bluuebunny/crossref_metadata_2025 huggingface dataset:
EISCAT observations of topside ionospheric ion outflows during auroral activity: Revisited
Authors: Wahlund J. ‐E. , Opgenoorth H. J. , Häggström I. , Winser K. J. , Jones G. O. L.
Published: February 2008

New EISCAT observations of large field‐aligned bulk ion outflows from the topside ionosphere during auroral activity are presented. The ions (mainly O+) start their outflows from a variable altitude and may reach field‐aligned outward velocities of up to 1500 m s−1 in the altitude region 900–1500 km. The observed ion fluxes are about a factor of 10 larger than previously observed reaching 2×1014 m−2 s−1, and in some cases is nonconstant with altitude. Two different types of ion outflows have been identified. The first type is related to periods of strong perpendicular electric fields, enhanced and anisotropic ion temperatures, and low electron densities below 300 km, indicating small amounts of auroral precipitation. The second type is related to auroral arcs and enhanced electron temperatures. The exact mechanism causing the ion outflows is still not yet understood, but additional mechanisms other than thermal expansion are required to explain the observations presented here.
Plasmasphere refilling: Recent observations and modeling
Authors: Singh N. , Horwitz J. L.
Published: February 2008

The phenomenon of plasmasphere refilling, and general considerations of plasmasphere structure and plasmasphere‐ionosphere coupling, have received increased attention in recent years in terms of both observational considerations and modeling investigations. New ideas arising from both observations and modeling efforts are revealing the refilling region to be rich in intriguing physical processes, including hot‐cold plasma interactions through wave‐particle effects and Coulomb collisions, electrostatic shocks, and differences in light and heavy ion behavior during refilling. “Global” observational and modeling studies of the plasmasphere structure are in the process of demonstrating the complex manner in which refilling and the spatial and temporal variations of convection interplay to produce complex structure in the plasmasphere; these studies are particularly timely as they could be extremely helpful for interpreting results from proposed imaging of plasmaspheric He+. This review will serve to highlight the observational and theoretical/modeling progress during the past few years as well as to introduce and place in context the particular contributions contained in this special section.
A statistical study of Pc 1–2 magnetic pulsations in the equatorial magnetosphere: 2. Wave properties
Authors: Anderson B. J. , Erlandson R. E. , Zanetti L. J.
Published: February 2008

A statistical study of narrowband transverse 0.1‐ to 4.0‐Hz magnetic pulsations, essentially Pc 1–2 (0.1–5.0 Hz), occurring from L=3.5 to L=9, |MLAT| < 16°, and all local times, has been made using data from the AMPTE CCE satellite. This work is reported in a pair of papers of which this is the second. In the first paper (Anderson et al., this issue) the occurrence distributions of Pc 1–2 were reported, and here we present statistical distributions of normalized frequency, X = F/FH+ (F and FH+ are the wave frequency and proton gyrofrequency, respectively); ellipticity, ε, and spectral power. Events occurring from 1000 to 1800 MLT are left‐hand to linearly polarized and exhibited properties corresponding to previous reports. The normalized frequency was found to depend systematically on MLT and L, decreasing from 0.5–0.45 at 1000 MLT to 0.30 at 1800 MLT and increasing from 0.28 at L = 3–5 to 0.39 at L = 8–9 (for 12–15 MLT). The average X dependence on MLT and L can be accounted for by several mechanisms including variations in cold or hot plasma density, He+ concentration, and hot proton parameters T⊥ and T⊥/T∥. A decrease (increase) in T⊥/T∥ with MLT (L) is shown to be necessary, and increases (decreases) in either T⊥ or plasma densities with MLT (L) are also required to reproduce the observed X variation. The role of He+ in causing the X variations is unclear. Events occurring at dawn, 0300–0900 MLT, and L > 7 have not previously been discussed and are found to exhibit remarkable polarization behavior. The A.M. events have the highest X observed in the data base, with X averaging 0.4 to 0.5, but most remarkably, they are linearly polarized at all magnetic latitudes sampled. Given the high normalized frequency, the equatorial linear polarization of the A.M. events cannot be explained in terms of a crossover from left‐ to right‐hand polarization occurring during propagation from low to high magnetic field strengths. Oblique propagation or the effects of multiple reflection through the wave growth region might lead to linear polarizations. The results suggest that a new examination of EMIC wave generation specifically addressing the properties of this A.M. population may be needed.
Solar cycle dependence of spacecraft charging in low Earth orbit
Authors: Frooninckx T. B. , Sojka J. J.
Published: February 2008

