bluuebunny/crossref_metadata_2025 huggingface dataset:
Heat transfer through the sediments of the Mounds Hydrothermal Area, Galapagos Spreading Center at 86°W
Authors: Becker Keir , Von Herzen Richard P.
Published: February 2008

Heat transfer processes at the mounds area of the Galapagos Spreading Center at 86°W are revealed by temperatures measured at ≈ 10‐m intervals in the 30±10m sediment at each of 12 holes at DSDP Leg 70 Sites 506–509 and by temperatures of up to five thermistors on eleven 8–12 m long piston cores. The 325 needle‐probe values show a significant linear increase of thermal conductivity with depth in each core. About half of the temperature‐thermal resistance profiles are nonlinear and are fit to a steady state, vertical pore water advection model. Results indicate high and variable total heat flow and localized hydrothermal discharge at ≈ 10−8 m/s, associated with individual mounds. Recharge is indicated at similar rates in the low heat flow belt ≈ 5 km south of the mounds and is suggested at slower rates in the intermediate heat flow (0.17–0.42 W/m2) belt surrounding the mounds heat flow high. Possible slow entrained recharge within ≈ 100 m of discharging mounds is suggested. Also suggested is strong local discharge along the major fault bounding the mounds crustal block to the north. About 95 km north of the spreading axis, at DSDP Site 510, temperatures in the 114‐m sediment cover on 2.7‐m.y. crust are linear, consistent with the suggestion that the hydraulic resistance of this layer is sufficient to seal off free hydrothermal exchange between basement and bottom water. The combination of heat flow data and the physical properties data of Karato and Becker (this issue) suggests that ≈ 50 m of sediment may be a threshold thickness for sealing of hydrothermal circulation within basement, where the topography is smooth. We suggest that the formation of mounds may be associated with the forced localization of hydrothermal discharge through the sediment, as its thickness approaches this threshold value.
Seismicity near Palmdale, California, and its relation to strain changes
Authors: Sauber Jeanne , McNally Karen , Pechmann James C. , Kanamori Hiroo
Published: February 2008

We evaluate the relationships between the spatio‐temporal patterns and faulting mechanisms of small earthquakes and the recent temporal changes in horizontal strain observed along the ‘big bend’ portion of the San Andreas fault near Palmdale, California. Microearthquake activity along the entire big bend of the San Andreas fault increased in November 1976 concurrent with the initiation of an earthquake swarm at Juniper Hills. This activity then decreased abruptly to the northwest and southeast of Juniper Hills during the beginning of 1979. This drop in seismic activity occurred around the time that crustal dilatation was observed on the U. S. Geological Survey Palmdale trilateration network. Focal mechanisms from the study region are predominantly thrust. There are two time periods when the mechanisms are closer to strike slip than to thrust. The first period (December 1976 to February 1977) corresponds to the beginning of the Juniper Hills swarm. The second period (November 1978 to April 1979) approximately coincides with a change in trend of the strain data from uniaxial N‐S compression to dilatation.
Paleomagnetism of Mesozoic and Tertiary rocks from East El Owenat Area, southwest, Egypt
Authors: Hussain A. Gouda , Aziz Y.
Published: February 2008

Ten sites (310 samples) from alkali syenite at East El Oweinat (23.2°N, 29.5°E) yield a stable mean direction of magnetization D = 340.3, I = 19.9 with α95 = 12.1, K = 13.4, N = 10 after alternating field cleaning resulting in a pole at 67.8°N. 268.8°E with A95 = 8.6°. This pole position agrees with the other Upper Cretaceous pole positions of Africa. From seven sites (82 samples) from Upper Cretaceous Nubian sandstone at East El Oweinat a mean direction D = 349.9, I = 27.1 with α95 = 10.1, K = 37.8 (mixed polarity) was obtained after thermal demagnetization. Combined with previous investigations on Nubian sandstone at other localities (Schult et. al., 1978, 1981), this yields a pole at 80.8°N, 238.9°E with A95 = 3.2, K = 435.9, N = 30. Five sites from Tertiary basalt yield a mean direction D = 12.4, I = 23.9 and α95 = 9.2, K = 46.6 after alternating field cleaning which yield a paleopole position at 74.1°N, 159.8°E with A95 = 6.0, K = 107.8, N = 5. The paleopole positions of East El Oweinant rocks confirm the apparent polar wander path of Africa established in previous papers (Hussain et. al., 1980; Schult et. al., 1981). More comparisons with rotated South American and Australian pole positions after proper reconstructions to Africa show agreements with the present results.
Triple seismic zone and the regional variation of seismicity along the Northern Honshu Arc
Authors: Kawakatsu Hitoshi , Seno Tetsuzo
Published: February 2008

