bluuebunny/crossref_metadata_2025 huggingface dataset:
Stress drop for three M∼4.3–4.7 (1992–1994) Parkfield, CA, earthquakes
Authors: Fletcher Jon B. , Guatteri Mariagiovanna
Published: December 2003

In a previous paper (Fletcher and Spudich, 1998) we determined the slip distribution for the three M∼4.3 to 4.7 Parkfield events that occurred between Oct. 20, 1992 and Dec. 20, 1994. The rupture characteristics of these events are of particular interest because they are the largest events to occur at Parkfield since the 1966 mainshock. To further investigate the rupture process of these events, we compute the static stress change or stress drop caused by the slip using a boundary integral method from Quin and Das (1989) adapted for the forward modeling case of computing stresses from a static slip distribution. We find that the initial phase at or very near the hypocenter has the largest stress change in all cases. This peak has a value of about 3.8 MPa for the Oct. 20, 1992 event (which also has the smallest magnitude of the three at 4.3) and about 45 MPa for the other Nov. 14, 1993 and 60 MPa for the Dec. 20, 1994 event. The rest of the stress release is comparatively small and smooth for the Nov. 14, 1993 event, which is the deepest event and the most impulsive. The pattern of stress release of the other two events (Oct. 20, 1992 and Dec. 20, 1994) is more complex with a greater spatial extent. The spatial distribution of stress is approximately similar to the spatial distribution of slip, but with the larger initial peak in stress drop at the hypocenter.
Wave‐field Tau‐p analysis for 2‐D velocity models: Application to western North American lithosphere
Authors: Gorman Andrew R. , Clowes Ron M.
Published: December 2003

The international Deep Probe refraction experiment is a continental‐scale study of the crust and upper mantle in western North America to determine well‐constrained mantle velocity‐depth structures for the stable Archean Hearne and Wyoming provinces and the tectonically modified Proterozoic lithosphere of the southern Rocky Mountains and Colorado Plateau. An initial 2‐D velocity interpretation for Deep Probe has been accomplished by the application of the downward‐continued wave‐field τ‐p transform method. This approach involves two linear transforms for each shot gather from the experiment: a τ‐p transform (or slant stack) of the input data followed by a downward continuation to z‐p space using an appropriate velocity function. The determination of this velocity function is an iterative process which converges when the downward‐continued wave field images a velocity function which is the same as that used to create it. The resulting 1‐D velocity function, υ(z), from each gather is converted to a 2‐D function, υ(x, z) using ray theory. Eleven profiles along the Deep Probe corridor have been analysed. The resulting functions have been combined, gridded and contoured to produce a velocity structure section for the project, which provides an initial model for the application of more advanced interpretation procedures. As a result, the lithospheric structures of three major tectonic provinces have been identified in an efficient and effective manner that did not require ‘picking’ of travel times.
Seismological and GPS evidence for the Aegean‐Anatolia Interaction
Authors: Papazachos C. B.
Published: December 2003

Seismological and GPS data are used in order to examine the seismotectonic setting and the plate motions in the Eastern Mediterranean. The main objective is to reevaluate the recently proposed idea of a single Anatolia‐Aegean microplate, which performs a rigid body rotation about an Euler pole located in northern Africa. Both data sets show a more complicated setting. South of the North Anatolia Fault (NAF) the plate motion of Anatolia relative to Europe can be adequately described by a simple rigid body rotation with a dominant East‐West component and an average velocity of ∼22mm/yr. However, both types of data show a significant increase of the deformation velocities in the Aegean area (30–35mm/yr) and exhibit a much larger N‐S component (up to 20mm/yr) than the one predicted from rigid body rotation. This deformation pattern is more consistent with models which consider the Aegean as a separate microplate.
First theoretical predictions for dynamic spectra of the outer heliospheric radio emissions
Authors: Cairns Iver H. , Zank G. P.
Published: December 2003

First theoretical predictions for dynamic spectra of radio emissions from the outer heliosphere are presented, based on a simple emission model and the global plasma density predicted by plasma‐neutral fluid simulations. Emission from the heliopause density ramp is not organized into slow‐drifting radiation consistent with the observed transient emissions, contrary to a widely accepted model. However, the dynamic spectra show emissions from the heliosheath beyond the heliopause reminiscent of the observed 2 kHz component. The results suggest that new emissions may be observed as spacecraft move beyond the termination shock and also place constraints on the generation processes and global plasma environment. Subject to caveats on the models used, the analysis shows that basic deficiencies exist in our understanding of the observed emissions.
Holocene variations in Asian monsoon moisture: A bidecadal sediment record from the South China Sea
Authors: Wang Luejiang , Sarnthein Michael , Erlenkeuser Helmut , Grootes Pieter M. , Grimalt Joan O. , Pelejero Carles , Linck Gudula
Published: December 2003

