bluuebunny/crossref_metadata_2025 huggingface dataset:
Spatial and temporal correlation between coda Q−1 and seismicity and its physical mechanism
Authors: Jin Anshu , Aki Keitti
Published: February 2008

From the analysis of local earthquake coda waves recorded by the Benioff short‐period vertical seismograph at Riverside, California, we found a surprisingly large and systematic temporal variation in coda Q−1 in the southern California during the 55‐year period from 1933 to 1987. Comparison of the coda Q−1 variation with the b value obtained for earthquakes with M≥3 occurred in the area within 180 km from Riverside (corresponding to the area for which Q−1 is estimated by the coda method), led to an extraordinary result that coda Q−1 correlates positively with b value with the correlation coefficient of 0.79. The significance of correlation cannot be rejected at the confidence level of 0.974. Since the data used for measuring Q−1 (namely, the coda part of seismograms) and the data used for measuring b (namely, the seismicity catalog) are totally independent, the observed correlation supports strongly the physical reality of temporal change in both coda Q−1 and b value. The correlation between coda Q−1 and b value has been reported for several other cases, but the correlation was positive in some cases and negative in others. Our current preferred model for explaining the observed correlation is the following. Aseismic creep is going on from time to time over various sizes of cracks in the ductile part and, in some cases, in the brittle part of lithosphere at and near the plate boundary. Creep activities tend to increase the crack density and coda Q−1 observed in the region. If the creek occurs over distributed cracks with a certain predominant scale length, spatial stress concentration may enhance seismicity for earthquakes with a similar size. If this characteristic size is in the lower part of the magnitude range for which the b value is estimated, the enhanced seismicity will increase b. On the other hand, if it is in the upper part of the magnitude range, it will decrease b. Thus we can explain the observed strong correlation between coda Q−1 and b value for both positive and negative cases. The existence of such a characteristic crack size in a seismic region has been supported by various observations on earthquake scaling.
Introduction to the special section on the EMSLAB‐Juan de Fuca Experiment
Authors: Booker John R. , Chave Alan D.
Published: February 2008

Electrical properties provide a window on processes that are of great interest in understanding Earth. Electromagnetic induction, which can image electrical properties, has undergone a major revolution in the last decade. The EMSLAB experiment, the largest electromagnetic induction study to date, has been a significant factor in this revolution. It has substantially advanced data processing and interpretation of electromagnetic induction data. It has also imaged conductive structure associated with subduction, volcanism, and continental accretion that provide important constraints on the role of fluids in these processes. Finally, EMSLAB has made significant contributions to the use of electromagnetic methods in physical oceanography. We outline the rationale and history of EMSLAB and briefly review the contributions of each of the papers in this special section in the overall context. In particular, we compare and contrast the implications of papers interpreting a densely sampled central transect from the Juan de Fuca Ridge to eastern Oregon.
Some comments on magnetotelluric response function estimation
Authors: Chave Alan D. , Thomson David J.
Published: February 2008

A new set of computational procedures are proposed for estimating the magnetotelluric response functions from time series of natural source electromagnetic field variations. These combine the remote reference method, which is effective at minimizing bias errors in the response, with robust processing, which is useful for removing contamination by outliers and other departures from Gauss‐Markov optimality on regression estimates. In addition, a nonparametric jackknife estimator for the confidence limits on the response functions is introduced. The jackknife offers many advantages over conventional approaches, including robustness to heterogeneity of residual variance, relative insensitivity to correlations induced by the spectral analysis of finite data sequences, and computational simplicity. These techniques are illustrated using long‐period magnetotelluric data from the EMSLAB Lincoln line. The paper concludes with a cautionary note about leverage effects by high power events in the dependent variables that are not necessarily removable by any robust method based on regression residuals.
Mesozoic magnetic anomaly lineations and seafloor spreading history of the northwestern Pacific
Authors: Nakanishi Masao , Tamaki Kensaku , Kobayashi Kazuo
Published: February 2008

We have identified Late Jurassic to Early Cretaceous magnetic anomaly lineations (M0 to M35 of the Japanese and Hawaiian lineation sets) and fracture zones in the northwestern Pacific more comprehensively than previous investigators. We fixed 3500 positions of magnetic anomalies identified from magnetic data collected along cruise tracks as well as 151 positions of fracture zones from bathymetric and seismic profiles. The resultant isochron map revealed the evolution of the triple junction of the Pacific, Izanagi, and Farallon plates and the intimate relationship between the triple junction and the origin of the Shatsky Rise. The triple junction stagnated on a hotspot (the Shatsky hotspot) from chron M21 (149.5 Ma) to chron M4 (126 Ma). The Shatsky Rise is a trace of the hotspot on the Pacific plate. A sudden appearance of the Shatsky hotspot between chrons M21 and M20 caused a regional reorganization of the Pacific‐Izanagi‐Farallon plate boundaries.
Crustal magnetization in the South Atlantic from inversion of magnetic anomalies
Authors: Wittpenn Nancy A. , Harrison Christopher G. A. , Handschumacher David W.
Published: February 2008

