bluuebunny/crossref_metadata_2025 huggingface dataset:
On the stability of perpendicular particle drifts in cold magnetoplasmas
Authors: Brinca A. L. , Romeiras F. J. , Gomberoff L.
Published: October 2002

Assessment of the stability of perpendicular particle drifts in cold magnetoplasmas shows that their free energy can stimulate wave activity under various circumstances that include nonoscillatory (aperiodic, purely growing) instabilities and growth with zero‐drift thresholds. The theoretical model uses gravity as a means of originating the perpendicular drifts but does not address the stability of the macroscopic plasma‐gravity system. It adopts a homogeneous zero‐order equatorial‐plane configuration with the gravity force perpendicular to the background magnetic field; the wave matrix and dispersion equations are derived for arbitrary directions of propagation and magnetized plasma populations, and the influence of the ignored inhomogeneity of the equilibrium medium on the discussed wave activity is shown to be negligible. Even for atomic hydrogen magnetoplasmas and low frequencies (much smaller than the upper hybrid frequency), instabilities are found for the three principal directions (magnetic field, gravity and drift), with some of them maximizing their growth rates away from these axes. The analysis, to be extended to hot plasmas, incorporates the influence of the background magnetic field on the current‐carrying particles, recovers, where appropriate, classical instability results, and can also provide insight to the phenomenology encountered in space environments whose perpendicular currents and particle drifts arise from alternative generating mechanisms. Evaluation of the influence of the neglected inhomogeneity of the zero‐order medium indicates that the discussed wave activity persists.
Equator‐S observations of ion cyclotron waves outside the dawnside magnetopause
Authors: Dunlop M. W. , Lucek E. A. , Kistler L. M. , Cargill P. , Balogh A. , Baumjohann W.
Published: October 2002

The Equator‐S satellite often remained close to the magnetopause for long periods on the morningside of the magnetosphere. The combination of this coverage, slow magnetopause crossings (∼1–2 km/s), and high‐resolution (64–128 Hz) magnetic field data allows very close inspection of the region adjacent to the magnetopause. Strong signatures consistent with mirror‐like structures were observed on a large minority (30%) of the orbits. For the majority of these passes the signatures start close, or adjacent, to the magnetopause, despite a variety of upstream solar wind conditions, suggesting that in most cases the plasma depletion layer (PDL) was either narrow or absent. Without extensive plasma data the PDL cannot be observed directly. Using high‐resolution magnetic field data, however, we have surveyed the occurrence of electromagnetic ion cyclotron (EMIC) waves in the magnetosheath, which have been used by previous authors as a proxy for PDL occurrence. Only a very small number (11) of the 130 documented crossings showed the presence of an ion cyclotron signature in the adjacent magnetosheath, supporting the hypothesis that the PDL is rare in this region. Two of these crossings had proton number density data available, although the wave signatures were exceptionally weak and short‐lived. In these cases we find no clear evidence for a region of plasma depletion, suggesting that the relationship between a PDL and short bursts of EMIC wave occurrence may not be straightforward.
Variability of magnetospheric storms driven by different solar wind perturbations
Authors: Huttunen K. Emilia J. , Koskinen Hannu E. J. , Schwenn Rainer
Published: October 2002

We have investigated the solar wind drivers of magnetic storms during the rising phase of solar cycle 23 from January 1996 to December 1999. We used observations of coronal mass ejections (CMEs) by the Large Angle and Spectrometric Coronagraph instrument on SOHO and in situ solar wind observations by Wind, IMP 8, and ACE spacecraft. The storms were determined from both the Dst and Kp indices, and the study was limited to storms with Dst ≤ −50 nT or Kp ≥ 5. We show examples of different behavior of Dst and Kp indices during magnetic storms caused by different types of solar wind drivers. Furthermore, we have investigated cross‐correlation between peak Dst and Kp values of storms organized according to the associated solar wind driver. It makes a difference whether a sheath region or the following ejecta causes the storm. We found that almost all intense and stronger magnetic storms (Dst ≤ −100 nT, or Kp ≥ 7−) were associated with shocks and CMEs, but for moderate storms, driver statistics were different in different phases of the solar cycle. We found different behavior of the Kp and Dst indices during different types of solar wind drivers. Intense and short‐time disturbances, like postshock streams and sheath regions, generated more Kp storms, and ejecta generated more Dst storms. Thus one should be careful when comparing studies based on any single activity index.
Geomagnetically trapped light isotopes observed with the detector NINA
Authors: Bakaldin A. , Galper A. , Koldashov S. , Korotkov M. , Leonov A. , Mikhailov V. , Murashov A. , Voronov S. , Bidoli V. , Casolino M. , De Pascale M. , Furano G. , Iannucci A. , Morselli A. , Picozza P. , Sparvoli R. , Boezio M. , Bonvicini V. , Cirami R. , Vacchi A. , Zampa N. , Ambriola M. , Bellotti R. , Cafagna F. , Ciacio F. , Circella M. , De Marzo C. , Adriani O. , Papini P. , Spillantini P. , Straulino S. , Vannuccini E. , Ricci M. , Castellini G.
Published: October 2002

