bluuebunny/crossref_metadata_2025 huggingface dataset:
Shear wave velocities and anisotropy in the Barbados accretionary complex
Authors: Peacock Sheila , Westbrook Graham K.
Published: February 2004

S wave velocities at the base of the Barbados accretionary wedge were determined from mode‐converted S waves in two vertical seismic profiles in Ocean Drilling Program boreholes. At Hole 949C the profile extends from 92 to 397 m below seafloor (mbsf), ending ∼17 m into the décollement zone separating the wedge from the underthrust sediment on the subducting plate beneath. At Hole 948D the profile extends from 105 to 471 mbsf, ending ∼19 m above the décollement zone. S wave velocities are 303–549 m/s at Hole 949C and 453–679 m/s at Hole 948D. The profiles show that the lowest 70 m of the accreted sediment, just above the décollement, has lower S wave velocity (379±26 m/s at Hole 949C and 453±11 m/s at Hole 948D) and higher ratio of P wave to S wave velocity (Vp/Vs) (4.59±0.31 at Hole 949C and 3.99±0.10 at Hole 948D) than in the overlying layer. This coincides with high clay content that has a high proportion of smectite. Shear wave splitting observed in records from the lowest 100 m of the accretionary wedge at both sites indicates little or no anisotropy in the low‐velocity zone. A single measurement at Hole 949C of the horizontal component of polarization of the leading split S wave from an ocean bottom shot is N68°E±20°. At Hole 949C, split shear wave delays in the downward propagating S wave originating at a fault at 275 mbsf indicate strong anisotropy in the range 275–306 mbsf and little or no anisotropy in the range 306–380 mbsf. The rate of increase of delay between 275 and 306 mbsf, 0.52±0.16 ms/m, can be modeled by either a Hudson cracked medium with crack density 0.186±0.054 or (preferably) a Hornby model of shale with 44% of clay particles aligned within 5° of the symmetry plane. This strong anisotropy is attributed to “scaly fabric,” alignment of clay particles by pervasive shear in the fault zone, which is observed in core. The low anisotropy in the remainder of the section shows that there is very little overall alignment of pore space in the lower part of the accreted sediments, supporting the hypothesis that the sediments are undercompacted rather than hydrofractured. The low S wave velocities and high Vp/Vs ratios indicate undercompaction and low effective stress despite the tectonic burial of the lowermost accreted sediments.
A stress inversion method requiring only fault slip sense
Authors: Lisle Richard , Orife Tobore , Arlegui Luis
Published: February 2004

In numerous cases, the use of standard methods of palaeostress inversion from fault slip data is prevented by the absence of structures that indicate the direction of slip on the fault. In such instances, however, the offset of planar markers, such as beds matched in the footwall and hanging wall, often allows the sense of dip separation (normal or reverse) to be observed. Where such displaced markers are horizontal or at least where their cut off lines are horizontal, it can be readily demonstrated that the sense of the dip‐slip component of net slip is the same as the sense of dip separation. Ways are explored for characterizing the stress tensor from data consisting of the orientations of a collection of faults, each with known dip‐slip sense (normal or reverse). On the basis of properties of the stress quadric the relationship between dip‐slip sense, fault plane orientation, and the stress tensor is discovered. This relationship is used as the basis of a grid search method of stress inversion, in which a large number of stress tensors differing in orientation and shape are systematically considered. Those tensors that successfully explain the senses of offset on the observed faults are considered potential solutions to the palaeostress problem. Examples of different applications of the new stress inversion method are described, including the analysis of fault data from boreholes. These examples illustrate the way the new method potentially broadens the scope of palaeostress analysis.
Geomagnetic paleosecular variation recorded in Plio‐Pleistocene volcanic rocks from Possession Island (Crozet Archipelago, southern Indian Ocean)
Authors: Camps Pierre , Henry Bernard , Prévot Michel , Faynot Liliane
Published: February 2004

Possession Island, in the Crozet Archipelago, consists of volcanic units erupted mainly between ∼5 and 0.5 Ma. A paleomagnetic sampling was carried out along several sections distributed near the northern, eastern, and southeastern coasts. A total of 45 independent flows were sampled (320 samples). For each flow a precisely defined characteristic remanence direction was usually isolated after a careful progressive cleaning in alternating fields. However, particularly complex remanence behavior is often observed. The magnetostratigraphy of the lava pile is quite simple, with reversed rocks in the lower part and normal units in the upper part of two sections. A third section is of normal polarity throughout its whole thickness, including three excursional directions. We did not find any intermediate directions between the normal and reverse magnetozones. Thus we have no evidence for the recording of the Matuyama‐Brunhes transition expected from a previous study [Watkins et al., 1972]. The amplitude of paleosecular variation, estimated from between‐flow dispersion from the field of an axial dipole, is 11.8° with 95% confidence limits between 9.3° and 14.0°. This value is consistent with the general anisotropic statistical model for paleosecular variation of Constable and Johnson [1999].
Microearthquake characteristics and crustal VP/VS structure at the Mid‐Atlantic Ridge, 35°N
Authors: Barclay Andrew H. , Toomey Douglas R. , Solomon Sean C.
Published: February 2004