A method for inverting magnetic anomalies in the presence of topography (Parker and Huestis, 1974) is used to analyze and describe crustal magnetization along 12 profiles in the South Atlantic. The profiles are approximately 2000 km long and are almost perpendicular to the trend of the magnetic anomalies. A bandpass filter is used to remove long‐wavelength anomalies unassociated with seafloor‐spreading processes. A filter that passes only wavelengths between 200 and 3 km produces a magnetization that approximates the positive and negative aspects of the geomagnetic reversal sequence, the positive aspects originating from normally magnetized portions of the crustal source and negative aspects originating from reversely magnetized portions of the crustal source. A 6‐km thick crust is assumed in order to produce magnetizations consistent with dredged oceanic rocks. A long‐wavelength pattern is evident in the magnetization solutions. The highest magnetizations are associated with the ridge crest (1.35–1.74 A/m). Within the western section of the study area, a sharp decrease in magnetization occurs within 100 km of the ridge crest (4.5 Ma). A minimum in magnetization is reached at 400 km from the axis or at 18 Ma (0.51–0.57 A/m). The magnetization then gradually increases out to 1100 km from the ridge axis or 50 Ma (0.91–0.93 A/m) and dramatically decreases to 1900 km or 86 Ma (0.34–0.43 A/m). Within the eastern section of the study area, a sharp decrease in magnetization occurs within the first 100 km of the ridge crest (5 Ma). A minimum in the magnetization occurs at approximately 400 km from the ridge axis or at 20 Ma (0.54–0.56 A/m). The magnetization then gradually increases out to 1200 km from the ridge axis or to 60 Ma (1.15–1.24 A/m). An overall pronounced decrease occurs to 1900 km from the ridge axis or to 95 Ma (0.36–0.40 A/m). In order to explain this behavior, the field intensity may have changed with a time scale of several millions of years. Alternatively, the higher magnetizations at 50–60 Ma could be explained by a uniform intensity of magnetization in a thicker crust.
Wide‐angle triangulation array study of simultaneous primary microseism sources
Authors: Cessaro Robert K. , Chan W. Winston
Published: February 2008

Persistent low frequency noise between 10 and 200 mHz due to storm systems is commonly observed as microseisms on seismic records from land and ocean bottom detectors. We report on an analysis of 20‐second microseisms recorded simultaneously on two land‐based long‐period arrays (the Alaskan Long Period Array and the Large Aperture Seismic Array) during November 1973. Azimuths of approach are determined by applying frequency wave‐number analysis and beam‐forming techniques to coherent bandpass‐filtered samples of the microseismic noise field recorded by the arrays. Microseismic source azimuths exhibit sufficient stability over periods of one hour to permit determination of reliable source locations by triangulation with the two arrays. Locations for two microseism noise sources associated with two separate Atlantic and Pacific pelagic storms were found simultaneously with these methods. In both cases, the microseismic noise source appears to be associated with nearshore processes. Although the gross spectral character of the microseisms displays the commonly observed primary‐ and double‐frequency microseism peaks, slight spectral differences are apparent for the two noise sources.
Infrared spectroscopy of the polymorphic series (enstatite, ilmenite, and perovskite) of MgSiO3, MgGeO3, and MnGeO3
Authors: Madon Michel , Price Geoffrey D.
Published: February 2008

The mid‐infrared spectra of the three polymorphs (clinoenstatite, ilmenite, perovskite) of MgSiO3, MgGeO3, and MnGeO3 have been recorded. The comparison between the different spectra shows that the isomorphic substitution of Si by Ge in MgSiO3 (or of Si by Ge and Mg by Mn) produces a shift of infrared bands toward lower wave numbers, the shift being larger for the substitution of larger cations. Assuming that the highest frequency in each spectrum is associated with the shortest bond (Si‐O or Ge‐O) in the corresponding structure, we find that the frequency associated with the stretching of the bond, is given by the following empirical law: ν r7/2 μ−1/5 = 3810, where ν is the stretching frequency (in cm−1), μ is the reduced mass (in grams) of the oscillator Si‐O (in silicate phases) or Ge‐O (in germanate phases), and r is the cation/oxygen distance (in angströms) in the given polyhedra. This relation can be extended to all frequencies of all the vibrational spectra studied. Such a relation, derived from harmonic oscillator theory, allows the prediction, to within 10 cm−1, of all frequencies of the germanate phases (52 bands) from those of the isomorphous silicates, or vice versa.
Spectral interpolation and downward continuation of marine magnetic anomaly data
Authors: Caress David W. , Parker Robert L.
Published: February 2008

