bluuebunny/crossref_metadata_2025 huggingface dataset:
Rupture characteristics of clustered microearthquakes and variations in fault properties in the New Madrid Seismic Zone
Authors: Xie Jiakang
Published: February 2004

Seismic waveforms from the Portable Array for Numerical Data Acquisition (PANDA) network were used to relocate 45 events, occurring in four spatial clusters in the northern New Madrid Seismic Zone (NMSZ). Source time functions of the larger (ML≥2.5) events were obtained and yield estimates of rupture radii between 130 and 175 m and static stress drops at, or below, the megapascal level. There is a tendency for closely located events, within ∼200 m from one another, to occur within short time intervals of minutes to days. Available focal mechanism solutions reveal that some of the clustered events have diverse slip directions, indicating that the ruptures occur on adjacent fault patches, rather than being repetitive over the same patch. On the basis of this and a previous study of events farther south, we infer that the properties of the faulting system in the NMSZ are laterally heterogeneous and are subject to effects of some temporally and spatially localized processes that drive the faults toward seismic failure. Comparison of this observation with that in the San Andreas Fault near Parkfield reveals that event clustering occurs in both fault systems on similarly short spatial and temporal scales. The inferred localized processes may occur in both fault systems, despite their different tectonic environment, far‐field loading rate, and geometric style of deformation. On the other hand, in the NMSZ we have not observed any quasiperiodic and repetitive ruptures that have been observed near Parkfield. This is consistent with a slower or absent aseismic creeping in the NMSZ.
A pressure‐independent geothermometer for primitive mantle melts
Authors: Gudfinnsson Gudmundur H. , Presnall Dean C.
Published: February 2004

Phase equilibrium data from the CaO‐MgO‐Al2O3‐SiO2, CaO‐MgO‐Al2O3‐SiO2‐Na2O, and CaO‐MgO‐Al2O3‐SiO2‐FeO systems on the melting behavior of plagioclase, spinel, and garnet lherzolite, are used to determine the molar partition coefficient of MgO between olivine and melt, DMgO*ol/liq, as a function of temperature, pressure, and composition. The data sets cover the pressure range 0.1 MPa to 5 GPa and temperatures of 1225°–1830°C. It is shown that the trend of DMgO*ol/liq is little affected by the transitions between different lherzolite assemblages and that one empirical equation, which has reciprocal temperature (in Kelvin) and mole fraction of NaO0.5 in the melt as independent variables, describes the variation of DMgO*ol/liq in pressure, temperature, and composition space and has the form lnDMgO*ol/liq = (4723/T) + 2.566CNaO05*liq − 1.729. This expression is tested by using it to calculate the temperatures of melting experiments containing olivine‐saturated basalt and picrite melts and is shown to yield good results for multiply saturated melts, consistent with the fact that the observed simple trend of DMgO*ol/liq is dictated by low‐variance phase relations. With fewer phases present, the equation systematically predicts temperatures that are too high. If the value of KDFe2+/Mgol/liq (KD) is known, the value of DMgO*ol/liq can be calculated from DMgO*ol/liq = 0.6667/(CFeO*liqKD + CMgO*liq), and hence the mole fractions of MgO, FeO, and NaO0.5 can be used to calculate the equilibrium temperature of multiply saturated olivine‐bearing melts. This geothermometer is mainly applicable to primitive tholeiitic and mildly alkalic basalt and picrite melts, with small amounts of volatiles only, in the 1150°–1800°C temperature range.
A broadband seismic study of the lowermost mantle beneath Mexico: Constraints on ultralow velocity zone elasticity and density
Authors: Havens Emily , Revenaugh Justin
Published: February 2004

Broadband seismic stacking is performed for PcP waves sampling the lowermost mantle and core‐mantle boundary beneath central Mexico and the western Gulf of Mexico. The data contain evidence of a 10 to 20‐km‐thick ultralow velocity zone on the western edge of the study area, tapering (or vanishing) to the east to less than 5‐km thickness over a horizontal distance of ∼200 km. Epicentral distance varies from 20° to 60°, a range over which the amplitude of reflections from the top of the ultralow velocity zone responds strongly to changes in the ratio of shear to compressional velocity reduction (τ=δlnυs/δlnυp) and to density within the zone. Where the zone is present and thick enough to separate PcP and the precursory reflection, our modeling is consistent with a value of τ near 3 and δln υp near 10%, assuming only a minor density increase (1 to 2%). The value of τ is reduced to 2 for a ∼14% density increase, but δln υp remains large (7%). Very low amplitude multiply converted phases are synthetically well reproduced with extreme attenuation within the zone, suggesting the presence of a melt component.
Time domain analysis of Earth's long‐period background seismic radiation
Authors: Ekström Göran
Published: February 2004

