bluuebunny/crossref_metadata_2025 huggingface dataset:
Tectonics of the western Gulf of Oman
Authors: White Robert S. , Ross David A.
Published: February 2008

The Oman line, running northward from the Strait of Hormuz separates a continent‐continent plate boundary to the northwest (Persian Gulf region) from an ocean‐continent plate boundary to the southeast (Gulf of Oman region). A large basement ridge detected on multichannel seismic reflection and gravity profiles to the west of the Oman line is probably a subsurface continuation of the Musandam peninsula beneath the Strait of Hormuz. Collision and underthrusting beneath Iran of the Arabian plate on which this ridge lies has caused many of the large earthquakes that have occurred in this region. Convergence between the oceanic crust of the Arabian plate beneath the Gulf of Oman and the continental Eurasian plate beneath Iran to the north is accommodated by northward dipping subduction. A deformed sediment prism which forms the offshore Makran continental margin and which extends onto land in the Iranian Makran has accumulated above the descending plate. In the western part of the Gulf of Oman, continued convergence has brought the opposing continental margin of Oman into contact with the Makran continental margin. This is an example of the initial stages of a continent‐continent type collision. A model of imbricate thrusting is proposed to explain the development of the fold ridges and basins on the Makran continental margin. Sediments from the subducting plate are buckled and incorporated into the edge of the Makran continental margin in deformed wedges and subsequently uplifted along major faults that penetrate the accretionary prism further to the north.
Evidence supporting the approximation γρ = const for the Grüneisen parameter of the Earth's lower mantle
Authors: Anderson Orson L.
Published: February 2008

There has been no uniform acceptance of one theoretical formulation of the Grüneisen parameter formula over another. The application of a Grüneisen function to the earth's interior for temperature calculations by one author has been to some extent arbitrary, or at least held in doubt by others. In this paper, using only the seismic data of the earth, an argument is presented which supports the contention that an acceptable empirical law for the lower mantle is

urn:x-wiley:01480227:media:jgrb2944:jgrb2944-math-0001

where γ is the Grüneisen parameter and ρ is the density, where q is close to unity. Because of the range of the compression, no great error is made by taking q to be exactly unity for simplicity in the empirical formulae. However, a value of q anywhere in the band 0.8 < q < 2.2 is acceptable on the basis of the seismic data. By assuming the rule γρ = const, it is shown that it follows that αKT is independent of temperature and pressure and therefore depth in the lower mantle. The term α is the volume thermal expansivity, and KT is the isothermal bulk modulus.
The Geos 3 Project
Authors: Stanley H. Ray
Published: February 2008

The Geodynamics Experimental Ocean Satellite (Geos 3), launched April 9, 1975, has provided over 3½ years of data useful for the determination of oceanographic and geophysical parameters. The quality and quantity of the data caused a rapid shift in emphasis from the experimental stage to an applications orientation, including areas of application not originally anticipated. Brief descriptions of the project organization, experiment package description, mission profile, and highlight results are presented to serve as background material for the detailed results presented in this special volume.
Plane shock wave studies of porous geologic media
Authors: Larson D. B. , Anderson G. D.
Published: February 2008

Plane shock wave experiments have been conducted on two highly porous rocks, Mount Helen tuff and Indiana limestone, in both dry and water‐saturated states up to stress levels of about 4 GPa. A light‐gas gun was used to load the sample in uniaxial strain, and the subsequent wave motion was monitored with particle‐velocity gages. All four materials studied show evidence of time‐dependent behavior. The time‐dependent behavior in the highly porous dry rocks is associated with the closing of pores. The strong time dependence observed in these materials would seem to preclude the use of quasi‐static data in constitutive models that are used to describe dynamic processes. In the water‐saturated rocks the time dependence is associated with the water, which shows no indication of transformation to the high‐pressure ice phases in the time frame of these shock wave experiments. This suggests the possibility of a metastable form of water existing under dynamic conditions.
Structural analysis of lunar grabens and the shallow crustal structure of the Moon
Authors: Golombek Matthew P.
Published: February 2008

Lunar grabens are bounded by ∼60°, inward dipping shear fractures. Although both pure bending and simple extension mechanical models for the formation of grabens are derived, geometric constraints rule out a bending origin for lunar grabens. The model used assumes simple extension of the lunar crust and includes a correction for the mass wasting of graben walls. Using this model, the crustal extension associated with each graben and the thickness of the faulted layer can be determined. The base of the faulted layer, defined as the depth of intersection of the faults bounding a graben, represents a mechanical discontinuity in the lunar crust. All major lunar grabens are used to map the thickness of the faulted layer, and results show excellent agreement with the findings of previous workers for the thickness of total ejecta. Hence the mechanical discontinuity at the base of the faulted layer probably corresponds with the base of the megaregolith. If moon‐wide expansion caused the formation of lunar grabens, then an increase in radius of only ∼18 m is necessary. This small an increase implies that the expansion needed for graben formation could be accounted for by local basin movements.
Structure under Mount Rainier, Washington, inferred from teleseismic body waves
Authors: Langston Charles A.
Published: February 2008