The regional variation of seismicity along the northern Honshu arc, Japan, is studied using accurate focal depths and focal mechanism types. We use focal depths determined from pP‐P time intervals reported in the ISC bulletins. For submarine earthquakes, depths are corrected by considering the pP phase reported in the bulletins as the pwP phase (the reflection from the ocean surface). Out of more than 600 well‐located earthquakes selected from the ISC bulletins, we determine the types of the focal mechanisms of 184 events using P wave first motion data. Based on historical seismicity of great and large earthquakes, we divide the zone of thrust type earthquakes at the plate interface into two regions: the shallow thrust zone (0–40km), where great earthquakes (MS∼8.0) occur, and the deep thrust zone (40–60km), where large earthquakes (MS∼7.4) occur. The activity of great or large earthquakes shows a variation along the arc; in some regions, both the shallow and deep thrust zones are active, and in other regions, only one of the thrust zones is active. The seismicity of recent moderate size earthquakes (mb>4) combined with the focal mechanism type shows a variation along the arc, which reflects the variation of the activity of great or large earthquakes. Where large earthquakes do not occur in the deep thrust zone, neither thrust type nor down‐dip compression/tension type events occur in and beneath the deep thrust zone. Where large earthquakes do occur in the deep thrust zone, we find a number of thrust type earthquakes. Further, in the latter case, in some regions, the down‐dip compression and tension type events of the double seismic zones extend seaward just beneath the deep thrust zone and form a triple‐planed structure of seismicity (the triple seismic zone). This study confirms the hypothesis of previous workers (Seno and Pongsawat, 1981) on the causal relation between the strong seismic coupling of two converging plates at the deep thrust zone and extension of the double seismic zone; i.e., the presence or absence of activity within the slab beneath the deep seismic zone occurs when the deep thrust zone has a strong or weak coupling, respectively. Here, the weak coupling could be interpreted as either aseismic slip or as low stress buildup since the last large event occurred at the deep thrust zone. Triple seismic zones are found offshore of Miyagi prefecture, where the deep thrust zone has been broken recently in 1978, and offshore of Fukushima prefecture. We expect a future large earthquake at the deep thrust zone offshore of Fukushima prefecture because the presence of the triple seismic zone suggests stress has been accumulating and 40 years have passed since the deep thrust zone was ruptured in 1938.
Spectral characteristics of high‐frequency PN, SN phases in the western Pacific
Authors: Walker Daniel A. , McCreery Charles S. , Sutton George H.
Published: February 2008

Pn and Sn phases from 25 selected earthquakes recorded since July of 1979 on ocean bottom hydrophones near Wake Island are used to complement and extend prior investigations of high‐frequency Pn, Sn spectra in the Western Pacific. At a distance of about 18° (≃2000 km), frequencies for Pn and Sn are as high as 30 and 35 Hz, respectively; at a distance of about 30° ( ≃3300 km), as high as 15 and 20 Hz, respectively. Pn phases lose their high‐frequency energy more rapidly than Sn phases do, yet Pn wavetrains are much longer than Sn wavetrains, Pn wavetrains of longer duration, more energy, and higher frequencies are found for travel paths primarily in the Northwestern Pacific Basin than for travel paths across the transition zone from the shallow Ontong‐Java Plateau to the deep Northwestern Pacific Basin. Sn phases are extremely weak or absent for travel paths crossing this transition zone from the shallower Ontong‐Java Plateau to the deeper Northwestern Pacific Basin, whereas Sn phases are well recorded for travel paths crossing the transition zone in the opposite direction. Although normal, mantle‐refracted P phases are well recorded beyond about 21° ( ≃2300 km), available data indicate that detectable normal, mantle‐refracted P phases may not exist at distances from about 17° to 21°.
Bedrock fracture parameters from the interpretation of well tides
Authors: Bower Don R.
Published: February 2008

This paper presents a theory to account for the main features of the tidal response of an isolated plane fracture as revealed by the water level in an intersecting open borehole. According to the theory the diurnal and semidiurnal tidal constituents (both amplitude and phase) of water level variations depend on the fracture aperture, the orientation (dip and strike) and the radial extent of the fracture, and the compressibility of the asperites. The theory is applied to water level observations made in six open boreholes located in crystalline rock at a test site at the Chalk River Nuclear Laboratories, Atomic Energy of Canada Limited, in Chalk River, Ontario, Canada. The boreholes were uncased and intersected many open fractures so that a definitive test of the theory could not be made. However, in four cases a fracture model could be found which fitted the observations within the experimental error.
Introduction to the Illinois Deep Hole Project
Authors: Coates M. S. , Haimson B. C. , Hinze W. J. , Van Schmus W. R.
Published: February 2008

Commonwealth Edison recently drilled three engineering test holes in northwestern Illinois. The three holes (UPH 1, 2, and 3) were 0.6, 1.6, and 1.6 km deep, respectively, and the latter two penetrated nearly 1 km of Precambrian granitic basement. In 1980, continuous core from all three holes and access to UPH 3 were made available for scientific investigations through the Illinois Deep Hole Project. The drill hole and cores provided an excellent opportunity for “piggyback” scientific investigations because (1) the basement geology of the region was poorly known, (2) a large vertical depth of continuous core was available, and (3) one of the holes was available for testing where corresponding core is available. The principal study of these holes was an integrated investigation of surface and in‐hole experiments, plus detailed petrologic, geochemical, and physical property studies of the core samples. The combined results of these studies contributed significatnly to our understanding of the regional geology of the Precambrian basement, the regional geophysics and tectonics, and the correlation between geophysical well logging and the true character of the rocks penetrated. Furthermore, this project has shown that substantial scientific benefit can be obtained from continental drilling “holes of opportunity” if members of the scientific community are able to respond in a timely and coordinated fashion.
Funnel functions in linear and nonlinear appraisal
Authors: Oldenburg D. W.
Published: February 2008