A time domain detector of long‐period (200–400 s) surface‐wave energy is developed and used to investigate Earth's background seismic radiation. Several recent studies have presented convincing evidence for an incessant low‐level excitation of Earth's normal modes in the frequency band 2–7 mHz. The detection of these oscillations has been accomplished through analysis of low‐frequency amplitude spectra, which provides good resolution of the excited frequencies but poor resolution of temporal variations and signal amplitude. The new time‐domain method is based on the autocorrelation function. A seismogram is correlated with a copy of itself to which a reverse great‐circle dispersion operator has been applied. The resulting correlogram is labeled the “autogram”. If multiple‐orbit surface waves are present in the seismogram, their energy will be phase equalized and concentrated at zero lag time in the autogram, allowing for identification of coherent radiation. Analysis of vertical‐component seismograms recorded at quiet stations leads to nearly continuous detections of coherent seismic energy using correlation time windows of only a few hours duration. Stacking of autograms from several stations enhances the detections. Five years of vertical long‐period data from the Global Seismographic Network are analyzed. All detections with an amplitude equivalent to that of an earthquake of Mw≥6.0 can be correlated with known earthquakes; no evidence is found for the existence of anomalously slow or “silent” earthquakes in the period band 200–400 s. The background excitation level is remarkably constant, corresponding approximately to the maximum amplitude detection generated by a Mw = 5.75 earthquake. Coherent Rayleigh wave energy is detected 94% of the time analyzed. A seasonal component with a period of 6 months is well resolved, with maxima in mid‐January and mid‐July, consistent with the hypothesis that atmospheric processes are the main cause of the phenomenon. Variations in the background signal level on timescales of several days to weeks are also documented.
Diurnal angular momentum budget of the atmosphere and its consequences for Earth's nutation
Authors: De Viron Olivier , Marcus Steve L. , Dickey Jean O.
Published: February 2004

The three‐dimensional diurnal atmospheric angular momentum (AAM) budget is investigated, and its geographical features and meteorological dynamics are explored. It is shown that the diurnal atmospheric torques are associated with mass effect (ellipsoidal torque for the equatorial component and mountain torque for the axial component). It is shown that there is an important difference between the torque and angular momentum approaches on diurnal timescales, with the AAM time derivative generally exceeding the calculated torques. For the retrograde diurnal band, however, which is of interest for Earth nutation, the torque approach predicts much larger AAM changes than those observed. The AAM budget equation is also investigated theoretically, in order to give condition on the torques and AAM in order to verify the equation. Alternatively to the condition proposed by Bell, [1994], a condition on the local torque is shown sufficient to verify the AAM budget equation in the retrograde band, that is, the local torque have to balance the ellipsoidal effect. This condition is shown to be verified qualitatively. However, when separating the AAM budget equation into prograde and retrograde part, this last part become very sensitive to the error on the computation of the local torque. This explains why the torque approach always give one order of magnitude larger than the AAM approach at the retrograde diurnal frequencies.
Microearthquake seismicity and focal mechanisms at the Rodriguez Triple Junction in the Indian Ocean using ocean bottom seismometers
Authors: Katsumata Kei , Sato Toshinori , Kasahara Junzo , Hirata Naoshi , Hino Ryota , Takahashi Narumi , Sekine Mayumi , Miura Seiichi , Koresawa Sadayuki , Wada Naoto
Published: February 2004

Hypocenters and focal mechanisms of microearthquakes have been investigated at the Rodriguez Triple Junction in the Indian Ocean. Little was known on microearthquake activity in this region. We deployed 18 ocean bottom seismographs during the KH93–3 cruise of the R/V Hakuho‐Maru (Ocean Research Institute, University of Tokyo) from July 30 to August 20, 1993. We obtained 579 well‐constrained hypocenters and 13 focal mechanisms. Microearthquakes were found to be active along all of the three ridges: the Central Indian Ridge, the Southeastern Indian Ridge, and the Southwestern Indian Ridge. Especially at the triple junction there was an earthquake swarm within narrow area of approximately 15×5 km2. All of the 13 focal mechanisms showed normal or strike‐slip faultings, which means that the extensional stress field characterizes this region.
The enigma of the Arthur's Pass, New Zealand, earthquake: 1. Reconciling a variety of data for an unusual earthquake sequence
Authors: Abercrombie Rachel E. , Webb T. H. , Robinson R. , McGinty P. J. , Mori J. J. , Beavan R. J.
Published: February 2004

The 1994 Arthur's Pass earthquake (MW6.7) is the largest in a recent sequence of earthquakes in the central South Island, New Zealand. No surface rupture was observed, the aftershock distribution was complex, and routine methods of obtaining the faulting orientation of this earthquake proved contradictory. We use a range of data and techniques to obtain our preferred solution, which has a centroid depth of 5 km, M0=1.3 × 1019 N m, and a strike, dip, and rake of 221°, 47°, 112°, respectively. Discrepancies between this solution and the Harvard centroid moment tensor, together with the Global Positioning System (GPS) observations and unusual aftershock distribution, suggest that the rupture may not have occurred on a planar fault. A second, strike slip, subevent on a more northerly striking plane is suggested by these data but neither the body wave modeling nor regional broadband recordings show any complexity or late subevents. We relocate the aftershocks using both one‐dimensional and three‐dimensional velocity inversions. The depth range of the aftershocks (1–10 km) agrees well with the preferred mainshock centroid depth. The aftershocks near the hypocenter suggest a structure dipping toward the NW, which we interpret to be the mainshock fault plane. This structure and the Harper fault, ∼15 km to the south, appear to have acted as boundaries to the extensive aftershock zone trending NNW‐SSE. Most of the ML ≥ 5 aftershocks, including the two largest (ML6.1 and ML5.7), clustered near the Harper fault and have strike slip mechanisms consistent with motion on this fault and its conjugates. Forward modeling of the GPS data suggests that a reverse slip mainshock, combined with strike slip aftershock faulting in the south, is able to match the observed displacements. The occurrence of this earthquake sequence implies that the level of seismic hazard in the central South Island is greater than previous estimates.
The enigma of the Arthur's Pass, New Zealand, earthquake: 2. The aftershock distribution and its relation to regional and induced stress fields
Authors: Robinson Russell , McGinty Peter J.
Published: February 2004

