bluuebunny/crossref_metadata_2025 huggingface dataset:
Stereological analysis of fracture network structure in geological formations
Authors: Berkowitz Brian , Adler Pierre M.
Published: February 2004

Relationships between three‐dimensional fracture networks consisting of poly disperse disks and the corresponding two‐dimensional trace maps are systematically analyzed. Bulk densities of disks and disk intersections are related to surface densities of traces and trace intersections; this results in a new relation for the average length of disk intersections. The probability densities of trace lengths and disk intersections are studied for several disk diameter distributions. The inverse problem of deriving the disk distribution from the trace distribution is then solved, assuming only that the disks have uniformly random locations and orientations. These results are applied to a variety of synthetically generated data, as well as to several sets of field data. Analysis of the field data suggests that fracture diameter distributions follow a power law, in agreement with previous conclusions based on trace length histograms, with exponents varying from 1.3 to 2.1.
Runout lengths of sturzstroms: The control of initial conditions and of fragment dynamics
Authors: Kilburn Christopher R. J. , Sørensen Søren‐Aksel
Published: February 2004

Sturzstroms are giant landslides that travel kilometers within minutes. Data from terrestrial examples of 0.001–10 km3 suggest that their runout lengths increase in proportion to the square root of their volume. This trend is investigated assuming that runout is controlled by fragmental flow. The results indicate that runout lengths depend on the potential energy available for motion after initial collapse (itself a function of the dimensions of the collapse zone), on the degree of rock fragmentation, and on the rate of momentum loss by fragment collisions in a basal boundary layer, assumed to thicken with time by diffusion. A dependence on initial conditions during collapse would explain claims that a minimum volume is required for sturzstroms to form: Beneath a critical volume, insufficient energy is available to initiate fragmental flow, and so the unstable mass slumps downslope. The drop height (H) is often similar to the vertical extent of the collapse zone and so is linked by geometry to sturzstrom volume. The ratio of H to L (the horizontal distance of effective transport), normally interpreted as a measure of frictional resistance, is thus reinterpreted as an inverse measure of the energy available for runout after collapse. By providing a physical basis for observed trends, the analysis justifies use of empirical limits for forecasting the runout lengths of major landslides.
Rift propagation, detachment faulting, and associated magmatism in Bransfield Strait, Antarctic Peninsula
Authors: Barker Daniel H. N. , Austin James A.
Published: February 2004

Multichannel seismic data from Bransfield Strait, a young (<4 Ma?) marginal basin located at the NW tip of the Antarctic Peninsula, reveal a complex array of rifting styles short of clearly defined seafloor spreading. The strait is undergoing basinwide extension. While enriched mid‐ocean ridge basalts have been dredged from topographic highs in the NE part of the basin, seismic evidence does not support the formation of oceanic crust there. Acoustic signatures of volcanic crust at known axial deep highs extend laterally subseafloor; their distribution and style support propagation of volcanism and associated extension from NE to SW. Active extensional structures along both margins control a strong physiographic asymmetry. The South Shetland Islands margin is composed of a few, closely spaced, large‐offset normal faults; the gentler Antarctic Peninsula margin exhibits broader, distributed extension. To the NE, normal fault polarity reversal structures (NFPRs) dominate; listric faults sole into NW dipping, low‐angle reflectors interpreted as detachments. To the SW, larger NFPRs and convex‐up geometries up to ∼20 km across suggest broad uplift rather than detachment‐based extension. Such uplift may precedes seafloor volcanism and rapid associated deflation along detachments. NW‐SE trending accommodation zones also segment the basin along‐strike. Bransfield Strait extension may result both from plate boundary tectonics at the South Shetland Trench and at the evolving Antarctic‐Scotia plate boundary. Extension was probably initiated by onset of subduction rollback at the trench, as a slab window opened from the SW. Augmented extension in the NE part of the basin may be a response to left‐lateral transtension associated with reorganization of the triple junction at the Shackleton Fracture Zone‐South Scotia Ridge intersection.
High‐resolution micromagnetic models of fine grains of magnetite
Authors: Williams W. , Wright T. M.
Published: February 2004

High resolution three dimensional models are presented of cubic grains of magnetite in the grain size range 0.03 μm to 4.0 μm, achieved using a resolution of up to 262,144 elements to represent a single grain. Previous models have been limited in the maximum grain size that could be investigated because of the lack of resolution and increase in complexity of domain structure with grain size. This paper confirms the simple flower and vortex states found previously, but the classical two‐domain (with closure domains) Kittel structure is shown not to be a local energy minimum state for magnetite. The concept of a single‐domain to two‐domain transition is now seen as a gradual unwinding of a flower state, and pseudo‐single domain grains are shown to be vortex and multiple‐vortex states. For grains greater than about 2 μm in size the center of vortices act a nucleation centers for the formation of domain walls. At the largest grains size modeled (4.0 μm), the expected features of multidomain grains are seen, with well‐defined domain walls, and body domains with their magnetization aligned along the easy magnetocrystalline anisotropy axis.
Emplacement of long lava flows on planetary surfaces
Authors: Zimbelman James R.
Published: February 2004

