bluuebunny/crossref_metadata_2025 huggingface dataset:
Brittle failure in the upper mantle during extension of continental lithosphere
Authors: Sawyer Dale S.
Published: April 2008

I present a model for the stress required to extend or stretch continental lithosphere. The lithosphere is considered to have four rheological fields: quartz in the brittle field, quartz in the ductile field, olivine in the brittle field, and olivine in the ductile field. As continental lithosphere extends, the required stress initially decreases due to nearly isothermal thinning of the strong upper mantle (olivine near the brittle‐ductile transition). The stress required to continue extending the crust may then increase due to cooling of the upper mantle. However, the required stress can increase to no more than about 1.3 times the stress required to start extension. This contrasts with an earlier study that required exponentially increasing stress to continue extension. My model improves on this earlier study by considering the effects of brittle failure in the upper mantle. The rheology of the continental lithosphere should be no barrier to its continued extension regardless of the rate of extension.
Radial variations of a satellite orbit due to gravitational errors: Implications for satellite altimetry
Authors: Wagner C. A.
Published: April 2008

The linear perturbations of the radius of a satellite orbit due to the geopotential are derived. From these, estimates are made of the radial orbit error due just to geopotential errors from current models employing only conventional satellite‐tracking data. The estimates show that about an order of magnitude improvement in the satellite geopotential will be necessary to utilize the full accuracy of independent satellite altimeter measurements. The linear perturbations are used to show that (1) a substantial improvement in the low‐degree geopotential is possible from the indirect use of Seasat altimetry at track crossovers, and (2) a separation of “permanent” sea topography and geoid may be possible from direct use of the heights along the track.
The relationship of the degree of interconnection to permeability in fracture networks
Authors: Long Jane C. S. , Witherspoon Paul A.
Published: April 2008

The problem of determining the permeability of a rock mass containing a system of finite fractures is highly dependent on both the degree of interconnection between fractures and the heterogeneity of individual fracture characteristics. This paper examines how degree of interconnection affects both magnitude and nature of the fracture permeability. The interconnection between given fracture sets is a complex function of (1) fracture density, i.e., the number of fractures per unit volume, and (2) the fracture extent or size. Unfortunately, neither the density nor the extent of fractures is easily determined from borehole data. However, fracture frequency can be directly measured in a borehole because it is simply the number of fractures intersected per unit length of the borehole. The frequency is a measure of the product of fracture density and size because the probability of a fracture intersecting a borehole is proportional to this product. The effect of the degree of interconnection was investigated by numerically simulating flow in fracture networks where fracture size and density varied inversely, while the product of these two parameters was held fixed. Directional permeabilities of a number of such networks were determined, and the hydraulic behavior of each fracture system was compared to that of an ideal porous medium. The permeability of the rock matrix between the fractures was assumed to be low enough to be negligible. The results show that as fracture length increases, the degree of interconnection increases. Thus, for a given fracture frequency as measured in a borehole, the permeability of the system increases as fracture length is increased, and density is proportionally decreased. Also, fracture systems with shorter, but more dense fractures, behave less like porous media than do systems with longer but less dense fractures. Knowledge of fracture frequency and orientation alone is inadequate when determining permeability or deciding the important question of whether a given system behaves like a porous medium. For many cases where these parameters can be measured in the borehole it is critical to know fracture length in order to predict the degree of fracture network interconnection. However, for certain cases where fractures are larger than some critical size, it may not be necessary to know fracture lengths exactly.
Compression‐induced microcrack growth in brittle solids: Axial splitting and shear failure
Authors: Horii H. , Nemat‐Nasser S.
Published: April 2008

Micromechanisms of rock failure (axial splitting and shear failure) are examined in light of simple mathematical models motivated by microscopic observations. The elasticity boundary value problem associated with cracks growing from the tips of a model flaw is solved. It is shown that under axial compression, tension cracks nucleate at the tips of the preexisting model flaw, grow with increasing compression, and become parallel to the direction of the maximum far‐field compression. When a lateral compression also exists, the crack growth is stable and stops at some finite crack length. With a small lateral tension, on the other hand, the crack growth becomes unstable after a certain crack length is attained. This is considered to be the fundamental mechanism of axial splitting observed in uniaxially compressed rock specimens. To model the mechanism of shear failure, a row of suitably oriented model flaws is considered and the elasticity boundary value problem associated with the out‐of‐plane crack growth from the tips of the flaws is solved. It is shown that for a certain overall orientation of the flaws the growth of the out‐of‐plane cracks may become unstable, leading to possible macroscopic faulting. On the basis of this model the variations of the “ultimate strength” and the orientation of the overall fault plane with confining pressure are estimated, and the results are compared with published experimental data. In addition, the results of a set of model experiments on plates of Columbia resin CR39 containing preexisting flaws are reported. These experiments are specifically designed in order to show the effect of confining pressure on the crack growth regime. The experiments seem to support qualitatively the analytical results.
Geomagnetic anomaly detected at hydromagnetic wave frequencies
Authors: Meloni A. , Medford L. V. , Lanzerotti L. J.
Published: April 2008

