bluuebunny/crossref_metadata_2025 huggingface dataset:
Geodetic monitoring of the southern San Andreas Fault, California, 1980–1991
Authors: Savage J. C. , Lisowski M.
Published: February 2004

Five geodetic arrays (10 to 40 km aperture) located along the San Andreas fault have been surveyed frequently (several times in most years) over the 1980–1991 interval to detect possible fluctuations in the deformation rate. In each survey of an array the distances between the same four to seven pairs of geodetic monuments were measured. The distances measured (with corresponding standard deviation) range from 8.4 km (3.4 mm) to 38 km (8.2 mm). The data are displayed as plots of measured distance as a function of time. Linear fits in such plots furnish a satisfactory representation of the data. That is, the scatter of the data about the linear fits is within the range expected for the estimated standard deviations in measurement. However, there are coherent low‐amplitude (within the observational error) fluctuations apparent in some of the measured distances. Those fluctuations need not be tectonic effects but rather may be either random patterns or artifacts introduced by systematic drift in instrument calibration or wander of the geodetic monuments. We conclude that the measurements are consistent with steady deformation of the arrays over the 1980–1991 interval, and we find no convincing evidence in the data for fluctuations in the rate of deformation.
Rapid extension recorded by cooling‐age patterns and brittle deformation, Naxos, Greece
Authors: John Barbara E. , Howard Keith A.
Published: February 2004

The metamorphic core complex exposed as the island of Naxos in the Aegean Sea records an unusually complete sequence of structures developed as a result of continental extension. The structures formed during Miocene rise and cooling from ductile, upper amphibolite facies and anatectic conditions to brittle near‐surface conditions beneath the Naxos detachment fault. Top‐to‐the‐north ductile fabrics in the footwall, which initially developed during amphibolite facies prograde metamorphism, were overprinted by a succession of north directed, normal sense lower‐temperature brittle structures as the footwall was tectonically unloaded and unroofed. Pseudotachylite and cataclasite formation, brittle faulting, alteration, and erosion of the footwall occurred during continued slip and tectonic denudation. Neogene conglomerate and megabreccia, in part derived from exhumation of this footwall, lie structurally above the peripheral Naxos fault. Published K‐Ar and 40Ar/39Ar ages for hornblende, white mica, and biotite in the footwall decrease northwestward; apparent ages 17–50 Ma in the southeast correspond to areas of low metamorphic grade where preextension argon was partially retained. Published ages 16–10 Ma in higher‐grade rocks of the domal core in the north are cooling ages that for each of the three minerals show a component of younging in the NNE direction of extension. Assuming this is the direction of unroofing, we interpret the rate of this younging as the fault slip rate as the footwall rocks moved >20 km SSW relative to their hanging wall along the base of the Naxos detachment fault. The calculated rates of slip average 5–8 mm/yr, comparable to maximum rates reported in the Basin and Range province.
Lower Ordovician reversal asymmetry: An artifact of remagnetization or nondipole field disturbance?
Authors: Torsvik Trond H. , Trench Allan , Lohmann Kyger C. , Dunn Shannon
Published: February 2004

Five stratigraphically linked polarity intervals (R1 → R5 and N1 → N5) from Late Arenig to Early Caradoc (Ordovician) times are recognized within the Gullhögen Limestone section, southern Sweden. The reversal frequency should be considered as a minimum given extremely low sedimentation rates (1–3 m/m. y.). Primary magnetite, probably detrital or biogenic in origin, is the prevailing remanence carrier in addition to early diagenetic hematite. A conspicuous reversal asymmetry, in which the remanence directions of the two lowermost normal polarity sections (N1 and N2) depart significantly from otherwise nearly antiparallel and typical Lower Ordovician directions, has been identified. This reversal asymmetry is mainly in declination. An artifact of remagnetization cannot be entirely ruled out, but we find this explanation rather unsatisfactory given the lack of systematic magneto‐mineralogical or isotope variations (δ18O and δ13C) within these anomalous normal polarity horizons. The anomalous strata involved do not exhibit a different degree or extent of diagenetic alteration or magnetomineralogical character when compared to horizons above and below carrying the reverse polarity directions. We therefore suggest nonzonal nondipole field disturbances, lasting some 1–1.5 m. y. as a likely source, in order to explain these normal polarity anomalies. If the observations appear to be global, the results question the fundamental palaeomagnetic concept of symmetric reversals throughout geological time.
Love numbers for diurnal tides: Relation to wobble admittances and resonance expansions
Authors: Mathews P. M. , Buffett B. A. , Shapiro I. I.
Published: February 2004

