bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of the 14 July 2000 solar flare on Earth's FUV emissions
Authors: Immel Thomas J. , Mende Stephen B. , Frey Harald U. , Østgaard N. , Gladstone G. R.
Published: May 2003

An X‐class solar flare occurring on 14 July 2000 (Bastille Day) marked the initiation of events which led to a powerful geomagnetic storm on 15 July. At the time of the flare the NASA‐IMAGE spacecraft was near apogee and observed a global‐scale transient increase in far ultraviolet (FUV) emissions of the thermosphere in both the N2 Lyman‐Birge‐Hopfield (LBH) band and the O I 135.6‐nm line and also in geocoronal emissions of H I at 121.6‐nm. Comparison with the solar x‐ray flux measured by GOES 10 shows the delayed response of the FUV emissions compared to the peak in x‐rays. This lag is attributed to a later peak in EUV emissions, which drive the photoelectron flux in the energy range which is, in turn, most effective in producing N2 LBH and O I 135.6‐nm emissions. Measurements of solar EUV input by Solar and Heliospheric Observatory (SOHO) Charge, Element, and Isotope Analysis System (CELIAS)/Solar Extreme‐Ultra‐Violet Monitor (SEM) confirm the close correlation between solar EUV and terrestrial FUV emissions. A comparison of the flare‐time variation in the signal in each channel of the instrument allows one to (1) determine the variation in resonantly scattered solar emissions and (2) differentiate between emissions created through photodissociation by solar EUV and those driven by terrestrial photoelectron impact. The latter provides a means for positively identifying the dayglow emissions observed in the SI‐12 instrument.
Ground‐based optical determination of the b2i boundary: A basis for an optical MT‐index
Authors: Donovan E. F. , Jackel B. J. , Voronkov I. , Sotirelis T. , Creutzberg F. , Nicholson N. A.
Published: May 2003

The equatorward boundary of the proton aurora corresponds to a transition from strong pitch angle scattering to bounce trapped particles. This transition has been identified as the b2i boundary in Defense Meteorological Satellite Program (DMSP) ion data [Newell et al., 1996]. We use ion data from 29 DMSP overflights of the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) Meridian Scanning Photometer (MSP) located at Gillam, Canada, to develop a simple algorithm to identify the b2i boundary in latitude profiles of proton auroral (486 nm) brightness. Applying this algorithm to a ten year set of Gillam MSP data, we obtain ∼250,000 identifications of the “optical b2i,” the magnetic latitude of which we refer to as b2iΛ. We intercompare ∼1600 near‐simultaneous optical and in situ b2iΛ, concluding that the optical b2iΛ is a reasonable basis for an optical equivalent to the MT‐index put forward by Sergeev and Gvozdevsky [1995]. Using ∼17,000 simultaneous measurements, we demonstrate a strong correlation between the optical b2iΛ and the inclination of the magnetic field as measured at GOES 8. We develop an empirical model for predicting the GOES 8 inclination, given the universal time, dipole tilt, and the optical b2iΛ, as determined at Gillam. We also show that in terms of information content, the b2i boundary is an optimal boundary upon which to base such an empirical model.
Resonant acceleration and diffusion of outer zone electrons in an asymmetric geomagnetic field
Authors: Elkington Scot R. , Hudson Mary K. , Chan Anthony A.
Published: May 2003

The outer zone radiation belt consists of energetic electrons drifting in closed orbits encircling the Earth between ∼3 and 7 RE. Electron fluxes in the outer belt show a strong correlation with solar and magnetospheric activity, generally increasing during geomagnetic storms with associated high solar wind speeds, and increasing in the presence of magnetospheric ULF waves in the Pc‐5 frequency range. In this paper, we examine the influence of Pc‐5 ULF waves on energetic electrons drifting in an asymmetric, compressed dipole and find that such particles may be efficiently accelerated through a drift‐resonant interaction with the waves. We find that the efficiency of this acceleration increases with increasing magnetospheric distortion (such as may be attributed to increased solar wind pressure associated with high solar wind speeds) and with increasing ULF wave activity. A preponderance of ULF power in the dawn and dusk flanks is shown to be consistent with the proposed acceleration mechanism. Under a continuum of wave modes and frequencies, we find that the drift resonant acceleration process leads to additional modes of radial diffusion in the outer belts, with timescales that may be appropriate to those observed during geomagnetic storms.
Estimation of E × B drift using a global assimilative ionospheric model: An observation system simulation experiment
Authors: Pi Xiaoqing , Wang Chunming , Hajj George A. , Rosen Gary , Wilson Brian D. , Bailey Graham J.
Published: May 2003

