bluuebunny/crossref_metadata_2025 huggingface dataset:
Seismic mapping of shallow fault zones in the San Gabriel Mountains from the Los Angeles Region Seismic Experiment, southern California
Authors: Fuis G. S. , Ryberg T. , Lutter W. J. , Ehlig P. L.
Published: February 2004

During the Los Angeles Region Seismic Experiment (LARSE), a reflection/refraction survey was conducted along a profile (line 1) extending from Seal Beach, California, northeastward to the Mojave Desert and crossing the Los Angeles and San Gabriel Valley basins and San Gabriel Mountains. In most shot gathers from the southern and central San Gabriel Mountains, clear secondary arrivals are seen that merge, or appear to merge, with first arrivals at three locations, including the location of the Vincent thrust fault, an exposed late Mesozoic/early Cenozoic megathrust. These secondary arrivals are interpretable as reflections in the shallow crust (<5 km depth) from a concave‐upward interface that projects to the surface in the north near the Vincent thrust fault, is offset in its central part at the San Gabriel fault (an old branch of the San Andreas fault), and terminates in the south at 1 to 2 km depth at the southern mountain front. The velocity structure above and below this interface strongly suggests it is the Vincent thrust fault: intermediate velocities (6.2 km/s), consistent with mylonites overlying the Vincent thrust fault, are observed above it; lower velocities (5.8 km/s), consistent with the Pelona Schist underlying the Vincent thrust fault, are observed below it. Problems arise, however, in attempting to match this reflector to the exposed Vincent thrust fault, which is seen in outcrops east of line 1. The Vincent thrust fault is shallower than the reflector in most places. An unmapped structure (steep fault, monocline, or thrust fault) is required between line 1 and the outcrops that either drops the Vincent thrust fault down to the depths of the reflector or repeats the Vincent thrust fault beneath line 1 in the footwall of another thrust fault. An alternative interpretation of the reflector is a deep greenstone horizon within the Pelona Schist, although this alternative is not favored by the velocity structure.
High‐resolution imaging using a high‐Tc superconducting quantum interference device (SQUID) magnetometer
Authors: Egli Ramon , Heller Friedrich
Published: February 2004

Most rock magnetometers measure the bulk remanent magnetization of 1‐inch cylindrical samples (centimeter scale) or even larger volumes of drill cores. High‐ Tc superconducting quantum interference device (SQUID) magnetometers are able to measure magnetic fields on rock surfaces at higher resolution on a millimeter scale: the fields of discrete rock‐forming magnetic mineral grains or fine magnetic rock textures and structures can be observed when scanning across selected surface areas. The stability problems of a commercially available high‐ Tc SQUID magnetometer have largely been solved by improving the magnetic shielding and reducing the noise due to turbulent boiling of liquid nitrogen. Magnetizations as weak as 5×10−4 A/m can now be discriminated with a resolution of 1 mm. A software package has been developed to eliminate measurement errors arising from instrumental drift. The program also calculates the downwards continuation of the field data and offers full inversion for the vertical magnetization component. A synthetic sample demonstrates the performance of the SQUID sensor and the inversion software. The potential of high‐resolution magnetic imaging is shown by measuring three rock samples with very different magnetic properties.
Multichannel analysis of shear wave splitting
Authors: Chevrot Sébastien
Published: February 2004

A multichannel analysis is introduced to constrain seismic anisotropy from the shear wave splitting of SKS and SKKS. This technique utilizes simultaneously a set of records coming from different azimuths. The splitting intensity of SKS waves, measured by the amplitude of the transverse component, depends on the angle between the back azimuth of the earthquake and the direction of the symmetry axis and on the delay time δt between the two quasi‐shear waves. It is shown that the splitting parameters can be determined from the azimuthal dependence of the splitting intensity, which is given by the first right eigenvector of the matrix containing the transverse components of all the records. Alternatively, the splitting intensity can be measured by projecting the transverse components on the radial components derivatives. Experiments on synthetic seismograms demonstrate that both approaches provide robust estimates of the splitting parameters. However, the projection approach gives measurements that are closer to the inputs and with smaller error bars, which suggests that it should be preferred when the signal‐to‐noise ratio is low. The new technique is applied to analyze shear wave splitting under stations BCAO (Bangui, Central African Republic) and BDFB (Brasilia, Brazil). The analysis of 75 seismograms recorded by station BCAO gives a fast polarization direction in very close agreement with the present‐day motion direction of the African plate. Comparisons with the results of previous studies, obtained with other techniques and smaller data sets, suggest that some of the published measurements may be strongly biased. The main limitation of the multichannel analysis is a good azimuthal distribution of the seismicity. Maps of the azimuthal coverage were determined, and it is found that the most favorable regions to study anisotropy with SKS waves are a latitudinal band that goes from South America to Africa, the eastern part of North America, and India.
A geological and geophysical investigation of Baby Bare, locus of a ridge flank hydrothermal system in the Cascadia Basin
Authors: Becker Nathan C. , Wheat C. Geoffrey , Mottl Michael J. , Karsten Jill L. , Davis Earl E.
Published: February 2004