Three long lava flows on Mars, Venus, and the Moon were examined in order to evaluate their possible emplacement rate and condition. On the Moon, flows of the last (phase III) effusion within the Imbrium impact basin were examined using Apollo photography. The longest phase III flow can be followed for 250 km, terminating ∼400 km from the probable source vent. This flow has a width of 10 to 25 km, thickness of 10 to 30 m, and a medial channel preserved in its proximal reach, and it was emplaced on a regional slope of ∼0.3°. In the Tharsis region of Mars, a well‐defined set of lava flows extends north from the topographic saddle between Ascraeus and Pavonis Montes. Viking Orbiter images show one flow that can be traced for 480 km, with a width ranging from 5 to 50 km, thickness of 30 to 100 m and a prominent medial channel in its proximal reach, and was emplaced on a regional slope of ∼0.5° to ∼0.1°. The Strenia Fluctus area on Venus consists of an array of intermixed radar‐bright and radar‐dark lobate flows, one of which can be traced for 180 km, with a width of 5 to 20 km, and an unknown thickness (but inferred to be ∼30 m), and was emplaced on the lowland plains where the regional slope is only ∼0.03°. When viewed at the full Magellan resolution, this flow contains several flow margins, indicating its compound nature. Effusion rates were calculated for the simple lunar and Martian flows using published empirical and theoretical relationships, resulting in a broad range of 500 to 108 (Moon) and 600 to 2×108 (Mars) m3/s, with most likely values of ∼5×104 to ∼105 for both flows. The compound Venus flow would have required 494 years for emplacement at the typical Kilauea rate of ∼5 m3/s, but the thermal balance of planetary tube systems could also be consistent with a rate at least an order of magnitude larger. The distinction between simple and compound flows is important to any evaluation of flow emplacement based solely on remote sensing data.
Thermal and compositional evolution of the shallow magma chambers of Vesuvius: Evidence from pyroxene phenocrysts and melt inclusions
Authors: Cioni Raffaello , Marianelli Paola , Santacroce Roberto
Published: February 2004

A large number of clinopyroxene (cpx) phenocrysts and hosted melt inclusions (MI) in pyroclasts from several Vesuvius eruptions were analyzed. Assuming that their temperature of homogenization (Thom) reflects that of crystallization of the host mineral and that after homogenization and quenching, composition of the MI represents the melt from which the host crystallized, each cpx‐MI pair records equilibrium conditions at Thom. These data were used to discuss the magma evolution within shallow magma chamber. They formed by means of the periodic arrival of mafic batches, recorded by the MI in diopside (Fs4–7) crystallized at 1150°–1200°C. Under open conduit conditions the chambers are small, and each magma pulse induces thermal and compositional variations recorded by oscillatory zoned pyroxenes (Fs4–15). Their MI and Thom summarize the evolution of the chamber resulting from cycles of magma injection, crystal settling, and magma extraction. The deposits of the explosive eruptions which reopen the conduit reflect variable mixing during magma withdrawal from zoned reservoirs, whose layering was deciphered through MI and Thom in salite (Fs15–30). We suggest that the Vesuvius magma chambers evolve from prolate toward subequant, changing their layering with increasing volume and age: (1) initial stage, high aspect ratio chamber, homogeneous mafic melt (T°C∼1100) crystal enriched downward; (2) young stage, medium aspect ratio, continuous gradation from mildly evolved (T°C∼1050) to felsic melt (T°C∼850–900); and (3) mature stage, low aspect ratio, twofold chamber with stepwise gradient separating lower, convective, mildly evolved portion (T°C∼1050) from upper, stratified, felsic portion (T°C∼800–950).
Stability limits of hydrous minerals in sediment and mid‐ocean ridge basalt compositions: Implications for water transport in subduction zones
Authors: Ono Shigeaki
Published: February 2004

Phase relationships in hydrated natural sediment and mid‐ocean ridge basalt (MORB) compositions were investigated experimentally from 6 to 15 GPa and 700°C to 1400°C in order to determine the stability of hydrous phases in subducting slabs and to constrain reactions resulting in the release of water from subduction zones to the mantle. Phengite and hydrous aluminum silicates (topaz‐OH and phase egg) are stable in the sediment composition at depths greater than 150 km and constitute H2O reservoirs. Similarly, lawsonite is an H2O reservoir in MORB composition at depths greater than 150 km. According to mass balance calculations based on experimental run products, the weight proportions of phengite and H2O at 6 GPa in the sediment are about 50% and 2%, respectively. At pressures from 8 to 12 GPa, topaz‐OH (Al2SiO4(OH)2) appears as the hydrous mineral, and the sediment retains about 0.5–0.7 wt % H2O. At higher pressures, phase egg (AlSiO3(OH)) appears as the hydrous phase with a large stability field. The H2O content in the sediment is about 0.5 wt% at pressures greater than 12 GPa. In natural MORB compositions, lawsonite is the hydrous phase at pressures lower than 10 GPa. About 1 wt%, H2O in the MORB composition is retained by lawsonite at 6 GPa and 800°C. Above 6 GPa, a lawsonite‐out reaction is controlled by compositional changes of garnet and clinopyroxene. If the oceanic crust descends with an overlying sediment layer, water can be carried down into the transition zone by the sediment. The amount of transported water is controlled by the chemical compositions of the subducting sediments. Water in the MORB layer can be stored to 300 km in lawsonite for a cold subduction zone geotherm. Water release at depths greater than 200 km through progressive lawsonite breakdown can hydrate the overlying mantle, causing the generation of a peridotite region including hydrous phases (chondrodite, clinohumite, phase A, and phase E). If this hydrous peridotite is dragged down by the descending oceanic crust, water can be transported into the transition zone by the subduction process. The amount of transported water is controlled by the thermal state of the slab and the chemical composition of MORB.
Heat flow and thickness of the lithosphere in the Canadian Shield
Authors: Jaupart C. , Mareschal J. C. , Guillou‐Frottier L. , Davaille A.
Published: February 2004