The detector New Instrument for Nuclear Analysis (NINA) aboard the satellite Resurs‐01‐N4 detected hydrogen and helium isotopes geomagnetically trapped, while crossing the South Atlantic Anomaly. Deuterium and tritium at L shell < 1.2 were unambiguously recognized. The 3He and 4He power law spectra, reconstructed at L shell = 1.2 and B < 0.22 G, have indices equal to 2.30 ± 0.08 in the energy range 12–50 MeV nucleon−1 and 3.4 ± 0.2 in 10–30 MeV nucleon−1, respectively. The measured 3He/4He ratio and the reconstructed deuterium profile as a function of L shell bring one to the conclusion that the main source of radiation belt light isotopes at Resurs altitudes (∼800 km) and for energy greater than 10 MeV nucleon−1 is the interaction of trapped protons with residual atmospheric helium.
Origin of the May 1998 suprathermal particles: Solar and Heliospheric Observatory/Charge, Element, and Isotope Analysis System/(Highly) Suprathermal Time of Flight results
Authors: Bamert K. , Wimmer‐Schweingruber R. F. , Kallenbach R. , Hilchenbach M. , Klecker B. , Bogdanov A. , Wurz P.
Published: October 2002

The Solar and Heliospheric Observatory/Charge, Element, and Isotope Analysis System/(Highly) Suprathermal Time of Flight ((H)STOF) mass spectrometer measures the elemental composition and the charge state distribution of ions with suprathermal energies in the range 10–4000 keV amu−1 from above the solar wind particle distribution up to low‐energy flare particle energies. We analyze the time period around the large coronal mass ejection event on 2–3 May 1998. Using data from (H)STOF, we study the charge state distribution of He, the CNO group, and Fe. In addition, we investigate the energy dependence of the mean ionic charge state of Fe in the energy‐per‐nucleon range 12–100 keV amu−1. Furthermore, we also report energy spectra of H+, He++, and He+ and the variations in the elemental abundance ratios He/H, He/CNO, and Fe/CNO. These observations greatly extend the energy range in which particles associated with this time period have been measured. Because of their elevated energies, suprathermal particles are the prime seed population for further acceleration in gradual events. We identify interstellar pickup ions as an important nonsolar seed population. The observed low mean ionic charge states of Fe and the small Fe/CNO ratio are typical for large gradual events. The energy dependence of the Fe charge states may be a result of the presence of different iron populations or of the thermal history of the accelerated material. The temporal variations of Fe/CNO and He/CNO indicate a stronger confinement of low‐rigidity particles at the acceleration site.
Testing global storm‐time electric field models using particle spectra on multiple spacecraft
Authors: Angelopoulos V. , Temerin M. , Roth I. , Mozer F. S. , Weimer D. , Hairston M. R.
Published: October 2002

During a magnetic storm on 17–18 February 1998, several spacecraft sampled the inner magnetosphere over a wide range of L shells simultaneously and provided a unique opportunity to obtain particle spectra as a function of L shells, latitude, and local time. We present phase space modeling in a dipole magnetic field and with model electric fields during the recovery phase of that storm to explain the ion spectrograms obtained on three spacecraft (POLAR, EQUATOR‐S, and FAST). The particle signatures studied are from the late recovery phase of the storm, but the particles are affected by the electric fields along their trajectories also during main phase. Our goal is to test and possibly improve global electric field models, which are crucial to the evolution of the storm‐time particle distributions. We backtrace ion distributions from the satellite locations and keep track of charge exchange losses. We use the Volland‐Stern, Weimer 96, Weimer 2000, and modifications of Weimer models to best fit instantaneous potential measurements made by the electric field instrument on POLAR and the ion drift meter instrument on the Defense Meteorological Satellite Program fleet of satellites. We incorporate corotation and a simple, global axisymmetric inductive electric field due to the ring current changes. Significant differences with ion spectral observations do exist and cannot be accounted for simply by modification of existing models. Explaining those differences requires addition of local inductive electric fields or nightside injections.
Net field‐aligned currents controlled by the polar ionospheric conductivity
Authors: Nakano Shin'ya , Iyemori Toshihiko , Yamashita Satoru
Published: October 2002

The results of an analysis of the east‐west magnetic variations at midlatitudes suggest that the net field‐aligned currents, which are the integral of the downward and upward field‐aligned currents along a meridian, are strongly controlled by the polar ionospheric conductivity. The east‐west magnetic disturbances at midlatitudes are expected to be the magnetic effects of the net field‐aligned currents in the polar region. We have examined the relation between the auroral electrojet intensity represented by the auroral electrojet (AE) index and the net field‐aligned current intensity represented by the east‐west magnetic disturbances at midlatitudes. It is deduced from this analysis that the net field‐aligned currents are upward on the nightside and downward on the dayside. It is also derived that the net upward field‐aligned currents on the nightside are stronger in winter than in summer. This seasonal dependence can be explained by the seasonal variation of the polar cap ionospheric conductivity.
Some expected impacts of a solar energetic particle event at Mars
Authors: Leblanc F. , Luhmann J. G. , Johnson R. E. , Chassefiere E.
Published: October 2002

