bluuebunny/crossref_metadata_2025 huggingface dataset:
Global disturbance of the transient magnetic field associated with thermospheric storm winds on March 23, 1979
Authors: Fambitakoye O. , Menvielle M. , Mazaudier C.
Published: February 2008

During large magnetic storms, wind disturbances produced by auroral phenomena can affect the whole thermospheric circulation and associated ionospheric dynamo currents for many hours after the end of the storm. This paper presents a morphological analysis of the ground magnetic field disturbance related to the storm winds observed on March 23,1979, the day after the Coordinated Data Analysis Workshop (CDAW) 6 events. The disturbance in the magnetic field variations is observed on a planetary scale, and its pattern does not correspond to that of the regular field variations related to the undisturbed atmospheric dynamo.
Concave‐outward slow shocks in coronal mass ejections
Authors: Steinolfson R. S. , Hundhausen A. J.
Published: February 2008

We consider the formation of slow shocks in a simplified model corona consisting of closed magnetic field lines near the coronal base with overlying open magnetic field lines. An increase in the magnetic field strength in the closed region is used to drive the corona outward, resulting in the generation of slow shocks for a suitable choice of parameters. The nature of the initial corona and the driver cause the field to be deflected around the expanding driver region and thereby produce a slow shock with a geometry that is concave upward (away from the Sun). The fastest‐moving response generated in the corona occurs across nonlinear, fast‐mode simple waves that are expansive over the driver region and compressive at the flanks. A nonlinear intermediate wave forms between the preceding fast wave and the following slow shock. This study not only demonstrates that concave‐upward slow shocks can be formed in a magnetic environment, such as our simple model of the solar corona; it also provides the first evidence that nonlinear manifestations of all three wave modes may occur as the corona adjusts from ambient conditions to those produced by the driver.
Ion acceleration at the contact surface of comet P/halley
Authors: Gurgiolo C. , Winningham J. D.
Published: February 2008

Suprathermal ions form a major component of the plasma within the inner coma of comet P/Halley. A close inspection of these ions shows them to contain at least two separate populations: one population covers the energy range below about 400 eV, and a second population exists in a narrow energy band centered between 3 and 4 keV. Similar distributions are found in two regions upstream of the contact surface. It is postulated that the suprathermal ions observed upstream of the contact surface were created at the contact surface. Sudden movements of the contact surface, in the cometary frame of reference, in response to changes in the magnetic field pressure, or cometary production rate, give rise to an induction electric field able to accelerate the local thermal cometary ions up into the tens of electron volts range. Higher energies can be attained with the reacceleration of these ions in subsequent movements of the contact surface. Besides creating the ion enhancements observed upstream of the contact surface, this acceleration mechanism will release large amounts of suprathermal ions into the inner coma.
Subionospheric VLF “imaging” of lightning‐induced electron precipitation from the magnetosphere
Authors: Inan Umran S. , Knifsend Frank A. , Oh Jane
Published: February 2008

High‐resolution measurements of subionospheric VLF signals at multiple sites are used as a new tool to assess the spatial distribution and occurrence of ionospheric disturbances associated with lightning‐induced electron precipitation. Simultaneous observations in California, Saskatchewan, and Quebec of VLF signals from multiple sources allow the monitoring of event activity over a coarse grid covering the continental United States. Association of the observed VLF signal perturbations with lightning is often made on the basis of time correlation with prominent radio atmospherics. Simultaneous observations of individual events on subionospheric paths that “cross” one another are used to locate the disturbed ionospheric region(s). Absence of perturbations on nearby paths permits assessment of the spatial extent of the region with a varying degree of accuracy, depending on the distribution of signal paths. In one case distinctly different onset delays (with respect to causative discharges) consistent with predictions of whistler‐particle scattering theory were found, and were interpreted as being due to two separate regions separated in L value by ∼0.4 L being excited in individual events. Occurrence statistics over the course of October 1987 exhibit generally higher levels of activity at the lower‐latitude end of the 2 < L < 4 range, although it is difficult to separately assess the role of the source lightning distribution. Occurrence statistics of simultaneous events on crossing paths are consistent with the spatial extent of the disturbed ionospheric regions being less than a few hundred kilometers. The occurrence statistics also suggest that scattering from disturbances located at distances of >100 km off the great circle paths is not significant.
Can ionospheric refraction and oblique reflection explain the Canadian 50 MHz IGY radio aurora observations?
Authors: McDiarmid D. R. , Watermann J. , McNamara A. G.
Published: February 2008

