bluuebunny/crossref_metadata_2025 huggingface dataset:
The effect of dislocation density on the aqueous solubility of quartz and some geologic implications: A theoretical approach
Authors: Wintsch Robert P. , Dunning Jeremy
Published: April 2008

The internal energy associated with dislocations is calculated in quartz as a function of dislocation density. These calculations show that at dislocation densities of 109, 1010, 1011, and 1012 cm dislocation/cm3 the energy stored per mole of quartz is 2.2, 19.7, 170, and 1426 J, respectively. The density of free dislocations in natural quartz rarely exceeds 109 line cm/cm3, and the resulting increase in free energy is probably too small to affect any geochemical processes significantly. This applies to the overwhelming volume of rocks in the earth's crust. The density of dislocations in tangles may exceed 1011 line cm−2 which is significant, but the volume of quartz containing tangles is a very small fraction of most quartz grains. Nevertheless, where fluid/rock ratios are low, the aqueous activity of SiO2 may be significantly increased by the dissolution of quartz containing dislocation tangles. Such dissolution could be geologically significant in active greenschist facies fault zones and mylonites where repeated cold working would produce a sustained source and large population of dislocation tangles. In such a setting the locally high internal energies of quartz containing dislocation tangles would lead to strain solution. The resulting high activity of aqueous silica would enhance the stability of feldspars and would establish chemical potential gradients of SiO2 which would drive the diffusive mass transfer of SiO2 out of the fault zone.
Experimental evaluation of a dynamic yield criterion for granular fluid flows
Authors: Hanes Daniel M. , Inman Douglas L.
Published: April 2008

Data obtained in an annular shear cell are interpreted to evaluate a dynamic Coulomb yield criterion at the boundary separating flowing and stationary grains in a steady granular fluid shear flow. The ratio of the shear stress to the normal stress is nearly constant at the depth of no motion, suggesting a dynamic balance between the mobilizing effect of the applied shear stress and the stabilizing effect of the normal stress which results from the immersed weight of the moving grains. We suggest that the yield criterion can be applied to predict the immersed weight of grains undergoing bed load transport.
Stress distribution and subduction of aseismic ridges in the Middle America Subduction Zone
Authors: LeFevre L. Victoria , McNally Karen C.
Published: April 2008

The regional distribution of stresses associated with the subduction of the Cocos plate is inferred from a synthesis of 190 earthquake focal mechanisms, body and surface wave analyses of large earthquakes, and seismicity distributions. Broad patterns of consistent behavior are found across the region, from the Rivera Plate boundary in the northwest to the Guatemala/El Salvador border in the southeast, and are used as a framework to evaluate evidence for variations in local stresses due to the subduction of two aseismic ridges, the Tehuantepec Ridge and the Orozco Fracture Zone. Information which bears on the seismic potential at locations of aseismic ridge subduction is particularly important in that no large (Ms ≥ 7.5) earthquakes have occurred historically. We identify three major zones with consistent patterns in focal mechanisms and hypocentral distributions of seismicity. The first, closest to the trench and reflecting the mechanical interaction of the converging plates, is a zone of shallow thrust earthquakes extending 100–150 km inland from the trench. The second is a zone of normal faulting, beginning at about 200 km inland from the trench, h ≥ 60 km, which extends continuously along the entire length of the descending plate throughout the region. The third distinct zone exhibits a relatively low level of activity and separates the zones of thrust and normal faulting at about 150–200 km inland from the trench. This zone extends from the Rivera plate boundary in the northwest to the Guatamala region in the southeast. At this point, the quiet region pinches out, and the thrust and normal faulting zones abut and overlap. Superimposed on this overall pattern, we find locally only minor changes in areas of aseismic ridge subduction, aside from the prominent seismic slip gaps. Furthermore, on October 25, 1981, the Playa Azul earthquake (Ms = 7.3) occurred in the midregion of the Orozco Fracture Zone. Body and surface wave analyses of this event show a simple source rupture and shallow thrust fault mechanism, as are found elsewhere in the region. The seismic moment is MO = 1.3 × 1020 N m; the calculated stress drop is 4.5 MPa, not extraordinarily high, as might be expected in these pervasive seismic gaps. An event in the Tehuantepec Ridge region, on January 24, 1983, Ms = 6.7, was a large normal faulting event, but further interpretation is ambiguous due to the proximity of other normal faulting. We conclude that while aseismic slip may be occurring in the areas of ridge subduction, the possibility of large thrust earthquakes cannot be ruled out, due to the overall similarities with adjacent regions.
Detection of open fractures with vertical seismic profiling
Authors: Beydoun W. B. , Cheng C. H. , Toksöz M. N.
Published: April 2008