Teleseismic long‐period P waves recorded at the World‐Wide Standard Seismograph Network station LON (Longmire, Washington) are shown to exhibit strong anomalous particle motion not attributable to instrument miscalibration or malfunction. In particular, a large and azimuthally smoothly varying tangential component is observed after vector rotation of horizontal P waves into the ray direction and after application of a deconvolution technique which equalizes effective source time functions and removes the instrument response. These tangential waves attain amplitudes comparable to the radial component and demonstrate wave form antisymmetry about a NNE azimuth. A model which contains a single high‐contrast interface dipping toward the NNE at a depth of 15–20 km can explain most of the characteristics of the long‐period P wave data, provided dips are greater than about 10° and only the interference of P and Ps generated at the interface is considered. The model breaks down for later arrivals which are presumably multiples or scattered waves. Examination of long‐period S waves from several deep teleseisms shows a prominent Sp arrival 18 s before S. The timing of this phase conversion suggests an interface at about 145‐km depth, and its sense of polarity suggests that the velocity contrast is from higher to lower velocities as depth decreases. This interface may correspond to the bottom of the upper mantle low‐velocity zone in the area.
The Galapagos Rift at 86°W: 2. Volcanism, structure, and evolution of the Rift Valley
Authors: van Andel Tjeerd H. , Ballard Robert D.
Published: February 2008

This paper presents part of the data obtained during a multidisciplinary study of the Galapagos Rift in the western Panama Basin. It is based on photographic traverses made by the Angus towed camera system, observations made from the DSRV Alvin, and bathymetric maps constructed by the U.S. Navy using a multi‐narrow‐beam sonar system. The area studied is a 9‐km section of the rift valley at 86°W on the crest of a long and uncomplicated segment of the Galapagos Rift. In contrast to the terraced rift valley of the Mid‐Atlantic Ridge, the Galapagos rift valley consists of a single trough, a few hundred meters deep, between faulted walls leading to low crestal ranges. The center of the rift valley contains a broad depression of sheet flows at the geometric axis and a lobate ridge of low volcanoes of pillow flows to the north. The volcanic ridge is highly fissured and contains several hydrothermal springs on pillowed terrain adjacent to depressions containing young sheet flows. The youngest sheet flows may be less than 100 years old, while the entire age range of the young volcanic complex can be estimated at no more than a few thousand years. The young volcanic complex is bordered to the north and south by block‐faulted marginal ridges that are significantly older. A model is proposed for the volcanic and structural evolution of the rift valley that begins with a zone of rapid extension, subsidence along boundary faults, and copious flows of sheet lavas from a shallow magma chamber (0.5–1.0 km). After the crust has thickened, subsidence, extension, and flow rates decrease, and a ridge of small pillowed volcanoes is formed. A new axis of extension then develops to one side or the other of the volcanic ridge. This area begins to subside and fill with sheet flows. The older volcanic ridge is accreted to the nearest marginal high and straightened by normal faults. This complex then undergoes uplift and evidently becomes the crestal range, as the previous crestal range is rafted outward and gradually subsides.
The Galapagos Rift at 86° W: 3. Sheet flows, collapse pits, and lava lakes of the Rift Valley
Authors: Ballard Robert D. , Holcomb Robin T. , van Andel Tjeerd H.
Published: February 2008

It has long been known that pillow basalts are a dominant feature of oceanic volcanism on midocean ridges. Recent studies in the Cayman Trough, East Pacific Rise, and Galapagos Rift valley have shown that sheet flows are also an important component, especially on ridges with intermediate and fast opening rates. This paper describes the surface forms of such sheet flows observed in the Galapagos Rift valley. The principal types are lobate, smooth, rippled, and wrinkled, as well as hackly and jumbled forms. Collapse pits of a wide range of sizes are common; they appear to result from subsidence of the lava as a consequence of distal spreading of the flow and/or headward drainback into the subsurface plumbing system. Lava pillars are numerous around collapse pits, and current evidence suggests that they most likely are spiracles produced by water trapped below an advancing flow and rising through it to the surface. From Hawaiian analogs we conclude that sheet flows can be considered a submarine equivalent of surface‐fed pahoehoe, while the pillow basalts are analogous to subaerial tube‐fed pahoehoe. We view the difference between sheet flows and pillow basalts to be the result of different degrees of channelization and rates of delivery of lava to flow fronts, associated with differences in duration and rate of eruption. In this model the sheet flows and pillowed ridges of the Galapagos Rift are the analogs of the early and the late products of an Hawaiian eruptive event, respectively. The sheet flows represent early, brief but voluminous eruptions. These are followed by more sustained, slower but steadier eruptive phases that produce pillow basalts after an internal plumbing system has been well established. Thus pillowed volcanics should normally overlie sheet flow complexes of only slightly greater age, and stratigraphie sections accumulated over long intervals should consist of alternating sequences of sheet and pillow flows. This model leads to several different ways of explaining the distinction between the apparent dominance of pillows on slow spreading ridges and the sheet flow and pillow complexes on faster spreading ridges.
A geophysical survey within the Mesozoic magnetic anomaly sequence south of Bermuda
Authors: Purdy G. M. , Rohr Kristin
Published: February 2008

