bluuebunny/crossref_metadata_2025 huggingface dataset:
Flux transfer events produced by the onset of merging at multiple X lines
Authors: Ku Hwar C. , Sibeck David G.
Published: September 2002

We present two‐dimensional magnetohydrodynamic model predictions for the signatures of flux transfer events (FTEs) at the dayside magnetopause produced by the onset of merging along two or more extended X lines. We consider three scenarios: (1) equal merging rates with identical resistivities south (η1) and north (η2) of the equator, (2) unequal merging rates with η1 > η2, and (3) equal merging rates in the presence of a background northward magnetosheath flow velocity (

). Realistic ratios of magnetosheath to magnetospheric parameters are chosen to conform with in situ observations and our previous simulations [Ku and Sibeck, 1997, 1998a, b]: ρmsh/ρsph = 10, Bmsh/Bsph = 0.5, and Tmsh/Tsph = 0.175. In case 1, a stationary magnetic island forms between two bulges accelerating away from the subsolar point. In case 2, the magnetic island formed between the two X lines pursues the bulge created near the line with smaller resistivity. In case 3 the island formed between the two X lines pursues the bulge by moving in the direction of the background flow. All three scenarios produce events with characteristics similar to those in the single X line model: strong asymmetric bipolar magnetic fields and plasma velocities normal to the magnetopause in the magnetosheath and no significant signatures in the magnetosphere (with the magnitude of the trailing pulse exceeding that of the leading edge). However, the islands produced in cases 2 and 3 also generate more symmetric signatures typical of FTEs in both the magnetosheath and magnetosphere. By comparison with observations, the transition parameter plots that we present can be used to discriminate between the merging scenarios.
Statistical analysis of F region and topside ionospheric ion field‐aligned flows at high latitudes
Authors: Wu X.‐Y. , Horwitz J. L. , Seo Y.
Published: September 2002

A statistical examination is made of ionospheric ion field‐aligned flow velocities, ion densities, and electron and ion temperatures as measured at high latitudes in the Northern Hemisphere with the Dynamics Explorer (DE) 2 spacecraft in the 300–1000 km altitude range. We explore the characteristic trends and morphology of these field‐aligned ion flows in terms of their variations with invariant latitude, altitude, solar zenith angle, plasma temperatures, and geomagnetic activity. Enhancements in the electron and ion temperatures, field‐aligned ion upflows, and ion densities are frequently observed above 600 km over the statistical cleft and auroral oval regions and over wide latitudes on the nightside. Intermittent, subsonic auroral upflows often reach bulk speeds of up to several hundreds meters per second. Upfluxes over the statistical auroral oval are generally in the range 108–109 ions cm−2 s−1, and occasionally up to 1010 ions cm−2 s−1. The nocturnal upflows have lower densities than those on the dayside. Statistical altitude profiles indicate that upflows accelerate above 450 km on the dayside, while detectable nightside upflows start in the lower altitudes. Upfluxes are more closely correlated to the electron temperatures than to the ion temperatures. The topside ion upflow fluxes generally are uncorrelated with Kp index. Downflows are more frequent over the statistical polar cap, especially above 600 km. The electron temperatures are usually 1000 K higher than the ion temperatures on the dayside; on the nightside the mean electron temperatures are comparable or occasionally even lower than those of the ions. The mean day‐night temperature asymmetry is more than 1000 K for electrons and is smaller for the ions. Plasma temperatures generally increase with Kp index. Field‐aligned ion flows of 0–100 m s−1 dominate the high‐latitude F region (300–500 km altitude), and are mostly down‐ward on the dayside but more consistently upward on the nightside. Density enhancements are frequently observed in the dayside statistical auroral region. Density depletions of about 30% are present in the nightside statistical auroral F region, and the depletions deepen with increasing magnetic activity.
Intense growth phase events of substorms
Authors: Shue J.‐H. , Kamide Y. , Elphinstone R. D. , Nishitani N.
Published: September 2002

Over the past few decades, several events of the intense growth phase preceding the expansion onset of substorms were reported. We reexamine these events using an extensive set of data, including ground‐based magnetograms at auroral latitudes, solar wind data from the Wind and IMP 8 satellites, and auroral UV images from the Polar and Viking satellites. Magnetograms show that the intense auroral electrojet is located in the early morning sector during these events. A common feature among the corresponding solar wind conditions for these events is the existence of a period of southward interplanetary magnetic field (IMF) preceded by a prolonged northward IMF. This intense growth phase, however, seems to be independent of the IMF By polarity. Enhanced auroras in the early morning sector appear during the growth phase. These auroras are consistent with relatively large geomagnetic disturbances during these events, indicating that enhanced conductances, combined with enhanced electric field caused by southward IMF, play an important role in the auroral electrojet during these intense growth‐phase events.
Latitudinal dependence of outflow velocities from O VI Doppler dimming observations during the Whole Sun Month
Authors: Strachan Leonard , Panasyuk Alexander V. , Dobrzycka Danuta , Kohl John L. , Noci Giancarlo , Gibson Sarah E. , Biesecker Douglas A.
Published: September 2002