A two‐ or three‐dimensional treatment of magnetic anomaly data generally requires that the data be interpolated onto a regular grid, especially when the analysis involves transforming the data into the Fourier domain. We present an algorithm for interpolation and downward continuation of magnetic anomaly data that works within a statistical framework. We assume that the magnetic anomaly is a realization of a random stationary field whose power spectral density (PSD) we can estimate; by using the PSD the algorithm produces an array incorporating as much of the information contained in the data as possible while avoiding the introduction of unnecessary complexity. The algorithm has the added advantage of estimating the uncertainty of every interpolated value. Downward continuation is a natural extension of the statistical algorithm. We apply our method to the interpolation of magnetic anomalies from the region around the 95.5°W Galapagos propagating rift onto a regular grid and also to the downward continuation of these data to a depth of 2200 m. We also note that the observed PSD of the Galapagos magnetic anomalies has a minimum at low wave numbers and discuss how this implies that intermediate wavelength (65 km < λ < 1500 km) magnetic anomalies are weaker than suggested by one dimensional spectral analysis of single profiles.
Prodigious submarine landslides on the Hawaiian Ridge
Authors: Moore J. G. , Clague D. A. , Holcomb R. T. , Lipman P. W. , Normark W. R. , Torresan M. E.
Published: February 2008

The extensive area covered by major submarine mass wasting deposits on or near the Hawaiian Ridge has been delimited by systematic mapping of the Hawaiian exclusive economic zone using the side‐looking sonar system GLORIA. These surveys show that slumps and debris avalanche deposits are exposed over about 100,000 km2 of the ridge and adjacent seafloor from Kauai to Hawaii, covering an area more than 5 times the land area of the islands. Some of the individual debris avalanches are more than 200 km long and about 5000 km3 in volume, ranking them among the largest on Earth. The slope failures that produce these deposits begin early in the history of individual volcanoes when they are small submarine seamounts, culminate near the end of subaerial shield building, and apparently continue long after dormancy. Consequently, landslide debris is an important element in the internal structure of the volcanoes. The dynamic behavior of the volcanoes can be modulated by slope failure, and the structural features of the landslides are related to elements of the volcanoes including rift zones and fault systems. The landslides are of two general types, slumps and debris avalanches. The slumps are slow moving, wide (up to 110 km), and thick (about 10 km) with transverse blocky ridges and steep toes. The debris avalanches are fast moving, long (up to 230 km) compared to width, and thinner (0.05–2 km); they commonly have a well‐defined amphitheater at their head and hummocky terrain in the lower part. Oceanic disturbance caused by rapid emplacement of debris avalanches may have produced high‐level wave deposits (such as the 365‐m elevation Hulopoe Gravel on Lanai) that are found on several islands. Most present‐day submarine canyons were originally carved subaerially in the upper parts of debris avalanches. Subaerial canyon cutting was apparently promoted by the recently steepened and stripped slopes of the landslide amphitheaters.
Reflectance spectra of “featureless” materials and the surface mineralogies of M‐ and E‐class asteroids
Authors: Cloutis Edward A. , Gaffey Michael J. , Smith Dorian G. W. , Lambert Richard St. J.
Published: February 2008

The 0.3‐ to 2.6‐μm reflectance spectra of meteoritic enstatite (nearly pure MgSiO3), iron meteorite metal, magnetite and amorphous carbon, and various mixtures of these materials with mafic silicates have been measured in order to search for diagnostic spectral parameters which can be used to differentiate among the different materials and to provide information on the detection limits for mafic silicates. Enstatite and metal can be discriminated on the basis of albedo and spectral slopes. Metal, magnetite, and amorphous carbon are variably effective at suppressing mafic silicate absorption bands. Metal‐silicate spectra suggest that >25 wt% olivine must be present to be spectrally resolvable, while the lower limit for orthopyroxene is ∼ 10 wt%. Magnetite‐silicate spectra indicate that magnetite is not an effective suppressor of mafic silicate absorption bands and that fine‐grained magnetite imparts a less red overall slope to mafic silicate spectra than coarse‐grained magnetite. Carbon‐silicate spectra indicate that suppression of olivine absorption bands is enhanced when the olivine is finely comminuted and is less effective at larger grain sizes. Carbon and magnetite both seem to be required to impart a blue overall slope to mafic silicate spectra and to suppress Fe2+ absorption bands. The small amount of carbon present in ordinary chondrites is probably not the cause of the flat reflectance spectra of metal‐rich ordinary chondrite separates. The observational data for a representative M‐class asteroid, (16) Psyche, are largely consistent with a fine‐grained metal‐rich surface assemblage, although an enstatite chondrite‐like assemblage cannot be ruled out. The observational data for the E‐class asteroid (44) Nysa indicate that its surface is composed of fine‐grained material similar to enstatite achondrites (aubrites), with a small amount of material comparable to the chondritic inclusions found in the Cumberland Falls aubrite.
Late Cretaceous ductile deformation, metamorphism and plutonism in the Piute Mountains, eastern Mojave Desert
Authors: Fletcher John M. , Karlstrom Karl E.
Published: February 2008

