bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct numerical testing of stationary shock model with low Mach number shock observations
Authors: Zilbersher D. , Gedalin M. , Newbury J. A. , Russell C. T.
Published: February 2004

Using mixed observational‐theoretical‐numerical analysis, we have tested to what extent the observed profile of a low‐Mach number quasi‐perpendicular shock is consistent with the shock model, in which the shock front is formed due to the ion interaction with the stationary and one‐dimensional field structure. Using observational data for the magnetic profile of the November 26, 1977, shock with “noise” removed and theoretically predicted cross‐shock electric field, we have numerically traced the ion trajectories in this shock structure, assuming an initial Maxwellian distribution of incident ions with the observed temperature. We found no reflected ions, although some fraction of ions were almost reflected, making loops inside the ramp and acquiring energies typical for ordinary reflected ions. The number of these quasi‐reflected ions is sensitive to the ion β and cross‐shock potential. We have applied the pressure balance condition to verify the consistency of the numerically derived ion pressure with the observed profile. The good agreement found shows that the shock is stationary and that its structure is indeed mostly due to the particle interaction with static fields in the shock front, while time‐dependent features (such as turbulence) may provide fine tuning.
Upper atmosphere potassium layer and its seasonal variations at 54°N
Authors: Eska V. , Höffner J. , von Zahn U.
Published: February 2004

We conducted measurements of the upper atmosphere potassium layer and its seasonal variations during 110 nights between June 1996 and June 1997 at Kühlungsborn, Germany (54°N, 12°E). The observations were performed by a ground‐based metal resonance lidar. We measured the following average properties of the nightly mean K layer: column density of 4.4×l07 cm−2; peak density of 47 cm−3; layer centroid height of 90.5 km and RMS layer width of 4.0 km. The nightly mean column and peak densities exhibit dominantly semiannual variations with maxima in summer and winter. The centroid height varies semiannually too, attaining highest altitudes during the equinoxes. The RMS layer width shows a strong annual variation, though, with a maximum width in winter. For the first time the seasonal variations of the K layer can be compared in detail with the variations observed previously in the Na and Fe layers. For all three metal layers their centroid heights are dominated by annual variations with quite similar phases. The column densities are dominated by a semiannual variation in the case of the K layer but are dominated by an annual variation for the Fe and Na layers. The layer RMS widths are dominated by annual variations in the case of the K and Fe layers. That is quite different to semiannual variations in the case of the Na layer.
A neural network prediction of solar cycle 23
Authors: Conway A. J. , Macpherson K. P. , Blacklaw G. , Brown J. C.
Published: February 2004

We examine the use of feed forward neural networks in the long term (i.e., years ahead) prediction of sunspot number. First, we briefly review the history of the time series and also some previous attempts to predict it. We outline our neural network method and discuss how the reliability of the data affects training. We conclude that earlier data should not be used to train neural networks that are intended to make predictions at the current epoch. We then use this understanding of the data in training neural networks, testing many different configurations to see which provides the best 1–6 year ahead prediction accuracies. By looking at the distribution of residuals, an estimate of the uncertainty is placed on the best networks' predictions. According to our predictions of yearly sunspot number, the maximum of cycle 23 will occur in the year 2001 and will have an annual mean sunspot number of 130 with an uncertainty of ±30–80% confidence. Finally, we discuss our result in relation to others and comment on how neural networks may be used in future work.
Stability of perpendicular propagation in time‐varying nongyrotropic plasmas: Simulations
Authors: Brinca Armando L. , Omura Yoshiharu , Matsumoto Hiroshi
Published: February 2004

Particle simulations of perpendicular (with respect to the ambient magnetic field) propagation in homogeneous time‐varying nongyrotropic magnetoplasmas show that the introduction of gyrophase organization in the (electron or ion) particle populations of otherwise stable media can bring about growth of (electron or ion) electrostatic cyclotron harmonic waves and destabilize the extraordinary mode. The kinetic code simulations demonstrate the occurrence of perpendicular instabilities for various types of nongyrotropic distributions, including those observed for heavy ions in the space shuttle environment, and show that gyrophase diffusion accompanies the evolution toward saturation.
Magnesium isotopic composition as observed with the CELIAS/MTOF experiment on the SOHO spacecraft
Authors: Kucharek H. , Ipavich F. M. , Kallenbach R. , Bochsler P. , Hovestadt D. , Grünwaldt H. , Hilchenbach M. , Axford W. I. , Balsiger H. , Bürgi A. , Coplan M. A. , Galvin A. B. , Geiss J. , Gliem F. , Gloeckler G. , Hsieh K. C. , Judge D. J. , Klecker B. , Lee M. A. , Livi S. , Managadze G. G. , Marsch E. , Möbius E. , Neugebauer M. , Ogawa H. S. , Reiche K.‐U. , Scholer M. , Verigin M. I. , Wilken B. , Wurz P.
Published: February 2004

