through the whole lithosphere which in the fissure swarm is only 4–6 km thick. During the rifting episode the general magma level in the fissure swarm rose to near the surface by 1980, but the loading increments appear to have decreased whith time from 1975 to 1980.
Inversion of phase velocities for the anisotropic elastic tensor
Authors: Harder Steven
Published: April 2008

A simple iterative technique has been devised to determine the elastic constants of a material of any symmetry with weak or strong anisotropy from phase velocity information. The method requires that multiple velocities and directions of propagation be observed and that crude estimates of the corresponding polarizations be made. The formulation of this procedure is exact and does not have the limitations of previous approximation methods.
A reexamination of the moment tensor solutions of the 1980 Mammoth Lakes Earthquakes
Authors: Wallace Terry C.
Published: April 2008

The moment tensor solutions determined from both long‐period body and surface waves for most of the large earthquakes in the 1980 Mammoth Lakes sequence have substantial deviations from a pure double‐couple mechanism. Julian (1983) and Aki (1984) argue that these deviations require nonshear faulting seismic sources such as a compensated linear vector dipole (CLVD) which could represent magmatic dike injection. Here we reexamine the moment tensor solutions and investigate the effects of the non‐double‐couple source on body wave synthetics. An error function which is based on the cross correlation of a synthetic and an observed waveform was used to assess the difference in fit between the complete moment tensor solution and one constrained to be a double couple. For the main shock (May 25, 1980; 1633) the sum of the errors differed by less than 5% for the two models. Similarly, the errors differed by only 3% for the aftershock on May 27, 1980. The conclusion is that the CLVD mechanism is not resolved in the long‐period data. In addition, the complete moment tensor solution has an extra parameter; a χ2 test was applied to normalize the number of parameters, and it was found that the complete solution was slightly less significant than the double‐couple solution. Finally, the complete solution was compared to a pure CLVD solution; for all the events examined, the total moment tensor is as far removed from a CLVD as it is a double couple. Although this does not rule out the possibility of dike injection, it strongly suggests that faulting can account for the long‐period seismic observations in the 1980 swarm.
Surface wave excitation and source mechanisms of the Mammoth Lakes Earthquake Sequence
Authors: Priestley Keith F. , Brune James N. , Anderson John G.
Published: April 2008

Spectral amplitudes for 24 events of the Mammoth Lakes earthquake sequence have been determined for the frequency range 0.1–10.0 Hz. The ML 6 earthquake that occurred at 1450 UT on May 27, 1980, is of particular interest because of the controversy surrounding its mechanism. A comparison of spectral levels determined from analysis of locally recorded strong motion and broadband digital data and spectral levels from regionally recorded surface waves extrapolated back to the source yields consistent results, indicating flat spectra in the band 0.1–1.0 Hz. The spectra observed in this study do not show pronounced spectral peaks predicted by theoretical studies of ground motion due to the jerky extension of a fluid‐driven tensile crack containing a low‐viscosity fluid. We have found nothing in the spectra of the ML 6 event that occurred at 1450 UT, on May 27, 1980, nor in the spectra of the aftershocks to distinguish them from spectra of “tectonic” earthquakes. However, the spectra themselves do not distinguish between various possible explanations for the non‐double‐couple source mechanism observed in moment tensor inversion and first motion data for the largest events of the Mammoth Lakes earthquake sequence.
Temporal and areal gravity investigations at Long Valley Caldera, California
Authors: Jachens Robert C. , Roberts Carter W.
Published: April 2008

Temporal gravity measurements over a broad network in the vicinity of Long Valley caldera, California, reveal significant gravity decreases over the interval June 1980 to July 1983 at five stations within or close to the caldera. The negative gravity changes reflect the late stages of an uplift of the caldera which began before the initial June 5–9, 1980, gravity survey (based on the gravity data) and after July 1979 according to the results of horizontal deformation measurements. Temporal gravity measurements at a more detailed network established within the caldera during June 1982 suggest that a low‐amplitude uplift occurred immediately after the January 1983 south moat earthquake swarm and that the uplift probably was accompanied by the addition of mass at depth. However, these data are complicated by gravity changes due to changes in depth to the water table. An areal gravity survey in the vicinity of the January 1983 swarm reveals two local gravity highs at 4–5 mGal amplitude and 2–3 km diameter. These local highs occur directly above the two areas that experienced the most intense earthquake swarm activity during the past few years and are believed to mark the locations of past intrusions into the south moat.
Evolution and present state of the hydrothermal system in Long Valley Caldera
Authors: Sorey Michael L.
Published: April 2008