Wobble‐related centrifugal and gravitational forces result in contributions to the Love numbers that are proportional to wobble admittances of the Earth and its core regions. We examine the nature of such forces on the basis of the recent work of Buffett et al. (1993) and present expressions relating the Love numbers to the wobble admittances. These expressions are qualitatively different from those of Sasao et al. (1980). By using a semianalytic theory for wobbles, we obtain a corresponding theory for the Love numbers and resonance formulae to describe their frequency dependence in the diurnal tidal band. In this formulation, the Earth's properties enter through a set of basic parameters (ellipticities, compliances, etc., of the whole Earth and its core regions). This analytic structure offers, for the first time, possibilities for the estimation of such parameters from data on nutation amplitudes and tides in a mutually consistent fashion. It also assures that use of a “modified preliminary reference Earth model (modifed PREM)” set of values for the basic parameters, which yields a free core nutation period in agreement with estimates from very long baseline interferometry (VLBI) data, will lead to resonances in the Love numbers at the corresponding nearly diurnal free wobble (NDFW) eigenfrequency. No other method is currently available for placing the resonance at the observed frequency. We present and discuss tables of values of (1) the parameters occurring in the resonance formulae for the wobbles and the Love numbers and (2) the wobble admittances and the Love numbers at various frequencies in the diurnal band, all computed using modified PREM in an approximation which takes account of all wobble‐related effects but treats the Earth otherwise as spherical and static. Such detailed information on the admittances of wobbles, of the outer and inner cores, in particular, is not available elsewhere in the literature. We present comparisons of other workers' and our values for the Love numbers, to help to elucidate the sources of the differences among them. The influence of the NDFW eigenfrequency implied by the model used is notable: for the ψ1tide which is nearest this eigenfrequency, we obtaink= 0.5382,h= 1.0812, andl= 0.0687, while the Wahr (1981b) values, for model 1066A, which has an NDFW eigenfrequency differing significantly from the VLBI estimate, arek= 0.466,h= 0.937,l= 0.0736.
Shear‐wave splitting beneath western United States in relation to plate tectonics
Authors: Özalaybey Serdar , Savage Martha K.
Published: February 2004

We have examined shear wave splitting in teleseismic shear waves from 26 broadband stations in the western United States. Fast polarization directions (ϕ) and delay times (δt) show spatial variations that are coherent within geologic provinces. Stations located near the San Andreas fault show clear evidence for fault‐parallel anisotropy in the crust and upper mantle (115–125 km thickness). This can be explained by the finite strain associated with the relative plate motion between the North American and Pacific plates. The lateral extent of this strain field is probably narrow to the west, because stations 55 km west of the San Andreas fault do not show fault‐parallel anisotropy in southern California. Station LAC located 80 km east of the San Andreas fault shows large fault‐parallel anisotropy. This suggests that the Pacific‐North American plate boundary in the mantle might be displaced to the east in southern California. A deeper E‐W oriented fast direction of anisotropy underlies the fault‐parallel anisotropic layer in the vicinity of the San Andreas fault. An E‐W fast feature is also present beneath the western Basin and Range and the foothills of the Sierra‐Nevada, although local variations are present. The magnitude of delay times suggests that this feature resides in the asthenosphere. We interpret this feature as the asthenospheric flow in the slabless window left behind the Farallon plate. The flow‐induced anisotropy may partially be frozen‐in at shallow depths. Station ORV is located near the southern edge of the Gorda slab where no anisotropy is detected. The absence of anisotropy at this location could therefore mark a boundary between Farallon associated flow and regions where E‐W oriented asthenospheric flow did not occur. The lack of evidence for NE‐SW fast orientation within the Walker Lane Shear Belt of western Nevada suggests that this crustal feature does not extend into the mantle or that is not as well developed as that beneath the San Andreas fault. Stations located over the young subducting Gorda plate mark a change in the fast direction to nearly NE‐SW. This direction aligns well with the maximum compressive stress direction in the overlying North American plate and the NE‐SW directed internal shearing of the Gorda plate. The anisotropic thicknesses calculated from delay times suggest roughly double that expected for purely lithospheric contributions. This implies that the anisotropic thickness may include some of the asthenosphere. Alternatively, using a higher anisotropy of 8% can bring thicknesses in line with other measures of lithospheric thicknesses. The correspondence between the fast directions and the present plate tectonic deformations suggest that mapping upper mantle deformation through seismic anisotropy is a viable method, and that asthenospheric flow may be a significant contributor to seismic anisotropy.
Inner core anisotropy in three dimensions
Authors: Su Wei‐jia , Dziewonski Adam M.
Published: February 2004

The purpose of this paper is the investigation of cylindrical anisotropy in the inner core based on the travel time anomalies of the PKIKP phase. We use the arrival times reported in the International Seismological Centre Bulletins for years 1964–1990. We select only earthquakes which have a good azimuthal coverage and a sufficiently large number of reporting stations. The earthquakes are relocated using corrections for lateral heterogeneity computed for our most recent three‐dimensional mantle model. We use a total of 313,422 observations of the DF branch of PKP travel time anomalies within the epicentral distance ranges of 120°–140° and 150°–180° reported by 2335 stations for 26,377 earthquakes. We process the data using an averaging procedure, which we call cylindrical anisotropy stacking, that enhances the effects of anisotropy, but is expected to suppress those due to lateral heterogeneity and random errors. The processed residuals show a remarkably consistent pattern. This confirms the dominance of the cylindrical elastic anisotropy in the inner core with, approximately, a constant axis of symmetry. This axis of symmetry is found to be tilted 10.5°±1° from the Earth's rotation axis in the direction 160°E±5° in the northern hemisphere. In this new coordinate system we determine a four‐layer axisymmetric model of transverse anisotropy with each layer approximately 300 km thick. The model shows that the anisotropy is strongest (>3%) within the innermost part of the core. The travel time anomalies show significant (±1.5 s) longitudinal variations, even when the tilt of the axis of symmetry is considered. There is a substantial increase in the amplitude of the longitudinal variations, which are dominated by the second and fourth harmonics, for rays with bottoming depths exceeding 400 km below the inner core boundary. The measurable tilt of the axis of symmetry and the presence of significant nonaxisymmetric signal may provide important clues with respect to the possible causes of anisotropy. The increase in the anisotropy in the innermost part of the core departs from earlier inferences that anisotropy may be limited to the outermost 200–300 km of the Earth's inner core.
Experimental, geological, and geochemical constraints on the origins of low‐K silicic magmas in oceanic arcs
Authors: Beard James S.
Published: February 2004