Uspensky and Williams (1988) have recently suggested that the aspect sensitivity of the radio aurora is universally near 10 dB/deg. They have further suggested that the Canadian measurement of 1–2 dB/deg at 48 MHz can be understood by correctly accounting for E region ionospheric refraction and oblique reflection in the case of equatorward directed radio waves. This hypothesis has an attractive simplicity. Moorcroft (1989) has subsequently proposed refraction by tilted layers to explain a number of puzzling features of Canadian Bistatic Auroral Radar System (BARS) data. We have therefore made an examination of the ionospheric conditions necessary to enable refraction/reflection to explain the Canadian International Geophysical Year (IGY) observations and have compared them with those expected in the E region ionosphere. It is established that refraction in extended and uniform E region plasma layers is insufficient to explain these observations using the Uspensky and Williams hypothesis. On the other hand, it may be possible that oblique reflection in a thin, dense, and localized sporadic E structure is sufficient to support the hypothesis. More information about these structures is required before a definitive conclusion can be drawn. The properties necessary for these structures to be consistent with an universal aspect sensitivity of about 10 dB/deg are presented.
Shock heating of the solar wind plasma
Authors: Whang Y. C. , Liu Shaoliang , Burlaga L. F.
Published: February 2008

We present a study of all shocks observed from Pioneers and Voyagers in 1973–1982. The average shock strength increases with the heliocentric distance outside 1 AU, reaches a maximum near 5 AU, and then decreases with the distance. The increase in the entropy of the solar wind protons across shocks also reaches a maximum near 5 AU. When an average shock propagates through the solar wind, the shock heating increases the entropy of the solar wind protons by approximately 0.8×0−23 J/K/proton. We also use plasma data from Voyagers and Pioneers between 1 and 30 AU and data from IMP at 1 AU to calculate the increase in the average entropy of solar wind protons with the heliocentric distance. When the distance increases by a factor of 10, the entropy increases by about 4×10−23 J/K/proton. In order to evaluate the role played by shocks for the heating of the solar wind, we use a MHD simulation model to calculate the entropy changes for the November, 1977 event. Shock heating is the only heating mechanism included in the model. The calculated entropy increase agrees reasonably well with that calculated from observational data. The simulation result suggests that shocks are chiefly responsible for the heating of the solar wind plasma between 1 and 15 AU.
Heating of minor ions by the coronal slow shock
Authors: Whang Y. C. , Zhao Xuepu , Ogilvie K. W.
Published: February 2008

The coronal slow shock has been predicted to exist embedded in large coronal holes at 4–10 solar radii. We use a three‐fluid model to study the jumps in minor ion properties across a slow shock such as the coronal slow shock. We formulate the jump conditions in the de Hoffmann‐Teller frame of reference. The Rankine‐Hugoniot solution determines the MHD flow and the magnetic field across the shocks. For each minor ion species, the fluid equations for the conservation of mass, momentum and energy can be solved to determine the velocity and the temperature of the ions across the shock. We also obtain a similarity solution for heavy ions. The results show that on the downstream side of the slow shock the ion temperatures are nearly proportional to the ion masses for He, O, Si, and Fe in agreement with observed ion temperatures in the inner solar wind. This indicates that the possibly existing coronal slow shock can be responsible for the observed heating of minor ions in the solar wind.
Re‐forming supercritical quasi‐parallel shocks: 2. Mechanism for wave generation and front re‐formation
Authors: Winske D. , Omidi N. , Quest K. B. , Thomas V. A.
Published: February 2008

One‐dimensional hybrid (particle ion, massless fluid electron) simulations of quasi‐parallel shocks and two stream interactions are carried out in order to study the mechanism by which large‐amplitude electromagnetic waves are generated at the shock and maintained in a quasi‐steady manner. It is concluded that a resonant interaction at the interface between the incoming ions and the heated downstream ions (“interface instability”) is the most likely source of the waves that ultimately comprise the quasi‐parallel shock. The effect of the generation of these waves at the shock transition results locally in a nonsteady shock ramp, which propagates downstream with respect to the average shock position and is then replaced by a new steepened ramp at the original front position. This process is complicated by several competing effects. First, backstreaming ions excite the beam cyclotron resonant instability at wavelengths longer than those generated at the shock interface which gives rise to compressive upstream perturbations that are then carried into the shock and interact with the shock‐generated waves. Second, the re‐formation process sometimes leads to the appearance of cold dense ion beams just upstream from the shock which gyrate in the upstream magnetic field to produce local density enhancements that have some effect on where the new shock front forms. Third, whistlers at wavelengths shorter than the waves associated with the re‐formation process are also present that can scatter the backstreaming ions into a hotter, more diffuse population. In order to separate these various processes, numerical experiments involving quasi‐parallel shocks and the interaction of two plasma streams have been carried out for various upstream parameters. In these studies the upstream conditions can be controlled to some extent by eliminating the backstreaming ions and suppressing the shorter wavelength modes, and in the case of the two stream interactions the upstream and downstream plasmas can also be distinguished. The interface instability that results from the interaction of the incident ion stream and a less dense population of heated downstream ions generates intermediate scale (kc/ωi ∼ 1) magnetosonic waves with group velocities pointing upstream, but phase fronts carried downstream, consistent with the waves observed in the shock simulations. A linear analysis indicates the wave numbers of the most unstable modes decrease with increasing upstream θBn and Mach number, in agreement with the simulations. Areas where further work is needed are also discussed.
Height and aspect sensitivity of large aspect angle coherent backscatter at 933 MHz
Authors: Moorcroft D. R. , Schlegel K.
Published: February 2008