Solar wind abundance ratios of magnesium isotopes measured with the high resolution Mass Time‐of‐Flight spectrometer (MTOF) of the Charge, Element, and Isotope Analysis System (CELIAS) experiment on board the Solar and Heliospheric Observatory (SOHO) are presented. MTOF, as part of CELIAS, is, because of its high time and mass resolution, an excellent tool for isotope abundance measurements in the solar wind. From the data analysis we have found that the isotopic composition of magnesium in the solar wind agrees with the terrestrial composition within the experimental uncertainty. We have obtained isotopic ratios of 24Mg/25Mg = 7.7 ± 0.4 and 24Mg/26Mg = 7.0 ± 0.5. These values are consistent with the terrestrial values of 24Mg/25Mg = 7.90 ± 0.01 and 24Mg/26Mg = 7.17 ± 0.03. Furthermore, these investigations also show that with the given uncertainties the abundance ratios do not vary significantly within a solar wind velocity range from 375 km/s to 530 km/s.
MHD wave conversion in plasma waveguides
Authors: Fedorov Evgeniy , Mazur Nikolai , Pilipenko Viacheslav , Yumoto Kiyofumi
Published: February 2004

A theoretical approach to the consideration of coupling between Alfvén and compressional modes in two‐dimensionally inhomogeneous plasma has been developed. The conversion of a compressional mode, propagating along a high‐density layer perpendicular to the magnetic field, into leaking shear Alfvén waves has been considered. The wave spatial structure along the layer is described in WKB approximation. A compressional‐type disturbance effectively emitts Alfvén waves when a local resonant condition within the layer is fulfilled. In contrast with the common field line resonance theory, the excited Alfvén modes are assumed to be running away waves but not standing oscillations. The physical situations that correspond to the considered model can be met in the outer near‐equatorial magnetosphere, in the exterior cusp region, and in the other magnetospheres such as Jupiter and Saturn.
Excitation of helium cyclotron harmonic waves during quiet magnetic conditions
Authors: Pokhotelov O. A. , Pokhotelov D. O. , Feygin F. Z. , Parrot M. , Kangas J. , Mursula K. , Shukla P. K. , Stenflo L.
Published: February 2004

A general approach to the generation of ion cyclotron harmonic waves observed on board the Akebono satellite in the deep plasmasphere is presented. It is shown that during quiet magnetic conditions the development of the hydrodynamic cyclotron instability with growth rate γ ∝ ni1/2, where ni is the number density of the hot heavy ions, is suppressed by the field‐aligned inhomogeneity of the dipole magnetic field. The instability is, in this case, controlled by the weak resonant interaction of the waves and the trapped particles with growth rate γ ∝ ni. The waves are generated by a kinetic instability involving hot helium ions with a ring‐like distribution. Such ions are present in the magnetosphere during quiet magnetic conditions. A simple analytical model of this instability accounting for the inhomogeneity of the ambient magnetic field is used. It is shown that the ULF wave observations during quiet times on board the Akebono satellite are in a reasonable agreement with the present theoretical approach.
Observations of electron distributions in magnetosheath mirror mode waves
Authors: Chisham G. , Burgess D. , Schwartz S. J. , Dunlop M. W.
Published: February 2004

The behavior of the electron distribution function within mirror mode waves in the Earth's magnetosheath is studied using observations made by the electron experiment on Active Magnetospheric Particle Tracer Explorers United Kingdom Subsatellite (AMPTE UKS). The results of a detailed pitch angle analysis show that in the mirror mode troughs the electron distribution displays two distinct features related to particle trapping: (1) The “deeply” trapped electrons (those with pitch angles close to 90° in the magnetic field troughs) are significantly cooled with respect to the rest of the electron population and (2) the “shallowly” trapped electrons (those with pitch angles just greater than the critical pitch angle for trapping) are significantly heated with respect to the rest of the electron population. This is explained as a result of the trapped electron population undergoing a combination of Fermi acceleration and deceleration and betatron deceleration. The untrapped electron population appears to behave adiabatically as it travels alternately through mirror mode peaks and troughs. This behavior is confirmed by the use of Liouville mapping techniques, which also allow us to estimate the electric potential associated with the mirror mode wave.
Modeling investigation of the evening prereversal enhancement of the zonal electric field in the equatorial ionosphere
Authors: Eccles J. Vincent
Published: February 2004

The neutral winds in the E and F regions are the drivers of the low‐latitude ionospheric current system and observed electric field structure. A flux‐tube‐integrated model of ionospheric electrodynamics is used to examine three proposed causes of the prereversal enhancement of the zonal electric field. All three mechanisms try to define the obscure relationship between the F region dynamo and the evening prereversal enhancement. The results show that the original mechanism proposed by Rishbeth [1971] is the fundamental mechanism. The rapid change in the vertical polarization electric field near local sunset, which is due to the accelerating neutral wind dynamo, produces a corresponding change in the zonal electric field through curl‐free requirements. The other two mechanisms do not produce the evening enhancement but alter the enhancement shape and magnitude.
Polar wind survey with the Thermal Ion Dynamics Experiment/Plasma Source Instrument suite aboard POLAR
Authors: Su Yi‐Jiun , Horwitz J. L. , Moore T. E. , Giles B. L. , Chandler M. O. , Craven P. D. , Hirahara M. , Pollock C. J.
Published: February 2004