Evidence for previous periods of hydrothermal activity in Long Valley caldera exists in the form of extensive deposits of hydrothermal alteration products at several locations within the caldera and saline deposits in Searles Lake which contain mineral assemblages contributed by hot spring discharge from Long Valley. Hydrothermal activity was more intense in the past and probably involved fluid circulation to depths of several kilometers or more with heat supplied by the Long Valley magma chamber. During the past 40,000 years the heat source may have shifted to the Inyo‐Mono magmatic system beneath the west moat, where deep fluid circulation supplied hot water to shallower zones of lateral flow within the Bishop Tuff beneath the resurgent dome. The present‐day hydrothermal system in Long Valley appears to consist of two principal zones in which hot water flows laterally from west to east at depths of less than 1 km within and around the resurgent dome. Maximum measured temperatures within these zones are near 170°C, but estimates from chemical geothermometers and extrapolation of a high‐temperature gradient measured in a recent drill hole indicate that a source reservoir at temperatures near 240° may exist at greater depths within the Bishop Tuff beneath the west moat. Regions possibly containing silicic melt detected by shear wave attenuation at depths of 4–5 km beneath the resurgent dome have probably not been in place long enough to influence sensibly the overlying thermal regime within the upper 2 km of caldera fill.
Shear waves (?) On COCORP Southern Appalachian Reflection Data
Authors: Gremell Paul E. , Cook Frederick A.
Published: April 2008

Analysis of data from the COCORP southern Appalachian seismic reflection survey in the Blue Ridge and Inner Piedmont of northeast Georgia indicates that an unusual seismic discontinuity at relatively shallow depth (about 7.0 km) produces a series of three distinct events arriving with increasing move out at increasing travel times. The events are most easily interpreted as P‐P, P‐SV (or SV‐P), and SV‐SV reflections. Hence the discontinuity, which has previously been interpreted as layered sedimentary or meta‐sedimentary strata beneath the crystalline Blue Ridge‐Inner Piedmont allochthon, is apparently a prominent shear wave reflector. The results further suggest that additional studies of shear wave propagation in the area may provide valuable information on the lithologic characteristics of the crust.
Magnetization of lower crust and interpretation of regional magnetic anomalies: Example from Lofoten and Vesterålen, Norway
Authors: Schlinger Charles M.
Published: April 2008

The interpretation of regional magnetic anomalies commonly observed over continental crust in both satellite and near‐surface magnetic data has remained enigmatic in its requirement for laterally extensive, thick, highly magnetic, and deep‐seated sources. Presented here are the results of a detailed field, analytical, and experimental study of the magnetic mineralogy and magnetic properties of rocks from Lofoten and Vesteralen, Norway, a province of deep‐seated origin with associated regional magnetic and gravity anomalies. These rocks have a bulk magnetic character of surprising uniformity, with susceptibilities, hysteresis, and Curie temperatures defined solely by coarse‐grained magnetite. Stable remanence is observed in samples from SW Lofoten but is not of significant magnitude. The thermal enhancement of magnetic susceptibility, the Hopkinson effect, is not important. Rocks from the deepest part of the section have the largest average susceptibilities, 6×10−2 (SI), and magnitudes of natural remanent magnetization (NRM), 5×10−4 A m2/kg. These values of susceptibility, together with the oft‐observed viscous character of the NRM, suggest that these granulite facies lithologies (particularly those of SW Lofoten) are perfect examples of lower crustal sources of typical regional magnetic anomalies. Furthermore, these intermediate‐pressure granulites define a unique magnetic petrology, especially for mafic compositions. At higher grades of metamorphism (high‐pressure granulites and eclogites) or lower grades (greenschist and amphibolite facies) such magnetite‐bearing assemblages will be unstable, and their high‐ or low‐grade equivalents will be only weakly magnetic. This evidence and these conclusions may be used to constrain models of lower crustal sources of regional magnetic anomalies over stable cratons. It portrays the lower crust as a highly magnetic unit, defined by metamorphic grade, with a Curie isotherm of 575°C (magnetite) and magnetization parallel to the ambient geomagnetic field (induced and viscous components).
Microstructure and the resistance of rock to tensile fracture
Authors: Peck Lindamae , Barton Christopher C. , Gordon Robert B.
Published: April 2008