During a special experiment in May 1988, coherent echoes were obtained with the UHF EISCAT radar system while the Tromsö antenna was directed to the north at elevation angles between 10° and 13°, with corresponding magnetic aspect angles between 5.8° and 7.6°. The echoes were found to be consistent with scattering from a layer having approximately a Gaussian height profile centered on about 103 km and with an average thickness of about 6 km. This height is considerably lower than most other reports of auroral backscatter height, and the thickness is also smaller than most other reported values. These unusually low heights were confirmed with observations at the remote sites Kiruna and Sodankyla. The heights seem to be independent of aspect angle over the range of aspect angles observed. By modeling the scattering layer and comparing echo intensities at Tromsö and the remote sites, estimates of aspect sensitivity of about 6 dB/deg were obtained, with an indication that the aspect sensitivity decreases with increasing aspect angle.
Low‐frequency ionization‐driven instability of a discrete auroral arc model
Authors: Cornwall John M.
Published: February 2008

The low‐frequency (time scales of tens of seconds) dynamics of the auroral ionosphere differs from that of the nonauroral ionosphere by the presence of strong, unstable space‐ and time‐dependent ionospheric ionization produced by precipitating auroral electrons. If recombination is relatively unimportant (as at high ionospheric heights), we show that in general transport processes cannot remove this ionization as fast as it is created, and no equilibrium is possible. We investigate these nonequilibrium phenomena in the context of a nonlinear adiabatic auroral model, which has previously been studied in static situations. We give a linearized local perturbation analysis of what amounts to a current‐driven E×B gradient‐drift instability with an ionization source, as well as some exact nonlinear solutions valid in a finite but limited spatial region. These solutions show continuing motion of auroral potential and plasma density, as the aurora tries to shift its ionization problems from one place to another. The analysis gives clues to the possibility of generation of chaos and of fine‐scale spatial structure.
Characterization of the dynamic variations of the dayside high‐latitude ionospheric convection reversal boundary and relationship to interplanetary magnetic field orientation
Authors: Ridley A. J. , Clauer C. R.
Published: February 2008

We present initial results from observations of the northern summer polar ionospheric convection reversal boundary using ground‐ and satellite‐based instrumentation. Ionospheric convection measurements obtained using the Sondrestrom radar are used to locate and observe the boundary. The flow around the reversal is compared to three different modeled flow patterns. The first is a shear reversal (oppositely directed flows with no flow across the boundary). The second is a shear reversal combined with uniform poleward flow. The final flow pattern observed is a rotational reversal, where flows gradually turn from one direction to the opposite over distances on the order of 200–400 km. The convection reversals observed were categorized into three different classes: (1) stationary and steady, (2) nonstationary, and (3) oscillating. A stationary and steady boundary remains at the same invariant latitude for long periods of time and demonstrates no observable motion. A nonstationary boundary will propagate northward or southward, generally remaining parallel to a line of invariant latitude. The oscillating reversal boundary (ORB) will have wave‐like motions of the local boundary location. A number of different reversals were classified and then studied further using other instrumentation, which include the Greenland coastal and MAGIC chains of magnetometers, the drift meter and particle precipitation instruments from the DMSP F9, F10, and F11 satellites, and interplanetary magnetic field (IMF) measurements from the IMP 8 satellite. A stationary and steady reversal has been observed during a time of magnitude variations in the By and Bz components of the IMF, but no sign changes in either of the components. The nonstationary reversals have been observed to be a response to both sign changes and magnitude changes with no sign changes in IMF components. Of the two ORBs reported here, one has been determined to be a ULF wave propagating along the convection reversal boundary, which maps to the magnetospheric low latitude boundary layer. The other ORB has little coherence between longitudinally spaced magnetometer stations, implying that the propagating waves are changing form between the stations.
Lithospheric flexure and uplifted atolls
Authors: McNutt Marcia , Menard H. W.
Published: February 2008

Contrary to the expectation that islands should monotonicalIy sink as the aging sea floor thermally contracts, some atolls that were formerly at sea level are now elevated as much as 70 m. A eustatic sea level high cannot explain the magnitude of the larger uplifts or the simultaneous occurrence of elevated and sea level atolls. The observation that recently active volcanoes are found near these raised atolls suggests that they have been tectonically uplifted by the loading effect of the volcanoes. By modeling the oceanic lithosphere as an elastic plate overlying a fluid asthenosphere we can both explain the varying amounts of elevation and bracket the effective flexural rigidity of the oceanic lithosphere between 1.7 and 2.5 ×1022 N m.
Coastal submarine hydrothermal activity off northern Baja California
Authors: Vidal Victor M. V. , Vidal Francisco V. , Isaacs John D. , Young David R.
Published: February 2008