The results of recent experimental and geochemical studies demonstrate that typical, low‐K, island arc dacites (IAD) and tonalites have liquidus water contents below 6 wt %, while many arc basalts, including evolved, low‐Mg high‐alumina basalt (HAB; H2O ≥ 4 wt%) and magnesian arc tholeiites (H2O ≥ 2 wt %) are water‐rich. If these water contents are typical of arc basalts, fractionation at pressures of 200 MPa or more would produce water‐rich magmas with major element chemical characteristics unlike >99% of observed IAD. In light of this, plausible mechanisms for IAD genesis include (1) dehydration melting of amphibolitized arc crust and (2) low‐pressure (less than 200 MPa) fractionation or assimilation/fractional crystallization (AFC) (accompanied by devolatilization) of hydrous arc basalts. In general, a partial melting origin is favored for tonalitic plutons emplaced at pressures ≥200 MPa, for bimodal suites where geochemistry rules out a genetic relationship between the mafic and silicic end‐members, and for IAD with isotopic characteristics distinct from potential basaltic or andesitic parents. Evidence for partial melting of amphibolite yielding IAD melts has been documented in several ancient island arc complexes. A fractionation origin is favored where there is isotopic homogeneity in a basalt‐dacite system and for dacites having very low concentrations of incompatible trace elements. Bimodal magmatism in general and high concentrations of incompatible elements in silicic magmas appear to favor a partial (batch) melting origin over Raleigh fractionation but can also result from convectively driven batch fractionation processes (Brophy, 1991). Although both fractionation/AFC and partial melting may be legitimately invoked to explain dacitic magmatism in a given situation, a general model must also account for the absence of high‐pressure fractionates of hydrous basalts. This observation seems to favor an important role for amphibolite melting in IAD genesis. It is also possible, however, that physical factors (e.g., neutral buoyancy) promote the formation of basaltic magma chambers in the upper crust or that convective processes, enhanced by large temperature gradients in the upper crust, may favor upper crustal over lower crustal fractionation.
Foreshock sequence of the 1992 Landers, California, earthquake and its implications for earthquake nucleation
Authors: Dodge Douglas A. , Beroza Gregory C. , Ellsworth W. L.
Published: February 2004

The June 28, 1992, Landers, California, earthquake (Mw = 7.3) was preceded for about 7 hours by a foreshock sequence consisting of at least 28 events. In this study we examine the geometry and temporal development of the foreshocks using high‐precision locations based on cross correlation of waveforms recorded at nearby stations. By aligning waveforms, rather than trying to obtain travel time picks for each event independently, we are able to improve the timing accuracy greatly and to make very accurate travel time picks even for emergent arrivals. We perform a joint relocation using the improved travel times and reduce the relative location errors to less than 100 m horizontally and less than 200 m vertically. With the improved locations the geometry of the foreshock sequence becomes clear. The Landers foreshocks occurred at a right step of about 500 m in the mainshock fault plane. The nucleation zone as defined by the foreshock sequence is southeast trending to the south and nearly north trending to the north of the right step. This geometry is confirmed by the focal mechanisms of the foreshock sequence, which are right‐lateral and follow the trend as determined by the foreshock locations on the two straight segments of the fault, and are rotated clockwise for foreshocks that occur within the step. The extent of the foreshock sequence is approximately 1 km both vertically and horizontally. Modeling of the Coulomb stress changes due to all previous foreshocks indicates that the foreshocks probably did not trigger each other. This result is particularly clear for the Mw = 4.4 immediate foreshock. Since stress transfer in the sequence appears not to have played a significant role in its development, we infer an underlying aseismic nucleation process, probably aseismic creep. Other studies have shown that earthquake nucleation may be controlled by fault zone irregularities. This appears to be true in the case of the Landers earthquake, although the size of the irregularity is so small that it is not detectable by standard location techniques.
Earthquake clustering in the eastern Mediterranean during historical times
Authors: Pirazzoli P. A. , Laborel J. , Stiros S. C.
Published: February 2004