We report on the results of a microearthquake experiment conducted at the along‐axis bathymetric high of the slow spreading ridge segment near 35°N on the Mid‐Atlantic Ridge (MAR). Of a total of 255 microearthquakes recorded during the 43‐day experiment, 31 were located near the Oceanographer transform at the northern end of the segment, 79 occurred at the nontransform offset at the southern end of the segment, and 145 were at the segment center. At the segment center, earthquake epicenters lay within the median valley inner floor and formed a ∼12‐km‐long trend paralleling a steep scarp bounding the western wall of the inner valley; focal depths were 3–4 km below the seafloor. Most (80%) of the focal mechanisms for 32 segment center earthquakes are consistent with normal faulting on faults approximately parallel to the axial trend. From a joint inversion for hypocenters and P and S wave velocity structures, we determined a horizontally averaged VP/VS ratio that decreases from 2.9 in the shallowmost 300 m to 1.7 at 2‐km depth, and we interpret this decrease as indicating a decreasing contribution of thin cracks to fracture porosity with depth. The maximum depth of seismicity, 4 km, is anomalously shallow compared with other MAR segments at which microearthquake experiments have been carried out. Cross‐axis relief is also anomalously low for this segment's center, and on the basis of this and other MAR microearthquake experiments, there appears to be a correlation between cross‐axis relief and maximum depth of seismicity. From the correlation of cross‐axis relief and inferred crustal thickness we suggest a relationship between thick crust, high crustal temperatures, and low cross‐axis relief, in qualitative agreement with thermomechanical models for the depth of the axial valley.
Experimental observations of pressure oscillations and flow regimes in an analogue volcanic system
Authors: Lane Stephen J. , Chouet Bernard A. , Phillips Jeremy C. , Dawson Phillip , Ryan Graham A. , Hurst Emma
Published: February 2004

Gas‐liquid flows, designed to be analogous to those in volcanic conduits, are generated in the laboratory using organic gas‐gum rosin mixtures expanding in a vertically mounted tube. The expanding fluid shows a range of both flow and pressure oscillation behaviors. Weakly supersaturated source liquids produce a low Reynolds number flow with foam expanding from the top surface of a liquid that exhibits zero fluid velocity at the tube wall; i.e., the conventional “no‐slip” boundary condition. Pressure oscillations, often with strong long‐period characteristics and consistent with longitudinal and radial resonant oscillation modes, are detected in these fluids. Strongly supersaturated source liquids generate more energetic flows that display a number of flow regimes. These regimes include a static liquid source, viscous flow, detached flow (comprising gas‐pockets‐at‐wall and foam‐in‐gas annular flow, therefore demonstrating strong radial heterogeneity), and a fully turbulent transonic fragmented or mist flow. Each of these flow regimes displays characteristic pressure oscillations that can be related to resonance of flow features or wall impact phenomena. The pressure oscillations are produced by the degassing processes without the need of elastic coupling to the confining medium or flow restrictors and valvelike features. The oscillatory behavior of the experimental flows is compared to seismoacoustic data from a range of volcanoes where resonant oscillation of the fluid within the conduit is also often invoked as controlling the observed oscillation frequencies. On the basis of the experimental data we postulate on the nature of seismic signals that may be measured during large‐scale explosive activity.
Stable isotope evidence for contrasting paleofluid circulation in thrust faults and normal faults of the central Apennines, Italy
Authors: Ghisetti F. , Kirschner D. L. , Vezzani L. , Agosta F.
Published: February 2004

We have investigated the structures and stable isotope geochemistry of fault rocks within thrust faults and normal faults of the central Apennines in order to understand the fluid circulation during late Miocene‐early Pliocene contraction and late Pliocene‐Pleistocene extension of the thrust belt. Stable isotope data were obtained for 70 carbonate host rocks and 80 fault rocks. Cataclasites, veins, and slickenfibers of the thrust faults have δ13C values ranging from 0 to 3‰, which are similar to the host rocks' values, and δ18O values from 25 to 34‰, which are slightly lower than the 28 to 34‰ δ18O values of the host rocks. Cemented cataclasites, veins, and slickenfibers of the normal faults have δ13C values ranging from −5 to 3‰ and δ18O values from 19 to 30‰, which are significantly lower than the host rocks' values. These data are consistent with the contraction‐related structures having formed in a semiclosed hydrologic system. In contrast, the extension‐related structures formed in semi‐open to open system conditions. During shortening, there was little to no advection of large quantities of isotopically distinct fluids from the siliciclastic wedge on top of the subducting Adriatic plate and/or from the overlying groundwater aquifers. Only during exhumation and extension of the orogen were fluids able to penetrate downward into large normal faults that at present are seismically active.
Travel times of P and S from the global digital seismic networks: Implications for the relative variation of P and S velocity in the mantle
Authors: Bolton Harold , Masters Guy
Published: February 2004