We report the discovery, in northwestern Illinois, of a geomagnetic anomaly, using hydromagnetic wave frequencies as the source spectrum. Three portable magnetometer stations with computer‐compatible digital data acquisition systems were operated in a longitude array at Piano and Ashton, Illinois, and Cascade, Iowa (total separation ∼200 km), in 1981–1982. Analysis of the natural geomagnetic field fluctuations in the hydromagnetic wave regime reveals that the vertical components of the detected fluctuations are essentially 180° out of phase between Plano/Ashton and Cascade for variations with periods ∼30–120 s. The observations can be modeled in terms of a shallow (∼10–20 km) north‐south oriented geomagnetic anomaly of enhanced conductivity located between Ashton and Cascade, approximately parallel to the Mississippi River valley.
Application of two‐dimensional synthetic seismogram modeling to interpretation of wide‐angle seismic data from Wind River, Wyoming
Authors: Wen Jing , McMechan George A.
Published: April 2008

In conjunction with the 1977 COCORP Wind River project, wide‐angle reflection‐refraction data were recorded on a fixed array. The goals of the present study were to model these wide‐angle data to provide independent constraints on the Wind River structure. A common‐receiver profile extracted from these data forms a pseudo split spread with maximum offsets of 13.3 km toward the southwest and 19.1 km toward the northeast. The first 10 s of these data are modeled by iterative fitting of synthetic seismograms computed by asymptotic ray theory. The result is a two‐dimensional velocity model that extends from the center of the Wind River Mountains into the Wind River Basin and from the earth's surface to a depth of 25 km. The major features of the model are (1) surficial sediments that thicken toward the center of the Wind River Basin, (2) Precambrian basement that underlies the sediments and outcrops in the Wind River Mountains, (3) an intrabasement reflector at ∼5 km depth, (4) a low‐velocity region from ∼6 to 15 km depth, (5) a local, high‐reflectivity laminar zone (possibly an intrusive feature) at ∼15 km depth, (6) a second‐low velocity region from ∼17 to 23 km depth, and (7) a laminar transition zone between 23 and 25 km depth that separates the middle crust (average velocity ∼6 km/s) from the lower crust (average velocity ∼7 km/s). This study clearly reveals the major crustal structures. The most interesting result is tentative evidence that they may be secondary local intrusion of high velocity material as well as the primary horizontal thrusting in the Wind River uplift.
Upper crustal stresses and vertical stress migration in eastern Canada
Authors: Hasegawa Henry S. , Adams John , Yamazaki Kensuke
Published: April 2008

Diversified phenomena are contributing to our knowledge of various aspects of the stress field and the origin of the crustal stresses in eastern Canada. Both in situ stress measurements (to 2100 m) and earthquake fault plane solutions in the upper crust (∼5–20 km) indicate that the maximum horizontal stress is greater than the vertical. Absence of earthquakes in the lower crust (>20 km) implies a comparatively lower deviatoric horizontal stress in this layer. Stress directions as determined from both in situ measurements and earthquake fault plane solutions indicate an overprint confined to the ENE octant, which correlates with the direction predicted by modeling of plate tectonic stresses. Thus spreading (Mid‐Atlantic) ridge stress is considered to be one of the important contributors to the stress field in eastern Canada. Viscoelastic relaxation of this stress is considered to enhance the deviatoric stress level in the upper crust, while simultaneously reducing that in the lower crust. The crust acts as a stress guide within which stresses in the upper crust that are lost through brittle failure (i.e., earthquake stress drop) can be replenished from the lower crust; moreover, the directionality predicted by modeling of spreading (Mid‐Atlantic) ridge stress is maintained. Incomplete postglacial rebound also contributes to deviatoric compression. Other possible contributors to the upper crustal stress field are basal drag and membrane stress, but the relative contributions from these sources are not properly understood.
Raman microprobe spectra of naturally shocked microcline feldspars
Authors: Velde B. , Boyer H.
Published: April 2008

Raman microprobe (RMP) spectra were made of naturally shocked (27–32 GPa) microcline from Lake Mistastin which showed the optical characteristics common to shocked minerals, i.e. loss of birefringence, strongly deformed twin planes in still birefringent material, etc. A detailed study of the optical properties of this material has been previously made. RMP spectra of different portions of shocked microcline grains show a progressive change from crystalline organization to a structure which maintains the tetrahedral configurations common to feldspars but lost the overall long‐range order of the crystalline state. The spectra obtained on shocked material are compared with those for thermal glass produced at various pressures and with feldspars containing three types of structural order in the tetrahedral positions (microcline, orthoclase, and sanidine). It is evident that the structure of shocked minerals and their amorphous shock products (diaplectic glass) differ from those of normal minerals and glasses. While there appears to be a preservation of the energetic and structural configuration of silica tetrahedra, there is total disorganization of the arrangement of these tetrahedra. This specific structural state can be used to explain the unusual characteristic of shocked minerals such as density crystals and high‐density amorphous material.
Thermoluminescence measurements on shock‐metamorphosed sandstone and dolomite from Meteor Crater, Arizona: 1. Shock dependence of thermoluminescence properties
Authors: Sutton S. R.
Published: April 2008