The effect of a typical solar energetic particle event on the Martian atmosphere is described. Sputtering, Lyman α emission, atmospheric heating, and the radiation flux at the surface are calculated using a model of the solar wind interaction with the Martian atmosphere. The effect of the crustal magnetic field is also investigated. We provide estimates of the associated escape flux of neutral atoms and molecules. We compare the atmospheric energy deposition due to the incident solar energetic particles with the energy deposition due to other sources such as solar EUV/IR, solar wind protons and Martian energetic neutral atoms derived from pickup ions. We show that atmospheric sputtering efficiency is small compared with that for pickup ions for the present epoch but that the energy deposition at the homopause is important compared with the EUV/IR energy deposition. While some areas of the atmosphere are shielded from the incident flux by the crustal magnetic fields, the energetic particle flux reaching the surface is virtually unattenuated. This study of “space weather” at Mars raises a number of questions that need to be addressed with new observations and models.
Nonlinear interactions in the upper atmosphere: The s = 1 and s = 3 nonmigrating semidiurnal tides
Authors: Angelats i Coll M. , Forbes J. M.
Published: October 2002

A semidiurnal oscillation with zonal wave number s = 1 has been observed over the South Pole, and longitudinal variations of the semidiurnal tide at northern midlatitudes have been ascribed to the presence of nonmigrating tides. A three‐dimensional nonlinear spectral model is used herein to study nonlinear wave‐wave interaction as a possible mechanism for the generation of nonmigrating tides in the mesosphere and lower thermosphere. In particular, the s = 1 and s = 3 nonmigrating semidiurnal tides are studied as a by‐product of the nonlinear interaction between the semidiurnal s = 2 migrating tide and a stationary s = 1 planetary wave in the lower and middle atmosphere. This process is found to yield significant amplitudes (∼10–30 m s−1) of the nonmigrating semidiurnal components with s = 1 and s = 3 in the upper atmosphere. Comparisons with monthly nonmigrating tidal amplitudes and phases at 95 km derived from High Resolution Doppler Imager and Wind Imaging Interferometer wind measurements from UARS show good agreement with the model simulations. The effect of the dissipating nonmigrating tides on the zonal mean circulation is found to reinforce that of the migrating component, contributing ∼10–20 m s−1 to the zonal wind field and ∼5–10 m s−1 to the mean meridional wind field. Several other wave components are generated through nonlinear interactions, including 6‐hour tides with zonal wave numbers s = 0, 1, 4 and a semidiurnal oscillation with s = 6. These wave components have amplitudes in the range 6–30 m s−1 in the 110–150 km height regime. The combination of all nonmigrating tides together with the migrating component suggests significant longitudinal variations in the semidiurnal tide in the E region which may be important for studies of the quiet‐time generation of ionospheric electric fields by the dynamo mechanism.
Nonlinear model for electron phase‐space holes in magnetized space plasmas
Authors: Jovanović D. , Shukla P. K. , Stenflo L. , Pegoraro F.
Published: October 2002

We formulate a drift‐kinetic theory of electron phase‐space vortices in magnetized space plasmas. It provides the theoretical explanation for the spatial and temporal scales of the electrostatic bipolar structures that have been observed by the Geotail and FAST spacecrafts in Earth's magnetotail and in the auroral zone. Our new nonlinear model accounts for the effects of both the electron polarization and the anisotropic electron temperature. Several different types of quasi‐three‐dimensional electron holes are found in the form of either cylinders or ellipsoids. Although topologically different, they produce similar signals on the spacecraft under typical FAST and Geotail observational conditions and cannot be distinguished with the use of the existing scarce satellite observational data.
Architecture of the volcanic layer from the Costa Rica Rift, constraints from core‐log integration
Authors: Haggas S. L. , Brewer T. S. , Harvey P. K.
Published: October 2002

An understanding of the architecture of the ocean crust's volcanic layer is critical since this records the variations in volcanic activity during formation and subsequently controls the movement of hydrothermal fluids through this layer. Unfortunately, core recovery in the majority of Ocean Drilling Program (ODP)/Deep Sea Drilling Program basement holes is generally low and biased toward more massive lithologies. Consequently, the identification and estimation of lithological diversity within the volcanic layer is only possible through the integration of core and wire line logging results. Wire line logging provides nearly continuous records of both physical and chemical properties of the borehole wall, which can be used to identify the various volcanic/sedimentary lithologies, which then can be calibrated in regions of enhanced core recovery. Using this approach, the architecture of the volcanics from ODP Hole 896A has been reconstructed. This new model illustrates a hiatus in volcanism marked by a thick breccia horizon, which probably marks the transition from axis to off‐axis volcanism.
Strike‐slip faulting, normal faulting, and lateral dike injections along a single fault: Field example of the Gljúfurá fault near a Tertiary oblique rift‐transform zone, Borgarfjörður, west Iceland
Authors: Khodayar Maryam , Einarsson Páll
Published: October 2002

