bluuebunny/crossref_metadata_2025 huggingface dataset:
Acoustic emission study of microfracturing during the cyclic loading of westerly granite
Authors: Sondergeld Carl H. , Estey Louis H.
Published: February 2008

A rectangular prism of Westerly granite was deformed in a uniaxial cycling experiment. Acoustic emission (AE) activity was recorded using eight transducers in real time. Hypocenters of 116 events were determined using a standard least squares approach. Event clustering was definitely evident and could easily be assessed using a stereographic projection of hypocenters. This clustering was observed at volumetric strains well below 0.03%. The distribution of hypocenters changed from cycle to cycle, suggestive of a spatial migration. The cumulative distribution of hypocenters documents the strongly anisotropic development of dilatancy in Westerly granite. Once the sample underwent the initial deformation and a zone of weakness was established, subsequent events clustered in and about the initial zone. Volumetric strain, differential stress, acoustic emission event rate, and cumulative numbers were continously measured during the experiment. The cumulative number of events per cycle generally decreased with the number of cycles. The majority of events occurred upon loading. Those that occured during unloading were few in number and not as systematically related to stress as were those for loading. In general, waveform quality deteriorated with increased cycling and in later cycles waveforms typically displayed low‐frequency components in initial onsets followed by a more typical higher‐frequency arrival.
Observations of flexure and the rheology of the oceanic lithosphere
Authors: Bodine J. H. , Steckler M. S. , Watts A. B.
Published: February 2008

Observations of flexure indicate that the effective flexural rigidity of oceanic lithosphere is a function of the age of the lithosphere at the time of loading, and hence temperature. We have used a yield stress envelope model constrained by data from experimental rock mechanics to determine how the flexural parameters and rheologic properties of oceanic lithosphere are related. The results of our model for seamounts and oceanic island loads in the interior of plates suggest that following loading, rapid stress relaxation occurs as the plate ‘thins’ from its short‐term to its long‐term (>106years) mechanical thickness. The mechanical thickness, which determines the effective flexural rigidity of the plate, is strongly dependent on temperature and weakly dependent on load size and duration (>1–10 m.y.). The results of our model for convergent plate boundaries suggest that changes in the shape of the Outer Rise along an individual trench system may be due to variations in the horizontal load acting across the boundary (<1 kbar). The model predicts a narrow zone of high strain accumulation seaward of a trench which is in agreement with variations in crustal velocities and seismicity patterns observed along some trench systems.
Propagating episodic creep and the aseismic slip behavior of the Calaveras Fault north of Hollister, California
Authors: Evans K. F. , Burford R. O. , King G. C. P.
Published: February 2008

A detailed kinematic study of fault slip occurring from the surface to a depth of about 7 km on the Calaveras fault north of Hollister was conducted during the summer of 1977. The observations coincided with a period of propagating episodic fault creep activity sensed along the fault trace. Data used in the investigation consist of creepmeter records, near‐field strainmeter observations, and high‐resolution geodetic measurements, all collected contemporaneously over a period of 4 months. Detailed descriptions and analyses of the creepmeter and geodetic data have been presented elsewhere. The near‐field strain measurements are here reported in detail, and their analysis draws upon the previous two data sets for support. The strainmeter observations are most sensitive to slip occurring in the upper 2 km; hence the emphasis of the paper is placed upon the role of propagating episodic creep in the broad‐scale behavior of the fault. The results suggest that propagating episodic fault creep as sensed along the fault trace is confined to the upper kilometer or so of the crust and represents the response of the surface layers to a longer‐term form of episodic aseismic slip occurring below. The mean form of the advancing rupture front within the upper kilometer is ostensibly the same as that indicated by records from the surface creepmeters. Evidence is presented, however, which suggests that propagating creep events may not always break the surface and may propagate at velocities much slower and at amplitudes significantly larger than those generally observed at the surface.
Hydromechanical behavior of a deformable rock fracture subject to normal stress
Authors: Tsang Y. W. , Witherspoon P. A.
Published: February 2008

A simple physical model is developed to understand the effect of normal stress on fluid flow through a single fracture. Roughness along the fracture walls plays a definite role in controlling the flow. In the usual parallel plate representation for a fracture, the flow is proportional to the cube of the constant aperture, b. However, when the effect of fracture roughness is taken into account, the flow follows an equivalent ‘cubic’ law where the cube of the single value for the aperture must be replaced by an appropriately weighted average 〈b3〉. To obtain this average value, a physical model was developed wherein the single fracture is represented by a collection of voids and the closure of the fracture results from a deformation of these voids. The model enables one to characterize the fracture roughness from a relationship between the stress‐displacement measurements of intact rock and those of jointed rock. This calculated value of 〈b3〉 leads to flow rate as a function of normal stress. Predicted flow rates using this model are in good agreement with results from laboratory data on granite and basalt. By making several simplifying physical assumptions, we have eliminated the necessity of incorporating fitting parameters to the flow data. In this manner, a basic understanding of the factors controlling the flow of fluids through fractures has been obtained.
Seismicity parameters preceding moderate to major earthquakes
Authors: von Seggern David , Alexander Shelton S. , Baag Chang‐Eob
Published: February 2008