The thermoluminescence (TL) properties of shocked Coconino sandstone and Kaibab dolomite from Meteor Crater, Arizona, have been studied. Equivalent dose (ED), a measure of stored TL, decreases with increasing shock (decreasing TL sensitivity) but becomes shock independent at high shock grades. The minimum shock grade at which ED becomes shock independent, the shock threshold for resetting preimpact TL to a negligible level, falls within petrographic shock class 2 as defined by Kieffer and corresponds to an equilibrated state at about 10 GPa and 700°C. Shocked Coconino fragments suitable for evaluation of the formation age of the crater using TL are those shocked in excess of the resetting threshold. The average postimpact temperature of the crater floor, about 190°C based on the IL of samples heated during burial, is consistent with impact energy estimates for the crater.
Microearthquake seismicity and fault plane solutions related to arc‐continent collision in the Eastern Sunda Arc, Indonesia
Authors: McCaffrey Robert , Molnar Peter , Roecker Steven W. , Joyodiwiryo Yoko S.
Published: April 2008

The collision of Australia with the eastern Sunda and Banda arcs of Indonesia represents a modern example of the early stages of arc‐continent collision. In order to obtain a more detailed view of the tectonics of the collision zone, a microearthquake survey was conducted around the Savu Sea, the region that encompasses the transition from subduction of Indian Ocean lithosphere on the west to collision or subduction of Australian continental lithosphere on the east. Intermediate depth earthquakes were concentrated near Pantar Island, the easternmost active volcano of the Sunda arc, and outline a northwest dipping zone that strikes N65°E from 70 to 150 km depth. The seismic zone probably marks the western edge of the collision zone and its more northerly strike, and the orientations of nodal planes from fault plane solutions indicate a 25° bend in the subducted slab to a depth of at least 150 km. Because the convergence rate and the timing of collision from geologic observations on Timor suggest that the part of the slab now at 150 km depth was at the trench when Australia first collided with Timor, we infer that continental crust (or at least rifted continental margin crust) caused the bend in the convergent margin and has been subducted to 150 km depth. Fault plane solutions of several events show nearly vertical nodal planes that trend parallel to the strike of the seismic zone, along which a northwest‐side‐down sense of displacement is indicated: these events are concentrated at the southern (top) side of the seismic zone and suggest that the subducted lithosphere is presently detaching in the 50–100 km depth range beneath the eastern Savu Sea. Fault plane solutions for earthquakes within the upper part of the detaching slab (70–100 km depth) have P and T axes whose trends lie parallel to the strike of the seismic zone and horizontal, southeast trending B axes. These earthquakes are interpreted as due to stresses caused by the bend in the subducting plate.
Seismicity associated with the Christmas 1965 event at Kilauea Volcano
Authors: Bosher Richard , Duennebier Frederick K.
Published: April 2008

A seismo‐volcanic event of unusual intensity and location occurred during a 6‐day period beginning on Christmas Eve 1965 at Kilauea Volcano. This event, centered on the Koae fault zone, was marked by thousands of small earthquakes, extensive ground cracking, harmonic tremor, and a minor eruption on the east rift. The Koae fracture zone lies between the east rift zone and the southwest rift zone of Kilauea, thus separating the south flank from the summit caldera. Historical records reveal a seismic gap between the rift zones. The Christmas 1965 activity filled this gap except near its eastern and western boundaries abutting the east and south‐west rift zones. This event is different from other seismic gap‐filling events in that (1) it was marked by an earthquake swarm rather than a main shock‐aftershock sequence, (2) it was partly volcanic in nature, and (3) it did not occur on a plate boundary. This paper discusses this event with emphasis on its seismic character and its significance in terms of Kilauea tectonics and volcanism.
Shear wave generation of near‐surface spall
Authors: Rinehart Eric
Published: April 2008

Spall, the result of tensional failure of material, has been observed in compressive, explosive surface tests. The configuration of these tests requires the spall mechanism to be different from the classical theory of tensional reflection of a compressive wave from a free surface. The data show that following the initial compression and partial rebound, acceleration wave forms have −1 g dwells followed by an impulsive rejoin which has been identified as spall. The spall appears to be caused by shear waves initiating from the finite edges of the explosive source. The shear waves tend to force the material, both under and beyond the explosive array, upward until the soil fails and falls with a −1 g acceleration. This is a new mechanism for the generation of spall due to explosions and is completely different from the classical theory. The new mechanism applies to the case of surface sources.
Correlation between time dependent variations of Doppler‐determined height and sunspot numbers
Authors: Tscherning C. Christian , Goad Clyde C.
Published: April 2008

