bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamical polar wind and its response to kinetic ion heating
Authors: Chen Margaret W. , Ashour‐Abdalla Maha , Holzer Thomas E.
Published: February 2008

The dynamic polar wind and its response to kinetic ion heating are examined in this paper. We used a time‐dependent hydrodynamic polar wind code to study the dynamic flow characteristics of an H+ and O+ polar wind from 200 km to ∼6 ‐ 7 RE. We found that the polar wind ion relaxation times are comparable to the time required for the ions to propagate at the ion speed of sound over the altitudinal range considered. Although there may be large transient O+ fluxes lasting ∼15 min during the early stage of expansion of the unheated polar wind, these fluxes cannot be sustained because of the large O+ gravitational potential. We subsequently studied how polar wind ion heating affects the ion outflow. Our previous work showed that O+ and H+ polar wind ions can become heated due to ion beam instabilities in the polar cap region. We incorporate these kinetic effects into our hydrodynamic model by specifying ion temperature profiles in the ion momentum equations. Two limiting cases were considered, (1) preferential O+ and (2) preferential H+ ion heating. We found that ion heating does not have much effect on the escape of H+ ions because of the limiting nature of the H+ escape fluxes. However, O+ heating can substantially increase the upward O+ pressure gradient in the heating region. This leads to increased O+ ion escape from the polar ionosphere. Thus O+ heating is a viable explanation for the unexpectedly large O+ fluxes which are frequently observed in the polar magnetosphere.
ISEE 1 and 2 observations of the high beta shock
Authors: Farris M. H. , Russell C. T. , Thomsen M. F. , Gosling J. T.
Published: February 2008

Measurements from the ISEE 1 and ISEE 2 spacecraft are used to examine the terrestrial bow shock under high beta conditions. On November 4, 1979, the large separation between the spacecraft, over 2500 km along the shock normal, was such to allow simultaneous upstream and downstream measurements of the solar wind plasma for a period of about 8 min. These measurements are compared with and found to be in agreement with the predicted values of the Rankine‐Hugoniot relations using the simple adiabatic approximation and a ratio of specific heats, γ, of 5/3. No additional terms, such as the heat flux, are needed to bring the measured and predicted values into agreement. Large magnetic field and density fluctuations were observed, but average downstream plasma conditions well away from the shock were relatively steady, near the predicted Rankine‐Hugoniot values. The magnetic disturbances persisted well downstream and a hot, dense ion beam was detected leaking from the downstream region of the shock. Both of these phenomena can be attributed to the excitation of the Alfvén ion cyclotron instability, which acts quickly to isotropize the ions close to the ramp of the shock. Additional shock crossings under high beta conditions having smaller separations allow us to confirm many of these observations.
Plasma transport in the equatorial ionosphere during the Great Magnetic Storm of March 1989
Authors: Rasmussen C. E. , Greenspan M. E.
Published: February 2008

We have modeled plasma transport in the low‐latitude and equatorial ionosphere during the great magnetic storm of March 1989. Our goal was to provide a consistent explanation for the DMSP (Defense Meteorological Satellite Program) observations of dramatic decreases in ion density and rapid ion drifts in the low latitude ionosphere over South America during the storm. The modeling effort supports the hypothesis that abnormally large upward drifts lifted F region plasma above the satellite's altitude and created the density depletions observed by DMSP. Modeled O+ densities at the satellite's altitude have a strong qualitative resemblance to DMSP observations. Both the model and the observations indicate a deep density trough with extremely sharp boundaries surrounding the equator. The widths of both the modeled and the observed equatorial troughs increase with time. Vertical ion drifts predicted by the model also have been compared with DMSP measurements. Like the observed vertical drifts, the modeled drifts reversed sign near the trough boundaries. The modeled vertical drifts are of the same order and direction as the vertical component of E × B convection near the equator, but of opposite direction (downward) near the trough boundaries and outside of the trough.
Remote sensing of two‐dimensional magnetopause structures
Authors: Walthour D. W. , Sonnerup B. U. Ö , Paschmann G. , Lühr H. , Klumpar D. , Potemra T.
Published: February 2008

A technique is developed for analyzing remote measurements made by a single spacecraft of two dimensional disturbances in the ambient magnetosheath or magnetospheric magnetic field, caused, for example, by flux transfer events or pressure pulses. The methodology is based on a recent linear theory for isentropic field‐aligned MHD flow over gently sloping two‐dimensional obstacles (Sonnerup et al., 1992). Using only magnetic field measurements, the analysis technique can provide information about the orientation and actual cross‐sectional shape of the event, as well as information about the spacecraft trajectory relative to the bulge. If three‐dimensional plasma velocity measurements are also available, the technique provides the velocity and size of the event as well, and it allows one to determine whether the current sources causing the disturbance were in fact located on the side of the spacecraft trajectory facing the magnetopause. Analysis of two sample events, one recorded by the spacecraft AMPTE/IRM (Active Magnetospheric Particle Tracer Explorer/Ion Release Module) in the magnetosheath and the other by AMPTE/CCE (Charge Composition Explorer) in the magnetosphere, indicates that the bulges on the magnetopause surface causing the magnetic field and flow perturbations for these events did not have the semicircular cross section suggested in previous work; instead they had a more elongated shape, the dimension tangential to the magnetopause being substantially larger than that normal to it. The calculated invariant axes of the two events were found to differ substantially from the corresponding minimum variance directions of the measured magnetic field. The IRM event was found to move at a speed of 227 km/s away from the subsolar region. For the CCE event, plasma flow data were not available, but it was deduced indirectly that the event moved at a speed of about 90 km/s, presumably away from the subsolar region.
First observations of Trimpi Events at Durban (L = 1.69) using an OMSKI Receiver
Authors: Friedel R. H. W. , Hughes A. R. W. , Dowden R. L. , Adams C. D. D.
Published: February 2008