In vertical seismic profiling surveys, tube waves are generated by compressional waves impinging on subsurface fractures or permeable zones. The problem of generation of these waves by a nonnormal incident P wave for an inclined borehole intersecting a tilted parallel‐wall fracture is formulated theoretically. The amplitude of tube waves depends on the permeability, the length of the fracture, and the frequency. The relative effects of these parameters are studied individually. The problem is also formulated for a thin oblate ellipsoidal (penny‐shaped) fracture. The results for the two fracture models are compared and contrasted. Field data from Tyngsboro, Massachusetts, are shown for open fractures in granite. From tube wave amplitudes normalized to P wave amplitudes, calculated permeabilities are of the order of 100 mdarcy.
Geodetic radio interferometric surveying: Applications and results
Authors: Carter W. E. , Robertson D. S. , MacKay J. R.
Published: April 2008

Very long baseline interferometry (VLBI) observations collected primarily under projects POLARIS (Polar‐Motion Analysis by Radio Interferometric Surveying) and IRIS (International Radio Interferometric Surveying) since 1980 have been used to derive polar motion and UT1 time series. The typical 24‐hour observing sessions using the multistation North America‐Europe network yield estimates of the X and Y components of the pole with uncertainties of 1–2 milliseconds of arc (mas) and UT1 with uncertainties of 0.05–0.1 ms of time. (Preliminary experimental results indicate that UT1 can be tracked by a single VLBI interferometer to the 0.1‐ms level in observing periods of about 1 hour.) The same observations have been used to estimate the relative rates of motion of the North American and European tectonic plates and investigate possible regional deformations of the North American plate, with uncertainties of a few millimeters per year; check the accuracy of the adopted nutation series to the millisecond of arc level; estimate earth tide Love numbers for each observatory site with uncertainties of about 1% in h and 25% in l; and determine the coordinates of 20 radio sources to a few milliseconds of arc. Temporal variations in the apparent positions of the radio sources caused by relativistic deflections of the radio signals in the solar gravitational field have been used to estimate the relativistic deflection parameter γ, yielding a value of 1.000±0.003.
On the geomagnetic jerk of 1969
Authors: McLeod Malcolm G.
Published: April 2008

French and British scientists have published reports describing a sudden change in the geomagnetic secular acceleration which took place around 1969. They claimed that this change, called an impulse or jerk, took place in a period of a year or two and that the sources for the jerk were internal. A recent paper by Alldredge questioned their method of analysis and concluded that abrupt changes in the observed field which occur in a year or two do not originate in the core of the earth. This paper questions Alldredge's conclusion and provides further evidence in support of the reports of the French and British investigators. The average centroid of the jerk is found to be 1969.5. Evidence is presented that indicates that the centroid of the jerk differs for the various spherical harmonics so that the centroid of the degree one spherical harmonic is significantly later than the centroids for the next three harmonics. New estimates for the spherical harmonic coefficients for the jerk and the prejerk and postjerk secular acceleration are given. Some possible further investigations are suggested.
Structure of the frontal part of the Andean Convergent Margin
Authors: Von Huene R. , Kulm L. D. , Miller J.
Published: April 2008

A multichannel seismic reflection record across the central Peru margin and trench was improved by processing 24 rather than the 12 channels previously processed and by thorough migrating to reject the strong diffractions that obscured weaker primary reflections. The increased resolution clarifies the structure of the 15‐km‐wide frontal accretionary complex and the adjacent truncated continental framework against which the trench sediment was imbricated. Resolved are individual thrust slices and packets adjacent to the trench axis, subducting sediment‐filled graben in the ocean crust beneath the lower slope, and a Tertiary stratigraphic section of the upper slope Yaquina Basin which is cut by normal faults. The resolution in this multichannel record provides criteria for reinterpreting single‐channel data off Chile with increased confidence. The previously proposed truncation of the South American continent along much of the Peru‐Chile Trench is confirmed and the development of an accretionary complex in front of the truncated continental crust appears to vary with the amount of sediment seen in the trench axis. The Andean margin frontal structure is similar to that off Central America, the Aleutian Trench, and the Japan Trench which suggests common truncation as well as accretion at the front of convergent margins.
Source solutions and station residuals from long‐period waveform inversion of deep events
Authors: Doornbos D. J.
Published: April 2008

Deep earthquakes of up to moderate size have been analyzed by a long‐period waveform inversion of Global Digital Seismographic Network data where the use of a point source model proved to be adequate, but the use of a standard earth model was inadequate without modification. Modifications include calculations in an earth model based on PREM but with different source and receiver structures and the introduction of time delays which were determined together with the moment tensor components of the source by an iterative correlation method. The moment tensor components are in accordance With the pattern of stress release in other work, but results for the seismic moment differ somewhat and there is no compelling evidence for deviations from a double couple mechanism. The time delays were inverted to find the source centroid depths and time shifts and the station residuals. There is no baseline effect in the times of source duration which could be explained by a simple circular source model, but there is such an effect in the station residuals together with a distance trend especially for S. Part of the baselines may be an effect of bias in origin times, but the pattern of S residuals in particular suggests also possible modifications of the standard model.
Stress‐induced borehole elongation: A comparison between the four‐arm dipmeter and the borehole televiewer in the Auburn Geothermal Well
Authors: Plumb Richard A. , Hickman Stephen H.
Published: April 2008