Baby Bare is one of three small basement outcrops on the eastern, sedimentburied Juan de Fuca Ridge flank that have localized heat loss and fluid movement within 3.5 Ma oceanic crust. Low‐temperature (25°C) hydrothermal vents near the summit of Baby Bare represent the highest‐temperature occurrence of off‐axis hydrothermal activity found in oceanic crust older than 1 million years. This site has been investigated with seismic reflection profiling, towed‐camera surveys, and an Alvin dive series that included heat flow measurements to document the detailed geological setting of these off‐axis vents. A new geologic map based on visual observations of Baby Bare shows that the distribution of rock, sediment, and biota appears to be controlled by seafloor slope and elevation, while specific vent locations are controlled by faulting and occur only in areas of thin or no sediment cover. Alvin heat flow data indicate that conductive heat loss from the edifice is ∼4.5 times greater than that from the sediment‐blanketed area around the outcrops. Although the outcrop is generally conical, seismic reflection profiles reveal that the sediment‐buried portions of the edifice have an asymmetric morphology, strongly suggesting that Baby Bare is a volcano built upon a preexisting, fault‐generated abyssal hill. This evidence, combined with previously published petrologic data and results of Ocean Drilling Program Leg 168 drilling, is consistent with the hypothesis that Baby Bare formed by off‐axis volcanism rather than at the adjacent ridge axis; sediment thickness and fossil assemblages indicate that it could be as young as 2.7 Ma. Off‐axis volcanoes such as Baby Bare increase the overall roughness of basement topography and thus delay complete sediment burial during normal lithospheric aging, particularly in areas where near‐axis sediment accumulation is rapid. Partially buried seamounts play an important role in focusing hydrothermal exchange between the oceans and young oceanic crust and, if Baby Bare is representative, may contribute as much as 85% of the heat flux from a sedimented ridge flank.
Crustal deformation near Hengill volcano, Iceland 1993–1998: Coupling between magmatic activity and faulting inferred from elastic modeling of satellite radar interferograms
Authors: Feigl Kurt L. , Gasperi Jérôme , Sigmundsson Freysteinn , Rigo Alexis
Published: February 2004

Tectonic activity in the Hengill volcanic area in southwestern Iceland accelerated in July 1994, when an unusually persistent swarm of moderate‐sized earthquakes began. Although the largest events were magnitude 5, the pattern of upward crustal deformation at 2 cm/yr indicates that most of the activity is related to inflation of a magma chamber at depth. To monitor this activity, we analyze synthetic aperture radar (SAR) images acquired by the ERS‐I and ERS‐2 satellites between July 1993 and September 1998 using interferometry. Interferograms composed of images acquired during the snow‐free summer months remain coherent on Holocene lava flows, even after 4 years. Some of the interferograms show a discontinuity in the fringe pattern, which we interpret as 8 mm of (aseismic) dip slip on a 3‐km‐long segment of a N5° striking normal fault, part of which had been mapped previously This slip must have occurred between July 31 and September 3, 1995 (inclusive), and has been confirmed by observations in the field. The predominant signature in all the interferograms spanning at least 1 year, however, is a concentric fringe pattern centered just south of the Hromundartindur volcanic center. This we interpret as mostly vertical uplift caused by increasing pressure in an underlying magma source. The volume source that best fits the observed interferograms lies at 7±1 km depth and remains in the same horizontal position to within 2 km. It produces 19±2 mm/yr of uplift. This deformation accumulates as elastic strain energy at a rate 2.8 times the rate of seismic moment release. Accumulated over 5 years, it increases the Coulomb failure stress by >0.6 bar in an area that includes some 84% of the earthquakes recorded between 1993 and 1998. Under our interpretation, magma is injected at 7 km depth, just below the seismogenic zone formed by colder, brittle rock. There the inflation induces stresses that exceed the Coulomb failure criterion, triggering earthquakes, possibly in a cyclical fashion.
In search of high‐fidelity geomagnetic paleointensities: A comparison of single plagioclase crystal and whole rock Thellier‐Thellier analyses
Authors: Cottrell Rory D. , Tarduno John A.
Published: February 2004