Heat flow and radioactive heat production data were obtained in the Canadian Shield in order to estimate the crustal heat production and the mantle heat flow. Several methods have been used to determine radioactive heat production in the crust. The analysis yields values for the mantle heat flow in the craton that are consistently between 7 and 15 mW m−2. Assuming that the lithosphere is in thermal equilibrium, we investigate the conditions for small‐scale convection to supply the required heat flux through its base. For a given creep law, the thickness of the lithosphere, the temperature at the base of the lithosphere, and the effective viscosity of the mantle are determined from the value of the mantle heat flow beneath the shield. The viscosity of the mantle depends on the creep mechanism and on the fluid content. Wet diffusion creep implies a viscosity between 1020 and 1021Pa s, corresponding to a mantle temperature of 1620 K at a depth of 250 km. The other creep mechanisms can be ruled out because they imply values for viscosity and temperature inconsistent with geophysical data. For a given creep law, there is a minimum mantle temperature below which equilibrium cannot be reached. For wet diffusion creep, this minimum mantle temperature (1780 K at 280 km depth ) is close to that of the well‐mixed ( isentropic ) oceanic mantle at the same depth. For a thermally stable lithosphere, our model requires the mantle heat flow to be at least 13 mW m−2 and the compositional lithosphere to be less than 240 km.
Constitutive mechanical relations of solid‐liquid composites in terms of grain‐boundary contiguity
Authors: Takei Yasuko
Published: February 2004

Macroscopic constitutive relations of solid‐liquid composites are derived as functions of microscopic geometry described by grain‐boundary contiguity. The model consists of solid grains, which make a framework through grain‐to‐grain contact, and an interstitial liquid phase. Contiguity is the essential geometric factor that determines the macroscopic mechanical properties of the granular composites, while the other factors such as liquid volume fraction affect the properties only indirectly through the contiguity. Contiguity is defined by the portion of the grain surface being in contact with the neighboring grains, and takes a value between 0 and 1. Total length of the solid‐solid and solid‐liquid phase boundaries observed in cross section can be quantitatively related to the contiguity. Contiguity ϕ can be extended to a tensor ϕij to cover anisotropic grain contact. The full anisotropic macroscopic elasticities are quantitatively derived as functions of contiguity. Also, as an attempt to apply the present formulation to viscous behaviors of solid‐liquid composites, bulk and shear viscosities are derived as functions of contiguity. The results successfully predict the large structural sensitivities of these properties; the bulk and shear moduli (or viscosities) of the skeleton are drastically reduced by decreased contiguity and vanish when contiguity vanishes. For partially molten media under thermodynamic equilibrium, geometrical variation according to melt fraction and dihedral angle is quantitatively related to the variation of contiguity, and the acoustic properties are derived as functions of melt fraction and dihedral angle.
Analysis of 11 Myr of geomagnetic intensity variation
Authors: Constable Catherine G. , Tauxe Lisa , Parker Robert L.
Published: February 2004

We have conducted a detailed exploratory analysis of an 11 million year long almost continuous record of relative geomagnetic paleointensity from a sediment core acquired on Deep Sea Drilling Project Leg 73, at Site 522 in the South Atlantic. We assess the quality of the paleointensity record using spectral methods and conclude that the relative intensity record is minimally influenced by climate variations. Isothermal remanence is shown to be the most effective normalizer for these data, although both susceptibility and anhysteretic remanence are also adequate. Statistical analysis shows that the paleointensity variations follow a gamma distribution, and are compatible with predictions from modified paleosecular variation models and global absolute paleointensity data. When subdivided by polarity interval, the variability in paleointensity is proportional to the average, and further, the average is weakly correlated with interval length. Spectral estimates for times from 28.77 until 22.74 Ma, when the reversal rate is about 4 Myr−1, are compatible with a Poisson model in which the spectrum of intensity variations is dominated by the reversal process in the frequency range 1–50 Myr−1. In contrast, between 34.7 and 29.4 Ma, when the reversal rate is about 1.6 Myr−1, the spectra indicate a different secular variation regime. The magnetic field is stronger, and more variable, and a strong peak in the spectrum occurs at about 8 Myr−1. This peak may be a reflection of the same signal as recorded by the small variations known as tiny wiggles seen in marine magnetic anomaly profiles.
Soil gas survey for tracing seismogenic faults: A case study in the Fucino Basin, central Italy
Authors: Ciotoli G. , Guerra M. , Lombardi S. , Vittori E.
Published: February 2004