The nature and origin of borehole elongation recorded by the four‐arm dipmeter calipers is studied utilizing information obtained from hydraulic fracturing stress measurements and borehole televiewer data taken in a well located in Auburn, New York. A preferred orientation N10°W‐S10°E, ±10° and a less prominant E‐W orientation of borehole elongation, was observed on two runs of the dipmeter. Comparisons of borehole geometry determined using the televiewer and the dipmeter show that both tools give the same orientation of borehole elongtion provided that the zone of elongation is longer than 30 cm. Comparisons of dipmeter caliper data with orientation of in situ stress and natural fractures, obtained from hydrofracturing tests and televiewer data show that the N10°W‐S10°E borehole elongations (1) are axisymmetric, (2) are aligned with the minimum horizontal stress Sh, and (3) are not associated with natural fractures intersecting the well. These elongations are interpreted as stress‐induced well bore breakouts. The E‐W elongation direction is characterized by an asymmetric borehole cross section in thinly bedded rocks and is not caused by breakouts. This asymmetric geometry can be discriminated from breakouts using the oriented electric measurements provided by the dipmeter. This study demonstrates that the dipmeter can be used to determine the orientation of Sh (by mapping breakouts), confirming the results of earlier less detailed studies, and provides a firm basis for mapping regional stress patterns using existing dipmeter data.
Closure of random elastic surfaces in contact
Authors: Brown Stephen R. , Scholz Christopher H.
Published: April 2008

The physical contact between two rough surfaces is referred to as a “joint,” and the deformation of such a joint under normal stress is called the “joint closure.” Toward better understanding of joint closure, we present a theory of contact between two random nominally flat elastic surfaces. This theory is a more general form of a theory presented previously by others for the elastic contact of a rough surface and a flat surface. In agreement with the previous theory we show that the joint closure property depends as much on the details of the surface topography as on the elastic properties of the material. To apply these results by using linear surface profiles requires mapping of profile information to three dimensions. The mapping techniques described here require the probability density function for the contacting surfaces to be approximated well by either a Gaussian distribution or an inverted chi‐square distribution. Laboratory experiments on ground surfaces of glass samples were done to test the theory. Both joint closure and surface topography were measured. In most cases, experimental results agreed quantitatively with predictions of the theory. However, in experiments on the smoothest surfaces, sample preparation problems often resulted in surfaces with a domed shape. These surfaces did not fit the assumptions of the theory, but the observed deviations from the theory were consistent with this domed shape. Surface topography measurements suggest that many surfaces are statistically similar. This implies that the success of the theory in predicting joint closure does not depend on a particular sample preparation technique. Therefore the theory should be valid for all nominally flat elastic surfaces. The form of the power spectrum implies that the surface topography and thus the joint closure depend on sample size.
Mineralogy and geochemistry of a sediment‐hosted hydrothermal sulfide deposit from the Southern Trough of Guaymas Basin, Gulf of California
Authors: Koski Randolph A. , Lonsdale Peter F. , Shanks Wayne C. , Berndt Michael E. , Howe Stephen S.
Published: April 2008

Samples dredged from a 15‐m‐high hydrothermal mound atop the flat turbidite pond in the Southern Trough of Guaymas Basin consist of pyrrhotite‐rich massive sulfide, barite, barite + calcite, talc, and opaline silica as well as substrate material composed of fossiliferous, clay‐rich ooze. An 11‐m‐long sediment core taken near the dredge site shows increasing hydrothermal alteration with depth; anhydrite‐filled fractures near the base of the core appear to be channels for hydrothermal discharge. Oxidation of the sulfide‐rich samples to an assemblage of geothite, lepidocrocite, and amorphous Fe oxyhydroxide is ubiquitous. Compared to other massive sulfide deposits on sediment‐starved oceanic ridges, the hydrothermal deposit dredged in Guaymas Basin has a high pyrrhotite/pyrite ratio, a low Zn sulfide and combined ore metal (Cu + Zn + Pb + Ag + Cd) content, and a greater abundance of sulfate, carbonate, and silicate phases. Venting hydrothermal solutions are alkaline with moderately high pH; high Ca, Ba, and SiO2 content; low ƒS2 and ƒo2; and very low transition metal content. Disequilibrium assemblages of pyrrhotite and sulfate minerals form during rapid mixing of this evolved vent fluid with ambient bottom waters at the discharge site. Talc is formed at a temperature near 270°C by mixing or entrainment of Mg‐rich bottom water or pore fluid with upwelling hydrothermal fluid that is saturated with silica. Calcite may precipitate from the alkaline, Ca‐rich fluid during degassing of CO2. The minimum temperature range for sulfide and nonsulfide deposition is approximately 190°–326°C. The composition of hydrothermal deposits, vent solutions, and altered sediment requires that circulating fluids evolve during deep penetration into the basaltic basement complex, further interaction with the organic‐and carbonate‐rich sediment pile, and near‐surface mixing with ambient seawater. Although the stable assemblage albite‐epidote‐clinochlore present at depth in the sediment pile requires very low dissolved Mg and Fe in the altering fluid, the addition of Mg to deeply buried sediment indicates significant recharge of the system by Guaymas Basin bottom water.
Seismic structure across the active subduction zone of western Canada
Authors: Spence G. D. , Clowes R. M. , Ellis R. M.
Published: April 2008