We present new data sets of P and S arrival times which have been handpicked from long‐period vertical and transverse component recordings of the various global seismic networks. Using events which occurred from 1976 to 1994 results in ∼38,000 globally well‐distributed measurements of teleseismic P and ∼41,000 measurements of S. These data are particularly useful for looking at the relative variation of S and P velocities in the lower mantle. We describe both the measurement techniques and the gross characteristics of the data sets. The size of our data sets allows us to exploit the internal consistency of the data to identify outliers using a summary ray analysis. Since the polarity of each arrival is also known, we can construct fault plane solutions and/or compare with polarities predicted by the Harvard centroid moment tensor solutions to further diagnose phase misidentification. This analysis results in ∼5% of the data being identified as outliers. An analysis of variance indicates that the S residual travel times are dominated by the effects of three‐dimensional structure but the P data have comparable contributions from noise and source mislocation effects. The summary ray analysis reveals the basic character of lower mantle structure, and there are large‐scale patterns in both the S and P data sets that correlate quite well with each other. This analysis suggests that on average, d ln vs/d ln vp is an increasing function of depth in the mantle going from a value of ∼1.7 at the top of the lower mantle to an apparent value of 4 near the base of the mantle. This latter extreme value of R seems to result mainly from data which sample one region in the lowermost mantle under the central Pacific, where large positive S residuals are associated with very small P residuals. Such an anomaly cannot be thermal in origin.
Evolution of dilatancy and permeability in rock salt during hydrostatic compaction and triaxial deformation
Authors: Popp Till , Kern Hartmut , Schulze Otto
Published: February 2004

Combined gas permeability and P and S wave velocity measurements were carried out on rock salt samples from the Gorleben salt dome and the Morsleben salt mine under hydrostatic and triaxial loading condions, mostly at room temperature. Permeabilities in the as‐received samples vary between 10−16 and 2×10−20 m2. The initial permeability is primarily due to decompaction induced by drilling, core retrieval and sample preparation. Hydrostatic loading gives rise to a marked decrease of permeability and a coeval significant increase of P and S wave velocities due to progressive closure of grain boundary cracks, tending to approach the in situ matrix permeability (<10−20 m2). The pore space sensitivity of P and S wave velocities is used to monitor the in situ state of the microstructure. Their reversals define the boundary in the state of stresses between dilatant and compactive domains (dilatancy boundary). Dilatancy during triaxial deformation of the compacted rock salt samples is found to evolve stress dependent in various stages. The crack initiation stress increases from ∼18 MPa differential stress at 10 MPa confining pressure to ∼30 MPa at confining pressures above ∼70 MPa. Dilatancy is due to the opening of grain boundary and (100) cleavage cracks and depends on the applied confining pressure. The orientation of the open cracks is primarily controlled by the loading geometry system (compression, extension). As a consequence, permeability increases dramatically with progressive dilatancy, followed by a period of plus/minus constant permeability during strain hardening up to 10% axial strain or even more. This suggests that the evolution of permeability is not only a function of dilatancy but also of microcrack linkage. Importantly, the anisotropic crack array within the samples causes a strong directional dependence of permeability.
Depth‐dependent mantle anisotropy below the San Andreas fault system: Apparent splitting parameters and waveforms
Authors: Hartog Renate , Schwartz Susan Y.
Published: February 2004

We measure apparent teleseismic shear wave splitting parameters at several stations of the Berkeley Digital Seismic Network (BDSN) and three temporary broadband stations of the University of California at Santa Cruz located near the San Andreas fault system in northern and central California. Previously proposed anisotropic models for the region include a two‐layered structure and a structure with a constant degree of anisotropy and a horizontal fast axis that gradually rotates as a function of depth. We significantly increase the number of observations and confirm the existence of depth‐dependent anisotropy beneath the San Andreas fault system. We also investigate the extent to which the enhanced data set can constrain the details of the depth‐dependence. Using theoretical expressions, we determine a suite of two‐layer models that fit our observations and result in practically indistinguishable apparent splitting parameters. The wide range of acceptable models indicates that apparent splitting parameters alone cannot constrain all four parameters that specify the two anisotropic layers (upper and lower fast polarization directions and delay times). Synthetic seismograms for three very different two‐layer models chosen from our suite of acceptable models and a model with a gradually rotating fast axis show subtle waveform variations, and apparent splitting parameters measured from these records do not overlap. Hence we conclude that waveforms indeed contain additional information about the anisotropic structure. We incorporate waveform data by searching for the four splitting parameters of two‐layer models that minimize the splitting of several records at each station simultaneously. The resulting models largely overlap with the models obtained using the theoretical expressions, which indicates that the addition of waveform data is not sufficient to uniquely determine all four parameters of a two‐layer model. Constraining the upper fast polarization direction to be parallel to the relative motion vector of the Pacific and North American plates, N35°W, allows the values of the other three model parameters to be uniquely determined. The lower layer fast polarization direction beneath all stations is approximately E‐W and the delay times vary from 0.85 to 1.70 s. The delay times in the upper layer are smaller and they vary between 0.50 and 1.25 s. Shear within the uppermost mantle due to the relative plate motion is a satisfactory explanation for the shallow anisotropic signature. The sublithospheric E‐W fast directions may be caused by small‐scale convection within a slabless window, or by motions of the lithosphere relative to the deeper mantle.
Channeled flow: Analytic solutions, laboratory experiments, and applications to lava flows
Authors: Sakimoto Susan E. H. , Gregg Tracy K. P.
Published: February 2004