This paper presents new methods for appraising the nonuniqueness inherent in linear and nonlinear inverse problems. The usual approach of constructing unique averages of the model is abandoned in favor of computing upper and lower bounds to inner products of the model with exact boxcar functions of variable width. The bounds are found by extremizing a suitable objective function in a linear programing problem. Information about the average value of the model is presented in terms of funnel functions and uncertainty functions. In an alternate approach, information about the model averages is summarized by global bound curves. These functions are upper and lower bounds to the indefinite integral of the model, and they provide, at a glance, insight about the resolving power of the data. Once computed, they can also be used to generate approximate funnel functions. The methods presented here are also applicable to the nonlinear problem, but first a reformulation is necessary so that a global norm of the model can be extremized. This has been done, but the global extremum is achieved only through iteration. Consequently, the potential difficulties arising from nonconvergence of a linearized algorithm and possible convergence to a local, rather than global, extremum are ever present. As an example of the nonlinear problem, I have analyzed the magnetotelluric responses taken near an ocean spreading center in the Pacific. The data are shown to possess very poor resolving power, and hence the large variability seen in acceptable models is now understood.
Modeling of large‐scale accretionary wedge deformation
Authors: Stockmal Glen S.
Published: February 2008

Instantaneous stress and velocity fields within an active accretionary wedge can be calculated using plastic slip line theory, given the assumptions of a perfectly plastic rheology and plane strain conditions. After selecting a plastic yield stress and an average density, a stress field (slip line field) can be computed for a wedge of known cross‐sectional configuration from known stress boundary conditions on the upper surface. Basal surface tractions are derived without basal boundary conditions. The geometry of the computed slip line field can then be used to constrain the lower limit of the yield stress. The known velocity boundary conditions are insufficient for calculation of a velocity field because the coupling between the wedge and the subducting plate is unknown. Consequently, basal velocity distributions must be assumed to permit the construction of velocity fields. These assumed distributions are constrained by known rates of surface uplift and rates of tilt as well as inferred patterns of instantaneous strain rates within a wedge. The Sunda accretionary wedge west of central Sumatra is used to illustrate the plasticity approach. A minimum average yield stress of 20–30 MPa and a weaker basal layer of variable effective strength are indicated for this example. Surface uplift rates may be affected significantly by regions of high basal strain rate, finite velocity discontinuities, and underplating. The known pattern of uplift rates near Nias Island, located on the outer arc ridge, is consistent with the constraint that 90% of the total incremental shortening of the wedge is concentrated within 20 km of the trench. Given a sufficient number of uplift and tilt rate observations across a particular wedge, the relative influence of underplating, tectonic erosion, and basal strain rate variations could be assessed using the perfectly plastic model.
Measurement of mantle wave velocities and inversion for lateral heterogeneity and anisotropy: 1. Analysis of Great Circle Phase Velocities
Authors: Nakanishi Ichiro , Anderson Don L.
Published: February 2008

Long‐period (100–330 s) fundamental‐mode Love and Rayleigh waves have been processed to measure the great circle phase velocities for about 200 and 250 paths, respectively. The observations are inverted for regionalized phase velocities and for an even‐order harmonic expansion of the lateral velocity heterogeneity. The regionalized inversions achieve a maximum variance reduction of about 65% and 85% for the Love and the Rayleigh wave data, respectively. The lmax = 2 inversions give a maximum variance reduction of about 60% and 90% for Love and Rayleigh waves, respectively. The lmax = 8 inversion does not make a large improvement in the fit. The Love wave phase velocities have more power in l = 4 and 6, relative to l = 2, than the Rayleigh waves. For both Love and Rayleigh wave data the sectoral component dominates the l = 2 harmonics, and this component is stable if we increase lmax from 2 to 6. Heat flow also has strong sectoral components (lm = 22), which are approximately in phase with those of the phase velocities. The l = 2 harmonics of the nonhydrostatic geoid are dominated by large zonal (lm = 20) and moderate sectoral components. The sectoral components are in phase with those of the phase velocities. The sectoral pattern of heat flow and phase velocity is controlled by high heat flow‐low velocity of the East Pacific Rise and western North America, which is reinforced by low velocities in the antipodal region (Red Sea‐Gulf of Aden‐East African Rift). By contrast the geoid l = 2 pattern is dominated by geoid highs over the western Pacific subduction zones. A spherical harmonic expansion of regionalized phase velocities shows that they have l = 2 variations similar to those of the lmax = 2 nonregionalized inversions. This means that the regionalization approach is appropriate as a first step for studying lateral heterogeneity of the earth. However, the great circle phase velocities are not sufficient by themselves to uniquely locate the lateral heterogeneity. The same is true for free oscillation data. Regions of convergence have the interesting property of being slow for short‐period waves and fast, faster than shields, for long‐period waves.
Direct determination of gravitational harmonics from low‐low GRAVSAT data
Authors: Wagner C. A.
Published: February 2008