Seismic events reported in the bulletins of the two large arrays, LASA and NORSAR, were merged with those from the NEIS bulletin for the period 1970–1977. Using a lower cutoff of mb= 5.8, 510 ‘main shocks’ within the P range of LASA or NORSAR were selected for this period; and various seismicity trends prior to them were investigated. A search for definite foreshocks, based on a significantly short time delay to the main shock, revealed that the true rate of foreshock occurrence was less than 20%. Foreshocks are almost exclusively associated with shallow (h < 100 km) main shocks. To establish common features, a method of averaging seismicity from many regions was used to suppress the randomness of the seismic behavior of each region. This averaging shows that the seismicity level around the main shock increases somewhat for 10 days before main shocks; this feature peaks in the last 3–4 hours prior to the main shocks. The averaging also reveals that the mean magnitude of events near the main shock increases prior to main shocks but only by a few hundredths of a magnitude unit. Again by averaging, the seismicity about main shocks is shown to tend with time toward the main shock as its origin time is approached, but the average effect is small (∼10% change). By expanding or contracting each region's time scale before averaging to relate to the magnitude of the main shock, these features are enhanced. Using a new variable to track the departures from both spatial and temporal randomness, the Poisson‐like behavior of deeper seismicity (>100 km) was demonstrated. For shallow events (<100 km) this variable reveals numerous instances of clustering and spatial‐temporal seismic gaps, with little tendency toward a uniformity of behavior prior to main shocks. A statistical test of the validity of seismic precursors was performed for approximately 90 main shock regions which had sufficient seismicity. Using a five‐variable vector (interevent time, interevent distance, magnitude, epicentral distance to main shock, and depth difference relative to main shock) for each event in a ‘precursory’ time window of 500 days before the main shock and for each event in a ‘normal’ time window of 500 days before that, the null hypothesis of equal vector means between the two groups was tested. At 90% confidence level, less than 30% of the main shock regions were thus found to exhibit precursory seismicity changes.

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Contrasting rhyolite suites in the Late Cenozoic of Papua New Guinea
Authors: Smith I. E. M. , Johnson R. W.
Published: February 2008

Rhyolite is found in several late Cenozoic volcanic areas in Papua New Guinea including (1) New Britain and islands to the north, where rhyolite compositions correspond to different depths to a Benioff zone, (2) the St. Andrew Strait area in the northern Bismarck Sea, where alkali rhyolites are the only rock types found on Tuluman, Lou, and Pam (TLP) islands, (3) the Moresby Strait area in the D'Entrecasteaux Islands, where alkali rhyolites similar in composition to the TLP rhyolites are found together with basalts, andesites, and dacites, and (4) the Dawson Strait area, also in the D'Entrecasteaux Islands, where the rhyolites are comenditic (peralkaline) and markedly different from all other rhyolite types known from Papua New Guinea. The TLP and Moresby Strait rhyolites are thought to have been formed by partial melting of mafic crust, whereas the peralkaline Dawson Strait suite has apparently originated by crystal fractionation. Crystal fractionation has also been proposed as the cause of rhyolite genesis on Willaumez Peninsula and at Rabaul in New Britain. However, the supporting evidence is not conclusive, and partial melting of young arc crust is a more acceptable alternative, at least for the abundant rhyolites of central Willaumez Peninsula.
Late Cenozoic volcanism at Twin Peaks, Utah: Geology and petrology
Authors: Crecraft H. R. , Nash W. P. , Evans S. H.
Published: February 2008

Bimodal volcanism that formed in the Twin Peaks volcanic complex, west‐central Utah, spanned a period from 2.7 to <1 m.y., and produced approximately 12 km3 of erupted material. Two sequences of differentiation can be discerned in the silicic lavas; both are characterized by increases in SiO2 from 71% to 76% and decreases in temperature from 875°C to 775°C. The first sequence erupted from 2.7 to 2.5 m.y., produced >10 km3 of aphyric lavas and pyroclastic deposits and resulted in a local subsidence of approximately 100 m. Tuffaceous lake deposits followed by voluminous olivine tholeiites subsequently filled the depression. Eruption of the second sequence of silicic rocks spanned a short time at 2.4 m.y. and produced a total volume of 0.7 km3 of strongly porphyritic felsites. At least two subsequent episodes of basaltic volcanism followed. Chemical variations displayed by the early differentiation sequence of silicic rocks are characterized by increases in volatiles, SiO2, Na2O, heavy rare‐earths, Rb, and highly charged cations and decreases in all other major components, light rare‐earths, Sr, and Ba. These variations in time closely resemble spatial variations in compositionally zoned ash flow sheets and are incompatible with crystal fractionation. The early differentiation sequence resulted in the development of a highly evolved, compositionally stratified roof zone, from which the ash and aphyric lavas were tapped. Conductive cooling models for the silicic magma chamber require thermal input into the system, probably by underplating a large volume of basalt, during the early eruptive sequence to sustain it beyond 50,000 years after initial emplacement. Chemical variations displayed by the second eruptive sequence exhibit an increase in K and decrease in Na and are consistent with crystal fractionation as the dominant control. Crystal fractionation and a short eruptive history indicate rapid cooling and a cessation of thermal input for the second sequence.
Three S‐type volcanic suites from the Lachlan Fold Belt, southeast Australia
Authors: Wyborn D. , Chappell B. W. , Johnston R. M.
Published: February 2008