Soil‐gas (He, Rn, CO2, and CH4) surveys were performed to test their sensitivity for locating fault or fracture systems when masked by unconsolidated lithologies and to investigate the gas‐bearing properties of seismogenic faults. The Fucino basin (Central Italy) was chosen as a test site because it displays a network of surface and shallow‐buried active faults within the valley floor which were partially reactivated during the 1915 Avezzano earthquake (Ms 7.0). The highest radon values were found aligned along the most important faults, especially along the San Benedetto‐Gioia dei Marsi fault (SBGMF) to the east and in the northwestern sector where the Avezzano‐Celano fault (ACF) meets the northwestern prolongation of the 1915 coseismic rupture. Moderately anomalous values of radon occur along the faults located in the depression of the historical lake (Trasacco fault, TF, and Ortucchio fault, OF). Anomalous values of CO2, Rn, and He were detected north of Borgo Ottomila in a zone where coseismic scarps and liquefaction phenomena were responsible for a pervasive brittle deformation during the 1915 earthquake. Highest helium values prevail in the western part of the plain, in correspondence with a horst structure bounded by the TF and considered to be the extension of the Vallelonga‐Trasacco ridge. Results of this research confirm the possible usefulness of soil gas methods for locating some tectonic faults when covered by unconsolidated deposits. The study provides some clues on the spatial influence of tectonics and geology on deeply‐seated gas migration toward the surface.
Joint estimation of afterslip rate and postseismic relaxation following the 1989 Loma Prieta earthquake
Authors: Pollitz Fred F. , Bürgmann Roland , Segall Paul
Published: February 2004

Global Positioning System (GPS) data from campaigns carried out over the 5 years following the 1989 Loma Prieta earthquake and leveling data measured in 1990 and 1992 define the postseismic velocity field around the Loma Prieta rupture zone. Subtraction of a background velocity field yields a residual velocity pattern which we interpret as the product of two physical processes: (1) slow afterslip along distinct planes in the upper crust and (2) viscoelastic relaxation of the lower crust and upper mantle. Bürgmann et al. [1997] previously derived an afterslip model involving uniform afterslip on two optimally determined planes, including oblique reverse slip on the coseismic rupture and reverse slip on a shallow thrust fault to the northeast of the San Andreas fault. We further consider models of distributed slip on these two fault planes plus a viscoelastic relaxation pattern which depends on a suitable coseismic rupture model and crust and mantle viscosities. Several fault models from the literature were considered for the 1989 coseismic rupture, with nearly identical impact on the results. Simultaneous maximum likelihood inversion of the GPS and leveling data for afterslip distribution and viscosity yields the following results: (1) A good fit to the data is obtained by smooth afterslip distributions without any viscoelastic relaxation being required. (2) Tangible broad‐scale viscoelastic relaxation of the lower crust and upper mantle are present in this data set at 97% confidence, and a lower crustal viscosity of ∼1019 Pa s is obtained; however, the viscosity of both the lower crust and upper mantle are poorly constrained. (3) For a given misfit, 20% less integrated afterslip and smoother afterslip distributions result when viscoelastic relaxation is included. (4) Maximum slip rates on the slip‐distributed models are 3–5 cm/yr, the dominant patches estimated on the two planes fill in the entire depth range 4–13 km without significant overlap, and deeper afterslip is not required. The afterslip distribution on the coseismic rupture plane is strongly dominated by reverse slip immediately southeast of the main center of coseismic reverse slip.
A joint study of experimental deformation and experimentally induced microstructures of pretextured peridotites
Authors: Wendt Anke S. , Mainprice David , Rutter Ernest , Wirth Richard
Published: February 2004