The East Asian monsoon system involves extensive transport of sensible/latent heat between land and sea and from low to high latitudes. Our high resolution, bidecadal marine records present a first detailed history of monsoon climate change over the Holocene. The high‐amplitude perturbation in monsoon moisture centered at 8,150 years ago and the monsoon maximum in the Early Holocene show inter‐hemispheric teleconnections to both a cool episode in Greenland and to the Indian monsoon monitored in the Arabian Sea. Periodicities of 84, 102 and, especially, near 775 years in monsoon variation suggest a climatic forcing both by long‐term oscillations in thermohaline circulation and (possibly) solar activity cycles.
Earth‐atmosphere angular momentum exchange and ENSO: The rotational signature of the 1997–98 Event
Authors: Dickey Jean O. , Gegout Pascal , Marcus Steven L.
Published: December 2003

The impact of the 1997–1998 ENSO event is presented in context of axial Earth‐atmosphere angular momentum exchange utilizing length of day (LOD), Southern Oscillation Index (SOI) and atmospheric angular momentum (AAM) data from 1970 to 1998; comparisons are made with previous events. The analysis of equal‐area latitudinally belted AAM from the NCEP reanalysis (1958–98) reveals slow global coherent poleward propagation of angular momentum. These structures originate in the equatorial regions, penetrate into high latitudes and are bimodal in nature with variations centered at low‐frequency (LF ∼4.7 yr) and quasi‐biennial (QB ∼2.4 yr) periods. Analyses utilize both a recursive filter and multichannel singular spectrum analysis (M‐SSA).
Modulated electron‐acoustic waves in auroral density cavities: FAST observations
Authors: Pottelette R. , Ergun R. E. , Treumann R. A. , Berthomier M. , Carlson C. W. , McFadden J. P. , Roth I.
Published: December 2003

We report on FAST observations of large amplitude (up to 500 mV m−1) envelope solitary waves at the edges of the AKR source region. These edges are characterized by the presence of two electron populations: a dominant hot (∼keV) component and a minority cold (<60 eV) component. The nonlinear waves are recorded when the spacecraft passes the base of the parallel auroral acceleration region. They form intense packets of electron acoustic waves. The modulation is due to ion acoustic waves. These structures are electrostatic and propagate along the magnetic field at speeds of a few 100 km s−1. They may play a crucial role in the acceleration processes taking place in these regions.
Linking the 10Be continental record of Lake Baikal to marine and ice archives of the last 50 ka: Implication for the global dust‐aerosol input
Authors: Aldahan A. , Possnert G. , Peck J. , King J. , Colman S.
Published: December 2003

We present here a 10Be profile from the continental sediments of Lake Baikal (the world's largest fresh water lake), which, for the first time, shows the ≈ 40 ka 10Be enhancement and a pattern that strongly matches those from the marine and ice records for the last 50 ka. This finding provides a new horizon for global and regional correlation of continental archives. Additionally, our VADM‐predicted 10Be production confirms and further strengthens a common global cause (geomagnetic field intensity) for the change in atmospheric 10Be over the last 50 ka. We also show that most of the 10Be inventory to the lake has been provided by riverine input, but with a significant addition from direct precipitation and dust‐aerosol fallout. We estimate a higher dust‐aerosol contribution of 10Be during the Holocene and interstadial stage 3 (22–50 ka) as compared with the glacial period (12–22 ka).
Estimate of global atmospheric organic aerosol from oxidation of biogenic hydrocarbons
Authors: Griffin Robert J. , Cocker David R. , Seinfeld John H. , Dabdub Donald
Published: December 2003

The results from a series of outdoor chamber experiments establishing the atmospheric aerosol‐forming potential of fourteen terpenoid hydrocarbons have been used to estimate the annual amount of secondary organic aerosol formed globally from compounds emitted by vegetation. Hydroxyl radical, ozone, and nitrate radical oxidation each contribute to aerosol formation in full‐photooxidation experiments; because oxidation by nitrate radical under ambient, remote conditions is likely to be negligible, parameters describing aerosol formation from hydroxyl radical and ozone reaction only are developed. Chamber results, temporally and spatially resolved, compound‐specific estimates of biogenic hydrocarbon emissions, and hydroxyl radical and ozone fields are combined to lead to an estimate for atmospheric secondary organic aerosol formed annually from biogenic precursors of 18.5 Tg, a number smaller than the previously published estimate of 30–270 Tg [Andreae and Crutzen, 1997].
Subsidence of Campi Flegrei (Italy) detected by SAR interferometry
Authors: Avallone A. , Zollo A. , Briole P. , Delacourt C. , Beauducel F.
Published: December 2003

Seven ERS SAR images of the Napoli area acquired from 1993 to 1996 were used to create six differential interferograms. Although the ambiguity height of the selected couples is high, because of the adverse characteristics of the area, the interferograms exhibit a relatively low level of coherence, even for couples sampling short time intervals. According to the estimated accuracy, no deformation is observed on Vesuvio volcano. The best interferogram, corresponding to the period Feb. 14, 1993–Apr. 3, 1996, shows that the centre of the Campi Flegrei caldera, near Pozzuoli harbour, subsided by about three fringes (84 mm) during the three years interval. This result is consistent with the value of 28±2 mm/year obtained from routine levelling surveys over the same period in the same area. The deflation that started after the 1983–1984 seismic and inflation crisis is thus still significant. The modelling of the fringe pattern, assuming a spherical deflating source within an elastic half‐space medium, predicts a source centred about 800 m Southwest of Pozzuoli at a depth of 2.7±0.3 km. For the studied time window, the maximum subsidence predicted by the model is 25 mm/year.
Separating natural and anthropogenic vertical movements in fast subsiding areas: The Po Plain (N. Italy) Case
Authors: Carminati E. , Di Donato G.
Published: December 2003