An efficient method is developed to compute gravitational harmonics from low‐low satellite‐satellite range rate measurements. The satellites are assumed to be in nearly the same low eccentricity orbits. The residual range rate signal is modeled with frequencies derived from linear perturbation theory to an accuracy of about 99%. Significant nonlinear effects involving J2, not currently modeled, require both J2 and J3 to be known in the reference trajectories. Each harmonic (l, m) generates l+ 1 principal frequencies, but they are not unique. Yet it appears possible to design a low‐altitude mission which keeps the pair at nearly constant separation and where the frequencies for all terms to (180,180) are separable after only about 4 weeks. A simple demonstration of the method is shown to recover (in two iterations) a complete (4,4) model (less J2 and J3) from 1 day of “perfect” measurements (every 7 min) generated by numerical integration. In this result, the effects of orbit determination are included in a crude way, but no other gravitational effects (of higher degree or from luni‐solar attraction) are present. Nevertheless, the method is easily extended to high degree with rapid new techniques (which are described) for calculating the required inclination functions of the orbits.
Seismicity and aseismic slip along the Eltanin Fracture Zone
Authors: Stewart Lisa M. , Okal Emile A.
Published: February 2008

Table Al is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue N.W., Washington, DC 20009. Document B83‐006; $2.50. Payment must accompany order.

The seismic history of the Eltanin Fracture Zone on the Pacific‐Antarctic Ridge for the years 1920–1981 shows that the seismic slip accumulated during this recording interval accounts for only a small fraction (less than 10%) of the amount of slip predicted by kinematic models of plate motion. We propose that the remainder of the plate motion along the transform faults making up the Eltanin system occurs as aseismic creep. This property singles out the Eltanin among transform systems, where previous studies have found good agreement between seismic slip and predicted rates of motion. The absence of record of any gigantic earthquakes and the geometry of the fault area reduce the likelihood of the Eltanin rupturing along its entire length. We interpret the Eltanin Fracture Zone in the light of an “asperity” model, involving small, well‐separated asperities. Its segmented nature and creeplike behavior may be due to the presence of excess magma and, possibly, local upwelling, which could also be involved in the mechanism of generation of the Louisville Ridge.
Geochemical studies of abyssal lavas recovered by DSRV Alvin from Eastern Galapagos Rift, Inca Transform, and Ecuador Rift: 2. Phase chemistry and crystallization history
Authors: Perfit Michael R. , Fornari Daniel J.
Published: February 2008

A diverse suite of lavas recovered by DSRV Alvin from the eastern Galapagos rift and Inca transform includes mid‐ocean ridge tholeiitic basalts (MORB), iron‐ and titanium‐enriched basalts (FeTi basalts), and abyssal andesites. Rock types transitional in character (ferrobasalts and basaltic andesites) were also recovered. The most mafic glassy basalts contain plagioclase, augite, and olivine as near‐liquidus phases, whereas in more fractionated basalts, pigeonite replaces olivine and iron‐titanium oxides crystallize. Plagioclase crystallizes after pyroxenes and iron‐titanium oxides in andesites, possibly due to increased water contents or cooling rates. Apatite phenocrysts are present in some andesitic glasses. Ovoid sulfide globules are also common in many lavas. Compositional variations of phenocrysts in glassy lavas reflect changes in magma chemistry, temperature of crystallization, and cooling rate. The overall chemical variations parallel the chemical evolution of the lava suite and are similar to those in other fractionated tholeiitic complexes. Elemental partitioning between plagioclase‐, pyroxene‐, and olivine‐glass pairs suggests that equilibration occurred at low pressure in a rather restricted temperature range. Various geothermometers indicate that the most primitive MORB began to crystallize between 1150° and 1200°C with fo2 < 10−7 atm. Coexisting iron‐titanium oxides in more evolved lavas yield temperatures ∼1025°C to as low as 910°C withfo2 from 10−8 to 10−12 atm. PH 2 o could have been as high as 1 kbar during andesite crystallization. Compositions of the lavas from the Galapagos rift follow the experimentally determined (1 atm‐QFM) liquid line of descent. Least squares calculations for the major elements indicate that the entire suite of lavas can be produced by fractional crystallization of successive residual liquids from a MORB parent magma. FeTi basalts represent 30–65 cumulative weight percent crystallization of plagioclase, augite, and olivine. An additional 30–50% fractionation of pyroxenes, plagioclase, titanomagnetite, and possible apatite is required to generate andesite from FeTi basalt liquids. The presence of partially resorbed mafic xenocrysts in some andesites, FeTi basalt inclusions in these xenocrysts, high‐silica glass inclusions in basaltic andesites, and the transitional chemistry of basaltic andesites are evidence that some magma mixing occurred during crystal fractionation. The diversity of lava types recovered at single dive sites suggests that low‐pressure fractional crystallization is a very efficient process beneath the eastern Galapagos rift and that isolated magma bodies must be present at shallow levels beneath the accretionary locus. Voluminous FeTi basalts erupted at the rift‐transform intersection are genetically related to the rift lavas, but their restricted chemistry reflects different thermal and tectonic controls on their petrogenesis.
Stillwater Anorthosites: A lunar analog?
Authors: Salpas Peter A. , Haskin Larry A. , McCallum I. S.
Published: February 2008