Late Cretaceous deformation in the Piute Mountains involved ductile thrusting on a network of anastomosing northeast and southwest directed shear zones. The shear zones often separate Proterozoic lithologies with strong competency contrasts, suggesting that Proterozoic crustal anisotropies were important in controlling the geometry of the shear zone network. Shear zones now divide the crust into wedge and lozenge‐shaped blocks. The Piute Mountains can be separated into two kinematic domains, each characterized by a dominant sense of shear. Thrusting in the southwest directed kinematic domain ended at 85±7 Ma, the age of the late synkinematic East Piute pluton. Thrusting in the northeast directed kinematic domain could have lasted until 74±3 Ma, the age of the Lazy Daisy pluton. Postthrusting northwest‐southeast shortening produced a second generation of upnght open folds with a northeast striking subvertical crenulation cleavage. F2 strain increases toward the south and is concentrated around the margins of the Lazy Daisy pluton. This deformation was synchronous with pluton emplacement. Late Cretaceous peak metamorphism outlasted all ductile deformation. Peak metamorphic grade increases from upper greenschist facies in the north to upper amphibolite facies as the Lazy Daisy pluton is approached. Temperature and pressure of approximately 450°C and 2.5–4.0 kbar were achieved during ductile thrusting. Temperatures of 500°‐540°C existed at the onset of upright open folding, and peak temperatures as high as 620°C near the Lazy Daisy pluton occurred after F2 upright folding. All penetrative deformation ended before 72–71 Ma, by which time the area had cooled to below 300°C (Foster et al., 1989).
Records of geomagnetic reversals from volcanic islands of French Polynesia: 1. Paleomagnetic study of a polarity transition in a lava sequence from the Island of Huahine
Authors: Roperch Pierrick , Duncan Robert A.
Published: February 2008

We report the behavior of the earth's magnetic field for a transition recorded in lavas at the volcanic island of Huahine in French Polynesia, south‐central Pacific Ocean. Detailed intermediate fields are observed that record an apparent N‐N excursion. The available K‐Ar ages indicate that most volcanic rocks of the southern part of Huahine erupted at a time close to the Kaena subchron. Gaps in the paleomagnetic sampling or a hiatus in the volcanic activity during the Kaena event may have led to the partial record of a complete polarity transition, missing the fully reversed state. Nevertheless, numerous flows have recorded intermediate fields that somewhat constrain reversal models. The low intensities of magnetization associated with the intermediate directions suggest that the transitional field has mainly low intensities. The beginning of the transition shows inclination steepening, consistent with a zonal model of geomagnetic field reversal based on an axial quadrupole. Large deviations from a north‐south planar distribution of the intermediate directions show that non axisymmetrical nondipolar fields are important in the middle of the transition.
Rock magnetic evidence for the dissolution and authigenic growth of magnetic minerals within anoxic marine sediments of the California continental borderland
Authors: Leslie Bret W. , Lund Steve P. , Hammond Douglas E.
Published: February 2008

The rock magnetic properties of marine sediments from the California continental borderland (San Pedro, Santa Catalina, and San Nicolas basins) have been studied in order to quantitatively assess the effects of sediment diagenesis on magnetic minerals. Previous studies have noted that the natural remanent magnetization in these sediments, primarily carried by detrital magnetite, decays to 10% or less of its surface value soon after deposition. This decrease is caused by magnetite dissolution related to sediment diagenesis and is unrelated to paleoclimatic variations or changes in the regional influx of detrital magnetic material. Detailed rock magnetic measurements show that shifts to softer remanent coercivity and differences in the rate and degree of magnetic intensity loss with depth can be related to the dissolution process. The shift to softer remanent coercivity is related to a coarsening of the magnetic mineral grain sizes with depth due to preferential dissolution of the finest‐grained magnetic material. The intensity decreases, which are linearly proportional to magnetite concentration decreases, indicate that dissolution occurs with rate constants ranging from 0.3 to 1.6 kyr−1. The rate constants, sulfide concentrations, and magnetite grain size estimates from the borderland are consistant with previous studies of magnetite dissolution. Our results demonstrate the importance of both sulfide and magnetite surface area in the dissolution process. Anomalous peaks in viscous remanence within the sediments suggest the authigenic growth of greigite and its subsequent transformation to pyrite.
Diagenesis in anoxic sediments from the California continental borderland and its influence on iron, sulfur, and magnetite behavior
Authors: Leslie Bret W. , Hammond Douglas E. , Berelson William M. , Lund Steve P.
Published: February 2008