In February 1996, the POLAR spacecraft was placed in an elliptical orbit with a 9 RE geocentric distance apogee in the northern hemisphere and 1.8 RE perigee in the southern hemisphere. The Thermal Ion Dynamics Experiment (TIDE) on POLAR has allowed sampling of the three‐dimensional ion distribution functions with excellent energy, angular, and mass resolution. The Plasma Source Instrument (PSI), when operated, allows sufficient diminution of the electric potential to observe the polar wind at very high altitudes. In this paper, we describe the results of a survey of the polar wind characteristics for H+, He+, and O+ as observed by TIDE at ∼5000 km and ∼8 RE altitudes over the polar cap during April–May 1996. At 5000 km altitude, the H+ polar wind exhibits a supersonic outflow, while O+ shows subsonic downflow, which suggests a cleft ion fountain origin for the O+ ions in the polar cap region. Dramatic decreases of the 5000 km altitude H+ and O+ ion densities and fluxes are seen as the solar zenith angle increases from 90° to 100° for the ionospheric base, which is consistent with solar illumination ionization control. However, the polar cap downward O+ flow and density decline from dayside to nightside in magnetic coordinates suggest a cleft ion fountain origin for the polar cap O+. Cleft ion fountain origin O+ density plumes could also be partially responsible for a similar day‐night asymmetry in H+, owing to the charge‐exchange reaction. At 8 RE altitude, both H+ and O+ outflows are supersonic and H+ is the highly dominant ion species. The average bulk ion field‐aligned velocities are in the typical ratio V0+ ∶ VHe+ ∶ VH+ ∼ 2 ∶ 3 ∶ 5, which may suggest a tendency toward comparable energy gains, such as via an electric potential layer.
Physical mechanism and statistics of occurrence of an additional layer in the equatorial ionosphere
Authors: Balan N. , Batista I. S. , Abdu M. A. , MacDougall J. , Bailey G. J.
Published: February 2004

A physical mechanism and the location and latitudinal extent of an additional layer, called the F3 layer, that exists in the equatorial ionosphere are presented. A statistical analysis of the occurrence of the layer recorded at the equatorial station Fortaleza (4°S, 38°W; dip 9°S) in Brazil is also presented. The F3 layer forms during the morning‐noon period in that equatorial region where the combined effect of the upward E×B drift and neutral wind provides a vertically upward plasma drift velocity at altitudes near and above the F2 peak. This velocity causes the F2 peak to drift upward and form the F3 layer while the normal F2 layer develops at lower altitudes through the usual photochemical and dynamical effects of the equatorial region. The peak electron density of the F3 layer can exceed that of the F2 layer. The F3 layer is predicted to be distinct on the summer side of the geomagnetic equator during periods of low solar activity and to become less distinct as the solar activity increases. Ionograms recorded at Fortaleza in 1995 show the existence of an F3 layer on 49% of the days, with the occurrence being most frequent (75%) and distinct in summer, as expected. During summer the layer occurs earlier and lasts longer compared to the other seasons; on the average, the layer occurs at around 0930 LT and lasts for about 3 hours. The altitude of the layer is also high in summer, with the mean peak virtual height being about 570 km. However, the critical frequency of the layer (f0F3) exceeds that of the F2 layer (f0f2) by the largest amounts in winter and equinox; f0F3 exceeds f0F2 by a yearly average of about 1.3 MHz.
Ground surface temperature history in central Canada inferred from 10 selected borehole temperature profiles
Authors: Frottier Laurent Guillou‐ , Mareschal Jean‐Claude , Musset Julien
Published: February 2004

Among the 57 temperature‐depth profiles recently measured in Manitoba and Saskatchewan (Canada), only 10 are suitable for inferring recent changes in ground surface temperature. Many of the rejected temperature profiles show an apparent climatic signal but are affected by topography, changes in vegetation, or the proximity of lakes. At the northernmost site, near Lynn Lake, Manitoba, shallow horizontal variations in temperature have been identified that are related to intermittent permafrost. Such variations can induce an apparent “climatic” perturbation. In areas where lateral thermal conductivity contrasts have been measured, heat refraction effects can also incorrectly be interpreted as ground surface temperature histories. The analysis suggests that forest fires that occurred at some of the sites have had little influence on the temperature profiles. The 10 selected temperature profiles cover a wide area in northern Manitoba and Saskatchewan. They have been independently and jointly inverted by a singular value decomposition method. The ground surface temperature history shows two main episodes. A cold period, tentatively identified as Little Ice Age, with a minimum around 1820 A.D., was followed by marked warming after 1920. These trends are similar to those recognized in eastern Canada.
Paleointensity of the Earth's magnetic field in Early Cretaceous time: The Paraná Basalt, Brazil
Authors: Kosterov Andrei A. , Perrin Mireille , Glen Jonathan M. , Coe Robert S.
Published: February 2004

