bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of water saturation on the distribution of partial melt in the Olivine‐Pyroxene‐Plagioclase System
Authors: Fujii Naoyuki , Osamura Kazumi , Takahashi Eiichi
Published: February 2008

Various dihedral angles were measured in partially molten, synthetic peridotite (Fo60 En35 Ab2.5 (wt%)), pressure‐annealed at 1.5 GPa (15 kbar), 1400°C (water free) for 192 hours and 1250°C (water saturated) for 212 hours; the melt fractions in the two specimens were 8.2% and 9.2%, respectively. The orthopyroxene‐melt interface shows the greatest difference in dihedral angles between the water‐free and saturated samples. At textural equilibrium the melt phase exists stably at grain edges except at those edges which include two or three orthopyroxene grains in the water‐free specimen. The melt phase is stable in all corners for both the water‐free and saturated samples. The equilibrium melt phase distribution for a system of two solids with coexisting melt is characterized by a newly proposed stability criteria. We conclude that the network of melt phase through edges and corners becomes more connected in the water saturated partial melt. It is suggested that the connectivity of the melt phase through grain edges is likely to be interrupted by the presence of orthopyroxene grains in dry ultramafic rocks. The increase of the melt fraction and/or the water content in the melt tends to diminish this “pyroxene effect.”
Three‐dimensional P and S wave velocity structures of southern Peru and their tectonic implications
Authors: Cunningham Paul S. , Roecker Steven W. , Hatzfeld Denis
Published: February 2008

In 1981 an 18‐station regional array was operated in southern Peru above the area where the dip of the subducted Nazca plate changes from virtually horizontal in the northwest to about 30° in the southeast. Arrival times of compressional (P) and shear (S) waves from microearthquakes recorded by this array are used in this paper to investigate the three‐dimensional velocity structure of the crust and upper mantle in the region between the coast and the Cordillera Occidental. The results suggest a crustal thickness of about 40 km beneath the coast, increasing to about 70 km beneath the Cordillera Occidental. This change in thickness occurs abruptly in the northwest but is gradual in the southeast. The inverse correlation between the dip of the Moho and the dip of the slab suggests a broad‐scale, causal relation between the two. At the same time, free‐air gravity anomalies suggest that the steepening of the Moho in the northwest may occur only in the region under the array. Because the crust is thicker in the southeast than originally surmised, a suite of earthquakes previously thought to occur in the mantle now appears to be confined to the crust. The association of these earthquakes with a shallow dipping Moho is suggestive of ongoing crustal deformation to the west of the Andes. However, other seismological and geological observations make the role of these earthquakes in crustal deformation unclear. S wave velocities in the mantle between 70‐ and 130‐km depth above the 30° dipping slab are low, suggesting the presence of partially melted asthenosphere that may be responsible for the magmatic activity observed in southern Peru.
Observations of anomalous splitting and their interpretation in terms of aspherical structure
Authors: Ritzwoller Michael , Masters Guy , Gilbert Freeman
Published: February 2008

The problem of inverting for the aspherical structure of the earth is complicated by the nonlinear dependence of low‐frequency seismic waveforms on aspherical structure. In an attempt to overcome this obstable, we report on the application of two complementary techniques. The first, a data space technique called singlet stripping, which linearly recombines seismic recordings to estimate singlet resonance functions, has been applied to 190 International Deployment of Accelerometers and Global Digital Seismographic Network recordings from five large events. More than 290 singlets from 34 low harmonic degree multiplets appear to have been resolved. A subset of these measurements has been compared with those produced from the second technique, a nonlinear regression, which iteratively estimates coefficients which are linear functionals of aspherical structure. Both techniques agree that most multiplets are normally split, with singlet frequency distributions insignificantly different from those predicted for a rotating, hydrostatic (RH) earth model. The main result of this paper is that both techniques also agree that approximately a third of the multiplets are anomalously split, some of which span frequency bands up to 2.5 times greater than predicted for an RH model. All of the anomalously split multiplets are SKS, PKP, or PKIKP equivalent. The observation of anomalously widely split multiplets is highly robust and provides compelling evidence for the existence of deep large‐scale, nonhydrostatic aspherical structure. The inverse problem for the axisymmetric part of aspherical structure has been performed in the hope of illuminating anomalous splitting. Unless a large amount of structure in the core is included, we are unable to construct a smooth axisymmetric model which accurately predicts the splitting characteristics for the anomalous multiplets while simultaneously fitting the normally split multiplets. The location and nature of this core heterogeneity are unclear, but we find that a simple outer core structure is sufficient to give a reasonable fit to the data. There are good theoretical reasons for believing that such nonhydrostatic outer core structure is geophysically unreasonable, yet differential travel time data sensitive to core structure apparently require similar large scale heterogeneity in the core. Although this dilemma remains unresolved, spectral fitting techniques like the nonlinear regression can be applied to many more multiplets than considered here and it is not unreasonable to predict that reliable large scale aspherical models of the deep earth soon will become available.
Effects of random horizontal variations in radiogenic heat source distribution on its relationship with heat flow
Authors: Vasseur G. , Singh R. N.
Published: February 2008