The resistance of rock to tensile fracture may be measured by its fracture energy GI, which is found to range from 40 to 200 J/m2 in tests on nine types of sedimentary and crystalline rock. Differences in microstructure among the rocks tested are the principal cause of differences in the steady state value of GI, in the distance that a crack must advance before steady state fracturing is attained, and in the amplitude of the fluctuation of GI that accompanies crack advance. When nearly continuous surfaces of weakness are present, as in the Salem limestone, GI is low and attains steady state after only a small amount of crack advance. When a preexisting, interconnected network of microcracks is exploited by the fracture process, GI is large, and steady state is attained only after extended crack propagation. The sensitivity of GI to crack speed and the presence of water is low under the test conditions used in all the rocks examined. However, the magnitude of GI measured in a given type of rock is found to depend on the configuration of the test specimen and on components of stress near the crack tip that do not influence crack growth in linearly elastic materials. The conditions under which GI can be considered a material property are therefore restricted.
Effects of temperature on remotely sensed mineral absorption features
Authors: Singer Robert B. , Roush Ted L.
Published: April 2008

Laboratory spectral reflectance of mafic silicates has been measured over a temperature range characteristic of solar system objects to study the variation of diagnostic mineral absorption bands. This information has become increasingly important with recent improvements in the quality of remotely sensed planetary data and the concurrent increase in sophistication of quantitative and qualitative spectrum analysis techniques. In this study, visible and near‐IR reflectance spectra (0.4–2.5 μm) of pyroxenes and olivine were obtained over a sample temperature range of 80–448 K. These are the first published reflectance data for these minerals at temperatures significantly below ambient (≈300 K). Additionally, thermal radiation emitted from the samples at elevated temperatures was effectively removed, disclosing for the first time the high‐temperature reflectance behavior of pyroxenes in the diagnostic 2‐μm spectral region. As expected from basic physical considerations, Fe2+ crystal field absorptions in all samples become narrower with decreasing temperature. The three components of the composite olivine band centered near 1 μm become better resolved with decreased temperature and show changes in relative absorption strength. There is not, however, any significant wavelength shift of the olivine reflectance minimum. Both orthopyroxene and clinopyroxene spectra show dramatic changes in band symmetry with temperature, with the greatest change in the longer‐wavelength edge. Despite these significant changes in shape, little if any change is observed in the wavelength position of reflectance minima of pyroxene absorptions near 1 μm. The “2‐μm” pyroxene absorptions, in contrast, demonstrate major shifts of reflectance minima but in different directions. In orthopyroxene spectra the band shifts to longer wavelengths with increased temperature, while for the clinopyroxene spectra the band shifts to shorter wavelengths. This behavior results in greatly improved spectral differentiation of orthopyroxene and clinopyroxene components in a mixture (such as a basalt) at reduced temperatures. Application of the results presented here to remote‐sensing mineral characterization techniques currently in use for determination of pyroxene composition can provide a moderate improvement to quantitative interpretations: from 5 to 20 wt % FeO for orthopyroxenes and ±5 wt % CaO for clinopyroxenes depending on planetary surface temperature.
High Rayleigh number convection with strongly variable viscosity: A comparison between mean field and two‐dimensional solutions
Authors: Quareni Francesca , Yuen David A. , Sewell Granville , Christensen Ulrich R.
Published: April 2008

We have studied single‐mode mean field equations associated with variable viscosity convection for both steady state and time‐dependent situations. Steady state mean field solutions can be obtained by treating the governing mean field equations as two coupled fourth‐order differential systems, which are solved as a series of coupled two‐point boundary value problems with underrelaxation. Steady solutions can be achieved in which there can exist viscosity contrasts exceeding 108 and interior Rayleigh numbers of 0(109). For comparison with two‐dimensional solutions we have employed results derived from two finite element methods. Both temperature‐dependent and temperature‐ and pressure‐dependent viscosity with the surface viscosity fixed have been studied. Our results show that the differences between the two approaches grow with both viscosity contrast and convective vigor. The steady state Nusselt numbers and interior temperatures are greater in the case of mean field solutions. The power law index β governing the relationship between the interior Rayleigh number and the Nusselt number is larger for the mean field. Comparison of time‐dependent solutions shows that one can monitor rather faithfully the evolution of the averaged interior temperature and surface heat flow over a long time scale with the mean field method. Initially, thermal instabilities originating from the mean field boundary layers are found to be correlated with two‐dimensional boundary layer instabilities and are much more violent in character and cause large oscillations of the surface heat flow. The time scales associated with the secular variations of the Urey number, representing the ratio of the heat production and the surface heat loss, agree well between the two approaches. These results suggest that the mean field equations may serve as an efficient vehicle for studying planetary convection with complicated physics.
Propagation of high‐frequency seismic waves inside the subducted lithosphere from intermediate‐depth earthquakes recorded in the Vanuatu Arc
Authors: Chiu Jer‐Ming , Isacks Bryan L. , Cardwell Richard K.
Published: April 2008