Solid phase and pore water profiles of compounds containing iron and sulfur have been determined by wet chemical, magnetic, and X‐ray diffraction techniques in three California continental borderland basins. The observed profiles have been fit by simple reaction‐diffusion models in order to determine reaction rates and constrain budgets for iron and sulfur. More than 95% of the solid phase reduced sulfur is pyrite, and down core profiles are well fit by a model in which net sulfate reduction rates decrease exponentially with depth. Net sulfate reduction rates determined from models fit to solid phase reduced sulfur measurements and pore water sulfate profiles yield results that are consistent. Depth integrated sulfate reduction rates for modern sediments in San Pedro, Santa Catalina, and San Nicolas Basins are, 11.4, 6.3, and 6.3–8.8 (μmol cm‐2yr‐1), respectively. Measurements of solid phase iron species indicate that surficial sediments are enriched in easily‐reducible ferric oxyhydroxides. The enrichment is maintained by a combination of oxidation of Fe2+ diffusing upward from underlying anoxic sediments, as well as input of fresh sediment enriched in ferric oxyhydroxides. The three primary sources for iron converted to pyrite and the sequence in which they are utilized are: ferric oxyhydroxides, magnetite and other crystalline oxides, and “exchangeable” iron in phyllosilicates. The majority (50–80%) of the iron converted to pyrite is from the silicates, and budgetary calculations indicate the amount of iron released from San Pedro basin silicates agrees within 35% with the amount of magnesium removed from pore water to solid phases. Fe2+ is enriched in near‐surface pore waters because rates of dissolved iron production by oxyhydroxide reduction exceed rates of sulfate reduction and pyrite formation. At depth, pore waters are sulfidic because rates of sulfate reduction exceed rates of iron release from silicates. Sulfide produced at depth diffuses upward until it reaches sediments with available iron, causing a step‐like increase in solid phase sulfur concentration. Over 90% of the magnetite present in surficial sediments is dissolved at depth due to reaction with H2S. A model is developed to predict the depth at which magnetite dissolution should occur, based on sulfate reduction rates and the flux of ferric oxyhydroxides. The results of this model predict the onset of dissolution at depths of 5–40 cm in different basins and agree well with the observed depths of magnetite dissolution.
Remanence in authigenic magnetite: Testing the hydrocarbon‐magnetite hypothesis
Authors: Elmore R. Douglas , Crawford Lisa
Published: February 2008

Paleomagnetic, rock magnetic, petrographic, and geochemical studies of hydrocarbon‐saturated speleothems in southern Oklahoma indicate there is a relation between hydrocarbons and a chemical magnetization that resides in magnetite. The speleothems, which are composed of light and dark calcite bands, occur in caves of karst origin in the Ordovician Kindblade Formation. Vertebrate fossils interbedded with the deposits indicate they are Permian in age. The dark bands contain primary hydrocarbon‐filled fluid inclusions. The dark calcites possess over an order of magnitude stronger natural remanent magnetization than the lighter bands which do not yield stable decay during demagnetization. Alternating field (AF) and thermal demagnetization of specimens of the dark bands reveal a Permian direction of magnetization (declination = 160, inclination = 3, k = 17, α95 = 5, n = 56). The results of rock magnetic experiments, and the fact that most maximum unblocking temperatures are below 580°C, suggest that the dominant component resides in magnetite. In some specimens stable decay to 640°C suggests the presence of a weak component residing in hematite. The presence of authigenic magnetite spheres in magnetic extracts of the dark calcites supports a chemical origin for the magnetization. Shallow burial depths probably eliminate the possibility of a thermoviscous magnetization. The occurrence of primary hydrocarbon inclusions suggests that hydrocarbons seeped into the caves during precipitation of the speleothems and were trapped in the calcite crystals. The relationship between intensity of magnetization and hydrocarbon abundance leads us to propose that chemical conditions created by the hydrocarbons caused precipitation of authigenic magnetite and acquisition of the associated chemical remanence.
Inversion for crustal structure using reflections from the PASSCAL Ouachita Experiment
Authors: Lutter W. J. , Nowack R. L.
Published: February 2008

An interface inversion has been tested and applied to wide‐angle reflection data from the 1986 PASSCAL Ouachita experiment. An interface corrugation with a relief of 10 km and a width of 30 km was successfully imaged in a test of the interface inversion. Velocity‐depth curves derived for shot points 14–19 using the tail‐sum method are similar to an average one‐dimensional velocity model from forward modeling using travel time correlations from southern profiles 14–19. The similarity of these velocity‐depth curves positioned over the southern half of the PASSCAL experiment suggests that crustal structure in this region is approximately laterally homogeneous. Lower crustal velocities are not well resolved due to the recording on most profiles of only small segments of the travel time triplications. A two‐dimensional velocity structure was derived by using the average one‐dimensional velocities for the deeper crustal layers and formally inverting for depth to interfaces. The final inversion model is found to be consistent with previous refraction interpretations south of the Ouachita orogenic trend and concurrent interpretations of the PASSCAL data set. Inversion results for the central and southern portion of the PASSCAL profile indicate a depth of 10–12 km for a midcrustal layer which thins southward from approximately 10 km to about 4 km. A lower crustal layer with an average thickness of 12 km and a Moho depth of approximately 29.5 km are also determined. Interface depths are in agreement with a normal moveout stack of the PASSCAL data set. In particular, the shallowing of the Moho to a depth of 30 km over the northern 50 km of the profile matches previous interpretations of the data set and has been interpreted here and in previous studies as the location of the Paleozoic continental margin. Geophysical studies of the modern Atlantic continental passive margin provide the simplest comparison to the crustal structure derived here. The lower crustal layer found south of the shallowing of the Moho to 30 km beneath the PASSCAL profile is analogous in thickness and position to rift stage lower crust. However, other tectonic interpretations are possible given the uncertainties in velocity and lithology relationships.
Global anisotropy in the upper mantle inferred from the regionalization of phase velocities
Authors: Montagner Jean‐Paul , Tanimoto Toshiro
Published: February 2008