The strength of Earth's magnetic field, on timescales greater than a million years, is known primarily from Thellier‐Thellier analyses of igneous rocks. In the Thellier‐Thellier method, the natural rémanent magnetization (NRM) is compared with a thermoremanent magnetization (TRM) imparted during heating in a known laboratory field. Unfortunately, the multiple heating steps required by the method often result in alteration of magnetic minerals in clay‐bearing groundmass of whole rock samples. This alteration may result in the creation of new magnetic minerals that could affect TRM acquisition. Analyses of single plagioclase crystals, which contain magnetic inclusions that retain paleofield values, may offer a way to retrieve paleointensity data that is less affected by alteration. We test this hypothesis through a study of whole rocks and single plagioclase crystals from a mid‐Cretaceous (113–115 Ma) basalt flow of the Rajmahal Traps (northeastern India). Magnetic hysteresis and thermal demagnetization data from plagioclase crystals and whole rock samples are similar, suggesting a common magnetic mineralogy. However, hysteresis data from heated whole rock samples indicate that a fine‐grained magnetic phase has formed. This effect was either absent, or much less pronounced in the single plagioclase crystals tested. Paleointensity data from these crystals suggest a paleofield value of 65.1±5.3 μT (n=15). Data from whole rock samples overlap these values but are on average lower. We attribute the difference in this study to the preferential growth of new magnetic minerals in the whole rock samples, resulting in an enhanced acquisition of TRM and shallowing of the slope of NRM‐lost versus TRM‐gained curves. These data suggest that subtle alteration effects may bias Thellier‐Thellier results from some whole rock samples toward low paleofield values. In these cases the single plagioclase crystal approach may yield more reliable estimates of geomagnetic paleointensity.
Local earthquake tomography of shallow subduction in north Chile: A combined onshore and offshore study
Authors: Husen Stephan , Kissling Edi , Flueh Ernst R.
Published: February 2004

Selected travel time data from the aftershock series of the great Antofagasta earthquake (Mw = 8.0) have been inverted simultaneously for both hypocenter locations and three‐dimensional VP and VP/VS structure. The data were collected with a dense 44‐station seismic network including ocean bottom hydrophones. We performed a series of inversions with increasing complexity: 1‐D, 2‐D, and 3‐D. This approach was chosen to account for the heterogeneous seismicity distribution and to obtain a smooth regional model in areas of low resolution. Special efforts were made to assess the solution quality including standard resolution estimates and tests with synthetic travel times. The subducted plate is imaged between 20 and 50 km in depth as an eastward dipping high VP feature. High VP/VS ratios within the oceanic crust possibly indicate elevated fluid content. Underplating of material eroded close to the trench is found beneath the Mejillones Peninsula. The lower crust of the overlying plate is characterized by an average VP of 6.8–6.9 km/s and an average to low VP/VS ratio. Large areas of anomalously high VP are found in the lower crust south of the city of Antofagasta; they are interpreted as remants of magmatic intrusions. A zone of high VP/VS ratios is found within the rupture area of the Antofagasta main shock, just above the subducted slab. Its location within the region of highest stress release from the main shock suggests that the main shock rupture causes the high VP/VS ratio. The high VP/VS ratio could indicate postseismic fluid migration from the subducted oceanic crust into the overlying lower crust.
P wave attenuation structure in the fault area of the 1995 Kobe earthquake
Authors: Rietbrock Andreas
Published: February 2004

The three‐dimensional (3‐D) P wave attenuation structure in the Kobe epicentral area is determined using aftershock records from the 1995 Kobe (Hyogo‐ken Nanbu) earthquake. Waveform data from both permanent seismic networks and portable stations, set up following the Kobe mainshock, were used. The observed P wave amplitude spectra were inverted for the spectral plateau value, the source corner frequency, and a frequency‐independent t* operator assuming an ω2‐type source model. The calculated t* operators were subsequently used for the computation of the 3‐D absorption structure using a 3‐D velocity model. Only high‐quality amplitude spectra were used for the estimation of reliable attenuation parameters, resulting in a final data set of ∼4100 t* operators. Two regions of high absorption were found in the upper part of the crust. One is below the northern part of Awaji Island and reaches down to a depth of 5 km. The other region, northeast of Kobe city, is along the Arima‐Takatsuki tectonic line and extends along the Rokko, Kashiodani, and Yamada Faults. It extends northward to the Hokusetsu‐Sanchi area. In both regions a high density of fault lineations is mapped, and high Poisson ratios are known. This suggests that partially saturated cracks cause the observed high attenuation.
The spectral decay parameter κ in the region of Umbria‐Marche, Italy
Authors: Castro Raul R. , Trojani Luca , Monachesi Giancarlo , Mucciarelli Marco , Cattaneo Marco
Published: February 2004

We analyze 44 earthquakes (M2.1–4.1) recorded by the Marchesan Seismograph Network during the 1997 Umbria‐Marche sequence. We calculated the spectral decay parameter κ [Anderson and Hough, 1984] using 33 events (M2.2–4.1) and ƒc < 10 Hz located in the middle of the rupture area. Although the source‐station paths have approximately the same distance (±2.5 km) and azimuth for a given site, the estimates of κ show great variability at all the stations. Even events with small standard deviation on the estimate of κ can have a value 2–3 times different from that estimated for other events located within the 2.5 km. This observation suggests that the attenuation near the source can change significantly in a small distance range. We separated source and path effects by performing a generalized spectral inversion. We dismissed events with source functions having corner frequencies between 10 and 20 Hz, the frequency band where κ was estimated. With the best estimates of κ we calculated the mean for each site, and we observed an apparent azimuthal variation of the attenuation. We used the observed changes of κ to estimate the near‐source attenuation, finding that in a distance of 2 km, Q may vary between 16 and 52, depending on the source‐station direction. The results of the analysis suggest that the most likely cause of the observed variability of κ is the spatial variation of Q near the seismogenic zone.
Mineral assemblages of basalt in the lower mantle
Authors: Funamori Nobumasa , Jeanloz Raymond , Miyajima Nobuyoshi , Fujino Kiyoshi
Published: February 2004