Recent experimental evidence has shown that Defense Meteorological Satellite Program (DMSP) polar orbiting spacecraft at 840 km can develop electric potentials as severe as −1430 V while at high magnetic latitudes. To explore this charging region, an analysis of DMSP F6, F7, F8, and F9 satellite precipitating particle and ambient plasma measurements taken during periods of high, medium, and low solar flux is performed. One hundred eighty‐four charging events ranging from −46 to −1430 V are identified, and an extreme solar cycle dependence is found as charging is most frequent and severe during solar minimum. Satellite measurements and time‐dependent ionospheric model (TDIM) output are used to determine the cause of the solar cycle dependence and to characterize the environments which generate and inhibit these potentials. The electron precipitation associated with various DMSP charging levels is analyzed; it is suggested that precipitating electrons as low as 2 to 3 keV may contribute to charging though higher‐energy electrons make greater contributions. Secondary electron production due to incident electrons below 1 keV is shown to inhibit charging. The energetic electron fluxes shown to generate charging do not vary significantly over the solar cycle. Instead, DMSP ambient plasma data and TDIM generated results identify a variation in plasma density over 1 or more orders of magnitude as the cause of the solar cycle dependence, and an ambient plasma density of less than 104 cm−3 is found necessary for significant negative charging (≥100 V) to occur.
Upper limits on spacecraft‐induced ultraviolet emissions from the space shuttle (STS‐61C)
Authors: Morrison D. , Feldman P. D. , Henry R. C.
Published: February 2008

Ultraviolet spacecraft‐induced emissions from low Earth‐orbiting satellites have been reported by several investigators. Several R/Å of ultraviolet emission were observed from the S3‐4 satellite at altitudes between 180 and 250 km and from the Spacelab 1 shuttle mission at an altitude of 250 km. Conway et al. (1987) showed that N2 Lyman‐Birge‐Hopfield (LBH) emissions observed by S3‐4 at night are probably the result of spacecraft interaction with the atmosphere. We have searched for band emission of N2, OH, O2, and NO in nightglow spectra obtained in January 1986 with the Johns Hopkins ultraviolet background experiment (UVX) flown on the space shuttle Columbia (STS‐61C) at an altitude of 330 km. The experiment consisted of two Ebert monochromators spanning the spectral range from 1200 to 1700 Å at 17 Å resolution and from 1600 to 3200 Å at 29 Å resolution. The spectra yield 3σ upper limits for the following total band emission rates: NO δ, 0.6 R; NO γ, 0.7 R; NO β, 3.5 R; O2 Herzberg I, 4.5 R; OH (A ²Σu+ ‐ X²Π) (0,0) and (1,0), 0.1 R, and N2 LBH, 5.3 R. The upper limits for the N2 LBH emissions are consistent with the recent models of spacecraft induced LBH glow of Kofsky (1988), Swenson and Meyerott (1988), and Cuthbertson and Langer (1989) and with a [N2]³ or [N2]²[O] altitude dependence.
Observations of plasma boundaries and phenomena around Mars with Phobos 2
Authors: Sauer K. , Roatsch T. , Motschmann U. , Schwingenschuh K. , Lundin R. , Rosenbauer H. , Livi S.
Published: February 2008

Magnetic field and plasma measurements on board the Soviet spacecraft Phobos 2 have been analyzed during five elliptical orbits around Mars. The existence of at least one separate plasma boundary and an adjacent plasma layer, called the planetopause and the transition region, between the bow shock and the ionopause seems to be a characteristic feature of the solar wind interaction with an almost nonmagnetized planetary ionosphere. It is suggested that the planetopause is a multiple‐ion discontinuity, where a large number of solar wind protons are deflected at an exospheric density ramp. Strong changes in magnetic field and plasma flow direction within the transition region are interpreted as signatures of current sheets or internal shocks. The detected eclipse boundary in the tail is perhaps an elongation of the ionopause found at Venus. New ideas concerning the formation of multiple‐ion flow boundaries by electrostatic plasma‐plasma interaction are discussed. Finally, a remarkable Deimos event has been detected during the fifth orbit. This is explained as an interaction of the subsonic solar wind with a thin cloud of charged dust particles.
Old Pacific crust near Hawaii: A seismic view
Authors: Lindwall Dennis A.
Published: February 2008

An expanding spread profile (ESP) over 80 Ma Pacific Ocean crust near Hawaii was modeled with a one‐dimensional solution for travel times, relative amplitudes, and sometimes frequency content. The resulting profiles of compressional and shear wave velocities and Poisson's ratio are similar to those determined by other ocean crustal studies except that the shear wave Q in the sediments is very low (10) and some unusual high‐frequency, high‐phase‐velocity arrivals require a laminated structure within the mid‐crustal section at about 3 km below the base of the sediment. The laminations are probably alternating layers of gabbro and ultramafic cumulates. The cumulates may be fractionation products from an upper level magma chamber that was present at the time of crustal formation. Reflections which may correspond to these laminations have been observed in reflection records both near this ESP and in the North Atlantic. Comparison of different ophiolite models for ocean crust reveal that the most commonly used velocity‐lithology model is probably in error for the pillow and dike basalts, resulting in an interpretation of overly thick layers in previous studies of refraction lines. The preferred interpretation is that the average total thickness of the pillow and dike sequences is between 1.0 and 1.5 km.
Contamination of basaltic magma by mafic crust at Amboy and Pisgah Craters, Mojave Desert, California
Authors: Glazner Allen F. , Farmer G. Lang , Hughes William T. , Wooden Joseph. L. , Pickthorn William
Published: February 2008