The geological and the anthropogenic components of vertical movement act on different time scales (millions to thousands years and hundreds to tens of years respectively). This peculiarity has been used in order to separate the factors controlling sea‐level changes in the Po plain area. The component due to long term geological processes is calculated from stratigraphic data (commercial wells and seismic lines). Adding to this long term component the contribution of Pleistocene deglaciation obtained from analytical models, an estimate of the total natural component of vertical velocities is obtained. The validity of these values is confirmed by comparison with velocities obtained from archaeological data and detailed palynological and paleontological analyses. The anthropogenic components can be inferred by comparison between the total natural vertical velocities we calculate and vertical velocities obtained from other data sets. Two applications are proposed using tide gauge records and geodetic data respectively.
The relationship between the Southern Oscillation Index and tropical cyclone tracks in the eastern North Pacific
Authors: Irwin Rossman P. , Davis Robert E.
Published: December 2003

Points of origin and downgradation of tropical cyclones in the eastern North Pacific (ENP) east of 160°W are related to the Southern Oscillation Index (SOI) during the hurricane season. All ENP tropical storms and hurricanes from 1966 through 1997 were grouped into three SOI‐based categories: those occurring during strong El Niño events (mean SOI < −0.6), strong La Niña events (mean SOI > 0.6), or near zero periods (−0.6 < mean SOI < 0.6). During El Niño storm seasons, ENP tropical cyclones originated approximately 5.7° (617 km) west and downgraded 7.5° (780 km) west of the long‐term mean longitudes for the positive SOI group. Near zero group storms also followed more northerly tracks than the negative SOI group storms. However, no significant differences in storm track are evident between the positive SOI and near zero groups, and the track length is not significantly different for any storm group.
High‐temperature dynamics in cristobalite (SiO2) and Carnegieite (NaAlSiO4): A Raman Spectroscopy Study
Authors: Richet Pascal , Mysen Bjorn O.
Published: December 2003

β‐cristobalite (SiO2) and β‐carnegieite (NaAlSiO4) have been investigated by Raman spectroscopy up to their melting points at 1999 and 1799 K, respectively. In both cases, the major spectral changes are observed at the α‐β transitions, near 540 and 970 K, especially at low frequencies where the modes, involving mainly intertetrahedral motion, merge to a single, broad envelope. These observations and comparisons made between the Raman spectra of the crystalline and liquid phases are consistent with the rigid‐unit model recently proposed for describing the dynamics of SiO4 and AlO4 tetrahedra in framework silicates. From a structural standpoint, an important consequence is that the atomic mobility associated with premelting sets in at the low temperatures of these transitions, not in the vicinity of the melting point as for other silicate minerals.
Stratosphere‐troposphere exchange at the subtropical jet: Contribution to the tropospheric ozone budget at midlatitudes
Authors: Langford A. O.
Published: December 2003

The interannual variability of the springtime maximum in free tropospheric ozone above Colorado is examined using 5‐1/2 years (1993–1998) of Differential Absorption Lidar (DIAL) data. Year‐to‐year differences of up to 20% in the monthly mean mixing ratios are shown to be correlated with changes in the 200 hPa wind speed over the eastern Pacific that are associated with the El Niño/Southern Oscillation (ENSO). This correlation is attributed to variations in the intensity of stratosphere‐troposphere exchange (STE) associated with the mean transverse circulation at the subtropical jet exit over the eastern Pacific.
Slip‐rate increase at Parkfield in 1993 detected by High‐Precision EDM and Borehole Tensor Strainmeters
Authors: Langbein J. , Gwyther R. L. , Hart R. H. G. , Gladwin M. T.
Published: December 2003

On two of the instrument networks at Parkfield, California, the two‐color Electronic Distance Meter (EDM) network and Borehole Tensor Strainmeter (BTSM) network, we have detected a rate change starting in 1993 that has persisted at least 5 years. These and other instruments capable of measuring crustal deformation were installed at Parkfield in anticipation of a moderate, M6, earthquake on the San Andreas fault. Many of these instruments have been in operation since the mid 1980s and have established an excellent baseline to judge changes in rate of deformation and the coherence of such changes between instruments. The onset of the observed rate change corresponds in time to two other changes at Parkfield. From late 1992 through late 1994, the Parkfield region had an increase in number of M4 to M5 earthquakes relative to the preceding 6 years. The deformation‐rate change also coincides with the end of a 7‐year period of sub‐normal rainfall. Both the spatial coherence of the rate change and hydrological modeling suggest a tectonic explanation for the rate change. From these observations, we infer that the rate of slip increased over the period 1993–1998.
Imaging P‐to‐S conversions with multichannel receiver functions
Authors: Neal Scott L. , Pavlis Gary L.
Published: December 2003