Samples of anorthosite from Stillwater Complex units AN‐I and AN‐II (terminology of McCallum et al (1980)) were analyzed by instrumental neutron activation analysis (INAA) for FeO, CaO, Na2O, REE, and other trace elements. No compositional trend with stratigraphic position was observed. Mean concentrations for most elements are about the same for three traverses (two for AN‐II) but CaO, FeO, and Sr concentrations are slightly different for the three traverses. The elements are not normally or lognormally distributed among samples from a single traverse. Strong interelement correlations can be understood in terms of sample mineralogy. Little evidence for compositional evolution of minerals during precipitation of the entire Middle Banded Zone is apparent. As in lunar anorthosites, the component of trapped residual liquid is relatively insignificant (less than 5%). In addition to plagioclase, which makes up, on average, 90% of the anorthosites, the rocks contain interstitial pyroxene (augite and inverted pigeonite) and, in some cases, quartz, titano‐magnetite, sulfides, and rare apatite and allanite. There is an inverse relationship between the amounts of pyroxene and quartz. General compositional trends, exemplified by La‐Sc systematics, indicate that the bulk of the poikilitic pyroxene appears to be cumulus‐adcumulus, presumably formed at the crystallization front from a magma saturated in plagioclase and pyroxene. A small fraction of the pyroxene and plagioclase plus all the quartz and other trace minerals formed from the minor trapped liquid component. The irregular distribution of pyroxene oikocrysts indicates that interstitial melt migrated substantially prior to final consolidation of the rock.
The lunar nearside highlands: Evidence of resurfacing
Authors: Andre C. G. , Strain P. L.
Published: February 2008

Comparative frequency distributions of aluminum, magnesium, titanium, iron, and thorium concentrations from orbital geochemical data show a chemical dichotomy between the lunar nearside and farside terra surfaces. Nearside terra rock types (indicated by modes in the histograms) are more mafic and less aluminous than those on the farside. To explore this chemical asymmetry, terra regions exhibiting either anomalously high or low Mg/Al concentration ratios were examined. Low Mg/Al material excavated from depth by basin and large crater impacts indicates that the highly ‘anorthositic’ farside crustal composition is also present in the subsurface of the near side at widely scattered locations. Study of high Mg/Al areas of the nearside terra suggests resurfacing of the preexisting terra, possibly by impact excavation of local mafic ‘pockets’ in the crust or early extrusion of volcanic materials.
Petrological and thermal histories of a lunar breccia 73217 as inferred from pyroxene crystallization sequences, exsolution phenomena, and pyroxene geothermometry
Authors: Ishii T. , McCallum S. , Ghose S.
Published: February 2008

Electron probe microanalyses, X ray diffraction studies, and pyroxene geothermometry of blue‐gray breccia 73217 indicate a record of an early plutonic period and a later thermal event. The breccia contains two domains; one (breccia A) contains coarse orthopyroxene clasts, whereas the other (breccia B) contains coarse pigeonite clasts. Pyroxene trends, deduced from pyroxene clasts from breccias A and B, are analogous to the early and late fractionation stages, respectively, of terrestrial basic layered intrusions. Minor element compositions of both augite and Ca‐poor pyroxenes from breccias A and B vary continuously with FeO/(MgO + FeO) ratio. Optical and X ray observations reveal that pyroxene clasts show a great variety of exsolution textures, but that individual pyroxene grains show relatively simple and less intensive exsolution patterns. There is no overlap in mineral compositions of pyroxenes as well as plagioclase and ilmenite clasts between breccias A and B. These characteristics are consistent with a model that the coarse mineral clasts of both breccias A and B possibly originated from the same relatively small layered intrusion, i.e., breccia A being derived from an early stage (1100°‐1060°C±20°C) and B from a later stage of fractionation (1060°‐1050°C). Both breccias experienced a later common annealing event (1040°‐1070°C).
Strangways Crater, Northern Territory, Australia: Siderophile element enrichment and lithophile element fractionation
Authors: Morgan John W. , Wandless Gregory A.
Published: February 2008