A global assimilative ionospheric model (GAIM) has been developed to improve the modeling of ionospheric weather. GAIM adopts a fixed three‐dimensional Eulerian grid following a geomagnetic dipole configuration. A four‐dimensional variational approach (4DVAR) with the adjoint technique is presented, which attempts to minimize the difference between modeled and measured line‐of‐sight total electron content (TEC) using nonlinear least squares minimization. The minimization is achieved by solving for corrections to the initial (climatological) model drivers so that the density state becomes consistent with the observations. The 4DVAR approach is exercised with GAIM in an observation system simulation experiment (OSSE) conducted for estimating the weather behavior of E × B drift at low latitudes. The OSSE takes the constellation of global positioning system (GPS) satellites and an existing global GPS receiver network as the observation system. The effectiveness of the 4DVAR technique with such an observation system is assessed in the experiment, which indicates that one can solve for the low‐latitude E × B drift and improve the density modeling using ground‐based, integrated line‐of‐sight (TEC) measurements from a relatively small number of stations.
Phase transition‐like behavior of magnetospheric substorms: Global MHD simulation results
Authors: Shao X. , Sitnov M. I. , Sharma S. A. , Papadopoulos K. , Goodrich C. C. , Guzdar P. N. , Milikh G. M. , Wiltberger M. J. , Lyon J. G.
Published: May 2003

Using nonlinear dynamical techniques, we statistically investigate whether the simulated substorms from global magnetohydrodynamic (MHD) models have a combination of global and multiscale features, revealed in substorm dynamics by Sitnov et al. [2000] and featured the phase transition‐like behavior. We simulate seven intervals of total duration of 280 hours from the data set used in the above works [Bargatze et al., 1985]. We analyze the input–output (vBs–pseudo AL index) system obtained from the global MHD model and compare the results to those inferred from the original set (vBs–observed AL index). The analysis of the coupled vBs–pseudo AL index system shows the first‐order phase transition map, which is consistent with the map obtained for the vBs–observed AL index system. Although the comparison between observations and global MHD simulations for individual events may vary, the overall global transition pattern during the substorm cycle revealed by singular spectrum analysis (SSA) is statistically consistent between simulations and observations. The coupled vBs–pseudo AL index system also shows multiscale behavior (scale‐invariant power law dependence) in SSA power spectrum. Besides, we find the critical exponent of the nonequilibrium transitions in the magnetosphere, which reflects the multiscale aspect of the substorm activity, different from power law frequency of autonomous systems. The exponent relates input and output parameters of the magnetosphere. We also discuss the limitations of the global MHD model in reproducing the multiscale behavior when compared to the real system.
A new model of solar EUV irradiance variability 2. Comparisons with empirical models and observations and implications for space weather
Authors: Lean J. L. , Warren H. P. , Mariska J. T. , Bishop J.
Published: May 2003

Motivated by the need for reliable specification of the Sun's electromagnetic radiation in the extreme ultraviolet (EUV) spectrum, we have developed a new model of solar EUV irradiance variability at wavelengths from 50 to 1200 Å. Solar images are used to quantify changes in the sources of EUV irradiance during the solar cycle. Optically thin EUV emission line fluxes are estimated from differential emission measures (DEMs) that characterize the properties of the solar atmosphere in the source regions, while fluxes for optically thick lines are modeled directly by specifying the source region contrasts. We compare the new model, NRLEUV, with three different empirical models of solar EUV irradiance since 1975. For solar cycles 21 and 22, NRLEUV predicts overall lower EUV irradiances and smaller solar cycle variability than the empirical models. The average total EUV energy at wavelengths from 50 to 1050 Å is 2.9 mW m−2, smaller than the HFG, EUVAC, and SOLAR2000 models for which average energies are 3.7, 4.3, and 5.6 mW m−2, respectively. These differences have distinct wavelength dependencies. The solar cycle variation in total EUV energy is 1.9 for NRLEUV compared with 2.7, 2.9, and 2.3 for HFG, EUVAC, and SOLAR2000. Here, too, the differences are wavelength dependent. We compare both the NRLEUV and the empirically modeled EUV irradiances with selected wavelength bands and emission lines measured during 4 years in cycle 21 by Atmospheric Explorer‐E (AE‐E) and two broad bands at 170–200 and 260–340 Å measured in cycle 23 by the Solar X‐Ray Photometer (SXP) on the Student Nitric Oxide Experiment (SNOE) and the Solar EUV Monitor (SEM) on the Solar and Heliospheric Observatory (SOHO), respectively. The NRLEUV model reproduces the variations observed during solar rotation better than, or as well as, the empirical models. Comparisons of solar cycle variations are more ambiguous because undetected instrumental drifts can cause spurious trends in the observations over these longer timescales. Drifts in the AE‐E instruments may explain why the HFG and EUVAC models, which are based on parameterizations of these data, have larger solar cycle variations than NRLEUV. We assess the implications for space weather of the significant differences among the modeled EUV irradiances by using the Atmospheric Ultraviolet Radiance Integrated Code (AURIC) to quantify corresponding differences in upper atmosphere energy deposition and photoionization rates.
Seismic velocity anomalies beneath SE Brazil from P and S wave travel time inversions
Authors: Schimmel M. , Assumpção M. , VanDecar J. C.
Published: May 2003