We report observations of transient ionospheric disturbances at VLF reflection heights, manifested as perturbations on sub‐ionospherically propagating VLF transmitter signals: “Trimpi events.” Four VLF transmitter signals were monitored at Durban using an OMSKI receiver. Both traditional Trimpi events and a new “dome” shaped event were observed at Durban. Occurrence of Trimpis at low L values can be interpreted as the precipitation of very high energy electrons which are capable of producing events during daylight, or as evidence of the “fast” Trimpi mechanism which could dominate at low L. The average winter event rate observed at Durban was quite low (7 per day on four transmitter signals) but during one very active period on the NAA‐Durban signal 88 Trimpi events were observed in 2 hours. This activity was linked to an isolated thunderstorm occurring over the signal path over South Africa.
An alternative look at the aspect sensitivity of auroral radar backscatter: A forward scatter effect
Authors: Uspensky M. V. , Kustov A. V. , Sofko G. J. , Koehler J. A. , Foster John C.
Published: February 2008

A simple model for diffuse forward scatter is presented in order to account for the aspect angle dependence of auroral radar backscatter over a wide range of aspect angles as can be observed by UHF radars. The basic assumption is that the backscatter itself is highly anisotropic but that the incident rays are partially scattered by quasi‐isotropic small to medium scale irregularities in the bottom/top part of the E/D regions. The model is developed to provide a theoretical forward scatter/aspect angle relationship, which is compared with the observations. The model exhibits reasonable agreement with recent 440‐MHz Millstone Hill radar observations over aspect angles ranging between 0 and 10 deg. Other supporting evidence is also presented.
On the latitude and solar cycle dependence of the interplanetary magnetic field strength
Authors: Wang Y. ‐M.
Published: February 2008

Using 20 years of solar magnetograph and in‐ecliptic interplanetary magnetic field (IMF) measurements, we verify that the radial IMF component (Br) can be approximated by a superposition of the Sun's inclined magnetic dipole moment and a current sheet normal to the dipole axis. The net field is found empirically to be ∼3 times stronger along the dipole axis than near the current sheet, whose effect is to redistribute flux toward the dipole equator. The radial IMF intensity at a given latitude and phase of the sunspot cycle is determined by the changing strength and inclination of the dipole, which attains its maximum amplitude when its axis is aligned with the solar poles near sunspot minimum. The model predicts that over the sunspot cycle, |Br| should undergo the least variation near the heliographic equator and the greatest variation above the Sun's poles, where it decreases by a factor of 10 between sunspot minimum and sunspot maximum. The latitudinal gradients in Br are expected to be steepest near sunspot minimum and flattest near maximum. The model suggests that Ulysses will encounter very strong fields when it flies over the solar poles during the declining phase of sunspot cycle 22.
The radial evolution of the amplitudes of “dissipationless” turbulent solar wind fluctuations
Authors: Verma M. K. , Roberts D. A.
Published: February 2008

We examine the evolution with heliocentric distance of the amplitude of fluctuations in the interplanetary magnetic and velocity fields assuming a model of homogeneous, steady state turbulence. Based on observations and recent results, both the Alfvén ratio and the normalized cross helicity are taken to be constant compared to other variations, and the turbulence is assumed to be nearly incompressive. The fluctuation amplitudes are found by taking the heating of the plasma by the fluctuations to be negligible; quasi‐steady state turbulence with damping balanced by large‐scale energy input will lead to the same conclusions for inertial range fluctuations. While the assumptions of this model contrast sharply with those for purely Alfvénic fluctuations, we find that the radial dependence of the amplitude of the fluctuations for reasonable parameters is very nearly that found from both WKB analysis and recent turbulence modeling. The robustness of this result suggests why some predictions of WKB theory are apparently correct in solar wind conditions where the theory is not expected to be valid.
Dual‐mode E region plasma wave observations from Millstone Hill
Authors: del Pozo C. F. , Foster J. C. , St‐Maurice J. ‐P.
Published: February 2008