Although channeled lava flows are common in basaltic volcanism, relationships between channel morphology, eruption and emplacement parameters, and lava properties are not well understood. Several models have commonly been used to constrain these relations, but they have not been well tested on natural or simulated lava flows over a wide range of parameter space. Here, we test the accuracy and assumptions of a moderately simple analytic rectangular channel solution by comparing the behavior of well‐controlled laboratory polyethylene glycol (PEG) channeled flows to the analytic solution for isothermal, steady Newtonian flow in a rectangular channel with constant dimensions. This analytic solution agrees well with laboratory measurements. Volumetric effusion rates (Q; m3 s−1) calculated from the analytical model using measured PEG flows as input yield ratios of Qcalculated/Qpumped of ∼0.2 to 3.6, and flow rates calculated from a best fit surface velocity profile to measured velocities give more accurate ratios of ∼0.8 to 1.2. We find a very weak dependence of solution accuracy on slope, attributable to flow front effects within the laboratory flows. We subsequently apply the solution to several subaerial and submarine terrestrial flows as well as extraterrestrial channeled flows over a wide range of flow parameters. Viscosity ranges and flow rates obtained using measured channel dimensions and assumed lava properties are plausible. Interestingly, the resulting extraterrestrial estimates of viscosities and flow rates tend to fall closer to known terrestrial measurements and estimates of channel flow than to previous planetary estimates. We therefore suggest that the analytic Newtonian rectangular channel flow model is a more appropriate physical model for many channeled terrestrial and planetary flows than the Newtonian infinite sheet flow and approximation to Bingham channel flow widely used previously.
A waveform migration for the investigation of P wave structure at the top of D″ beneath northern Siberia
Authors: Freybourger Marion , Chevrot Sébastien , Krüger Frank , Achauer Ulrich
Published: February 2004

We present the first waveform migration for the lower mantle, focusing on the lowermost 400 km of the mantle beneath northern Siberia. We migrate vertical component P wave seismograms recorded in western Europe for earthquakes located in the Kuril Islands region. We focus on the time window between the direct P wave and the PcP wave, reflected at the core‐mantle boundary, and we investigate the origin of PcP precursors. The data, selected for their good signal‐to‐noise ratio, are deconvolved by the source time function determined by the beam of the P wave. We observe the signature of a finite size scattering object located around 75°N, 85°E, at an apparent depth of ∼2650 km with respect to the IASP91 model. Its scale length is probably of the order of 100 km. This object is characterized by an anisotropic scattering diagram and may be related to the presence of an ancient subducted plate near the core‐mantle boundary.
Lithospheric structure of a nascent spreading ridge inferred from gravity data: The western Gulf of Aden
Authors: Hébert Hélène , Deplus Christine , Huchon Philippe , Khanbari Khaled , Audin Laurence
Published: February 2004

The Aden spreading ridge (Somalia/Arabia plate boundary) does not connect directly to the Red Sea spreading ridge. It propagates toward the East African Rift through the Afar depression, where the presence of a hot spot has been postulated from seismological and geochemical evidence. The spreading direction (N37°E) is highly oblique to the overall trend (N90°E) of the ridge. We present and interpret new geophysical data gathered during the Tadjouraden cruise (R/V L'Atalante, 1995) in the Gulf of Aden west of 46°E. These data allow us to study the propagation of the ridge toward the Afar and to discuss the processes of the seafloor spreading initiation. We determine the lithospheric structure of the ridge using gravity data gathered during the cruise with the constraint of available refraction data. A striking Bouguer anomaly gradient together with the identification of magnetic anomalies defines the geographical extent of oceanic crust. The inversion of the Bouguer anomaly is performed in terms of variations of crustal thickness only and then discussed with respect to the expected thermal structure of the mantle lithosphere, which should depend not only on the seafloor spreading but also on the hot spot beneath East Africa. Our results allow us to define three distinct lithospheric domains in the western Gulf of Aden. East of 44°45′E the lithosphere displays an oceanic character (thermal subsidence recorded for the last 10 Ma and constant crustal thickness). Between 43°30′E and 44°10′E the lithosphere is of continental type but locally thinned beneath the axial valley. The central domain defined between 44°10′E and 44°45′E is characterized by a transitional lithosphere which can be seen as a stretched continental crust where thick blocks are mixed with thinned crust; it displays en echelon basins that are better interpreted as extension cells rather than accretion cells.
Monitoring Pacific Ocean seismicity from an autonomous hydrophone array
Authors: Fox Christopher G. , Matsumoto Haruyoshi , Lau Tai‐Kwan Andy
Published: February 2004