A series of deformation tests was performed on samples from natural xenoliths and dunites with well‐defined textures and microstructures. The rock samples were deformed perpendicular to the foliation, 45° to the foliation, and parallel to the foliation at temperatures from 1000°C to 1200°C, at a confining pressure of 300 MPa, and a constant strain rate of 10−5/s to obtain insight into the rheological behavior of anisotropic lithospheric mantle material. The experiments showed strong work hardening for all samples with a large increase of the flow stress at relatively low strain. In all samples, both brittle and plastic mechanisms were operative. We observed nonlocalized‐dilatant and localized‐dilatant deformation features. Nonlocalized and localized plastic deformation created subgrain boundaries, deformation lamellae, and an increase in dislocation density in the olivine crystals. Heterogeneous dislocation activities occurred on the [100] (0kl) system at 1000°C, and on the [100] (010) and [101] (010) systems at 1200°C. Strong crystallographic fabrics developed in samples with olivines that were initially oriented with low resolved shear stresses on their slip systems. The postdeformation fabric intensity was lower in specimens with strong initial petrofabrics. Interactions between dislocations of different type led to high work hardening rates during high‐temperature deformation, favoring intragranular microcracking. Fractures opened parallel to the [100] screw dislocations in the (010) plane, and in the {hk0} planes oblique to the (100) plane. The strength of the deformed samples was temperature and orientation dependent. Rocks deformed perpendicular to the foliation are strongest, and those deformed parallel to the foliation have intermediate rock strength similar to those deformed at 45° to the foliation. Based on the laboratory results, we find important implications for the rheology of the lithosphere: (1) Peridotites develop semibrittle rheology even at high temperature, and show characteristic microstructures and fabric changes. (2) The orientation of compressive stress and initial petrofabric are important parameters for defining a deformation regime if pressure, temperature, and strain rate are fixed. (3) Interaction of slip systems produces strong work hardening and intragranular microcracking.
Magnetic record of industrial pollution in forest soils of Upper Silesia, Poland
Authors: Heller Friedrich , Strzyszcz Zygmunt , Magiera Tadeusz
Published: February 2004

The organic top horizons of forest soils in the vicinity of industrial centers in Upper Silesia, Poland, are characterized by remarkably high magnetic susceptibility. The unusually strong soil susceptibility does not result from weathering or pedogenesis or from deposition of natural detrital ferrimagnetic minerals but is due rather to the influx of anthropogenic magnetic particles contained in industrial dusts and fly ashes. The magnetic particles are iron oxide spherules which originate during the combustion of fossil fuels (brown and hard coals) and during iron or steel production. Heavy metals, such as zinc, lead, or cadmium which stem from the same pollution sources, are often associated with the magnetic and other dust particles and cause soil contamination, which is potentially hazardous for plants, animals, and humans. Because the regional magnetic susceptibility distribution pattern is closely correlated to measured dust fall and largely parallels the distribution of heavy metals, measurements of magnetic susceptibility, which can be conducted easily both in the laboratory and in the field, can be used as a fast and sensitive tool to trace and monitor soil contamination in industrialized regions.
Global surface heat flux anomalies from seismic tomography‐based models of mantle flow: Implications for mantle convection
Authors: Pari Giovanni , Peltier W. Richard
Published: February 2004

We investigate the very long‐wavelength, global pattern of surface heat flux anomalies within the context of whole‐mantle and layered‐mantle anelastically compressible internal loading theories. Since the internal loading framework does not yield a direct estimate of the geotherm, we argue that accurate predictions for the surface heat flux may nevertheless be obtained by assuming that it is linearly related to the radial component of flow velocity at shallow depth in the mantle. The mantle convective circulation is assumed to be driven by density heterogeneity inferred from global seismic tomography models. Best results for the pattern of surface heat flux anomalies are obtained for models that significantly impede the circulation at a depth of 670 km. Total variance reductions of 60–65% (degree 1–5) are obtained when the viscosity profile includes a low‐viscosity asthenosphere. Within the context of our modeling assumptions, however, whole‐mantle circulation models provide best descriptions of the long‐wavelength nonhydrostatic gravity data. In order to resolve the gravity‐heat flux impasse that is revealed herein, we consider the possibility of modifying the a priori global seismic models employed in the calculations. We show that the rigidly layered‐mantle internal loading theory is equivalent to a theory in which no explicit flow‐blocking boundary condition is imposed at 670 km but in which the buoyancy field inferred from the a priori tomographic model is supplemented by flow‐blocking heterogeneity in the form of an appropriately constrained sheet mass load. We develop a general mathematical formalism describing how the introduction of appropriately constrained sheet mass loads allows the exact reconciliation of a number of a priori constraints or hypotheses concerning the structure of the circulation. Using this formalism, we explore the extreme nonuniqueness that not only characterizes internal loading theory inferences of the depth profile of mantle viscosity but also inferences of the radial style of the circulation. On this basis, we suggest that great caution is warranted with respect to tomography‐based inferences of mantle properties. Based on a viscosity profile whose depth dependence is close to that independently inferred within the context of postglacial rebound studies, we present plausible resolutions of the gravity‐heat flux impasse effected either within the framework of whole‐mantle or layered‐mantle circulation models.
Contemporary kinematics of the western United States determined from earthquake moment tensors, very long baseline interferometry, and GPS observations
Authors: Shen‐Tu Bingming , Holt William E. , Haines A. J.
Published: February 2004