The unique capabilities of the Massachusetts Institute of Technology Millstone Hill Radar facility allow for high‐sensitivity, dual‐mode probing of the unstable auroral ionosphere. From almost simultaneous observations of the E and F regions, we have determined both the local electric field and the spectral properties of the plasma wave backscatter. We present observations and analysis of the unstable auroral E region during local afternoon to early morning hours (respectively, the eastward and the westward electrojet conditions) and identify modified two‐stream (Farley‐Buneman) waves as well as two kinds of waves associated with the turbulence created by such waves. One of these types has properties similar to those of type II waves reported in the literature, whereas the second type has a broader spectrum, is detected in all directions, and appears to be new. From our observations, onset of turbulence corresponds to an electric field threshold of the order of 20 mV/m. We have detected the presence of an azimuthal asymmetry in both the threshold field and the observed phase velocities with systematically greater values which we attribute to the presence of a 50 m/s E‐W neutral wind at E region heights. The maximum observed volume reflectivity for type I waves ranges exceeded 10−11 m−1 (60 dB above the stable thermal fluctuations) and that of the secondaries was of the order of 20 dB smaller.
Ion and electron heating at the low‐Mach‐number, quasi‐parallel bow shock
Authors: Thomsen M. F. , Gosling J. T. , Onsager T. G. , Russell C. T.
Published: February 2008

The magnetic structure and energy dissipation at low‐Mach‐number, quasi‐parallel collisionless shocks are examined through observations obtained by the ISEE 1 and 2 spacecraft at seven crossings of the Earth's dayside bow shock. The Alfvén Mach number of the shocks in this set ranges from 2.0 to 3.7, and the angle θBn between the shock normal and the average upstream magnetic field direction ranges from 2° to 39°. The shocks all exhibit a fairly short‐scale‐length ramp where the principal jumps in the electron temperature, ion temperature, and magnetic field strength take place. Large‐amplitude, low‐frequency, transverse waves in the magnetic field are present in and downstream from the ramp, with more modest waves upstream. The amplitude of the downstream waves is typically larger than can be accounted for by shock compression of the upstream waves. The electron heating represents about 6% of the dissipated bulk flow energy, consistent with observations in other parameter regimes, and suggesting that the dominant electron heating process is the same. The electron share of the dissipated energy appears to increase somewhat with increasing θBn, and the ion share appears to decrease. Coherent reflection of incident ions does not appear to be very important for ion heating at these low Mach numbers, although a low level of reflection may contribute significantly to the generation of the large‐amplitude waves. Finally, the downstream ion distributions do not show the strong variability previously interpreted as evidence for a cyclic re‐formation process at higher‐Mach‐number, quasi‐parallel shocks. It is possible that the low‐Mach‐number shocks are re‐forming but do not exhibit the downstream variability because of the relatively low levels of reflected ions.
Double shock pairs in the solar wind
Authors: Hu Y. Q. , Habbal S. R.
Published: February 2008

This paper presents a numerical study of the evolution of a velocity enhancement disturbance in the solar wind in terms of a one‐dimensional, isentropic MHD flow model. It is shown that the disturbance steepens and evolves into a double shock pair while propagating outward away from the Sun. The double shock pair consists of a reverse fast shock, a reverse slow shock, a forward slow shock, and a forward fast shock in order of distance away from the Sun. The formation time of the double shock pair is nearly inversely proportional to the average velocity gradient of the disturbance. When the double shock pair is fully developed, the strength of the fast shocks is essentially determined by the disturbance amplitude, while the slow shocks behave differently. Their strength increases first with the disturbance amplitude but starts to decrease once the disturbance amplitude exceeds a certain value. However, the fully developed slow shocks will retain their identity up to 1 AU and even farther, though their propagation speed in the solar wind frame and the jump in velocity and total pressure across them decrease substantially with heliocentric distance. Theoretically, double shock pairs would occur frequently in the inner heliosphere, since the solar wind there is characterized by various large‐scale structures and disturbances, which provide an appropriate ground for the formation of double shock pairs. Such a prediction remains to be confirmed by observations and data interpretation.
Preferential phase velocities for type 4 irregularities in the auroral E region plasma
Authors: Haldoupis C. , Koehler J. A. , Sofko G. J. , Danskin D. W. , McKibben M. J. , André D.
Published: February 2008

This paper presents additional evidence on the nature of 50‐MHz type 4 auroral backscatter. By using a large number of events, recorded during several periods of continuous wave Doppler radar operation in the past few years, and a data sequence of long lasting type 4 echoes, the mean phase velocity of type 4 waves and its relation to type 1 phase velocity have been studied statistically. On the average, type 4 phase velocities range between 800 and 1200 m/s with the great majority grouping near 1000 m/s. The mean velocities of simultaneous type 4 and type 1 spectrum components are well separated in the Doppler shift range with an approximate factor of 2 in the type 4 to type 1 velocity ratio. Also, both spectrum components have the same Doppler polarity, which is indicative of a direct or indirect role for the electric field in type 4 wave generation. On the basis of the widely accepted conviction that type 1 waves have velocities saturating near the ion acoustic speed, it is concluded that type 4 waves propagate with velocities at about twice the acoustic speed in the plasma. The evidence presented here strongly suggests that type 4 waves propagate at preferential velocities near 1000 m/s, an experimental fact which must be explained in theoretical models.
Plasma wave signatures in the magnetotail reconnection region: MHD simulation and ray tracing
Authors: Omura Yoshiharu , Green James L.
Published: February 2008