The Vancouver Island Seismic Project was conducted in 1980 to study the structure of the subducting oceanic Juan de Fuca plate and the overriding continental America plate. The principal seismic refraction line (line I) was a 350‐km onshore‐offshore profile perpendicular to the continental margin. An array of 32 receivers was located on the America plate on the mainland and across Vancouver Island and extended offshore with three ocean bottom seismometers (OBS's). Two shots were fired at the eastern end of the line, and 17 shots were located along the westernmost 100 km of the profile. Control for the interpretation of the onshore‐offshore profile was provided by a reversed refraction profile along the length of Vancouver Island and by a marine refraction profile recorded on the OBS'S. Modeling of the seismic structure of this complex region utilized an iterative inversion method for travel times from explosions in which shots at several locations are recorded on the same set of receivers and utilized an algorithim based on asymptotic ray theory for the calculation of synthetic seismograms through two‐dimensional media. The major features of the refraction structural model are that (1) the oceanic lithosphere dips at 3° or less beneath the continental slope, so the bend in the subducting slab occurs landward of the foot of the slope. (2) the oceanic lithosphere dips at 14°–16° beneath the continental shelf until it passes beneath the continental Moho at 37 km depth below western Vancouver Island, (3) an upper mantle reflector may correspond to the base of the subducting lithosphere, and (4) a segment of high‐velocity material above the downgoing crust, with velocity 7.7 km/s and depth range 20–25 km, may represent a remnant of subducted lithosphere, perhaps detached when the subduction zone jumped westward to its present position.
The rupture process of the 1976 Mindanao Earthquake
Authors: Beck Susan L. , Ruff Larry. J.
Published: April 2008

The large Mindanao earthquake (Mw = 8.1) of August 16, 1976, presents a complex rupture history. The epicenter of this earthquake is located at the southern end of the 160×80 km2 aftershock area, and the thrust mechanism with a shallow NE dipping plane indicates the subduction of the North Celebes Sea beneath Mindanao. We have characterized both temporally and spatially the rupture process of this event by deconvolving the source time functions from long‐period P wave seismograms at 20 azimuthally well‐distributed stations. The seismic moment is released in a jagged fashion in two main pulses. The observable directivity associated with these two pulses of moment release defines three segments on the fault: (1) from 0 to 54 km N‐NW of the epicenter with ≈½ of the moment release and an apparent rupture velocity of 2.0 km/s, (2) from 54 to 72 km N‐NW of the epicenter with low to no resolvable moment release and an apparent rupture velocity of 0.8 km/s, and (3) from 72 to 157 km N‐NW of the epicenter with ≈½ of the moment release and a rupture velocity of 3.3 km/s. Although the overall moment release is comparable in the first and second pulses, the second pulse has a substantially higher rupture velocity and a higher level of short‐period radiation. The sharp truncation of the second pulse generates the largest amplitudes in the P wave seismograms and is interpreted as the abrupt termination of the rupture front against Mindanao Island, 160 km N‐NW of the epicenter. An additional third pulse of moment release after the dominant truncation is identified at stations to the NW and SE. This additional moment release is located on a separate fault to the west which is probably strike‐slip in nature and related to a large strike‐slip aftershock along the coast of Mindanao. The jagged, multiple event moment release of the Mindanao earthquake is in sharp contrast to the smooth rupture of the 1979 Colombia subduction zone earthquake (Mw = 8.2), which resulted from the rupture of a single large asperity ( ≈60 km). Although these two earthquakes are both underthrusting events, they occur in very different tectonic settings. The Mindanao earthquake occurred at a relatively young subduction zone in a region with rapidly evolving plate boundaries, whereas the Colombia earthquake occurred at a well‐established plate boundary.
Nature of seismic reflections from the crystalline basement: COCORP Wind River Line, Wyoming
Authors: Jones Terry D.
Published: April 2008

Seismic reflection experiments designed to explore the lower continental crust have raised the problem of distinguishing multiples generated within the shallow sedimentary section and arriving at long travel times from primary events generated deep in the crystalline basement. This problem is investigated for the COCORP Wind River profile in Wyoming, where numerous reflections from 6 to 14 s (two‐way travel time) have been suspected of being multiples. One‐dimensional synthetic seismograms are generated using an appropriate Q and velocity structure and compared to true amplitude records. The amplitudes of multiples generated from within the sedimentary section are far too small to account for the observed deep reflections, since multiples are traveling entirely in the much more attenuating section. The deep reflection horizons in the Wind River profile are primary events from laminar impedance contrasts deep in the crystalline basement.
A numerical model of compaction‐driven groundwater flow and heat transfer and its application to the paleohydrology of intracratonic sedimentary basins
Authors: Bethke Craig M.
Published: April 2008