Ellipsoidal heightsh′ have been determined using Doppler‐satellite data collected between 1971 and 1982 in nearly 800 points in North America where the heightHabove mean sea level is also known. The approximate geoid heightNwas computed for each point using Rapp's (1978) set of potential coefficients complete to degree and order 180. An estimate of the ellipsoidal heighthwas then obtained usingh=H+N. The differencesh′ ‐h(which numerically were less than 4.0 m) were used to form mean values of height differences for periods from 1 month to 1 year. Yearly mean values, for which the error contribution from the geoid is small owing to the geographical distribution of the points, were then studied with respect to their time dependence. A strong correlation was observed. Yearly mean (h′ ‐h) ≈ 0.95s ‐ 86 (cm), wheresis the mean smoothed sunspot number. Using this relationship as an estimate of the error inh′ for the individual points due to uncompensated ionospheric effects, the standard deviation (1 sigma) of the yearly means of the differences decreased from 55 to 20 cm. A statistical test of the corrected differences showed that they can be represented as white noise at the 95% significance level.
Paleomagnetism of the Late Ordovician Thouars Massif, Vendee Province, France
Authors: Perroud Hervé , Van der Voo Rob
Published: April 2008

A paleomagnetic study of the Late Ordovician Thouars Massif reveals very steep components of magnetization, directed upward and downward, and with thermally discrete blocking temperatures of 550° to 580°C. This characteristic magnetization yields a mean direction with declination/inclination = 340°/−83°, with k = 27 and α95 = 7°, based on 15 sites. The south paleopole is located at 34°N, 5°E. A partial magnetic overprint of Hercynian age is observed in sites close to the South Armorican Shear Zone. The 15 sites are spread over some 20 by 30 km, and the internal consistency of the site‐mean directions indicates that differential tilting of parts of the massif has not occurred. Since it appears to be unlikely that the entire massif was tilted, we infer that the characteristic direction is primary and that the paleopole adequately represents the local magnetic field in the Late Ordovician. The paleopole for the Thouars Massif supports previously obtained results from other Ordovician rocks in Hercynian Europe, but it is the first result to pass modern reliability criteria. The high Ordovician paleolatitudes of the Armorican Massif, other areas in Hercynian Europe, and north‐western Africa, indicate that Gondwana and Armorica were adjacent until Late Ordovician time. Low paleolatitudes for Late Devonian time suggest that Armorica underwent rapid northward motion, at a rate of about 10 cm/yr, during the Silurian and earliest Devonian. Armorica is thought to have collided with North America and the Baltic shield/Russian Platform before or during the Early Devonian, forming the Old Red Continent. The collisions produced the Caledonian and Acadian orogenic belts.
Paleomagnetism of Jurassic rocks in the Western Sierra Nevada Metamoprhic Belt and its bearing on the structural evolution of the Sierra Nevada Block
Authors: Bogen Nicholas L. , Kent Dennis V. , Schweickert Richard A.
Published: April 2008

The western metamorphic belt of the Sierra Nevada consists of two eugeosynclinal terranes separated by the Melones and Sonora faults. Subvertical, bedded Mesozoic volcanic rocks metamorphosed to low greenschist facies predominate to the west, whereas Paleozoic metamorphic rocks of higher grade and greater structural complexity predominate to the east. In order to study the structural development of the faults, 121 samples of basalt and diabase were collected for paleomagnetic analysis from three Jurassic formations, the Logtown Ridge and Penon Blanco formations west of the Melones fault and the Sonora dike swarm to the east of the Sonora fault. A northwesterly, downward directed magnetization occurs in each unit. Three fold tests and a conglomerate test on the two formations west of the faults show that the magnetization is secondary, postdating Nevadan (Late Jurassic) folding and is probably coeval with peak metamorphism. An average of five paleomagnetic poles from the Sierra Nevada, three derived from the secondary magnetizations given herein and two previously published, all of probable Kimmeridgian age, yields λ′ = 67.2°N, ϕ′ = 161.2°E, and α95 = 6.5°. Southeasterly magnetizations also occur in the Logtown Ridge Formation and Sonora dike swarm. Directions from the Sonora dikes are approximately antipodal to the secondary directions and are reversed; magnetizations from the Logtown Ridge Formation yield similar results only if corrected for the tilt of bedding. The Logtown Ridge magnetizations (tilt‐corrected) yield a pole position near to that expected for North America. The data from the Sonora dikes require a tilt correction of 25°–30° toward the south‐southwest about a horizontal axis parallel to the regional structure in order to yield a North American pole position. We conclude that the eastern wall rocks of the Melones and Sonora faults have been rotated 25°–30° in response to Nevadan deformation in contrast to the western wall rocks, which have been rotated about 90°.
Multicomponent magnetizations from the Mississippian Mauch Chunk Formation of the central Appalachians and their tectonic implications
Authors: Kent Dennis V. , Opdyke Neil D.
Published: April 2008