A data set of 2600 paths for Rayleigh waves and 2170 paths for Love waves was selected in order to resolve azimuthal anisotropy of surface waves in the period range 70–250 s. The epicenters were chosen so that the paths provide uniform spatial and azimuthal coverage of Earth. A synthetic seismogram is computed for each path and compared to the observed seismogram in order to infer the phase velocity along each path. To process such a large data set, a new tomographic technique is designed. A good correlation is found with surface tectonics in the whole period range for Love waves and at shorter periods for Rayleigh waves. The directions of maximum velocities are also well correlated with plate tectonics motions for the 2ψ azimuthal term of Rayleigh waves. When the power spectrum of heterogeneities is computed, it is found that for Love waves the even orders are usually larger than the odd orders. For Rayleigh waves the degrees 2 and 6 dominate only at long periods, suggesting a deep origin for these components.
Right‐lateral displacements and the Holocene slip rate associated with prehistoric earthquakes along the Southern Panamint Valley Fault Zone: Implications for southern Basin and Range tectonics and Coastal California deformation
Authors: Zhang Peizhen , Ellis Michael , Slemmons D. B. , Mao Fengying
Published: February 2008

The N20°W‐trending Panamint Valley fault zone is linked to the N60°W‐trending Hunter Mountain strike‐slip fault and the Saline Valley fault system, which represents one of the three major fault systems accommodating active crustal extension in the southern Great Basin. A 25 km‐long zone of fault scarps along the southern Panamint valley fault zone is recognized as the surface rupture zone associated with the most recent prehistoric earthquake. The displacement associated with the most recent event, determined through six detailed topographic maps of offset features, is 3.2±0.5 m, and a number of larger offsets, in range of 6–7 m and 12 m, are also observed. If the larger displacements represent, respectively, two and three events, each of ∼3 m, then the fault zone appears to be associated with a characteristic earthquake, which we estimate from the length of the rupture zone and the displacement to be between (Ms) 6.5 and 7.2. The Holocene slip rate is 2.36±0.79 mm/yr, is determined from the displacement of two alluvial features whose maximum age is estimated from pluvial shorelines. Assuming a characteristic earthquake model, the recurrence interval is between 860 and 2360 years. The Holocene slip rate appears to be similar to the 4 million year slip rate of 2–2.7 mm/yr (determined from the Hunter Mountain fault), which we suggest reflects the relatively constant tectonics in this region over the last 4 million years. We further speculate that this supports the San Andreas discrepancy in that the Holocene slip rate of the San Andreas fault probably represents its very‐long term (several Ma) slip rate. The total slip vector of the southern Panamint Valley fault system is oriented toward ∼N35°W, making this a predominantly strike‐slip fault. In conjunction with the N60°W orientation of the Hunter mountain strike‐slip fault, we suggest that the displacement vector for the southern Great Basin is toward the NW, consistent with results from VLBI data, rather than WNW as determined by combining VLBI and geological data. This in turn suggests that the coastal California deformation component involves, respectively, less shortening and more strike‐slip displacement perpendicular and parallel to the San Andreas fault than is currently proposed.
A finite amplitude necking model of rifting in brittle lithosphere
Authors: Lin Jian , Parmentier E. M.
Published: February 2008

We formulate a mechanical model describing the formation of rifts as finite amplitude necking of an elastic‐plastic layer overlying a fluid substrate. A perfectly plastic rheology is a continuum description of faulting in rift zones. Two important aspects of rift evolution are illustrated by this model: the evolution of the rift width as extension proceeds and the finite strain that occurs. A region at yield initially develops with a width determined by the thickness of the brittle layer, and the internal deformation within this yield zone is proportional to the topographic slope. As extension proceeds, the surface within the rift subsides, and the width of the subsiding yield zone decreases. At any stage of rifting, material in regions just outside the yield zone is deformed but no longer deforming. The width of these deformed regions increases with increasing extension. Vertical forces due to the mass deficit of the rift depression will flex the elastic layer outside the yield zone, creating flanking uplifts. The external force required to maintain active rifting increases with the amount of lithospheric stretching, indicating that rifting is a quasi‐static, stable process. Because the yield zone will revert to elastic behavior if the external force causing extension is removed, the model predicts that the rift depression and flanking uplifts will be preserved after extension stops. Our simple mechanical model demonstrates the inherent relationship among graben formation, lithospheric thinning, and rift shoulder uplift in rift zones.
Earthquake swarms on the Mid‐Atlantic Ridge: Products of magmatism or extensional tectonics?
Authors: Bergman Eric A. , Solomon Sean C.
Published: February 2008