The crustal block of Borgarfjörður in western Iceland was caught between the propagating Reykjanes‐Langjökull Rift Zone (RLRZ) and the receding Snæfellsnes Rift Zone (SRZ) during a Tertiary rift jump at ∼7 Ma. This Tertiary rift jump configuration presents an analogy with the presently unstable plate boundary in south and southwest Iceland. Field examination of the Gljúfurá fault in the Tertiary lavas of Borgarfjörður reveals a complicated history of movements and magmatic activity. During its evolution since Tertiary time, this N‐S fault acted as follows: (1) The fault acted as a possible dextral strike‐slip fault, as judged from Riedel fractures in the oldest associated fault breccia. (2) Then the fault acted as a normal fault, resulting in vertical displacement, new brecciation of part of the oldest breccia, hydrothermal activity, and mineralization dominantly in N‐S mode I veins. (3) The fault acted again as a dextral strike‐slip fault, when the fault was injected laterally by dikes. Magma was injected mainly into the N‐S strike‐slip fault and secondarily into adjacent NNE normal faults. Evidence of this stage are the en échelon geometry of the northerly dikes, cooling cracks oblique to dike edges, flow lineation, elongated vesicles, and soft striations on dike edges, as well as bending and displacement of preexisting dikes. (4) After the emplacement and partial erosion of the northerly dikes, the fault acted dominantly as a strike‐slip fault, with possible reactivation as an open fissure. The Gljúfurá fault strikes obliquely to the NNE trending rifting structures active during crustal formation in this area. Its activity was initially related to a transform zone or an oblique rift connected to the SRZ. Then a shift in the plate boundary occurred around 7–6 Ma when the RLRZ became active. During this shift the Gljúfurá fault probably played a role similar to the N‐S strike‐slip faults of the presently active plate boundary within the South Iceland Seismic Zone and the oblique rift of the Reykjanes Peninsula. These faults may, similar to the Gljúfurá fault, change mode and be injected by dikes if they are sufficiently close to a magma source.
A theory of the Earth's background free oscillations
Authors: Fukao Yoshio , Nishida Kiwamu , Suda Naoki , Nawa Kazunari , Kobayashi Naoki
Published: October 2002

It has recently been established that the Earth's free oscillations are continuously excited by phenomena other than earthquakes and that these oscillations constitute the background noise in the normal mode band at quiet sites. On the basis of evidence that the excitation source is at or just above the Earth's surface, a normal mode theory of the Earth's free oscillations excited by random atmospheric loading is developed. The displacement field is expressed in the frequency domain in general terms of the cross‐spectral density of air pressure disturbance. For spatially homogeneous and isotropic disturbance the cross‐spectral density is approximated by the power spectral density and the coefficient of coherence with a coherence length much shorter than the wavelengths of normal modes. With this approximation the spectrum of ground acceleration is represented as the product of the pressure force term and the Earth response term. The final expression of the acceleration spectrum includes the effect of the gravity attraction of a disturbed air mass. A synthetic spectrum is calculated, using a power law decaying air pressure spectrum consistent with observations, assuming a frequency‐dependent coherence length of air pressure fluctuation, taking into account the effect of the gravity attraction of a disturbed air mass. This synthetic spectrum exhibits distinct peaks of fundamental modes and complex troughs consisting of overtone modes, in quantitative agreement with the peaks and troughs of the observed spectrum.
Topographic outcomes predicted by stream erosion models: Sensitivity analysis and intermodel comparison
Authors: Tucker G. E. , Whipple K. X.
Published: October 2002

Mechanistic theories of fluvial erosion are essential for quantifying large‐scale orogenic denudation. We examine the topographic implications of two leading classes of river erosion model, detachment‐limited and transport‐limited, in order to identify diagnostic and testable differences between them. Several formulations predict distinctly different longitudinal profile shapes, which are shown to be closely linked to terrain morphology. Of these, some can be rejected on the basis of unrealistic morphology and slope‐area scaling. An expression is derived for total drainage basin relief and its apportionment between hillslope and fluvial components. Relief and valley density are found to vary with tectonic forcing in a manner that reflects erosion physics; these properties therefore constitute an additional set of testable predictions. Finally, transient responses to tectonic perturbations are shown to depend strongly on the degree of nonlinearity in the incision process. These findings indicate that given proper constraints, fluvial erosion theories can be tested on the basis of observed topography.
Deformation associated with the 1997 eruption of Okmok volcano, Alaska
Authors: Mann Dörte , Freymueller Jeffrey , Lu Zhong
Published: October 2002

Okmok volcano, located on Umnak Island in the Aleutian chain, Alaska, is the most eruptive caldera system in North America in historic time. Its most recent eruption occurred in 1997. Synthetic aperture radar interferometry shows deflation of the caldera center of up to 140 cm during this time, preceded and followed by inflation of smaller magnitude. The main part of the observed deformation can be modeled using a pressure point source model. The inferred source is located between 2.5 and 5.0 km beneath the approximate center of the caldera and ∼5 km from the eruptive vent. We interpret it as a central magma reservoir. The preeruptive period features inflation accompanied by shallow localized subsidence between the caldera center and the vent. We hypothesize that this is caused by hydrothermal activity or that magma moved away from the central chamber and toward the later vent. Since all historic eruptions at Okmok have originated from the same cone, this feature may be a precursor that indicates an upcoming eruption. The erupted magma volume is ∼9 times the volume that can be accounted for by the observed preeruptive inflation. This indicates a much longer inflation interval than we were able to observe. The observation that reinflation started shortly after the eruption suggests that inflation spans the whole time interval between eruptions. Extrapolation of the average subsurface volume change rate is in good agreement with the long‐term eruption frequency and eruption volumes of Okmok.
Modeling of nutation‐precession: Very long baseline interferometry results
Authors: Herring T. A. , Mathews P. M. , Buffett B. A.
Published: October 2002