Previous paleomagnetic study of Mississippian red beds of the Mauch Chunk Formation from the central Appalachians reported shallow inclination directions. A positive fold test was obtained after thermal demagnetization, typically to only 550°C. The near‐equatorial position for North America indicated by these results has been used to support the idea of a circa 15° latitudinal tectonic offset of the Acadia displaced terrain in the northern Appalachians. The present study of 153 samples from 29 sites occupied in the Mauch Chunk Formation of eastern and southern Pennsylvania reveals multicomponent magnetizations. A characteristic component (Declination (D) = 161.0°, Inclination (I) = 27.8°, alpha95 (a95) = 7.9°) with unblocking temperatures usually concentrated above 650°C and with dual polarity (10 sites normal, 13 sites reversed) is isolated in 23 sites and passes a fold test at the 99% confidence level. It is interpreted as an early acquired hematitic (detrital or chemical) magnetization of pre‐Alleghanian orogeny and pre‐Kiaman Reverse Polarity Interval age; a southern hemisphere paleolatitude of 15° for the sampling area is indicated. Another component has generally lower unblocking temperatures which, however, can extend up to 650°C or more and therefore is also likely to be carried by hematite. This secondary magnetization (D = 170.6°, I = −5.3°, a95 = 5.0°) isolated in 25 sites is of uniform reverse polarity, and application of fold tests suggests that it is of synfolding origin; a near‐equatorial paleolatitude (3°N) at the time of acquisition is obtained, consistent with Permo‐Carboniferous Kiaman directions from North America. Incomplete removal of the secondary component can explain the previous results from the Mauch Chunk. Comparison of the present paleomagnetic results with those from similar age rocks in the Canadian Maritimes does not support a latitudinal offset of Acadia with respect to interior North America in the Early Carboniferous. In fact, according to these and other paleomagnetic data thought to be representative of this age, the Atlantic‐bordering continents already were near to a Pangea configuration. The Alleghanian and particularly the Hercynian orogenies therefore may not be associated with the closure of a large (latitudinal) ocean later in the Carboniferous and Permian. Alleghanian deformation, however may have resulted in the partial bending of the Pennsylvanian salient on the basis of apparently systematic deviations in paleomagnetic declinations.
A spectral analysis of the Earth's angular momentum budget
Authors: Eubanks T. M. , Steppe J. A. , Dickey J. O. , Callahan P. S.
Published: April 2008

Changes in the circulation of the earth's atmosphere cause fluctuations in the length of day (LOD) by exchanging angular momentum with the surface of the earth. In this paper we examine the earth's polar angular momentum budget using earth rotation data from optical astrometry and lunar laser ranging together with simultaneous estimates of the atmospheric angular momentum Matm derived from meteorological data. The atmospheric excitation can explain most of the observed LOD fluctuations at periods between 700 and 40 days. The observed Matm spectral power is proportional to the inverse frequency squared for periods between 1 year and 3 days and flattens out at higher frequencies. The LOD and Matm spectra are very similar except at periods of <40 days, where the LOD measurement noise dominates. The Matm ‐ LOD coherence is highly significant between periods of 700 and 40 days. There is no evidence for phase shifts between the Matm and LOD data at periods between 600 and 40 days. A broad peak is clearly evident at periods of 40–60 days in the spectra of all three time series. The LOD data have a long‐term drift, not matched by the Matm data, that is probably the result of a core‐mantle angular momentum exchange. There are significant imbalances in the angular momentum budget. Most of the seasonal imbalance can be attributed to the effects of systematic errors in the meteorological data, but some of this discrepancy may be caused by exchanges of angular momentum with the oceans. The semiannual, monthly, and fortnightly tidal variations in the earth rotation were detected, and the observed amplitudes of these variations are consistent with theoretical estimates.
Microearthquakes beneath the Median Valley of the Mid‐Atlantic Ridge near 23°N: Hypocenters and focal mechanisms
Authors: Toomey Douglas R. , Solomon Sean C. , Purdy G. M. , Murray Mark H.
Published: April 2008

We report hypocenters and focal mechanisms of microearthquakes located by an ocean bottom seismic network deployed in the median valley of the Mid‐Atlantic Ridge near 23°N during a 3 week period in early 1982. The network consisted of seven ocean‐bottom hydrophones and three three‐component ocean bottom seismometers. The instrument coordinates were acoustically determined to within 25 m at the 1 standard deviation level of confidence. The hypocentral parameters of the 26 largest microearthquakes are reported; 18 of these events have epicenters arid focal depths which are resolvable to within ±1 km formal error at the 95% confidence level. Microearthquakes occur beneath the inner floor of the median valley and have focal depths generally between 5 and 8 km beneath the seafloor. Composite fault plane solutions for two spatially related groups of microearthquakes beneath the inner floor indicate normal faulting along fault planes that dip at angles of 30° or more; these solutions are similar to the mechanisms of nearby large earthquakes. Microearthquakes also occur beneath the steep eastern inner rift mountains. The rift mountain earthquakes have nominal focal depths of 5–7 km and epicenters as distant as 10–15 km from the center of the median valley, but these hypocenters have larger uncertainties because of the possible effects of large topographic relief and associated lateral heterogeneity in velocity structure. We interpret the depth distribution and source mechanisms of these microearthquakes to indicate that this segment of ridge axis is undergoing brittle failure under extension to a depth of at least 7–8 km. We infer that the entire crustal column has been cooled to temperatures within the brittle field of behavior and that significant time has elapsed since the most recent episode of volcanic or shallow magmatic activity along this ridge segment.
Second‐order moment of central California seismicity, 1969–1982
Authors: Reasenberg Paul
Published: April 2008