Empirical determinations of outflow velocities in the solar corona provide a much needed constraint, along with density and temperature determinations, of the acceleration and heating mechanisms in the extended corona. Much progress has been made on density determinations from white light polarized brightness observations but outflow velocities have been more difficult to determine. We present the first determinations of outflow velocities versus height and latitude based on a three‐dimensional (3‐D) reconstruction of the O VI 1032 and 1037 À emissivities. The Doppler dimming (and pumping) of the local emissivities give true localized outflow velocities at the selected locations in the extended corona from ∼1.75 to 2.75 solar radii. The velocities are based on an empirical model of the corona which is constrained by the reconstructed O VI emissivities derived from the SOHO Ultraviolet Coronagraph Spectrometer (UVCS) synoptic observations and by electron density determinations based on white light measurements from the SOHO Large Angle Spectroscopic Coronagraph (LASCO) and Mauna Loa Solar Observatory (MLSO) Mk III coronameter.
Driving dayside convection with northward IMF: Observations by a sounding rocket launched from Svalbard
Authors: Maynard N. C. , Burke W. J. , Pfaff R. F. , Weber E. J. , Ober D. M. , Weimer D. R. , Moen J. , Milan S. , Måseide K. , Sandholt P.‐E. , Egeland A. , Søraas F. , Lepping R. , Bounds S. , Acuña M. H. , Freudenreich H. , Machuzak J. S. , Gentile L. C. , Clemmons J. H. , Lester M. , Ning P. , Hardy D. A. , Holtet J. A. , Stadsnes J. , van Eyken T.
Published: September 2002

The first sounding rocket flight into the dayside cusp with dark ground and northward interplanetary magnetic field (IMF) conditions was launched from the new SvalRak range at Ny‐Ålesund in the Svalbard archipelago in early December 1997. Extensive ground observations of auroral emissions and radar backscatter provided contexts for in situ rocket measurements. Real‐time interplanetary measurements from the Wind satellite aided launch selection with foreknowledge of impending conditions. NASA rocket flight 36.153 was launched near local magnetic noon while the IMF was dominated by positive BX and had lesser northward BZ and negative BY components. The rocket's westward trajectory carried it toward auroral forms associated with morningside boundary layers. The rich set of vector dc electric and magnetic fields, energetic particles, thermal plasma, plasma waves, and optical emissions gathered by the rocket reveal a complex electrodynamic picture of the cusp/boundary‐layer region. Four factors were important in separating temporal and spatial effects: (1) Near the winter solstice the Earth's north magnetic pole tilts away from the Sun, (2) at the UT of the flight the dipole axis was rotated toward dawn, (3) the variability of solar wind driving was low, and (4) BX was the dominant IMF component. We conclude that no signatures of dayside merging in the Northern Hemisphere were detected in either the rocket or ground sensors. Electric field variations in the interplanetary medium directly correlate with those observed by the sounding rocket, with significantly shorter lag times than estimated for simple propagation between Wind and the ionosphere. The correlation requires that the observed Northern Hemisphere convection structures were stirred in part by merging of the IMF with closed field lines in the Southern Hemisphere, thereby adding open flux to the northern polar cap. Subsequent motions of adiaroic polar cap boundaries were detected in the rocket electric field measurements. The observations indicate that IMF BX significantly affected the location and timing of merging interactions.
Diffusion of energetic particles in focusing fields
Authors: Kóta József
Published: September 2002

We address the modification of the effective spatial diffusive coefficient of energetic charged particles in magnetic field configurations where the divergence of the field lines and the consequent weakening of the field strength lead to adiabatic focusing. Particles propagating along the magnetic field lines and undergoing pitch angle scattering and adiabatic focusing are considered. In the presence of significant focusing the conditions for the applicability of a diffusive description are not strictly valid, thus focusing modifies the effective parallel diffusion coefficient κ‖. We calculate the appropriate spatial diffusion coefficients from a method based on the use of adjoint Green functions. The correspondence between this method and the method based on the velocity correlation function < νi(0)νj(t) > developed by Kubo [1957] is discussed. We target the modulation of galactic and anomalous cosmic rays, which can be best approximated by the assumption of a constant, or slowly varying, spatial gradients, for which the method of adjoint Green functions is most suitable. We show that this assumption leads to an effective κ‖ identical to that derived by Bieber and Burger [1990] from a Born approximation. The derivation also results in a nonsteady analytical solution to the Fokker‐Planck equation, which describes a distribution of constant spatial gradient moving at a constant speed. We also discuss the case of hemispherical scattering, when scattering is effective within each of the μ < 0 and μ > 0 hemispheres (where μ is the cosine of pitch angle) but is restricted between the two hemispheres. A refinement to the hemispherical equation of Isenberg [1997] and Schwadron [1998] is suggested.
A study of the Jovian “energetic magnetospheric events” observed by Galileo: role in the radial plasma transport
Authors: Louarn Philippe , Roux Alain , Perraut Sylvaine , Kurth William S. , Gurnett Donald A.
Published: September 2002