We present a new methodology in the direct imaging of P‐to‐S converted phases recorded on broadband seismic arrays. Our approach is based on conventional three‐component array processing and receiver function techniques with the key addition of a weighted stack based on an aerial smoothing function. This creates synthetic arrays with a specified aperture whose image points vary continuously across the array. With this approach, it is possible to interpolate data from an array of broadband stations onto an arbitrarily fine grid. We have applied this technique to a single deep event recorded by the Lodore broadband array, located in northern Colorado and southern Wyoming. The resulting images show distinct differences in crustal structure across the array, and also image major upper mantle discontinuities.
Plate interface properties in the Northeast Hikurangi Subduction Zone, New Zealand, from converted seismic waves
Authors: Eberhart‐Phillips Donna , Reyners Martin
Published: December 2003

Arrival time inversion of local earthquakes in the northeast Hikurangi subduction zone has indicated high Vp/Vs at the plate interface, but the thickness of the anomalous zone is poorly constrained by the P and S travel‐times. We investigate this plate interface zone further by modeling S to P converted phases, using synthetic seismograms for upgoing rays of earthquakes in the subducting plate mantle. The modeling indicates a low velocity layer at the plate interface, which is generally 1–2 km thick, and has Vp of 5.0–5.35 km/s and Vp/Vs of 2. This is consistent with a subducting sediment channel with near‐lithostatic fluid. The low rigidity indicates a weak zone, which will have a strong influence on the distribution of deformation. The modeling approach shows promise in defining rheological parameters at the plate interface, and mapping their variation across the seismogenic zone of the subduction thrust.
No spreading across the Southern Juan de Fuca Ridge axial cleft during 1994–1996
Authors: Chadwell C. David , Hildebrand John A. , Spiess Fred N. , Morton Janet L. , Normark William R. , Reiss Carol A.
Published: December 2003

Direct‐path acoustic measurements between seafloor transponders observed no significant extension (−10±14 mm/yr) from August 1994 to September 1996 at the southern Juan de Fuca Ridge (44°40′ N and 130°20′ W). The acoustic path for the measurement is a 691‐m baseline straddling the axial cleft, which bounds the Pacific and Juan de Fuca plates. Given an expected full‐spreading rate of 56 mm/yr, these data suggest that extension across this plate boundary occurs episodically within the narrow (∼ 1 km) region of the axial valley floor, and that active deformation is occurring between the axial cleft and the plate interior. A cleft‐parallel 714‐m baseline located 300 m to the west of the cleft on the Pacific plate monitored system performance and, as expected, observed no motion (+5±7 mm/yr) between the 1994 and 1996 surveys.
Ice formation by black carbon particles
Authors: DeMott P. J. , Chen Y. , Kreidenweis S. M. , Rogers D. C. , Sherman D. E.
Published: December 2003

Ice formation by black carbon particles was studied using a continuous flow thermal diffusion chamber. Submicron particles were treated to uptake H2SO4 in amounts estimated to range from zero to several percent by weight. The particles were processed at constant temperatures (−40 to −60°C), and humidity from ice saturation to values exceeding water saturation. Conditions required for ice nucleation were determined for different acid exposures. Untreated soot particles showed activity as deposition/sorption ice nuclei. Particles with approximate monolayer equivalent coverage by H2SO4 nucleated ice at humidities for which it was inferred that the acid was dilute enough to freeze homogeneously on soot surfaces. Particles with multi‐layer equivalent H2SO4 coverage froze as solution droplets most readily. Heterogeneous freezing of the latter particles was observed to occur in preference to the homogeneous freezing of an externally mixed population of H2SO4 droplets at lower temperatures.
Solar wind quasi‐invariant as a new index of solar activity
Authors: Osherovich Vladimir A. , Fainberg J. , Stone R. G.
Published: December 2003

Solar wind parameters, such as magnetic field strength B, plasma density ρ and bulk speed υ correlate with solar cycle measured by sunspot numbers SSN. Using hourly data in the solar wind near the Earth, we show that the energy density ratio QI = (B²/8π)/(ρυ²/2) has a correlation coefficient (cc) with SSN which is appreciably higher than for each of B, ρ and υ taken separately. The proposed quasi‐invariant is related to magnetic Mach number MA (QI = MA−2). The yearly median QI for the last 28 years holds a linear relation with SSN with high cc = 0.98, which makes it the best index of solar activity derived from solar wind parameters so far.
Venus‐like interaction of the solar wind with Mars
Authors: Cloutier P. A. , Law C. C. , Crider D. H. , Walker P. W. , Chen Y. , Acuña M. H. , Connerney J. E. P. , Lin R. P. , Anderson K. A. , Mitchell D. L. , Carlson C. W. , McFadden J. , Brain D. A. , Rème H. , Mazelle C. , Sauvaud J. A. , d'Uston C. , Vignes D. , Bauer S. J. , Ness N. F.
Published: December 2003

The magnetometer and electron reflectometer experiment (MAG/ER) on the Mars Global Surveyor (MGS) spacecraft has obtained magnetic field and electron data which indicates that the solar wind interaction with Mars is primarily an ionospheric‐atmospheric interaction similar to that at Venus. However, the global‐scale electric currents and resulting magnetic fields due to the interaction at Mars are locally interrupted or perturbed over distance scales of several hundred kilometers by the effects of paleomagnetic fields due to crustal remanence. In this paper we compare the Mars‐solar wind interaction with the Venus‐solar wind interaction by selecting MGS orbits which do not show significant magnetic perturbations due to crustal magnetic anomalies, and demonstrate that a number of phenomena characteristic of the Venus‐solar wind interaction are also observable at Mars.
A probable signal of the 11‐year solar cycle in the troposphere of the northern hemisphere
Authors: van Loon Harry , Shea Dennis J.
Published: December 2003