This geophysical survey of an approximate 1° square covers Mesozoic magnetic anomalies M0, M2, and M4 south of Bermuda. Bathymetry, magnetics, seismic reflection profiling, and seismic refraction data are presented. The isochron trend within the survey area at magnetic anomaly M4 time is 025°. Two left lateral fracture zones exist: the southern fracture zone has an offset of <10 km at M4 time and 33 km at M0 time. The northern fracture zone has an offset of 37 km at M4 time and 26 km at M0 time. These changes in offset are accounted for by asymmetric spreading, an 11° change in trend of anomaly M0 relative to M4, and by M0 time, growth of a small right lateral fracture zone. Seismic refraction data provide poor control on the shallow crustal structure but suggest the presence of significant lateral inhomogeneities within layer 2.
Propagation of high frequency Pn waves at great distances in the central and South Pacific and its implications for the structure of the lower lithosphere
Authors: Talandier Jacques , Bouchon Michel
Published: February 2008

Using 17 seismic stations in French Polynesia, we study the propagation of high‐frequency Pn waves in the Central and South‐Central Pacific. This study is prompted by the presence of striking Pn wave phases on the seismograms of the Hawaii earthquakes recorded in the Tuamotu and Society islands (Tahiti), almost 40° away. The Pn velocity inferred from these observations (7.95±0.03 km/s) is in good agreement with the values obtained in the North Pacific by Walker (1977a, b) and shows the existence over most of the Pacific basin of a remarkably efficient Pn wave guide of relatively low velocity. Compared to Pn, Sn waves are much less efficiently transmitted. The Pn velocities measured from intraplate earthquakes in the South‐Central Pacific suggest the presence of a 2–3% anisotropy in the wave guide, the direction of maximum velocity being parallel to the trend of the island chains. All these observations are interpreted as evidence of the presence of minimum velocity channels within the subcrustal lithosphere and a discussion of the models which can account for these features is presented.
An intraplate thrust earthquake in the South China Sea
Authors: Wang Shi‐Chen , Geller Robert J. , Stein Seth , Taylor Brian
Published: February 2008

To investigate the state of stress in marginal seas, we have studied the October 7, 1965 (mb = 5.9, Ms = 5.6), earthquake in the center of the South China Sea. This is one of the largest events in the interior of a marginal sea since the installation of the World‐Wide Standard Seismograph Network. Using body and surface wave analysis, the focal mechanism is found to be almost pure thrust faulting at a very shallow depth (about 5 km) with a seismic moment of 1×1025 dyn cm. The mechanism indicates horizontal compression oriented N38°W. Since this event does not occur on a bathymetric feature, its mechanism may indicate the overall state of stress in the South China Sea.
Structure of the Laramide Wind River Uplift, Wyoming, from Cocorp deep reflection data and from gravity data
Authors: Smithson S. B. , Brewer J. A. , Kaufman S. , Oliver J. E. , Hurich C. A.
Published: February 2008

The question of the structure of the Wind River uplift, a Laramide foreland structure in Wyoming, has been answered by Consortium for Continental Reflection Profiling (Cocorp) deep crustal reflection profiling and by gravity interpretation. Wyoming uplifts are asymmetrical anticlines whose steep limb is commonly cut by a reverse fault or thrust. If the thrust continues into the crust with low dip, the uplift is caused by horizontal movements; and if the fault steepens into a high‐angle reverse fault in depth, the uplifts are caused by vertical movement. The Wind River uplift is the largest of the Laramide structures in Wyoming, and the thrust fault has 14 km of vertical separation and 26 km of horizontal separation. About 168 km of crustal reflection profiling has been completed by Cocorp across the uplift. The Wind River thrust fault generates a continuous reflection from near the surface to a depth of about 24 km, and the thrust appears as a complex zone of faults. Apparent dip of the thrust is 30°–38°, and true dip may be up to 48°. Interpretation of deep crustal reflections is complicated by the presence of multiple reflections. A structure whose complexity approaches that of basement outcrops is found in the deep crust. Gravity modeling also suggests that the fault dips moderately and that dense rocks are uplifted in the deep crust. The Moho does not seem to be presently displaced by faulting. The crust in the Wind River uplift began to deform by large‐scale folding and then broke and faulted as a rigid slab. Faulting is caused by crustal shortening from compression that is related to plate movements.
Geochemical modeling of mantle differentiation and crustal growth
Authors: O'Nions R. K. , Evensen N. M. , Hamilton P. J.
Published: February 2008