A new numerical method allows calculation of compaction‐driven groundwater flow and associated heat transfer in evolving sedimentary basins. The model is formulated in Lagrangian coordinates and considers two‐dimensional flow in heterogeneous, anisotropic, and accreting domains. Both the continuity of the deforming medium and aquathermal pressuring are explicitly taken into account. A calculation of compaction‐driven flow during evolution of an idealized intracratonic sedimentary basin including a basal aquifer predicts slow groundwater movement over long time periods. Fluids in shallow sediments tend to move upward toward the sedimentation surface, and deeper fluids move laterally. The hydraulic potential gradient with depth reverses itself near the basal aquifer, and fluids in this area have a tendency to migrate obliquely into stratigraphically lower sediments. Only small excess pressures develop, suggesting that intracratonic basins are not subject to overpressuring during their evolutions. Owing to the small fluid velocities, heat transfer is conduction‐dominated, and the geothermal gradient is not disturbed. Variational studies show that excess hydraulic potentials, but not fluid velocities, depend on assumptions of permeability and that both excess potentials and velocities scale with sedimentation rate. Aquathermal pressuring is found to account for <1% of the excess potentials developed during compaction. These results cast doubt on roles of compaction‐driven flow within intracratonic basins in processes of secondary petroleum migration, osmotic concentration of sedimentary brines, and formation of Mississippi Valley‐type ore deposits. Results might also be combined with chemical models to investigate the relationship of compaction flow to cementation in sediments.
Source mechanism of the May 18, 1980, Mount St. Helens eruption from regional surface waves
Authors: Burger Roy W. , Langston Charles A.
Published: April 2008

A reference source equalization method using regional surface waves from the Mount St. Helens eruption of May 18, 1980, and the Elk Lake event of February 14, 1981, was used to determine the St. Helens eruption source time history. The time domain deconvolution signals display a complex series of pulses lasting 250 s. An initial pulse was isolated and was found to have a smoothly varying two‐lobed radiation pattern for both Love and Rayleigh waves, suggesting a near‐horizontal point force directed south consistent with previous results. The initial southward force corresponds to the acceleration of the landslide in a northern direction about 5–10 s after the triggering earthquake. Lateral forces were isolated 70–140 s later, consistent with the occurrence of the large lateral blast and the slide changing its direction down the Toutle River Valley. There also appears to be significant source activity between 140 and 250 s after the initial pulse. This later activity exhibits forces in the northwest‐southeast azimuth and may correspond to the final deceleration of the landslide. Several vertical pulses were located between 10–60 and 100–120 s after the eruption began. Much of the vertical force represents the two sets of explosions which occurred during the eruption.
Waveform synthesis of surface waves in a laterally heterogeneous Earth by the Gaussian Beam Method
Authors: Yomogida Kiyoshi , Aki Keiiti
Published: April 2008

Combining the traditional normal mode theory for vertical structure with the newly developed Gaussian beam approach for laterally heterogeneous structure, a new method has been developed for synthesizing waveforms of surface waves for the laterally heterogeneous seismic velocity structure of the earth. In the frequency domain the wave field is constructed from single‐mode surface waves which propagate along ray paths on the surface of the earth following the phase velocity mapping for that frequency. Then the wave field at the observation point is constructed using the Gaussian beam method, a high‐frequency asymptotic solution. We use the ray‐centered coordinate system previously developed for body wave synthesis to derive parabolic equations. The method for surface wave synthesis differs from body wave synthesis by the following: (1) the speed of the wave packet along the ray is the local group velocity even though the ray path itself is determined by the phase velocity distribution in an isotropic or transversely isotropic medium, (2) the energy flow density of a ray is determined by the product of geometrical spreading, group velocity, and energy integral of the eigenfunction of the normal mode; spreading may be an important factor in most cases, and (3) surface waves traveling on a spherical earth may be mapped into Cartesian coordinates (two‐dimensional) by using the Mercator transformation including the effect of ellipticity. We found that the weighting factors of each Gaussian beam for a moment tensor representation of an earthquake are equivalent to those of a far‐field radiation pattern in laterally homogeneous model. Synthetic seismograms of narrow bandwidth with several different center frequencies are compared to real band‐pass‐filtered data to delineate the anomalies of three‐dimensional structures. Although there are some ambiguities in the selection of parameters used to synthesize seismograms by the Gaussian beam method, physically appropriate values may be estimated, and the choice of these parameters is not critical to the results. The amplitudes of surface waves are determined mainly from the spatial second derivative of the phase velocity distribution, and several forward tests on regionalized models with periods 20–40 s show that this waveform synthesis is sensitive to slight variations of laterally heterogeneous structure which conventional methods using only phase information cannot resolve. Results of tests for heterogeneous structure in the Pacific Ocean imply that this method may help to resolve weak and small‐scale velocity anomalies such as the Hawaiian hot spots or details of lateral changes in seismic velocities near spreading ridges.
Establishment of three‐dimensional geodetic control by interferometry with the Global Positioning System
Authors: Bock Yehuda , Abbot Richard I. , Counselman Charles C. , Gourevitch Sergei A. , King Robert W.
Published: April 2008