Using the Galileo Plasma Wave Subsystem (PWS) experiment, we analyze the large‐scale energetic events that recurrently occur in the Jovian magnetosphere. As described by Louarn et al. [1998], these sporadic phenomena are associated with enhancements in the flux of the various auroral radio emissions, with the creation of new sources of radiation in the Io torus, and correspond to large fluctuations in the magnetodisc density. These events have been interpreted as sudden releases of energy in the Jovian magnetosphere. In order to better characterize them we study an extended PWS data set (orbits G2, G7, and G8) corresponding to 130 days of observations during which 32 energetic events have been unambiguously identified. We conclude the following: (1) The periods of enhanced energy releases in the magnetosphere (as indicated by increases in the auroral radio flux) are almost systematically initiated by an energetic event. (2) The periodicity of the events changes from one orbit to the other and it is shown that the more frequent they are, the denser is the plasma sheet. (3) In a large majority of cases the events occur as the disc is thin and relatively depleted in plasma. A few hours after the events, a thick and heavily populated magnetodisc is observed. It then thins, and its density progressively decreases over a timescale of a few tens of hours. Our interpretation is that the events correspond to sequences of rapid plasma loading of the magnetodisc that are followed by much more progressive evacuations of the magnetodisc plasma. The global plasma content of the magnetodisc would thus increase with their frequency. This study suggests that the energetic events are related to an instability developing in the external part of the Io torus or in the close magnetodisc that sporadically injects new plasma populations in the more distant magnetodisc. The associated transport process appears to be efficient enough to explain the outward evacuation of a significant fraction of the plasma created at the Io orbit (a few 1028 ions/s).
Effect of magnetopause leakage on the lifetime of magnetospheric cavity modes
Authors: Freeman M. P.
Published: September 2002

Magnetospheric cavity modes can decay through various loss processes. Energy leakage across the magnetopause is one such process that is commonly neglected. Here we consider what the actual effect of the neglected magnetopause leakage might be on cavity mode lifetime and hence on coupled transverse mode wave growth. First, we review the linear theory of fast mode wave reflection at a tangential discontinuity to show that magnetopause leakage is significant. For realistic subsolar magnetopause conditions we estimate that 20–60% of magnetospheric fast mode wave Poynting flux incident on the magnetopause is transmitted through the magnetopause and out of the magnetospheric cavity. Second, we demonstrate the effect of magnetopause leakage on magnetospheric cavity modes using an MHD simulation. The simulation is of a box‐like cavity with a dipolar variation of the magnetospheric magnetic field and realistic conditions at the subsolar magnetopause, based upon in situ spacecraft observations. The simulation results show that a purely fast mode cavity oscillation decays due to leakage of fast mode wave energy across the magnetopause. The decay rate that arises from magnetopause leakage is comparable to that found in previous simulations from coupling of fast mode energy into transverse mode field line resonances. Thus we conclude that magnetopause leakage is likely to enhance the decay rate of fast and transverse cavity mode oscillations and should be taken into account in field line resonance theory and especially its application to observations.
Cusp observations of high‐ and low‐latitude reconnection for northward IMF: An alternate view
Authors: Russell C. T. , Le G. , Petrinec S. M.
Published: September 2002

During high solar wind dynamic pressure, northward interplanetary magnetic field (IMF) intervals, the POLAR/TIMAS instrument observes angular distribution functions, that have been interpreted in terms of reconnection, below the latitude of the cusp, of nearly parallel magnetic field lines from the magnetosheath with those in the magnetosphere. If true, this process would lead to erosion of the dayside magnetosphere by northward IMF, a process not consistent with the observed control by IMF Bz of the location of the dayside magnetopause, of the polar cusp, or of geomagnetic activity. However, we note that the TIMAS observations do not require such an interpretation and that a simpler interpretation exists, consistent with the current paradigm of IMF effects on the magnetosphere.
Spatial variability of seismicity parameters in aftershock zones
Authors: Wiemer Stefan , Katsumata Kei
Published: September 2002

The spatial variability of thebvalue of the frequency‐magnitude relationship and the decay rate of aftershocks as described by thepvalue of the modified Omori law is investigated. By using dense spatial grids we map out the distribution ofbandpvalues within the Landers, Northridge, Morgan Hill, and Kobe aftershock sequences. Considerable spatial variability is found, withbvalues of independent subvolumes ranging from 0.6 to 1.4, andpvalues ranging from 0.6 to 1.8. These systematic and statistically highly significant differences argue that it is an oversimplification to assign one singlepandbvalue to an aftershock sequence that extends up to 100 km. The spatial distribution of these two parameters is compared with the slip distribution during the mainshock, suggesting that the areas of largest slip release correlate with highbvalue regions. We hypothesize that the frictional heat created during the event may influence thepvalue distribution within an aftershock zone, while applied shear stress, crack density and pore pressure govern the frequency‐magnitude distribution. By investigating the frequency‐magnitude distribution separately for preseismic and postseismic periods for the Morgan Hill mainshock, we find that only the volume in the vicinity of the highest slip release shows a significant increase in thebvalue, which decays to premainshock values within a year. Surrounding areas of the aftershock zone show an approximately constantbvalue with time. Because the aftershock hazard after a mainshock depends strongly on both thebandpvalue, we propose that aftershock hazard assessment can be improved by taking into account the spatial distribution of the parameters.
Anisotropy in the oceanic lithosphere from the study of local intraplate earthquakes on the west flank of the southern East Pacific Rise: Shear wave splitting and waveform modeling
Authors: Hung Shu‐Huei , Forsyth Donald W.
Published: September 2002