Using moment tensors from earthquakes between 1850 and 1995, we determine the horizontal velocity gradient tensor field associated with the seismic deformation in the western U.S. plate boundary zone. The velocity vectors obtained from the integration of the seismic strain rates across the entire plate boundary lie within 5° of the NUVEL‐IA Pacific‐North American plate motion direction. The magnitude of the earthquake‐related velocity is 62% of the NUVEL‐1A total Pacific‐North American plate motion. If earthquake occurrence is a random, Poisson‐like process, then the large formal errors associated with the model velocity estimates are an indication that the 144‐year interval is too short to define the long‐term seismic moment release rate to within an uncertainty less than the deficit between the observed seismic moment release rate and the total long‐term moment release rate associated with both the seismic and aseismic deformation. To spatially resolve the deficit of the earthquake moment release in the last 144 years, we have also estimated the total long‐term horizontal velocity gradient tensor field using seismic strain rate tensors and recent geodetic velocities, both with and without the NUVEL‐1A Pacific‐North American rigid plate motion constraint. We find a systematic difference between the Southern California Earthquake Center (SCEC) geodetic velocity map and NUVEL‐1A North American reference frames with a pole at (38.8°N, 124.2°W, 0.33° m.y.−1). This difference is significant at the 99% confidence level and indicates that 4–7 mm/yr of NE‐SW convergence is absorbed in offshore areas of southern California if both the SCEC velocity map and NUVEL‐1A correctly describe the relative motion between the Pacific and North America plates. Deficit strain rates calculated from comparison of observed seismic strain rates over the last 144 years with long‐term model strain rate tensors are largest in areas east of San Francisco Bay, in the Transverse Ranges, and along the southern San Andreas fault and San Jacinto fault. The deficit in each 100 by 50 km grid area within these regions is equivalent to an accumulation of strain in the last 144 years that would be released by an Mw>7 earthquake. Elsewhere the accumulated strain in the last 144 years is small, except in northern Mojave Desert, along the Garlock fault, and in the region southwest of the southern San Andreas‐San Jacinto fault zone. Within each 100 by 50 km grid area in these regions the accumulated strain is equivalent to that released by an Mw=6.5–7.0 earthquake.
Introduction to Special Section: Long Lava Flows
Authors: Cashman Katharine , Pinkerton Harry , Stephenson Jon
Published: February 2004

Long lava flows are traditionally considered to form when low‐viscosity lava is erupted at high effusion rates. However, this view has recently been challenged. Detailed field measurements on active lava flows on Kilauea have shown that inflation of lava flows after emplacement can result in reactivation and continued lengthening of flows. Inflated sheets can thus act a insulated conduits (lava tubes) that permit the transport of lava over great distances at near isothermal conditions. Detailed observations of long lava flows in the Columbia River Basalt Group and in the Cenozoic Volcanic Provinces in northern Queensland confirm that this mechanism is not restricted to recent flows on Hawai'i. These findings have led to a search for evidence of inflation in flows in other parts of the world and have stimulated theoretical and laboratory research on the emplacement and cooling of lava in flows and in tubes. Understanding the formation of long submarine and planetary lava flows presents an additional challenge. Current evidence supports high effusion rates for some, possibly all, long planetary lava flows, and improved resolution from the Mars Global Surveyor will undoubtedly lead to either a confirmation or a rejection of this view. In this review, we discuss the geological importance and distribution of long lava flows, we investigate diametrically opposed views on the formation of long lava flows, and we stress the need for an interdisciplinary approach to improve our understanding of these enigmatic geological features.
An examination of along‐axis variation of magma chamber width and crustal structure on the East Pacific Rise between 13°30′N and 12°20′N
Authors: Babcock Jeffrey M. , Harding Alistair J. , Kent Graham M. , Orcutt John A.
Published: February 2004

We investigate the along‐axis variations of magma chamber width and crustal structure along the East Pacific Rise (EPR) from 13°30′N to 12°20′N through reprocessed common depth point (CDP) reflection profiles. The magma lens is, predominantly, a continuous feature in the study area with an average width of ∼500 m as determined from migrated cross‐axis CDP profiles. This value is similar to widths estimated elsewhere along the EPR, suggesting that the axial magma chamber (AMC) width is not spreading rate dependent once the threshold for a steady state magma chamber is reached. The axial morphology of the 13°N area is generally not a good predictor of magma lens width or continuity. A fairly continuous melt lens is imaged where the triangular axial topography might suggest waning magma supply. In fact, between 13°05′N and 13°01′N a shallow melt lens has been imaged which may be indicative of recent or impending eruptive activity. This shoaling is similar to that observed near the 17°26′S region of the EPR where the rise axis summit is domed and highly inflated. Generally, the thickness of seismic layer 2A beneath the ridge crest is uniform and comparable to that estimated for 9°N, 14°S, and 17°S on the EPR, suggesting that the axial extrusive layer is invariant along fast spreading ridges. Uniformity of layer 2A thickness along‐axis implies that variations in magma chamber depth are directly attributed to changes in thickness of the sheeted dike complex (seismic layer 2B). Contrary to expectations of decreasing melt sill depth with increasing spreading rate, the average thickness of seismic layer 2B is slightly less (∼165 m) at 13°N than at the faster spreading, more robust 9°N area. Finally, geochemical/petrologic boundaries, which may delineate separate melt supply regions, occurring at the 13°20′N and 12°46′N devals (deviation in axial linearity) are observed to coincide with subtle changes in AMC and layer 2A reflection characteristics.
Crustal structure of the northwestern Vizcaino block and Gorda Escarpment, offshore northern California, and implications for postsubduction deformation of a paleoaccretionary margin
Authors: Leitner Beate , Tréhu Anne M. , Godfrey Nicola J.
Published: February 2004