Quaternary alkali basalts from Pisgah Crater and Amboy Crater in southern California exhibit unusual chemical and isotopic variations which probably result from assimilation of mafic crust. Although lavas from both volcanoes are alkali basalts and hawaiites with isotopic and chemical characteristics that are similar to ocean island basalts (OIB) (e.g., εNd=2.6 – 5.9, 87Sr/86Sr = 0.7038 – 0.7049, Hf/Ba=0.014 – 0.017), they display highly correlated and unusual variations in their chemical and isotopic compositions. At each volcano, MgO decreased during the eruptive sequence from ≈8.5 wt % to ≈4.5 wt %. Incompatible elements are positively correlated with MgO and therefore also decreased during the eruptive sequence. Nd, Sr, and Pb isotope ratios correlate strongly with MgO. Compositional and isotopic data cannot be explained by any combination of closed‐system fractionation, partial melting of the mantle, or silicic contamination. These data indicate that the basalts represent mixing between a high‐MgO, high‐εNd component, common to both Pisgah and Amboy craters, with lower‐MgO and ‐εNd components that are unique to each center. The high‐MgO component most likely is a primitive mantle‐derived magma, based in part on its similarity to nearby xenolith‐bearing Quaternary basalts. The low‐MgO components are interpreted to be partial melts of mafic crust. If our model is correct, then both volcanoes evolved from eruption of nearly pure mantle melts early in their history to eruption of nearly pure remelted mafic crust late in their history. The crustal source could not have been underplated Mesozoic or younger oceanic crust, but its age is otherwise unconstrained; nearby Mesozoic gabbros and Proterozoic diabases have appropriate isotopic compositions. The basalt data provide no evidence that ancient enriched lithospheric mantle currently underlies the Mojave Desert. If such mantle was present at any time beneath this region, it must have been removed during one or more of the many tectonic events that affected the Mojave Desert during the Phanerozoic. Regional variability of isotope ratios in basalts is commonly interpreted to reflect variability of the underlying mantle. Data from this study raise the possibility that some of this variability may result from cryptic contamination of OIB‐like basalts by mafic crust.
Eocene plutonic rocks in south central Idaho
Authors: Lewis Reed S. , Kiilsgaard Thor H.
Published: February 2008

Large areas previously thought to be underlain by Cretaceous granodiorite of the Idaho batholith are now known to be underlain by plutonic rocks of the Eocene Challis magmatic episode (50–44 Ma). Recent mapping conducted in south central Idaho as part of the U.S. Geological Survey's Conterminous United States Mineral Assessment Program (CUSMAP) indicates that roughly 30% of the exposed plutonic rocks are part of this younger event. The Eocene plutonic rocks are subdivided into a pink granite suite, consisting of varieties of biotite granite, and a quartz monzodiorite suite, which includes hornblende‐biotite granite, granodiorite, quartz monzodiorite, diorite, and gabbro. The members of each suite form discrete epizonal stocks, and some exposures of pink granite are of batholithic size. Silica content of the quartz monzodiorite suite ranges from 48 to 70%; the rocks vary considerably in texture and mineralogy. The most mafic rocks of the quartz monzodiorite suite are cumulates. These are layered intrusive phases containing pyroxene, olivine, and plagioclase. Other mafic phases include hornblende‐ and pyroxene‐rich diorites, which contain plagioclase feldspar as an interstitial phase and are “cumulates” in the sense that they probably represent rocks that were enriched in early formed mafic minerals by some fractionation process. Stocks of the pink granite suite are more compositionally restricted and range from 70 to 77% in SiO2 content. The quartz monzodiorite suite appears to be older than the pink granite suite, but age data are limited. Field relations, initial 87Sr86Sr ratios, CaO content, and MgO/(MgO + FeO*) ratios demonstrate that the two suites are not related by fractional crystallization. Rather, the respective magmas probably originated from different sources. Large ranges in MgO, Sc, Cr, and Co content in the quartz monzodiorite suite can be explained by crystal‐liquid fractionation of olivine, pyroxene, and hornblende, but variations in composition of parental magmas are also likely. Large variations of Ba content in the pink granite suite are probably a result of removal or accumulation of variable amounts of K‐feldspar. Mixing between the two Eocene suites has not been extensive, but some varieties of hybrid pink granite were generated that contain mafic inclusions and have less than 72% SiO2. The pink granite has geochemical affinities with A‐type granite, but unlike typical A‐type granite, it is not uniformly enriched in the highly charged cations such as Zr, Nb, Y, and Ce. By analogy to other A‐type granites the pink granite suite probably originated from partial melting of lower crustal rocks under high‐temperature, vapor‐absent conditions.
Isostatic compensation of tectonic features of the Mid‐Atlantic Ridge: 25–27°30′S
Authors: Blackman Donna K. , Forsyth Donald W.
Published: February 2008