A simple two‐reservoir. model with time‐dependent transport coefficients between the reservoirs has been used to model the abundances of K, Ar, Rb, Sr, Sm, Nd, U, Th, and Pb and the isotopic compositions of Ar, Sr, Nd, and Pb in the earth's mantle and continental crust. The transport coefficients, like heat production, are considered to decay exponentially with time. Models which involve the whole mantle in generating the continental crust yield 87Sr/144 and 143Nd/144Nd ratios for the residual mantle which are higher and lower than midocean ridge basalts, respectively. A model which involves only half of the mantle in the production of continental crust produces a residual mantle with isotope ratios similar to those of midocean ridge basalt. The 40Ar/36Ar ratios in the atmosphere and suboceanic mantle are reproduced by this model without any inequality in the upward transport coefficients of K and Ar but with a smaller downward transport coefficient for Ar than for K. The results imply that the earth's crust may have developed by extraction of material from only half of the mantle and constrain the possible convective regimes that have existed in the mantle.
Mechanical and thermodynamic constraints on fluid distribution in partial melts
Authors: Bulau J. R. , Waff H. S. , Tyburczy J. A.
Published: February 2008

In this paper we examine the mechanical and thermodynamic criteria which are necessary for equilibrium to exist in partial melts that are subject to hydrostatic stress. Existing theory satisfactorily demonstrates (1) that crystal‐liquid interfaces will have no sharp edges and will approach constant curvature and (2) that the liquid will form a continuously interconnected network by distributing itself in channels along intergranular edge intersections provided that the liquid wetting angle is less than 60°. However, the full textural implications of the thermodynamics have apparently not been realized previously. Currently accepted theoretical developments have been extended to show how equilibrium melt distribution varies with wetting angle. We find that contrary to a widely accepted view a homogeneous fluid phase cannot completely wet intergranular faces, regardless of the exact wetting angle. As a result the intercrystalline faces will remain in mechanical contact throughout the assemblage even when it is characterized by a 0° dihedral angle.
Geodetic leveling and the sea level slope along the California Coast
Authors: Balazs Emery I. , Douglas Bruce C.
Published: February 2008

Comparisons between ‘free’ adjusted normal orthometric leveling elevations and mean sea level at tide stations are given by Braaten and McCombs (1963) for the United States coastlines. Their report indicated an apparent systematic difference between the 1963 free adjustment, which had a mean epoch of 1939, and mean sea level along the coasts. Since that time, new leveling surveys, performed primarily by the National Ocean Survey's National Geodetic Survey, have been accomplished between San Francisco and San Pedro tide stations for the epochs 1968–1969, 1968–1971, 1971–1972, 1973–1975, and 1977–1978. The observed elevations at tide stations are compared to mean sea levels of the 1941–1959 epoch. Leveling surveys show very good consistency with a steady trend from negative to positive between 1968 and 1978. This trend indicates that San Pedro is rising with respect to San Francisco or that San Francisco is subsiding with respect to San Pedro at an average rate of about 70 mm/yr. However, an examination of the tidal records does not reveal large changes in the trend of mean sea level with respect to the land at the tide stations. Hicks and Crosby (1972) report that the mean sea level trend with respect to land (1940–1972 series) is −0.3 mm/yr at San Pedro and +1.8 mm/yr at San Francisco. Thus the indicated relative movement rate (70 mm/yr) from leveling is about 30 times greater than the rate indicated by tidal observations. The reasons for the large discrepancy between relative movement rates from repeat levelings and tidal observations are presently unknown.
Detailed gravity anomalies from Geos 3 satellite altimetry data
Authors: Gopalapillai Gopal S. , Mourad A. George
Published: February 2008

Detailed gravity anomalies are computed from a combination of Geos 3 satellite altimeter and terrestrial gravity data using least‐squares principles. The mathematical model used is based on the Stokes' equation modified for a nonglobal solution. Using Geos 3 data in the calibration area, the effects of several anomaly parameter configurations and data densities/distributions on the anomalies and their accuracy estimates are studied. The accuracy estimates for 1° × 1° mean anomalies from low density altimetry data are of the order of 4 mgal. Comparison of these anomalies with the terrestrial data and also with Rapp's data derived using collocation techniques show rms differences of 7.2 and 4.9 mgal, respectively. Indications are that the anomaly accuracies can be improved to about 2 mgal with high density data. Estimation of 30′ × 30′ mean anomalies indicates accuracies of the order of 5 mgal. Proper verification of these results will be possible only when accurate ground truth data become available.
Bulk modulus—volume relationship for cation‐anion polyhedra
Authors: Hazen Robert M. , Finger Larry W.
Published: February 2008