The concept that granitoids of the Lachlan Fold Belt in southeast Australia, are derived from either igneous or sedimentary source rocks (I‐ or S‐type), can be extended to volcanic rocks of the same age. Three suites of late Silurian S‐type volcanics are described, two of which can be matched quite closely with plutonic equivalents. A large part of the Paleozoic continental margin volcanic activity in southeast Australia consisted of the magmatic recycling of old metasedimentary crust, probably of late Proterozoic age. The three volcanic suites are moderately to strongly peraluminous, with the corresponding presence of Al‐rich minerals. Variation within the volcanic suites is ascribed chiefly to progressive removal of restite, or source material residual from partial melting. The most mafic suite, the Hawkins Suite, contains plagioclase, cordierite, orthopyroxene, biotite and quartz as restite components, with less abundant almandine. These rocks are chemically equivalent to mafic biotite‐rich and cordierite‐bearing granitoids from parts of the Berridale and Murrumbidgee Batholiths. Garnet is absent from the Goobarragandra Suite volcanics, which are a little more felsic and close in compositon to granitoids of the Young and Maragle Batholiths. The S‐type character of the Laidlaw Suite is less pronounced although a sedimentary source seems to be established. Such a source would be less mature than those for the other two volcanic suites. The Laidlaw Suite resembles, but cannot be closely identified with, felsic S‐type granitoids of the Murrumbidgee and Berridale Batholiths. Low‐grade regional metamorphism in most of the volcanic rocks has resulted in much mineralogical alteration and mobility of alkali and alkaline earth elements. Despite this alteration, unaltered rocks are present in all three suites. Compositional data for the phenocryst phases within the unaltered rocks has allowed estimation of some intensive parameters. Hawkins Suite volcanics were extruded directly from their source region and preserve phenocryst equilibria established at 5–6 kbar at 800°C. The Laidlaw and Goobarragandra Suites reequilibrated at lower pressures than their source before extrusion. The Laidlaw Suite reequilibrated at estimated temperatures of 725°–730°C, ƒO2 of 10−15.5 to 10−14 bars, and ƒ(H2O) of 2 to 2.5 kbar. Differing mineral compositions in the three suites are related to differing source rock compositons and oxygen fugacities. Relative biotite and orthopyroxene mg is possibly pressure dependent.
Contrasting evolution of calc‐alkalic volcanic and plutonic rocks of western Chihuahua, Mexico
Authors: Bagby W. C. , Cameron K. L. , Cameron M.
Published: February 2008

Two sequences of continental arc igneous rocks occur in the Batopilas area of the Sierra Madre Occidental of northwestern Mexico. The older includes late Cretaceous to (?) early Cenozoic granitoids, and the younger is a mid‐Tertiary andesite to rhyolite series that is represented largely by voluminous ignimbrites. The latter sequence contains rocks varying from 54 to 78 wt.% SiO2 whereas the granitoids have a more restricted range in SiO2, 57–68 wt %. The major element chemistry of the two sequences is generally similar; however, the older granitoids have lower Zr/Nb, and higher Nb/Y and K/Rb compared to volcanic rocks. In addition, the granitoids have generally smaller Eu anomalies, lower concentrations of REE, and higher (Ce/Yb)N (4.6–7.2 versus 3.8–4.0) than the volcanic rocks. The intermediate rocks of the two sequences, quartz diorites and andesites, had similar petrogenetic histories, and both may have originated by fractionation of mantle‐derived basaltic magmas. In contrast, the more silicic rocks, granodiorites on one hand and dacites and rhyolites on the other, evolved very differently. The granodiorites may have originated from a crustal source leaving a residue of amphibolite or garnet granulite, whereas the rhyolites were the product of protracted fractional crystallization of mantle‐derived melts.
Implications of the mechanical and frictional behavior of serpentinite to seismogenic faulting
Authors: Dengo Carlos A. , Logan John M.
Published: February 2008