Analysis of over 20 years of very long baseline interferometry data (VLBI) yields estimates of the coefficients of the nutation series with standard deviations ranging from 5 microseconds of arc (μas) for the terms with periods <400 days to 38 μas for the longest‐period terms. The largest deviations between the VLBI estimates of the amplitudes of terms in the nutation series and the theoretical values from the Mathews‐Herring‐Buffett (MHB2000) nutation series are 56 ± 38 μas (associated with two of the 18.6 year nutations). The amplitudes of nutational terms with periods <400 days deviate from the MHB2000 nutation series values at the level standard deviation. The estimated correction to the IAU‐1976 precession constant is −2.997 ± 0.008 mas yr−1 when the coefficients of the MHB2000 nutation series are held fixed and is consistent with that inferred from the MHB2000 nutation theory. The secular change in the obliquity of the ecliptic is estimated to be −0.252 ± 0.003 mas yr−1. When the coefficients of the largest‐amplitude terms in the nutation series are estimated, the precession constant correction and obliquity rate are estimated to be −2.960 ± 0.030 and −0.237 ± 0.012 mas yr−1. Significant variations in the freely excited retrograde free core nutation mode are observed over the 20 years. During this time the amplitude has decreased from ∼300 ± 50 μas in the mid‐1980s to nearly zero by the year 2000. There is evidence that the amplitude of the mode in now increasing again.
Regional wave propagation in Tanzania, East Africa
Authors: Langston Charles A. , Nyblade Andrew A. , Owens Thomas J.
Published: October 2002

Crust and upper mantle structure under Tanzania, East Africa, is investigated using broadband waveforms and phase travel times collected from regional events recorded by the 1994–1995 Tanzania Broadband Seismic Experiment. Broadband displacement waveforms from the mL 5.9 Rukwa graben earthquake (18 August 1994) are modeled using a wave number integration algorithm and a new “phase time” inversion technique to infer a very simple crustal and upper mantle model that can be used for waveform studies of other regional earthquakes in the area. Displacement amplitudes for all phases in the data are predicted quite well using source parameters determined from a previous study of teleseismic waveforms. The crust is parameterized by a linear gradient in velocity defined by surface and Moho P velocities, Poisson's ratio, and crustal thickness. This simple parameterization is sufficient to explain arrival times and waveshapes of the three‐component waveforms recorded between 200 and 800 km at stations located on the craton and Mozambique belt. The S wave train consists of Sn, multiple Sn, and critically reflected S wave multiples in the crustal waveguide, in addition to the fundamental mode Rayleigh wave. High phase velocity, critical S wave reflections are seen to form the low group velocity Lg phase. Prominent sP depth phases are seen in the regional waveform data which can be used to infer accurate source depths for events in the region. The Tanzania craton and adjacent Mozambique belt crustal structure is simple and laterally homogeneous, showing few effects of rift disruption. High wave velocities in the uppermost mantle suggest little thermal modification from rifting processes of the East African Rift.
Response of the San Andreas fault to the 1983 Coalinga‐Nuñez earthquakes: An application of interaction‐based probabilities for Parkfield
Authors: Toda Shinji , Stein Ross S.
Published: October 2002

The Parkfield‐Cholame section of the San Andreas fault, site of an unfulfilled earthquake forecast in 1985, is the best monitored section of the world's most closely watched fault. In 1983, the M = 6.5 Coalinga and M = 6.0 Nuñez events struck 25 km northeast of Parkfield. Seismicity rates climbed for 18 months along the creeping section of the San Andreas north of Parkfield and dropped for 6 years along the locked section to the south. Right‐lateral creep also slowed or reversed from Parkfield south. Here we calculate that the Coalinga sequence increased the shear and Coulomb stress on the creeping section, causing the rate of small shocks to rise until the added stress was shed by additional slip. However, the 1983 events decreased the shear and Coulomb stress on the Parkfield segment, causing surface creep and seismicity rates to drop. We use these observations to cast the likelihood of a Parkfield earthquake into an interaction‐based probability, which includes both the renewal of stress following the 1966 Parkfield earthquake and the stress transfer from the 1983 Coalinga events. We calculate that the 1983 shocks dropped the 10‐year probability of a M ∼ 6 Parkfield earthquake by 22% (from 54 ± 22% to 42 ± 23%) and that the probability did not recover until about 1991, when seismicity and creep resumed. Our analysis may thus explain why the Parkfield earthquake did not strike in the 1980s, but not why it was absent in the 1990s. We calculate a 58 ± 17% probability of a M ∼ 6 Parkfield earthquake during 2001–2011.
A statistical scaling model for fracture network geometry, with validation on a multiscale mapping of a joint network (Hornelen Basin, Norway)
Authors: Bour Olivier , Davy Philippe , Darcel Caroline , Odling Noëlle
Published: October 2002