By analogy with the microearthquake swarms which accompany terrestrial volcanic eruptions, teleseismically observed earthquake swarms along mid‐ocean ridges are commonly thought to be indicators of volcanism, particularly dike emplacement. We test this assumption by investigating the spatial and temporal patterns and other characteristics of earthquakes in 34 swarms on the Mid‐Atlantic Ridge. Resolution of the relative locations of swarm events is improved by the use of a multiple‐event relocation technique. Most swarms extend 5 to 15 km across the ridge axis and 5 to 20 km along it. None of the swarms displays the large length‐to‐width ratio and temporal migration of epicenters along axis which are characteristic of earthquake swarms in the well‐studied rift zones of Hawaii and Iceland. The relative locations, magnitudes, and source mechanisms of swarm events are most easily explained if teleseismically observable swarms are expressions of extension across a substantial portion of the median valley and perhaps into the rift mountains. The possibility that some of these swarms include events directly associated with magmatic intrusion cannot be excluded, however. Like their terrestrial counterparts, purely volcanic earthquakes on the mid‐ocean ridges probably occur dominantly at magnitudes below the teleseismic detection threshold of global seismic networks (mb < 4.5). The largest normal‐faulting earthquakes on mid‐ocean ridges and many of the teleseismically observed swarms likely occur on the faults marking the inner rift wall, where the crust first experiences significant faulting and is broken into numerous closely‐spaced normal faults. We speculate that the high degree of order in the dimensions, spacing, throws, and mechanical properties of these faults, combined with stress concentrations arising naturally from the mechanical behavior of the lithosphere at rifted, slow‐spreading ridges, may account for the common occurrence of earthquake swarms.
Origins of the 1954–1960 Lavas, Kilauea Volcano, Hawaii: Major element constraints on shallow reservoir magmatic processes
Authors: Russell James K. , Stanley Clifford R.
Published: February 2008

In the 1954–1960 period, Kilauea Volcano, Hawaii, erupted four times: summit eruptions occurred in 1954 (Halemaumau) and 1959 (Kilauea Iki) and flank eruptions occurred in the lower East rift zone in 1955 and 1960. Our analysis demonstrates that the chemical compositions of lavas from each eruption are related solely through fractionation and sorting of the observed phenocryst assemblages. We have used Pearce element ratio diagrams to make the following observations and inferences: 1) The conserved element ratio P/K for each eruption has less variance than is expected from analytical uncertainty. Thus, we cannot reject the hypothesis: Each set of lavas represents a comagmatic suite of compositions. 2) Where each of the four groups of lava analyses is plotted on a Pearce element ratio diagram with axes of Si/K and [0.5(Mg+Fe)+1.5Ca+2.75Na+0.25Al]/K, the data fit a slope of 1.0. Therefore, we cannot reject the hypothesis: The chemical variation in each data set is consistent with sorting of OL±PL±AU. 3) Each of the 4 groups of lavas has the same mean P/K ratio. As a result, we cannot reject the hypothesis: The 1954–1960 lavas originated from a single magma batch or several magmas with identical P/K values. Finally, using the most magnesian glass analysis in the Kilauea data suite (1959: SG‐05), we have modelled the differentiation path with thermodynamic‐based calculations. These calculations predict a liquid line of descent that is consistent with the path inferred from rock and glass analyses. Therefore, the simulations provide an additional test of our hypotheses and corroborate the postulated differentiation model. Additionally, these calculations suggest that differentiation took place at less than 0.3 Gpa and extended over a temperature range of between 100°C and 150°C.
Permanent uplift in magmatic systems with application to the Tharsis Region of Mars
Authors: Phillips Roger J. , Sleep Norman H. , Banerdt W. Bruce
Published: February 2008

There has been considerable debate as to the importance of permanent structural uplift in attaining the immense topographic relief of the Tharsis rise on Mars. The presence of ancient terrain high on the rise suggests uplift took place, and this idea is supported by the presence of graben and fractures in the Claritas Fossae region that are orthogonal to extensional stress directions predicted by flexural uplift models of Tharsis. We derive expressions for total elevation and crustal displacement resulting from the partial melting associated with an upper mantle magmatic system. The effect of lateral mass loss is included in the derivation, as is the difference between mass balance and isostatic balance. This “isostatic effect” is due mainly to membrane stress support. The total elevation requirement for Tharsis is rather easy to meet by the choice of source region vertical dimension, but significant upward displacement of the crust requires a special condition. While most of the buoyancy for uplift is provided by a low density residuum, the crustal extrusive load will strongly counteract this effect unless it is only a small fraction of the melt products. A lower bound estimate of the fraction of intrusives necessary for any uplift at all is about 85% of the total magmatic products at Tharsis. Thus we propose that most of the magmas associated with Tharsis evolution ended up as intrusive bodies in the crust and upper mantle.
P and S wave travel time inversions for subducting slab under the island arcs of the northwest Pacific
Authors: Zhou Hua‐Wei , Clayton Robert W.
Published: February 2008