The widespread occurrence of serpentinite along major strike slip, seismogenic faults warrants systematic investigation to determine how its frictional characteristics may affect slip along the fault. Different locations along the Motagua fault zone in Guatemala were sampled to investigate the sliding mode as a function of composition and texture, confining pressure, and displacement rate. Air‐dried, right circular cylinders, 7.1 cm in length and 3.3 cm in diameter, with a precut 35° to the cylinder and load axes were deformed at confining pressures up to 200 MPa, room temperature, and displacement rates of 10−4 and 10−7 cm/s. Compositional analyses of specimens cored from five blocks of serpentinite that were tested show that the serpentinite can be divided into two groups. One is a mesh‐textured serpentinite containing up to 70% serpentine, mostly lizardite, 11% enstatite phenocrysts, 19% oxides, and minor amounts of olivine and carbonate (undifferentiated). The other is a flare‐textured serpentinite containing up to 84% serpentinte, mostly antigorite, 10% oxides, 6% magnesite and dolomite, and almost no enstatite or olivine. The flare serpentinite undergoes the transition from stable sliding to stick slip sliding at confining pressures as low as 10 MPa. The mesh serpentinite results only in stable sliding up to confining pressures of 200 MPa and a displacement rate of 10−4 cm/s. At a displacement rate of 10−7 cm/s only one of the three mesh blocks exhibits stick slip. The shear stress required to initiate sliding for the flare‐textured serpentinites is given by τ = 0.77σn. For the mesh‐textured serpentinites at a displacement rate of 10−4 cm/s it is given by τ = 0.56σn, and at a displacement rate of 10−7 cm/s by τ = 0.50σn. Differences in fracture strength between the five specimen blocks correlate with differences in frictional strength. Differences in the sliding behavior and in the deformation observed along the sliding surfaces can be related to the mineralogical components of each serpentinite type. Extensive deformation occurs in the mesh serpentinites as a result of hard oxide and enstatite indenters ploughing through a soft lizardite substrate. Lizardite‐lizardite contacts prevail, and these deform by plastic flow. The stick slip behavior in the flare‐textured serpentinites is explained by brittle failure of antigorite‐antigorite contacts. The smaller grain size of the flare serpentinites, relative to the mesh serpentinites, may also contribute to the higher frictional strength of the flare serpentinites. We discuss the implications of the antigorite‐rich, flare‐textured serpentinite to seismogenic faulting. In particular, we consider the case of antigorite forming by shear‐heating‐induced progressive metamorphism of lizardite‐chrysotile serpentinites. If serpentine dehydration temperatures are approached, talc may form. Having a lower coefficient of friction than serpentine, talc may act as a ‘lubricant’ along the fault plane.
Geodetic measurement of crustal deformation on the San Andreas, Hayward, and Calaveras Faults near San Francisco, California
Authors: Prescott W. H. , Lisowski M. , Savage J. C.
Published: February 2008

Analysis of a geodetic network of 115 lines crossing the San Andreas, Hayward, and Calaveras faults in the vicinity of San Francisco Bay and measured repeatedly between 1970 and 1980 has revealed details about the accommodation of relative plate motion in this area. The most striking result is that the deformation is not uniformly distributed across the area. In the east bay, along the Hayward and Calaveras faults, all motion appears to take place as slip directly on the fault, with no accumulation of strain in the adjacent crust. On both the Calaveras and the Hayward faults the rate obtained for the 1970 to 1980 period agrees with geologic rates spanning a few million years and with creep rates spanning a few decades. The Hayward fault slip rate is 7 ± 1 mm/yr. The Calaveras fault slip rate is 7 ± 1 mm/yr, with perhaps half of this slip distributed across a zone a few kilometers wide, probably as inelastic deformation of weak near‐surface material. The absence of strain accumulation in the east bay is surprising since the Hayward and Calaveras faults have been the site of large earthquakes in the past. A block located east of the Calaveras fault and south of the Las Positas fault has been rotating clockwise at a rate of 0.3 ± 0.1 μrad/yr with very little internal deformation. Along the San Francisco peninsula no detectable slip occurs (less than 1.5 mm/yr) at the surface, but appreciable strain is accumulating. Near fault shear strain rates are 0.6 ± 0.1 μstrain/yr (engineering) with direction N47°W ± 9. The slip rate near the San Andreas fault is 12.2 ± 3.9 mm/yr distributed across a broad zone. The relative motion across the whole region during the period 1970–1980 is 32.1 ± 7.4 mm/yr.