In situ observations of submarine hydrothermal activity have been conducted in Punta Banda, Baja California, Mexico, approximately 400 m from the coast and at a seawater depth of 30 m. The hydrothermal activity occurs within the Agua Bianca Fault, a major transverse structure of Northern Baja California. Hot springwater samples have been collected and analyzed. Marked differences exist between the submarine hot springwater, local land hot springwaters, groundwater, and local seawater. SiO2, HCO3−, Ca, K, Li, B, Ba, Rb, Fe, Mn, As, and Zn are enriched in the submarine hot springwater, while Cl, Na, SO42−, Mg, Cu, Ni, Cd, Cr, and perhaps Pb are depleted in relation to average and local seawater values. Very high temperatures, at the hydrothermal vents, have been recorded (102°C at 4‐atm pressure). Visible gaseous emanations rich in CH4 and N2 coexist with the hydrothermal solutions. Metalliferous deposits, pyrite, have been encountered with high concentrations of Fe, S, Si, Al, Mn, Ca, and the volatile elements As, Hg, Sb, and Tl. X ray dispersive spectrometry (1500‐ppm detection limit), X ray diffraction, and scanning electron microscopy of the isolated metalliferous precipitates indicate that the principal products of precipitation are pyrite and gypsum accompanied by minor amounts of amorphous material containing Si and Al. Chemical analyses and XRD of the reference control rocks of the locality (volcanics) versus the hydrothermally altered rocks indicate that high‐temperature and high‐pressure water‐rock interactions can in part explain the water chemistry characteristics of the submarine hydrothermal waters. Tritium dating of the hydrothermal solutions places them in the prebomb period of 1953 with an undetermined old age. Their long residence time, the occurrence of an extensive marine sedimentary formation, their association with CH4, and their similarities with connate waters of oil and gas fields suggest that another component of their genesis could be in cation exchange reactions within deeply buried sediments of marine origin. The approximate volume discharge of the hydrothermal system has been measured (330,000 m3/yr), and the overall convective heat flux over the mapped submarine hydrothermal area has been calculated, the value of which exceeds 105 times the reported average conductive flux through the sea floor. Our data and observations indicate that metal‐rich marine sediments and marine ore‐forming processes are a concurrent reality and that ores are being formed today in submarine areas of high convective heat flow where hydrothermal activity is the precursor for their occurrence. At the same time the results of our investigations have demonstrated that submarine hydrothermal activity and submarine hot springs exist in the ocean in regions characterized by relative quiescence and that technically ‘active’ ridge environments are not an exclusive prerequisite for their existence.
Convection models having a multiplicity of large horizontal scales
Authors: Richter Frank M. , Daly Stephen F.
Published: February 2008

A distinctive property of plate tectonics as a natural form of convection is the variety of large horizontal scales exhibited by the plates in contrast to the marked regularity of scale seen in other natural convecting systems and laboratory experiments. Two idealized model problems are studied to gain some insight into the physical properties of a convecting layer that can lead to a multiplicity of large horizontal scales. The first model determines the most unstable wavelength for convection in a fluid having anisotropic viscosity. When the resistance to horizontal motions and the associated dissipation is made small by means of the anisotropy, large horizontal scales become most unstable. The second model investigates convection with a horizontal low‐viscosity layer within the body of fluid. The effect of such a low‐viscosity zone is to make a broad band of large wavelengths most unstable. The general conclusion is that convection cells of large horizontal size compared to the layer depth are possible whenever the dissipation associated with horizontal motions is small, regardless of what material property of the system is responsible for this reduction in dissipation. The models suggest that the low‐viscosity zone believed to exist at the base of the lithospheric plates may play an important role in allowing plate tectonics to exhibit a variety of horizontal scales.
Precise relocations of earthquakes and seismotectonics of the New Hebrides Island Arc
Authors: Pascal Georges , Isacks Bryan L. , Barazangi Muawia , Dubois Jacques
Published: February 2008

Earthquakes for the period 1962–1973 are relocated by the method of joint hypocenter determination in order to resolve better the configuration and structure of the inclined seismic zone in the New Hebrides island arc. Twelve new focal mechanism solutions are reported and, together with previously published solutions, are integrated with the new information on the spatial distribution of hypocenters. At intermediate depths the seismic zone has a uniformly steep dip of about 70° and exhibits no resolvable contortions or disruptions along at least 700 km of the subduction zone. The thickness of the zone, about 20 km, may be in part due to seismically active fault zones which cut across a portion of the descending lithosphere. Features associated with the anomalous central region of the arc, where the d'Entrecasteaux fracture zone is being subducted and where the islands of Santo and Malekula appear to be in positions normally occupied by the oceanic trench, include the following: (1) an inclined zone of shallow earthquakes with very much smaller dip than is found elsewhere in the arc, (2) a pronounced gap in seismic activity at depths between about 50 and 120 km, (3) evidence for features in the upper or overthrust plate with trends transverse to the arc and parallel to the east‐west trends of the topographic feature being subducted, including nodal planes of shallow focal mechanism solutions, and (4) two features which appear to coincide with the downdip projection of the northern scarp of the d'Entrecasteaux fracture zone, including a well‐defined boundary between two adjacent zones of plate slippage along the main plate boundary and a faultlike feature in the intermediate depth seismic zone which also has a strike parallel to the fracture zone.