Paleointensity experiments were carried out with the Coe and Thellier methods on Early Cretaceous basalt (133 My old) from the Paraná basin in southern Brazil. Paleointensity estimates could be obtained from only six lava flows: three of normal and three of reversed polarity, of the 71 sampled. Moreover, the quality of the determinations is fairly low, with q ratio usually less than 10, mainly as a result of chemical and/or physical changes of the magnetic oxides during heating. This study strongly underscores the difficulty of finding natural rocks suitable for paleointensity experiments even so the directional results previously obtained were good. The most reliable result give an intensity of 28±1 μT, while the others range from 20.8 μT to 37.7 μT. The corresponding virtual dipole moments (VDMs) range from 4.7×1022 to 7.9×1022 Am2. This dispersion is attributed mainly to inaccurate estimates of the ancient field rather than to secular variation. Although the number of cooling units for which paleointensity estimates could be obtained is limited, the results nevertheless indicate that the early Cretaceous VDM was significantly lower than that for the recent field but greater than has been reported for this portion of the Mesozoic Dipole Low period.
Thermoremanent magnetization of nonuniformly magnetized grains
Authors: Dunlop David J.
Published: February 2004

A simple and elegant interpretation of thermoremanent magnetization (TRM) in uniformly magnetized single‐domain (SD) grains was given by Néel 50 years ago, but the TRM acquisition processes in larger, nonuniformly magnetized grains are more varied and difficult to describe theoretically. SD TRM is a frozen high‐temperature partition between two microstates: spins parallel or antiparallel to an applied magnetic field. Nonuniformly magnetized grains have a much greater choice of microstates (local energy minimum or LEM states), and partitioning among various LEM states continues to change during cooling. These changes may involve Barkhausen jumps of domain walls between positions of minimum local energy or nucleation of new domains and walls. Because of the lower remanence capacity of nonuniform microstates compared to the uniform SD state, TRM intensity decreases as grain size increases, although certain microstates, e.g., single‐vortex states, seem to contribute little to TRM. Thermal demagnetization of TRM begins just above room temperature and continues to the Curie point, quite unlike the sharp “unblocking” of SD TRM. This continuous demagnetization, resulting from changes in microstates driven by the changing internal demagnetizing field during heating, profoundly affects the separation of different components of natural remanent magnetization and the determination of paleomagnetic field intensity.
Anomalous surface waves from Lop Nor nuclear explosions: Observations and numerical modeling
Authors: Pedersen Helle A. , Avouac Jean‐Philippe , Campillo Michel
Published: February 2004

Surface waves from the Chinese test site of Lop Nor are analyzed using long‐period and broadband stations located at regional and teleseismic distances and at different azimuths. For most azimuths, strong Love waves between 0.02 and 0.045 Hz are observed with an amplitude of up to 10 times that of the Rayleigh waves. In addition, an anomalous early Rayleigh wave train is observed at some stations in western Europe. Due to a particularly favorable station and source configuration, it is possible to isolate the areas where the anomalies are created. The high‐amplitude Love waves must be attributed to either source effects or path effects immediately north of Lop Nor. The early wave train is shown to be due to a partial energy conversion between Love and Rayleigh waves, probably at the Tornquist Zone. To estimate the possible contribution from surface wave conversions to the observed anomalies, numerical simulations are carried out with the indirect boundary element method. The simulations show that a relatively small variation of crustal thickness can induce Rayleigh to Love wave conversions between 0.02 and 0.1 Hz frequency. The calculated amplitudes of the Love waves are significant (up to 35% of the amplitude of the incoming Rayleigh waves), but they are too small to fit the observed amplitude anomaly. The observed converted waves and the numerical results nevertheless indicate that surface wave conversions can be significant across strong lateral crustal heterogeneities. In particular, the conversions due to changes in crustal thickness are located in the period interval which is routinely used for estimation of Ms.
Stability of phase D at high pressure and high temperature
Authors: Frost Daniel J. , Fei Yingwei
Published: February 2004

We have determined the stability of the dense hydrous magnesium silicate phase D in a Mg2SiO4 + 20.5 wt % H2O composition between 16 and 25 GPa at 900°–1400°C. Phase D coexists with superhydrous phase B and a Mg‐rich liquid to temperatures of 1000°C at 17 GPa and 1400°C at 26 GPa. Experiments in an externally heated diamond‐anvil cell confirm that phase D is stable to pressures of at least 50 GPa at 930°C. From static compression measurements, the zero‐pressure bulk modulus of phase D was determined as 200±7 GPa. We also present evidence that phase D is identical to phase F [Kanzaki, 1991] and phase G [Kudoh et al., 1997a; Ohtani et al, 1997]. The high‐pressure and high‐temperature stability of phase D makes it eminently suitable as a hydrous phase which is stable within subducting lithosphere, in the transition zone, and in the lower mantle. The positive pressure‐temperature stability slope of phase D, determined in this study, indicates that it is potentially stable at temperatures much greater than 1400°C in the lower mantle.
Effect of subductions and trends in seismically induced Earth rotational variations
Authors: Alfonsi Laura , Spada Giorgio
Published: February 2004