We present the results of an innovative and powerful new technique to be used in the interpretation of Geotail plasma wave data when the spacecraft is in the deep magnetotail during potential reconnection events. To provide the best possible magnetotail structure of magnetic fields and plasma densities, an MHD simulation is performed. At precisely the moment of reconnection, during the simulation, the magnetotail structure is used in the ray tracing calculations of various plasma waves expected to be generated in the X point region. Several wave modes are studied in detailed (L mode ion cyclotron, R mode ion cyclotron, lower hybrid waves, whistler mode waves, and L‐O and R‐X mode waves). Ray tracing calculations are performed in these modes as generated from a point source at all wave normal angles in the reconnection region. When we assume each ray is generated at the same intensity, a ray density distribution is derived for the entire spatial reconnection region for several frequencies that each of the plasma wave modes can support in the MHD magnetotail model. As a hypothetical spacecraft transverses the tail reconnection structure along a typical trajectory, a pseudofrequency time spectrogram can then be constructed. It is clear that each of these wave modes has a distinctive pattern of propagation, along the magnetic field, transverse to the magnetic field, or trapped within the resulting plasmoid or reconnection region and should be easily distinguished in Geotail measurements.
Waves generated in the vicinity of an argon plasma gun in the ionosphere
Authors: Cahill L. J. , Arnoldy R. L. , Lysak R. L. , Peria W. , Lynch K. A.
Published: February 2008

Wave and particle observations were made in the close vicinity of an argon plasma gun carried to over 600 km altitude on a sounding rocket. The gun was carried on a subpayload, separated from the main payload early in the flight. Twelve‐second argon ion ejections were energized alternately with a peak energy of 100 or 200 eV. They produced waves, with multiple harmonics, in the range of ion cyclotron waves, 10 to 1000 Hz at rocket altitudes. Many of these waves could not be identified as corresponding to the cyclotron frequencies of any of the ions, argon or ambient, known to be present. In addition, the wave frequencies were observed to rise and fall and to change abruptly during a 12‐s gun operation. The wave amplitudes, near a few hundred Hertz, were of the order of 0.1 V/m. Some of the waves may be ion‐ion hybrid waves. Changes in ion populations were observed at the main payload and at the subpayload during gun operations. A gun‐related, field‐aligned, electron population also appeared.
Use of the relative amplitude method to compute “best fit” earthquake moment tensors
Authors: Rogers Richard M. , Pearce Robert G.
Published: February 2008

The method of specifying the amplitudes of teleseismic phases (P, pP, and sP, or three‐component direct S) used in the relative amplitude moment tensor method (Pearce and Rogers, 1989) is modified to provide for a most likely, or “best fit,” moment tensor. Instead of imposing uniform probability on all amplitudes lying between upper and lower bounds which are specified for each phase, a Gaussian probability function related to these bounds is assumed. This modification acknowledges that an amplitude value nearer to the midpoint of the range is more likely to represent the true value, and results in a probability density function in moment tensor space which is itself peaked, yielding a single most likely solution. By contrast, a uniform likelihood within specified amplitude bounds can yield only a range of solutions acceptable with uniform likelihood. This Gaussian relative amplitude method (GRAM) is applied to several earthquakes previously studied using the original procedure (relative amplitude moment tensor program, RAMP). GRAM yields results which agree closely with those of RAMP, but in which the source type is more tightly defined, particularly in its deviatoric component. Previously, any single solution had to be chosen arbitrarily from near the centroid of the compatible range, but it is shown that, as expected, the most likely solution may lie anywhere within this range.
Tomographic image of the axial low‐velocity zone at 12°50′N on the East Pacific Rise
Authors: Caress David W. , Burnett Mark S. , Orcutt John A.
Published: February 2008