We present the result from teleseismic travel time inversions for P and S wave data mainly recorded at portable broadband stations in SE Brazil. The stations were deployed at 45 sites within an area 1000 × 1700 km during the years 1992–1999. More than 10,000 relative P and S wave arrival times, including core phases, were obtained from the waveforms using a new coherence functional. These P and S relative phase times are independently inverted for slowness perturbations, earthquake relocations, and station corrections. The final models represent the least amount of structure required to explain the residuals within a defined standard error. The robust and consistent features in the velocity anomaly models are interpreted and their resolution is tested with synthetic case inversions. We confirm the existence of a cylindrical low‐velocity anomaly beneath the Paraná basin, which has been interpreted by VanDecar et al. [1995] as the fossil conduit through which the initial Tristan da Cunha plume head traveled to generate the Paraná–Etendeka flood basalts about 130 Ma. We now show that this low‐velocity cylindrical structure seems to be confined to the upper mantle. Beneath the upper mantle, the velocity anomalies show a N‐S oriented pattern, which we interpret as due to the Nazca plate subducted slab. At lithospheric depths, the Archean, southern part of the São Francisco craton, shows high velocities down to 200–300 km. All areas with Late Cretaceous postrift alkaline intrusions are characterized by low velocities at lithospheric depths.
Three‐dimensional, prestack, plane wave migration of teleseismic P‐to‐S converted phases: 1. Theory
Authors: Poppeliers Christian , Pavlis Gary L.
Published: May 2003

We present the theoretical foundations for a prestack migration technique to image teleseismic P‐to‐S converted phases. The method builds on teleseismic P wave deconvolution, pseudostation stacking [Neal and Pavlis, 1999] and on the idea of using a plane wave decomposition for imaging as introduced by Treitel et al. [1982]. Deconvolution operators are constructed by pseudostation stacking of the array aligned to the incident P wave arrival times to produce a space‐variable deconvolution operator. The resulting data are then muted to remove the deconvolved direct P wave pulse and pseudostation stacked over a grid of feasible slowness vectors. The pseudostation stack interpolates the wave field onto a regular grid along Earth's surface producing a series (one per slowness vector) of uniformly sampled three‐dimensional data cubes (two space variables and time). The plane wave components can be propagated downward using a form of approximate ray tracing with a three‐dimensional Earth model. This yields a series of distorted cubes topologically equivalent to the original uniformly sampled data cubes. These data volumes are summed as a weighted stack with the weights derived from an integration formula for inverse scattering based on the generalized Radon transform. This allows an image of the subsurface to be constructed on an event by event basis beneath the array. We apply this technique to data from the Lodore array that was deployed in northwestern Colorado. The results suggest the presence of a major lithospheric‐scale discontinuity defined by a south dipping boundary.
Multiscale electrical impedance tomography
Authors: Pessel Marc , Gibert Dominique
Published: May 2003

Electrical impedance tomography aims to recover the electrical conductivity underground from surface and/or borehole apparent resistivity measurements. This is a highly nonlinear inverse problem, and linearized inverse methods are likely to produce solutions corresponding to local minima of the misfit function to minimize. In the present paper, electrical impedance tomography is addressed through a nonlinear approach, namely, simulated annealing, in order to escape from local minima and to produce conductivity distributions independent of the starting models. Simulated annealing belongs to the Monte Carlo family which needs numerous forward modelings, and particular attention is paid both to the forward numerical solution of the Poisson equation and to the parameterization of the inverse problem, i.e., the way the conductivity distribution is mathematically represented. The 2.5‐dimensional forward problem is solved through a multigrid approach which iteratively solves the Poisson equation from large to small scales. In the same spirit, the conductivity model is parameterized in a multiscale way by representing the conductivity distribution with a superimposition of block whose sizes (in suitable units) are integer powers of 2. The multiscale inversion is implemented in a sequential way by first inverting for the conductivity of the coarser blocks and by progressively incorporating finer blocks in the conductivity model. A decision stage based on a sensitivity analysis is performed before incorporating finer blocks in order to optimize the parameterization by not adding unnecessary details into the conductivity model. Both a synthetic example and a field experiment are used to explain the method, and comparisons with a linearized inversion technique are done.
Great explosive eruptions on Kamchatka during the last 10,000 years: Self‐similar irregularity of the output of volcanic products
Authors: Gusev A. A. , Ponomareva V. V. , Braitseva O. A. , Melekestsev I. V. , Sulerzhitsky L. D.
Published: May 2003

Temporal irregularity of the output of volcanic material is studied for the sequence of large (V ≥ 0.5 km3, N = 29) explosive eruptions on Kamchatka during the last 10,000 years. Informally, volcanic productivity looks episodic, and dates of eruptions cluster. To investigate the probable self‐similar clustering behavior of eruption times, we determine correlation dimension Dc. For intervals between events 800 and 10,000 years, Dc ≈ 1 (no self‐similar clustering). However, for shorter delays, Dc = 0.71, and the significance level for the hypothesis Dc < 1 is 2.5%. For the temporal structure of the output of volcanic products (i.e., for the sequence of variable‐weight points), a self‐similar “episodic” behavior holds over the entire range of delays 100–10,000 years, with Dc = 0.67 (Dc < 1 at 3.4% significance). This behavior is produced partly by the mentioned common clustering of event dates, and partly by another specific property of the event sequence, that we call “order clustering”. This kind of clustering is a property of a time‐ordered list of eruptions, and is manifested as the tendency of the largest eruptions (as opposed to smaller ones) to be close neighbors in this list. Another statistical technique, of “rescaled range” (R/S), confirms these results. Similar but weaker‐expressed behavior was also found for two other data sets: historical Kamchatka eruptions and acid layers in Greenland ice column. The episodic multiscaled mode of the output of volcanic material may be a characteristic property of a sequence of eruptions in an island arc, with important consequences for climate forcing by volcanic aerosol, and volcanic hazard.
Modeling fluid transfer along California faults when integrating pressure solution crack sealing and compaction processes
Authors: Gratier Jean‐Pierre , Favreau Pascal , Renard François
Published: May 2003