Appendix table and entire article are available on microfiche.Order from American Geophysical Union, 1909 K Street, N.W.,Washington, D. C., 20006. Document J78‐006; $1.00. Payment mustaccompany order.
Rodriguez, Darwin, Amsterdam, ..., A second type of Hotspot Island
Authors: Morgan W. Jason
Published: February 2008

Darwin and Wolf Islands are about 200 km north of the main group of Galapagos Islands and are aligned on a 315° trend. Rodriguez Island is on a long 090° trending ridge about 900 km east of Reunion Island. These trends are very different from the fracture zone or hotspot trends in these areas. The Reykjanes ridge has been interpreted as the result of channeled asthenosphere flow from the Iceland hotspot (which produces new asthenosphere) down the rise axis (where asthenosphere is converted into lithosphere). We propose similar channeling for hotspots not exactly on a rise crest—a ‘pipeline’ from a hotspot to a nearby rise crest, with this second type of island forming at the intersection of the rise and pipeline. Predicted trends for these features may be found by vector addition of the absolute motion of a plate over a hotspot and the relative motion of a rise crest away from a plate.
Mars: Near‐infrared spectral reflectance and compositional implication
Authors: McCord Thomas B. , Clark Roger N. , Huguenin Robert L.
Published: February 2008

Several distinct absorption features, some recognized for the first time, are evident in a newly obtained reflectance spectrum (λλ = 0.62‐2.6 μm, λ/Δλ = 83) of the integral disc of Mars. The effects of Martian atmospheric CO2 have been removed from the spectrum to arrive at a reflectance spectrum that we believe is due mostly to surface material. Absorptions, at 1.22, 1.55, and 2.05 μ are interpreted to indicate the presence of H2O ice plus highly desiccated mineral hydrate, although an H2O ice phase with strongly shifted fundamental frequencies cannot be ruled out. Ferrosilicate and ferric oxide bands near and shortward of 1.0 μm are confirmed. The new observations were made on April 21–23, 1976, universal time, at the 2.24‐m telescope on Mauna Kea, Hawaii, using a newly developed infrared spectrometer.
Simulation gravity modeling to spacecraft‐tracking data: Analysis and application
Authors: Phillips R. J. , Sjogren W. L. , Abbott E. A. , Zisk S. H.
Published: February 2008

It is proposed that line‐of‐sight gravity measurements derived from spacecraft‐tracking data can be used for quantitative subsurface density modeling by suitable orbit simulation procedures. Such an approach avoids complex dynamic reductions and is analogous to the modeling of conventional surface gravity data. This procedure utilizes the vector calculations of a given gravity model in a simplified trajectory integration program that simulates the line‐of‐sight gravity. Solutions from an orbit simulation inversion and a dynamic inversion on Doppler observables compare well (within 1% in mass and size), and the error sources in the simulation approximation are shown to be quite small. An application of this technique is made to lunar crater gravity anomalies by simulating the complete Bouguer correction to several large young lunar craters. It is shown that the craters all have negative Bouguer anomalies.
Faulting caused by groundwater extraction in southcentral Arizona
Authors: Holzer Thomas L. , Davis Stanley N. , Lofgren Ben E.
Published: February 2008

Modern surface faulting, 4.8 km southeast of Picacho, Arizona, has created a scarp ranging from 0.2 to 0.6 m high and approximately 15 km long. The scarp, which has been steadily increasing in height since it began to form in 1961, occurs along the eastern margin of the Eloy‐Picacho subsidence bowl where more than 2.9 m of land subsidence, caused by declines of groundwater levels, occurred from 1934 to 1977. Faulting is concluded to be related to groundwater withdrawal. First, the scarp is restricted to an area underlain by alluvium in which groundwater levels have declined. Second, faulting postdates the beginning of water level declines and associated land subsidence. Third, observed vertical displacements associated with faulting are compatible with results from a model of subsurface faulting in which rupture does not extend beneath the zone affected by stresses related to declines of water levels. The model is based on elastic dislocation theory. And fourth, analysis of levelings of bench marks unaffected by man‐induced subsidence indicates minor regional crustal movements that do not appear to be compatible with the magnitude of fault offset.
Counterclockwise precession of the geomagnetic field vector and westward drift of the non‐dipole field
Authors: Dodson R. E.
Published: February 2008

A prominent feature in the secular variation of the present geomagnetic field is the clockwise precession of the field vector observed in records from several locations. It is now known that this phenomenon results from the slow westward drift of a number of features in the non‐dipole field. Although there is no unambiguous evidence of counterclockwise motion of the present field vector, several examples of such motion have been inferred in the paleomagnetic record. It has been assumed that clockwise precession is the only possible result of westward drift during periods of normal polarity, and that counterclockwise precession is indicative of eastward drift. It will be shown that a westward drifting eccentric dipole, under certain conditions, can generate a counterclockwise precession of the local field vector and virtual geomagnetic pole (VGP). This apparent violation of the Runcorn rule is shown to result from the combination of spherical harmonics of differing degree and from the orientation of the perturbing (time‐varying) component of the field vector with respect to the stationary component.
Strain softening prior to two‐dimensional strike slip earthquakes
Authors: Stuart William D.
Published: February 2008