A bulk modulus—volume relationship is demonstrated for cation coordination polyhedra in a variety of structure types including oxides, silicates, halides, sulfides, phosphides, and carbides: Kp 〈d〉3/S2zcza = 7.5 (Mbar Å3), where KP is the polyhedral bulk modulus (in megabars), 〈d〉 is the mean cation‐anion separation (in angstroms), zc and za are the cation and anion formal charge, and S2 is an empirical ionicity term, defined as 0.50 for oxides and silicates and calculated to be 0.75 for halides; 0.40 for sulfides, selenides, and tellurides; 0.25 for phosphides, arsenides, and antimonides; and 0.20 for carbides. The bulk modulus of a substance depends on the bulk moduli of component polyhedra and the manner in which these polyhedra are linked. In corner‐linked structures, such as quartz and framework silicates, mineral bulk moduli are significantly less than those of constituent corner‐linked polyhedra, because of accommodations resulting from changes of the metal‐oxygen‐metal angles. In layer structures such as micas, with extensive edge sharing within the layers but weak bonds between the layers, compression is highly anisotropic. Structures such as periclase, spinel, and garnet, with extensive edge sharing between polyhedra in three dimensions, have large bulk moduli similar to those of constituent polyhedra. Bulk moduli of mantle mineral phases approximate the moduli of component polyhedra because most mantle mineral structure types have edge sharing in three dimensions. Compression for a given polyhedron does not appear to depend upon the linkage topology of the structure.
Martian channel morphology: Maja and Kasei Valles
Authors: Baker Victor R. , Kochel R. Craig
Published: February 2008

Detailed geomorphic mapping from Viking imagery of selected portions of Kasei Vallis, Maja Vallis, and vicinity reveals numerous similarities of channel morphology to erosional and depositional features of the Channeled Scabland. Characteristic scabland landforms which occur in Kasei and Maja Valles include erosional grooves, streamlined uplands and hills, scour zones around flow obstacles, inner channels with erosional head cuts, breached ridges and basin (crater) rims, pendant forms (bars?), erosional terracing of streamlined hills and channel margins, and possible midchannel bars. These features constitute an assemblage of landforms which on earth is most characteristic of catastrophic flood channeling in jointed bedrock. Prominent mass wastage and sapping features are associated with the high‐wall relief in lower Kasei Vallis. Many cliffs along the channel margins exhibit steep upper slopes and gentler lower talus slopes which form the spur‐and‐gully topography that has also been described along chasma walls in the Valles Marineris. Landslides, debris fans, and debris cones can also be recognized. Much less wall modification occurs in the shallower Maja Vallis. Probably, the steep escarpments of Kasei were created by tectonic processes and subsequent channel incision. These escarpments later receded by mass wasting and sapping.
MARS: The North Polar Sand Sea and related wind patterns
Authors: Tsoar Haim , Greeley Ronald , Peterfreund Alan R.
Published: February 2008

Viking Orbiter 2 images of the north polar region reveal an enormous sand sea (erg) covering an area of >5×105 km2 around the perennial ice cap. All dunes are either transverse or barchan. The various dune morphologies and modification of primary dune types reflect a wind regime having more than one wind direction. In the summer, two major wind directions prevail: (1) off‐pole winds that become easterly due to coriolis forces and (2) on‐pole winds that become westerly. During the winter and/or spring, only the on‐pole winds exist. Strong winds (>75 m/s) are required for sand accumulation to form the thick transverse dunes. The strongest winds in the north polar region are thought to exist during summer over the transverse dune field between 110°W and 220°W; this area is a relatively warm belt (temperature >230 K) between two ice zones (temperature <220 K). A frequent cyclogenetic process in this area may cause sand storms. The dunes seem to be currently active. The lack of well‐developed longitudinal dunes implies that the dune field is young. The relationship of the present dune field to the perennial ice indicates that the dunes began to form after the formation of the present ice cap.
The formation of Martian lee waves generated by a crater
Authors: Pickersgill Adrian O. , Hunt Garry E.
Published: February 2008

The vertical wave displacement, generated by a Martian crater, propagating upward from a lower atmospheric layer into an upper layer is investigated in this paper. An examination is made of the leakage of this displacement into the upper Martian atmosphere (≈20 km) for conditions where the atmospheric interface between the two layers is the level at which a discontinuity occurs, either in the stability profile or in the velocity profile. A comparison is made with the atmospheric conditions which exist when the stability and velocity profiles are continuous at all levels. Observed lee wave phenomena are compared with those predicted by this model. It is found that airflow in the upper layer depends critically on the atmospheric stability in the lower layer, the ‘ship‐wake’ pattern predominates for a decrease in stability with height.
Sediment sound velocities from sonobuoys: Arabian Fan
Authors: Bachman Richard T. , Hamilton Edwin L.
Published: February 2008

Eight variable‐angle seismic reflection stations in the Arabian Fan, Northwestern Indian Ocean, provided 40 determinations of sound velocity in sediment and sedimentary rock. Sound velocity in the homogeneous, largely terrigenous fan increases smoothly with depth. Regression analysis yielded the velocity‐time relationship V (km/s) = 1.510 + 1.863t, where V is instantaneous velocity and t is one‐way travel time below the sea floor to 1 s. The velocity‐depth function is V (km/s) = 1.510 + 1.200h; − 0.253h2 + 0.034h3, where h is subbottom depth in km.
Shallow earthquakes and active tectonics in eastern Afghanistan
Authors: Prevot R. , Hatzfeld D. , Roecker S. W. , Molnar P.
Published: February 2008