The Vizcaino block is an anomalously shallow region of the western U.S. continental margin located southwest of the Mendocino triple junction. It originated as part of the accretionary prism of the North America plate and was transferred to the Pacific plate in the Miocene as the Pacific‐North America plate boundary migrated ∼130 km eastward, forming the Gorda Escarpment at its northern boundary. We present hybrid crustal models for the northwestern part of the Vizcaino block derived from marine seismic and gravity data. The velocity and density structure of the northwestern Vizcaino block are compatible with paleoaccretionary complex material similar to San Simeon/Patton terrane overlying oceanic crust or a mafic layer. The most significant result of our modeling is an abrupt increase in Moho dip from ∼5° to ∼20–30° beneath the western edge of the Oconostota ridge along the northwestern margin of the Vizcaino block. This Moho dip is steeper than observed anywhere along the Cascadia subduction zone, indicating postsubduction deformation. We suggest that the paleotrench was deformed by compression, which reactivated preexisting thrust faults in the upper crust and thickened the crust within this apparent weak zone. At least part of the deformation predates late Pliocene Pacific‐North America plate convergence and may result mainly from north‐south compression between the Pacific‐Juan de Fuca plates across the Mendocino transform fault. North‐south compression continues today and may dynamically support the uplifted northern margin of the Vizcaino block, although the primary locus of deformation shifted to the relatively weak Gorda plate sometime prior to 3 Ma.
Electrical conductivity in shaly sands with geophysical applications
Authors: Revil A. , Cathles L. M. , Losh S. , Nunn J. A.
Published: February 2004

We develop a new electrical conductivity equation based on Bussian's model and accounting for the different behavior of ions in the pore space. The tortuosity of the transport of anions is independent of the salinity and corresponds to the bulk tortuosity of the pore space which is given by the product of the electrical formation factor F and the porosity ϕ. For the cations, the situation is different. At high salinities, the dominant paths for the electromigration of the cations are located in the interconnected pore space, and the tortuosity for the transport of cations is therefore the bulk tortuosity. As the salinity decreases, the dominant paths for transport of the cations shift from the pore space to the mineral water interface and consequently are subject to different tortuosities. This shift occurs at salinities corresponding to ξ/t(+)f ∼ 1, where ξ is the ratio between the surface conductivity of the grains and the electrolyte conductivity, and t(+)f is the Hittorf transport number for cations in the electrolyte. The electrical conductivity of granular porous media is determined as a function of pore fluid salinity, temperature, water and gas saturations, shale content, and porosity. The model provides a very good explanation for the variation of electrical conductivity with these parameters. Surface conduction at the mineral water interface is described with the Stern theory of the electrical double layer and is shown to be independent of the salinity in shaly sands above 10−3 mol L−1. The model is applied to in situ salinity determination in the Gulf Coast, and it provides realistic salinity profiles in agreement with sampled pore water. The results clearly demonstrate the applicability of the equations to well log interpretation of shaly sands.
Velocity‐porosity relationships for water‐saturated siliciclastic sediments
Authors: Erickson Stephanie N. , Jarrard Richard D.
Published: February 2004

Seeking a global empirical relationship between compressional wave velocity and porosity for siliciclastic sediments, we have brought together an extensive suite of both new and published log‐ and core‐based data. We undertook a detailed statistical analysis of Ocean Drilling Program data from Amazon Fan to examine variables affecting compressional velocity in shallow, unconsolidated sediments. We identify three dominant variables (porosity, shale fraction, and consolidation history) and present two empirically determined boundary curves (one for normally consolidated sediments and a second for highly consolidated environments (e.g., accretionary prisms)). These two empirical relationships predict the compressional velocity of siliciclastic sedimentary rocks with water‐filled pores as a function of porosity and clay content for the full range of observed porosities. Velocities of siliciclastic sedimentary rocks decrease rapidly with both increasing porosity and increasing clay content. At fractional porosities higher than about 0.4, fluid dominates the elastic properties, and velocity exhibits a subtle dependence on porosity. Remarkably, the Amazon Fan data show that clay content has no direct influence on velocity at high porosities. Both clay content and sorting do indirectly affect velocity, through their control of porosity. Burial affects velocity not only by compaction‐related porosity decrease but also by pressure‐induced increase of intergrain coupling. Because of the sensitivity of velocity to consolidation history, particularly at intermediate fractional porosities of about 0.30–0.40, no single velocity‐porosity relationship can apply to all high‐porosity sediments. The two proposed relationships fit the majority of published and new data. They are applicable, however, only for normally pressured, in situ conditions and water‐filled pores.
Earthquakes, volcanoes, and rectified diffusion
Authors: Brodsky E. E. , Sturtevant B. , Kanamori H.
Published: February 2004