A model for two‐dimensional strike slip faulting in which the fault friction is displacement softening is analyzed to estimate preearthquake crustal deformation and conditions for an earthquake instability. The friction law is initially displacement hardening but becomes softening after the peak stress is surpassed. The peak stress increases with depth to a maximum before decreasing again. The material surrounding the fault is represented by elastic plates. Several deformation histories are calculated by solving numerically the quasi‐static, nonlinear problem subject to displacement boundary conditions simulating relative plate motion. When the crust is compliant or the fault is rapidly softening, an inertia‐limited instability results. Prior to the instability the point of maximum fault slip rate moves upward toward the greatest peak stress position. The slip causes a rapid increase in shear strain rate at the free surface near the fault trace. Although fault slip is monotonic, the average fault stress reaches a maximum and then decreases before the instability. When instability is not possible, a similar but smoother deformation episode results. Qualitative extrapolation of the computed results suggests that crustal earthquakes may be preceded by accelerating fault slip near the focus, particularly below, and that the enhanced slip rate may cause recognizable strain and tilt anomalies at the free surface.
A mechanical model for thrust sheet deformation at a ramp
Authors: Wiltschko D. V.
Published: February 2008

A theoretical model for deformation of a thrust sheet at a ramp which takes into account (1) viscous deformation within the upper plate, (2) plastic deformation within the fault zone, and (3) gravity shows that the upper plate's resistance to deformation can be an important impediment to thrust sheet movement. Using representative parameters for the Pine Mountain block of Kentucky, Virginia, and Tennessee, upper plate bending resistance is shown to be of the same order as fault zone drag. Gravity can be either an important resisting force or a moderate aid in the motion of the thrust sheet depending on how the paleotopography at the time of thrust sheet motion is reconstructed. Two end‐member cases are considered. (1) In the ‘no erosion’ case the surface of the block mimics the shape of the fault zone. In this case, gravity is the largest resistance to movement, greater than either internal deformation or fault zone drag. (2) In the ‘stacking’ case, erosion is assumed to alter the topographic profile of the block and overlying sheets in such a way as to provide a surface slope in the direction of movement. Gravity in this case aids movement of the Pine Mountain block but cannot provide all the work necessary to move the upper plate at a ramp. A tectonic push is also required. The bending stresses are a significant portion of the total stress field and are responsible for the existence of zones of both potential normal and potential reverse faults in the model. An element of the sheet situated on the lower or upper surface of the block will, in traveling from the bottom to the top of the ramp, experience both normal and reverse faulting regimes. This is perhaps an explanation for the complex association of normal and reverse faults found in some major thrust sheets.
Polymorphism in MnF2 (rutile type) at high pressures
Authors: Yagi Takehiko , Jamieson John C. , Moore Paul B.
Published: February 2008

X ray powder patterns have been obtained on MnF2 at 35 kbar and 200°C. Original rutile type patterns have disappeared completely, and the new patterns may be indexed as tetragonal with a = 5.122 ± 0.002 Å and c = 5.256 ± 0.003 Å. Intensity measurements are best fit by the space group (P42m) but do not preclude other space groups of still lower symmetry. This structure is a distortion of the fluorite type (Fm3m). The density 4.478 is some 14% greater than that of the rutile type.
Magnetic fabric of the Exeter Pluton, New Hampshire
Authors: Birch Francis S.
Published: February 2008

Although it is massive in outcrop and compositionally variable, the Exeter pluton in southeastern New Hampshire has a systematic, penetrative magnetic fabric. The direction of maximum susceptibility is parallel to the long axis of the pluton and to the axial trace of the anticline into which the pluton was emplaced. The direction of minimum susceptibility is normal to the fold surface of the anticline and to the contacts of the pluton. Magnetic foliation is most intense near the margins. The magnetic fabric is due to secondary magnetite and may mimic a petrofabric caused by magma flow or postemplacement strain.
Steam‐water counterflow in porous media
Authors: Schubert Gerald , Straus Joe M.
Published: February 2008

A one‐dimensional model of a counterflowing steam‐water mixture above quiescent water in a porous medium is developed to study two‐phase flow processes in vapor‐dominated geothermal systems. Only if the temperature gradient in the underlying water lies between specified limits is it possible for a counterflowing steam‐water mixture to overlie the water. In general, there is an abrupt jump in the steam fraction and the temperature gradient at the interface between the two‐phase region and the underlying water. These sudden changes in steam fraction and temperature gradient together with the depth profiles of temperature, pressure, steam fraction, and steam flow in the two‐phase region depend on the magnitude of the temperature gradient in the water Γ, the temperature at the interface between the water and the steam‐water mixture Td, and the permeability to thermal conductivity ratio K/k; these dependences are studied quantitatively. For small K/k, the thickness of the two‐phase region is essentially independent of K/k, whereas for large K/k the thickness of the region increases dramatically with K/k. For fixed K/k the thickness of the steam‐water region also increases substantially with decreasing Γ. Very deep regions (kilometers thick) of counterflowing steam and water are possible with physically realistic values of K/k and Γ. Normally, condensation of rising steam occurs in the two‐phase region resulting in a steam flow which decreases with proximity to the surface and a water flow which increases with depth. However, downward flowing water can boil and add steam to the upflow.
Earthquake instability on a strike‐slip fault
Authors: Stuart William D. , Mavko Gerald M.
Published: February 2008