Three seismic phases associated with the slab of subducted lithosphere are identified on seismograms of intermediate‐depth earthquakes (depths >70 km) recorded in the distance range up to 350 km by the Vanuatu (formerly New Hebrides) seismograph network. They are interpreted as (1) a P wave refracting into the inner portion of the slab, (2) a P wave traveling mainly through the outer(upper) portion of the slab, and (3) a P to S converted wave in which the conversion occurs across the upper surface of the slab. The existence of the two P arrivals from intermediate‐depth earthquakes suggests that these earthquakes are not located in the strong and cold inner core of the slab but within an outer zone where the velocity is lower than that within the inner portion of the slab. The low‐frequency character of the shear wave converted from the P wave across the outer “boundary” of the slab indicates that either the boundary is a transition layer with a finite thickness or the material above the slab has high attenuation for high‐frequency shear waves. Large spatial variations in the transmission of high‐frequency shear waves also indicate that there is a localized zone of anomalous shear wave attenuation in the mantle wedge lying just above the subducted slab and beneath the active volcanos.
Frequency of ball lightning reports
Authors: Barry J. D.
Published: April 2008

The frequency distribution by publication date of ball lightning reports and papers from 1651 to 1978 does not reflect a positive correlation with the intensity of solar activity nor indicate a cyclic characteristic. It is recognized that skewing of the distribution may have occurred because of human factors, such as publication bias, possible quasi‐periodic public interest, misinterpretation and misidentification of the actual event, and hesitancy of reporting by the observer. A natural dependence is not apparent.
Latitudinal variations of stratospheric aerosols
Authors: Farlow Neil H. , Ferry Guy V. , Lem Homer Y. , Hayes Dennis M.
Published: April 2008

We have obtained stratospheric aerosols from tropical to northern latitudes using special collectors on U‐2 aircraft during 1976 and 1977. Aerosols characterized by large numbers of small particles are found in the tropical zone suggesting this is a region of particle growth; whereas aerosols containing mostly larger particles are distributed throughout the Northern Hemisphere indicating a well‐mixed, mature population. We find the aerosol layer extends from higher altitudes near the equator to lower ones toward the pole. Although this gradient suggests mature aerosols may leave the stratosphere at high latitudes, the data are, as yet, inconclusive. Comparisons of our data with those of other investigators using different instruments are generally encouraging, suggesting that if similar populations were sampled, the results would be similar. When our calculated sulfate mass mixing ratios are compared with those measured directly by others, we find better agreement is achieved if we assume more dilute sulfate and water mixtures than previously proposed.
Spray in the atmospheric surface layer: Review and analysis of laboratory and oceanic results
Authors: Wu Jin
Published: April 2008

Previous measurements of spray, conducted with various techniques, are reviewed; the results are reanalyzed and compared to provide parametric descriptions of spray in the atmospheric surface layer. The number of droplets is shown to decrease in a power law form with elevation above the water surface. The size distribution of droplets peaks around 200 μm; on the side of larger droplets the frequency of occurrence of droplets decreases with increasing size and appears to approach asymptotically an equilibrium shape at high winds. These results are used along with calculations of droplet motions to determine the mean velocity and the mean distance of traveling of droplets and, subsequently, the concentration of droplets and the effects of spray on momentum, heat, and mass transfers across the air‐water interface. Finally, the results appear to indicate that the spray in laboratory tanks is produced by bubble bursting.
Infrasonic signals from thunder
Authors: Balachandran Nambath K.
Published: April 2008