The second‐order moment (cross‐correlation function) of earthquakes in the U.S. Geological Survey central California catalog between 1969 and 1982 was calculated with respect to a magnitude thresholdM≥ 4.0 over interevent distances up to 80 km and interevent times up to 320 days. The statistical procedure results in a representation of the spatial‐temporal structure of the catalog associated withM≥ 4.0 earthquakes and is capable of revealing patterns too weak to be detected in the space‐time distribution of seismicity for individual earthquake sequences. A method is introduced for identifying aftershocks based on a physical two‐parameter model of the earthquake interaction process. The results show that the aftershock process dominates the second‐order moment and may even obscure the statistical expression of a precursory process. A concentration of foreshocks within 15 km and 3 days ofM≥ 4.0 main shocks exhibits an apparent migration toward the main shock loci with velocity 2.6–5.3 km/d. This concentration may be related to an observed tendency forM≥ 4.0 events to cluster (auto‐correlate) over this interevent range. With the identified aftershocks removed, the residual catalog is Poissonion in space and time. When twoM≥ 4.0 earthquakes occur within 80 km and 40 days of each other, aftershock productivity appears to be relatively enhanced in the earlier sequence. This suggests that aftershock populations are not solely dependent on their main shocks and that unusually productive aftershock sequences may be predictors of future moderate earthquakes.
Dispersion analysis of velocity and attenuation in Berea sandstone
Authors: Winkler Kenneth W.
Published: April 2008

Ultrasonic velocity and attenuation measurements were made on dry, brine‐ and oil‐saturated Berea sandstone and fused glass beads. The results for fused glass beads are consistent with the predictions of Biot theory. They indicate that as predicted, the Biot absorption/dispersion mechanism shifts to higher frequencies as the fluid viscosity increases. Similar data for Berea sandstone are not consistent with Biot theory, since observed velocities are generally higher than predicted. Using the Biot theory, we calculate low‐ and high‐frequency velocities for the liquid‐saturated samples. “Biot dispersion” is then defined as the percent difference between the low‐ and high‐frequency limits. “Apparent dispersion” is defined as the percent difference between the measured ultrasonic velocity and the low‐frequency Biot limit. Comparison of these two measures of dispersion gives insight into the presence of a non‐Biot absorption/dispersion mechanism. Whenever the apparent dispersion is larger than the Biot dispersion, the extra dispersion is interpreted as being caused by a local flow relaxation. To be consistent with attenuation data, this relaxation must be distributed over at least five to six decades in frequency.
Stress and subsidence resulting from subsurface fluid withdrawal in the epicentral region of the 1983 Coalinga Earthquake
Authors: Segall Paul
Published: April 2008

The proximity of the May 2, 1983, Coalinga earthquake to active oil fields on Anticline Ridge led to speculation that the earthquake might have been triggered by oil field operations. Elsewhere, earthquakes have been associated with pore pressure increases resulting from fluid injection and also with subsidence resulting from fluid extraction. Simple calculations show that shales, which underlie the oil producing strata, hydraulically isolate the oil field from the earthquake focal region. The large volumes of fluid extracted from the oil fields caused a 50% decline in reservoir pressures from 1938 to 1983. These observations independently rule out substantial increases in pore pressure at focal depths due to fluid injection. A theoretical method, based on Biot's constitutive theory for fluid‐infiltrated elastic media, is used to evaluate the change in stresses acting in the focal region resulting from fluid extraction in the overlying oil fields. As an independent check on the method, the subsidence of the earth's surface in response to fluid withdrawal is calculated and compared with measured elevation changes of Anticline Ridge. The producing horizons are taken to be horizontal permeable layers, bounded above and below by impermeable horizons. Strains within the producing layers are related to extraction‐induced changes in pore fluid mass. Contraction of the producing layers causes the free surface to subside and strains the elastic surroundings. The calculated subsidence rate of Anticline Ridge between 1933 and 1972 is 3 mm/yr, in good agreement with the measured subsidence rate of 3.3±0.7 mm/yr. Calculated pore pressure changes in the deepest producing zone also compare well with observed changes in reservoir pressure. Although the shear stresses induced by extraction favor reverse slip on either the northeast or southwest dipping nodal plane, the induced normal stresses are compressive, inhibiting fault slip. The driving stress (shear stress minus frictional resistance) acting across the northeast dipping plane increased by 0.01 MPa (0.1 bar) between 4 and 9 km depth, weakly favoring slip, and decreased by half that amount at depths of 9 to 11 km, weakly inhibiting slip. The driving stress on the southwest dipping plane increased by 0.02 MPa (0.2 bar) at 10 km, slightly favoring slip. The sign and magnitude of the pore pressure and stress changes at hypocentral depths do not support the hypothesis that the earthquake was induced, although knowledge of the rate of tectonic stress accumulation is required to assess properly the significance of the extraction‐induced stresses.
In situ stress estimates from hydraulic fracturing and direct observation of crack orientation
Authors: Warren William E. , Smith Carl W.
Published: April 2008