Shear wave splitting is observed on ocean bottom seismometer records from local, intraplate microearthquakes on the west flank of the East Pacific Rise at 18°S. Split times reach a maximum of about 0.2 s. For most of the incoming waves with long mantle paths (∼20 km), the polarization direction of the fast arrival is subparallel to the spreading direction, which we attribute to anisotropy caused by the strain‐induced preferential orientation of olivine. In contrast, for some records with short paths or shallower sources or propagation along the spreading direction, the fast direction is nearly parallel to the ridge axis. These polarizations are probably caused by seismic anisotropy from aligned cracks in the uppermost crust. In addition, for some events, the apparent splitting is frequency dependent. To explore the pattern of shear wave splitting that would be expected for nonvertical paths in an oceanic lithosphere with two distinct anisotropic layers, we generate synthetic seismograms for a variety of source depths and mechanisms. We employ a multidomain, pseudospectral method to simulate the elastic wave fields from point sources in an inhomogeneous, anisotropic medium. Splitting parameters measured from synthetic S waves demonstrate that the apparent fast direction is not always parallel to the symmetry axes and that, in some cases, fast directions at higher frequencies will be more characteristic of the shallower crustal anisotropy while fast directions at lower frequencies are dominated by the mantle portion of the path. Most of the observed characteristics of splitting can be reproduced if there is approximately 8% S wave anisotropy in the mantle and an average of about 6% S wave anisotropy in the upper crustal, seismic layer 2.
Dynamics of a phase change at the base of the mantle consistent with seismological observations
Authors: Sidorin Igor , Gurnis Michael , Helmberger Don V.
Published: September 2002

The phase change model for the origin of the D″ seismic discontinuity is tested by comparing the results of convection modeling with seismic observations. We compute a number of global dynamic models that incorporate a phase change at the base of the mantle with different characteristics and transform the resulting temperature field and the distribution of phases to seismic velocities. Over 900 two‐dimensional synthetic waveforms are computed for each of the models from which S, ScS, and Scd phases are picked. The distribution of the relative amplitudes and differential travel time residuals for these phases are statistically compared with the distribution of data from four well studied regions (northern Siberia, Alaska, India, and Central America) in a search for the characteristics of a phase transition that best match these seismic observations. We find that the best fit among the models considered is obtained for phase transitions characterized by a Clapeyron slope of ∼6 MPa K−1 and an elevation above the core‐mantle boundary of ∼150 km under adiabatic temperature or 127 GPa and 2650 K on a (P,T) diagram. Dynamic models demonstrate that the value of Clapeyron slope and the density difference between the phases can have significant influence on the dynamics of plumes but probably only a minor influence on the dynamics of subducted slabs. We find that the thermal structure of subducted slabs can be important in giving rise to the seismic triplication; the strongest Scd arrivals in our models are observed in the area of subduction. The folding of the slab at the base of the mantle leads to patterns in differential travel time distributions consistent with seismic observations and suggests that the largest heterogeneity occurs at the top of the D″ layer or just above it. Analysis of the spatial autocorrelation functions of the differential travel time residuals suggests that their characteristic peaks reflect the patterns of slab folding and may provide constraints on the rheology of slabs at the base of the mantle.
Contrasting styles of seafloor spreading in the Woodlark Basin: Indications of rift‐induced secondary mantle convection
Authors: Martínez Fernando , Taylor Brian , Goodliffe Andrew M.
Published: September 2002

The Woodlark Basin in the southwest Pacific is a young ocean basin which began forming by ∼6 Ma following the rifting of continental and arc lithosphere. The N‐S striking Moresby Transform divides the oceanic basin into eastern and western parts which have contrasting characteristics. Seafloor spreading west of Moresby Transform began after ∼2 Ma, and although spreading rates decrease to the west, the western basin has faster spreading characteristics than the eastern basin. These include (1) ∼500 m shallower seafloor; (2) Bouguer gravity anomalies that are >30 mGals lower; (3) magnetic anomaly and modeled seafloor magnetization amplitudes that are higher; (4) a spreading center with an axial high in contrast to the axial valleys of the eastern basin; (5) smoother seafloor fabric; and (6) exclusively nontransform spreading center offsets in contrast to the eastern basin, which has transform faults and fracture zones that extend across most of the basin. Overall depth contrasts and Bouguer anomalies can be matched by end‐member models of thicker crust (∼2 km) or thinner lithosphere (<1/3) in the western basin. Correlated with these contrasts, the surrounding rifted margins abruptly thicken westward of the longitude of Moresby Transform. We examine alternative explanations for these contrasts and propose that rift‐induced secondary mantle convection driven by thicker western margin lithosphere is most consistent with the observations. Although rift‐induced convection has been cited as a cause for the voluminous excess magmatism at some rifted margins, the observations in the Woodlark Basin suggest that this mechanism may significantly affect the morphology, structure, and geophysical characteristics of young ocean basins in alternate ways which resemble increased spreading rate.
Two‐dimensional anisotropic tomography of lithosphere beneath France using regional arrival times
Authors: Judenherc S. , Granet M. , Boumbar N.
Published: September 2002