Infrasonic signals from thunder have been observed at Palisades, New York, over a number of years. The signals are, in general, dominated by sharp rarefaction pulses with periods in the range of 0.4–1.0 s and peak‐to‐peak amplitude up to 10 dynes/cm2 (1 N/m2). The signals are found to be highly directional traveling almost vertically downward. Some aspects of these observations are in agreement with the theory of generation of sound by the sudden reduction of electrostatic field within a thundercloud after a lightning discharge, originally proposed by Wilson (1920) and later refined by Dessler (1973). The spectra of the signals also indicate frequencies as low as 0.1 Hz. These low frequencies may be associated with other electrostatic stresses in the thundercloud. A low‐amplitude compressional wave at the beginning of the signal is also observed. Sample computations of the thickness of the charged region and electric field strength are given.
Long‐term current and temperature observations on the Middle Atlantic Shelf
Authors: Mayer Dennis A. , Hansen Donald V. , Ortman Donald A.
Published: April 2008

Nearly 2200 days of current and temperature data were collected at a midshelf location in the Middle Atlantic Bight between June 1974 and March 1977. These data were examined for the average conditions and seasonal cycles of water circulation and temperature and some statistical properties of their variation at higher frequency. The average flow is found to be toward the south‐southwest at about 5 cm/s near the surface, diminishing to about 1 cm/s near the bottom. The occurrence of energetic wind‐driven transient current events which can exceed a 2‐month duration makes it impossible to determine a clear seasonal pattern in the sequence of monthly mean flows. There is, however, a clear seasonal pattern in the distribution of higher‐frequency fluctuations. Storm wind‐induced transient currents of 3‐ to 10‐day duration appear prominently in winter records. Inertial currents appear selectively in the summer, that is, in parts of the water column well insulated from the bottom by a strong thermocline. For low‐frequency motions (periods from 3 to 10 days) in both summer and winter records the preferred directions of motion throughout the water column appear to be consistent with equilibrium (mean) Ekman veering arguments. The water column response was found also to be most sensitive to the component of wind stress parallel to the bathymetry. The temperature follows a well‐known seasonal cycle of heating and stratification. During the unstratified seasons, very little high‐frequency temperature variation is observed, but during summer, thermal oscillations due to daily heating and inertial and semidaily tidal frequencies (3/3, 4/3, and 6/3 cpd) and a 5/3‐cpd oscillation appear prominently.
Beta dispersion of inertial waves
Authors: Anderson David L. T. , Gill Adrian E.
Published: April 2008

If a wind stress is suddenly imposed on an ocean at rest, inertial oscillations are produced according to the traditional ƒ‐plane solution. However, if the inertial period varies with latitude, this is true only for a short time, and the solution is strongly modified after only five periods or so because of ‘β dispersion.’ Some comparisons between ƒ‐plane and β‐plane solutions are made to illustrate the effect. In particular, the way in which energy can move latitudinally, in the latter case, away from the region in which it is produced by the wind is illustrated. The main features of the β‐plane results can be explained using WKB ray‐tracing techniques. Although the model is very simple, it is not inconsistent with the internal wave experiment spectrum, which shows low‐frequency energy propagating toward the southeast. It is suggested that this directionality in the spectrum at low frequencies may be associated with β dispersion.
Dip in the atmospheric CO2 level during the mid‐1960's
Authors: Bacastow Robert
Published: April 2008

Removal of the southern oscillation effect from the CO2 records at Mauna Loa, Hawaii, and the South Pole reveals corresponding decreases following the Agung eruption in 1963. The period of the decreases roughly corresponds to the period of reduced solar transmittance, as measured at Mauna Loa. It is suggested that the decrease in CO2 level is due to reduced sea surface temperatures, for which there is some direct evidence. The temperature anomaly required to produce the CO2 level dip is calculated on the basis of several simple models and found to be close to that observed.
Simultaneously measured vertical profiles of H2, CH4, CO, N2O, CFCl3, and CF2Cl2 in the mid‐latitude stratosphere and troposphere
Authors: Fabian P. , Borchers R. , Weiler K. H. , Schmidt U. , Volz A. , Ehhalt D. H. , Seiler W. , Müller F.
Published: April 2008

Results from balloon‐borne experiments carried out in southern France at a latitude of about 44° are presented. Four successful flights were performed with a neon‐cooled cryogenic air sampler designed to collect large air samples at eight different stratospheric heights. Supplementary tropospheric air samples were taken aboard a chartered aircraft. The samples, which cover an altitude range from 2.8 km to 35.3 km, were analyzed gas chromatographically for H2, CH4, CO, N2O, CFCl3, and CF2Cl2.