The aftershock distribution of the 1994 Arthur's Pass earthquake, MW6.7, is unusual for a reverse faulting event in that it extends 12 km NNW and 30 km SSE of the actual fault plane, which strikes NE‐SW. We have used several methods to infer the regional stress field in the region, including geodetic results, earthquake mechanisms, and inversion of P wave polarity data for the stress tensor orientation. The inversion method is new and does not require the focal mechanisms of the events used. It also incorporates the Coulomb failure criterion. All results point to a stress field favoring strike‐slip faulting, not thrusting, with near‐horizontal σ1 and σ3 principal axes striking at 298° and 28°. Using dislocation theory, we calculate the stress induced by the Arthur's Pass earthquake and its largest aftershock (a strike‐slip event) and add this to the regional field. There is a fair correspondence between the hypocenters of aftershocks away from the mainshock fault plane and regions of high induced Coulomb Failure Stress (CFS) on optimally oriented fault planes. However, there are regions of high induced CFS that are devoid of aftershocks. It appears that earthquake slip in this region of oblique (19°) plate convergence is, as observed elsewhere, partitioned into components parallel and perpendicular to the plate margin. Most of the slip is parallel, as occurs on the nearby dextral Alpine fault, the boundary between the Pacific and Australian plates. However, occasional reverse events, such as the Arthur's Pass earthquake, account for at least some of the perpendicular component of slip and the uplift that produced the Southern Alps.
Thermal conductivity of unconsolidated sediments with geophysical applications
Authors: Revil A.
Published: February 2004

A theoretical model for the prediction of the thermal conductivity of unconsolidated granular sediments, with application to sand shale mixtures, is presented. The sediment is modeled as an assemblage of grains of thermal conductivity λS immersed in a pore fluid of thermal conductivity λƒ. In order to take into account for the thermal interactions between the grains a differential effective medium approach is used to provide a relationship between the effective thermal conductivity of the isotropic mixtures of grains saturated by the pore fluid, the porosity, and the thermal conductivities of the grains and pore fluid. The influence of the topology of the interconnected pore space is accounted for through the use of the electrical cementation exponent m, which is related to the electrical formation factor F by F = ϕ−m, where ϕ is the interconnected porosity. The main assumption of the model lies in the small contiguity between the grains. This assumption holds well for unconsolidated sand shale mixture sediments as demonstrated by comparing the model to available experimental data. The model offers the possibility to derive thermal conductivity profiles from downholes measurements of natural radioactivity, electrical resistivity, and bulk density.
Hypocenter migration as an explanation for temporal changes in coda Q
Authors: Chen Xiuqi , Long L. T.
Published: February 2004

In the 1993 earthquake swarm near the Norris Lake Community, Georgia, Qc (coda Q) and b value exhibit systematic temporal variations on a scale of months and hours. The long‐term monthly variations correlate with the swarm's growth and decay. Examination of the hypocenters demonstrates that Qc is a strong function of hypocenter and the temporal variations of Qc reflect a systematic migration of hypocenters. The b value appears to vary in this swarm because the method of magnitude determination is dependent on Qc. Throughout the duration of the swarm, the active areas migrated from a high‐gc area eastward to a low‐Qc area. Variations of Qc over periods of hours were explained by a shifting of hypocenters between high‐ and low‐Qc areas. Significant changes in Qc occur for hypocenters that are separated by distances <1 km even though the width of the volume sampled by Qc is >10 km. The nonuniform (fractal) distribution of relative scattering coefficients plays a major role in determining the spatial dependence of Qc. By using forward coda modeling on the distribution of relative scattering coefficients derived for this earthquake swarm, we can reproduce seismic coda with the same spatial dependence on Qc as found in the earthquakes throughout the swarm. The relative locations of earthquakes associated with different Qc values and our ability to model different Qc values from a known distribution of relative scattering coefficients confirm that the migration of hypocenters, and not a temporal change in attenuation properties, determines the temporal variation of Qc observed during the Norris Lake Community earthquake swarm.
Strain accumulation across the Eastern California Shear Zone at latitude 36°30′N
Authors: Gan Weijun , Svarc J. L. , Savage J. C. , Prescott W. H.
Published: February 2004