Since May 1996, an array of autonomous hydrophone moorings has been continuously deployed in the eastern equatorial Pacific to provide long‐term monitoring of seismic activity, including low‐level volcanic signals, along the East Pacific Rise between 20°N and 20°S and the Galapagos Ridge. The instruments and moorings were designed to continuously record low‐frequency acoustic energy in the SOFAR channel for extended periods and produce results comparable to those previously derived by using the U.S. Navy Sound Surveillance System (SOSUS) in the northeast Pacific. The technology and methodology developed for this experiment, including instrument design, mooring configuration, analysis software, location algorithms (with an analysis of errors), and a predicted error field, are described in detail. Volcanic activity is observed throughout the Pacific, along with seismicity along transform faults, subduction zones, and intraplate regions. Comparison data sets indicate detection thresholds and accuracy better than the land networks for open ocean areas and results comparable to, or better than, SOSUS. Volcanic seismicity along the fast spreading East Pacific Rise appears similar to documented examples in the northeast Pacific but with much shorter durations. One example from the intermediate spreading Galapagos Ridge is comparable to northeast Pacific examples, and several episodes of activity were observed in the Wilkes Transform Fault Zone. A site of continuing off‐axis seismicity is located near 18°S and 116°W. Isolated intraplate earthquakes are observed throughout the study area. Earthquake information from this experiment and future observations will be provided through the World Wide Web and earthquake data centers.
Size scaling relationships in the active fault networks of Japan and their correlation with Gutenberg‐Richter b values
Authors: Öncel Ali Osman , Wilson Thomas H. , Nishizawa Osamu
Published: February 2004

Fractal properties of active fault systems in Japan are evaluated and compared to Gutenberg‐Richter b values. Properties of the active fault network and seismicity were evaluated at 20 km intervals along three lines oriented along the length of Honshu, the main island of Japan. Fractal dimensions for the active fault network are calculated using the box counting method. The box curves often reveal the presence of an abrupt transition in slope at ∼8 km scales. This transition separates linear regions in the box curves that span box sizes of 17.5 to 8.5 km and 7.75 to 2 km. Power law coefficients were computed for the 17.5 to 8.5 km (DL) and 7.75 to 2 km (DS) range of box sizes from overlapping 70×70 km regions of the active fault complex. The maximum likelihood method is used to estimate b value from earthquakes occurring in the seismogenic zone (upper 20 km). The correlation between D and b value are found to vary considerably throughout Japan. In general, D and b are negatively correlated, which suggests that increased complexity in the active fault network accommodates rupture along fault planes of relatively larger surface area. Areas of significant positive correlation are also observed in Japan. The positive correlations arise through joint increases in b and D. In such areas the probability of large magnitude earthquakes decreases in response to increased fragmentation of the active fault network and increased possibility that stress release will take place along faults of smaller surface area. Negative correlations between b and D generally occur when increases in D are paralleled by decreases in b. Negative correlation areas bound the intensely faulted region of central Japan. An area of negative correlation also occurs in the intensely faulted high D area of central Japan where continued increases in D are paralleled by decreases in b. In general, these observations suggest that as fault complexity increases (as measured by D), interconnected planes of larger and larger surface area will accommodate strain release. However, with continued rise in fault complexity, strain release begins to occur on smaller fragments resulting in higher b and lower‐magnitude seismicity. The analysis may provide insights into the relationship between seismicity associated with large‐scale faults populations and earthquake hazard assessment.
Submarine landslides around the Canary Islands
Authors: Krastel Sebastian , Schmincke Hans‐Ulrich , Jacobs Colin L. , Rihm Roland , Le Bas Timothy P. , Alibés Bárbara
Published: February 2004

The morphology and structure of the submarine flanks of the Canary Islands were mapped using the GLORIA long‐range side‐scan sonar system, bathymetric multibeam systems, and sediment echosounders. Twelve young (<2 Ma) giant landslides have been identified on the submarine flanks of the Canary Islands up to now. Older landslide events are long buried under a thick sediment cover due to high sedimentation rates around the Canary Islands. Most slides were found on the flanks of the youngest and most active islands of La Palma, El Hierro, and Tenerife, but young giant landslides were also identified on the flanks of the older (15–20 Ma) but still active eastern islands. Large‐scale mass wasting is an important process during all periods of major magmatic activity. The long‐lived volcanic constructive history of the islands of the Canary Archipelago is balanced by a correspondingly long history of destruction, resulting in a higher landslide frequency for the Canary Islands compared to the Hawaiian Islands, where giant landslides only occur late in the period of active shield growth. The lower stability of the flanks of the Canaries is probably due to the much steeper slopes of the islands, a result of the abundance of highly evolved intrusive and extrusive rocks. Another reason for the enhanced slope instability is the abundance of pyroclastic deposits on Canary Islands resulting from frequent explosive eruptions due to the elevated volatile contents in the highly alkalic magmas. Dike‐induced rifting is most likely the main trigger mechanism for destabilization of the flanks. Flank collapses are a major geological hazard for the Canary Islands due to the sector collapses themselves as well as triggering of tsunamis. In at least one case, a giant lateral blast occurred when an active magmatic or hydrothermal system became unroofed during flank collapse.
Reconstruction of eruption column dynamics on the basis of grain size of tephra fall deposits: 2. Application to the Pinatubo 1991 eruption
Authors: Koyaguchi Takehiro , Ohno Marekazu
Published: February 2004