The Strangways Crater, Northern Territory, Australia (15°12′S, 133°35′E), has a central core, about 10 km in diameter, of shocked granitic gneiss and amphibolite, and some remnants of a melt rock sheet, surrounded by outer rings of quartzite and siltstone to a diameter of 20–25 km. Seven samples of melt rock (six granitic melts, one shale melt clast) and four samples of country rock (granitic gneiss, amphibolite, shale, quartzite) were analyzed by neutron‐activation analysis: for Sc, Cr, Fe, Co, Zn, Rb, Zr, Sb, Cs, Ba, rare earth elements, Hf, Ta, Th, and U, the samples were analyzed instrumentally; and for Ni, Se, Pd, Ag, Cd, Re, Os, Ir, and Au, they were analyzed radiochemically. Siderophile elements are significantly enriched in the granitic melt rocks relative to country rocks; for example, the Ir enrichments range from 0.6 to 2.8 ppb. The low Ir/Ni ratio (∼0.16 relative to C1 chondrites) excludes a chondritic impacting body, and Cr enrichment argues against impact by an iron meteorite. The Strangways Crater may have been formed by the impact of an olivine‐rich achondrite and melt rocks appear to contain about 3 wt.% of projectile material. The composition of the granitic melt rocks cannot be reproduced by any simple mixture of analyzed country rock types and chemical fractionation by selective shock melting appears to have taken place.
Crustal structure of the Lomonosov Ridge and the Fram and Makarov Basins near the North Pole
Authors: Forsyth D. A. , Mair J. A.
Published: February 2008

Reversed refraction surveys were conducted along and across the Lomonosov Ridge as part of the 1979 Lomonosov Ridge Experiment. Interpretation of the strike profiles indicates a 5‐km‐thick upper crustal layer with a velocity of 4.7 km/s overlying a 15‐ to 20‐km‐thick layer of 6.6 km/s material. An upper mantle velocity of 8.3 km/s is indicated by a few pn arrivals. High‐amplitude reflection events recorded from this boundary can be successfully modeled by a transition zone of rapidly changing velocity over a depth interval of 5 km. Ray trace modeling of the dip profiles suggests a root structure extending to about 28‐km depth flanked by crust thinning to a depth near 13 km beneath the Makarov Basin and a more gradual thinning to near 16 km beneath the Fram Basin. The similarity between the crust of the Lomonosov Ridge and that beneath the Barents and Kara seas supports the suggestion that the ridge is a slice rifted from the Baltic Shelf.
Pn travel times in southern California
Authors: Hearn Thomas M.
Published: February 2008

The Pn arrivals recorded from the southern California array have been used to demonstrate regional variations in crustal thickness and Pn velocity and to investigate the presence of anisotropy in the mantle. Over 2800 travel times were used in the analysis. The classical time term method was extended to allow the Moho velocity to vary in a block‐type structure. The statistical F test shows these effects to be significant. Crustal delays determined depend on both crustal velocities and Moho depth. The Ventura and Los Angeles basins have sediment delays which indicate sediment thicknesses near 10 km. The Moho depth itself varies by 10 km over southern California with a mean of about 29 km. Slightly thicker crust is prevalent in the northwestern corner of the array near the Southern Coast Ranges. Five kilometers of rapid thinning between the coast and the Channel Islands is shown by a sharp gradient in the delay times. Normal crustal thicknesses occur over the rest of the array except for the area east of the Sadton Trough, where an anomalously thin crust of 22 km exists. This area extends from near the plate boundary into Arizona. While the Peninsular Ranges and the San Bernadino Mountains have small roots, the San Gabriel Mountains have none. Pn velocities range from 7.6 to 8.2 km/s within the array. High velocities are found in the Mojave and offshore regions. Low velocities are found in the Peninsular Ranges and the Transverse Ranges. The analysis supports a mean anisotropy of 0.15 km/s with the fast direction at N 75° W. This value represents an average of a parameter that varies considerably in the region.
Analysis of seismic body waves excited by the Mount St. Helens eruption of May 18, 1980
Authors: Kanamori Hiroo , Given Jeffrey W. , Lay Thorne
Published: February 2008

Seismic body waves which were excited by the May 18, 1980, eruption of Mount St. Helens and recorded by the Global Digital Seismographic Network stations are analyzed to determine the nature and the time sequence of the events associated with the eruption. The polarity of teleseismic p waves (period ∼20s) is identical at six stations, which are distributed over a wide azimuthal range. This observation, together with a very small S to P amplitude ratio (at 20 s), suggests that the source is a nearly vertical single force. A simple model shows that for seismic radiation a volcanic eruption can be represented by a single,force applied in the direction opposite to the blast direction. The time history of the vertical force suggests two distinct groups of events, about 2 min apart, each consisting of several subevents with a duration of about 25 s. The magnitude of the force is approximately 2.6×1012 N. This vertical force is in contrast with the long‐period (∼150 s) southward horizontal single force which has been determined by a previous study and interpreted to be due to the massive landslide. An Ms = 5.2 earthquake initiated the eruption sequence. The direction of the P wave first motion of this event observed at two nearby stations is consistent with the radiation pattern expected for the landslide and suggests that it represents the onset of the landslide. The ground motions observed at station LON (Δ = 67 km) are dominated by Rayleigh waves (i.e., Lamb pulse) and provide tight constraints on the time sequence of the events.
Interpretation of Earth tide response of three deep, confined aquifers
Authors: Narasimhan T. N. , Kanehiro B. Y. , Witherspoon P. A.
Published: February 2008