The motion of a linear array of monuments extending across the Eastern California Shear Zone (ECSZ) has been measured from 1994 to 1999 with the Global Positioning System. The linear array is oriented N54°E, perpendicular to the tangent to the local small circle drawn about the Pacific‐North America pole of rotation, and the observed motion across the ECSZ is approximated by differential rotation about that pole. The observations suggest uniform deformation within the ECSZ (strike N23°W) (26 nstrain yr−1 extension normal to the zone and 39 nstrain yr−1 simple right‐lateral shear across it) with no significant deformation in the two blocks (the Sierra Nevada mountains and southern Nevada) on either side. The deformation may be imposed by right‐lateral slip at depth on the individual major fault systems within the zone if the slip rates are: Death Valley‐Furnace Creek fault 3.2±0.9 mm yr−1, Hunter Mountain‐Panamint Valley fault 3.3±1.6 mm yr−1, and Owens Valley fault 6.9±1.6 mm yr−1. However, this estimate of the slip rate on the Owens Valley fault is 3 times greater than the geologic estimate.
Rapid deformation of Kilauea Volcano: Global Positioning System measurements between 1990 and 1996
Authors: Owen Susan , Segall Paul , Lisowski Michael , Miklius Asta , Denlinger Roger , Sako Maurice
Published: February 2004

Campaign Global Positioning System (GPS) measurements from 1990 to 1996 are used to calculate surface displacement rates on Kilauea Volcano, Hawaii. The GPS data show that the south flank of the volcano, which has generated several large earthquakes in the past 3 decades, is displacing at up to ∼8 cm/yr to the south‐southeast. The summit and rift zones are subsiding, with maximum subsidence rates of ∼8 cm/yr observed a few kilometers south of the summit caldera. Elastic dislocation modeling of the GPS data suggests that the active sources of deformation include deep rift opening along the upper east and east rift zone, fault slip along a subhorizontal fault near the base of the volcano, and deflation near the summit caldera. A nonlinear optimization algorithm was used to explore the parameter space and to find the best fitting source geometry. There is a broad range of model geometries that fit the data reasonably well. However, certain models can be ruled out, including those that have shallow rift opening or shallow fault slip. Some offshore, aseismic slip on a fault plane that dips between 25° northnorthwest and 8° south‐southeast is required. Best fitting slip and rift opening rates are 23–28 cm/yr, although rates as low as 10 cm/yr are permitted by the data.
Using thermochronometry and low‐temperature demagnetization to accurately date Precambrian paleomagnetic poles
Authors: Warnock Andrew C. , Kodama Kenneth P. , Zeitler Peter K.
Published: February 2004

Dating magnetizations in Precambrian rocks is increasingly important in the attempt to unravel Precambrian plate configurations and supercontinent assemblages. We used low‐temperature demagnetization and modern thermochronometric methods to dissect a multicomponent magnetization of the Glamorgan Gabbro, Ontario, previously studied by Buchan and Dunlop [1976] and Berger and York [1981]. We found that the HbA component is a primary thermoremanent magnetization carried by single‐domain magnetite. The new paleomagnetic pole position (32.6°S latitude, 141.9°E longitude) is not significantly different from the published pole; however, the cooling history suggests that the 1015±15 Ma magnetization age is older than previously thought (980 Ma). The new age produces a better fit in Rodinia supercontinent reconstructions. The refined HbB pole (29.9°N latitude, 169.9°E longitude) is carried by multidomain‐type magnetite and pyrrhotite. A possible ∼175°C thermal event at ∼600 Ma recorded by K‐feldspar could be responsible for remagnetizing the multidomain grains. The new age for Hbß is 220 m.y. younger than the previous estimate, raising questions concerning the ages of similar poles from the Canadian Grenville Province.
Numerical modeling of reaction‐induced cavities in a porous rock
Authors: Ormond Anne , Ortoleva Peter
Published: February 2004

The interaction between mineral reaction and mass transport in a rock can lead to reaction front instability and the development of channel‐like voids. This phenomenon is studied with a two‐dimensional model accounting for the nonlinear feedback between flow, reaction, and matrix porosity‐permeability evolution. In our model we calculate the flow field in both the porous medium and the reaction‐induced voids, using the Brinkman equation. While a linear analysis cannot determine the length scale of the channels which can develop in a typical geological system, our simulations indicate that the channel size is actually unique and well characterized. While the onset of instability is favored at a preexisting heterogeneity, the channel growth and orientation is governed by the global flow pattern, even in an initially heterogeneous system.
No regional anisotropic domains in the northeastern U.S. Appalachians
Authors: Levin Vadim , Menke William , Park Jeffrey
Published: February 2004