We consider the theoretical and the practical aspects of using radiointerferometric observations of the Global Positioning System (GPS) satellites to establish a three‐dimensional, multistation, geodetic control network. We discuss various ways of processing GPS data and try them with observations from a 35‐station network in Germany. We examine the reproducibility of interferometric determinations of individual baseline vectors and the three‐dimensional vector closure of subnetworks and the whole network. We compare these measures of precision with corresponding predictions based on statistics of interferometric phase residuals. We conclude that the relative positions of stations in the network were determined by GPS interferometry within about 1 part per million (ppm) in both horizontal coordinates and about 1.6 ppm in the vertical. As an external test of horizontal accuracy, we compare baseline lengths with electro‐optical distance measurements. The vertical accuracy is tested against determinations, by means of spirit leveling and gravimetry, of the height differences between stations. The differences between the interferometric and the independent determinations are consistent with the uncertainties of the latter. Interferometry with GPS is by far the most efficient method of establishing geodetic control on local and regional scales. This is already true, even though the constellation of satellites is incomplete. The accuracy of regional control by GPS should improve to about 0.1 ppm when interferometry is also used to determine the satellite orbits.
Shock wave properties of anorthosite and gabbro
Authors: Boslough Mark B. , Ahrens Thomas J.
Published: April 2008

Shock wave experiments have been conducted on San Gabriel anorthosite and San Marcos gabbro to peak stresses between 5 and 11 GPa using a 40‐mm‐bore propellant gun. Particle velocity wave profiles were measured directly at several points in each target by means of electromagnetic gauges, and Hugoniot states were calculated by determining shock transit times from the gauge records. The particle velocity profiles yielded sound velocities along the release adiabats which indicate a retention of shear strength upon shock compression for anorthosite, with a loss of strength upon release to nearly zero stress. Sound velocities of anorthosite shocked to peak stresses between 6 and 10 GPa were measured to be between 5.1 and 5.3 km/s upon release to nearly zero stress as compared to ∼6.9 and 5.4 km/s for the expected longitudinal and bulk wave speeds. Stress density release paths in the anorthosite indicate possible transformation of albite to Jadeite + (quartz or coesite), with the amount of albite transformed ranging from as low as 0.05 to as much as 0.19 mass fraction in the 6–10 GPa shock stress range. Electrical interference effects precluded the determination of accurate release paths for San Marcos gabbro. Because of the apparent loss of shear strength during unloading from the shocked state, the fluidlike rheology of anorthosite which is indicated implies that calculations of energy partitioning due to impact onto planetary surfaces based on elastic‐plastic models will underestimate the amount of internal energy deposited in the impacted surface material.
Complex resistivity measurements of confined rock
Authors: Lockner David A. , Byerlee James D.
Published: April 2008

Complex resistivity was measured for samples of Westerly granite and Berea sandstone saturated with 0.01 M KClaq. A four‐electrode technique was employed to make measurements over a frequency range of 10−3 to 106 Hz and a confining pressure range of 0.2 to 200 MPa. When the electrical response was analyzed in terms of equivalent conductivity σ′ and relative permittivity K′, both of these quantities varied more with confining pressure for the granite samples than they did for the sandstone. In addition, both rock types exhibited large low‐frequency dispersion of permittivity. While previous studies have measured the response of near‐dc conductivity to changes in pressure, measurements of pressure dependence of the full complex resistivity ρ*(ω) provide greater insight into earthquake‐related phenomena as well as induced polarization measurements and other remote sensing techniques.
A plate flexure approximation to postseismic and interseismic deformation
Authors: Savage J. C. , Gu Guohua
Published: April 2008

The rather large postseismic deformation that is associated with two‐dimensional dip‐slip faulting in the lithosphere is related to the bending of a free plate generated by dip‐slip faulting. In the absence of gravity, asthenosphere relaxation eventually permits the faulted lithosphere to assume the dihedral configuration of a faulted free plate. For thrust faulting, the faulted area is depressed into the asthenosphere, and the flanks of the plate slope uniformly upward. In the presence of gravity, buoyancy forces act upon the plate, and the ultimate (t→∞) postseismic uplift is approximated (y/α > 0.2) by w1 exp (−y/α) (cos y/α ‐ sin y/α), where w1 is an explicit function of the fault parameters and lithosphere properties, y is the horizontal distance from the downdip end of the fault, and α is the flexural parameter for the lithosphere plate. The relaxed‐asthenosphere response is the sum of the coseismic deformation (unrelaxed‐astheriosphere response) and this ultimate postseismic deformation. The annual, steady deformation associated with strain accumulation at a subduction zone is simply the relaxed‐asthenosphere response to virtual, normal slip on the main thrust zone of an amount equal to the annual plate convergence. From these relations we have estimated the deformation (strain and uplift) expected along the Pacific coast of northern Honshu. The estimated deformation is about twice the observed deformation, but the predicted geographical distribution of the deformation is similar to the observed deformation.
Isostatic response of the lithosphere with in‐plane stress: Application to central Australia
Authors: Stephenson Randell , Lambeck Kurt
Published: April 2008