Several models pertaining to earthquake cycles imply intermittent fluid flow through fault. During the interseismic period, increase in fluid pressure from hydrostatic to lithostatic values is a crucial parameter in mechanisms leading to earthquakes. To achieve such pressures, geodynamic processes (gouge compaction, fluid flow) and changes in permeability are required. Previous models have postulated that changes in permeability (by self‐healing) are faster than the effects of geodynamic processes. We consider the different mechanisms and rates of crack sealing near active fault on the examples of uplifted Californian faults. We find that natural crack sealing is normally not achieved by a rapid self‐healing process. Pressure solution, with mass transfer from solution cleavage to cracks, appears to be a more important mechanism for crack sealing and creep during postseismic deformation. The geometry of transfer path and experimental data have been used to model crack sealing rates by pressure solution which are estimated to be rather slow, similar to the recurrence time of some earthquakes. Such slow changes in permeability may be crucial factors in controlling the increase in fluid pressure and, consequently, the mechanism of critical failure in faults. Then, numerical modeling of fluid pressure and transfer around active faults has been performed integrating a slow change in permeability by crack sealing, gouge compaction, and fluid flow from depth. This modeling shows various location and evolution of fluid overpressure during the interseismic period depending on these processes and allows one to estimate the amount of fluid transferred from depth during interseismic periods.
Subsurface crevasse formation in glaciers and ice sheets
Authors: Nath P. C. , Vaughan D. G.
Published: May 2003

Crevasses form in response to tensile stresses in glaciers and ice sheets. It has been widely assumed that crevasses initiate at, or near, the surface of the ice, from starter cracks up to a few centimeters long. If the tensile stress is sufficient, these cracks propagate downward into the ice to form a crevasse, until the weight‐induced lithostatic stress prevents them penetrating deeper. We present ground‐penetrating radar data acquired on the Rutford Ice Stream, Antarctica, which indicate that crevasses occur at depths of several meters beneath the ice surface and were formed in areas where surface crevassing is absent. The data support the hypothesis that these are examples of subsurface crevasse formation. Using linear elastic fracture mechanics (LEFM), we investigate the feasibility of crevasse initiation at depth. We consider the initiation of an isolated crevasse from a subsurface crack, subject to a “dynamic tensile stress” which results from deformation associated with ice movement and a weight‐induced lithostatic stress. The LEFM approach allows us to estimate ainit, the minimum length a crack must be before crack propagation will occur. In earlier models of crevasse formation, it was assumed that the dynamic tensile stress is constant with depth. We consider a more realistic scenario, where the dynamic tensile stress varies with depth, in such a way that the tensile strain rate remains constant. We show that in this scenario, crevasse initiation from centimeter‐scale starter cracks is feasible at depths of 10–30 m, as well as at the surface. At present, the formulation of a reliable predictive model is limited by an incomplete knowledge of the mechanical properties of firn. In previous studies, the depth of buried crevasses has been used to estimate the time elapsed since ice was exposed to higher stresses and different flow regimes. In the light of the results presented here, those estimates may need to be reviewed.
Implications of the shear stress river incision model for the timescale of postorogenic decay of topography
Authors: Baldwin Julia A. , Whipple Kelin X. , Tucker Gregory E.
Published: May 2003

The reason for the survival of mountainous topography in ancient orogenic belts is a long‐standing problem in geomorphology and geodynamics. We explore the geomorphologic controls on the timescale for the postorogenic decay of topography to address the question of whether there is a viable geomorphologic explanation for the persistence of topography in ancient orogenic belts or whether alternative geodynamic explanations must be sought. Using both approximate analytical solutions and numerical simulations, we show that the standard detachment‐limited stream power river incision model predicts, for reasonable initial topographies, relatively short postorogenic decay times of 1–10 Myr. Additional layers of complexity are introduced to this simplest model including isostasy and flexural strength, a transition to transport‐limited conditions during decline, and the incorporation of the combined effects of a critical threshold for erosion and the stochastic variability of flood magnitudes. Each of these additional factors acts to lengthen the decay timescale. Pure Airy isostatic rebound of a thick crustal root results in decay times that are at most a factor of 6 longer than the detachment‐limited result. The transition to transport‐limited conditions involves partial protection of the bed by a thin layer of alluvium, which inhibits erosion and therefore increases decay times by a factor of 2–3 in our analysis. Finally, including critical shear stress results increases decay time by approximately a factor of 20 for the parameter values in our example calculation. More importantly, however, a significant portion of topography remains after predicted lowering rates have dropped to values less than measured rates of denudation in weathering‐limited, low‐relief environments. Thus a model combining isostasy, a transition to transport‐limited conditions, and a critical shear stress for erosion could account for the presence of residual topography for hundreds of million years. All three of these factors can be expected to play a role in natural settings.
Episodic dike swarms inferred from near‐bottom magnetic anomaly maps at the southern East Pacific Rise
Authors: Shah Anjana K. , Cormier Marie‐Helene , Ryan William B. F. , Jin Wen , Sinton John , Bergmanis Eric , Carlut Julie , Bradley Al , Yoerger Dana
Published: May 2003