In the past, the influences of large earthquakes on Earth rotation and on changes of the gravity field have been the subject of extensive investigations. Though it is now recognized that earthquakes cannot excite the Chandler wobble and that they only drive a very modest secular drift of the Earth's spin axis, their rotational signatures present some intriguing peculiarities. In fact, they preferentially drive the rotation axis toward a well‐defined direction (i.e., ∼ 140°E) and systematically induce negative variations of the Earth's oblateness. In the first part of this work the static dislocation theory is employed to ascertain the origin of these two remarkable trends. This task is accomplished by considering the effects of seismicity on Earth rotation during the last two decades. We find that the above tendencies are operating even on a yearly time scale, and we explain their nature by means of simple geometrical and physical arguments. We also find that the trend of the pole toward 140°E is essentially due to the seismicity which characterizes the western Pacific subduction zones. The seismic excitation function takes for shallow thrust earthquakes a direction roughly opposite to the excitation function associated with mass injection along active subduction zones; this point highlights the potential effects of aseismic mass rearrangement at subduction zones on the secular drift of the pole. Our findings, based on a simple model of steady state subduction, show that in the last two decades this process substantially acts to counterbalance the polar drift due to global seismic activity. We verify that this balance between earthquake‐and subduction‐induced rotational signatures also characterizes the time variations of the dynamic oblateness, which is intrinsically independent of the inertia changes that drive polar motion. From these results, which may potentially have some impact on the interpretation of long‐term polar motion, we conclude that the pole of rotation cannot be significantly perturbed by tectonic motions on a decade time scale.
Site displacement due to variation in Earth rotation
Authors: Gipson John M. , Ma Chopo
Published: February 2004

We study site displacement caused by changes in the rate of rotation of the Earth and in the orientation of the Earth's pole. We include small effects which have been previously neglected, most notably those caused by polar motion induced ocean loading. Using very long baseline interferometry (VLBI) data we estimate the real and imaginary Love numbers associated with vertical and horizontal deformations caused by polar motion. The estimates of the real part of the Love numbers are consistent with theoretical predictions, although the imaginary parts are too large. Including the loading effect improves the agreement of the real part.
Joint seismic tomography for bulk sound and shear wave speed in the Earth's mantle
Authors: Kennett B. L. N. , Widiyantoro S. , van der Hilst R. D.
Published: February 2004

High‐quality P and S travel times are now available from careful reprocessing of data reported to international agencies. A restricted data set has been extracted for which comparable ray coverage is achieved for P and S, and used for a joint inversion to produce a three‐dimensional model for shear and bulk sound velocities represented in terms of 2° × 2° cells and 18 layers in depth through the mantle. About 106 times for each of P and S are combined to produce 312,549 summary rays for each wave type. Linearizing about the ak135 reference model, 583,200 coupled tomographic equations are solved using an iterative partitioned scheme. Clear high‐resolution images are obtained for both bulk‐sound speed and shear wavespeed. The bulk and shear moduli have differing sensitivity to temperature and mineral composition, and so the images of the two velocity distributions help to constrain the nature of the processes which produce the variations. Different heterogeneity regimes can be recognised in the upper mantle, the transition zone, most of the lower mantle, and the lowermost mantle. In the upper mantle, many features can be explained by thermal effects; but in some orogenic zones (e.g. western North America), the opposite sense of the bulk‐sound and shear wave speed variation requires compositional effects or volatiles to outweigh any thermal effects. In the lower mantle, pronounced narrow structures which may represent remnant subduction are most marked in shear. The level of large‐scale variations in bulk sound speed compared to shear diminishes with depth in the lower mantle reaching a minimum near 2000 km. Below this depth, the variability of both wave speeds increases. Near the core‐mantle boundary the variations of the two wave speeds show little concordance, suggesting the presence of widespread chemical heterogeneity.
Megamullions and mullion structure defining oceanic metamorphic core complexes on the Mid‐Atlantic Ridge
Authors: Tucholke Brian E. , Lin Jian , Kleinrock Martin C.
Published: February 2004

In a study of geological and geophysical data from the Mid‐Atlantic Ridge, we have identified 17 large, domed edifices (megamullions) that have surfaces corrugated by distinctive mullion structure and that are developed within inside‐corner tectonic settings at ends of spreading segments. The edifices have elevated residual gravity anomalies, and limited sampling has recovered gabbros and serpentinites, suggesting that they expose extensive cross sections of the oceanic crust and upper mantle. Oceanic megamullions are comparable to continental metamorphic core complexes in scale and structure, and they may originate by similar processes. The megamullions are interpreted to be rotated footwall blocks of low‐angle detachment faults, and they provide the best evidence to date for the common development and longevity (∼1–2 m.y.) of such faults in ocean crust. Prolonged slip on a detachment fault probably occurs when a spreading segment experiences a lengthy phase of relatively amagmatic extension. During these periods it is easier to maintain slip on an existing fault at the segment end than it is to break a new fault in the strong rift‐valley lithosphere; slip on the detachment fault probably is facilitated by fault weakening related to deep lithospheric changes in deformation mechanism and mantle serpentinization. At the segment center, minor, episodic magmatism may continue to weaken the axial lithosphere and thus sustain inward jumping of faults. A detachment fault will be terminated when magmatism becomes robust enough to reach the segment end, weaken the axial lithosphere, and promote inward fault jumps there. This mechanism may be generally important in controlling the longevity of normal faults at segment ends and thus in accounting for variable and intermittent development of inside‐corner highs.
Starch columns: Analog model for basalt columns
Authors: Müller Gerhard
Published: February 2004