The relation between gravity and topography in central Australia cannot be described by a conventional isostatic response function, whether it assumes local or regional compensation or whether the crust behaves as an elastic or viscoelastic medium. In particular, the response function exhibits considerable anisotropy. Hence a theory for isostatic response has been developed for a crust in mechanical equilibrium of surface loads, buoyancy, and in‐plane forces and subject to erosion, sedimentation, and stress relaxation. The observed north‐south response function exhibits extreme isostatic overcompensation at some wavelengths, and this can be fully explained by the Viscoelastic plate model with in‐plane compression. The observed east‐west response function is consistent with models of regional or local isostatic compensation. The response function implies an effective flexural rigidity of (5–10)×1022 N m, an effective Maxwell relaxation time of 25–50 m.y., an in‐plane compression of 125–150 MPa, and an erosion time constant of about 200 m.y. These parameters assume that the model evolved over a period of about 700 m.y. and are wholly consistent with the evolutionary model developed from geological observations.
The East Pacific Rise in cross section: A seismic model
Authors: McClain James S. , Orcutt John A. , Burnett Mark
Published: April 2008

In 1982 we undertook a seismic refraction experiment, known as the MAGMA expedition, to examine the detailed structure of the East Pacific Rise near 12°50′N. This segment of the rise, where the full spreading rate is about 110 mm/yr, is near the projected trace of the O'Gorman fracture zone and is the site of an overlapping spreading center as well as “black smoker” hydrothermal activity. In this paper we describe the analysis of a subset of the travel time data collected during the MAGMA expedition, namely the data from profiles which were oriented normal to the rise axis. These profiles provide a data set roughly equivalent to those collected on other experiments and sample the cross‐sectional structure of the rise. We have modeled these data using a two‐dimensional ray‐tracing program. We have found that the seismic velocities in the young oceanic crust are rather high, with velocity gradients of 4.0–5.5 s−1 in the uppermost crust. The highest velocities at the seafloor occur beneath the rise axis itself and seem to decrease as the crust ages to 0.1 Ma. This decrease in velocity must result from an increase in porosity in the upper crust and may coincide with the development of abundant surface fissures as the crust spreads. The decrease in velocity does not appear to penetrate deeper than about 0.5 km and may reverse itself as hydrothermal alteration fills the pores and cracks. The fact that the highest velocities occur under the rise axis suggests that the hydrothermal circulation responsible for the black smokers is confined to seismically unresolved conduits, a result consistent with the high temperatures and discrete nature of the vents. Our best model includes a magma chamber some 4 km wide and extending from the Moho to within about 1.1 km of the seafloor. This magma chamber is far smaller than many models for the rise axis have predicted but larger than those inferred from seismic refraction experiments at other sites on the East Pacific Rise. These discrepancies probably arise because the magma chamber under the East Pacific Rise is not a steady state feature but changes with time because of hydrothermal cooling and perhaps because of an episodic supply of magma from the mantle or along the rise axis.
Seismicity and attenuation in the central Vanuatu (New Hebrides) Islands: A new interpretation of the effect of subduction of the D'entrecasteaux Fracture Zone
Authors: Marthelot J.‐M. , Chatelain J.‐L. , Isacks B. L. , Cardwell R. K. , Coudert E.
Published: April 2008

The spatial distribution of the intermediate‐depth earthquakes in the central Vanuatu (New Hebrides) islands includes a remarkable gap in seismic activity located between depths of 100 and 200 km and having a lateral extent of about 150 km. No well‐located earthquakes determined from teleseismic data during a 16‐year period nor any microearthquakes located by a local seismograph network during a 5‐year period have occurred in the gap. A zone of attenuation of high‐frequency shear waves overlaps the seismicity gap. No contortion of the Benioff zone in the region of the gap indicative of major disruption of the subducted plate is apparent. However, the gap is close to the extrapolated location of the subducted part of the D'Entrecasteaux Fracture Zone (DFZ). The DFZ is a ridgelike bathymetric feature on the oceanic plate being subducted beneath the island arc. A positive thermal anomaly associated with the DFZ may cause the gap of intermediate‐depth seismicity and produce anomalous absorption of high‐frequency shear waves. Alternatively, a zone of strong scattering associated with the subducted part of the DFZ may also account for the anomalous attenuation.
Apparent Q for upper crustal rocks of the Central Rio Grande Rift
Authors: Carpenter Philip J. , Sanford Allan R.
Published: April 2008