Fracture patterns are characterized by a complex geometry which involves a large length distribution and nonhomogeneous density distributions. Here we address the issue of the modeling of this complex geometry over scales through a first‐order model providing the characterization of the number of fractures of a given length and at a given scale. We propose that the simplest model is a power law in both space and fracture length, with three main parameters: the exponent a of a power law length distribution, the fractal dimension D which fixes the scale dependence of the number of fractures, and the fracture density α. We verify the applicability of this model on seven fracture patterns mapped from the metric scale up to almost the kilometric scale in the Hornelen basin. The model efficiently describes fracture network properties at all scales with a single set of parameters. For the Hornelen fracture networks we found a simple relationship between basic exponents, a = D + 1, implying that the fracture network is self‐similar. Through this analysis, we present different methodological developments for deriving the basic exponents a and D and for verifying the consistency of the model. Overall, the main methodological development is about the normalization of the measurements made from different scales of observation. Finally, we discuss both the limitations and the uses of such a model for analyzing the hydraulic and mechanical properties of fracture networks.
Evidence of harmonic tremor from a submarine volcano detected across the Pacific Ocean basin
Authors: Dziak Robert P. , Fox Christopher G.
Published: October 2002

Since the 1950s seismic activity associated with submarine volcanic eruptions has been detected throughout the Pacific Ocean basin using hydroacoustic methods. However, narrowband harmonic tremor with multiple overtones typically associated with subaerial volcanic eruptions has not commonly been observed. This paper presents a series of recently recorded hydroacoustic signals that originated in the Volcano Islands arc south of Japan, a region that frequently experiences submarine eruptions. The signals are characterized by a narrowband, long‐duration, high‐amplitude fundamental centered at 10 Hz with three harmonics at 20, 30, and 40 Hz and are consistent with harmonic tremor signals observed using traditional seismic methods at active subaerial volcanoes throughout the world. These hydroacoustic signals were detected 21 different times over the period from April 1998 through December 1999, typically lasting from several days to several weeks. After a 6‐month hiatus, the signals were detected 26 more times from June 2000 through August 2001. The signals are qualitatively interpreted as a result of resonance of a magma‐gas mixture within a large chamber/conduit near the surface of a shallow (<1 km) submarine volcanic edifice, allowing for efficient release of acoustic energy into the ocean sound channel. During periods of peak energy radiation the tremor could be clearly detected on hydrophones in the western Aleutians, Hawaii, west coast of the United States, and eastern equatorial Pacific and was detected by the French Polynesian seismic network, covering a maximum distance of ∼14,000 km.
Fluid generation and pathways beneath an active compressional orogen, the New Zealand Southern Alps, inferred from magnetotelluric data
Authors: Wannamaker Philip E. , Jiracek George R. , Stodt John A. , Caldwell T. Grant , Gonzalez Victor M. , McKnight J. Donald , Porter Allen D.
Published: October 2002

Forty‐one wideband magnetotelluric (MT) soundings were collected in a 150‐km‐long transect across the Southern Alps of the central South Island of New Zealand, an active compressional orogen. Decomposed MT impedance tensors, vertical magnetic field relations, and reconnaissance soundings at two locations off line imply an approximately two‐dimensional geometry here with average regional geoelectric strike of ∼N40°E, similar to surface geologic trends. Two independent, two‐dimensional inversion algorithms were applied to the MT data, and both imply a concave‐upward (U‐shaped), middle to lower crustal conductive zone beneath the west central portion of the island. The average conductivity of this zone in the strike direction appears to be much higher than that required across strike and may represent anisotropy or along‐strike conductive strands narrower than the transverse magnetic (cross‐strike) mode MT data can resolve. The deep crustal conductor under the Southern Alps is interpreted to represent mainly a volume of fluids arising from prograde metamorphism within a thickening crust. Fluid interconnection and electrical conduction are promoted by shear deformation. The conductor rises northwestward toward the trace of the Alpine Fault but attains a near‐vertical configuration at a depth of ∼10 km and reaches close to the surface 5–10 km inland of the fault trace itself. The transition to vertical orientation at this depth is interpreted to occur as fluids ascend across the brittle‐ductile transition in uplifting schist and approach the surface through induced hydrofractures. The high‐grade schist becomes resistive after depletion of fluids and continues to extrude toward the Alpine Fault. Shallow extensions of the deep high conductivity are coincident with modern, hydrothermal veining and gold mineralization interpreted to be of deep crustal provenance. To the southeast, high conductivity also reaches the surface coincident with a major back thrust fault zone of the doubly vergent Southern Alps orogen, which also exhibits evidence for expulsion of high‐temperature fluids. The higher conductivity inferred along strike (possible anisotropy) could reflect more efficient fluid interconnection in this higher‐strain direction, as well as possible contributions by sheared, fluid‐deposited graphite. Conductivity of the uppermost mantle of the South Island is low, consistent with advection of cold mantle lithosphere into the underlying asthenosphere as suggested by P wave delay studies.
Crust and upper mantle discontinuity structure beneath eastern North America
Authors: Li Aibing , Fischer Karen M. , van der Lee Suzan , Wysession Michael E.
Published: October 2002