The 1982 MAGMA seismic refraction experiment yielded a large set of accurate P wave travel times corrected for bathymetry and anisotropy which sample the structure of the East Pacific Rise (EPR) at 12°50′N. The arrivals were recorded using ocean bottom seismographs at three sites: on axis, and at 7 km and 16 km east of the axis. We invert 2320 travel times for the twodimensional crustal seismic velocity structure across the EPR axis, assuming that the velocity structure is invariant along strike. The travel times provide strong evidence for compressional velocity anisotropy in the upper crust corresponding to ∼10% faster velocities for propagation parallel to the axis than perpendicular to the axis; the travel tunes used for the tomography are corrected for the effects of this azimuthal anisotropy. Our preferred model contains only the structure clearly required by the data (structure which is stable under excessive smoothing) and achieves a variance reduction of 81% relative to the laterally homogeneous starting model. We resolve a substantial zone beneath the rise axis in which the velocity is reduced by 0.4 to 0.7 km/s; this low‐velocity zone (LVZ) is about 7 km wide and extends from a depth of about 1.5–2.0 km down to Moho at a depth of 5.5 km. The LVZ is slightly asymmetric, extending 1 km further to the east than to the west of the axis. In the shallow (<1.0 km depth) crust, a pattern of velocity variations is imaged in which velocities are high at the spreading axis, decrease between 3 km and 7 km east of the axis, and then increase again between 10 and 15 km east of the axis. We investigate the resolution of the inversion using an impulse response method; the LVZ and off‐axis upper crustal variations are well resolved. In addition, the travel time data indicate that an axial high‐velocity anomaly less than 2 km wide exists in the upper crust but is not resolved by the inversion. The small velocity reductions of the LVZ are consistent with hot rock containing only a small quantity of melt. These results, combined with the multichannel seismic reflection lines and expanded spread profiles from the northern EPR, suggest that the zone of high melt fraction under the spreading center is confined to a narrow, thin lens capping a broad zone of hot plutonic rocks. The upper crustal velocity reduction within 1 km of the axis reflects near‐axial thickening of the extrusive layer and the later reduction probably reflects porosity increases due to near‐axial tectonism; the upper crustal velocity increase beyond 15 km off axis is attributed to porosity decreases associated with hydrothermal alteration.
The structure of the core‐mantle boundary from diffracted waves
Authors: Wysession Michael E. , Okal Emile A. , Bina Craig R.
Published: February 2008

Diffracted P and S waves (Pd, Sd) traveling around the core‐mantle boundary (CMB) of the Earth give us information about the velocity structure and therefore the thermochemistry of D″, the base of the Earth's mantle. By examining Pd and Sdarrivals we determined the apparent ray parameter for different regions at the base of the mantle. By comparing the data slownesses to those found from reflectivity synthetic seismograms we were able to quantify D″ average velocities. Using these averaged velocities with a thermochemical modeling of lower mantle minerals using a Birch‐Mumaghan equation of state, we have been able to make chemical and physical inferences as to the causes of lateral variations at the CMB. Examinations found significant lateral heterogeneity at the base of the mantle, amounting to ≈ 4% for both P and S velocities. These velocities did not always vary in parallel, and the Poisson ratio varied regionally by almost 6%. The most unusual region of the CMB found was under Indonesia, where velocities 3% slower than the preliminary reference Earth models were found adjacent to a region of faster than average velocities. These regions currently correspond to areas of core up welling and down welling (respectively) found by Voorhies (1986), which if mostly held in place by core‐mantle coupling might cause a flux of heat and iron into the mantle, making the anomaly both thermally and chemically derived. At the CMB under the northern Pacific rim the fastest shear velocities were found, but the same region yielded slower than average P velocities. While the presence of fast shear velocities here would support the idea that we are seeing the cold dregs of mantle convection, perhaps continuing down from the North Pacific subduction zones, the presence of slow P velocities suggests additional complications. Our thermochemical modeling suggests that the D″ Poisson ratio is very sensitive to variations in the silicate/oxide ratio and that a decrease in the amount of perovskite relative to magnesiowüstite may play an important role in this region.
Electrical conductivity variations around the Palmdale Section of the San Andreas Fault Zone
Authors: Madden T. R. , LaTorraca G. A. , Park S. K.
Published: February 2008

Our analysis of a decade of observations from the Palmdale California telluric array is presented. The array is a system of large‐scale telluric field measurements used for the passive monitoring of electrical conductivity variations over a large region centered over the San Andreas Fault Zone northeast of Los Angeles. Since 1985, systematic changes in the relationships between the telluric field measurements around the array have been observed. These changes amounted to 1% to 2% by the time the array was closed down in 1989, with the fields measured along the fault and northeast of the fault showing signal decreases relative to the fields southwest of the fault. Because of the possible implications of these changes for conductivity variations related to tectonic activity on the San Andreas, we have attempted to sort out the geography of the zones that could cause the changes. The simplest, though nonunique, interpretation of the observed variations in the telluric relationships is a steady increase in the conductivity of the lower crustal region of the fault zone. From two‐dimensional models of the telluric response around the array, we infer that an increase of about 60 milli‐Siemens in the conductance of the lower crustal fault zone could account for the changes seen over the period 1985 to 1989. Assuming changes were in a kilometer‐wide zone and that the lower crustal fluids were very conductive, this could represent a strain of about 0.4 × 10−6. The surface strain would be much smaller and would be lost in surface strain noise. Because of the possibility that fluid pressure changes are involved, these small strains could still be significant for earthquake cycle phenomena. Alternatively the observed changes could have been produced by increases in the conductivity of the upper crust of the Mojave Desert, northeast of the fault, or by decreases in the conductivity of the upper crust in the San Gabriel Mountains southwest of the fault To produce the observed telluric changes, however, the additional conductivity and the associated strain changes would have to be much larger. The strain needed to accommodate the increased conductivity on the Mojave side would have to be of the order of 6 × 10−4, which seems unrealistically high. Smaller strains could account for conductivity changes in the San Gabriel Mountains, but these strains would have to be compressive. Water‐level changes are not considered to be a possible cause of the conductivity changes because the measurement scale is much larger than the typical scale of water level changes, and no seasonally correlated changes in the telluric fields were observed.
Convective momentum and mass transport in porous sloping layers
Authors: Ludvigsen Arild , Palm Enok , McKibbin Robert
Published: February 2008