A fabric “frozen” within the continental lithosphere should lead to “regional anisotropic domains” with close correlation between tectonics, seismic anisotropy, and shear wave travel time delays. Alternatively, seismic anisotropy could be maintained by active deformation in the sub‐lithospheric mantle. This scenario would predict broad spatial consistency in shear wave splitting and weak correlation between the splitting, surface tectonics, and travel time delays. A coherent pattern of shear wave splitting is observed in the northeastern United States, covering large parts of New York and New England, spanning the Appalachian Orogen and areas underlain by the Grenvillian basement. The direction of fast shear wave propagation, as measured from core‐refracted SKS, SKKS, and PKS phases, varies systematically with event back azimuth at all sites examined, and is fit well by two‐layers of anisotropy with subhorizontal symmetry axes. Relative travel time delays of shear waves that sample the region vary over 100 km length scale, and correlate with surface geology. Tomographic images suggest lateral shear velocity variations ∼3%, including a slow anomaly beneath the Grenvillian basement exposed in the Adirondack Mountains. There is little correlation between this horizontally rough seismic velocity and the horizontally smooth anisotropic model consistent with shear wave splitting. We therefore conclude that in the northeastern US the concept of “regional anisotropic domains” (i.e. distinct regions within the lithosphere characterized by coherent anisotropic properties reflective of present or past deformation) does not apply.
High‐temperature viscoelastic relaxation in iron and its implications for the shear modulus and attenuation of the Earth's inner core
Authors: Jackson Ian , Gerald John D. Fitz , Kokkonen Harri
Published: February 2004

The viscoelasticity of mildly impure polycrystalline iron has been studied over the temperature range 20–1300°C through a combination of seismic‐frequency torsional forced oscillation and microcreep tests. For all temperatures above ∼400°C, linear absorption band behavior is observed, both strain energy dissipationQ−1and shear modulus dispersion providing evidence of substantial departures from ideal elastic behavior. For 3 s oscillation period the temperature sensitivity of the shear modulus |∂G/∂Taverages 0.04 GPa K−1. An even larger derivative applies to the highest temperatures within the bcc field (600–800°C) and at longer periods. The isothermal variation ofQ−1with periodTois generally adequately described byQ−1∼Toα. Within the bcc field the exponent α, and hence the distributionD(τ) ∼ τα−1of relaxation times, are temperature‐independent, allowing the parameterizationQ−1∼ [Toexp(−E/RT)]α, with α = 0.20±0.02 and activation energyE= 280±30 kJ mol−1. Within the fcc field, the exponent α increases systematically with increasing temperature from ∼0.1 to ∼0.3 across a wide temperature interval, indicating that the distribution of relaxation times within the absorption band is strongly temperature‐dependent. Steady‐state viscosities inferred mainly from microcreep records for temperatures between 1000 and 1300°C, lie within the range (0.2–2)×1013Pa s. The observed mix of recoverable anelastic and viscous behavior (the latter becoming progressively more important with increasing temperature and time/period) is tentatively attributed to diffusional processes operative at grain boundaries in the relatively fine‐grained bcc‐Fe and to processes involving dislocations in the coarser‐grained specimens of the fcc phase. Relaxation of internal stresses caused by pronounced single‐crystal elastic anisotropy probably dominates the anelastic response of both phases. The marked elastic anisotropy of fcc‐Fe makes it an attractive alternative to the hcp structure widely favored for the dominant crystalline phase of the Earth's inner core. At seismic frequencies and homologous temperatures approaching unity in the inner core, solid‐state viscoelastic relaxation probably accounts for much of the observed seismic wave attenuation and contributes through the associated dispersion to the unusually high value of Poisson's ratio.
Comparisons of water and argon permeability in natural clay‐bearing fault gouge under high pressure at 20°C
Authors: Faulkner D. R. , Rutter E. H.
Published: February 2004

Quantification of fluid transport through fault zones is critical for the understanding of fault mechanics and prediction of subsurface fluid flow. The permeability of clay‐bearing fault gouge has been determined using first argon then water as pore fluids under total confining pressures ranging up to 200 MPa and pore pressures of 40 MPa at room temperature. Use of the two pore fluids allows interactions between the gouge and pore fluids to be examined. Natural clay‐bearing fault gouge recovered from surface exposures of the Carboneras fault zone in southeastern Spain was used and was collected in such a way that the in situ microstructure was preserved. Cores were collected in directions relative to the well‐developed planar fabrics seen in these types of fault rock. The mineralogy of the gouges included muscovite/illite, chlorite, and quartz, with minor amounts of gypsum, albite, and graphite. Glycolation of the gouge showed no discernible amounts of swelling phases. Grain size analyses revealed a bimodal grain size distribution, with the <2 μm fraction dominant (>50 wt%). This fraction contained predominantly clay phases. Permeabilities in the range of 10−17 m2 to 10−22 m2 were measured. Experimental results show that the previous highest in situ effective pressure to which the fault gouge had been subjected (overconsolidation pressure) could not be determined from changes in permeability. Differences between water and argon permeabilities determined on the same sample amounted to ∼1 order of magnitude, even if the sample had been pressure cycled (reduction to zero and reapplication of both confining and pore pressure) using argon as pore fluid until asymptotic values for permeability had been attained. Volumetric strain measurements showed no enhanced compaction due to the introduction of water as the pore fluid, leading to the conclusion that the reduction in permeability must be due to physicochemical interactions of the water with the fault gouge. The low permeabilities measured support models invoking high fluid pressure weakening of large faults with minimal fluid loss. The stability of structured water films with varying temperature, water pressure and water chemistry may produce a heterogeneous permeability profile with depth in fault zones.
Diffuse hydrothermal fluids from Lucky Strike hydrothermal vent field: Evidence for a shallow conductively heated system
Authors: Cooper M. J. , Elderfield H. , Schultz A.
Published: February 2004