The granulometric methods to reconstruct eruption column dynamics developed in paper 1 are applied to the tephra fall deposits of the climactic Plinian phase of the 1991 eruption at Pinatubo. The tephra fall deposits are composed of two units: layer C1, which corresponds to the first half of the climactic phase, and layer C2 the second half. The granulometric estimates of the expansion rate of the umbrella cloud for layers C1 and C2 are 7×1010 and 3×1010 m3/s, respectively, which agree with the observations of satellite images. These estimates indicate that the magma discharge rate decreased from 9×108 to 3×108 kg/s during the eruption. The grain‐size distribution at the top of the eruption column is characterized by depletion of coarse clasts, suggesting a distinct deceleration in the gas thrust region down to several tens of meters per second, particularly during the second half of the climactic phase. The total amount of layers C1 and C2 is estimated to be 3×1012 kg, and the very fine particles, which did not deposit in the accessible on‐land area, may occupy up to 60% of the total ejecta. The effective duration of the eruption estimated by the present methods (a few hours or less) is substantially shorter than a previous estimate from the real‐time observation based on infrasonic data (∼10 hours). This discrepancy may be attributed to the overestimation of the magma discharge rate due to the entrainment of the ambient air at the cloud top and/or to the underestimation of the total amount of the ejecta due to the effect of the contemporaneous Plinian activity and generation of pyroclastic flows. It is also suggested that the intensity of eruption was fluctuating with time so that the instantaneous observations such as cloud height in satellite images do not always show a quantitative agreement with the time‐averaged features predicted by the present methods.
A two‐phase model for compaction and damage: 1. General Theory
Authors: Bercovici David , Ricard Yanick , Schubert Gerald
Published: February 2004

A theoretical model for the dynamics of a simple two‐phase mixture is presented. A classical averaging approach combined with symmetry arguments is used to derive the mass, momentum, and energy equations for the mixture. The theory accounts for surficial energy at the interface and employs a nonequilibrium equation to relate the rate of work done by surface tension to the rates of both pressure work and viscous deformational work. The resulting equations provide a basic model for compaction with and without surface tension. Moreover, use of the full nonequilibrium surface energy relation allows for isotropic damage, i.e., creation of surface energy through void generation and growth (e.g., microcracking), and thus a continuum description of weakening and shear localization. Applications to compaction, damage, and shear localization are investigated in two companion papers.
Modeling of the influence of gas hydrate content on the electrical properties of porous sediments
Authors: Spangenberg Erik
Published: February 2004

The geophysical investigation of gas hydrate accumulations within marine sediments and the sediments of permafrost regions suffer from a lack of information on the influence of the hydrate content on the physical sediment properties. The estimation of the hydrate content using downhole electrical measurements based on Archie's law requires the knowledge of the saturation exponent. The saturation exponent is an empirical parameter that includes influences from the internal rock structure such as pore shape, connectivity and constrictivity of the pore network, and the distribution of the conducting phase. Based on different models that account for the different morphological forms of gas hydrates found during gas hydrate sampling in various research wells, the influence of gas hydrate content on the electrical properties of the hydrate bearing sediment was investigated. For all studied forms of hydrate occurrence, disseminated in the pore space, nodular, and layered, the saturation exponent depends on the sediment properties and on saturation itself. The growth of gas hydrate nodules, lenses, and layers is a process that is assumed to result in the displacement and compaction of the surrounding sediment. Because of this change of sediment properties during hydrate generation, the saturation exponent for these forms of hydrate occurrence depends strongly on the relationship between porosity and formation resistivity factor, expressed in the form of Archie's cementation exponent. For the case that hydrate occurs disseminated in the pore space and the assumption that capillary effects are important for hydrate generation, the saturation exponent depends on grain size and grain size sorting. For the parameters chosen for these model calculations, the saturation exponent varies between 0.5 and 4. The use of a constant mean value for the saturation exponent of approximately 2 can result in both underestimation and overestimation of the hydrate content.
Resolution analysis of finite fault source inversion using one‐ and three‐dimensional Green's functions: 2. Combining seismic and geodetic data
Authors: Wald David J. , Graves Robert W.
Published: February 2004