Desiccation of starch‐water mixtures produces tensile‐crack patterns which appear to be interesting, but largely unknown study objects for fracture mechanics, structural geology, and volcanology. This paper concentrates on columnar jointing and on columns in starch. Starch columns have polygonal cross sections and are very similar to basalt columns. They are produced by lamp drying starch specimens with dimensions of several centimeters and have diameters in the millimeter range. The columns develop behind a crack front which propagates from the surface into the interior. The experiments, supported by X ray tomograms, show that polygonal regularity of the crack pattern is not present at the surface but develops during penetration. This transition is steered by a minimum‐fracture‐energy principle. The analogy between basalt cooling and starch desiccation is far reaching: water concentration in starch is analogous to temperature in basalt, both quantities obey diffusion equations, water loss is equivalent to heat loss, the resulting contraction stresses have similar dependences on depth and time, and in both cases the material strength is exceeded. The starch experiments show that column diameters are controlled by the depth gradient of water concentration at the crack front. High (low) gradients are connected with thin (thick) columns. By analogy, a similar relation with the temperature gradient exists for basalt columns. The (normalized) starch gradients are about 3 orders of magnitude larger than the (normalized) gradients in basalt. This explains why starch columns are much thinner than basalt columns. The gradients are so different, because the crack front speeds differ by a factor of about 10: after 3 days the speed is about 10 mm/d in starch but about 100 mm/d in basalt [Peck, 1978]. The speed difference, in turn, results from the difference of the diffusion constants: the hydraulic diffusivity of starch is 2 orders of magnitude lower than the thermal diffusivity of basalt.
Postrifting seismicity and a model for the 1993 diking event on the Coaxial segment, Juan de Fuca Ridge
Authors: Sohn Robert A. , Hildebrand John A. , Webb Spahr C.
Published: February 2004

Microearthquake data from two deployments of ocean bottom seismographs made shortly after the June/July 1993 rifting episode on the CoAxial segment of the Juan de Fuca Ridge constrain the propagation path of the magmatic dike and provide insight into the effect of the dike injection on the axial stress field. The first deployment, consisting of five instruments in a small array (aperture of ∼2 km) at the eruption (Flow) site (46°30′N), was made 3 months after the episode. The second deployment, consisting of two larger arrays (apertures of ∼9 km) of five instruments each, one at the eruption site and one at a site of vigorous venting of bacterial flocculent material (Floe) site (46°18′N), was made 11 months after the eruption. Epicenters from nine posteruption earthquakes near the Floe site at the south end of the segment are confined to a narrow zone on the rift axis. Epicenters from 61 earthquakes observed near the Flow site are located throughout the rift valley and include an event cluster just south of the observed lava flows. We combine these posteruption earthquakes with T wave data recorded by SOSUS hydrophones during the rifting episode to develop a model for the diking event. In this model, magma propagated at depth along the axial rift zone until reaching the eruption site near 46°31′N. During the next few days the dike swelled as magma accumulated behind the arrested tip, elevating local stress levels and generating a swarm of seismic activity. About 3 days after the magma reached the north end of the segment, the top of the dike penetrated the seafloor surface, resulting in eruptions along the rift zone at the Flow site.
Displacements and stresses induced by a point source across a plane interface separating two elastic semi‐infinite spaces: An analytical solution
Authors: Tinti Stefano , Armigliato Alberto
Published: February 2004

The problem of computing the static deformations and stresses produced by a point source in a homogeneous infinite medium was solved by Volterra [1907] in a closed analytical form at the beginning of this century. The similar problem of computing fields generated by point sources in a homogeneous half‐space bounded by a free surface was later studied by Steketee [1958a,b] and several others [see Okada, 1985, 1992], who focused on point as well as on rectangular fault sources of interest in seismology. Here the model taken into account consists of two elastic half‐spaces characterized by different elastic properties (rigidity modulus μ and Poisson coefficient v ) and separated by a planar interface: assuming that a point source is active in one half‐space, static deformations and stresses generated by the source in the whole space are computed. The similar problem of two half‐spaces welded together was solved by Heaton and Heaton [1989], but they imposed the simplifying constraint that both materials are Poissonian ( i.e., both have the same Poisson coefficient v=0.25). The present approach, which is based on the Galerkin vector method, is general and applicable to an arbitrary point source. In this paper the computations have been carried out explicitly only for the special case of a dislocation source having the form of a strike‐slip double couple. The solution is provided in a closed analytical form by means of expressions involving the source descriptors (position and intensity) as well as the elastic parameters of the heterogeneous medium. The solutions have been illustrated and discussed with special attention given to the dependence of the displacement and stress components on the elastic parameters of the model. One interesting finding concerns the limiting case when the rigidity modulus of the half‐space not containing the point source is equalized to zero. Although the solution in this half‐space no longer makes sense, the solution in the other reduces exactly to the one computed for a half‐space with a free surface, that is, to the solutions computed by Steketee [1958a] and Okada [1985] following an alternative approach.
The static stress change triggering model: Constraints from two southern California aftershock sequences
Authors: Hardebeck Jeanne L. , Nazareth Julie J. , Hauksson Egill
Published: February 2004