Near‐bottom, high‐resolution magnetic field data gathered at the southern East Pacific Rise near 17°28′S, 18°14′S, and 18°37′S, using the autonomous underwater vehicle Autonomous Benthic Explorer (ABE) echo various geologic structures, including void space within lobate caverns, recent pillow mounds, and hydrothermal vent activity. This study is focused on a magnetic field low extending several kilometers along axis, coincident with a trough created by the draining of a lava lake during a highly effusive fissure eruption at 17°28′S. Similar lows are observed at three other drained lava lake troughs, including one which is at least 1800 years old, residing 400 m away from the present‐day axis. We attribute these lows to the presence of shallow dike swarms. The degree to which other geologic features may contribute to the lows is constrained using geologic, geophysical, and geochemical observations and forward modeling. Compositional analyses of Alvin samples at 17°28′S do not support Fe or Ti variations as a primary source. Hypotheses requiring hydrothermal alteration and porosity variations are both inconsistent with geologic observations and near‐bottom gravity data analysis from similar areas. Previous mappings between paleointensity variations and the observed magnetic field over distances of several kilometers from the axis suggest that such variations do not create the field low. The dominant source of the magnetization low is most likely the presence of a 100–200 m wide region of shallow dikes which are poorly magnetized relative to extrusives, or a region heated above magnetic blocking or Curie temperatures by intrusions during the most recent eruption (though the latter interpretation cannot explain the low at the fossil trough). In the first case, this extrusive thinning implies a change in eruptive behavior over the last 750–1500 years given the local spreading rate. For the latter case, thermal models suggest the anomaly had to have been created by a dike swarms totaling at least 45 m width during the most recent eruption(s), corresponding to ∼300 years of plate spreading. Models indicate that the source of the low is centered slightly east of the axial trough. This offset suggests that the axis has been progressively migrating westward over the past millennium, consistent with other studies covering greater length and timescales. Westward migration provides an explanation for the preferential emplacement of recent lavas flows west of the axis, evident in ABE bathymetry and submersible observations.
Earth tide and tilt detection by a ring laser gyroscope
Authors: Schreiber K. Ulrich , Klügel Thomas , Stedman Geoffrey E.
Published: May 2003

An Earth tide signal at the lunar tidal period of 12 hours 25 min has been detected in the Sagnac frequency record of the C‐II ring laser 30 m underground at Christchurch, New Zealand. Its amplitude, one part per million of the Earth rotation signal, is much greater than the value of 40 parts per billion expected. Tiltmeter records show that a substantial part of this amplification is geophysical, the lunar component of tilt having an amplitude of the order of 0.1–0.2 μrad, principally because of ocean loading of Banks Peninsula. The joint records also show effects on the Sagnac frequency associated with cavern deformation under ambient pressure and temperature change and with long‐period waves in cavern tilt.
Fracture populations on the Reykjanes Peninsula, Iceland: Comparison with experimental clay models of oblique rifting
Authors: Clifton Amy E. , Schlische Roy W.
Published: May 2003

We have used high‐resolution scanned air photos and field measurements to analyze fracture population systematics for the Reykjanes fissure swarm in order to determine the effect of rift obliquity on the evolution of fracture populations on the Reykjanes Peninsula, SW Iceland. The peninsula is oriented approximately 30° oblique to the direction of plate motion. Data show significant differences between the strike, length, and degree of development of fractures along the margin of the fissure swarm and those in the center of the zone of active volcanism. Along the margin, fractures are long, highly segmented extension fractures, and normal faults with offset on the order of several meters and a predominant strike 20° oblique to the trend of the plate boundary (rift axis). In the center of the zone of active volcanism, fractures are generally shorter, straighter, fewer in number, and strike approximately perpendicular to the direction of maximum horizontal extension and parallel to the trend of eruptive fissures. Right‐lateral oblique‐slip faults striking approximately perpendicular to the spreading direction are present in the center of the rift zone. Scaled experimental models of oblique extension predict a significant change in fracture strike at the rift margins due to the presence of a secondary stress field; on the Reykjanes Peninsula, this corresponds to the boundary between thicker and thinner brittle crust. The models confirm that right‐lateral oblique‐ or strike‐slip faults accommodate significant left‐lateral shear where deformation is distributed in a rift zone.
Modeling of highly anisotropic crust and application to the Altiplano‐Puna volcanic complex of the central Andes
Authors: Leidig Mark , Zandt George
Published: May 2003