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The first decade of Gloria
Authors: Laughton A. S.
Published: February 2008

In 10 years of operation, GLORIA has been used on 26 cruises and has inonified more than 1% of the ocean floor. The paper reviews the scientific achievements of this new tool and assesses the contributions which it has made to understanding the geological features and the processes creating them. A complete bibliography of GLORIA papers is included in the references, and the best examples of sonographs obtained over different terrain are illustrated.
Slowness techniques in seismic interpretation
Authors: Kennett B. L. N.
Published: February 2008

Wave propagation in a horizontally stratified medium may be characterized by the slowness p of the waves. In ray theory the travel time curves are parameterized by the slowness p, and this dependence is exploited in modern methods of tau inversion for travel times. The complete response of a stratified medium may also be represented as a superposition of cylindrical waves of varying slowness. This representation leads to the possibility of presenting the response of the stratification in a slowness time map containing both travel time and amplitude information. Such an approach requires copious good quality data but offers advantages in interpretation. Even in the presence of lateral variations of structure, slowness methods can provide useful information.
Serpentinization faults and their role in the tectonics of slow spreading ridges
Authors: Francis T. J. G.
Published: February 2008

Two major families of faults dominate the tectonics of slow spreading ridges: the transform faults paralleling the spreading direction and the ‘GLORIA’ faults, observed by large‐scale side‐scan sonar, normal to it. The observation of microearthquakes to 8 km depth beneath the Mid‐Atlantic Ridge axis implies that not only the transform faults but the GLORIA faults as well penetrate into the mantle. Since large horizontal temperature gradients are often associated with them, both types of fault provide the means of circulating large volumes of sea water through the oceanic mantle. Under these conditions, serpentinization of upper mantle rocks will occur in a predictable way. Its influence on the tectonic development of slow spreading ridges is discussed.
Zonal harmonic models of reversal transition fields
Authors: Williams I. , Fuller M.
Published: February 2008

Synthetic reversal records for different latitudes have been generated for model transition fields with various zonal harmonic contents. The model fields are based upon a redistribution of energy from an exponential decay of the dipole field to g2°, g3°, and g4°.The records emphasize the dependence of their characteristics upon the latitude of the observation site. Both intensity and inclination changes, the relationship between these two aspects of the records, and estimates of the time to complete the reversal are all strongly dependent upon latitude. A particular model in which the dipole energy is redistributed to g2°, g3°and g4° according to the ratio 2:3:5 is used to simulate the last reversal.
An analysis of geoid anomalies across the Mendocino Fracture Zone: Implications for thermal models of the lithosphere
Authors: Detrick Robert S.
Published: February 2008

Geoid height increases in a steplike fashion across oceanic fracture zones owing to the juxtaposition of lithosphere with different ages and density structures. At a ridge/transform intersection, the initial geoid offset depends on the age difference across the fault and is approximately equal to 0.15 m/m.y. of age offset. The amplitude and shape of this anomaly will change along the length of a fracture zone depending on how the lithosphere cools. If the lithosphere thickens indefinitely (boundary layer model), then the geoid offset will remain constant although the width of the transition zone from one side of the fracture zone to the other will increase owing to both heat transfer across the fault and an increase in the average depth of compensation. If, however, the lithosphere eventually approaches a constant thickness (plate model), the total geoid offset will decrease with time. An analysis of 18 SEASAT altimetry profiles across the Mendocino fracture zone (offset 25–30 m.y.) indicates geoid anomaly amplitudes systematically decrease from more than 4 m across the eastern (younger) part of the fracture zone to about 1 m across the western (older) part of the fault. Although the geoid offsets across the older portion of the Mendocino fracture zone may have been underestimated by as much as 20–25%, this large decrease in geoid offset strongly suggests that the lithosphere approaches a constant thickness along the older part of the fracture zone. A plate thickness of ∼100 km fits the observed change in geoid offset along the Mendocino, but with uncertainties in the offset determination, the age difference across the older part of the fault, and the presence of areas of anomalous depths on both sides of the fracture zone, this probably represents a minimum estimate of the total plate thickness.
Shear fracture energy of Westerly granite from post‐failure behavior
Authors: Wong Teng‐fong
Published: February 2008

We consider in this paper the energetic relationship among four different types of measurements: evaluation of shear fracture energy from seismological data, rock mechanics study of post‐failure behavior in triaxial compression, measurement of specific surface energy of minerals in a tensile mode, and scanning electron microscope (SEM) observation of stress‐induced microstructures. Shear fracture energy can be estimated from post‐failure deformation data following an integration scheme recently suggested by Rice. Values so determined for Westerly granite at pressures up to 250 MPa and temperatures up to 700°C are of the order of 104 J m−2, which is about 2 orders of magnitude higher than the tensile values. Analysis of our data, together with previous room temperature post‐failure data, show that the influence of temperature, pressure, and rock type can change the shear fracture energy by an order of magnitude. Microcracking energy can be estimated by multiplying the stress‐induced crack area (obtained by stereological technique) and the single‐crystal specific surface energy. Energy for the extension of microcracks resolvable under SEM can account for a major portion of the total energy input for pre‐failure deformation. However, for post‐failure deformation the estimate of microcracking energy is smaller than the total by at least an order of magnitude.
The Pelusium Megashear System across Africa and associated lineament swarms
Authors: Neev David , Hall John K. , Saul John M.
Published: February 2008