Natural convection, caused by nonhorizontal isolines for water density in sloping sandstone layers, is studied. The flow field is calculated for a fluid‐filled three‐layer model composed of two identical sandstone strata separated by a low‐permeability layer (shale). The condition for recirculation within the sandstone is found. It is shown that for layers of sufficiently small aspect ratio, the shale layer has negligible effect on the flow. With the assumption that the fluid density is a function only of temperature, the effect of the flow on the change of porosity in the sandstone layers is examined. The spatial change of porosity is calculated for small values of time. It is shown that the maximum change takes place at the midpoints of the end walls if the angle between the isotherms and the slab is less than 36.5° and at the midpoints of the lateral boundaries if this angle is greater than 36.5°. This result is independent of the angle the slab makes with the gravity vector. Also discussed is how the angle the isotherms make with the horizontal depends on the thermal conductivities of the sandstone layers and the bounding rock.
Inference in infinite‐dimensional inverse problems: Discretization and duality
Authors: Stark Philip B.
Published: February 2008

Many techniques for solving inverse problems involve approximating the unknown model, a function, by a finite‐dimensional “discretization” or parametric representation. The uncertainty in the computed solution is sometimes taken to be the uncertainty within the parametrization; this can result in unwarranted confidence. The theory of conjugate duality can overcome the limitations of discretization within the “strict bounds” formalism, a technique for constructing confidence intervals for functionals of the unknown model incorporating certain types of prior information. The usual computational approach to strict bounds approximates the “primal” problem in a way that the the resulting confidence intervals are at most long enough to have the nominal coverage probability. There is another approach based on “dual” optimization problems that gives confidence intervals with at least the nominal coverage probability. The pair of intervals derived by the two approaches bracket a correct confidence interval. The theory is illustrated with gravimetric, seismic, geomagnetic, and helioseismic problems and a numerical example in seismology.
Postseismic deformation and stress diffusion due to viscoelasticity and comments on the Modified Elsasser Model
Authors: Cohen Steven C.
Published: February 2008

Finite element techniques have been used to investigate the far‐field deformation and stress changes due to asthenospheric viscoelastic relaxation following a dip‐slip earthquake. The diffusion of extensional stress toward the subduction zone following a thrust earthquake on land is qualitatively consistent with the modified Elsasser model as proposed by Rydelek and Sacks (1988, 1990) to explain the coupled occurrence of land and sea earthquakes near Japan. However, the magnitude of the diffusing tensional signal is significantly smaller. The nominal model consists of a partially faulted elastic lithosphere overlying a viscoelastic substrate. Other models consider thin channel flow, rupturing of the entire elastic lithosphere, and changes in the depth of faulting. While some of these changes have significant impact on the magnitude and spatial features of the stress and deformation field, the far‐field stress remains small. Numerical experiments demonstrate that the assumptions of the modified Elsasser model accentuate uniaxial deformation. When these assumptions are replicated in the finite element calculations, reasonable agreement between the models is achieved.
Magmatism and extension: The thermal and mechanical effects of the Yellowstone Hotspot
Authors: Anders Mark H. , Sleep Norman H.
Published: February 2008

Currently, there is a parabolic‐shaped pattern of seismicity and latest Quaternary faulting which centers on the axis of the eastern Snake River Plain (SRP) and has its apex at the Yellowstone Plateau. The pattern of activity surrounds a region of aseismicity that includes the eastern SRP. Geologic evidence suggests that this pattern has migrated in tandem with rhyolitic volcanism induced by the Yellowstone hotspot during its 16 m.y. trek across western North America. Similarly there is a migration in the onset of basaltic lava flows and the emplacement of up to 12 km of midcrustal mafic intrusions that follows the track of rhyolitic volcanism. These observations suggest that the intrusion of magmas has a significant influence on the pattern of seismicity and faulting. We present a one‐dimensional, finite‐difference, thermomechanical model that accounts for the observed pattern of increased faulting followed by fault quiescence within the circum‐eastern SRP. In this model, mafic magmas are intruded into a lithosphere that is already extending. The intrusions heat the surrounding rocks resulting in locally increased strain rates. As the intruded magmas solidify, the length of time required to return strain rates to their pre‐intrusion level is then determined. The model assumes constant horizontal tectonic forces and maps strain rate as a function of yield strength and time since intrusion. We vary model parameters such as crustal thickness, initial geothermal gradient, and amount of magma intruded, in order to assess how they affect turnaround time for strain rates. The amount and timing of intruded material allowed by the model are constrained by seismic refraction data as well as by regional geochronologic and petrologic studies. We assume intruded rock rheology to be a Maryland diabase; mantle rheology we base on dry olivine; we represent the crust by both dry granite and dry anorthosite rheologies. Our results suggest that 2 to 3 m.y. appears to be a reasonable length of time for strain rates to return to levels present before a midcrustal mafic intrusion equivalent in magnitude to the Yellowstone intrusion. This corresponds closely to the length of time between the onset of along‐axis accelerated faulting and subsequent fault quiescence, assuming a hotspot velocity of 3.5 to 4.0 cm/yr relative to the North American Plate.
Shortening within underplated oceanic crust beneath the Central California Margin
Authors: Miller Kate C. , Howie John M. , Ruppert Stanley D.
Published: February 2008