Brief studies of microearthquakes in four separate parts of eastern Afghanistan reveal a high level of seismicity over a broad area. In general, the activity is not concentrated on well‐defined faults, nor does it define new faults, but seismicity on or close to the Chaman and Sarubi faults attests to their activity. First motions of P waves are consistent with left‐ and right‐lateral strike‐slip motion, respectively, on these two faults. Fault plane solutions and composite solutions of events in different areas throughout the region differ from one another, but in general, the P axes are parallel to the north‐northwest direction of relative motion between India and Eurasia. Several earthquakes beneath Kabul and its immediate surroundings emphasize a need for further study of its seismic hazard.

The appendix is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J80‐001; $1.00. Payment must accompany order.
Unstable slippage across a fault that separates elastic media of different elastic constants
Authors: Weertman J.
Published: February 2008

In this paper it is demonstrated that relatively slow, quasi‐static slippage on a fault that separates two half spaces of different elastic constants can become unstable if the slippage is governed by the Amontons‐Coulomb friction law (the shear stress across a fault required for slipping motion is proportional to the normal compressive stress across the fault). If the two half spaces have identical properties, unstable slippage is not possible under this friction law. The unstable slippage that is investigated in this paper is a consequence of the existence of a short‐range normal traction stress that gliding edge dislocations produce across an interface between two half spaces of different elastic constants, This normal traction stress does not exist if the two half spaces have identical properties. (Recent work of Dundurs and Comninou and their co‐workers has revealed the importance of the short‐range traction stress components to crack problems.)
Fault zone controlled charging of a liquid‐dominated geothermal reservoir
Authors: Goyal K. P. , Kassoy D. R.
Published: February 2008

A mathematical model is developed for the fault zone controlled charging of a geothermal reservoir. The model is used to describe a reservoir of finite vertical extent with an impermeable upper boundary. A quasi‐analytic theory is developed for high Rayleigh number convection of liquid in a rigid porous medium. In this approximation, liquid rises up the fault and spreads into the near regions of the reservoir isothermally. The cooling effect of the surface on the fault flow is confined to a thin layer near the surface. This layer grows with distance from the fault. In the far field of the aquifer the full depth of the reservoir is cooled by the surface. A study is made of the effect of various parameters such as mass flow rate, Rayleigh number, and fault width on the pressures, velocities, temperatures, and their gradients at different locations in the fault and in the aquifer. This analysis can be applied to geothermal systems where the thickness of the impermeable reservoir cap is quite small compared to the reservoir depth.
Distribution of Quaternary rhyolite domes of the Coso Range, California: Implications for extent of the geothermal anomaly
Authors: Bacon Charles R. , Duffield Wendell A. , Nakamura Kazuaki
Published: February 2008

Thirty‐eight separate domes and flows of phenocryst‐poor, high‐silica rhyolite of similar major element chemical composition were erupted over the past 1 m.y. from vents arranged in a crudely S‐shaped array atop a granitic horst in the Coso Range, California. Most of the extrusions are probably less than about 0.3 m.y. old. The area is one of Quaternary basaltic volcanism and crustal extension. The central part of the rhyolite field is characterized by high heat flow, low apparent resistivity, and substantial fumarolic activity indicative of an active geothermal system. The immediate source of heat for the surficial geothermal phenomena is probably a silicic magma reservoir that may still contain molten or partially molten material at a depth of at least 8 km beneath the central part of the rhyolite field. Outlying rhyolite extrusions probably reflect the presence of feeder dikes emanating from the reservoir beneath the central region. Azimuths of dikes appear to be parallel to the regional tectonic axis of maximum horizontal compression, analogous to some dike‐fed flank eruptions on basaltic shields and andesitic stratovolcanoes. The areal extent of a magma reservoir and the present total heat content of the silicic magma system at Coso may be less than was previously estimated. However, the area is still considered to be one of significant geothermal potential.
Changes in subsurface radon concentration associated with earthquakes
Authors: Mogro‐Campero A. , Fleischer R. L. , Likes R. S.
Published: February 2008