The Pelusium megashear system (Neev, 1977) extends from the ‘border zone’ of Anatolia, along the eastern Mediterranean and across Africa from the Nile Delta to the delta of the Niger in the Gulf of Guinea. It appears to continue beyond Africa, via the Atlantic equatorial fracture zones, into the Amazon Basin. It has functioned as a system of en echelon left‐lateral megashears since Precambrian times. At least four other geosutures also cross Africa, paralleling the Pelusium system and similarly continuing into the Atlantic as fracture zones. Swarms of lineaments observed in LANDSAT image mosaics of central and northeast Africa are associated with the Pelusium megashear system. They are interpreted to be of tectonic origin. Linear dunes and yardangs oriented parallel to the lineaments appear to have been formed by winds deviated and channeled by the tectonic lineaments. These eolian features should not be considered therefore as reliable indicators for higher‐altitude atmospheric circulation. Whereas the patterns of the swarms are affiliated to that of the Pelusium megashear system, they are not related to most of the other nearby geologic features. Both the Pelusium megashear system and its associated lineament swarms exhibit northeastward convergence patterns, which suggest counterclockwise rotation of the crust. These probably result from left‐lateral shear movements within the asthenosphere which have continued since the Precambrian.
Parametric analyses of the transient method of measuring permeability
Authors: Lin Wunan
Published: February 2008

Analyses are made to compare the two approaches of using the transient method to determine permeability of rocks: the simplified version and the numerical version. The simplified version assumes that when the sample volume is much smaller than the reservoir volume, the fluid storage in a rock sample can be neglected and the pressure decay in the upstream reservoir can be approximated by an exponential function of time. The numerical version uses finite difference method to solve the differential equation of pressure decay and matches the observed pressure decay with the calculated decay curves. The pressure decay calculated by the numerical version is not generally a simple exponential function of time, as suggested by the simplified version. Our analyses show that the numerical version fits to the observed data very well. The permeability value determined by the simplified version tends to be greater than that determined by the numerical version. The difference in apparent permeability between the two depends on many factors such as rock properties (porosity and compressibility), sample size, reservoir volumes, etc. A relative fluid storage is used to illustrate the difference of these two approaches. For a relative fluid storage value of greater than 0.03, the difference in permeability between the two is more than 30%. For a system with a fluid storage of less than 0.01, these two versions agree with each other well.
Shear wave structure of the Southern Volcanic Plateau of Oregon and Idaho and the Northern Great Basin of Nevada from surface wave dispersion
Authors: Priestley Keith F. , Brune James N.
Published: February 2008

Fundamental mode Rayleigh wave phase velocities (to 100‐s period) have been measured for two paths across the northern Great Basin of Nevada and Utah and southern Volcanic Plateau of Oregon and Idaho. The average dispersion for these regions is similar to that previously observed for the central Great Basin of Nevada and Utah. Average structural models were derived for these paths from the observed dispersion data and existing refraction data. The model for the southern Volcanic Plateau and western Snake River consists of a three‐layer, 40‐km‐thick crust and a 10±10 km thick high‐velocity mantle lid above a broad low‐velocity zone for shear waves. Higher phase velocities are observed at long periods (>40 s) for the path through the northern Great Basin than for parallel paths in the central Great Basin or Volcanic Plateau, indicating higher average mantle shear wave velocities. The model for the northern Great Basin consists of a three‐layer 30‐km‐thick crust and a 30±10 km thick high‐velocity mantle lid above a low‐velocity zone for shear waves. The average shear wave velocity within the low‐velocity zone beneath the northern Great Basin is 4.3 km/s compared with 4.1 km/s within the low‐velocity zone beneath the central Great Basin. These higher mantle shear wave velocities postulated for the northern Great Basin are at odds with the observed heat flow but in agreement with teleseismic P wave results. Both the higher velocities and the high heat flow may be the result of upper mantle differentiation during a recent volcanic event.
On isostasy at Atlantic‐type continental margins
Authors: Karner G. D. , Watts A. B.
Published: February 2008

Cross‐spectral techniques have been used to analyze the relationship between free air gravity anomalies and topography of approximately 100 profiles over the continental margins of eastern North America, southwest Africa, and the Coral Sea/Lord Howe Rise. The resulting admittance functions for these margins have been interpreted in terms of the flexure model of isostasy. The relatively young Coral Sea/Lord Howe rise margin is associated with the lowest value of the effective elastic thickness (Te < 5 km), while the relatively old eastern North American margin is associated with the highest value (10 < Te < 20 km). Thus Te appears to increase with the age of the margin. These results, which are consistent with flexure studies from the major ocean basins, have been interpreted in terms of a simple model in which the flexural strength of the lithosphere increases with age. The model suggests that a low‐rigidity plate (or Airy‐type model) is most applicable early in rifting history, while a high‐rigidity plate is more applicable later in margin evolution. Thus sediments at a margin initially load a relatively thin and hot plate, while sediments later in margin evolution load a relatively thick and cold plate. The model explains a number of features of the tectonic‐stratigraphic evolution of margins. These include a change in flexural style from a narrow and deep basin to a broad and shallow basin, coastal plain onlap, and uplift in the continental interior. Furthermore, the flexure model of isostasy contributes in a major way to observed free air gravity anomalies at continental margins. In particular the model explains the existance of (Airy) isostatic gravity anomalies at margins. The amplitude and wavelength of these anomalies are a function of the sediment supply, the type of mechanisms controlling sediment deposition (up building and/or out building), and the thermal structure, history, and age of the margin.
Crack‐induced velocity anisotropy in the White Mountains, New Hampshire
Authors: Park Stephen , Simmons Gene
Published: February 2008