Spectra from 130 digitally recorded microearthquakes in the magnitude range −0.9 to 0.3 were used to compute apparent seismic quality factors (Q) for upper crustal rocks within 45 km of Socorro, New Mexico. In this study, apparent Q was defined as a measure of seismic wave attenuation due to intrinsic absorption and scattering and was computed over the frequency range 0–50 Hz (most of the recorded energy was in the 3–30 Hz band). It was found that apparent Q increases with event distance for the eight stations used in this study. This increase was successfully modeled with a varying thickness, low Q, low‐velocity layer lying above a relatively high Q high‐velocity half‐space. It was found that (1) near‐surface low Q regions lying directly beneath the recording sites vary from 0.3 to 2.0 km in thickness and exhibit Qp and Qs values less than 50. (2) The average Qs for the upper crust in the central Rio Grande rift, excluding the near‐surface low Q region, is 535 (95% confidence interval 374–938). (3) The observed Qp/Qs ratio for events recorded at eight stations varied from 0.34 to 1.39. For earthquakes recorded at three stations, the Qp/ Qs ratio decreased with increasing event distance. For two other stations, no systematic increase or decrease in Qp/Qs with distance was evident, and for the remaining three stations, events were too closely clustered to draw any conclusions. These changes in the Qp/Qs ratio with increasing event distance may reflect varying degrees of fluid saturation in upper crustal rocks. (4) At least three anomalously low Qs regions exist within 20 km southwest of Socorro. These abnormally low Q regions may reflect small magmatic intrusions or high frature densities at depth and may lie in close proximity to microearthquake swarms at depths of 7.2–10.3 km.
Effect of cracks on the pressure dependence of P wave velocities in crystalline rocks
Authors: Carlson Richard L. , Gangi Anthony F.
Published: April 2008

To test the “bed of nails” model, we have made detailed measurements of P wave velocities in five low‐porosity, crystalline rocks at effective pressures to 500 MPa and fit two equations based on the model to the laboratory data. The first equation, V(P) = V0(1 + P/ Pi)(1 ‐ m)/2, applies at relatively low pressures because it assumes that the grain modulus is very much larger than the crack modulus. It can be fit to four of the five data sets. The fit to the data for a monomineralic epidote yields values for V0, Piand m of 8.02±0.02 km/s, 1.2±0.5 MPa, and 0.9845±0.0004, respectively, with a rms error of 6.28 m/s. The second equation, 1/V2 (P) = (1/Vc2 ‐ Vg2)/(1 + P/Pi)1 ‐ m + 1/Vg2 assigns a constant velocity to the grains and applies when the modulus of the cracks is of the order of the grain modulus at high pressures. This equation can be fit to three of the data sets; the fit to data for a diopside pyroxenite yields values of Vc, Vg, Pi, and m of 6.20±0.04 km/s, 8.28±0.02 km/s, 7±1 MPa, and 0.20±0.05, with a rms error of 17.9 m/s. For all seven fits to the laboratory data the rms errors range from 0.1 to 0.3% and are of the order of the limits of precision of the measurements. The “bed of nails” model explains the pressure dependence of P wave velocities in the samples remarkably well, as evidenced by the small rms errors. The variation with pressure of P wave velocities in these rocks clearly reflects the increasing stiffness of cracks. The fact that the first equation fits four of five data sets is one of several indications that cracks significantly affect the mechanical properties of the rocks even at 500 MPa. Finally, we note that different kinds of cracks have markedly different mechanical properties; the best fitting model parameters reflect the nature of the cracks which populate the samples.
Cubic approximations of definitive geomagnetic reference field models
Authors: Alldredge Leroy R.
Published: April 2008

The definitive geomagnetic reference field (DGRF) models for 1965, 1970, and 1975 and the international geomagnetic reference field (IGRF) model for 1980 with linear interpolation between the four epochs is defined as the DGRF linear model. It has 480 spherical harmonic coefficients. This model contains a piecemeal linear representation of coefficients, and hence of the field itself, that results in discontinuities in the secular variation which may lead to difficulties if the model is used at different epochs to subtract values from observations to obtain anomaly fields. A new DGRF cubic model is defined by the coefficients of a cubic curve for the time dependence of each spherical harmonic coefficient. It exactly fits the DGRF linear model coefficients at 1965, 1970, 1975, and 1980 and has continuous first, second, and third derivatives throughout the interval 1965–1980. This DGRF cubic model also contains 480 coefficients which can replace the 480 coefficients of the DGRF linear model. Differences in field components computed using the two models for epochs between the key epochs of the DGRF linear model are considerable, amounting to over 60 nT in 1977.5 for the Z component. No significant differences were found in the comparison of the two models with observatory data. This comparison is difficult because of observatory anomaly biases which appear to be variable with time because of observatory measurement errors or inadequate models. Because the DGRF cubic model provides a model of continuous secular variation, it is recommended that it replace the DGRF linear model.
Static compression of wüstite (Fe0.98O) to 120 GPa
Authors: Yagi Takehiko , Suzuki Toshihiro , Akimoto Syun‐Iti
Published: April 2008

An X ray diffraction study on wüstite (Fe0.98O) was performed up to 120 GPa at room temperature and up to 160°C at 24 GPa using a diamond‐anvil‐type, high‐pressure apparatus. Wüstite transforms from the cubic rock salt structure into a distorted structure with a rhombohedral unit cell at about 16 GPa at room temperature, and retrogressive transition occurs when the distorted phase is heated to 160°C at 24 GPa. This transition can be explained as the paramagnetic to antiferromagnetic transition resulting from the increase of the Néel temperature of wüstite under high pressure. No further discontinuous change in the X ray diffraction pattern was observed up to 120 GPa at room temperature. This result is in disagreement with shock compression experiments, which have indicated the existence of a large discontinuous volume decrease at approximately 70 GPa.