Most coastal sectors which show evidence of Holocene coseismic uplift in Greece and the Eastern Mediterranean were raised during a short period called here the Early Byzantine tectonic paroxysm (EBTP) between the middle of the fourth and the middle of the sixth century A.D. The areas uplifted at that time include Cephalonia and Zante in the Ionian Islands, Lechaion and the Perachora Peninsula in the Gulf of Corinth, the Pelion coast of Thessaly, Antikythira and the whole of western Crete, a coastal sector near Alanya in southern Turkey, and the entire Levant coast from Hatay (Turkey) to Syria and the Lebanon. The amount of the EBTP uplift was generally between 0.5 m and 1.0 m but reached a maximum of about 9 m in southwestern Crete. In several areas (Zante, Pelion coast, Antikythira, western Crete, Alanya), the EBTP uplifted shoreline is the only evidence of Holocene emergence. In other areas, however, a similar uplift occurred earlier in the Holocene (Levant coast), or more recently (Cephalonia). Evidence of preseismic subsidence prior to the EBTP uplift has been reported from Thessaly, Antikythira, and Crete; in both the latter islands, the EBTP uplift was preceded by a series of about 10 coseismic small subsidence movements, each measuring some tens of centimeters, which took place in the preceding 3000 years. No evidence was observed of postseismic vertical displacements.
Experimental deformation of fine‐grained anhydrite: Evidence for dislocation and diffusion creep
Authors: Dell'Angelo Lisa N. , Olgaard David L.
Published: February 2004

Deformation experiments on two fine‐grained anhydrite aggregates have revealed two high‐temperature flow regimes: (1) twinning and dislocation creep at high stresses (σ), and (2) diffusion creep accompanied by grain boundary sliding at low stresses. Each regime is characterized by a power law constitutive equation, diagnostic microstructures, and crystallographic preferred orientations (CPO). Experiments were performed at 400°–800°C, strain rates of 10−3–10−6 s−1, and confining pressures of 150–300 MPa on natural (d = 30 μm) and synthetic (d = 12 μm) anhydrites. The data for both anhydrites, corrected for grain size (d), are combined into a single composite flow law. The anhydrites show strain hardening for σ > 150 MPa and steady state flow for σ ≤ 150 MPa. Regime 1 is characterized by a flow law with a stress exponent (n) = 5. The deformation microstructures are indicative of twinning and dislocation creep: twins, undulatory extinction, grain flattening, serrate grain and twin boundaries, high dislocation densities, and newly recrystallized strain‐free grains. Recrystallization occurs by grain and twin boundary migration and subgrain rotation. The CPO reflects twinning at low strains and a combination of slip systems at high strains. In regime 2 the rheology is indicative of diffusion creep with n = 1. There is very little microstructural evidence of the bulk deformation. Grains are equant and have very low dislocation densities, and grain boundaries are smooth and gently curved. Split cylinder experiments reveal significant grain boundary relief attributed to grain boundary sliding. The CPO within this regime is random. The rheology and microstructures of regime 2 strongly suggest that diffusion creep accompanied by grain boundary sliding is the dominant deformation mechanism.
Shallow and peripheral volcanic sources of inflation revealed by modeling two‐color geodimeter and leveling data from Long Valley Caldera, California, 1988–1992
Authors: Langbein John , Dzurisin Daniel , Marshall Grant , Stein Ross , Rundle John
Published: February 2004

We refined the model for inflation of the Long Valley caldera near Mammoth Lakes, California, by combining both geodetic measurements of baseline length and elevation changes. Baseline length changes measured using a two‐color geodimeter with submillimeter precision revealed that the resurgent dome started to reinflate in late 1989. Measurements between late 1989 and mid‐1992 revealed nearly 13 cm of extension across the resurgent dome. Geodetic leveling surveys with approximately 2‐mm precision made in late 1988 and in mid‐1992 revealed a maximum of about 8 cm of uplift of the resurgent dome. Two ellipsoidal sources satisfy both the leveling and two‐color measurements, whereas spherical point sources could not. The model's primary inflation source is located 5.5 km beneath the resurgent dome with the two horizontal axes being nearly equal in size and the vertical axis being 4 times the length of the horizontal axes. A second ellipsoidal source was added to improve the fit to the two‐color measurements. This secondary source is located at a depth between 10 and 20 km beneath the south moat of the caldera and has the geometry of an elongated ellipsoid or pipe that dips down to the northeast. In addition, the leveling data suggest dike intrusion beneath Mammoth Mountain during the 1988–1992 interval, which is likely associated with an intense swarm of small earthquakes during the summer of 1989 at that location. Our analysis shows the dike intrusion to be the shallowest of the three sources with a depth range of 1–3 km below the surface to the top of the intrusion.
A tomographic study of the lithosphere beneath the Pyrenees from local and teleseismic data
Authors: Souriau Annie , Granet Michel
Published: February 2004

A tomographic image of the Pyrenean lithosphere from the surface down to 200 km is deduced from the propagation of the seismic waves recorded at the regional Pyrenean seismograph network. Crustal and subcrustal models are derived independently from the inversion of two different data sets. P and S velocity models for the crust are obtained from the propagation of local earthquakes. The upper crust appears very heterogeneous and exhibits sharp velocity contrasts. In the uppermost 7 km, an east‐west trending zone with velocities up to 6% higher than the surrounding material is observed all along the Pyrenees. This zone coincides in most places with the exposure of the Paleozoic rocks of the Axial Zone. Low velocities are observed on both sides of this high‐velocity zone. At the surface, they coincide with well‐known sedimentary basins but dip down to about 11 km depth toward the axis of the range. In the central and eastern Pyrenees, two large, high‐velocity bodies stand out in the North Pyrenean Zone and extend down to the base of the upper crust. They coincide with large positive gravity anomalies and are interpreted as lower crust or upper mantle blocks uplifted into the upper crust. The lower crust is poorly resolved but appears to be less heterogeneous than the upper crust. Moreover, the lower crustal structure is almost uncorrelated with the upper crustal structure. The major feature of the lower crust is a low‐velocity anomaly which stretches along the North Pyrenean Fault in the western Pyrenees and beneath the Axial Zone in the central Pyrenees; its location nearly coincides with the distribution of the seismicity. In the upper mantle, P models are derived from the travel times of the remote events recorded at the Pyrenean stations. The most important feature is the presence, in the eastern and central Pyrenees, of a vertical, low‐velocity heterogeneity south of the North Pyrenean Fault down to 80–100 km. This structure exhibits a 2% velocity contrast with respect to the surrounding structures and is interpreted as lower crustal material subducted during the Pyrenean convergence. This heterogeneity is not observed in the western Pyrenees, suggesting that the crustal shortening has been completely accommodated by several thrusts in the thinned Aquitaine crust.
Nonlinear teleseismic tomography at Long Valley Caldera, using three‐dimensional minimum travel time ray tracing
Authors: Weiland Charles M. , Steck Lee K. , Dawson Phillip B. , Korneev Valeri A.
Published: February 2004