Estimates of in situ stress in G Tunnel, Rainier Mesa, Nevada Test Site, have been obtained with hydraulic fracturing techniques. This work represents a nontraditional use of hydraulic fracturing in that it was performed primarily in horizontal boreholes drilled into the formation from access drifts tunneled into the mesa rather than the usual operation performed at depth in vertical boreholes drilled from the surface. Several operations were performed in essentially orthogonal triads of boreholes located at a point. A significant feature of this work is the mineback operation in which the borehole is mined out to reveal the actual fracture. During the hydraulic fracturing operation, colored dye was added to the fracturing fluid which left an easily detectable stain on the fracture surface. Direct observation of the fracture orientation away from the borehole establishes the direction of the minimum compressive in situ stress and the plane of the other two principal stresses. In one borehole, which was not aligned along a principal stress direction, a modified mineback operation revealed a fracture plane that twisted as it grew away from the borehole, aligning itself finally in a plane determined by the in situ stresses. This twisting is consistent with theoretical predictions of borehole stresses under these conditions. This same mineback also showed that fracture of the formation initiated at or under one of the packers, indicating that the packers may have an effet on in situ stress estimates. General observations of over 100 tests performed in G Tunnel show that under the sloping portion of the mesa, fracture planes are not vertical but dip in a direction tending to parallel the mesa slope. Deep into the tunnel and well under the flat part of the mesa, fractures are essentially vertical with strikes approximately N45°E, which substantiates the usual vertical fracture assumption in traditional fracture operations performed at depth in vertical boreholes. Advantages, limitations, and problem areas associated with extracting in situ stress fields from hydraulic fracture pressure records are discussed in detail.
Localization of dilatancy in Ohshima granite under constant uniaxial stress
Authors: Yanagidani Takashi , Ehara Shoji , Nishizawa Osamu , Kusunose Kinichiro , Terada Makoto
Published: April 2008

The localization of dilatancy during creep of Ohshima granite under uniaxial compression was observed. Hypocenters of 3933 acoustic emission (AE) events were accurately located. It was found that the mechanical behavior of Ohshima granite was controlled by the localization of microcracks. During the stage of loading up to the creep stress, which is 83% of the average short‐term fracture strength, the hypocenters of AE events were randomly distributed throughout the specimen. As soon as the primary creep began, abrupt migration and clustering of AE hypocenters into several near‐surface zones were observed. AE events formed volumetric concentrations. This migration and clustering strongly suggested the rapid localized development of dilatancy at the very beginning of the primary creep stage. The distribution of AE hypocenters observed in this stage was unchanged until final faulting. By the end of the primary creep stage, AE events began to concentrate into one of these clusters, while the activities of other clusters gradually decayed. This change spread broadly and continuously in time during the creep. In the most active cluster, clustering of AE events by itself gave rise to more AE events. The shape of this cluster was spheroidal with the long axis parallel to the loading axis. No evidence directly related to planar focusing of dilatancy was found. Surface strains were mapped. The axial strain distributions in the loading interval showed that the state of stress within the sample was homogeneous. A large change in both axial and circumferential strain fields occurred during the early stage of the primary creep. After this drastic change, the pattern of strain distribution remained unchanged in the subsequent stage of the creep. The accelerated increase in one of the circumferential strain gauges during the tertiary creep stage showed strongly localized deformation preceding faulting. The development of localized dilatancy identified by hypocenter locations was confirmed by the surface strain mapping. The position where the anomalous acceleration in circumferential strain was observed was close to the active, remaining cluster. The migration and clustering of AE hypocenters to the circumferential surface at the beginning of primary creep in the uniaxial unjacketed specimen were explained in terms of stress corrosion. Since the surface is the most favorable for accessing atmospheric moisture, stress corrosion in the vicinity of the surface was facilitated.
Surface characteristics of Venus derived from Pioneer Venus altimetry, roughness, and reflectivity measurements
Authors: Head James W. , Peterfreund Alan R. , Garvin James B. , Zisk Stanley H.
Published: April 2008