We have observed slablike high P and S velocity anomalies around the Wadati‐Benioff zone under island arcs of the northwest Pacific through travel time tomographic inversions. Nineteen years of International Seismological Centre travel time residuals for events and stations in this large region are used. Analyses of resolution and noise show that the images are generally well resolved. The images illustrate that slab anomalies are continuous along strike in most parts of the upper mantle of the region and become contorted and generally broadened with depth. Near the bottom of the upper mantle, fingering of the slabs, including segmenting and spreading, is indicated. The fast anomalies associated with the Japan, Izu‐Bonin, and Mariana subduction zones tend to flatten to subhorizontal at depth, while downward spreading may occur under parts of the Mariana and Kurile arcs. The fast anomalies below 700 km are not in the shape of a single coherent sheet. The principal compressional axes of focal mechanisms in the region consistently follow the downdip direction of the high‐velocity slab, even when it bends to subhorizontal at depth. The depth at which compression begins to dominate the downdip stress regime in the slab apparently depends on bending of the slab and its dip. Slab fingering and intense deep seismicity probably are the consequence of the slab encountering a barrier of some form around the “670‐km” discontinuity.
Composition and isotopic constraints on the petrogenesis of alkaline arc lavas: Lihir Island, Papua New Guinea
Authors: Kennedy A. K. , Hart S. R. , Frey F. A.
Published: February 2008

The SiO2‐undersaturated lavas from Lihir island, Papua New Guinea, like most arc lavas are highly enriched in Sr, Ba, K, Rb, and Cs and depleted in Hf, Ta, Nb, and Ti relative to ocean floor basalts and oceanic island basalts. These alkali‐rich lavas have arc trace element signatures and Nd, Sr, and Pb isotopic systematics. However, they are not a product of present‐day subduction, as this volcanism has tapped mantle which was enriched by prior subduction episodes. The narrow range of Pb (206pb/204pb, 18.74–18.76) and Nd (143Nd/144Nd, 0.51297–0.51304) isotopic compositions suggest a cogenetic origin for these lavas. During the fractionation of the primitive Lihir lavas, elements normally considered incompatible (i.e., the light rare earth elements (LREE), Rb, Th, and P) have high bulk solid/melt partition coefficients (0.15–1.5). Relatively higher partition coefficients during forma‐tion of the evolved lavas produced crossing rare earth element (REE) patterns, and primitive lavas have higher incompatible elements abundances than evolved lavas. This results from (1) changes in the amount of apatite in the fractionating assemblage and (2) the abnormal partitioning of trace elements into apatite microphenocrysts which nucleate at the crystal‐liquid interface of rapidly growing clinopyroxene phenocrysts. The Lihir lavas have lower alkali, Sr, Ba, K, Rb, Cs, and LREE abundances than other Tabar‐Feni lavas. They are derived from a less enriched mantle source rather than by a higher degree of melting of a source similar to that of the other islands. The similarity of Sm/Nd ratios of these undersaturated arc lavas to those of tholeiitic and calc‐alkaline arc lavas and the moderate chondrite‐normalized La/Yb (La/Ybcn = 3–7) indicates that there has been limited enrichment of the LREE relative to the heavy REE during generation of the arc‐modified source mantle. The alkaline nature of these lavas reflects their generation, in a tensional tectonic environment, from a “fossil” arc mantle region that has undergone extreme arc enrichment of alkali and alkaline earth elements during two earlier subduction episodes.
Differential stress‐induced melt migration: An experimental approach
Authors: Cooper Reid F.
Published: February 2008

A gradient in the dilatational component of a differential state of stress will cause migration of the melt phase in a texturally (quasi)equilibrated partial melt. An experimental approach to image such deformation‐induced melt migration is presented. Two‐phase, solid‐liquid aggregate beams (prepared by a glass‐ceramic technique) having a primary crystalline phase of MgSiO3 (orthoenstatite with a limited amount of clinoenstatite intergrowths) in chemical and textural equilibrium with a sodium aluminosilicate glass are subjected to four‐point flexure; a first‐order thermodynamic analysis, based on the energy balance between grain boundaries (solid‐solid interfaces) and solid‐liquid interfaces, indicates that the melt phase flows from that side of the specimen under a compressive principal stress to the specimen side under a tensile principal stress. When the solid‐liquid aggregate is characterized by a Newtonian rheology (i.e., the deformation occurs via a solution‐precipitation‐enhanced diffusional creep mechanism), the melt migration is easily observed as a large deformation transient accompanying the flexural flow of a specimen. The melt migration is thus characterized as a completely recoverable, anelastic strain in the two‐phase system; the rheology of the partially molten beams is well modelled by
where εT is the total inelastic strain, ε0 is the total anelastic strain due to melt migration,
is the steady‐state strain rate for the two‐phase aggregate, t is time and B is a function of either the viscosity of the liquid phase or of the rheology (viscosity) of the two‐phase aggregate. In the experiments reported here, the melt migration is shown to be rate limited by the kinetics of compaction and/or dilation of the crystalline residuum. The impact of the experimental approach on compaction‐based models of melt transport and segregation is discussed.