The response of a confined, areally infinite aquifer to external loads imposed by earth tides is examined. Because the gravitational influence of celestial objects occurs over large areas of the earth, the confined aquifer is assumed to respond in an undrained fashion. Since undrained response is controlled by water compressibility, earth tide response can be directly used only to evaluate porous medium compressibility if porosity is known. Moreover, since specific storage Ss quantifies a drained behavior of the porous medium, one cannot directly estimate Ss from earth tide response. Except for the fact that barometric changes act both on the water surface in the well and on the aquifer as a whole while stress changes associated with earth tides act only in the aquifer, the two phenomena influence the confined aquifer in much the same way. In other words, barometric response contains only as much information on the elastic properties of the aquifer as the earth tide response does. Factors such as well bore storage, aquifer transmissivity, and storage coefficient contribute to time lag and damping of the aquifer response as observed in the well. Analysis shows that the observation of fluid pressure changes alone, without concurrent measurement of external stress changes, is insufficient to interpret uniquely earth tide response. In the present work, change in external stress is estimated from dilatation by assuming a reasonable value for bulk modulus. Earth tide response of geothermal aquifers from Marysville, Montana; East Mesa, California; and Raft River Valley, Idaho, were analyzed, and the ratio of Ss to porosity was estimated. Comparison of these estimates with independent pumping tests shows reasonable agreement.
Fault friction, regional stress, and crust‐mantle coupling in southern California from finite element models
Authors: Bird Peter , Baumgardner John
Published: February 2008

Ongoing deformation of southern California is simulated in a set of 63 models in an attempt to empirically determine fault friction. To a previous code for simulation of anelastic crustal strain, we introduce a new finite element to represent a curved fault of arbitrary rheology. We also add several selfconsistent patterns of horizontal drag on the base of the crust caused by independent upper mantle flow. A grid of isoparametric finite elements represents the regional fault geometry and the known variations in elevation and heat flow. Plate tectonic velocity boundary conditions are applied. Solutions include the time‐averaged flow velocities and fault slip rates and vertically averaged stresses. Each model is scored by comparing its slip rate predictions with 56 rates from dated offset geologic features. In membrane stress models the optimal fault friction is 0.4–0.6. Frictional self‐heating of faults is not a major factor determining strength. Addition of crust‐mantle coupling improves results if convergence of North American and Pacific lithospheres is assumed to occur under the Transverse Ranges. Among such models, those with fault friction of about 0.3 score best by most norms of the prediction error. These have over 80% of regional strain in faults and only a small amount of block deformation. However, one model with friction of 0.85 scores better by the percent of correct predictions norm. This model has only half the regional strain as fault slip and matches slow rates well but underpredicts slip rates over 1 cm/yr. Furthermore, its predictions are degraded considerably if hydraulic lubrication of thrust faults is assumed; the low‐friction models are unaffected. We conclude that some source of variation in fault friction remains unmodeled but that at least the fastslipping faults in California have “low” friction, similar to that of hydrated clay gouges. Appendix is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B84‐001 ; $2.50. Payment must accompany order.
An extensive region of off‐ridge normal‐faulting earthquakes in the southern Indian Ocean
Authors: Bergman Eric A. , Nábělek John L. , Solomon Sean C.
Published: February 2008

An unusually large number of off‐ridge earthquakes have occurred within a broad region astride the Southeast Indian Ridge. The source mechanisms of nine of these earthquakes, in lithosphere from 4 to 27 m.y. old, have been determined with a formal inversion technique based on matching observed and synthetic P and SH waves. All events occur in the oceanic mantle, from 8 to 22 km below the seafloor. The seismic moment and duration of faulting obtained from the inversion constrain the stress drop to lie between a few tens of bars and a few hundred bars for most of the events. All of the off‐ridge earthquakes are characterized by normal‐faulting mechanisms, with the T axes nearly parallel to the nearby ridge axis, in sharp contrast to typical mid‐ocean ridge or intraplate earthquakes. A variety of possible tectonic processes which might produce normal faulting in young oceanic lithosphere are considered as possible causes, including bending stresses, thermal stresses of various origins, differential subsidence at a fracture zone, off‐ridge volcanism, latitudinal plate motions on a nonspherical earth, basal shear stress from small‐scale convection, and plate‐wide perturbations of the stress field due to the continental collision between India and Asia. No single mechanism is adequate to explain all the observed off‐ridge normal faulting earthquakes. An apparent asymmetry in the observed seismicity with respect to the ridge suggests that the earthquakes on the Antarctic and Indian plates are caused by different processes. The Antarctic plate events can be satisfactorily attributed to the release of thermal and bending stresses associated with a residual depth anomaly between the Kerguelen Plateau and the Southeast Indian Ridge, a region which has been suggested as the site of a horizontal “pipe” for flow in the asthenosphere from the Kerguelen hot spot to the vicinity of Amsterdam and St. Paul islands on the Southeast Indian Ridge. The off‐ridge earthquakes in the Indian plate are speculatively attributed to part of the plate‐wide response of the lithosphere to the ongoing continental collision.
Propagation of PL and implications for the structure of Tibet
Authors: Shaw Peter , Orcutt John
Published: February 2008