This paper presents an anisotropic tomography study beneath France. Travel times of Pn waves were inverted in order to simultaneously retrieve lateral variations of both seismic velocity and seismic anisotropy within the subcrustal lithosphere. The parameterization scheme uses a “classical” approach. The subcrustal lithosphere is modeled using a two‐dimensional horizontal grid of nonoverlapping cells (0.5° × 0.5°), each cell being characterized by an average slowness (the isotropic term of slowness) and an azimuthal anisotropy term. The velocity and anisotropy perturbation amplitudes do not exceed 2% in the Paris Basin and in the Aquitaine Basin. More significant perturbations can be observed in the tectonically active regions. The images of the modeled subcrustal lithosphere emphasize the not recently reactivated areas of the Hercynian Range by high velocities: +2 to +5% from the Armorican Massif to the west part of the Massif Central and from the northeast Massif Central to the Vosges Mountains and Black Forest Mountains regions. These areas are also characterized by an anisotropy of 2–5% with the fast directions parallel to the Hercynian structures. Low velocities (−2 to −4%) are observed in the Neogene volcanic areas of the Massif Central which confirm the results obtained from a short‐wavelength teleseismic tomography. These low velocities are most probably caused by high temperatures. Another low‐velocity region (−3 to −4%) roughly follows the Alpine belt and is interpreted as the image of the crustal root of the range. In the inner part of the Alps, fast Pn directions lie perpendicular to the belt (with an anisotropy amplitude of ∼3%), while, in the outer part, they are parallel to the range (with amplitudes ranging from 3 to 5% to the east and from 2 to 3% to the west). The anisotropy feature of the Pyrenean Range follows a simple pattern: One observes high amplitudes (up to 5%) and a fast direction which is parallel with the strike of the range. The observations show seismic anisotropy for most of the tectonically active zones across France (recently formed structural units and old Hercynian structures as well), and the amplitudes reach the same order as the amplitudes of the velocity perturbations.
Airborne gravity and precise positioning for geologic applications
Authors: Bell R. E. , Childers V. A. , Arko R. A. , Blankenship D. D. , Brozena J. M.
Published: September 2002

Airborne gravimetry has become an important geophysical tool primarily because of advancements in methodology and instrumentation made in the past decade. Airborne gravity is especially useful when measured in conjunction with other geophysical data, such as magnetics, radar, and laser altimetry. The aerogeophysical survey over the West Antarctic ice sheet described in this paper is one such interdisciplinary study. This paper outlines in detail the instrumentation, survey and data processing methodology employed to perform airborne gravimetry from the multi‐instrumented Twin Otter aircraft. Precise positioning from carrier‐phase Global Positioning System (GPS) observations are combined with measurements of acceleration made by the gravity meter in the aircraft to obtain the free‐air gravity anomaly measurement at aircraft altitude. GPS data are processed using the Kinematic and Rapid Static (KARS) software program, and aircraft vertical acceleration and corrections for gravity data reduction are calculated from the GPS position solution. Accuracies for the free‐air anomaly are determined from crossover analysis after significant editing (2.98 mGal rms) and from a repeat track (1.39 mGal rms). The aerogeophysical survey covered a 300,000 km2region in West Antarctica over the course of five field seasons. The gravity data from the West Antarctic survey reveal the major geologic structures of the West Antarctic rift system, including the Whitmore Mountains, the Byrd Subglacial Basin, the Sinuous Ridge, the Ross Embayment, and Siple Dome. These measurements, in conjunction with magnetics and ice‐penetrating radar, provide the information required to reveal the tectonic fabric and history of this important region.
Constraints on the origin of slab and mantle wedge anomalies in Tonga from the ratio of S to P velocities
Authors: Koper Keith D. , Wiens Douglas A. , Dorman LeRoy , Hildebrand John , Webb Spahr
Published: September 2002

We examine two prominent upper mantle velocity anomalies in the southwest Pacific, the Tonga slab anomaly and the corresponding overlying mantle wedge anomaly, using data collected during a combined land‐sea deployment of temporary seismometers. The linear geometry and small interstation spacing of the instruments yield high‐resolution data along a cross section of the Tonga subduction zone, including the actively spreading Lau back arc basin. We estimate the relative variation of P and S velocity, often described as ν = δlnVs/δlnVp, for the slab and mantle wedge anomalies using two distinct methods: a linear regression of the P and S travel time residuals, and detailed modeling of the velocity structure using a three‐dimensional finite difference travel time algorithm. The two methods yield similar results, with ν of the slab being 1.1–1.5 and ν of the mantle wedge being 1.2–1.3. These values are consistent with experimental data concerning the effect of temperature on P and S wave velocities in the upper mantle and are lower than what is expected for velocity anomalies generated by the presence of partial melt. These observations imply that either the theoretical estimates of ν for partial melt are too large or very little partial melt is present beneath the Lau basin. In the latter case, melt must be quickly removed from the rock matrix, such that the velocity anomalies are due to increased temperature, and not melt. The bulk of the velocity anomaly in the mantle wedge can be explained by temperature anomalies of 400–600°C because of the amplification of temperature derivatives of seismic velocity by anelastic effects. Such large thermal anomalies, generated by decreased lithospheric thickness and mantle upwelling beneath the fast spreading Lau back arc basin, can still leave the mantle near the solidus, even after accounting for the effect of increased volatile content in the mantle wedge. The lower‐amplitude velocity reductions in the deeper wedge are likely related to an increased concentration of volatiles from the subducting slab.
Paleointensity record from Pleistocene sediments (1.4–0 Ma) off the California Margin
Authors: Guyodo Yohan , Richter Carl , Valet Jean‐Pierre
Published: September 2002