The three primary data sets for the Pioneer Venus orbiter radar experiment (topography, roughness, and reflectivity) contain important information about the geological and textural characteristics of the surface of Venus. We have subdivided the range of roughness and reflectivity values into three categories as follows: roughness, in degrees rms slope: relatively smooth (1°–2.5°), transitional from smooth to rough (2.5°–5°), and relatively rough (>5°); and Fresnel reflectivity: surfaces dominated by soil or porous material (<0.1), surfaces dominated by rock material (0.1–0.2), and surfaces with a significant percentage of anomalously high dielectric material (>0.2). We have analyzed each of these data sets and their relationships to each other in order to define areas of the surface that are characterized by distinctive properties (e.g., rough rocky surfaces, smooth soil surfaces). We then describe the abundance and areal distribution of such areas and locally calibrate the geological significance of some of the surface types by examining high‐resolution images from spacecraft and earth‐based observatories. We find that the majority of Venus is covered by regionally contiguous rock and bedrock surfaces. Many of the smooth surfaces we interpret to be of volcanic origin, most likely lava flows, while rougher surfaces are locally characterized by tectonic deformation of several types. Soil surfaces cover less than about 27% of the planet and are generally patchy in their distribution. On the basis of the distribution of these surfaces we see no evidence for the extensive preservation of an ancient global regolith or for widespread, topographically controlled erosion, lateral transport, and sedimentation. The small percentage of the surface of Venus characterized by high‐dielectric material appears to originate from several processes including primary lava flows probably containing enrichments of high‐dielectric materials, such as metal or metal oxides (e.g., Theia Mons in Beta Regio), and exposure of high‐dielectric materials by tectonic deformation (e.g., Maxwell Montes in Ishtar Terra). These global data set correlations provide a fundamental frame‐work for understanding the nature of the surface of Venus and will permit extrapolation of local and regional findings from future geochemical and imaging experiments to a global context.
MIneralogic variation in a layered ultramafic cumulate sequence at the North Arm Mountain Massif, Bay of Islands ophiolite, Newfoundland
Authors: Komor Stephen C. , Elthon Don , Casey John F.
Published: April 2008

In order to investigate magmatic processes operative within magma chambers underlying oceanic spreading centers, a suite of 59 samples was collected over an 8‐m stratigraphic interval at the base of the cumulate ultramafic unit exposed on the North Arm Mountain Massif in the Bay of Islands ophiolite complex, Newfoundland. Interlayered dunites and wehrlites are present in the 8‐m section. Low abundances of incompatible elements (Ti, Y, Zr) and other evidence suggest that these rocks originally formed as adcumulates. Olivine compositions are in the range Fo84.5–89.9 and clinopyroxene Mg # (= (Mg/Mg+Fe2+) × 100) range from 89.7 to 93.3. Spinel compositions are highly variable, and Mg # and Al2O3 contents of spinels have been modified by subsolidus reequlibration with olivine and clinopyroxene. Cryptic variation patterns defined by the Mg # of olivine and clinopyroxene are characterized by successive cryptic regressions that have a repeat interval averaging ∼1 m. The cryptic regressions are believed to record influxes of fresh, relatively unfractionated magma into a magma chamber or conduit. The gradual decrease in the Mg # of olivine and clinopyroxene and the decrease in olivine Ni contents between successive cryptic regressions indicate that ∼10% crystallization of each batch of magma occurred before a new batch of magma entered the crystallization site of these cumulate rocks. In some cases, the cryptic regressions in mineral chemistry correlate with abrupt lithologic changes (from wehrlite to dunite), suggesting rapid displacement of the fractionated magma by more primitive incoming magma or rapid mixing of fresh incoming magma and resident magma. In other cases, inverse fractionation trends and fine scale interlayering of the cumulate rocks appear to represent zones of less rapid mixing between fractionated and fresh magmas. Comparison of cryptic variation patterns at the base and the top of the cumulate ultramafic section on North Arm Mountain indicates that in some cases the liquids that crystallized to form the upper part of the unit were more primitive (higher MgO/FeO) than the liquids that crystallized to form the base of the unit. This feature is an indication of the complexity of processes that may operate in magma chambers underlying present‐day oceanic spreading centers.
True polar wander: An analysis of Cenozoic and Mesozoic paleomagnetic poles
Authors: Andrews Jean A.
Published: April 2008

A true polar wander path is presented for the past 180 Ma tracing the relative motion between the hotspot framework and the geomagnetic poles deduced from worldwide paleomagnetic data. A relative displacement of 22°±10° is observed. The motion between the two reference frames has not been smooth. Episodes of rapid motion, on the order of 80–100 mm/yr, include the present, 65–50 Ma, 115–85 Ma, and 180–160 Ma. Periods of slow or little polar motion, less than 20 mm/yr, are observed at 50–5 Ma and 160–115 Ma. The effect of plate motions is removed using Morgan's plate motion model A19. This model is tied to a hotspot reference frame assumed to be fixed in the mantle. The rotated paleopoles are grouped by ages and then averaged. They are found to cluster about points distinct from the north pole as defined by the hotspots. These points trace out a path of motion between the rotation axis and the hotspots or mantle. This is interpreted as a shifting of the entire earth in response to a change in the principal axes of the moment of inertia of the mantle. Paleomagnetic poles of the past 7 m.y. are examined independently. This eliminates a dependence on the fixed hotspot hypothesis, since plate motions are essentially negligible for this time span. There is a deviation of 5°±2° between the present geographic north pole and the average of the paleopoles at 5 Ma. This is too great to attribute to plate motion. Interpreted as true polar wander, the rate of this polar motion is about 1°/m.y. Astronomical measurements for the past 80 years have recorded actual motion of the earth's rotational axis up to 1°/m.y. in the direction of eastern Canada. This agrees in rate with paleomagnetic polar motion of the past 7 m.y. and in both rate and direction for the past 2 m.y.