Static stress change has been proposed as a mechanism of earthquake triggering. We quantitatively evaluate this model for the apparent triggering of aftershocks by the 1992 MW 7.3 Landers and 1994 MW 6.7 Northridge earthquakes. Specifically, we test whether the fraction of aftershocks consistent with static stress change triggering is greater than the fraction of random events which would appear consistent by chance. Although static stress changes appear useful in explaining the triggering of some aftershocks, the model's capability to explain aftershock occurrence varies significantly between sequences. The model works well for Landers aftershocks. Approximately 85% of events between 5 and 75 km distance from the mainshock fault plane are consistent with static stress change triggering, compared to ∼50% of random events. The minimum distance is probably controlled by limitations of the modeling, while the maximum distance may be because static stress changes of <0.01 MPa trigger too few events to be detected. The static stress change triggering model, however, can not explain the first month of the Northridge aftershock sequence significantly better than it explains a set of random events. The difference between the Landers and Northridge sequences may result from differences in fault strength, with static stress changes being a more significant fraction of the failure stress of weak Landers‐area faults. Tectonic regime, regional stress levels, and fault strength may need to be incorporated into the static stress change triggering model before it can be used reliably for seismic hazard assessment.
Absence of earthquake correlation with Earth tides: An indication of high preseismic fault stress rate
Authors: Vidale John E. , Agnew Duncan Carr , Johnston Malcolm J. S. , Oppenheimer David H.
Published: February 2004

Because the rate of stress change from the Earth tides exceeds that from tectonic stress accumulation, tidal triggering of earthquakes would be expected if the final hours of loading of the fault were at the tectonic rate and if rupture began soon after the achievement of a critical stress level. We analyze the tidal stresses and stress rates on the fault planes and at the times of 13,042 earthquakes which are so close to the San Andreas and Calaveras faults in California that we may take the fault plane to be known. We find that the stresses and stress rates from Earth tides at the times of earthquakes are distributed in the same way as tidal stresses and stress rates at random times. While the rate of earthquakes when the tidal stress promotes failure is 2% higher than when the stress does not, this difference in rate is not statistically significant. This lack of tidal triggering implies that preseismic stress rates in the nucleation zones of earthquakes are at least 0.15 bar/h just preceding seismic failure, much above the long‐term tectonic stress rate of 10−4 bar/h.
On the interpretation of subevents in teleseismic waveforms: The 1994 Bolivia deep earthquake revisited
Authors: Ihmlé Pierre F.
Published: February 2004

We evaluate the methodologies that aim at resolving the space‐time slip distribution during large earthquakes. In one common approach, the source process of an event is assumed to be composed of a series of subevents. Critical points in teleseismic body waves, such as peaks and coherent arrivals, are viewed as discrete points in the spatiotemporal history of an earthquake. We show that these features can be interpreted as the instantaneous centroid locations of the moment release history. We demonstrate this result with an analysis of the Mw=8.2 Bolivia 1994 deep event. We reconstruct the slip distribution in two contrasting ways. First, broadband P waves are interpreted as being composed of subevents. Five peaks in the moment release rate are located relative to the onset of the rupture using the azimuthal variations of onset‐to‐peak times. Second, we invert broadband P waves for the slip distribution on a subhorizontal fault. We use a nonlinear tomographic imaging technique called simulated annealing and an L1 misfit norm. Rupture velocities appear to be <2 km s−1, and largest slip values are found to the east and north of the onset in a 40 by 50 km region. The instantaneous centroids of the moment release history correspond closely to the locations found using the subevent analysis. However, especially near the middle and the end of the rupture, the instantaneous centroid locations are in regions of little moment release. We discuss how the subevent analysis and the instantaneous centroid approach lead to dissimilar interpretations of rupture processes, especially for events with length to width ratios near unity. Another result of this study is that the rupture of the 1994 Bolivia event appears to end 50 km to the northwest of the onset, implying that the slip area is larger that previously thought. Comparing the slip distribution with the aftershock distribution suggests that more than one physical mechanism is necessary to account for the rupture process of this great deep event.
The Hatton Basin and continental margin: Crustal structure from wide‐angle seismic and gravity data
Authors: Vogt Ulrike , Makris Jannis , O'Reilly Brian M. , Hauser Franz , Readman Peter W. , Jacob A. W. Brian , Shannon Pat M.
Published: February 2004