High resolution magnetic measurements of 45 m of sediment from the Delgada Fan area at Ocean Drilling Program Site 1021 (39°5′N/127°46′W) were performed in order to extract a reliable signal of relative paleointensity. A detailed chronology was established by correlation of the magnetic susceptibility variations to the insolation curve for the past 1.4 Myr covered by the record. The remanence carrier is pseudo‐single‐domain‐sized magnetite that varies in concentration by a factor <3. Stepwise alternating field demagnetization isolated a very stable natural remanent magnetization (NRM) which showed the succession of the Cobb and Jaramillo Subchrons and directional changes associated with the Punaruu event (1.105 Ma). Two techniques have been used to extract the signal of relative paleointensity. A significant climatic component remained present in the curve obtained from the slopes of the NRM versus the anhysteretic remanent magnetization(ARM). In contrast the NRM and the susceptibility (klf) both being driven by orbital forcing, normalization by susceptibility is not contaminated by climatically driven changes and is thus taken as our best estimate of relative paleointensity. Comparison with other high‐resolution records and compilations reinforces the actual geomagnetic origin of the variations displayed by the NRM/k signal. Overall, this record confirms the succession of distinct highs and lows which are concomitant with full field reversals and short‐term geomagnetic events present during the Matuyama Chron. The presence of large oscillations of the field with no periodicity confirms that the geomagnetic field (at least his dipolar part) is highly unstable and may have remained in this mode for a long period of time.
Air blasts generated by rockfall impacts: Analysis of the 1996 Happy Isles event in Yosemite National Park
Authors: Morrissey M. M. , Savage W. Z. , Wieczorek G. F.
Published: September 2002

The July 10, 1996, Happy Isles rockfall in Yosemite National Park, California, released 23,000 to 38,000 m3 of granite in four separate events. The impacts of the first two events which involved a 550‐m free fall, generated seismic waves and atmospheric pressure waves (air blasts). We focus on the dynamic behavior of the second air blast that downed over 1000 trees, destroyed a bridge, demolished a snack bar, and caused one fatality and several injuries. Calculated velocities for the air blast from a two‐phase, finite difference model are compared to velocities estimated from tree damage. From tornadic studies of tree damage, the air blast is estimated to have traveled <108–120 m/s within 50 m from the impact and decreased to <10–20 m/s within 500 m from the impact. The numerical model simulates the two‐dimensional propagation of an air blast through a dusty atmosphere with initial conditions defined by the impact velocity and pressure. The impact velocity (105–107 m/s) is estimated from the Colorado Rockfall Simulation Program that simulates rockfall trajectories. The impact pressure (0.5 MPa) is constrained by the kinetic energy of the impact (1010–1012 J) estimated from the seismic energy generated by the impact. Results from the air blast simulations indicate that the second Happy Isles air blast (weak shock wave) traveled with an initial velocity above the local sound speed. The size and location of the first impact are thought to have injected <50 wt% dust into the atmosphere. This amount of dust lowered the local atmospheric sound speed to ∼220 m/s. The discrepancy between calculated velocity data and field estimated velocity data (∼220 m/s versus ∼110 m/s) is attributed to energy dissipated by the downing of trees and additional entrainment of debris into the atmosphere not included in the calculations.
Body waveform modeling of east Mediterranean earthquakes at intermediate distances (17°–30°) with a Gaussian beam summation method
Authors: Leborgne Sébastien , Madariaga Raúl , Farra Véronique
Published: September 2002

We study eastern Mediterranean earthquakes in the magnitude range 5.5–6.5 by body waveform modeling at distances between 17° and 30°. We compute synthetics at these distances using the Gaussian beam summation (GBS) method, taking into account all waves refracted and reflected by the upper mantle discontinuities. Modeling depends crucially on the choice of an appropriate upper mantle model. Using the well‐known May 13, 1995, Kozani‐Grevena event in Greece, we choose a representative column of the 3SMAC upper mantle model as an average velocity structure for modeling eastern Mediterranean earthquakes recorded in Europe. We studied three events with different tectonic settings, location, and magnitude. For the July 20, 1996, Dodecanese earthquake we determined a depth of 12.5 km, a source duration of 6 s, and a seismic moment of 1.20×1018 Nm. Most of the seismograms for this event are very well modeled. Using only seismograms recorded at the epicentral range 17°–30°, we modeled the October 1, 1995, Turkey event. We find a depth of 9 km and a normal fault mechanism in agreement with the regional tectonic setting. Finally, we studied the February 23, 1995, earthquake in Cyprus for which we retrieved depth, source time function, and a seismic moment of 2.04×1018 Nm. The upper mantle phases for all these events are well identified and modeled. We conclude that GBS can be used to model seismograms at intermediate distances, retrieving information about both fault plane solution, depth and source time functions. Propagation effects are also observed, but they can not be inverted with this limited data set.
A simple parameterization of strain localization in the ductile regime due to grain size reduction: A case study for olivine
Authors: Braun Jean , Chéry Jean , Poliakov Alexei , Mainprice David , Vauchez Alain , Tomassi Andrea , Daignières Marc
Published: September 2002