Chemical analysis of 18 diffuse hydrothermal fluids collected from the Lucky Strike hydrothermal vent site reveals the presence of less than 7% high‐temperature hydrothermal end‐member. Temperature‐Li and temperature‐Mg relationships for the diffuse fluids indicated two sample groups. One group of three samples had temperatures which fell on or below the temperatures expected for conservative mixing between seawater and high‐temperature vent fluids, consistent with mixing and cooling. The second grouping had diffuse fluid temperatures greater than expected from conservative mixing of high‐temperature end‐member fluids and seawater. This indicates that the diffuse hydrothermal effluent was heated conductively during its formation. The Lucky Strike site is characterized by a hydrothermal slab of basaltic fragments indurated by silica and barite. It is proposed that the diffuse fluids circulate within the cracks of the slab, forming a shallow short‐pass hydrothermal convection system. The source of heat is likely to be high‐temperature hydrothermal fluids circulating beneath the hydrothermal slab. The chemistry of the diffuse fluids shows no evidence for silica precipitation, suggesting that the slab is not actively forming from the diffuse fluids at the present.
Dynamics of intracontinental extension in the north Baikal rift from two‐dimensional numerical deformation modeling
Authors: Lesne Olivia , Calais Eric , Deverchère Jacques , Chéry J. , Hassani R.
Published: February 2004

We model the recent deformation along a lithospheric‐scale cross section perpendicular to the northern part of the Baikal rift zone (BRZ) with two‐dimensional finite element models. Using realistic lithospheric structure and rheological properties and imposing extension as a far‐field boundary condition, we find a model that matches reasonably well the topography, observed deformation pattern, gravity anomalies, and age of formation of the northern BRZ. Our results suggest that (1) extensional strain can occur away from the main rift basin in the Sayan Baikal range and create, depending on the rheological properties of the lithosphere, a large “off‐rift” basin analogous to the Barguzin basin, or a series of basins and ranges, as observed in the southern part of the Sayan Baikal range, (2) anelasticity plays a major role by participating in the uplift of rift shoulders and hanging wall deformation, and allowing the subsidence and tilting of the hanging wall along high‐angle planar normal faults, (3) the lower crust accommodates differential strain between the brittle upper crust and upper mantle by horizontal shear and lateral flow towards the regions of crustal and mantle thinning, (4) far‐field extensional stress and a lithospheric discontinuity inherited from Paleozoic tectonic events are sufficient to initiate rifting and basin subsidence, and (5) there is no, or very little, dynamic contribution of a hypothetic asthenospheric plume under the BRZ, at least in the recent phase of its evolution (“fast rifting stage”, 3.5 Myr).
Large‐scale relief development related to Quaternary tectonic uplift of a Proterozoic‐Paleozoic basement: The Armorican Massif, NW France
Authors: Bonnet Stéphane , Guillocheau François , Brun Jean‐Pierre , Van Den Driessche Jean
Published: February 2004

The topography of basements located in intraplate domains has often been interpreted in terms of planation theory by examining the remnants of paleosurfaces. In this paper, we address the significance of the present‐day drainage networks that erode such topographies by looking at the relief development on the Armorican Massif basement (northwestern France). Using digital elevation model (DEM) analysis, the topography of this massif is characterized by large‐scale (∼250 km) relief variations that define high elevation domains (HEDs). These domains control the pattern of drainage networks by setting the location of the main drainage divides and the polarity of the regional slopes. Large‐scale relief is strongly associated with the existence of scarps along inherited fault zones, suggesting that relief development is mainly controlled by tectonics through fault reactivation. A surface evolution study shows that drainage networks developed during the Quaternary, which corresponds to the timing of the tectonic activity which also led to relief development. The study of Quaternary fluvial erosion through a new method for measuring incision shows that the drainage basins of the HEDs record greater amounts of base level fall during Quaternary times. This implies that differential uplift is responsible for the large‐scale relief development of the Armorican Massif during the Quaternary. Our study suggests that as seen in active settings, the topography of basements corresponds to a dynamic signal between tectonics and erosional processes which can be used, for example, in the study of intraplate deformation.
Electrical double‐dipole experiment in the German Continental Deep Drilling Program (KTB)
Authors: Stoll Johannes B. , Haak Volker , Spitzer Klaus
Published: February 2004