The existence of a signal of the 11‐year solar cycle in the geopotential heights of the lower half of the stratosphere implies, through hydrostatic argument, an association with the temperatures in the troposphere. We have defined this association for the layer between 750hPa and 200hPa by means of the NCEP/NCAR re‐analyses for the period 1958–1997. In the zonal average it is estimated to be 0.15°C–0.2°C for the whole of the Northern Hemisphere in summer, and about the same for the tropics‐subtropics in March–December.
On the relationship between bursty flows, current disruption and substorms
Authors: Angelopoulos V. , Mozer F. S. , Mukai T. , Tsuruda K. , Kokubun S. , Hughes T. J.
Published: December 2003

A current disruption (CD) event was observed by the Geotail spacecraft at (X, Y)GSM=(−10.0, 1.9) RE within 1 min from the onset of a substorm. The event was selected as one of four near‐Earth current disruption events identified in a search of three years of Geotail data. Fast ion flows accompanied the magnetic field dipolarization, rendering this event a near‐Earth bursty bulk flow (BBF) event. The gradient anisotropy of the 30–44 keV ions at the onset of the flow is consistent with an Earthward motion of the heated plasma and agrees with the direction of flux and energy transport. The aurora and the associated electrojet moved from low to high latitudes during substorm expansion. Our observations show that CDs and BBFs are qualitatively similar phenomena in the near‐Earth tail, both associated with poleward‐moving (classical) auroral substorms.
Holocene stratigraphy of the Dead Sea: Correlation of high‐resolution seismic reflection profiles to sediment cores
Authors: Ben‐Avraham Zvi , Niemi Tina M. , Heim Christoph , Negendank Joerg , Nur Amos
Published: December 2003

The northern basin of the Dead Sea is occupied by a ∼300‐m‐deep lake. A series of cores in the deep‐water part of the lake provide information about the top 365 cm of the sediments. The cores were correlated with high‐resolution 3.5‐kHz seismic profiles from this area and provide lithologic and age constraints for the high‐resolution seismic reflection data. Visual comparison of the two data sets shows that strong surface and shallow subsurface reflectors (A and B) correlate to the massive salt at the seafloor surface and the indurated salt at the base of the cores, respectively. Calculations of an average seismic velocity based on the interval between these reflectors and the corresponding sedimentary thickness yield an average 3500 m/s velocity. This agrees closely with velocities determined from direct measurements of compressional velocities for sediment samples. Ultrasonic wave velocity measurements of salt samples from the cores and dry rock salt cores from the southern basin of the Dead Sea indicate that wave velocities are independent of the burial depth at shallow depths; however, velocities show strong dependence on porosity. At low hydrostatic pressure a reduction in porosity as well as closure of microcracks in the crystals cause an increase in the velocities. This increase disappears at higher stress levels. Synthetic seismograms of the upper 3 ms and the entire 25 ms penetrated by the seismic profiles reinforce the lithologic and seismic stratigraphic correlation and confirm that prominent reflectors in the basin represent the top boundary of halite layers which are separated by laminated sequences of evaporites and elastics. The salt in the upper salt sequence is deposited at a very fast rate of more than 20 mm/yr. However, at shallow depths, considerable compaction takes place. Variations in appearance and velocities of the upper salt sequence and middle salt sequence indicate that the porous, granular, and fine‐grained precipitates of the surface salts are diagenetically altered to a coarse and compact crystalline aggregate by re‐solution and crystallization with burial. The sedimentary sequences recovered in the cores suggest that significant lake level fluctuations took place in the past in response to climatic changes. The detailed correlation of the cores and seismic profiles makes it possible to extrapolate climatic data from earlier periods beneath the maximum core penetration by analyzing the seismic stratigraphic sequences of the seismic reflection data.
Streaming potential in porous media: 1. Theory of the zeta potential
Authors: Revil A. , Pezard P. A. , Glover P. W. J.
Published: December 2003

Electrokinetic phenomena are responsible for several electrical properties of fluid‐saturated porous materials. Geophysical applications of these phenomena could include the use of streaming potentials for mapping subsurface fluid flow, the study of hydrothermal activity of geothermal areas, and in the context of earthquake prediction and volcanic activity forecasting, for example. The key parameter of electrokinetic phenomena is the ξ potential, which represents roughly the electrical potential at the mineral/water interface. We consider silica‐dominated porous materials filled with a binary symmetric 1:1 electrolyte such as NaCl. When in contact with this electrolyte, the silica/water interface gets an excess of charge through chemical reactions. Starting with these chemical reactions, we derive analytical equations for the ξ potential and the specific surface conductance. These equations can be used to predict the variations of these parameters with the pore fluid salinity, temperature, and pH (within a /pH range of 6–8). The input parameters to these equations fall into two categories: (1) mineral/fluid interaction geochemistry (including mineral surface site density and surface equilibrium constants of mineral/fluid reactions), and (2) pore fluid /pH, salinity, and temperature. The ξ potential is shown to increase with increasing temperature and pH and to decrease with increasing salinity. The proposed model is in agreement with available experimental data. The application of this model to electric potentials generated in porous media by fluid flow is explored in the companion paper.
Crustal structure of the Filchner‐Ronne Shelf and Coats Land, Antarctica, from gravity and magnetic data: Implications for the breakup of Gondwana
Authors: Studinger Michael , Miller Heinz
Published: December 2003