We investigate the isostatic compensation of tectonic features along the Mid‐Atlantic Ridge 25°–27°30′S through a detailed examination of the relationship between bathymetry and gravity anomalies The study area includes three ridge segments, spreading at an average full rate of 35 mm/yr, their flanks out to about 6 m.y. old crust, and the intervening Rio Grande and Moore fracture zones. In a three‐dimensional analysis of gravity and Sea Beam bathymetry data, we focus on crustal thickness variations and mantle density anomalies by removing from the observed fields the predicted contribution of simple crustal and mantle models. Positive residual gravity anomalies over the northern wall of the Rio Grande fracture zone indicate that in spots the crust is 2–3 km thinner than average there, in contrast to the Moore fracture zone where little thinning is observed. The greater than average depths of the fracture zones as a whole are not locally compensated by thin crust, but the deepest basins within the active transform parts of the fracture zones are partially compensated by thin crust or cooler mantle. A narrow linear ridge that crosses one of the inactive branches of the Moore fracture zone had been suggested to be the product of an episode of excess volcanism but is found to be underlain by thinner, not thicker crust. There is no indication of thicker crust beneath topographic highs at the inside corners of ridge‐transform intersections. Presumably, these highs are dynamically maintained as must be the median valleys characteristic of most the length of the ridge segments judging from the absence of axial residual gravity anomalies. There is a residual gravity low associated with an unusually shallow 15‐ to 20‐km section of the ridge segment between the Rio Grande and Moore offsets where the median valley virtually disappears. Our analysis suggests that while part of the anomalous elevation of this section of the ridge axis is attributable to excess volcanism and a thicker crust, much of the elevation contrast is simply caused by the diminution of the dynamic mechanism responsible for median valley formation. Two‐dimensional Fourier transforms of the gravity and bathymetry fields show that seafloor topography is strongly aligned parallel to either ridges or transforms while density anomalies are more randomly oriented. This implies that the tectonic processes that control the gross ridge‐transform topography are not the dominant control on the magmatic processes that determine the upper mantle and crustal density structure. Statistical analysis of the coherence between the gravity and bathymetry fields indicates that the average, effective, elastic plate thickness in the study area is about 6 km.
Gravity anomalies over extinct spreading centers: A test of gravity models of active centers
Authors: Jonas Jason , Hall Stuart , Casey John F.
Published: February 2008

Gravity fields over active mid‐ocean ridges display long wavelength free‐air gravity highs with superimposed short wavelength anomalies over the spreading axis. These axial anomalies are negative over slow spreading active ridges with amplitudes of typically 30–70 mGal and wavelengths of 40–60 km but decrease in amplitude and wavelength with increasing spreading rate until, at fast spreading centers, the anomalies are positive (10–20 mGal) with wavelengths of about 20–30 km. Similar free air anomalies are observed over extinct spreading ridges and typically show the same paleo‐spreading rate dependency, suggesting that a large component of the anomaly is not dynamically maintained. Calculation and removal of gravitational effects from basement relief results in significant residual gravity lows at the extinct ridges analyzed here, even though the thermal effects associated with active spreading have diminished and no longer contribute to such short wavelength gravity lows. The amplitudes of the residual lows at slow paleo‐spreading rates are 25–45 mGal with half widths of about 20–45 km, and decrease with increasing spreading rate to about 5 mGal amplitude and 10–15 km half‐width over fast paleo‐spreading ridges. The amplitudes and wavelengths of these residual anomalies are nearly identical to those of the residual anomalies observed over active spreading centers. Most previously proposed models of mid‐ocean ridge structure are unable to explain these anomalies and many are inconsistent with a least some aspect of the large body of knowledge currently available for mid‐ocean ridges. This research provides a further constraint on geometric models of accretion, as these models predict vastly different gravity fields. Results indicate that a frozen‐in low density accretionary structure is probably confined to within about 10 km of either side of the extinct axis. This conclusion is consistent with the presence of a low‐density gabbroic root situated at shallow mantle depths beneath the ridge axis. This root tends to narrow with increasing spreading rate, producing the required residual gravity field variation.
Application of a percolation model to flow in fractured hard rocks
Authors: Balberg I. , Berkowitz B. , Drachsler G. E.
Published: February 2008

The “backbone” (i.e., conducting part of the network) of a computer‐generated two‐dimensional percolation model of fractured formations is examined. The lengths of the conducting elements (line segments) are found to follow a power law distribution defined by N1 ∝ 1−1.9, where 1 is the length of the line segment sections, and N1 is the number of elements of length 1 in the backbone. The measured electrical resistivity of the model is found to yield a power law behavior defined by R ∝ (N/Nc − 1)−1.3, where R is the overall resistance of the network, Nc is the threshold for the onset of electrical conduction, and N is the number of line segments in the system. Both the threshold and the electrical resistivity exponent are in agreement with the theoretical values expected for percolation systems in the continuum. Examination of available geological data supports these results, although additional field data are required. The model presented here provides a useful reference for comparison and suggests the nature of geological‐geophysical data that should be obtained in future field studies. An analysis of the effect of a distribution of fracture apertures in the network on its transport properties is also presented. Based on the limited data available, it is suggested that the so‐called universal behavior predicted by percolation theory is in general to be expected for both the fluid hydraulic conductivity and the electrical conductivity in common systems of fractures.
Fractal properties of tremor and gas piston events observed at Kilauea Volcano, Hawaii
Authors: Chouet Bernard , Shaw Herbert R.
Published: February 2008