Crust and mantle discontinuities across the eastern margin of the North American craton were imaged using P to S converted phase receiver functions recorded by the Missouri to Massachusetts Broadband Seismometer Experiment. Crustal structure constrained by modeling Moho conversions and reverberations shows a variation of Moho depth from a minimum of 30 km near the Atlantic coast to depths of 44–49 km beneath the western Appalachian Province and 38–45 km beneath the Proterozoic terranes in the west. The variation in crustal thickness is substantially greater than that required for local isostasy, unless lower crustal densities are >3110 kg/m3. In the upper mantle, Ps phases corresponding to a discontinuity at depths of 270–280 km were clearly observed beneath the eastern half of the array. Beneath the western third of the array, the receiver function stacks indicate more complex scattering, but weak Ps phases may be generated at depths of roughly 320 km. The transition between these two regions occurs across the eastern edge of the North American lithospheric keel imaged by tomography. The observed phases may be interpreted as conversions from the base of a low‐velocity asthenosphere.
Two‐dimensional magnetotelluric inversion of blocky geoelectrical structures
Authors: Mehanee S. , Zhdanov M.
Published: October 2002

This paper demonstrates that there are alternative approaches to the magnetotelluric (MT) inverse problem solution based on different types of geoelectrical models. The traditional approach uses smooth models to describe the conductivity distribution in underground formations. In this paper, we present a new approach, based on approximating the geology by models with blocky conductivity structures. We can select one or another class of inverse models by choosing between different stabilizing functionals in the regularization method. The final decision, whose approach may be used for the specific MT data set, is made on the basis of available geological information. This paper describes a new way of stabilizing two‐dimensional MT inversion using a minimum support functional and shows the improvement that it provides over traditional methods for geoelectrical models with blocky structures. The new method is applied to MT data collected for crustal imaging in the Carrizo Plain in California and to MT data collected for mining exploration by INCO Exploration.
Effects of depth‐dependent viscosity and plate motions on maintaining a relatively uniform mid‐ocean ridge basalt reservoir in whole mantle flow
Authors: Stegman Dave R. , Richards Mark A. , Baumgardner John R.
Published: October 2002

Mid‐ocean ridge basalts (MORBs) exhibit relatively uniform and depleted rare earth element concentrations compared with ocean island basalts (OIBs). Previous researchers have focused on long‐term (billion‐year timescale) preservation of an enriched and heterogeneous OIB reservoir within the convecting mantle. Such studies commonly conclude that the OIB reservoir must exist in an area which remains isolated from convection, i.e., D″. Here we investigate the maintenance of MORB reservoir homogeneity over shorter timescales in the face of vigorous upper/lower mantle mass exchange (deep subduction), which may be due to two effects: (1) a high‐viscosity lower mantle and/or (2) chaotic mixing due to toroidal flow generated by surface plate motions. We explore this conceptual model using three‐dimensional spherical numerical models that include surface plate motions, radial viscosity variation, and a geophysically plausible model of mantle density contrasts. A correlation dimension method is used to characterize mixing of passive tracers. For a uniform viscosity mantle the upper and lower mantles mix on essentially the same timescales. A factor of 100 viscosity contrast results in a relative mixing time for the lower mantle only ∼30–60% longer than that of the upper mantle. Therefore neither a strong viscosity contrast nor toroidal mixing significantly affects the relative mixing times of the upper and lower mantle. We conclude that return flow from the lower mantle is of similar (depleted) composition and that the depleted MORB source reservoir constitutes most of the mantle, except for a convectively isolated OIB source region at the base of the mantle.
Seismic coda attenuation after the Mw = 6.2 Armenia (Colombia) earthquake of 25 January 1999
Authors: Ugalde Arantza , Vargas Carlos A. , Pujades Lluís G. , Canas José A.
Published: October 2002

Seismic wave attenuation in the central region of the Colombian Andes is studied using coda waves. Most of the events used occurred in the region on the occasion of the Armenia 25 January 1999 earthquake. The estimation of the decay rate of coda amplitudes (called coda Q−1 or Qc−1) is performed by means of the single isotropic scattering method of Sato [1977]. The attenuation parameters Qi−1 (intrinsic absorption), Qs−1 (scattering loss), and Qt−1 (total attenuation) are also estimated using the multiple lapse time window method of Hoshiba et al. [1991]. The frequencies of interest lie between 1 and 15 Hz. Results show that scattering attenuation is predominant for the frequency bands 1.5 ± 0.5 and 13.5 ± 1.5 Hz, whereas the intrinsic absorption and scattering contribute in equal shares to total attenuation for the 3 ± 1, 5 ± 1, 7.5 ± 1.5, and 10.5 ± 1.5 Hz frequencies. A comparison among the estimated attenuation parameters indicates that Qc−1 is close to total attenuation for all the studied frequency bands. On the other hand, no evidence in support of temporal changes of Qc−1 before and after the main shock has been found. Finally, the regionalization of Qc−1 values seems to correlate well with the geotectonic characteristics of the region.
Insights into the mechanism for orogen‐related carbonate remagnetization from growth of authigenic Fe‐oxide: A scanning electron microscopy and rock magnetic study of Devonian carbonates from northern Spain
Authors: Weil Arlo B. , Van der Voo Rob
Published: October 2002