High‐pressure X‐ray diffraction and analytical transmission electron microscopy of laser‐heated diamond cell samples show temperature‐dependent phase assemblages in mid‐ocean ridge basalt (MORB) at lower mantle pressures. The assemblage observed below ∼2250 K consists of only stishovite plus an orthorhombic perovskite phase (space group Pbnm or Cmcm) having a composition close to that of MORB but slightly depleted in SiO2. With increasing temperature, the assemblage transforms to orthorhombic and cubic perovskites plus stishovite and then at ∼3250 K to orthorhombic and cubic perovskites plus stishovite and the CaFe2O4 phase. All of these assemblages could be present in slabs that have penetrated deep into the mantle and can therefore contribute to the seismological heterogeneity of the lower mantle.
Slip rates on San Francisco Bay area faults from anelastic deformation of the continental lithosphere
Authors: Geist Eric L. , Andrews D. J.
Published: February 2004

Long‐term slip rates on major faults in the San Francisco Bay area are predicted by modeling the anelastic deformation of the continental lithosphere in response to regional relative plate motion. The model developed by Bird and Kong [1994] is used to simulate lithospheric deformation according to a Coulomb frictional rheology of the upper crust and a dislocation creep rheology at depth. The focus of this study is the long‐term motion of faults in a region extending from the creeping section of the San Andreas fault to the south up to the latitude of Cape Mendocino to the north. Boundary conditions are specified by the relative motion between the Pacific plate and the Sierra Nevada‐Great Valley microplate [Argus and Gordon, 2000]. Rheologic‐frictional parameters are specified as independent variables, and prediction errors are calculated with respect to geologic estimates of slip rates and maximum compressive stress directions. The model that best explains the region‐wide observations is one in which the coefficient of friction on all of the major faults is less than 0.15, with the coefficient of friction for the San Andreas fault being approximately 0.09, consistent with previous inferences of San Andreas fault friction. Prediction error increases with lower fault friction on the San Andreas, indicating a lower bound of μSAF > 0.08. Discrepancies with respect to previous slip rate estimates include a higher than expected slip rate along the peninsula segment of the San Andreas fault and a slightly lower than expected slip rate along the San Gregorio fault.
Size‐dependent microearthquake initiation inferred from high‐gain and low‐noise observations at Nikko district, Japan
Authors: Nakatani Masao , Kaneshima Satoshi , Fukao Yoshio
Published: February 2004

We deployed a temporary seismic station at a low‐noise site in the Nikko district, Japan, in November 1993, and analyzed velocity waveforms of 17 microearthquakes (0.3 ≤ M ≤ 2.1) with small focal distances (<11 km). We compared the initial parts of the P wave velocity seismograms on the same absolute scale, after correcting for the effects of geometrical spreading and radiation pattern. We find that the P wave radiated from an earthquake of a larger final magnitude has a greater amplitude than that from a smaller earthquake, even at a very early stage when the P wave from the smallest event is still rising with an increasing slope. This observation does not necessarily mean that the initiation of the rupture is dependent on the eventual size because anelastically attenuated waveforms of earthquakes with short time duration could show such an apparent trend. Hence we computed synthetic seismograms to test a group of source scaling models in which source time function grows independently of its eventual size, taking into account anelastic attenuation and instrument response. We find that such scaling models do not easily reproduce simultaneously the size dependence of the initial rise and the pulse duration of the observed waveforms, though poor data coverage above M1.5 prevents us from deriving a firm conclusion. A model with size‐dependent growth rate is able to match our observations.
Variation in slip on intersecting normal faults: Implications for paleostress inversion
Authors: Maerten Laurent
Published: February 2004

Numerical models based on linear elasticity theory predict asymmetric slip distribution with a steep slip gradient near the line of intersection of intersecting normal faults. They also predict a discrepancy between the direction of slip on the fault plane and the direction of resolved shear stress. Both variations in slip magnitude and direction are due to mechanical interaction between the faults with intersecting patterns. These interactions cause local perturbations of the shear stress field acting on the plane of the adjacent fault. Field observations from the Chimney Rock area of central Utah show that slickensides on normal faults cutting the Navajo Sandstone change rake away from the expected downdip direction as the intersection line with adjacent faults is approached. The sense and magnitude of this change in orientation are similar to those computed by using the numerical models. The good correspondence between field observations and theoretical results from this paper not only provides insight into the mechanics of intersecting faults, but suggests that care is required when using standard inverse methods to compute paleostresses from slickenside data. The slickenside orientation near intersection lines will generally not be in the direction of the remote maximum shear stress as resolved on the fault plane. A parameter study of this change in orientation provides helpful results for evaluating field data prior to a paleostress analysis.
A simple and testable model for earthquake clustering
Authors: Console Rodolfo , Murru Maura
Published: February 2004