Among the most important rationales to drill the German Continental Deep Drilling Program (KTB) borehole was the necessity to calibrate geophysical methods. Deep and hitherto inaccessible seismic reflectors, high‐conductivity layers, and temperature belong to this group of deep crustal properties which can be predicted from surface measurements, but whose depth and nature are a matter of dispute. One problem is the unknown influence of inhomogeneous superficial layers on the determination and resolution of the model parameters. In the case of electrical resistivity a number of presite experiments had detected a high‐conductivity layer of regional extent at a mean depth of ∼10 km. Distorting superficial layers were expected to cause severe ambiguity in the interpretation of the specific properties of this layer, even feigning its existence at all. The drilling yielded direct evidence of high‐conductivity material within the range of 8 km depth. After completion of the KTB a large‐scale dipole‐dipole experiment was carried out using a vertical electric receiver dipole with one of the electrodes in the main drill hole at 9065 m depth and a second in the earlier drill hole at 4000 m depth. The idea was to find out whether the buried electrode was close to a high‐conductivity layer of regional extent. The surprising result was that the two apparent resistivity curves measured with the transmitter spread perpendicular and parallel to the NNW striking very highly conductive fracture zones are almost overlapping, even though these fracture zones are the cause of a strong structural anisotropy of the apparent resistivity measured with magnetotellurics. Such a strong anisotropy should also show up in the buried electrode experiment except when a high‐conductivity layer close but above the buried electrode at 9000 m depth is introduced in the model, As a result, the interpretation of this experiment suggests a NE dipping electrically conductive fault system soling out into a high‐conductivity horizontal layer at 7–8 km depth. The conductivity is increased due to graphite and high‐salinity fluids, in a depth near the fossil Cretaceous brittle‐ductile transition zone for quartz‐rich rocks.
Pine Island Glacier, Antarctica: A study of the catchment using interferometric synthetic aperture radar measurements and radar altimetry
Authors: Stenoien Mark D. , Bentley Charles R.
Published: February 2004

We have mapped the catchment region of Pine Island Glacier, Antarctica, using ERS‐1 and ERS‐2 radar altimetry and synthetic aperture radar (SAR) interferometry. The radar altimeter data were converted to a digital elevation model (DEM), using an algorithm we developed that corrects for slopes and grids the surface elevation estimates in a single step. We used the DEM and precise (postprocessed) orbital ephemerides to generate terrain‐corrected and geolocated complex radar imagery. Interferometric pairs on both ascending and descending orbits, corrected by means of the DEM, yielded components of ice surface motion in the radar look direction. Where possible, we combined velocity components from ascending and descending tracks to produce vector velocities; elsewhere, the velocity was calculated by assuming the direction of motion is down the surface slope. The velocity map reveals a system of tributaries that channel ice from the basin‐like catchment area into the fast flowing downstream trunk of Pine Island Glacier. None of the tributaries shows a clear onset region. Instead, the ice speed gradually increases, along with a gradual decrease in driving stress, along each tributary. We combined the velocity data with sparse information on ice thicknesses and snow accumulation rates to calculate an approximate mass balance. We find that the region is in balance within an uncertainty of ∼30% of the total mass throughput.
Acoustic and elastic characterization of marine sediments by analysis, modeling, and inversion of ultrasonic P wave transmission seismograms
Authors: Breitzke Monika
Published: February 2004

Ultrasonic P wave transmission seismograms recorded on sediment cores have been analyzed to study the acoustic and estimate the elastic properties of marine sediments from different provinces dominated by terrigenous, calcareous, and diatomaceous sedimentation. Instantaneous frequencies computed from the transmission seismograms are displayed as gray‐shaded images to give an acoustic overview of the lithology of each core. Centimeter‐scale variations in the ultrasonic waveforms associated with lithological changes are illustrated by wiggle traces in detail. Cross‐correlation, multiple‐filter, and spectral ratio techniques are applied to derive P wave velocities and attenuation coefficients. S wave velocities and attenuation coefficients, elastic moduli, and permeabilities are calculated by an inversion scheme based on the Biot‐Stoll viscoelastic model. Together with porosity measurements, P and S wave scatter diagrams are constructed to characterize different sediment types by their velocity‐ and attenuation‐porosity relationships. They demonstrate that terrigenous, calcareous, and diatomaceous sediments cover different velocity‐ and attenuation‐porosity ranges. In terrigenous sediments, P wave velocities and attenuation coefficients decrease rapidly with increasing porosity, whereas S wave velocities and shear moduli are very low. Calcareous sediments behave similarly at relatively higher porosities. Foraminifera skeletons in compositions of terrigenous mud and calcareous ooze cause a stiffening of the frame accompanied by higher shear moduli, P wave velocities, and attenuation coefficients. In diatomaceous ooze the contribution of the shear modulus becomes increasingly important and is controlled by the opal content, whereas attenuation is very low. This leads to the opportunity to predict the opal content from nondestructive P wave velocity measurements at centimeter‐scale resolution.
Isostatic response of the Australian lithosphere: Estimation of effective elastic thickness and anisotropy using multitaper spectral analysis
Authors: Simons Frederik J. , Zuber Maria T. , Korenaga Jun
Published: February 2004