We explore the impact of three‐dimensional minimum travel time ray tracing on nonlinear teleseismic inversion. This problem has particular significance when trying to image strongly contrasting low‐velocity bodies, such as magma chambers, because strongly refracted and/or diffracted rays may precede the direct P wave arrival traditionally used in straight‐ray seismic tomography. We use a simplex‐based ray tracer to compute the three‐dimensional, minimum travel time ray paths and employ an iterative inversion technique to cope with nonlinearity. Results from synthetic data show that our algorithm results in better model reconstructions compared with traditional straight‐ray inversions. We reexamine the teleseismic data collected at Long Valley caldera by the U.S. Geological Survey. The most prominent feature of our result is a 25–30% low‐velocity zone centered at 11.5 km depth beneath the northwestern quadrant of the caldera. Beneath this at a depth of 24.5 km is a more diffuse 15% low‐velocity zone. In general, the low velocities tend to deepen to the south and east. We interpret the shallow feature to be the residual Long Valley caldera magma chamber, while the deeper feature may represent basaltic magmas ponded in the midcrust. The deeper position of the prominent low‐velocity region in comparison to earlier tomographic images is a result of using three‐dimensional rays rather than straight rays in our ray tracing. The magnitude of our low‐velocity anomaly is a factor of ∼3 times larger than earlier models from linear arrival time inversions and is consistent with models based on observations of ray bending at sites within the caldera. Our results imply the presence of anywhere from 7 to 100% partial melt beneath the caldera.
Effective elastic thickness and crustal thickness variations in west central Africa inferred from gravity data
Authors: Djomani Y. H. Poudjom , Nnange J. M. , Diament M. , Ebinger C. J. , Fairhead J. D.
Published: February 2004

The west central African region is characterized by various geological features: Cretaceous rifts (Benue), Tertiary domal uplift (Adamawa volcanic uplift), Tertiary‐Recent volcanoes (Cameroon Volcanic Line or CVL), Tertiary sedimentary basins (Chad basins), and cratonic region (Congolese craton). In this study, we investigate the relationship between these tectonic features and the flexural rigidity of the lithosphere in Cameroon, in terms of effective elastic thickness (Te), by the use of the coherence function analysis. For that purpose, we use a new dataset of ∼32,000 gravity and topography points from Cameroon and the adjacent countries. The Te contour map deduced from this study shows a good relationship between the tectonic provinces and the rigidity of the lithosphere, the minima (14–20 km) are beneath the active rifted and volcanic areas (Benue, CVL, and Adamawa), and the maxima (∼40 km) correspond to the Archean reworked unit in Congo. A spectral analysis of the gravity data is performed to determine the crust‐mantle boundary in these tectonic provinces. The crustal thickness (Tc) contour map shows a variation from 14 km to about 45 km, consistent with other geophysical data. The lower values (14–20 km) are obtained in central Cameroon on the Adamawa uplift, and the highest values are found in southern Cameroon (Archean reworked Congolese craton). Comparing Te and Tc values shows that there is generally a positive correlation between the two parameters, with an exception in Chad where this correlation is rather negative.
Northeastern Pacific mantle conductivity profile from long‐period magnetotelluric sounding using Hawaii‐to‐California submarine cable data
Authors: Lizarralde Daniel , Chave Alan , Hirth Greg , Schultz Adam
Published: February 2004