The compressional wave velocity of rock in situ measured at four sites in the White Mountains of New Hampshire varies as a function of azimuth and is anisotropic. The anisotropy [(Vmax − Vmin)/Vmax], which ranged from 18% to 42%, is attributed to two sets of cracks with different azimuths and with total crack density of order of 0.2 at each site. The anisotropy in velocity correlates only approximately with the orientation of mapped joints at each site.
Paleolatitudes versus time for southern Alaska
Authors: Stone D. B. , Panuska B. C. , Packer D. R.
Published: February 2008

Paleomagnetic determinations of the inclination of the ancient geomagnetic field for localities in the Alaska Peninsula, Wrangellia, Chugach and Kuskokwim tectonostratigraphic terranes of Alaska show systematic northward motion from late Cretaceous time until the present. Prior to this, the ambiguity of polarity of the ancient geomagnetic field allow several interpretations; however, the preferred interpretation involves southerly motion of the terranes from Triassic to Early Jurrassic time, and steady northward motion from then up to the present. The latitudinal rate is about 6 cm/yr.
Terranes and suture zones in east central Alaska
Authors: Churkin M. , Foster H. L. , Chapman R. M. , Weber F. R.
Published: February 2008

East central Alaska, with its 17 terranes, forms a part of the mosaic of allochthonous terranes that join the North American and Siberian plates. These terranes range from continental and continental margin, such as the Tatonduk with its thick well‐bedded succession of marine shelf rocks, to seamount, arc, and ocean floor terranes. The Yukon crystalline terrane, the largest described here, is a composite of at least four subterranes juxtaposed across the Tintina fault with the Tatonduk terrane, a northwestern extension of the North American plate in Alaska. Inboard of the Yukon crystalline terrane are packets of closely appressed microterranes separated from the Tatonduk and other terranes belonging to North America by major suture zones. These microterranes lie between North America and the mosaic of accretionary terranes that form the more southerly part of Alaska. The most obviously allochthonous microterranes within the suture zones are the Woodchopper Canyon, an Early Devonian basaltic seamount, and the White Mountains, an Ordovician volcanic arc terrane capped by Silurian and Devonian carbonate bank deposits. The nearest counterpart of these terranes is the Alexander terrane in southeastern Alaska. The Tintina fault of Mesozoic and Cenozoic age, like the Denali fault, primarily follows old suture zones that separate terranes. Strike slip faulting developed after collision in places where further convergence was oblique to the terrane margins. Where terranes met head‐on, their leading edges lie along a multiple set of high‐angle faults that outline microterranes in accretion zones.
Frequency characteristics of acoustic emission in rocks under uniaxial compression and its relation to the fracturing process to failure
Authors: Ohnaka Mitiyasu , Mogi Kiyoo
Published: February 2008

Acoustic emissions generated during the uniaxial compression of brittle rocks were counted after band‐pass filtering. Throughout the whole process from application of load to failure, the count rate monitored through a low pass window was compared statistically with that monitored through a high pass window. The emission rate monitored through a low pass window increases more rapidly than that monitored through a high pass window as rock approaches failure. Two possible explanations for this effect are (1) generation of larger cracks (or coalescence of cracks into larger sizes) and (2) the relative attenuation of higher‐frequency wave components. This finding is consistent with past observations, i.e., the relative number of large‐amplitude events increases (i.e., a decrease in b value) as rock approaches failure. We suggest that larger cracks tend to generate events with larger amplitudes and containing lower‐frequency components. The frequency characteristics of acoustic emissions are used to characterize the microfracturing processes leading to failure. The beginning of a sequence of acoustic emission activity is discussed in relation to the stress‐strain curve and the onset of dilatancy. The present observations may be utilized in understanding structural instability of a region where rockbursts or earthquakes can potentially occur. If a sizable microseismic population is observed through sensors placed near the fault, then a potential exists for predicting major earthquakes.
Uncertainties in the relative positions of the Australia, Antarctica, Lord Howe, and Pacific Plates since the Late Cretaceous
Authors: Stock Joann , Molnar Peter
Published: February 2008