Gravity and topography provide important insights regarding the degree and mechanisms of isostatic compensation. The azimuthally isotropic coherence function between the Bouguer gravity anomaly and topography evolves from high to low for increasing wavenumber, a diagnostic that can be predicted for a variety of lithospheric loading models and used in inversions for flexural rigidity thereof. In this study we investigate the isostatic response of continental Australia. We consider the effects of directionally anisotropic plate strength on the coherence. The anisotropic coherence function is calculated for regions of Australia that have distinctive geological and geophysical properties. The coherence estimation is performed by the Thomson multiple‐Slepian‐taper spectral analysis method extended to two‐dimensional fields. Our analysis reveals the existence of flexural anisotropy in central Australia, indicative of a weaker N‐S direction of lower Te. This observation is consistent with the suggestion that the parallel faults in that area act to make the lithosphere weaker in the direction perpendicular to them. It can also be related to the N–S direction of maximum stress and possibly the presence of E–W running zones weakened due to differential sediment burial rates. We also demonstrate that the multitaper method has distinct advantages for computing the isotropic coherence function. The ability to make many independent estimates of the isostatic response that are minimally affected by spectral leakage results in a coherence that is more robust than with modified periodogram methods, particularly at low wavenumbers. Our analysis elucidates the reasons for discrepancies in previous estimates of effective elastic thickness Te of the Australian lithosphere. In isotropic inversions for Te, we obtain values that are as much as a factor of 2 less than those obtained in standard inversions of the periodogram coherence using Bouguer gravity and topography but greater than those obtained by inversions that utilize free‐air rather than Bouguer gravity and ignore the presence of subsurface loads. However, owing to the low spectral power of the Australian topography, the uncertainty on any estimate of Te is substantial.
Variations in P wave speeds and outboard earthquakes: Evidence for a petrologic anomaly in the mantle transition zone
Authors: Brudzinski Michael R. , Chen Wang‐Ping
Published: February 2004

We investigate the nature of laterally varying P wave speeds (Vp) and their relationship to a group of unusual deep earthquakes beneath the backarc of Tonga. This subhorizontal swath of “outboard” earthquakes occurs within the transition zone of the mantle but is several hundred kilometers away from the inclined Wadati‐Benioff zone. Surprisingly, the seismogenic region is not associated with fast Vp. It follows that the effect of cold temperature, as implied by the presence of large earthquakes, must have been counteracted by other factors such as compositional or mineralogical effects. Our results are based on a sequence of seismic profiles in a fan‐shot geometry. For a given azimuthal sector, we seek the simplest model of radially varying Vp that explains observed broadband waveforms and travel times. The lateral extent of anomalous Vp is then constrained by comparing the results from a wide range of azimuths. This approach fully utilizes information from triplicate P arrivals that are sensitive to seismic wave speeds at the depths of interest. Our results identify prominent anomalies of fast Vp (∼3% over average Earth models) both in the transition zone and in the lower mantle beneath the Tonga backarc. However, the region of fast Vp in the transition zone is offset by ∼300 km to the south of that in the lower mantle, and the region of offset coincides with the outboard earthquakes. A straightforward interpretation is that a large flux of subducted Pacific lithosphere lingers in the transition zone before descending into the lower mantle. Within this broad remnant of subducted lithosphere, the outboard earthquakes occur in a region where impounding of subducted material is most significant, a condition favorable for the accumulation of met astable olivine or volatiles. Such materials reduce Vp and are candidates for triggering earthquakes, thus accounting for the absence of fast Vp in the source zone of deep earthquakes.
Aftershock distribution of the 1994 Sanriku‐oki earthquake (Mw 7.7) revealed by ocean bottom seismographic observation
Authors: Hino Ryota , Ito Shinobu , Shiobara Hajime , Shimamura Hideki , Sato Toshinori , Kanazawa Toshihiko , Kasahara Junzo , Hasegawa Akira
Published: February 2004

The 1994 Sanriku‐oki earthquake (Mw 7.7) ruptured the entire seismogenic zone of the Pacific plate subduction beneath northeastern Japan arc. We deployed 18 ocean bottom seismographs and recorded its aftershocks, which span the whole seismogenic zone, with unprecedented accuracy. We calculate hypocenters and mechanisms to improve our understanding of the plate subduction geometry and plate coupling. The rupture of the mainshock initiated at the updip end of the seismogenic zone defined as the trenchward limit of the aftershock and background seismicity. The rupture continued to slip along a shallow dipping (<10°) interface for ∼60 km, and then slipped across an asperity where the dip angle starts to increase at ∼143°E. Here aftershocks are few, suggesting complete release of strain. The rupture continued another 100 km or so to the downdip end beneath the coastline, where the plate dip angle reaches 30° to smoothly connect to the dip angle of the Wadati‐Benioff zone. Coseismic slips are larger westward of 143°E, where the mantle wedge comes into contact with the subducting plate. The stress redistribution due to the mainshock seems to have caused aftershocks with normal faultings to occur offset above and below the plate boundary.
Electromagnetic core‐mantle coupling for laterally varying mantle conductivity
Authors: Wicht Johannes , Jault Dominique
Published: February 2004

Electromagnetic coupling between the Earth's core and mantle is one of the proposed mechanisms to explain length of day (LOD) variations on decadal timescales. Mantle conductivity, a determining parameter in this process, is only poorly known. Earlier work on core‐mantle coupling mostly assumed laterally homogeneous mantle conductivities. However, the lower mantle is a highly inhomogeneous region. Seismic evidence is growing that many of the inhomogeneities must have chemical, compositional, or thermal origins. We consider the effect of different laterally varying conductivity models on electromagnetic mantle torques. Torque amplitudes for a specific epoch can depend strongly on the assumed conductivity distribution. However, when comparing time series of the electromagnetic torque to decadal LOD variations, none of the examined conductivity models improve the agreement significantly. As in the simplest case of a homogeneous mantle conductivity, a minimum average conductance of 108 S is always required to make electromagnetic coupling efficient.