Structures within a tectonically underplated layer of early Miocene age oceanic crust indicate that the lower crust of offshore central California has been deformed under compression prior to the inception of the San Andreas transform system along the continental margin. Shortening in the lower crust is inferred from the geometry of two structures, the Santa Lucia monocline and the Santa Maria antiform, delineated by an integrated interpretation of multichannel and wide‐angle seismic reflection data in conjunction with regional gravity data and teleseismic receiver functions. The 700 km of multichannel seismic data and 300 km of wide‐angle seismic data from the 1986 Pacific Gas and Electric/EDGE seismic experiment form the primary data base used to establish the northwesterly trend and extent of the monocline and the WNW trend of the antiform. In cross‐sectional view, the Santa Lucia monocline is a 30° kink in the oceanic crust, while the Santa Maria antiform is characterized by local thickening of the oceanic crust. These structures are probably the result of convergence between the Monterey and Arguello microplates that took place as subduction along the margin waned. The multichannel seismic reflection data image two Miocene age shelf basins, the Santa Lucia and offshore Santa Maria basins that formed within a transtensional stress regime. The offshore Santa Maria basin directly overlies the Santa Maria antiform, suggesting that either the upper and lower crustal stress regimes were completely decoupled in Miocene time or that the two features formed in different places. Differential slip between the upper crust and lower crust may have been accommodated along a detachment zone located above oceanic crust. Convergence within the oceanic crust and apparent differential motion between upper and lower crust are indicative of the processes that can affect the transition from subduction to an oceanic‐continental transform zone.
The effects of steam injection on the electrical conductivity of an unconsolidated sand saturated with a salt solution
Authors: Vaughan Peter J. , Udell Kent S. , Wilt Michael J.
Published: February 2008

The spatial and temporal variation of electrical conductivity in saturated sands during steam injection has been measured and modelled. Experiments consisted of introducing steam into one end of a tube filled with sand saturated with a slightly saline solution. A steam condensation front formed, separating the mixed‐phase steam zone from the liquid zone. Measurements of electrical conductivity were made at 10 locations along the tube using a four‐electrode technique. Results show that conductivity starts at a constant value, decreases before the steam front arrives and then, immediately prior to the steam front arrival, goes through a maximum before dropping by a factor of about 25. These variations can be explained by first, a dilution of the interstitial solution causing the initial drop in conductivity; second, an increase in temperature of the solution immediately prior to the arrival of the steam front causing the conductivity maximum; and finally, the large drop in conductivity due to the combined effects of a decrease in saturation and dilution of the residual liquid in the two‐phase zone. Mathematical solutions of a set of differential equations that take into consideration all of these effects are presented. These solutions reproduce the significant features of the conductivity data. This study suggests that the measurement of changes in the subsurface conductivity field during steam injection operations may indicate the location of ionic concentration, temperature, and steam saturation fields.
Interaction of the San Andreas Fault Creeping Segment with Adjacent great rupture zones and earthquake recurrence at Parkfield
Authors: Ben‐Zion Yehuda , Rice James R. , Dmowska Renata
Published: February 2008

Three‐dimensional finite element calculations are employed to study interactions in space and time between the creeping segment of the San Andreas fault in central California and the adjacent currently locked zones of the 1857 and 1906 great earthquakes. Vertically, the model consists of an elastic upper crust over a Maxwell viscoelastic region, representing the entire lower crust or a narrower horizontal detachment layer, and a stiffer and more viscous upper mantle. The crust has a single vertical fault extending to the top of the mantle at 25 km depth. In zones along strike corresponding to the 1857 and 1906 events, the top 12.5 km of the fault is locked against slip, except in great earthquakes. Below the locked zones and everywhere along the creeping region between them, the fault is freely slipping. The model parameters are compatible with seismological and geological observations, and with a ratio of Maxwell relaxation time to the relaxing layer thickness in the range 1 to 2 yr/km, as established by Li and Rice (1987) and Fares and Rice (1988) based on fits to geodetic data along the San Andreas fault. An imposed constant far field shear motion and periodic 1857‐ and 1906 ‐ type earthquakes generate slip rates along the creeping fault segment that evolve in time throughout the entire earthquake cycle. Shortly after an adjacent great earthquake, slip rates in the creeping zone are higher than the far field velocity, while later in the cycle they are lower. Hence, time dependency should be accounted for when measurements of fault slip are used to estimate the plate motion. If Parkfield earthquakes are a response to a time dependent loading of the type simulated here, their recurrence interval would tend to lengthen with time since the 1857 event. Thus, the hypothesis of characteristic periodic earthquakes at Parkfield may not provide the best estimate of the occurrence time of the next event. Using, for example, the statistics of past events and assuming that Parkfield earthquakes are a response to a slip deficit near Middle Mountain, and that the elastic crustal layer is 17.5 km thick, we find that the next event is predicted for about 1992 ± 9 years if the lower crust is a 7.5 km thick layer having a material relaxation time of 15 years, and 1995 ± 11 years if the 7.5 km thick lower crust is characterized by a relaxation time of 7.5 years. These values may be compared to the 1988 ± 7 years estimate based on periodicity in time. The modeling results also indicate that the interaction between the 1857 and 1906 rupture zones is small.
Attenuation of crustal waves across the Alpine Range
Authors: Campillo Michel , Feignier Bruno , Bouchon Michel , Béthoux Nicole
Published: February 2008