Results from a wide‐angle seismic and gravity study between the Rockall Bank and the Iceland Basin in the North Atlantic are presented. Crustal and sedimentary structures are resolved in the Hatton Basin and across the Hatton continental margin (HCM) east of magnetic anomaly 24. The structure of the oceanic crust west of the anomaly is also determined. Gravity data support the seismic model in areas of good seismic coverage and are used to control the model where the wide‐angle seismic data are poor. A two‐layer sedimentary sequence is present both in the Hatton Basin and across the continental margin. The lower layer, with P wave velocity of about 4 km/s, is interpreted as pre‐Eocene synrift sediments and is up to 3.5 km thick. A younger and thinner (1–2.5 km) postrift sequence, with a velocity of about 2 km/s, defines a strong velocity contrast, which suggests an erosional unconformity surface. The sedimentary structure is distinctly different from that in the Rockall Trough, where a third intermediate layer (Vp ≈ 3 km/s) occurs. The three‐layer crust, characterized by two intracrustal reflections (PiP1 and PiP2) varies from 30 km thick under the Rockall Bank to about 15 km below the Hatton Basin, where it is stretched by a factor of 2 relative to onshore Ireland. The crust is thinnest below the Hatton Bank, where the presence of a single intracrustal reflection indicates that the lower crustal layer thins to below the seismic resolution limit. Below the HCM a region of thick lower crust with anomalously high velocity (Vp ≈ 7.2 km/s) is resolved by the seismic and gravity data. It is connected (west of anomaly 24) to a region of oceanic crust, which is thicker than in the Iceland Basin. These relationships between the thick lower crust below the HCM and the oceanic crust in the Iceland Basin are interpreted as evidence for magmatic underplating, consistent with previous models for the HCM. The inferred unconformity surface between the synrift and postrift layers may be due to regional uplift driven by upwelling of hot asthenosphere before anomaly 24 (early Eocene) time.
Thermochemical regime of the early mantle inferred from numerical models of the coupled magmatism‐mantle convection system with the solid‐solid phase transitions at depths around 660 km
Authors: Ogawa Masaki , Nakamura Hidenori
Published: February 2004

A numerical model is presented for the coupled magmatism‐mantle convection system in the entire mantle to study how the coupled system controls the thermochemical state of the mantle under the influence of the solid‐solid phase transitions at depths around 660 km depending on the internal heating rate. The solid‐state convection in the mantle is modeled by a convection of a binary eutectic material with constant viscosity in a two‐dimensional square box uniformly heated by an internal heat source. One of the end‐members of the material stands for an olivine‐rich material in the uppermost mantle and is transformed into its high‐pressure phase at depths greater than a threshold around 660 km. The phase boundary has a negative Clausius‐Clapeyron slope. Another end‐member stands for a garnet‐rich material in the uppermost mantle and is gradually transformed into its high‐pressure phase with increasing depth in a depth range around 660 km. The density of the material depends on its composition, phase, melt content, and temperature. Magmatism is modeled by a permeable flow of melt generated by a pressure release partial melting of the material. The permeable flow is driven by the buoyancy of the melt. There are two regimes in the thermochemical state of the mantle regardless of the strength of the barrier against mass exchange between the upper mantle and the lower mantle due to the solid‐solid phase transitions. On one regime called the TC branch, magmatism occurs only mildly at most, the mantle remains chemically homogeneous as a whole, and the solid‐state convection occurs dominantly as a thermal convection. The convective circulation is mantle‐wide or layered depending on the strength of the barrier due to the solid‐solid phase transitions. The TC branch is stable only when the internal heating rate is lower than a threshold. A bifurcation occurs on the TC branch at the threshold, and the thermochemical state falls on another regime called the CS branch above the threshold. An episodic magmatism actively occurs to make the mantle chemically stratified with the upper mantle largely occupied by olivine‐rich residual materials and the deeper part of the lower mantle occupied by magmatic products of basaltic composition on the CS branch. The solid‐state convection occurs as a whole mantle convection when the barrier is weak, while it occurs as a layered convection punctuated by flushing events that induce particularly vigorous magmatic activities when the barrier due to the phase transition of the garnet‐rich material is sufficiently strong. The overall features of the thermochemical state on the CS branch mesh in many observations from the Archean and early Proterozoic continents.
Determination of oceanic dynamic barometer corrections to atmospheric excitation of Earth rotation
Authors: Dickman S. R.
Published: February 2004

The excitation of Earth's rotation by atmospheric pressure fluctuations, on timescales from days to years, is modified by the oceanic response to the pressure load. That response has often been approximated as a static, “inverted barometer” (IB) change in sea level; but evidence from theory and both tide gauge and altimetric data suggests that the IB poorly models the sea level response at short periods. Static models also fail to account for the currents induced by the pressure loading. The actual, dynamic response of the oceans to barometric loads depends on frequency and on the spatial structure of the loading. This work presents a Green's function approach to determining the rotational effects of that oceanic response. The Green's functions characterize the rotational effects of the oceanic response to pressure loads described spatially by individual spherical harmonics. For each harmonic of forcing they were computed at nine periods, ranging from 4 days to 18 years, and interpolated at intermediate periods. Computations were based on solutions to the author's dynamic spherical harmonic ocean tide model. The effects on rotation of the “dynamic barometer” oceanic response are obtained by a convolution‐type combination of the Green's functions with actual atmospheric pressure data. Results for data spanning the past decade will be illustrated using harmonically decomposed National Center for Environmental Prediction (NCEP) and European Centre for Medium‐Range Weather Forecasts (ECMWF) pressure fields.