We propose a simple parameterization of the transition between dislocation creep and grain‐size‐sensitive creep under conditions characteristic of the lithospheric mantle and derived from the results of laboratory experiments on olivine‐rich rocks. Through numerical modeling and linear stability analysis, we determine the conditions under which this transition takes place and potentially leads to strain localization. We pay particular attention to the effect of cooling rate and strain rate which are likely to be dominant parameters in actively deforming tectonic areas. We conclude that at constant temperature, strain localization can only take place if the rheology of the material is nonlinearly related to grain size; that strain localization is facilitated by syndeformation cooling; that there is only a narrow region in the strain rate versus cooling rate parameter space where localization is likely to take place; and that grain growth inhibits strain localization at fast cooling rates but may lead to “grain growth localization” at low cooling rates. We draw attention to the potential consequences of our analysis of strain localization for the style of plate motions at the Earth's surface.
Noncharacteristic behavior and complex recurrence of large subduction zone earthquakes
Authors: Schwartz Susan Y.
Published: September 2002

The last few years have been remarkable with respect to the number of large underthrusting earthquakes in subduction zones that reruptured plate boundary segments that failed in previous great events. Availability of modern seismic data for two consecutive large earthquakes rupturing the same portion of the plate interface provides the opportunity to compare the spatial distribution of moment release for both events. Such comparisons have been made for the plate boundary segments that failed in (1) the 1957 (Mw = 8.6), 1986 (Mw = 8.0), and 1996 (Mw = 7.9) Aleutian Islands earthquakes; (2) the 1963 (Mw = 8.5) and 1995 (Mw = 7.9) Kuril Islands earthquakes; (3) the 1971 (Mw = 8.0) and 1995 (Mw = 7.7) Solomon Islands earthquakes; and (4) the 1968 (Mw = 8.2) and 1994 (Mw = 7.7) northern Honshu earthquakes. The spatial distribution of moment release for all four of the initial great earthquakes and two of the repeat events has been determined in previous studies. Here, slip distributions for the three most recent events are determined from inversion of source time functions, derived by empirical Greens function analysis of long‐period surface waves and broadband body waves. Comparisons of the spatial distribution of moment release for sequential earthquake ruptures reveal considerable differences in the pattern of recurrent fault slip. The 1994 northern Honshu and 1995 Solomon Islands earthquakes primarily fill in areas of slip deficit left by their preceding events rather than rerupture identical asperities. The 1995 Kuril Islands and the 1996 Aleutian Islands earthquakes both rerupture portions of an asperity distribution defined by preceding events but with variable amounts of slip. This study provides the first direct evidence that recurrence of large circum‐Pacific plate boundary events is more complex than repeat rupture of dominant asperities. Recurrent fault slip for the four plate boundary segments studied does not support characteristic slip models either where failure on an entire fault segment occurs repeatedly in events with nearly identical rupture lengths, locations, and slip magnitudes or where failure of individual asperities occurs with identical slip functions through consecutive earthquake cycles. These sequential slip patterns are not consistent with physical models of earthquake rupture where slip complexity is exclusively controlled by invariant geometric and/or material heterogeneity but suggest that dynamic considerations are also important.
Densification and permeability reduction in hot‐pressed calcite: A kinetic model
Authors: Zhu Wenlu , Evans Brian , Bernabé Yves
Published: September 2002

Laboratory studies on hot isostatically pressed (HIP) calcite reveal that the evolution of porosity and permeability during mechanical compaction can be divided into two distinct regimes. At high porosities, permeability is related approximately to porosity raised to the third power. However, below a porosity called the crossover porosity, the power law relationship no longer applies, and permeability reduction is accelerated. At a porosity of ∼4%, permeability becomes too low to be measured, indicating that a percolation threshold has been reached. In previous studies the time evolutions of porosity and permeability were not predicted, and further, the crossover porosity was introduced as an empirical input parameter. In this study we developed a unified model combining crack healing with densification by power law creep to reproduce porosity evolution as a function of time. Both the healing and the creep are deterministically controlled by the pressure and temperature. Permeability can then be calculated by incorporating quantitative microstructural data (i.e., pore size distribution) into a three‐dimensional cubic network model. We were able to reproduce the permeability‐porosity relationship in hot‐pressed calcite aggregates in both high‐ and low‐porosity regimes. In particular, our model predicted a crossover porosity of ∼7% and a percolation threshold of ∼4%, both in a good agreement with the experimental data. However, we generally overestimated the absolute values of permeability. Because the model yielded correct absolute permeability values in the case when the pore size distribution was known, we suppose that at least part of the error arises from inadequate data for microstructure.
Chemical systematics of an intermediate spreading ridge: The Pacific‐Antarctic Ridge between 56°S and 66°S
Authors: Vlastélic Ivan , Dosso Laure , Bougault Henri , Aslanian Daniel , Géli Louis , Etoubleau Joël , Bohn Marcel , Joron Jean‐Louis , Bollinger Claire
Published: September 2002