We present results of a long‐period magnetotelluric (MT) investigation of the electrical structure beneath the eastern North Pacific. The electric field data consist of ∼2 years of continuously recorded voltages across an unpowered, ∼4000‐km‐long submarine telephone cable (HAW‐1) extending from Point Arena, California, to Oahu, Hawaii. The electric field measurements are coherent to some degree with magnetic field measurements from Honolulu Observatory at periods of 0.1 to 45 days. This coherence is enhanced at long periods over that observed with point electric field sensors due to horizontal averaging of the motional electric fields of spatial scale smaller than the cable length, significantly diminishing their effect. Robust, controlled leverage MT response estimates and their jacknife confidence limits are computed for the HAW‐1 to Honolulu data. An equivalent scalar MT response obtained from Honolulu magnetic variations data is used to correct the HAW‐1 MT response for static shift and to extend the MT response estimate to periods of 100 days. The composite response function satisfies necessary and sufficient conditions for consistency with a one‐dimensional conductivity structure and is most sensitive to structure between 150 and 1000 km. Inversion of the MT response reveals a conductive zone (0.05–0.1 S/m) between 150 and 400 km depth and a positive gradient below 500 km; these observations are consistent with previous MT studies in the North Pacific. This upper mantle conductivity is too high to be explained by solid‐state conduction in dry olivine using reasonable mantle geotherms. Calculations based on measurements of hydrogen solubility and diffusivity in olivine indicate that H+ dissolved in olivine, possibly combined with a lattice preferred orientation consistent with measured seismic anisotropy, provide sufficient conductivity enhancement to explain the inversion results. The high conductivity may also be explained by the presence of gravitationally stable partial melt. Comparison of the HAW‐1 results with long‐period MT studies conducted on land reveals differences in upper mantle conductivity between different tectonic regimes. In particular, the upper mantle beneath the Pacific Ocean is considerably more conductive than that beneath the Canadian shield and similar in conductivity to that beneath the Basin and Range.
Modeling of radon transport in unsaturated soil
Authors: Chen Chuan , Thomas Donald M. , Green Richard E.
Published: February 2004

This study applies a recently developed model, LEACHV, to simulate transport of radon through unsaturated soil and compares calculated soil radon activities against field‐measured values. For volatile and gas phase transport, LEACHV is modified from LEACHP, a pesticide version of LEACHM, a well‐documented one‐dimensional model for water and chemical movement through unsaturated soil. LEACHV adds consideration of air temperature changes and air flow driven by barometric pressure change to the other soil variables currently used in LEACHP. It applies diurnal barometric pressure and air temperature changes to reflect more accurately the typical field conditions. Sensitivity analysis and simulated results have clearly demonstrated the relative importance of barometric pressure change, rainfall events, changes in water content, gas advection, and radon source term in radon transport process. Comparisons among simulated results illustrated that the importance of barometric pressure change and its pumping phenomenon produces both fluctuation in soil gas radon activities and an elevation of the long‐term average radon activity in the shallow soil. Barometric pressure pumping was found to produce an effect on radon activity in shallow soils of an equal magnitude to the distributed source parameter. Comparison between measured and simulated soil radon activities showed that LEACHV can provide realistic estimates of radon activity concentration in the soil profile.
Segmentation and rift propagation at the Valu Fa Rridge, Lau Basin: Evidence from gravity data
Authors: Sinha Martin C.
Published: February 2004

Gravity and bathymetry data from a detailed ship‐based survey of a 40‐km length of the Valu Fa ridge allow an analysis of the fine‐scale segmentation of this young, back arc spreading center, where the primary morphological segmentation takes the form of small, overlapping spreading centers (OSC) separating ridge segments typically 30 km long. The gravity survey covers one complete segment and the overlap at its northern end and part of the overlap at its southern end. The mantle Bouguer anomaly shows a small (<5 mGal) negative anomaly associated with the ridge axis along much of its length. However, the northern OSC is characterized by a larger (>10 mGal), subcircular negative anomaly. The pattern of anomalies implies that the northern OSC is the site of particularly vigorous magmatism, with either increased crustal thickness or density reduction in the mantle (or both) accounting for the observed gravity low. This interpretation confirms the results of seismic analyses of this spreading center, which also imply that the OSC is the site of enhanced magmatism. The observations appear to contrast with widely accepted models of segmentation at mature, ocean basin spreading centers, where morphological offsets are believed to coincide with a relatively weak supply of magma. On a longer (130 km) gravity profile crossing the ridge, the mantle Bouguer anomaly shows a large (∼50 mGal), longer‐ wavelength (∼100 km) positive anomaly centered on the spreading axis. This can be modeled by an increase in crustal density and/or a decrease in crustal thickness over a 40‐ to 50‐km‐wide area centered on the axis. This is consistent with recent models for the evolution of the basin, involving recent southward propagation of seafloor spreading through preexisting, but stretched, thinned, and intruded island arc lithosphere.
Comparison of along‐track resolution of stacked Geosat, ERS 1, and TOPEX satellite altimeters
Authors: Yale Mara M. , Sandwell David T. , Smith Walter H. F.
Published: February 2004

Cross‐spectral analysis of repeat satellite altimeter profiles was performed to compare the along‐track resolution capabilities of Geosat, ERS 1 and TOPEX data. Geophysical Data Records were edited, differentiated, low‐pass‐filtered, and resampled at 5 Hz. All available data were then loaded into three‐dimensional files where repeat cycles were aligned along‐track (62 cycles of Geosat/Exact Repeat Mission; 16 cycles of ERS 1, 35‐day orbit; 73 cycles of TOPEX). The coherence versus wave number between pairs of repeat profiles was used to estimate along‐track resolution for individual cycles, eight‐cycle‐average profiles, and 31‐cycle‐average profiles (Geosat and TOPEX only). Coherence, which depends on signal to noise ratio, reflects factors such as seafloor gravity amplitude, regional seafloor depth, instrument noise, oceanographic noise, and the number of cycles available for stacking (averaging). Detailed resolution analyses are presented for two areas: the equatorial Atlantic, a region with high tectonic signal and low oceanographic noise; and the South Pacific, a region with low tectonic signal and high oceanographic variability. For all three altimeters, along‐track resolution is better in the equatorial Atlantic than in the South Pacific. Global maps of along‐track resolution show considerable geographic variation. On average globally, the along‐track resolution (0.5 coherence) of eight‐cycle stacks are approximately the same, 28, 29, and 30 km for TOPEX, Geosat, and ERS 1, respectively. TOPEX 31‐cycle stacks (22 km) resolve slightly shorter wavelengths than Geosat 31‐cycle stacks (24 km). The stacked data, which are publicly available, will be used in future global gravity grids, and for detailed studies of mid‐ocean ridge axes, fracture zones, sea mounts, and seafloor roughness.
Observations of ridge‐hotspot interactions in the Southern Ocean
Authors: Small Christopher
Published: February 2004