We study the fractal properties of shallow volcanic tremor and gas piston events associated with magma degassing at Kilauea Volcano, Hawaii, using data from two dense short‐baseline arrays of seismographs deployed near the active crater of Puu Oo on the east rift of the volcano. We found an upper bound on the fractal dimension of a strange attractor common to phase portraits of both tremor and gas piston activities in the range of 3.1–4.5 with a mean of 3.75 over a scale length of the wave field of the order of 100 m. The dimension of the attractor fluctuates over the course of tremor and gas piston episodes within a range comparable to the spatial variations of fractal dimensions found in the wave field. The existence of a categorically stable attractor characterizing both types of activities strongly suggests that the excitation mechanism of tremor is similar to that of gas piston events, which in turn are correlated with visual observations at the volcanic vent. The low value derived for the dimension of the attractor in phase space points to significant self‐organization in the process of generation of tremor and offers general constraints on the dimensionality of attractors derived from models of acoustic emission associated with magma flow, vesiculation, and degassing.
Alteration processes at Deep Sea Drilling Project/Ocean Drilling Program Hole 504B at the Costa Rica Rift: Implications for magnetization of oceanic crust
Authors: Pariso Janet E. , Johnson H. Paul
Published: February 2008

Magnetics properties and oxide petrography results are presented from the most recent penetration at hole 504B during Ocean Drilling Program leg 111. Our results, combined with those from previous studies, show abrupt first‐order changes in magnetic properties at alteration boundaries. Within the 504B crustal section, changes in style and degree of alteration fall near the boundaries of the three well‐defined lithologic units: the extrusive basalts, the transition zone, and the sheeted dike complex. This postemplacement alteration heavily influences magnetic properties and is observed to change, in both style and degree, with depth within each lithologic unit. Our results indicate that, at 504B, the extrusive crustal section deeper than 600 m below seafloor has become magnetically less stable due to postemplacement reheating and alteration. The upper, more permeable basalts above this critical depth have not experienced this reheating. Within the sheeted dike complex, the subsolidus cooling rate and the degree of hydrothermal alteration of the opaque minerals both decrease with depth. The changes in alteration of the oxide minerals occur in parallel with the decrease in bulk permeability associated with increasing depth. Overall, these observations suggest that the effective penetration of water into layer 2C decreased with increasing depth and resulted in a lower rate of convective cooling. Magnetic properties of rock samples from the sheeted dike complex suggest that as a result of this gradient in hydrothermal alteration, the upper dikes have become magnetically more stable than the lower dikes. A review of all magnetic properties indicate that the dike section at 504B carries a lower remanent magnetization than its intrinsic rock magnetic properties and mineralogy would predict. We suggest that this lower remanent magnetization is a result of the long and complex thermal and alteration history which involves the acquisition of magnetic components in different directions. Despite a magnetization which is lower than expected, it appears that the sheeted dike complex at hole 504B is capable of making a substantial contribution to the overlying marine magnetic anomaly. Because of the systematic decrease in hydrothermal alteration of magnetic minerals, the ability of the dike section to contribute decreases as a function of depth.
Rheological nonlinearity and flow instability in the deforming bed mechanism of ice stream motion
Authors: Kamb Barclay
Published: February 2008

Contrary to what has recently been assumed in modeling the proposed deforming bed mechanism for the rapid motion of Antarctic ice streams, the rheology of water saturated till is probably highly nonlinear, according to information from soil mechanics and preliminary experiments on till from the base of Ice Stream B. The equivalent flow law exponent n is probably as high as ∼100, and the nonlinearities of the shear stress and effective pressure dependences are closely linked. The high nonlinearity has important consequences for the deforming bed mechanism. A flow system operating by this mechanism can be unstable as a result of feedback from the generation of basal water by shear heating of basal till. The short‐term feedback effect is analyzed for a perturbation in a model ice stream in which the basal meltwater is transported through a distributed system of narrow gap‐conduits at the ice‐till interface. Although the analysis is approximate and some of the system parameters are poorly known, the results suggest that the deforming bed mechanism is unstable for n > ∼ 20. The apparent lack of such an instability in the currently active ice streams implies that their motion is controlled not by the deforming bed mechanism but by some other as yet unidentified mechanism.
Compressional study of katoite [Ca3Al2(O4H4)3] and grossular garnet
Authors: Olijnyk H. , Paris E. , Geiger C. A. , Lager G. A.
Published: February 2008

End‐member synthetic katoite and grossular were studied by energy‐dispersive X ray diffraction in a diamond anvil cell to 42.3 GPa and 18.4 GPa, respectively. A Birch‐Murnaghan fit to the katoite data gives K0 = 66 ± 4 GPa and K0′ = 4.1 ± 0.5. For grossular we determined K0 = 168 ± 25 GPa with K0′ = 6.2 ± 4. A better constrained fit is obtained using K0 = 168.4 GPa as determined by Brillouin spectroscopy (Bass, 1989) and the present data to give K0′ = 6.1 ± 1.5. The low bulk modulus of katoite is attributed largely to the highly compressible tetrahedral O‐O distances in the structure.
A new look at the U.S. Geological Survey 1970–1980 horizontal crustal deformation data around Hollister, California
Authors: Schneider Dieter , Vaníček Petr
Published: February 2008