Understanding the tectonic evolution of the Weddell Sea embayment provides valuable insights on the breakup of Gondwana. Existing geodynamic models for this region differ mainly in the nature of the crust beneath the Filchner‐Ronne shelf and the movement of the Ellsworth‐Whitmore Mountains (EWM) crustal block. New gravity data presented here constrain the area and timing of this EWM movement from its prebreakup position to its present‐day position. The crustal thickness estimates of 29±5 km and 27±4 km derived from power spectral analysis as well as forward gravity modeling make the existence of oceanic crust beneath the Filchner‐Ronne shelf unlikely. A new crustal unit consisting of Precambrian crust has been identified with gravity and magnetic data and extends ∼175 km off the coast of Coats Land. This crustal unit conflicts with the late Mesozoic movement of the EWM in this area. The effective elastic thickness of Te = 35±5 km has been estimated from coherence spectra for the Ronne shelf. Together with the crustal thickness of 27 km from refraction seismic studies and comparison with theoretical models, the thermal age is estimated to be between 165 and 230 Ma for the Ronne shelf. This age marks the last significant tectonic event in this area and is in good agreement with the breakup of Gondwana. It further makes later movements of microplates like the EWM in this region unlikely. Thus a Filchner block consisting of continental crust should be considered in any geodynamic model of the early Weddell Sea evolution.
Characterization of in situ elastic properties of gas hydrate‐bearing sediments on the Blake Ridge
Authors: Guerin Gilles , Goldberg David , Meltser Aleksandr
Published: December 2003

During Ocean Drilling Program Leg 164, shear sonic velocity and other geophysical logs were acquired in gas hydrate‐bearing sediments on the Blake Ridge to characterize the very distinct seismic signature of such formations: anomalous low amplitudes overlying a strong bottom‐simulating reflector (BSR). A comparison of the bulk moduli derived from the logs to standard elastic consolidation models shows that the sediments are overconsolidated above the BSR at 440 meters below seafloor (mbsf) because of the presence of hydrates. Below the bottom of the thermodynamic hydrate stability zone at ∼520 mbsf, the high compressibility of the formation and the attenuation of the monopole sonic waveforms are typical of sediments partially saturated with free gas. Between these two depths, gas hydrate and free gas seem to coexist. Within the Gas hydrate stability zone, we estimate the amount of gas hydrates using different models based on theories for wave scattering in multiphase media and for grain cementation. In close agreement with measurements made on discrete in situ samples, the latter describes most accurately the interactions between the matrix, the pore fluids, and the hydrates. This model indicates that 5 to 10% of the pore space is occupied by hydrates deposited uniformly on the surface of the grains. The comparison with Gassmann's model also show that the amount of free gas below the BSR never exceeds 5% of the pore space but is high enough to generate the BSR. The coexistence of free gas and gas hydrates below the BSR may be explained by capillary effects in the smaller pores or by remaining crystalline structures after partial hydrate decomposition.
Dynamics of sediment subduction‐accretion at convergent margins: Short‐term modes, long‐term deformation, and tectonic implications
Authors: Beaumont Christopher , Ellis Susan , Pfiffner Adrian
Published: December 2003

A conceptual model is proposed for the short‐term operation of subduction‐accretion systems at convergent margins. The operation is described in terms of the mass fluxes among the four system components, namely, the accretionary (or prowedge) wedge (P), an uplifted plug (U), a retrowedge (R), and the subduction zone. The latter consists of a conduit (C) of slowly moving and deforming material and an underlying subduction channel where material moves and deforms more rapidly. A broad range of convergent margin types can be accommodated within the conceptual model framework, with differing short‐term modes that depend on which of the P‐U‐R‐C components that are active. Finite element geodynamical model experiments are used to investigate whether these modes are dynamically feasible. These experiments determine excitation of modes under controls that include model entry and exit mass fluxes; flexural loading by accreted material and slab pull forces; boundary velocities corresponding to subduction and subduction zone advance/retreat; distribution, thickness, and density of accreted sediment; and internal frictional properties of the Coulomb sediments. That all modes, with the exception of P (in which only a prowedge is created), are seen in the models provides support for the concepts and allows conditions that favor particular modes to be recognized. In addition, the finite deformation predicted by the models is used to infer the long‐term tectonic styles associated with both single‐mode evolutions and those that exhibit mode switching. A steady state mass balance can occur for modes 0 (pure subduction), P‐C (i.e., where components P and C are active), or other modes when the surface mass denudation plus the subduction flux balances accretion plus tectonic underplating. The P‐C mode is favored by subduction zone retreat and/or an increasing subduction load and creates an apparent landward dipping backstop. P‐U‐C is predicted to be the most common mode, and it creates an apparent seaward dipping backstop. P‐U‐R‐C is an inefficient mode but is favored by easily detached retrocrust or sediment and/or a decreasing subduction load.
What controls the along‐strike slopes of volcanic rift zones?
Authors: Fialko Yuri A. , Rubin Allan M.
Published: December 2003