The evolution of ridge‐hotspot systems is not well understood. In this investigation, satellite‐derived marine gravity data are used in conjunction with underway bathymetric and magnetic anomaly profiles to investigate the nature of ridge‐hotspot interaction at four sparsely explored systems in the Southern Ocean. These systems illustrate three different stages of ridge‐hotspot interaction in which a migrating spreading center approaches a hotspot (Pacific‐Antarctic/Louisville), passes over or is captured by the hotspot (Mid‐Atlantic/Shona‐Discovery), and ultimately migrates away from the hotspot (Southeast Indian/Kerguelen). All of these systems show some evidence of discrete ridge jumps in the direction of the hotspot as the spreading center attempts to relocate toward the hotspot by asymmetric spreading. Interestingly, these ridge jumps show no evidence of propagating offsets as have been seen on many other ridge‐hotspot systems. A simple model predicts that typical plume excess temperatures can weaken the lithosphere sufficiently to promote asymmetric spreading and possibly allow a discrete ridge jump. The presence of previously uncharted, obliquely oriented aseismic ridges and gravity lineations between the ridge and the hotspot supports the notion of asthenospheric flux from the plume to the spreading center both before and after the time when the hotspot is ridge centered. The azimuths of the aseismic ridges cannot be explained by plate kinematics alone; they consistently extend from the ends toward the centers of the adjacent spreading segments suggesting some interaction between plume derived asthenospheric flux and local lithospheric structure. The features discussed here also indicate that the transfer of asthenospheric material from the plume to the spreading center is influenced by the local plate boundary configuration and interaction with transform offsets.
Geochemistry and geochronology of ancient southeast Indian and southwest Pacific seafloor
Authors: Pyle D. G. , Christie D. M. , Mahoney J. J. , Duncan R. A.
Published: February 2004

Within the Australian‐Antarctic Discordance (AAD), a boundary exists between isotopically defined “Pacific‐type” and “Indian‐type” mid‐ocean ridge basalt (MORB) erupted along the Southeast Indian Ridge (SEIR). This boundary has migrated westward beneath the easternmost AAD spreading segment at a minimum rate of 25 mm/yr since 4 Ma; however, its long‐term history remains a matter of speculation. To determine if Pacific‐type upper mantle has migrated westward beneath the eastern Indian Ocean basin as Australia and Antarctica drifted apart during the last 70 m.y., we present new Sr‐Nd‐Pb isotope data, combined with trace element and 40Ar‐39Ar radiometric age determinations, for samples from Legs 28 and 29 of the Deep Sea Drilling Project (DSDP). Basaltic basement at these DSDP sites provides a record of their upper mantle source composition and shows regional variations consistent with upper mantle flow in this region. East of the South Tasman Rise, all DSDP basalts have 87Sr/86Sr (0.7025–0.7029) and 206Pb/204Pb (18.80–19.48) ratios typical of Pacific‐type MORB indicating that Pacific‐type upper mantle existed east of the Australian‐Antarctic continental margin and beneath the Tasman Sea during the early stages of seafloor spreading in this region. Basalts from DSDP sites west of the AAD have high 87Sr/86Sr (0.7030–0.7035), low 206Pb/204Pb (17.99–18.10) and trace element characteristics typical of present day Indian‐type SEIR MORB. Between these two regions, DSDP basalts recovered along the western margin of the South Tasman Rise have isotopic characteristics that are, in one case consistent with an Indian‐type MORB source (Site 280A) and, in the second case, transitional between Pacific‐type and Indian‐type mantle sources. The occurrence of seafloor basalts with transitional or Indian‐type isotopic characteristics well to the east of the present Indian‐Pacific MORB isotopic boundary within the AAD strongly implies that Pacific‐type upper mantle has migrated westward into the region since the South Tasman Rise separated from Antarctica circa 40 Ma.
Structure of the mobile south flank of Kilauea Volcano, Hawaii
Authors: Denlinger Roger P. , Okubo Paul
Published: February 2004