Axial bathymetry, major/trace elements, and isotopes suggest that the Pacific‐Antarctic Ridge (PAR) between 56°S and 66°S is devoid of any hotspot influence. PAR (56–66°S) samples have in average lower 87Sr/86Sr and 143Nd/144Nd and higher 206Pb/204Pb than northern Pacific mid‐ocean ridge basalts (MORB), and also than MORB from the other oceans. The high variability of Pb isotopic ratios (compared to Sr and Nd) can be due to either a general high μ (HIMU) (high U/Pb) affinity of the southern Pacific upper mantle or to a mantle event first recorded in time by Pb isotopes. Compiling the results of this study with those from the PAR between 53°S and 57°S gives a continuous view of mantle characteristics from south Pitman Fracture Zone (FZ) to Vacquier FZ, representing about 3000 km of spreading axis. The latitude of Udintsev FZ (56°S) is a limit between, to the north, a domain with large geochemical variations and, to the south, one with small variations. The spreading rate has intermediate values (54 mm/yr at 66°S to 74 mm/yr at 56°S) which increase along the PAR, while the axial morphology changes from valley to dome. The morphological transition is not recorded by the chemical properties of the ridge basalts nor by the inferred mantle temperature which displays few variations (30–40°C) along the PAR. Contrary to what is observed along the South‐East Indian Ridge, PAR morphology appears to be controlled more by spreading rate rather than by mantle temperature. Much of the major and trace element variability results from segmentation control on the shallowest thermal structure of the mantle. The cold edge of a fracture zone seems to be more efficient when occurring in an axial dome context. It is expressed as an increase of the magnitude of the Transform Fault Effect along the valley‐dome transition, resulting in a clear increase of trace element ratio variability (such as Nb/Zr). There is no strong evidence for the previously proposed southwestward asthenospheric flow in the area. However, this flow model could explain the intrasegment asymmetric patterns.
Active faulting and paleoseismology along the Bree fault, lower Rhine graben, Belgium
Authors: Meghraoui Mustapha , Camelbeeck Thierry , Vanneste Kris , Brondeel Marijke , Jongmans Denis
Published: September 2002

Paleoseismic analysis of the 10‐km‐long Bree fault scarp in the lower Rhine graben yields numerous lines of evidence of earthquake activity in the Holocene and late Pleistocene. This active normal fault, a part of the Feldbiss fault system, dips 70°NE and is expressed at the surface by a prominent NW‐SE trending 7 to 20 m high scarp, formed since the deposition of the Maas River main terrace <700 kyr. B.P. Trenches and geophysical prospecting show that the fault, which is known to have ∼100 m of vertical offset since the late Pliocene, breaks late Pleistocene and Holocene deposits. Ground‐penetrating radar, seismic refraction, and electric tomography suggest that at shallow depth the amount of displacement is larger than the youngest vertical offset visible in the trenches and corresponds to cumulative fault displacements. The analysis of 36 leveling profiles across the scarp indicates that its height can be classified into three groups, likely corresponding to different events. A morphologic dating gives approximate ages of 2±1.5 kyr B.P., 14±5 kyr B.P., and 41±6 kyr B.P. for the past three surface‐faulting earthquakes. Analysis of faulted stratigraphy and earthquake‐induced deformation structures exposed in trenches suggests the occurrence of three large earthquakes during the past 45×l03 years and yields 0.07 mm/yr of relative vertical deformation rate. The most recent seismic event occurred between A.D. 610 and 890. The first identification of an active fault with surface ruptures in the lower Rhine graben area emphasizes that large earthquake sources exist within intraplate Europe and that at least some of these events are preserved in the geologic record.
Effects of the extrusion of fault gouge on frictional sliding
Authors: Sleep Norman H.
Published: September 2002

Thick gouge is extruded from between sample contacts of simulated fault zones. Thin gouge is locally extruded by irregularities of the fault surfaces. Extrusion causes the simple shear strain rateto vary as a function of position within the gouge and with time. The local shear traction thus varies from its macroscopic value. The case of thick gouge is quantitatively modeled. Extrusion does not significantly affect experiments measuringaandbin the traditional formula for the instantaneous coefficient of friction, where μ0is the steady state coefficient of friction at the reference strain rateand the state variable ψ depends on the previous history of the sample. Other effects are modest and most likely to be observable whena>b, implying that the strain rate does not spontaneously localize. The macroscopic coefficient of friction fora>bis decreased by an amount which is less than the short/long aspect ratio of the gouge zone. Measurments of frictional dilatancy in velocity‐stepping experiments and of the effects of sudden changes of normal traction on shear traction are not compromised if the plates bounding the fault gouge behave as rigid parallel planes. The changes in shear traction after a sudden change in normal traction are strongly affected if the walls confining the gouge are compliant and do not remain parallel. Then the local ratio of the new normal traction to the old normal traction varies with position on the fault surface.