We investigate the dynamics of viscous pressure losses associated with lateral magma transport in volcanic rift zones by performing (1) coupled elastic‐hydrodynamic simulations of downrift magma flow in dikes and (2) analog experiments mimicking lateral dike propagation in the presence of an along‐rift topographic slope. It is found that near‐source eruptions are likely to be favored by shallow slopes while distant downrift eruptions may be encouraged by steeper slopes, provided that along‐rift variations in the tectonic stress are negligible or uncorrelated on the timescale of multiple dike intrusions. This implies the existence of a critical slope to which a volcanic rift zone would naturally evolve. Such behavior is produced by three‐dimensional (3‐D) elastic effects and is controlled by the ratio of the driving pressure gradient due to the along‐strike topographic slope to the vertical gradient in the excess magma pressure in the dike. This model may be viewed as complementary to commonly cited mechanisms that appeal to magma viscosity and the dynamics of freezing of lava flows at the surface to explain the low profiles of basaltic shield volcanoes. Our estimated values of the critical slopes are in general agreement with observations in Hawaiian rift zones, but further development of fully 3‐D models is required for more accurate predictions.
Streaming potential measurements: 1. Properties of the electrical double layer from crushed rock samples
Authors: Lorne Benoit , Perrier Frédéric , Avouac Jean‐Philippe
Published: December 2003

The ξ potential has been inferred from streaming potential measurements with crushed rock samples as a function of pH and electrolyte concentration for various salts. The value obtained for crushed Fontainebleau sandstone at pH = 5.7 and a KCl solution with a resistivity of 400 Ω m is −40±5 mV, where the error is dominated by sample to sample variations. The sensitivity of the ξ potential to the electrolyte resistivity for KCl is given experimentally by ρf0.23±0.014 where ρf is the electrolyte resistivity. The point of zero charge (pzc) is observed for pH = 2.5±0.1, and the ξ potential is positive for pH < pzc and negative for pH > pzc. For pH > 5 the variations of the ξ potential with pH can be approximated by ξ(pH)/ξ(5.7) = 1 + (0.068±0.004)(pH‐5.7) for ρf = 100 Ω m. The ξ potential has been observed to be sensitive to the valence of the ions and is approximately reduced by the charge of the cation, unless specific adsorption takes place like in the case of Al3+. The experimental results are well accounted for by a three‐layer numerical model of the electrical double layer, and the parameters of this model can be evaluated from the experimental data. The sensitivity of the ξ potential to the rock minerals has also been studied. The ξ potential obtained for granitic rocks is comparable to that obtained for Fontainebleau sandstone but can be reduced by a factor of 2–4 for sandstones containing significant fractions of carbonates or clay. To take into account the effect of the chemical composition of the electrolyte, a chemical efficiency is defined as the ratio of the ξ potential to the ξ potential measured for KCl. This chemical efficiency is measured to be ∼80% for typical groundwater but can be as low as 40% for a water with a high dissolved carbonate content. The set of empirical laws derived from our measurements can be used to assess the magnitude of the streaming potentials expected in natural geophysical systems.
P wave anisotropy, stress, and crack distribution at Coso geothermal field, California
Authors: Lees Jonathan M. , Wu Huatao
Published: December 2003

A new inversion method for P wave anisotropy [Wu and Lees, 1999a] has been applied to high‐precision, microseismic traveltime data collected at Coso geothermal region, California. Direction‐dependent P wave velocity and thus its perturbation, are represented by a symmetric positive definite matrix A instead of a scalar. The resulting anisotropy distribution is used to estimate variations in crack density, stress distribution and permeability within the producing geothermal field. A circular dome‐like structure is observed at the southwestern part of the geothermal region southwest of Sugarloaf Mountain. Using a linear stress‐bulk modulus relationship, deviatoric stress is estimated to be 3–6 MPa at geothermal production depths (1–2 km), assuming all the anisotropy is related to stress. The stress field is compressional NNE‐SSW and dilational WNW‐ESE, coinciding with a previous, independent study using earthquake focal mechanisms. Following a theory on flat, elliptic cracks, residual crack density estimated from P anisotropy is ∼0.0078 assuming crack aspect ratios ≫ 1:60 and is ∼0.041 when crack aspect ratios are close to 1:60. Residual crack orientation distribution is related to velocity anisotropy. On the basis of anisotropic part of crack density distribution function, the anisotropic part of permeability distribution may be calculated by a statistical approach via simple parallel fluid flow along cracks.
Magnetostratigraphy of the Miocene Corque Basin, Bolivia: Implications for the geodynamic evolution of the Altiplano during the Late Tertiary
Authors: Roperch Pierrick , Hérail Gérard , Fornari Michel
Published: December 2003