Earthquakes are regarded as the realization of a point process modeled by a generalized Poisson distribution. We assume that the Gutenberg‐Richter law describes the magnitude distribution of all the earthquakes in a sample, with a constant b value. We model the occurrence rate density of earthquakes in space and time as the sum of two terms, one representing the independent, or spontaneous, activity and the other representing the activity induced by previous earthquakes. The first term depends only on space and is modeled by a continuous function of the geometrical coordinates, obtained by smoothing the discrete distribution of the past instrumental seismicity. The second term also depends on time, and it is factorized in two terms that depend on the space distance (according to an isotropic normal distribution) and on the time difference (according to the generalized Omori law), respectively, from the past earthquakes. Knowing the expected rate density, the likelihood of any realization of the process (actually represented by an earthquake catalog) can be computed straightforwardly. This algorithm was used in two ways: (1) during the learning phase, for the maximum likelihood estimate of the few free parameters of the model, and (2) for hypothesis testing. For the learning phase we used the catalog of Italian seismicity (M≥3.5) from May 1976 to December 1998. The model was tested on a new and independent data set (January‐December 1999). We demonstrated for this short time period that in the Italian region this time‐dependent model has a significantly better performance than a stationary Poisson model, even if its likelihood is computed excluding the obvious component of main shock‐aftershock interaction.
Pelagic sedimentation on rough seafloor topography 1. Forward Model
Authors: Webb Helen F. , Jordan Thomas H.
Published: February 2004

This is the first paper in a four‐part sequence exploring the factors that govern the distribution of pelagic sediments in regions of the world mid‐ocean ridge system where basement outcrops and sediment deposits both contribute to local topography. In this paper, we present a simple numerical model of pelagic sedimentation in which sediments travel from topographic highs to lows, with flux regulated by a diffusivity parameter κ. By applying the forward model to realistic models of basement topography we investigate how various basement structures, sediment rain rates, and values of κ affect the morphology of the seafloor through time. We quantify the effect of sedimentation as a nonlinear smoothing filter on topography, which acts to reduce spectral power at high wavenumber, to decrease the root‐mean‐square variability of topography, to increase the spacing between hills, and to lower the relative proportion of high‐slope seafloor. We find that local accumulation rate is highly dependent on local topography and varies greatly with time. We also find that sediment compaction does not contribute directly to the surface concavity exhibited by many intrahill sediment ponds.
Physics of friction and strain rate localization in synthetic fault gouge
Authors: Sleep Norman H. , Richardson Eliza , Marone Chris
Published: February 2004

Data on synthetic fault gouge previously collected by Richardson and Marone [1999] were compared with the predictions of a unified theory for rate‐ and state‐dependent friction compiled by Sleep [1997]. The theory treats the gouge as a continuum one‐dimensional fluid sheared between parallel plates. It is predicted that the strain rate localized into a shear band of width called Wss during steady state sliding from the nominal width of the gouge zone Wnom. The critical displacement during velocity stepping tests is predicted to be Wss εint, where εint is the critical strain, an intrinsic material property. It is predicted that the strain rate for renewed sliding after holds delocalizes to a width Wnew which is greater than Wss and for long holds approaches the full gouge zone width Wnom. The displacement for recovery of the shear traction to its steady state value is predicted to be Wnewεint, which for long holds is much greater than the critical displacement obtained by velocity stepping. Only the macroscopic effects of this process could be studied using the laboratory data in hand. Compaction during the hold and the difference between peak shear traction upon restart and the steady state shear traction during sliding (healing) were measured. To simulate more complex normal traction variations on real faults, the normal traction was varied sinusoidally about its previous value during some holds. The theory reasonably predicts the observed relationship between healing and compaction and healing versus hold time. It predicts the slip needed for recovery of shear traction following holds but poorly predicts the shear traction versus time during recovery. We attribute this failure to the fact that the laboratory gouge is a heterogeneous three‐dimensional substance. Qualitatively, the delocalized width Wnew varies with position within the gouge plane, and slip is required for localized shear to organize in three dimensions. As strain rate was not observed as a function or time and position within the gouge, other explanations for the observed long recovery times following holds remain viable, including consolidation strengthening.
Temperature‐dependent permeability and bifurcations in hydrothermal flow
Authors: Germanovich Leonid N. , Lowell Robert P. , Astakhov Dmitriy K.
Published: February 2004

We investigate the effects of temperature‐dependent permeability in a hydrothermal upflow zone on the evolution of a seafloor hydrothermal system. Our mathematical modeling of a seafloor hydrothermal system with temperature‐dependent permeability suggests that the system can be in one of two stable regimes of heat transfer. In one regime, heat conduction and thermoelastic effects do not play a significant role because the rock thermal expansion coefficient is too low or the rock porosity is too high. In the other regime, thermoelastic stresses reduce the permeability by orders of magnitude, and some fraction of the heat is transferred by conduction. In both regimes, essentially the same amount of heat is transported by the upflow but with different flow parameters. When thermoelastic stresses reduce the permeability, discharge occurs at relatively high temperature and low flow velocity, whereas when thermoelastic stresses have little effect on the permeability, discharge occurs at much lower temperature and higher flow velocity. The existence of two stable states of the hydrothermal system results from the nonlinearity of the dependency of permeability upon temperature and from specifying the heat flux entering the system. Consequently, a hydrothermal system can be on one or another solution branch, depending on small variations in system parameters, its history, and boundary conditions. At the top of the hydrothermal system the temperature difference between the branches can reach several hundred degrees. Relatively small changes in basal heat flux or rock permeability in the upflow zone can rather quickly switch the system from one stable branch to another. Such permeability changes may result from magmatic events, earthquakes, or chemical dissolution or precipitation. The calculations show that it is easier to drive the system from the high‐temperature branch to the low‐temperature one than vice versa.
Acoustic velocity in fractured rock
Authors: Monsen Karstein
Published: February 2004