Using numerical tests for a prescribed heterogeneous earthquake slip distribution, we examine the importance of accurate Green's functions (GF) for finite fault source inversions which rely on coseismic GPS displacements and leveling line uplift alone and in combination with near‐source strong ground motions. The static displacements, while sensitive to the three‐dimensional (3‐D) structure, are less so than seismic waveforms and thus are an important contribution, particularly when used in conjunction with waveform inversions. For numerical tests of an earthquake source and data distribution modeled after the 1994 Northridge earthquake, a joint geodetic and seismic inversion allows for reasonable recovery of the heterogeneous slip distribution on the fault. In contrast, inaccurate 3‐D GFs or multiple 1‐D GFs allow only partial recovery of the slip distribution given strong motion data alone. Likewise, using just the GPS and leveling line data requires significant smoothing for inversion stability, and hence, only a blurred vision of the prescribed slip is recovered. Although the half‐space approximation for computing the surface static deformation field is no longer justifiable based on the high level of accuracy for current GPS data acquisition and the computed differences between 3‐D and half‐space surface displacements, a layered 1‐D approximation to 3‐D Earth structure provides adequate representation of the surface displacement field. However, even with the half‐space approximation, geodetic data can provide additional slip resolution in the joint seismic and geodetic inversion provided a priori fault location and geometry are correct. Nevertheless, the sensitivity of the static displacements to the Earth structure begs caution for interpretation of surface displacements, particularly those recorded at monuments located in or near basin environments.
Nontidal oceanic contributions to gravitational field changes: Predictions of the Parallel Ocean Climate Model
Authors: Johnson Thomas J. , Wilson Clark R. , Chao Benjamin F.
Published: February 2004

This study examines the nontidal contributions of the oceans to the Earth's gravitational field variations as predicted by a global ocean general circulation model: the Parallel Ocean Climate Model. Such variations in the gravitational field Stokes coefficient are determined up to spherical harmonic degree and order 20 and compared with satellite laser ranging (SLR) data from LAGEOS I and LAGEOS II. For most Stokes coefficients except the lowest‐degree ones, this investigation indicates that the application of sea level adjustment to reduce the effects of the model's lack of mass conservation due to the Boussinesq approximation has a negligible effect on timescales less than a few years. Predicted gravitational changes show strong seasonal variability and account for a portion of the variations estimated from SLR. We conclude that, in addition to the atmosphere, the oceans are an important contributor to the temporal variations in the Earth's gravitational field. The Stokes coefficients are useful in examining oceanic mass transport between hemispheres and ocean basins. The estimated oceanic power spectrum has a spectral shape similar to the atmosphere and is well above the noise level of planned satellite missions like the Gravity Recovery and Climate Experiment (GRACE).
A geomagnetic record over the last 3.5 million years from deep‐tow magnetic anomaly profiles across the Central Indian Ridge
Authors: Pouliquen Gaud , Gallet Yves , Patriat Philippe , Dyment Jérôme , Tamura Chiori
Published: February 2004

High‐resolution records of the geomagnetic field intensity over the last 4 Myr provided by paleomagnetic analyses of marine sediments have shown the occurrence of short‐lived low field intensity features associated with excursions or short polarity intervals. In order to evaluate the ability of marine magnetic anomalies to record the same geomagnetic events, we have collected six deep‐tow (∼500 m above the seafloor) and several sea surface magnetic anomaly profiles from the Central Indian Ridge across the Brunhes, Matuyama, and Gauss chrons (i.e., from the ridge axis to anomaly 2A). After removal of topography, latitude, and azimuth effects, we converted distances into time sequences using well‐dated polarity reversal anomalies as tie points. We calculated the average signal to test the robustness of the short‐wavelength anomalies. The resulting stacked profile is very similar to stacked sea surface and downward continued profiles from the Central Indian Ridge, the East Pacific Rise, and the Pacific‐Antarctic Ridge. Our results suggest that in addition to polarity reversals, to previously suggested geomagnetic events (subchrons or excursions) within the Brunhes and Matuyama chrons. A new small‐scale magnetic anomaly, likely generated by several closely spaced geomagnetic field intensity variations represent the major contributor to the detailed shape of recent marine magnetic anomalies in investigated areas. We observe a dense succession of microanomalies that are correlated excursions (Ontong Java 1 and 2, and Gilsa), is found after the Olduvai chron. The near‐bottom results support the existence of three geomagnetic features between the Gauss‐Matuyama boundary and Olduvai. They also suggest three geomagnetic events during the C2A.1n subchron within the Gauss chron. This study emphasizes the potential of deep‐tow magnetic surveys in detecting fluctuations in geomagnetic field intensity and, in particular, short‐lived excursions, a poorly constrained part of the geomagnetic field temporal variation spectrum.
Rheology of ice I at low stress and elevated confining pressure
Authors: Durham William B. , Stern Laura A. , Kirby Stephen H.
Published: February 2004

Triaxial compression testing of pure, polycrystalline water ice I at conditions relevant to planetary interiors and near‐surface environments (differential stresses 0.45 to 10 MPa, temperatures 200 to 250 K, confining pressure 50 MPa) reveals that a complex variety of rheologies and grain structures may exist for ice and that rheology of ice appears to depend strongly on the grain structures. The creep of polycrystalline ice I with average grain size of 0.25 mm and larger is consistent with previously published dislocation creep laws, which are now extended to strain rates as low as 2×10−8 s−1. When ice I is reduced to very fine and uniform grain size by rapid pressure release from the ice II stability field, the rheology changes dramatically. At 200 and 220 K the rheology matches the grain‐size‐sensitive rheology measured by Goldsby and Kohlstedt [1997, this issue] at 1 atm. This finding dispels concerns that the Goldsby and Kohlstedt results were influenced by mechanisms such as microfracturing and cavitation, processes not expected to operate at elevated pressures in planetary interiors. At 233 K and above, grain growth causes the fine‐grained ice to become more creep resistant. Scanning electron microscopy investigation of some of these deformed samples shows that grains have markedly coarsened and the strain hardening can be modeled by normal grain growth and the Goldsby and Kohlstedt rheology. Several samples also displayed very heterogeneous grain sizes and high aspect ratio grain shapes. Grain‐size‐sensitive creep and dislocation creep coincidentally contribute roughly equal amounts of strain rate at conditions of stress, temperature, and grain size that are typical of terrestrial and planetary settings, so modeling ice dynamics in these settings must include both mechanisms.
A new model for the evolution of the volcanic island of Réunion (Indian Ocean)
Authors: Lénat Jean‐François , Gibert‐Malengreau Barbara , Galdéano Armand
Published: February 2004