A rock magnetic and SEM study of Devonian carbonates from the Cantabria‐Asturias Region, northern Spain, was undertaken to further our understanding of the pervasive remagnetization of carbonate rocks during the Late Paleozoic, and the mechanism by which these remagnetizations occur. These rocks contain three ancient Late Paleozoic magnetizations. The rock magnetic properties of mineral extracts were compared with those of whole rock chips and “nonmagnetic” residue to deduce magnetic carrier(s) and grain sizes. Hysteresis measurements for rock chips show “typical” wasp‐waisted loops, whereas extract shows typical pseudosingle‐domain‐like (PSD) unrestricted loops. Within all sites, there is a noticeable contribution of superparamagnetic (SP) grains seen in hysteresis properties and low‐temperature magnetization measurements of whole rock chips, whereas a trend away from a strong SP contribution is seen when hysteresis properties of whole rock are compared with those of residue and extract. Consequently, our extraction process (predictably) removes SP grains, while preserving the characteristic fraction of remanence‐carrying material, which behaves like a typical mixture of single‐domain (SD) and PSD magnetite. Paradoxically, the typical “fingerprint” of remagnetized carbonates, as seen in the whole rock data, seems to be a response to abundant SP grains associated with the acquisition of chemical remanent magnetizations (CRM), and not the actual remanence carrying population itself. Scanning electron microscopy (SEM) observations of magnetic extract reveal abundant authigenic Fe‐oxides, characterized as either 10–100 μm Ni‐free spherules or individual 0.1–10 μm euhedral grains. SEM observations of thin sections reveal abundant evidence of fluid flow driven chemical reactions that resulted in formation of new Fe oxide. Such reactions occurred along cracks and grain boundaries and within void space, and are associated with Fe‐rich clay and calcite‐dolomite reactions or as oxidation of Fe‐sulfide framboids. Together, the SEM observations and rock magnetic experiments reveal that the three Late Paleozoic remagnetizations experienced by Cantabria‐Asturias Paleozoic carbonates are CRMs facilitated by the presence of fluids activated during Late Paleozoic Variscan deformation.
An inversion method to analyze rupture processes of small earthquakes using stopping phases
Authors: Imanishi Kazutoshi , Takeo Minoru
Published: October 2002

We propose a method to obtain source parameters, most importantly the rupture velocity, of small earthquakes on the basis of stopping phases. It is assumed that a rupture begins at one focus of an ellipse and spreads radially at a constant rupture velocity on the fault surface, eventually terminating on the elliptical boundary. This model allows us to investigate a wide variety of rupture configurations, ranging from a circular to a unilaterally propagating rupture. In this model we find that two high‐frequency stopping phases, Hilbert transformations of each other, are radiated and that the difference in arrival times between the two stopping phases is dependent on the average value of the rupture velocity, the radius of the major semiaxes of the ellipse, and its ellipticity. These parameters can be estimated by a nonlinear least squares inversion method. We perform simulation tests, which allow us to detect a Hilbert transform pair whose arrival times coincide with ones predicted by an assumed source model from noisy data. These simulation tests show that our inversion method is useful. Since our method does not require an a priori assumption of the average rupture velocity, the orientation of the fault plane, and the circular crack model, it provides us with a tool for estimating accurate source dimensions as well as rupture processes of small earthquakes.
Electrical impedance measurement of plastically deforming halite rocks at 125°C and 50 MPa
Authors: Watanabe Tohru , Peach Colin J.
Published: October 2002

Electrical impedance measurements have been performed on deforming fine‐grained (∼300 μm) synthetic halite rocks containing small quantities of water in order to study the evolution of fluid distribution. Experiments were carried out on four wet samples (H2O ~ 30 ppm) and one “dry” sample (H2O ~ 5 ppm) under nondilatant condition (125°C, 50 MPa confining pressure, and strain rate of ∼5 × 10−7 s−1), using a triaxial deformation apparatus. The mechanical behavior of wet and dry samples is strikingly different. Only ∼30 ppm water greatly weakens halite rocks through grain boundary migration recrystallization. Wet experiments show oscillating stress‐strain curves, reflecting the competition between work hardening and weakening due to dynamic recrystallization. The dry experiment shows neither weakening nor recrystallized microstructure. The measured impedance shows that an interconnected network of water is formed even in the dry sample and that the connectivity in bulk is attained through the very thin (1–10 nm) elements of fluid. The resistivity change with deformation suggests the competition between thinning and thickening of connected fluid paths. The thinning is caused by fluid squirt due to the axial compression, and the thickening is caused by fluid squirt due to the grain growth due to fluid‐assisted grain boundary migration. In wet samples the diffusivity through grain boundary water is estimated to be lower than the bulk water by 5–6 orders of magnitude. In the dry sample, grain boundary water is thinner, and its diffusivity might not be high enough to promote grain boundary migration. Observed migration recrystallization can occur in natural conditions and greatly weaken salt rocks. The impedance measurement is very useful to track the evolution of fluid distribution in deforming materials. For further understanding of mechanical and electrical properties of halite rocks it is essential to clarify properties of thin water films.