The concentration of radon at shallow subsurface depths (∼1 m) changes in response to the vertical flow of fluids past the measuring site. Thus locally produced radon serves as a tracer gas to detect the flow of subsurface fluids which may occur in association with earthquakes. Therefore it is not necessary to invoke either changes in the local rate of radon production or the long‐distance transport of radon to explain changes in the concentration of radon, although these phenomena may also occur. We have measured radon concentration at 60‐cm depth by using solid state nuclear track detectors to measure weekly values and electronic detectors to measure hourly values. An automated system records the outputs of the electronic detectors of radon, as well as the values of various atmospheric parameters. The radon variations we observe at Blue Mountain Lake, New York, are not controlled by changes in atmospheric parameters. From a 3‐year record of weekly radon readings we conclude that the largest seismic event in New York State during this period (M = 3.9), which occurred at the beginning of our record at a distance of 14 km from our site, may be responsible for the highest radon readings, which were also recorded at this time. The seismicity and the radon concentrations have decreased jointly since then. The automated system shows a tenfold increase in radon concentration in association with an unusual burst of seismic activity. Radon was also measured in a grid of 54 sites near Thoreau, New Mexico. The largest nearby earthquake (M = 4.6) occurred during the most anomalous monthly pattern of radon.
Some recent studies on groundwater radon content as an earthquake precursor
Authors: Teng Ta‐Liang
Published: February 2008

Some recent studies of groundwater radon content in relation to seismic activities are reviewed. Results from China and Japan are presented, including laboratory experiments and the development of continuous groundwater radon monitoring systems. In addition, groundwater radon monitoring studies conducted at the University of Southern California (USC) since 1974 are presented in some detail. The USC project includes a groundwater radon monitoring network of 14 sampling sites at cold springs, hot springs, deep irrigation wells, and artesian wells that are distributed along a ‘locked’ stretch of the San Andreas fault from Gorman to San Bernardino in California. Radon contents in weekly samples from these sites are analyzed to the precision of a few percent. No moderate‐to‐large earthquake has yet occurred in the vicinity, and the results show no clear association of radon anomalies with small (M ∼ 3) earthquakes to provide better test conditions. In contrast to earlier reports from Russia showing a long‐term buildup of radon emission before earthquake occurrence, several recent reports from China and Japan show that groundwater radon anomalies can be short in duration and occur only within a few days before the main shocks. Observations of such effects are made possible with the use of continuous monitoring systems. In general, available data suggest that groundwater radon monitoring could sometimes yield precursor information. However, criteria have yet to be established by which sampling sites and sampling frequencies are selected.
Postseismic viscoelastic surface deformation and stress: 1. Theoretical considerations, displacement, and strain calculations
Authors: Cohen Steven C.
Published: February 2008

This paper, the first of two related articles, presents a model of viscoelastic deformations associated with earthquakes. A strike slip fault is represented by a rectangular dislocation in a viscoelastic layer (lithosphere) lying over a viscoelastic half space (asthenosphere). Deformations occur on three time scales. The initial response is governed by the instantaneous elastic properties of the earth. A slower response is associated with viscoelastic relaxation of the lithosphere, and a yet slower response is due to viscoelastic relaxation of the asthenosphere. The major conceptual contribution of this paper is the inclusion of lithospheric viscoelastic properties into a dislocation model of earthquake‐related deformations and stresses. Numerical calculations using typical fault parameters reveal that the postseismic displacements and strains are small compared to the coseismic ones near the fault but become significant farther away. Moreover, the directional sense of the deformations attributable to the elastic response, the lithosphere viscoelastic softening, and the asthenospheric viscoelastic flow may differ and depend on location and model details. The results and theoretical arguments suggest that the stress changes accompanying lithospheric relaxation may also be in a different sense than and be larger than the strain changes.
Net tectonic rotation, apparent tectonic rotation, and the structural tilt correction in paleomagnetic studies
Authors: MacDonald William D.
Published: February 2008

The conventional tilt correction in paleomagnetic studies is intended to correct for bedding tilt. It is assumed that tilt took place about the line of strike of bedding. The basic assumption is incorrect where tectonic rotations have taken place about nonhorizontal, inclined axes. In such cases the conventional tilt correction produces declination anomalies in paleomagnetic vectors. These resemble the effects of tectonic rotation but are more aptly interpreted as apparent tectonic rotation. The challenge of paleomagnetic studies in orogenic zones is to distinguish between plate tectonic rotation, local structural rotation about vertical axes, and apparent tectonic rotation. Some criteria for recognizing apparent tectonic rotation are given, with examples. Methods for determining axes of net tectonic rotation and of relative tectonic rotation are developed by combining rotations. These have many potential applications to structural and tectonic analyses.
The paleomagnetism of certain Late Precambrian and Early Paleozoic rocks from the Red Sea Hills, Eastern Desert, Egypt
Authors: Davies J. , Nairn A. E. M. , Ressetar R.
Published: February 2008

A stable remanence was isolated from three groups of rocks collected in the Eastern Desert of Egypt. Two groups of dikes, one collected north of the Qena‐Safaga road and the other about 200 km to the south near the Um Rus gold mine, gave paleomagnetic pole positions of 87°N, 304°E and 86°N, 185°E, respectively. Both have K/Ar ages in the range 480–530 m.y. The results are compatible with data reported from other Gondwana cratonic areas for this time period. The third group of rocks were samples of the Dokhan Volcanic Series collected at seven sites along the Qena‐Safaga road. The paleomagnetic pole position of the higher‐coercivity component, 36°S, 17°E, acquired about 600 m.y. ago is not easily reconciled with established polar wander paths. The younger component, however, with a paleomagnetic pole lying at 54°N, 327°E, suggests a Cambro‐Ordovician age.
The inverse problem of electromagnetic induction: Existence and construction of solutions based on incomplete data
Authors: Parker Robert L.
Published: February 2008