The propagation of acoustic waves through a fractured one‐dimensional medium is examined theoretically with the purpose of determining how the velocity depends on the fracture properties and wave frequency. Wave propagation has been modeled by a modified version of the propagator matrix method. The welded boundary conditions normally assumed for layer interfaces have been replaced by linear slip conditions to represent seismic fracture response. By using a propagator formulation, all intrafracture reflections are included in the calculation of the propagating wave field. Eigenvalue analysis of the propagator matrix allowed for derivation of an analytical solution of the group velocity in fractured rock. Examination of this solution shows that the propagation in fractured rock is strongly dispersive at wavelengths comparable to the fracture spacing. It is also shown how the velocity of a fractured medium is affected by the stiffness and spacing of fractures, at different frequencies. The transition from low to high frequency has been defined by the ratio between the wavelength and fracture spacing. By relating the phase delay between fractures with the phase delay across fractures, it was possible to define an upper limiting frequency for wave propagation.
Laboratory‐based electrical conductivity in the Earth's mantle
Authors: Xu Yousheng , Shankland Thomas J. , Poe Brent T.
Published: February 2004

Recent laboratory measurements of electrical conductivity of mantle minerals are used in forward calculations for mantle conditions of temperature and pressure. The electrical conductivity of the Earth's mantle is influenced by many factors, which include temperature, pressure, the coexistence of multiple mineral phases, and oxygen fugacity. In order to treat these factors and to estimate the resulting uncertainties, we have used a variety of spatial averaging schemes for mixtures of the mantle minerals and have incorporated effects of oxygen fugacity. In addition, to better calculate lower mantle conductivities, we report new measurements for electrical conductivity of magnesiowüstite (Mg0.89Fe0.11)O. Because the effective medium theory averages lie between the Hashin‐Shtrikman bounds for the whole mantle, a laboratory‐based conductivity‐depth profile was constructed using this averaging scheme. Comparison of apparent resistivities calculated from the laboratory‐based conductivity profile with those from field geophysical models shows that the two approaches agree well.
The relation of crustal scattering to seismicity in southern California
Authors: Revenaugh Justin
Published: February 2004

Dense regional array recordings of 377 teleseismic events are used to map geographic variability in upper crustal P to S scattering in southern California. Scattered‐wave energy is sensitive to short‐wavelength heterogeneity and maps of scattering potential offer a powerful complement to travel time tomography in characterizing seismic crustal heterogeneity. The scale length of resolved scattering variability is commensurate with the scale lengths of surface fault‐trace length and microseismicity variations, and both mapped faults and seismicity are concentrated in regions of strong scattering. Fault‐proximal scattering averaged across‐strike of predominately strike‐slip fault zones is highly correlated with microseismicity levels and is predictive of the pattern of microseismicity variation but not of absolute scale. North‐south profiles through the Transverse Ranges reveal coincident strong gradients in microseismicity and scattering, whereas mapped segment bounds along the San Andreas fault zone from San Francisco to the Salton Sea coincide with strong along‐strike gradients in the level of scattering. The overall correlation of microseismicity and scattering potential is consistent with structural control of aftershock and background microseismicity production, strain energy control of scattering, or both. Prior evidence for fault offset in the pattern of scattering intensity [Revenaugh and Reasoner, 1997] favors the former.
Reexamination of log periodicity observed in the seismic precursors of the 1989 Loma Prieta earthquake
Authors: Huang Y. , Saleur H. , Sornette D.
Published: February 2004