The Altiplano‐Puna volcanic complex (APVC) is located in the central Andes and covers an area over 50,000 km2. The style of volcanism is predominately caldera‐forming ignimbrite eruptions active between 10 and 1 Ma. Chmielowski et al. [1999] used teleseismic events, recorded from seven portable broadband seismic stations deployed in the APVC, to identify a 1‐km‐thick very low‐velocity zone at a depth of 19 km. Based on the correlation with surface volcanics, the extremely low shear velocities of 1 km/s or less, and the depth of the layer, the low‐velocity zone was interpreted to be a “magma body.” Using local events recorded during the same seismic deployment, we have improved the crustal model for the APVC and detected seismic anisotropy in the crust above the low‐velocity zone. Using a hexagonal symmetry anisotropy code, we computed synthetic receiver functions for many models of layered anisotropic media and found two different models consistent with the data. In both models, approximately 20–30% anisotropy is present in a 3‐km‐thick surface low‐velocity layer, and 15–20% anisotropy is present in the remaining crust above the buried low‐velocity zone. The first model has the 1‐km‐thick low‐velocity zone at 17 km, while the alternate model has the low‐velocity zone at 14‐km depth overlain by a 4‐km‐thick transition zone of intermediate velocities. This anisotropy may be due to the fracture system by which magma migrates to the surface from the midcrustal sill‐like magma body.
Geological perspectives on consolidation of clay‐rich marine sediments
Authors: Karig D. E. , Ask M. V. S.
Published: May 2003

Experimental consolidation of uncemented clay‐rich marine sediments provides information concerning their stress history. A main finding is that some of the well‐known behavior of soft sediment deformation in geotechnical applications cannot validly be extrapolated to sediments that have been subjected to higher stresses and longer times of geologic conditions. This study confirms that the yield stress of the uncemented sediment accurately reflects its previous maximum consolidation state. Furthermore, we have identified a new phase of post‐yield strain that is associated with higher values of the modified compression index (the slope of the porosity versus the logarithm of effective vertical stress) than that of elastic deformation, but with much lower values than that for primary consolidation. This post‐yield behavior is a linear, non‐elastic deformation, and is termed tertiary consolidation. Yield stress appears independent of creep time or strain rate, whereas the tertiary‐primary consolidation transition is sensitive to these parameters. During post‐yield creep (secondary consolidation) the slope of the porosity versus the logarithm of time curve, or the secondary consolidation index, is generally assumed constant. However, this is not valid for claystones at effective vertical stresses above about 1 MPa, where the secondary consolidation index increases with stress. At a given effective vertical stress, the secondary consolidation index also increases with creep times greater than about 105 s (≈28 h).
The June 2000 Mw 7.9 earthquakes south of Sumatra: Deformation in the India–Australia Plate
Authors: Abercrombie Rachel E. , Antolik Michael , Ekström Göran
Published: May 2003

Two large (Mw 7.9) earthquakes occurred on 4 and 18 June 2000, south of Sumatra, beneath the Indian Ocean. Both earthquakes were predominantly left‐lateral strike‐slip on vertical N‐S trending faults that we interpret to be reactivated fracture zones. The 4 June Enggano earthquake occurred at the edge of the rupture area of the 1833 subduction earthquake. The first strike‐slip subevent within the subducting plate triggered a thrust subevent on the plate interface, which comprised at least 35% of the total moment and ruptured SE away from the 1833 earthquake. The 18 June earthquake in the Wharton Basin is one of the largest shallow strike‐slip faulting earthquakes ever recorded. A small second subevent with reverse slip is required to fit the body waves. The orientation of both subevents in our preferred model is consistent with the current stress field in the region. Both the June 2000 earthquakes are consistent with recent models of distributed deformation in the India–Australia composite plate. The occurrence of the Enggano earthquake implies that the stress field within the Indian plate continues to a depth of 50 km in the subducting slab. The purely strike‐slip source model of the Wharton Basin earthquake obtained by Robinson et al. [2001] matches the P waves very poorly and fits the S waves no better than our preferred model. The strike‐slip subevents of both earthquakes had few aftershocks and higher stress drops than the subduction thrust subevent of the Enggano earthquake. This difference is consistent with previous observations of oceanic and subduction earthquakes.
Brittle‐ductile transition and associated seismicity: Experimental and numerical studies and relationship with the b value
Authors: Amitrano David
Published: May 2003

The acoustic emission (AE) and the mechanical behavior of granite samples during triaxial compression tests have been analyzed. The size of AE events displays power law distributions, conforming to the Gutenberg‐Richter law observed for earthquakes, which is characterized by the b value. As the confining pressure increases, the macroscopic behavior becomes more ductile. For all different stages of the rock mechanical behavior (linear, nonlinear prepeak, nonlinear postpeak, shearing), there is a systematic decrease of the b value with increasing confining pressure. A numerical model based on progressive elastic damage and the finite element method allows simulations of the main experimental observations on AE and of a wide range of macroscopic behaviors from brittleness to ductility. The model reproduces a decrease in the b value that appears to be related to the type of macroscopic behavior (brittle‐ductile) rather than to the confining pressure. Both experimental and numerical results suggest a relationship between the b value and the brittle‐ductile transition. Moreover, these results are consistent with recent earthquake observations and give new insight into the behavior of the Earth's crust.
High mantle heat flow in a Precambrian granulite province: Evidence from southern India
Authors: Ray Labani , Kumar P. Senthil , Reddy G. K. , Roy Sukanta , Rao G. V. , Srinivasan R. , Rao R. U. M.
Published: May 2003