A compact mathematical formulation is presented for the estimation of crustal strain and fault displacement parameters from repeated geodetic observations. On the basis of the theory of Hubert space optimization, complex linear forms are used to model spatially and temporally piecewise continuous relative horizontal displacement fields. A mathematical model for the simultaneous network adjustment and strain approximation is elaborated. In an application of the method to the Hollister network reobserved several times during the 1970s, several approximation models are evaluated. The final model, which incorporates a third degree complex algebraic polynomial with four block translation terms in space and a fifth degree algebraic polynomial with three episodic terms in time, is presented in detail. This approximation estimates coseismic fault slip and strain release associated with three moderate earthquakes which occurred in the Hollister area within the time interval spanned by the observations.
The Grenville Front Tectonic Zone: Results from the 1986 Great Lakes Onshore Seismic Wide‐Angle Reflection and Refraction Experiment
Authors: Epili Duryodhan , Mereu Robert F.
Published: February 2008

The Grenville Front, which marks the orogenic boundary between the Archean Superior Structural Province and the much younger Grenville Province to the southeast, is one of the major tectonic features of the Canadian Shield. Within Canada, it is approximately 1900 km in length extending from the north shore of Lake Huron across Ontario and Quebec to Labrador. In 1986, a major coincident onship near‐vertical reflection and onshore wide‐angle reflection/refraction experiment (GLIMPCE–Great Lakes International Multidisciplinary Program on Crustal Evolution) was conducted along a series of lines across the Great lakes. One of the lines, line J, ran across Georgian Bay and Lake Huron for a distance of 350 km and crossed the Grenville Front Tectonic Zone (GFTZ). The seismic signals from the air gun array source were well recorded by the onshore stations up to distances of 250 km with a seismic trace spacing of 50–62.5 m. The GFTZ had a profound effect on the nature of the reflector patterns observed on the onshore seismic sections. Data recorded by the stations on the east end of the line indicate that the crustal P phases are very complex and form a “shinglelike” pattern of reflected waves. Data recorded by stations at the center and at the western end of the line show that the Pg phases are normal and lack the shinglelike appearance. This character of arrivals was also observed on the corresponding S wave sections. A combined P and S wave forward modeling analysis shows that the GFTZ is composed of bands of reflectors dipping at angles of 20°–35° extending to the lower crust. These reflectors were also well imaged on the coincident near‐vertical reflection data. Reflectors under the Britt domain to the east of the GFTZ have a shallower dip than those along the zone. The structure of the crust under the Manitoulin terrane to the west of the GFTZ is laterally homogeneous with a major intracrustal reflector at a depth of 17–20 km below the surface. Poisson's ratio is slightly higher to the east of the Grenville Front compared to the region to the west. The travel times of the PmP signals indicate that the Moho may be deeper under the GFTZ than under the surrounding regions. Our results give added support to tectonic theories that the Grenville Front owes its origin to a continental collision process.
Teleseismic coda analysis with simultaneous modeling of single and multiple scattering
Authors: Soroka William L. , Stump Brian W.
Published: February 2008

Spectra from teleseismic coda waves are used to estimate Earth and source properties through generalized linear inversion (GLI) techniques. High‐frequency teleseismic (66°–98°) P wave codas recorded by the Regional Seismic Test Network (RSTN) are analyzed. The spectra of the teleseismic coda were found to be described by the following two models:(A/t2 + B) exp (−Ct) and (A/t + B) exp ( − Ct). A two‐term model consisting of a time dependent and time independent part were required to fit the single and multiple scattering effects observed in the data. Two near‐surface explosions (eastern Kazakh) of magnitude 6.1 and one earthquake (northern Argentina‐Chile, 559 km depth) of magnitude 5.8 are analyzed to evaluate near‐source scattering. The five three‐component North America RSTN receiving stations are used to evaluate the effect of near‐receiver scattering. Coda Q estimates from the inversions increased with frequency in the approximate range from 800 at 1 Hz to 1600 at 4 Hz. The same range of coda Q values were obtained for both of the models tested; however, the (A/t + B) exp (−Ct) model results had higher variances, suggesting poorer fit of this model to the data. Coda Q estimates are higher at stations RSCP and RSNY compared to the other stations. The coda Q results from the GLI analyses are in agreement with Q values determined from the analysis of local coda. These results suggest that teleseismic coda Q are dominated by near‐receiver scattering effects. The coda analysis method outlined in this paper represents an alternative to normal single scattering analysis methods performed over small time windows early in the coda. This method, which accounts for multiple scattering effects, models the entire coda (500–600 s) where signal is above background noise. Good results were obtained for both earthquake and explosion data. While coda analysis on multiple scattered waves at teleseismic distances was the emphasis in this paper these methods should also work at local and regional distances.
Multicycle slip distribution along a laboratory fault
Authors: King Chi‐Yu
Published: February 2008