Based on several empirical evidence, a series of papers has advocated the concept that seismicity prior to a large earthquake can be understood in terms of the statistical physics of a critical phase transition. In this model, the cumulative seismic Benioff strain release ∈ increases as a power law time‐to‐failure before the final event. This power law reflects a kind of scale invariance with respect to the distance to the critical point: ∈ is the same up to a simple reseating λz after the time‐to‐failure has been scaled by a factor λ. A few years ago, on the basis of a fit of the cumulative Benioff strain released prior to the 1989 Loma Prieta earthquake, Sornette and Sammis [1995] proposed that this scale invariance could be partially broken into a discrete scale invariance, defined such that the scale invariance occurs only with respect to specific integer powers of a fundamental scale ratio. The observable consequence of discrete scale invariance takes the form of log‐periodic oscillations decorating the accelerating power law. They found that the quality of the fit and the predicted time of the event are significantly improved by the introduction of log periodicity. Here we present a battery of synthetic tests performed to quantify the statistical significance of this claim. We put special attention to the definition of synthetic tests that are as much as possible identical to the real time series except for the property to be tested, namely, log periodicity. Without this precaution, we would conclude that the existence of log periodicity in the Loma Prieta cumulative Benioff strain is highly statistically significant. In contrast, we find that log‐periodic oscillations with frequency and regularity similar to those of the Loma Prieta case are very likely to be generated by the interplay of the low‐pass filtering step due to the construction of cumulative functions together with the approximate power law acceleration. Thus the single Loma Prieta case alone cannot support the initial claim, and additional cases and further study are needed to increase the signal‐to‐noise ratio, if any. The present study will be a useful methodological benchmark for future testing of additional events when the methodology and data to construct reliable Benioff strain function become available.
Quasi‐hydrostatic compression of magnesium oxide to 52 GPa: Implications for the pressure‐volume‐temperature equation of state
Authors: Speziale Sergio , Zha Chang‐Sheng , Duffy Thomas S. , Hemley Russell J. , Mao Ho‐kwang
Published: February 2004

Room temperature static compression of MgO (periclase) was performed under nearly hydrostatic conditions using energy dispersive synchrotron X‐ray diffraction in a diamond anvil cell with methanol‐ethanol (to 10 GPa) or helium (to 52 GPa) as a pressure‐transmitting medium. Highly precise cell parameters were determined with an average relative standard deviation 〈Δa/a〉 = 0.0003 over all the experimental pressure range. Fixing the bulk modulus K0T = 160.2 GPa, a fit of the data to the third‐order Birch‐Murnaghan equation of state yields: V0 = 74.71±0.01 Å3, (∂K0T/∂P)T = 3.99±0.01. A fit of different P‐V‐T datasets, ranging to 53 GPa and 2500 K, to a Birch‐Murnaghan‐Debye thermal equation of state constrained the Grüneisen parameter γ0 = 1.49±0.03, but not its volume dependence q, which was constrained to 1.65±0.4 by thermodynamic theory. A model based on a constant value of q cannot explain the ultrahigh pressure (P = 174–203 GPa) shock compression data. We developed a model in which q decreases with compression from 1.65 at 0.1 MPa to 0.01 at 200 GPa. This model, within the framework of the Mie‐Grüneisen‐Debye assumptions, satisfactorily describes the low‐pressure static data (〈ΔV/V〉 = 0.4% to 53 GPa) and the high‐pressure Hugoniot data (〈ΔV/V〉 <1% to 203 GPa). Average values of the thermal expansion coefficient α range between 14.1±2.8 and 16.3 ± 2.7 × 10−6 K−1 at P = 174–203 GPa. The pressure dependence of the melting temperature yields an initial pressure derivative ∂Tm/∂P = 98 K/GPa. Our analysis shows that it is possible to develop a simple model of the volume dependence of the Grüneisen parameter that can successfully describe the P‐V‐T equation of state of MgO from ambient conditions to 203 GPa and 3663 K.
Experimental studies of seismoelectric conversions in fluid‐saturated porous media
Authors: Zhu Zhenya , Haartsen Matthijs W. , Toksöz M. N.
Published: February 2004

When seismic waves generate a relative fluid‐solid motion in a fluid‐saturated porous medium, the moving charges (streaming current) in the electric double layer induce an electromagnetic (EM) field. This paper first experimentally confirms that the coupling between the seismic wave and the electromagnetic field in the kilohertz range is electrokinetic in nature. Seismoelectric signals are measured in homogeneous cylindrical porous rock samples and multilayered models. The seismoelectric signals in homogeneous rock are electric fields that move along with the acoustic wave. The mechanism of the seismoelectric conversion is completely different from the piezoelectric effect of quartz grains. The seismoelectric sensitivity with respect to salinity of the saturant has been experimentally determined. The amplitude of seismoelectric signals increases as the saturant conductivity decreases. The seismoelectric effects are generated by two different mechanisms. Both the EM radiation and the electric potential generated at an interface and within a porous medium, respectively, were measured as the P wave, at ultrasonic frequencies, passes through the layered models. Our experimental results demonstrate that seismoelectric effects exist and are measurable in the kilohertz range. The paper concludes with a comparison of experimental data and modeled data in a three‐layer porous model. Seismoelectric measurements could be an effective means of obtaining transport coefficients such as hydraulic permeability and other porous rock properties.
Accretional curvature of lithosphere at magmatic spreading centers and the flexural support of axial highs
Authors: Buck W. R.
Published: February 2004