Twelve new heat flow values determined at nine sites and heat production estimated from radioelemental measurements at 330 sites in the southern granulite province (SGP) bring out contrasting crustal and subcrustal thermal characteristics between the SGP and the adjacent Archaean Dharwar greenstone‐granite‐gneiss province (DP) in south India. A two‐layer granulitic crust of Late Archaean charnockites and gneisses characterizes the northern block (NB), north of the Palghat‐Cauvery lineament (PCL). The heat production of the upper, 7–10 km thick, metasomatized granulitic layer ranges between 0.2 and 0.75 μW m−3(mean 0.5 ± 0.3 (SD) μW m−3). This layer overlies radioelement‐depleted granulites characterized by very low heat production ranging from 0.14 to 0.2 μW m−3(mean 0.16 ± 0.07(SD) μW m−3). In a large sector of the NB, erosion of the upper metasomatized granulite layer has laid bare the depleted granulitic rocks, which represent one of the lowest heat‐producing crustal sections. The mean heat flow in the NB is 36 ± 4 mW m−2(N= 10). The southern block (SB), south of PCL, in contrast to the NB, comprises complexly interlayered charnockites, gneisses, granites, khondalites, and leptynites, which have variable and much higher levels of heat production ranging between 1.11 and 2.63 μW m−3. The heat flow in the SB is 47 ± 8 mW m−2(N= 3). Overall, the range of heat flow values in the SGP is within the range for the DP. Mantle heat flow in the NB, both from the lowest heat‐producing sector and other areas, is deduced in the light of heat production and heat flow data, at 23–32 mW m−2, whose values are distinctly higher than 11–16 mW m−2for the adjacent DP. The higher mantle heat flow in the NB appears to be a consequence of higher heat production in the subjacent mantle.
A method for measuring the solid particle permittivity or electrical conductivity of rocks, sediments, and granular materials
Authors: Robinson D. A. , Friedman S. P.
Published: May 2003

Measurement of the effective dielectric permittivity of porous media provides an elegant method of estimating both the porosity and water content of soils, sediments, and rocks. Obtaining accurate estimates of water content can only be achieved if the models relating the measured permittivity to water content work well. One of the difficulties with testing physical models describing two‐ and three‐phase dielectric mixtures (saturated and unsaturated systems) is obtaining an accurate value for the permittivity of the mineral matrix. No direct measurement method has been presented for rocks and soils, so the solid permittivity value is often an estimate of 5, essentially remaining a fitting parameter. We present a measurement method requiring no preassumed theoretical model. The method requires the measurement of the effective permittivity of a granular material immersed in fluids of differing permittivity and interpolation between the range of data points. Measurements of the permittivity of the mineral matrix of a number of materials are presented, including glass beads (7.6), quartz sand (4.7), calcareous seashell fragments (8.9), hematite ore (18.1), tuff (6.0), and a sandy loam soil (5.1). Estimates based on using mixing models to obtain the solid permittivity are demonstrated to underestimate the permittivity of hematite ore by between 0% and 45%, depending on the model and packing. The method is also suitable for measuring the electrical conductivity of granular minerals by changing the electrical conductivity of the bathing solution. Results for the electrical conductivity of hematite ore (0.040 dS m−1) and carborundum (10.9 dS m−1) are presented.
Experimental deformation of synthetic aragonite marble
Authors: Rybacki E. , Konrad K. , Renner J. , Wachmann M. , Stöckhert B. , Rummel F.
Published: May 2003

We deformed synthetic polycrystalline aragonite aggregates in a Griggs‐type apparatus using a molten salt cell at temperatures between 600 and 900°C, confining pressures between 1.6 and 2.8 GPa, and strain rates between 3 × 10−6 and 5 × 10−3 s−1. At temperatures above 600°C, triaxial compression tests to ∼15% axial strain are characterized by a constant flow stress. The strength of the aragonite marble is comparable to strength data previously published for calcite rocks of comparable grain size, i.e., 50 to 100 μm. The mechanical data and microstructures of the deformed aragonite specimens indicate dislocation creep as the dominant deformation mechanism. A power law flow law, = Aexp(−Q/RT)σn, fits the mechanical data with a preexponential factor ln(A[MPa−5.2 s−1]) = −0.6 ± 2.5, an activation energy Q = 249 ± 29 kJ mol−1, and a stress exponent n = 5.2 ± 0.6. Extrapolated to natural strain rates, the flow law provides an upper bound to the strength of carbonate rocks at (ultra‐) high‐pressure metamorphic conditions.
Magnitude dependence of radiated energy spectra: Far‐field expressions of slip pulses in earthquake models
Authors: Shaw Bruce E.
Published: May 2003