Slip distribution along a laboratory fault, which consists of eight spring‐connected blocks that are elastically driven to slide on a frictional surface, has been examined for a “long” sequence of slip events to test the applicability of some conceptual models proposed recently in the literature. The distributions of large slip events are found to be quite variable and do not fit the uniform slip or characteristic earthquake models. The rupture initiation points are usually not near the corresponding maximum slip points, in contrast to observations by Thatcher (1990) and by Fukao and Kikuchi (1987) that earthquake hypocenters are commonly near corresponding regions of maximum slip in the fault planes. This contrast may suggest that either the present observations or theirs are not representative or the teleseismically determined hypocenters may not always be true rupture initiation points as usually assumed. Large slip events are also found to be a stress‐roughening process. They are triggered by some small events after the stresses have been adjusted by some earlier small‐to‐moderate events to be near the critical levels at most locations along the fault. This suggests that earthquake prediction monitoring efforts should not be limited to a small region near an asperity but should be spread out to cover the entire fault segment in a seismic gap in order to detect the condition of simultaneous strain buildup.
Upper mantle anelasticity and tectonic evolution of the western United States from surface wave attenuation
Authors: Al‐Khatib Hanan H. , Mitchell Brian J.
Published: February 2008

Rayleigh wave phase and group velocities and attenuation coefficients in the period range 18 to 120 s have been determined for three regions along nine paths across the western United States using a two‐station technique. The attenuation coefficients were found to increase from east to west between the Rocky Mountains and the Pacific coast. Rayleigh wave group and phase velocities were inverted, using a differential procedure, to obtain shear wave velocity models for three regions. These velocity models were then used in an inversion process where Qβ−1 as a function of depth was obtained from observations of Rayleigh wave attenuation. The inversion results show that Qβ values in the upper mantle of the western United States are lowest for the coastal regions and westernmost Basin and Range and highest for the Rocky Mountains and western Great Plains. Intermediate Qβ values appear to occur in the upper mantle beneath the Basin and Range province and the Columbia and Colorado plateaus, although uncertainties in the data prevent a clear separation between that region and coastal regions. Low values of Qβ occur in the upper crust, higher values in the lower crust, and highest values in the uppermost 20 km of the mantle of all these regions. These overlie an upper mantle low‐Q zone in which Qβ values decrease from east to west, lowest values (15–20) being similar to those observed above descending slabs in the western Pacific. The regional pattern of upper mantle shear wave Q values is similar to that observed for upper crustal shear wave Q values inferred from surface wave studies and for Lg coda Q values. The patterns of upper crustal and upper mantle Q variations correlate with the temporal sequence of tectonic and magmatic activity in the western United States, lowest Q values in both depth ranges occurring where tectonic activity has been most recent. Partial melting and/or enhanced dislocation motion in the mantle and variations in the volume of interstitial hydrothermal fluids in cracks in the upper crust may both be ultimately due to processes which occurred during plate consumption and interaction near the western margin of the United States.
Carbonate stability in the Earth's mantle: A vibrational spectroscopic study of aragonite and dolomite at high pressures and temperatures
Authors: Kraft Susan , Knittle Elise , Williams Quentin
Published: February 2008

The Raman spectra of aragonite and dolomite have been measured to pressures of 23 and 28 GPa, respectively, and the infrared spectra of aragonite measured to 40 GPa. Results have also been obtained probing the spectra of aragonite at high pressures following laser‐heating to temperatures in excess of 2000 K, and of compressed dolomite during external heating to 800 K. Our spectral range, between 100 and 1400 cm−1, encompasses vibrations characteristic of both stretching and bending motions of the carbonate group, as well as lower frequency bands associated with lattice vibrations. The pressure shifts of the aragonite bands vary from −0.3 cm−1/GPa for the carbonate out‐of‐plane bend to 3.4 cm−1/GPa for a lattice mode, while those of dolomite lie between 1.1 cm−1/GPa (in‐plane carbonate bend) and 4.4 cm−1/GPa (lattice mode). Bands associated with vibrations of the carbonate ions have systematically smaller Grüneisen parameters than those associated with cation displacements, and we infer from the mode shifts of these low frequency vibrations that the bulk of the compression of these crystals is produced by the deformation and compaction of the metal ion environment. Indeed, the carbonate group remains stable and relatively undistorted throughout this pressure range, as manifested by the amount of splitting of the carbonate asymmetric stretching vibration (∼90 cm−1) at the highest pressures of these measurements (41 GPa). Neither aragonite nor dolomite are observed to undergo phase transitions over the pressure and temperature range of this study, documenting that these phases are notably stable under compression. Thus, carbon is likely to remain stably bound, possibly within carbonate phases, over a pressure range spanning those present in the upper mantle and shallow lower mantle.
Mapping of low P wave velocity structures in the subducting plate of the central New Hebrides, southwest Pacific
Authors: Prevot R. , Roecker S. W. , Isacks B. L. , Chatelain J. L.
Published: February 2008