A theory is described for the inversion of electromagnetic response data associated with one‐dimensional electrically conducting media. The data are assumed to be in the form of a collection of (possibly imprecise) complex admittances determined at a finite number of frequencies. We first solve the forward problem for conductivity models in a space of functions large enough to include delta functions. Necessary and sufficient conditions are derived for the existence of solutions to the inverse problem in this space. The approach relies on a representation of real‐part positive functions due to Cauer and an application of Sabatier's theory of constrained linear inversion. We find that delta‐function models are fundamental to the problem. When existence of a solution has been established for a given set of data, actual conductivities fitting the measurements may be explicitly constructed for various special classes of functions. For a solution in delta functions or homogeneous layers a development as a continued fraction is the essential idea; smoothly varying models are found with an adaption of Weidelt's analytic solution.
Static elastic‐gravitational deformation of a layered half space by point couple sources
Authors: Rundle John B.
Published: February 2008

A method for computing the deformation in a layered, self‐gravitating half space due to internal point sources is formulated. The coupled elastic‐gravitational field equations appropriate to the first‐order perturbation problem are presented. Although the first‐order perturbation problem is not strictly coupled to a physically reasonable zeroth‐order problem, the perturbation equations together with boundary conditions present a well‐posed problem. The general homogeneous solution for deformation in a uniform infinite medium is found in the form of eight independent solution vectors. It is easily shown that the solution to the coupled elastic gravitational equations reduces to that for the uncoupled equations in the limit of vanishing density. The solutions for various internal point sources in a layered half space are constructed out of the solution found for the infinite medium by the use of well‐known techniques. The half space is divided into a series of horizontal layers, and a matrix method is used to propagate the solution from one layer to the next. The point source is represented by a discontinuity in several of the various propagated quantities across a plane z = constant. The solution is ultimately determined by the requirements of vanishing normal tractions on the free surface and vanishing of all perturbation quantities at infinite depth. Finally, expressions are given for the direct calculation of surface displacements from several types of point sources.
Power dissipation and stress levels on faults in the upper crust
Authors: Sibson R. H.
Published: February 2008

Rock deformation textures from deeply exhumed fault zones in quartzo‐feldspathic crust are considered in relation to likely rates of energy dissipation and hence the levels of shear resistance operative during seismic slip in the upper, factional regimes of major crustal dislocations. Available evidence suggests that both low‐stress (τ ≤100 bars) and, less commonly, high‐stress (τ≥1 kbar) faulting occur, depending on local conditions of which the most important is the ratio of fluid to overburden pressure. The higher stresses are usually associated with immature fault systems, especially reverse faults developed in crystalline host rocks. As a result of initial power dissipation at the onset of slip, feedback mechanisms involving either friction melting or the creation of high transient fluid pressures may drastically diminish kinetic shear resistance over all or part of the rupture surface, the effects becoming more pronounced the greater the initial shear stress. For shallow strike slip earthquakes, values of radiant flux (the wave power radiated per unit area of a fault) range from 0.1 to 10 MW/m2. This suggests that there are considerable variations in seismic efficiency and/or total energy release, even for events of similar magnitude occurring on faults of the same type.
Geochemical evidence for water‐rock interaction along the San Andreas and Garlock Faults of California
Authors: O'Neil James R. , Hanks Thomas C.
Published: February 2008

Mesozoic granitoid rocks adjacent to the San Andreas fault in central California have retained their radiogenic Ar for the last 70 m.y. but have, generally, the highest 18O and H2O+ contents and the lowest D contents of all the granitoid rocks in California. The geographical coincidence of the D, 18O, and H2O+ patterns with the present trace of the San Andreas fault leave little doubt that some kind of groundwater circulation system has operated in the vicinity of the fault in central California. Similar isotopic patterns exist in rocks along the Garlock fault. These water‐rock interactions probably took place at temperatures <200°C, although depth, extent, and timing are not resolved. Stable isotope compositions of rocks from many localities of the earth have provided unambiguous proof that meteoric waters have descended to depths at least as great as 8 to 10 km when a heat source such as a cooling pluton was present. Massive circulation of groundwater is common in the upper crust of tectonically active areas and may affect the frictional stress state along major faults. The geochemical evidence for extensive deep circulation of groundwater and the relatively high permeabilities for most rocks of the upper crust recently compiled by Brace (1980) argues against nearly lithostatic fluid pressures as an ambient condition of the upper crust. Groundwater circulation along the San Andreas provides an efficient mechanism for diffusing the heat flow anomaly that arises in heat transport calculations that are based on thermal conduction alone.