Fluid magma should tend to rise up to the level of local isostatic equilibrium at an axis of plate spreading. The magma and underlying partial melt accretes to the side of the separating plates and gets denser as it cools and freezes. This load bends down the lithosphere and additional accretion forms a plate with concave upward curvature. The topography resulting from a curved plate passively moving off axis can be described analytically for accretion on the vertical side of uniform lithosphere when its flexural response is that of a thin elastic plate. Model predictions are consistent with the shape of the axial highs at fast spreading centers (typically ∼400m high and ∼20 km wide) as well as related gravity anomalies. This model requires at least several kilometers of lithospheric thickening within a few kilometers of the ridge axis. Seismic data appear consistent with this lithospheric structure. The magnitude and lateral distribution of faulting on the flanks of axial highs is crudely consistent with formation during plate unbending as lithosphere moves away from the axis. According to this model abandoned fast spreading centers should retain an elastically supported axial high, while for previous models abandoned highs should collapse.
A numerical model for the formation of gas hydrate below the seafloor
Authors: Davie Matthew K. , Buffett Bruce A.
Published: February 2004

We develop a numerical model to predict the volume and distribution of gas hydrate in marine sediments. We consider the environment of a deep continental margin where sedimentation adds organic material to the region of hydrate stability. Conversion of the organic material to methane by bacteria promotes hydrate formation and depletes the supply of organic carbon. We derive mass balance equations for the volume of hydrate and gas bubbles in the sediments and account for the changing concentration of dissolved methane and salts in the pore fluid. The effects of sediment compaction and the associated fluid flow are explicitly modeled. Allowances for deeper sources of fluid are also described, though we focus on the case of an idealized passive margin where carbon is input solely through sedimentation. The numerical calculations indicate that the key parameters in this model are the rate of sedimentation, the quantity and quality of the organic material, and a rate constant that characterizes the vigor of biological productivity. Model predictions for conditions that are representative of the Blake Ridge are compared with observations from Ocean Drilling Program Leg 164. We obtain a very good match to the observed chlorinity profile, including the region below the stability zone, without invoking any extraneous sources of freshening. We also predict that hydrate is unlikely to occupy more than 7% of the pore volume, in good agreement with observed estimates.
Role of ocean‐continent contrast and continental keels on plate motion, net rotation of lithosphere, and the geoid
Authors: Zhong Shijie
Published: February 2004

Three‐dimensional spherical shell models of mantle convection have been formulated to investigate the effects of ocean‐continent contrast (i.e., different viscosity for continental and oceanic lithospheres) and continental keels on plate motion, net rotation of lithosphere (i.e., degree 1 toroidal plate motion), and the geoid. The models include relatively realistic plate rheology (i.e., strong plate interiors and weak plate margins) and continental keel structure based on seismic models. The mantle flow in the models is driven by slab buoyancy. The models demonstrate that the ocean‐continent contrast and continental keels have minor effects on the long‐wavelength geoid (for degrees 2–5). However, continental keels overriding subducted slabs, for example in North America, can produce large negative gravity anomalies at a regional scale. This may have important implications to interpreting the gravity anomalies in North America. Our models show that when plates bounded by weak plate margins have the same thickness, neither weak plate margins nor the ocean‐continent contrast efficiently excite net rotation of lithosphere, although they excite toroidal motion at higher harmonic degrees. However, plate thickness variations like continental keels excite the net rotation. In spite of their ability to excite the net rotation, continental keels as thick as 300 km cannot provide the necessary coupling to the deep mantle to produce the observed net rotation of lithosphere and slow continental motion if there is a weak asthenosphere underlying them. If the thermomechanic structure of North American continental upper mantle derived from seismic and heat flow studies is representative, our models suggest that the temperature‐ and pressure‐dependent mantle rheology may not produce sufficiently high viscosity below continental keels that is needed to explain the observed plate motion. Our study hints the necessity to include more realistic treatment of plate boundaries in future studies to assess possible effects of plate‐plate coupling on plate motion.
High‐temperature embrittlement of extensional Alpine mylonite zones in the midcrustal ductile‐brittle transition
Authors: Axen Gary J. , Selverstone Jane , Wawrzyniec Tim
Published: February 2004

Structural and fluid inclusion analyses of two large‐displacement extensional Alpine shear zones show that embrittlement occurred at anomalously high temperatures and indicate that factors other than gradually changing temperature and pressure can exert primary control on the ductile‐brittle transition. Most rocks within the Brenner and Simplon mylonite zones, including abundant weak schists, failed brittlely by distributed faulting, fracturing, and brittle‐ductile shearing at T = 450°–575°C and P = 400–750 MPa, conditions in which plastic or semibrittle flow rather than brittle deformation is expected, even in strong orthogneiss. Embrittlement was caused by transiently(?) high fluid pressure and local bending strain rather than by temperature or pressure decrease. Mylonitization shut off permanently in the embrittled parts of the shear zones despite continued high‐T denudation of the footwalls. However, mylonitization apparently did continue in the structurally highest ∼50 m of the shear zones where brittle structures are absent or rare. A strength contrast evolved between these late, thin mylonite zones and the stronger, deeper parts where mylonitization ended. This contrast probably reflects both weakening of the late mylonite zones and strengthening of the deeper embrittled parts, although differential stress may have ultimately increased in the former due to strain rate increase as the shear zones thinned. The shear zones probably evolved to discrete frictional faults by T ≈ 450°C and P ≈ 400 MPa (∼15 km).