We examine the radiated waves emitted by events on a model fault. The model deterministically produces a complex sequence of events, with a wide range of sizes, from a uniform frictional instability. The spontaneous rupture events emit a rich spectrum of radiated waves as they nucleate, propagate, and decelerate within the complex stress field left by previous events. Two model innovations, a new driving boundary condition on the fault and a new radiatingboundary condition which allows a spatially varying prestress away from the fault, allow us to directly measure the radiation without problems from boundary reflections in our two‐dimensional model. We quantify the radiation by first measuring the energy spectral density and then averaging over events of a similar size to examine the magnitude dependence. Assuming only a physics of the tractions on the fault, we obtain a full spectra of radiated waves for a complex population of events with a wide range of sizes. To quantify the resulting spectra, we consider two different spectral measures. One, the peak amplitude of the spectral energy density, occurs at a period which scales with the rupture length and corresponds with the classical corner frequency measurement. The other, the peak amplitude of the spectral average acceleration or the low‐frequency corner in the case of a flat acceleration spectrum, occurs at a period that scales with the duration of slip of points on the fault. The period of the peak spectral acceleration saturates for large events. Looking at the rupture motions on the fault, we find that this spectral behavior corresponds with the behavior of slip pulses in the model. Intense narrow pulses of slip develop for very long rupture events. We quantify this by measuring the mean slip duration as a function of rupture length and show that it is has the same behavior as the peak period of spectral acceleration. Thus the duration of the slip pulses in these ruptures is directly expressed in their radiated spectra. Moreover, we find that these corner periods exhibit a nontrivial dependence on event magnitude for the different frictional instabilities that we have examined, suggesting that any observed dependence of these corner periods on earthquake magnitude might provide insight into the frictional instability of earthquakes.
Thermal models of flat subduction and the rupture zone of great subduction earthquakes
Authors: Gutscher Marc‐André , Peacock Simon M.
Published: May 2003

In subduction zones, the size of the seismogenic zone that ruptures during a great thrust earthquake may be thermally controlled. We have constructed finite element thermal models of six transects across three different subduction zones in order to determine the temperature distribution along the plate interface and to predict the size of the seismogenic zone. These models incorporate the complex plate geometries (variable dip downsection) necessary to model a flat slab subduction style. We focus on the rupture zones of the great earthquakes of Nankai (SW Japan) 1946 M8.3 and Alaska 1964 M9.2 as well as on the Cascadia margin. Subduction zone segments with moderate to steep dips exhibit rupture zones of 100–150 km downdip width, consistent with earlier elastic dislocation models and thermal models. For shallow dipping flat slab segments, our models predict larger locked zones, of 150–250 km width, in good agreement with aftershock and geodetic studies. The wider seismogenic zone predicted for flat slab segments results from an uncommonly wide, cold, forearc region, as corroborated by surface heat flow observations. A global analysis of great M > 8 interplate earthquakes of the 20th century reveals that more than a third of these events occurred in flat slab segments, whereas these segments represent only 10% of modern convergent margins. This implies substantially higher interplate coupling for flat slab segments, likely due to the increased downdip extent of the seismogenic zone, and suggests that the seismic risk near such regions may be higher than previously thought.
Nucleation and early seismic propagation of small and large events in a crustal earthquake model
Authors: Lapusta Nadia , Rice James R.
Published: May 2003

Earthquake nucleation and early seismic propagation are studied in a two‐dimensional strike‐slip fault model with depth‐variable properties. The fault is governed by the Dieterich‐Ruina rate and state friction law. We use an efficient and rigorous numerical procedure for elastodynamic analysis of earthquake sequences on slowly loaded faults developed by Lapusta et al. [2000]. We find that for decreasing values of the characteristic slip distance of the friction law, small events appear at the transition from the locked to creeping behavior toward the bottom of the seismogenic zone. Small and large events have very similar nucleation phases in our simulations. Here, by “nucleation phase” we mean gradually accelerating aseismic slip in a small slowly expanding zone before the breakout of the dynamic, seismically detectable event. Moment acceleration (to which velocity seismograms are proportional) in early stages of seismic propagation exhibits irregular fluctuations, in the form of speedups and slowdowns in the moment release rate, consistently with observations as reported by Ellsworth and Beroza [1995]. Our simulations show that such irregular moment acceleration can, at least in part, be due to the heterogeneous stress distribution imprinted on the fault by the arrest of previous small events and by stress concentrations at the borders of creeping regions and to partial arrest of the rupture in velocity‐strengthening fault regions which inhibit seismic slip.
A reassessment of the rupture characteristics of oceanic transform earthquakes
Authors: Abercrombie Rachel E. , Ekström Göran
Published: May 2003

We investigate the long‐period source spectra of oceanic transform earthquakes and find that previously proposed slow rupture components can be explained as artifacts generated by the modeling procedure. We use low‐frequency (≤20 mHz) Rayleigh and Love waves to calculate the amplitude spectra of five earthquakes on the Romanche and Chain transform faults in the equatorial mid‐Atlantic Ocean. We find that errors and approximations in the centroid depth, focal mechanism, and earth structure at the source have significant effects on the shape of the source spectra. If global catalog values and an average crustal model are assumed, the spectra exhibit apparent anomalous energy at long periods which has previously been interpreted as a result of slow rupture. We recalculate the source spectra using precise, independently determined depths and moment tensors and a more realistic oceanic crustal structure in the source region. The resulting source spectra are flat at long periods with no indication of anomalous long‐period energy. Our results imply that oceanic transform earthquakes do not commonly have detectable slow rupture components.