How do huge landslides occur in Hawaii? We can begin to address this question by examining an incipient landslide structure on the mobile south flank of Kilauea volcano. Between 1970 and 1989 this part of the south flank moved in a southeast direction (seaward by 4 m on its eastern end and by 10 m on its western end), parallel to the western boundary of a large oifshore topographic bench. We show that abrupt decreases in cumulative flank movement correspond to abrupt decreases in hypocenter concentration. Most of the earthquakes occur in a 7‐ to 10‐km‐deep band beneath the south flank, and relative relocation of a subset of these events shows that they are derived from a single horizontal plane which appears to be a decollement. Focal mechanisms for this deep seismicity are dominated by thrust fault mechanisms, with the overlying block moving seaward. A comparison of south flank bathymetry with decollement structures on other volcanoes suggests that the distal end of this decollement coincides with the distal end of the offshore topographic bench (30–50 km from shore and 5 km below sea level). The fault system forming the Hilina pali is connected with this decollement, so that slip on the Hilina pali's normal faults coincides with slip of the decollement and with reverse faulting of the distal end of the bench. (This interpretation is supported by studies of the events associated with the M7.2 Kalapana earthquake in 1975.) When the Hilina pali is connected to the outer bench with a decollement inferred from seismicity, a wedge 8‐km thick adjacent to Kilauea's magma system and 1–2 km thick along the edge of the outer bench (an area over 3000 km2) is defined. The thickness adjacent to the magma system decreases eastward along Kilauea's East Rift Zone and southwestward from Kilauea caldera, whereas the thickness of the lip of the outer bench decreases only eastward. Geologic observations and geophysical studies of the subaerial flank reveal an extensive magma system beneath the Koae fault system as well as beneath Kilauea caldera and its East Rift Zone. We infer that the boundary of the proximal end of the mobile flank is the southern boundary of this magma system and that this boundary coincides with the southern boundaries of both the Koae fault system and Kilauea's East Rift Zone.
Paleomagnetism, polar wandering, and the rejuvenation of crustal mobility
Authors: Opdyke Neil D.
Published: February 2004

The decade from 1951 to 1961 witnessed the birth of a new geophysical subdicipline, paleomagnetism. Early studies in Europe, North America, and Australia led to the following conclusions: (1) rocks could preserve directions of magnetiziation for hundreds of millions of years in red beds, (2) late Cenozoic lavas had directions of magnetiziation that led to the conclusion that the mean geomagnetic field was a geocentric dipole aligned along the axis of rotation, (3) rocks of Triassic age and older yield directions which depart widely from the present axis of rotation, (4) if these directions are used to calculate pole positions, then poles for older and older rocks fall farther and farther from the present pole of rotation, (5) these data may be used to construct polar wander curves, (6) polar wander curves from different continents do not coincide with one another, (7) they may be reconciled if the continents move with respect to each other, and (8) the distribution of climatic indicators show that the pole of rotation of Earth and the paleomagnetic pole for the same periods coincide for Phanerozoic time. These observations changed the perspectives of many Earth scientists and paved the way for seafloor spreading and plate tectonics.
A 300‐ to 550‐year history of slip on the Imperial fault near the U.S.‐Mexico border: Missing slip at the Imperial fault bottleneck
Authors: Thomas Andrew P. , Rockwell Thomas K.
Published: February 2004

Subsurface investigation of the Imperial fault zone approximately 50–60 m south of the U.S.‐Mexico International Border in Mexicali Valley reveals that only the 1940 Imperial Valley earthquake (ML7.1) has produced significant surface slip in the past 300 years. The 1940 earthquake produced about 5 m of slip in this vicinity, as indicated by an offset tree line. We excavated a buried channel adjacent to these trees and determined that it is also offset 5 m. These data indicate that at least the upper meter of sediments has only been displaced by the 1940 event. One other slip event of undetermined magnitude is recorded within the lacustrine highstand deposits of the last lake about 300 years ago. There is no evidence of earlier faulting events in sediments that possibly may correlate to the penultimate lake approximately 550 years B.P. Below an unconformity, ∼4000‐year‐old deltaic deposits are intensely faulted by previous slip along the Imperial fault. These data suggest that the slip rate on the Imperial fault for the past 300–550 years is only about 15–20 mm/yr, substantially less than that expected from geodetic measurements and regional structural models. This in turn suggests that either slip is accommodated by other faults in the border region or that strain release on the Imperial fault is characterized by episodic (clustered) behavior.
Diamond anvil cell study of the transformation mechanism from the olivine to spinel phase in Co2SiO4, Ni2SiO4, and Mg2GeO4
Authors: Burnley Pamela C. , Bassett William A. , Wu Tzy‐chung
Published: February 2004

Transformation textures and kinetics of the olivine to spinel phase transformation were observed in situ in ungasketed samples using a diamond anvil cell (DAC). The low‐temperature kinetic limit for reconstructive transformation from olivine (α) to spinel phase (γ) observed in DAC experiments is approximately 150°C lower than when observed under more hydrostatic conditions. The spinel phase, which in many of the samples is distributed in an annular pattern, exhibits reconstructive textures including grain boundary nucleation, and lack of topotaxy; in some cases it forms lenses similar to those associated with transformation‐induced mechanical failure. Although spinel phase lamellae, formed by a martensiticlike mechanism, are observed in the specimens, the lamellae remain extremely thin (∼10 nm) and do not produce enough spinel to be optically visible. The observation of reconstructive textures within the annular transformed regions leads us to conclude that high shear stress and plastic strain enables reconstructive transformation at temperatures where transformation rates would otherwise be virtually zero. High transient differential stresses and rapid deformation accompany deep earthquakes. Therefore knowledge of the kinetics of this transformation under these conditions is important for understanding the connection between phase transformation and deep earthquakes in subducting lithospheric slabs.