Arrival times of compressional (P) and shear (S) waves generated by earthquakes located in the New Hebrides subduction zone and recorded by local and regional arrays of seismographs are used to determine large‐scale one‐ and three‐dimensional elastic wave velocity structures of the subduction zone between 15° and 20°S and from the surface to about 250 km depth. The results obtained from inverting the locally and regionally recorded arrival times individually corroborate each other, and they are inverted jointly in order to improve the resolution of shallow to intermediate depth structures. The results for one‐dimensional structure indicate a gradual increase of velocity with depth until a 9% reversal appears between 60 and 100 km depth. The three‐dimensional structure determined from the joint inversion shows that these low velocities lie within a sizable seismic gap in the descending Benioff zone. Taken with other observations such as the attenuation of high‐frequency shear waves travelling across this gap and the locations of active volcanoes at the surface, we infer that the low‐velocity region represents a thermal anomaly of about 750°C which alters the physical properties of the descending plate in this region. At shallower depths there is evidence of low‐velocity structures included in the descending plate: one beneath north Malekula island and another between Malekula and Efate islands corresponding to the large embayment of the leading edge of the upper plate. The locations of these structures combined with previous investigations of the area lead us to infer that these low velocities are due to the subduction of small‐scale features such as seamounts and accretionary wedges.
Characteristics of shut‐in curves in hydraulic fracturing stress measurements and determination of in situ minimum compressive stress
Authors: Hayashi Kazuo , Haimson Bezalel C.
Published: February 2008

Characteristics of pressure decay curves obtained after shut‐in in hydraulic fracturing stress measurements are studied in detail in an effort to enhance the reliability of the minimum compressive in situ stress determination. The analysis utilizes linear theory of elasticity, fracture mechanics, and global mass balance of fracturing fluid after shut‐in. A small amount of crack growth takes place almost instantaneously just after shut‐in due to equilibration of injected‐fluid pressure within the fracture. Thereafter, the fracture gradually closes commensurate with the amount of fluid leakage into the rock and the net compliance of the pressured system consisting of the rock, the fracture, and the tubing conveying pressurized fluid from the surface to the depth of testing. Theoretical considerations and laboratory and field data suggest the closure process after shut‐in can be considered to consist of three major stages: from cessation of fracture growth until fracture tip closure (stage I), from just after fracture tip closure until complete fracture closure (stage II), and from just after complete fracture closure until the test is stopped (stage III). An analysis of these stages reveals that the inverse of the pressure decrease rate is linear with respect to the fluid pressure in stages I and III. It is also shown that the far‐field minimum compressive stress can be determined on the basis of these characteristics. The method of determination of the in situ minimum compressive stress is successfully applied to a sampling of shut‐in curves obtained in laboratory and field experiments.
Deformation under extension of assemblies of steel balls in contact: Application to sandbox models
Authors: Boerner Sean T. , Sclater John G.
Published: February 2008

Experiments on sandboxes provide a useful tool for understanding the deformation of continental crust. To understand the mechanics at the grain to grain level, we idealized the sand as an assembly of steel balls in contact. We performed a carefully constrained suite of experiments in two and three dimensions by stretching various assemblies of steel balls placed on a rubber sheet attached to moveable boundaries. Though dilation occurred in all the experiments, careful observation supplemented by theoretical treatments of both the two‐ and three‐dimensional cases demonstrated that only the balls which started in a close‐packed or approximately close‐packed hexagonal arrangement deformed by failure along well‐defined slip planes. In the two‐dimensional close‐packed state, preferential dilation gave rise to linear “ghosts” over single gaps or complex arches above the rubber sheet. Dropping of either close‐packed or approximately close‐packed triangular blocks in two or three dimensions created approximately 60° faults, triangular horst, and trapezoidal grabens. Under certain conditions we also observed crosscutting faults, faults which died out along strike, and rotating, internally deforming fault blocks. Our experiments duplicate many of the features of sandbox models. We use the predicted behavior of an idealized arrangement of like spheres to explain the mechanical process by which the steel balls deform and compare our results directly with observations on similar experiments on sandboxes. Though sand grains and steel balls differ in size, unit density, interparticle coefficient of friction, roughness and angularity, we conclude that both the sand and the steel balls dilate as a result of deformation and form triangular fault blocks which move as coherent units. Further, we argue that close packing determines both the appearance and angle of faulting. Sandbox models are used to visually illustrate the deformation of both soft sediments and the much harder continental crust. The implicit assumption is that sand, soft sediments, and continental crust all deform by the same mechanism. We believe that the sandbox models may represent close to true scale models of the upper brittle continental crust if the mechanical process of deformation is the same. In addition, they may provide useful analogues for the deformation of sedimentary sequences. Finally, our model of sand deformation which stresses dilation and pervasive antithetic faulting provides a physical basis to current “pseudo‐continuum” models for determining amounts of extension from fault dips and bed geometries in sedimentary basins.
A damage mechanics model for fault zone friction
Authors: Steacy Sandra J. , Sammis Charles G.
Published: February 2008

A damage mechanics model for shear failure under compressive loading is used to calculate the shear strength of a fault. Based on field studies of the structure of natural fault zones, the distribution of starter flaws (initial damage) is assumed to be fractal with dimension Df = 2.6. For this fractal dimension, the largest flaws dominate the fracture process at low confining stress and the shear strength scales as the inverse square root of the largest flaw size (as in tensile loading). At higher levels of confining stress appropriate to the base of the seismogenic zone, the strength becomes independent of the distribution of flaw sizes and depends only on the density of starter flaws. When the initial damage is sufficiently high, the damage initiation surface coincides with the failure surface, and the fault zone appears to obey the same friction law which controls slip on the individual microfractures. The initial damage corresponding to the fractal distribution of flaws measured in a natural fault zone is large enough for this to occur.