We determined parameters that describe finite rotations and their uncertainty regions for relative plate motion at the spreading centers between the Pacific and Antarctica plates, between Australia and Antarctica, and between the Lord Howe Rise and Australia. We combined these to yield a range of possible finite rotations describing the relative positions of the Pacific, Australia, Antarctica, and Lord Howe plates since the Late Cretaceous. If the Pacific‐Australia plate boundary has had its present trend since anomaly 18 time, reconstructions show 330±110 km of motion of the Pacific plate relative to the Lord Howe Rise since anomaly 5 time (9.8 m.y.), 420±110 km since anomaly 6 time (19.5 m.y.), 770±330 km since anomaly 13 time (35.6 m.y.), and 820±260 km since anomaly 18 time (43.0 m.y.). We examined two cases for times prior to anomaly 18, assuming a Late Cretaceous age of Australia‐Antarctica separation. If a plate boundary existed between the Lord Howe Rise and Pacific plates since the Late Cretaceous, with no plate boundary in Antarctica, reconstructions with the Lord Howe Rise fixed predict 610 ± 200 km of westward motion of the Pacific plate between the times of anomalies 31 and 22, followed by 260±100 km of northward motion between the times of anomalies 22 and 18. If the Lord Howe Rise was fixed to the Pacific plate until the Eocene, but a plate boundary existed between East and West Antarctica, reconstructions show very little motion across this boundary between the times of anomalies 31 and 22, followed by convergence between the times of anomalies 22 and 18. This second case also brings 70–80 m.y. paleomagnetic poles from the Pacific and East Antarctica plates into better agreement than the first case, but large uncertainties in the reconstructions do not allow the first case to be conclusively eliminated.

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A note on the correction of ocean floor depths for sediment loading
Authors: Le Douaran Sylvie , Parsons Barry
Published: February 2008

The measured physical properties of sediments recovered in a number of deep‐sea drill holes in the North Atlantic have been used to derive expressions for the correction to basement depths required to account for loading by sediments. The average density and porosity over a given thickness of sediment show an approximate linear dependence on sediment thickness. This behavior can be explained as primarily the result of compaction. If the compaction of the sediments is neglected in correcting basement depths for sediment loading, errors of a few hundred meters in the corrected depths can be made. A sediment loading correction based on the observed variation of density with depth can give corrected depths to better than 100 m, with remaining uncertainties due to variability in sediment properties and rebound effects on the measured densities. A relation between sediment thickness and vertical travel time was obtained from sonic velocity measurements on samples from drill holes, sonobuoy wide‐angle reflection experiments, and direct correlation of seismic reflectors with downhole lithology. The sediment loading correction can then be expressed directly in terms of the vertical travel time measured along seismic reflection profiles. The loading correction obtained here for the North Atlantic may not be applicable in areas with different sedimentary environments. When direct measurements of physical properties in drill holes are not available in such areas, it is shown that it is preferable to assume a compaction curve for the appropriate lithology rather than to use observed seismic velocities together with a velocity‐density relationship.
Creep in polycrystalline aggregates by matter transport through a liquid phase
Authors: Raj Rishi
Published: February 2008

Polycrystalline aggregates which contain some liquid in grain interfaces can deform by matter transport, through the liquid phase. If the applied stress is multiaxial, then the deviatoric component of the stress will produce creep, that is, a change in shape of the aggregate without any change in volume, while the mean stress will cause the aggregate to densify. Constitutive equations for creep rate and densification rate are derived. It is shown that the rate may be controlled either by the interface kinetics of dissolution/precipitation of the crystal from the liquid or by the diffusion rate of matter through the liquid. The first process can be related to the kinetics of crystal growth from solution, and the second, to the viscosity of the liquid. The results of the analysis are used to interpret experiments on densification rate of a rock salt polycrystalline aggregate immersed in saturated brine.
The single‐crystal elastic moduli of stishovite
Authors: Weidner Donald J. , Bass Jay D. , Ringwood A. E. , Sinclair William
Published: February 2008

The single‐crystal elastic moduli of stishovite have been determined experimentally from measured Brillouin scattering spectra. They are C11 = 4.53 ± 0.04, C33 = 7.76 ± 0.05; C44 = 2.52 ± 0.02, C66 = 3.02 ± 0.03; and C12 = 2.11 ± 0.05, C13 = 2.03 ± 0.04 in units of mbar. The estimated aggregate (Voigt‐Reuss‐Hill) adiabatic moduli are K = 3.16 ± 0.04 and μ = 2.20 ± .03 mbar. Previous values of the isothermal bulk modulus, obtained from hydrostatic compression experiments, are in agreement with the Reuss value of KT = 3.06 ± .04 mbar (as corrected to isothermal conditions). Taken together, these results suggest that the pressure derivative of the bulk modulus for stishovite is low relative to other rutile structure oxides.