Rectified diffusion is a mechanism by which a strain wave can rapidly pump volatiles into a bubble and therefore increase the pressure in a closed system. The dynamic strain of either distant regional tectonic earthquakes or local volcanic tremor can be translated to static strain inside a magma chamber via this process. We formulate a theory appropriate to the conditions of a magma chamber and calculate the increased pressure using realistic physical parameters. For a basaltic system initially at 130 MPa pressure, the excess pressure from rectified diffusion is between 0.4 and 4 MPa for a regional M≥8 earthquake. The pressure from rectified diffusion is often significantly above the static stress caused by deformation for documented cases of triggered eruptions and thus presents a more viable mechanism for triggering. Prolonged tremor can have a similar effect since the total pressure added increases linearly with the duration of the excitation.
Thermodynamic conditions for the stability of gas hydrate in the seafloor
Authors: Zatsepina O. Ye. , Buffett B. A.
Published: February 2004

Suitable pressures and temperatures for methane hydrate are found over most of the seafloor but thermodynamic equilibrium imposes an additional condition on the concentration of dissolved gas. We quantify the thermodynamic conditions for hydrate stability using a simulated annealing algorithm to minimize the free energy of a mixture of methane gas and seawater. The equilibrium state includes a description of the composition of all stable phases as a function of pressure, temperature, and salinity. When the hydrate phase is stable, we find that the equilibrium concentration of dissolved gas (solubility) decreases sharply with temperature. The gas solubility is also lowered for typical values of salinity in seawater. Since lower solubilities reduce the amount of gas required to form hydrate, the presence of salts in seawater can actually promote hydrate formation. Changes in salinity that accompany hydrate formation add a thermodynamic degree of freedom, which permits a three‐phase zone to develop, where hydrate, seawater, and free gas coexist over a range of temperatures at a constant pressure. We apply our calculations to determine the location of stable phases in the seafloor. The calculated profile of gas solubility permits hydrate to crystallize directly from dissolved gas in seawater. Diffusion of gas along the gradient in the equilibrium concentration implies a continual transport of gas through the hydrate layer into the overlying ocean. In order to maintain hydrate in the seafloor sediments, a persistant source of methane is required to overcome the losses due to diffusion. Rates of hydrate growth and loss are estimated using simple models of physical conditions in marine sediments.
Crustal deformation of the Marlborough Fault Zone in the South Island of New Zealand: Geodetic constraints over the interval 1982–1994
Authors: Bourne S. J. , Árnadóttir T. , Beavan J. , Darby D. J. , England P. C. , Parsons B. , Walcott R. I. , Wood P. R.
Published: February 2004

Crustal deformation across the Marlborough fault zone in the South Island of New Zealand has been investigated by resurveying with the Global Positioning System (GPS) a triangulation and trilateration network across part of the zone. The principal strain rates, which mostly have errors between 5% and 10%, vary systematically across the region. The principal axis of horizontal contraction gradually swings from SE‐NW southeast of the Hope fault to almost east‐west in the west. The component of velocity parallel to the strike of the faults can account for 95% of the relative motion between the Australian and Pacific plates. This component exhibits little variation west of the Alpine (Wairau) fault, and shows an almost linear variation from the Wairau fault to the east coast. The horizontal strain rate northwest of the Wairau fault is a small east‐west uniaxial contraction (∼0.1 ppm yr−1). If it is assumed that the crustal blocks bounded by the major faults are primarily driven by basal shear tractions, with stresses on the faults being relatively insignificant, then the slip rates expected on the faults may be derived from the observed velocity variations across the fault zone. These predicted slip rates are in good agreement with geologically observed slip rates. The larger slip rates on the Hope fault are related to the observation that the strain field extends southeast of the fault over a distance large compared with the spacing between the faults. The agreement between predicted and geological slip rates lends support to the idea that the observed pattern of deformation reflects that in the lower lithosphere.
Grid search modeling of receiver functions: Implications for crustal structure in the Middle East and North Africa
Authors: Sandvol Eric , Seber Dogan , Calvert Alexander , Barazangi Muawia
Published: February 2004

A grid search is used to estimate average crustal thickness and shear wave velocity structure beneath 12 three‐component broadband seismic stations in the Middle East, North Africa, and nearby regions. The crustal thickness in these regions is found to vary from a minimum of 8.0±1.5 km in East Africa (Afar) region to possibly a maximum of 64±4.8 km in the lesser Caucasus. Stations located within the stable African platform indicate a crustal thickness of about 40 km. Teleseismic three‐component waveform data produced by 165 earthquakes are used to create receiver function stacks for each station. Using a grid search, we have solved for the optimal and most simple shear velocity models beneath all 12 stations. Unlike other techniques (linearized least squares or forward modeling), the grid search methodology guarantees that we solve for the global minimum within our defined model parameter space. Using the grid search, we also qualitatively estimate the least number of layers required to model the observed receiver functions' major seismic phases (e.g., PSMoho). A jackknife error estimation method is used to test the stability of our receiver function inversions for all 12 stations in the region that had recorded a sufficient number of high‐quality broadband teleseismic waveforms. Five of the 12 estimates of crustal thicknesses are consistent with what is known of crustal structure from prior geophysical work. Furthermore, the remaining seven estimates of crustal structure are in regions for which previously there were few or no data about crustal thickness.