A magnetostratigraphic study of a thick Miocene continental red bed sequence (Totora Formation) located in the north central Altiplano (the Corque syncline) yielded quantitative determinations of the sedimentation rate and better constraints of the timing of the deformation within the Altiplano. Paleomagnetic results obtained in 653 samples from a composite 4.5 km thick red bed sequence across the Corque syncline indicate that most of the Totora sequence was deposited in the middle Miocene from 14 to 9 Ma with high sedimentation rates up to 970 m/Myr from 12 to 9 Ma. The high rate of infill of the Corque basin demonstrates active subsidence of the northern Altiplano during the middle Miocene. Deformation ceased in the northern Altiplano domain only at the end of the Miocene. Two major tuff beds within the sequence have been previously dated by 39Ar/40Ar of sanidine minerals [Marshall et al, 1992]. The excellent agreement between the magnetostratigraphic dating of the tuffs and the radiometric dating independently supports the age calibration of the geomagnetic reference timescale proposed by Cande and Kent [1995]. The numerous paleomagnetic results obtained in middle Miocene sediments within the Corque syncline, a structure which can be traced for more than 100 km along its axis, demonstrate that the Corque syncline rotated counterclockwise by 10.8°±2.9° since 9 Ma as a consequence of the internal deformation of the Altiplano. Inclination flattening of 17° is recorded in these red bed sediments. This result provides additional evidence that detrital magnetizations, especially in red beds, should be taken with caution when estimating paleolatitudes or long‐term variations of the geomagnetic field.
Modeling local crustal isostasy caused by ductile flow in the lower crust
Authors: Bott Martin H. P.
Published: December 2003

Some continental regions display local isostatic uplift on a much shorter scale than that of classical isostasy, such as high ground associated with granites. This is best explained by compensatory channel flow in the lower crust. To investigate the mechanism, the uplift caused by an existing granitic body 30 km wide, extending to 10 km depth and of density deficiency −120 kg/m3 has been modeled by finite element analysis, as it rises toward isostatic equilibrium over 5 Myr by inward creep in the ductile lower crust. The upper crust is treated as elastoplastic with a specified strength envelope. The lower crust (15 to 36 km depth) is viscoelastic with either Newtonian or power law quartz rheology and the strong upper mantle down to 60 km is either elastoplastic or has olivine rheology. Models with channel flow produce much higher and sharper topography than those without it. Faulted models produce the highest and sharpest elevations; when preexisting faults are present, the uplift is strongly affected by the state of stress in the upper crust, with compression suppressing uplift on outward dipping faults and tension accentuating it (the opposite for inward dipping faults). The surface topography approaches isostatic equilibrium over 5 Myr at a decaying rate dependent on the type of rheology. The Moho is pulled upward as the surface rises during the earlier stages, but it eventually subsides toward its equilibrium level much more slowly than the surface approaches equilibrium. This linked deformation of surface and Moho allows approximate isostatic equilibrium of the upper crust to be established without inflow at the edges of the model, and it may produce long‐lasting bumps on the Moho.
Premonitory slip and tidal triggering of earthquakes
Authors: Lockner David A. , Beeler Nick M.
Published: December 2003

We have conducted a series of laboratory simulations of earthquakes using granite cylinders containing precut bare fault surfaces at 50 MPa confining pressure. Axial shortening rates between 10−4and 10−6 mm/s were imposed to simulate tectonic loading. Average loading rate was then modulated by the addition of a small‐amplitude sine wave to simulate periodic loading due to Earth tides or other sources. The period of the modulating signal ranged from 10 to 10,000 s. For each combination of amplitude and period of the modulating signal, multiple stick‐slip events were recorded to determine the degree of correlation between the timing of simulated earthquakes and the imposed periodic loading function. Over the range of parameters studied, the degree of correlation of earthquakes was most sensitive to the amplitude of the periodic loading, with weaker dependence on the period of oscillations and the average loading rate. Accelerating premonitory slip was observed in these experiments and is a controlling factor in determining the conditions under which correlated events occur. In fact, some form of delayed failure is necessary to produce the observed correlations between simulated earthquake timing and characteristics of the periodic loading function. The transition from strongly correlated to weakly correlated model earthquake populations occurred when the amplitude of the periodic loading was approximately 0.05 to 0.1 MPa shear stress (0.03 to 0.06 MPa Coulomb failure function). Lower‐amplitude oscillations produced progressively lower correlation levels. Correlations between static stress increases and earthquake aftershocks are found to degrade at similar stress levels. Typical stress variations due to Earth tides are only 0.001 to 0.004 MPa, so that the lack of correlation between Earth tides and earthquakes is also consistent with our findings. A simple extrapolation of our results suggests that approximately 1% of midcrustal earthquakes should be correlated with Earth tides. Triggered seismicity has been reported resulting from the passage of surface waves excited by the Landers earthquake. These transient waves had measured amplitudes in excess of 0.1 MPa at frequencies of 0.05 to 0.2 Hz in regions of notable seismicity increase. Similar stress oscillations in our laboratory experiments produced strongly correlated stick‐slip events. We suggest that seemingly inconsistent natural observations of triggered seismicity and absence of tidal triggering indicate that failure is amplitude and frequency dependent. This is the expected result if, as in our laboratory experiments, the rheology of the Earth's crust permits delayed failure.