A strain softening model for earthquake instability on a long, vertical, strike‐slip fault is analyzed using continuously distributed screw dislocations. The fault zone constitutive law is represented by a Gaussian‐shaped friction law in which slip softening occurs beyond the peak stress. The peak stress varies as the Gaussian of depth, and the greatest peak stress occurs near the inferred earthquake focus. Material surrounding the fault is modeled by elastic plates with stress‐free surface and bottom; forcing is by a growing displacement applied at the remote plate ends. Inertia‐limited instability (an earthquake analog) occurs when the fault zone weakens with strain faster than the nearby elastic stress can decrease. Thus a ratio of two stiffnesses determines if an instability will occur. Surface strain fields are less uniform for unstable cases than for stable cases.
Earthquake precursory effects due to pore fluid stabilization of a weakening fault zone
Authors: Rice James R. , Rudnicki John W.
Published: February 2008

We report the analysis of two mechanisms by which pore fluids could partially stabilize the earthquake rupture process in natural rock masses. These mechanisms are based on dilatancy strengthening and on the increase of elastic stiffness for undrained as opposed to drained conditions. Both are studied in relation to an inclusion model in which a zone of strain weakening material, possibly representing a highly stressed seismic gap zone, is embedded in nominally elastic surroundings subjected to steadily increasing tectonic stress. Owing to the coupling between deformation and pore fluid diffusion, the inclusion does not exhibit an abrupt rupture instability; rather, a period of self‐driven precursory creep occurs which ultimately accelerates to dynamic instability. The precursory time scale is reported for a wide range of constitutive parameters, including fluid diffusivity, ratio of undrained to drained stiffness, and factors expressive of strain softening and dilatancy. Our conclusions are that the precursory times for a spherical inclusion of 1‐km radius are of the order of 15–240 days for a range of constitutive parameters that we suggest are representative. The predicted times are shorter by a factor of approximately 10 for a flattened ellipsoidal inclusion that we analyze with an 18 : 1 aspect ratio. It is suggested that perhaps only toward the latter part of the precursory period are the effects of accelerating inclusion strain detectable in terms of surface deformation or alteration of transport or seismic properties.
A complete solution of a one‐dimensional propagating fault with nonuniform stress and strength
Authors: Israel Moshe , Nur Amos
Published: February 2008

We have investigated details of the dynamics of faults with strongly variable stress and frictional strength. These factors are in all likelihood responsible for nonuniform fault slip, multiple events, random ground acceleration, and the earthquake frequency‐magnitude relation. In order to obtain a full dynamic solution to greatly heterogeneous faulting we used a one‐dimensional continuum model in which stress and strength varied along the fault. The results show that the process of rupture in a heterogeneous system is closely linked to the stopping phase of the motion, or the healing process of faults. The process, which may involve strong nonlinear effects such as discontinuous motion, cannot be seen in static models and requires a dynamic approach with a numerical scheme which remains stable at high frequencies. We have used the method of characteristics to obtain the results in this paper. We found that heterogeneous rupture can be due to either initially nonuniform stress or spatially nonuniform frictional strength. However, nonuniform stress drop—the difference between tectonic stress and frictional stress—tends to become smoother with increasing slip. After some initial heterogeneous sliding, fault slip in the model becomes uniform and repetitious, a condition which is clearly not observed in nature. Spatially variable strength, however, can produce and maintain a heterogeneous stress drop. We found that when the strength was sufficiently variable from place to place, the final stress was statistically at least as variable as the initial stress. This means that the character of fault activity can remain heterogeneous with time; this would conform to the behavior observed in situ. We conclude therefore that spatially variable fracture energy may in fact be responsible for the heterogeneous nature of earthquake faulting. Nonuniform stress by itself cannot maintain with time the heterogeneous nature of faults. The model is applied to the irregular slip distribution of the Borrego Mountain (Coyote Creek), California, earthquake of April 1968 and reveals links among irregular rupture propagation, multiple events, fault slip, and stress drop.
Systematic inversion of geodetic data in central California
Authors: Thatcher Wayne
Published: February 2008