The island of Réunion has been studied using data from airborne and shipborne magnetic surveys. The subaerial history of Réunion spans the last 2.1 million years. The Brunhes‐Matuyama geomagnetic reversal enables differentiation of volcanic rocks older and younger than 0.78 Ma. The lower submarine flanks are poorly magnetized and are interpreted as landslide deposits. The core of the island is composed of highly magnetized rocks. Piton des Neiges volcano is composed mostly of rocks older than 0.78 Ma. Only its western flank and central area include thick piles of younger rocks. Piton de la Fournaise is a highly and normally magnetized edifice, but its northern and eastern flanks are underlain at shallow depth by reversely magnetized formations. The latter are regarded as remnants of Les Alizés volcano, associated with the Grand‐Brûlé hypovolcanic complex. At the Matuyama‐Brunhes transition the island was composed of at least two main volcanoes (Piton des Neiges and Les Alizés) and perhaps also of a third old volcano (Takamaka) at the center‐north of the island. Piton de la Fournaise grows on the flank of Les Alizés and is a relatively young focus of volcanism. These volcanoes have had successive phases of construction and destruction. The analysis of magnetic anomalies over Réunion was decisive in defining a new coherent model for the evolution of the island.
Landslide tsunami
Authors: Ward Steven N.
Published: February 2004

In the creation of “surprise tsunami,” submarine landslides head the suspect list. Moreover, improving technologies for seafloor mapping continue to sway perceptions on the number and size of surprises that may lie in wait offshore. At best, an entirely new distribution and magnitude of tsunami hazards has yet to be fully appreciated. At worst, landslides may pose serious tsunami hazards to coastlines worldwide, including those regarded as immune. To raise the proper degree of awareness, without needless alarm, the potential and frequency of landslide tsunami have to be assessed quantitatively. This assessment requires gaining a solid understanding of tsunami generation by landslides and undertaking a census of the locations and extent of historical and potential submarine slides. This paper begins the process by offering models of landslide tsunami production, propagation, and shoaling and by exercising the theory on several real and hypothetical landslides offshore Hawaii, Norway, and the United States eastern seaboard. I finish by broaching a line of attack for the hazard assessment by building on previous work that computed probabilistic tsunami hazard from asteroid impacts.
Strain transfer between disconnected, propagating rifts in Afar
Authors: Manighetti I. , Tapponnier P. , Courtillot V. , Gallet Y. , Jacques E. , Gillot P.‐Y.
Published: February 2004

We showed before that both the Aden and Red Sea plate boundaries are currently rifting and propagating along two distinct paths into Afar through the opening of a series of disconnected, propagating rifts. Here we use new geochronological, tectonic, and paleomagnetic data that we acquired mostly in the southeastern part of Afar to examine the geometry, kinematics, and time‐space evolution of faulting related to strain transfer processes. It appears that transfer of strain is accommodated by a bookshelf faulting mechanism wherever rifts or plate boundaries happen to overlap without being connected. This mechanism implies the rotation about a vertical axis of small rigid blocks along rift‐parallel faults that are shown to slip with a left‐lateral component, which is as important as their normal component of slip (rates of ∼2–3 mm/yr). By contrast, where rifts do not overlap, either a classic transform fault (Maskali) or an oblique transfer zone (Mak'arrasou) kinematically connects them. The length of the Aden‐Red Sea overlap has increased in the last ∼0.9 Myr, as the Aden plate boundary propagated northward into Afar. As a consequence, the first‐order blocks that we identify within the overlap did not all rotate during the same time‐span nor by the same amounts. Similarly, the major faults that bound them did not necessarily initiate and grow as their neighboring faults did. Despite these variations in strain distribution and kinematics, the overlap kept accommodating a constant amount of strain (7 to 15% of the extension amount imposed by plate driving forces), which remained distributed on a limited number (seven or eight) of major faults, each one having slipped at constant rates (∼3 and 2 mm/yr for vertical and lateral rates, respectively). The fault propagation rates and the block rotation rates that we either measure or deduce are so fast (30–130 mm/yr and 15–38°/Myr, respectively) that they imply that strain transfer processes are transient, as has been shown to be the case for the processes of tearing, rift propagation, and strain jumps in Afar.