We present observational evidence of an anomalous propagation of Lg waves across the south‐western part of the Alpine range, and we use numerical simulations to model these observations. We consider a set of 48 earthquakes which occurred in Switzerland, northern Italy, and southeastern France and were recorded by the French (Laboratoire de Detection et de Geophysique) and northwest Italian (Istituto Geofisico di Genova) seismic networks. While the amplitude of the Pn phase is stable throughout the region studied, Lg wave amplitude undergoes strong variations. We map this anomaly in Lg wave propagation by dividing the region into a grid and attributing to each cell a value equal to the mean value of the Lg/Pn amplitude ratios computed for all the paths which cross this cell. The image obtained shows that the extinction of Lg waves occurs in a limited region of the western Alps which corresponds to the zone of highest positive Bouguer anomaly. This zone located to the east of the high peaks of the Massifs Cristallins Externes does not correspond either to the region of the highest topographies or to the one of the deepest Moho. At a frequency of 2 Hz, the crustal waves that cross this anomalous region have amplitudes divided by more than 10 with respect to waves that propagate along other paths. In order to investigate the cause of the anomaly we perform numerical simulations of SH wave propagation through a model of the western Alps which includes the main characteristics of the geological structure. Our results indicate that the geometrical effect of the lateral variations of the medium does not account entirely for the actual vanishing of crustal waves. The simulation predicts a decay of the amplitude by only a factor between 2 and 3. The introduction of corrugation in the interface shapes further adds to the decay. However, other sources of attenuation such as anelasticity or severe heterogeneity must be invoked to explain fully the observations.
Effect of loading rate on crack propagation under compressive stress in a saturated porous material
Authors: Atkinson C. , Cook J. M.
Published: February 2008

A model has been produced which is an initial attempt to predict the effect of loading rate and other parameters on the propagation of a tensile crack in a fluid‐saturated porous material under compression. The model is based on an existing treatment for a nonsaturated brittle solid but includes (in a simplified way) the influence of internal fluid pressure within the propagating crack and the way that this is changed by fluid flow into the crack. The predictions of the model are that at very low loading rates the loading rate has little effect on the crack length; that as loading rate is increased, the crack growth rate is decreased as the fluid pressure within the crack falls; and that at high loading rates the growth rate is lowered. Increasing fracture toughness, decreasing far‐field pore pressure, and decreasing permeability have similar effects to those of increasing loading rate. For materials with very low permeability, such as shales, the loading rates at which such effects occur are within the range of laboratory experiments; suppression of tensile cracking and its replacement by shear cracking do appear to be seen.
Observation and analysis of the tidal gravity variations at Asuka Station located on the Antarctic Ice Sheet
Authors: Shibuya Kazuo , Ogawa Fumio
Published: February 2008

Tidal gravity observations were made at Asuka Station (71.5°S, 24.1°E), Antarctica. The LaCoste‐Romberg model G‐805 gravity meter was altered to integrate an electrostatic feedback amplifier and was installed in a snow pit. Records for tidal analysis were obtained from June 26 to November 13, 1987, at a sampling interval of 30 min. From the estimated tidal amplitudes, corrections for the oceanic effects were made using Schwiderski's cotidal maps. The ocean tide corrected δ factors, which define the response of the deformable Earth to lunar‐solar tidal forces, were found to be 1.169 ± 0.008 (O1), 1.127 ± 0.017 (P1), 1.135 ± 0.006 (K1), 1.261 ± 0.033 (M2), and 1.302 ± 0.039 (S2). The diurnal δ factors agree with those obtained from statistically derived formulas by Melchior and De Becker. However, the semidiurnal δ factors (M2 and S2) are 10% larger than the theoretical values predicted by Wahr. This is much larger than the typical 1–2% discrepancy between the statistical model and theory. Similar tidal gravity observations made at Syowa Station (69.0°S, 39.6°) yielded consistent results with those monitored at Asuka Station. The observed M2 δ factors have a tendency to increase from the equator toward the southern high latitudes. The increasing effect of inaccurate ocean load tidal corrections may be due to Schwiderski's cotidal chart. Tidal analysis resulted in a gravity admittance of −0.24 ± 0.04 μGal/mbar for the air pressure change at Asuka Station. The smaller magnitude compared with the typical value of −0.35 μGal/mbar for a normal crust may be explained by the loading deformation of the ice sheet under the air mass covering an extent of 30 km over the site.