An extensive set of geodetic observations from the 170‐km‐long central creeping segment of the San Andreas fault is used to constrain the broadscale deformation pattern, determine the distribution of aseismic slip along the fault, and estimate the rate of relative plate motion across central California. Although small but significant departures appear to exist, rigid block displacements across the major faults provide a good first‐order approximation to the deformation, as concluded previously by Savage and Burford. Geodetic measurements of ∼60‐km aperture that cross the fault zone provide a firm lower bound of 33±1 mm/yr for the rate of right‐lateral plate motion parallel to the San Andreas fault system since 1885. Measurements near the fault indicate rates of movement close to but still significantly less than this lower bound rate, ranging from 25±1 mm/yr near San Juan Bautista‐Hollister to 29±1 mm/yr farther southeast on the fault. There is no evidence for changes in the gross movement rate since 1885; in particular, no signficant rate changes spanned the occurrence time of the great 1906 earthquake to the north, indicating that the creeping zone largely isolates the episodic movements occurring on the two adjacent locked sections of the fault. Further refinements in modeling the deformation are provided by making plausible assumptions on the fault geometry of the creep zone and the conditions appropriate for the locked portions of the fault, and using a simple dislocation model, all available data can be inverted simultaneously. Shallow slippage is close to the maximum values of observed surface creep but is smoother as a function of distance along the fault, and rates vary from about 20 to 30 mm/yr. The deep slip rate, the relative plate motion rate for this model geometry, is uniquely determined in the inversion, but its precise value is sensitive to the model assumptions. However, off‐fault data do constrain its value to be no greater than about 45 mm/yr. Although the data do not require it, modest amounts of elastic strain accumulation on the creeping segment of the fault cannot be unambiguously precluded. The difference between the near‐fault and longer‐range movement rates, or equivalently, the deficit between deep and shallow slip rates, is not definitively explained, although it may be due to slip on minor faults or an effect of the locked ends on the fault. These differences suggest that small amounts of deformation occur off the major faults, and although this straining is too faint or inhomogeneous to be resolved with available data, its presence may be reflected in the diffuse regional off‐fault seismicity.
Strike slip faulting in a downward varying crust
Authors: Mahrer Kenneth D. , Nur Amos
Published: February 2008

Both laboratory measurements and in situ velocity data show that (with the exception of anomalous low‐velocity zones) the rigidity μ of the upper crust increases monotonically with depth. Fits to these data show that this variation can be approximated quite reasonably with μ(x) = μ∞(1 ‐ θe −x/x). Best fit to the data gives 0.45 ≤ θ ≤ 0.95 and x0, the characteristic depth, approximately equal to 2.5 km. To determine the effect it produced on faulting characteristics, this rigidity profile was used in conjunction with the theory for a screw dislocation in an inhomogeneous elastic medium (Barnett, 1972). A model for a very long strike slip fault in a crust with a downward varying shear modulus was developed. The free‐surface deformation and fault stress change were calculated for typical faulting events and compared with the uniform half‐space model. Results were obtained for faults which broke the surface and faults which were completely buried. For surface faults the free‐surface displacements and strains were practically identical for both the variable and uniform modulus cases, but the accompanying stress drop for the variable modulus crust was substantially less than the uniform crust. By contrast, for the buried faults the stress changes were practically identical for crustal models, while the free‐surface displacements were substantially larger for the variable modulus model. Further, the corresponding surface strains were amplified by a factor of 2 or more depending on the depth of the source. By a Poisson's ratio effect this amplification would also increase the magnitude of the tilt. These results suggest that the rigidity change with depth will, in general, modify faulting characteristics enough to effect conclusions inferred from data inversions assuming a uniform half space.
Shock wave compression of single‐crystal forsterite
Authors: Jackson Ian , Ahrens Thomas J.
Published: February 2008

Hugoniot equation of state measurements have been performed on pure synthetic single‐crystal forsterite (Mg2SiO4) in the pressure range 70–160 GPa (0.7–1.6 Mbar). These and earlier data for polycrystalline forsterite are compared with theoretical Hugoniots for the assemblages 2MgO (rocksalt) + SiO2 (stishovite) and MgO (rocksalt) + MgSiO3 (perovskite). The densities attained by single‐crystal forsterite at pressures in excess of 120 GPa are greater than those expected in the event of shock‐induced transformation to the isochemical oxide mixture. A similar test of the hypothesis of shock‐induced transformation to the perovskite‐bearing assemblage is sensitive to the choice of MgSiO3 (perovskite) bulk modulus. Recent static compression measurements of Yagi et al. (1978) yield a K0T of 286 GPa (for K0T′ = 5), which, along with other elastic and thermodynamic parameters, suggests that shocked forsterite may be more dense than the perovskite‐bearing assemblage. Crystalline phases of up to 5% greater zeropressure density or equally dense short‐range‐order‐only phases may well be involved. Alternatively, the use of an isentropic bulk modulus of 250 GPa (estimated by Liebermann et al., 1977) for MgSiO3 (perovskite) allows consistency between the data and the calculated MgO + MgSiO3 (perovskite) Hugoniot for a reasonable choice (∼3.8) of K0s′ for the latter phase. The new forsterite data along with high‐pressure Hugoniot data for other olivines and olivinitic rocks define a smooth isobaric variation of Hugoniot density with composition. It is shown that an estimated pyrolite (Ringwood, 1975) Hugoniot density of 5.31 g/cm3 at 120 GPa is ∼2% less dense than inferred from typical lower mantle density profiles.