PL is a long‐period (20 s or more) wave train beginning just after the P arrival in seismograms and continuing until the S arrival or the Rayleigh wave. The wave train can be observed at epicentral distances of about 5°–20° with instruments sensitive to these low frequencies. PL propagates as a partially trapped P‐SV wave in the crust; S wave energy is lost to the mantle during propagation making PL a “leaky mode.” We study PL propagation for a variety of earth models using the synthetic seismogram algorithm wave number integration and find that the vertical travel time in the crust is the most important parameter controlling PL's oscillation period. This period can vary by more than a factor of two between oceanic and continental paths. P‐SV leaky mode propagation includes many different modes; the low‐frequency motion termed “PL” is only the first, or fundamental mode, in this family. A second, higher‐frequency mode roughly equal in amplitude to the fundamental appears for models without a net positive velocity gradient in the crust. We use these results to match an observed PL wave train whose propagation path consisted almost entirely of the Tibetan Plateau. By considering first and second PL mode behavior we find that the Tibetan crust is about 85 km thick, with an uncertainty of about 20 km, and possesses a significant (≥0.01 s−1) positive velocity gradient. The presence of a large, low‐velocity zone in the lower Tibetan crust thus seems unlikely. Much higher‐frequency PL modes also appear in the Green's functions for layer over half space models and appear to be responsible for the high‐frequency Pg phase, making PL and Pg different members of the same type of leaky mode propagation.
Stress and relaxation time in the viscoelastic lithosphere inferred from the outer topographic rise
Authors: Ida Yoshiaki
Published: February 2008

The deformation of a thin viscoelastic plate is usually analyzed by use of an equation that is derived with an artificial assumption of the Poisson's ratio equal to 1/2. It is shown in this paper that a simple treatment applicable to an arbitrary value of Poisson's ratio is possible by introducing a new variable that represents the pressure gradient between the top and bottom of the plate. In this treatment a one‐dimensional stationary deformation of moving plate is given by a simple analytic equation. This equation is used to analyze the bathymetry of the outer topographic rise seaward of the deep‐sea trenches. The analysis reveals that the lithosphere in the western Pacific trenches is subject to an extensional horizontal force of 100 MPa or greater. The horizontal force may be much less or even compressive in the eastern Pacific trenches. To prevent horizontal force and bending stress from being extraordinarily large, the Maxwell relaxation time of the plate must be 1 m.y. or shorter, and the effective viscosity must be 1024 Pa s or less.
The effects of aqueous chemical environments on crack propagation in quartz
Authors: Dunning J. D. , Petrovski D. , Schuyler J. , Owens A.
Published: February 2008

The chemical role of water and other aqueous environments in crack propagation was examined in a series of crack propagation tests in natural and synthetic quartz. The potential roles of surface free energy reduction and zeta potential (surface electrostatic potential) in chemical weakening were specifically examined by running the crack propagation tests in the presence‐of six surface active aqueous environments which varied sub‐stantially in these parameters with respect to quartz. Calorimetric and electrophoresis tests were also undertaken in order to determine the degree of reduction of the surface energy of quartz produced by each chemical environment and the zeta potential between each environment and quartz. It was found that there was a moderate correlation between reduction of the crack propagation stress of quartz and the degree by which the surface energy was reduced in the presence of a particular environment. No such correlation was found with respect to zeta potential. It was also observed that some of the chemical environments appeared to produce highly branched cracks in quartz. This branching effect is probably related to the velocity of crack propagation.
Plasticity and hydrolytic weakening of quartz single crystals
Authors: Blacic James D. , Christie John M.
Published: February 2008

The historical record and experimental data on the plasticity of quartz crystals are reviewed, including the discovery of the important phenomenon of hydrolytic weakening. Quartz deforms by slip on numerous planes, generally with Burgers vectors 1/3<
> and [0001];  slip may be activated when resolved shear stresses on a and c slip systems are low. Dry natural crystals deformed in an anhydrous environment are very strong, with strengths approaching the intrinsic “theoretical” values, at temperatures up to 1000°C. Synthetic crystals with high OH concentrations and dry crystals heat‐treated in a hydrous environment are anomalously weak above a critical temperature (Tc) that varies inversely with OH content. There is a transition in the preferred slip system in crystals equally stressed for a and c slip that coincides with the hydrolytic weakening temperature (Tc). Yielding and flow in all crystals below their respective critical weakening temperatures appear to be controlled by a lattice resistance mechanism, characterized by a low activation energy and very small activation area. Above the critical temperatures, it appears that crystal flow is facilitated by diffusion of “water,” or some related hydroxy defect, to the dislocations, causing hydrolysis of the strong Si‐O‐Si bonds and aiding bond exchange. This process may assist glide or climb of dislocations–or both–and appears to be rate‐limiting. There is evidence that hydrolytic weakening is strongly influenced by confining pressure, possibly through the solubility or diffusivity of H2O or its components in quartz. The change in slip systems at the weakening temperature is attributable to anisotropy of the diffusivity. Similarly, a discontinuity in the creep rates of synthetic crystals, with a concomitant change of activation energy, at the α‐β transition is consistent with observed changes in diffusivity at the transition temperature. The experimental data are not yet complete enough to construct an adequate microdynamical model of the flow mechanism.