Lateral conduction resulting from variations in the radiogenic heat generation is known to modify the heat flow‐heat generation relation and to bias the evaluation of thickness of radiogenic heat sources. In this paper a statistical approach is used to account for a random horizontal variation in radiogenic heat sources, whereas their vertical variation is simulated by simple models. Relationships between spectral densities and autocorrelation functions of heat generation and heat flow are obtained. When the horizontal scale length of heat generation distribution is not much larger than the vertical one, lateral heat conduction modifies drastically the statistical properties of heat flow distribution, and the apparent thickness obtained from the heat flow‐heat generation plot is highly reduced. The procedure is applied to a set of actual heat flow and heat generation data recorded over New Hampshire, U.S.A. Heat production data and especially heat flow data are affected by different types of noise and errors which can be analyzed through their autocorrelation functions. The depth scale of heat generation deduced empirically does not seem to have much relation with any geophysical dimension.
Theory of pressure dependent resistivity in crystalline rocks
Authors: Johnson David Linton , Manning Howard J.
Published: February 2008

We calculate the changes in the electrical conductivity of crystalline rocks due to the closure of cracks as pressure is applied. We have modeled the system as a random array of pore conductors, crack conductors, and nonconducting bonds on a network. There are several different ways of doing this; the inputs to these theories are the high‐pressure behavior of the conductivity (where the cracks are presumed to be closed and only the pores are conducting) and the amount of crack porosity, Φc(P), which is derived from the dilatancy data. In all cases where there is a clear demarcation between a region of conductivity dominated by crack closure and that due to residual porosity, we have been able to successfully model this dependence with one free parameter. The conclusion is that the pores and cracks are arrayed on a lattice having an average coordination number in the range 2 < Z < 2.5 and that at random, they each close completely as pressure is increased; our operational definition of coordination numbers is quite different from that deduced by visual inspection of micrographs. The free parameter is the occupancy of nonconducting bonds, expressed as a fraction of the maximum available number of such bonds. Other plausible models based on other ways of distributing crack and pore porosities are strictly ruled out for all values of the relevant parameters.
Geochemical study on volcanic gases at Sakurajima Volcano, Japan
Authors: Hirabayashi Jun‐ichi , Ossaka Joyo , Ozawa Takejiro
Published: February 2008

The chemical composition of volcanic gases from the summit crater of Sakurajima Volcano was observed by a variety of gas collection techniques and analytical methods, and the H2 content in gas from a hot spring was continuously measured by an automatic gas chromatograph. Increases in the molar ratio of HCl to SO2 were found to coincide with, and sometimes to precede, rises in volcanic activity. An increase in H2 content of the hot spring gas was also found to precede a period of increased volcanic activity.
Chemistry of gases associated with geothermal activity and volcanism in Iceland: A review
Authors: Arnórsson Stefán
Published: February 2008

The concentrations of CO2, H2S, and H2 in geothermal reservoir fluids in Iceland are fixed by equilibria with alteration minerals. CH4 concentrations are typically very low, and gas‐gas equilibria involving CO2, H2, and CH4 are generally not closely approached. In some geothermal systems the N2 is atmospheric only, but a magmatic N2 contribution is significant in others. N2/Ar ratios lie mostly between that of the atmosphere (85) and the N2/Ar ratio in cold water in contact with the atmosphere (37). Fluids with a strong magmatic N2 contribution have N2/Ar ratios of 40–60. During several volcanic eruptions in Iceland, magmatic gases have emerged at the surface beyond the eruptive vents and mixed with surrounding groundwaters. CO and H2 can be detected closest to the vent, but only CO2 is detected at greater distances. During the present volcanic episode at Krafla, fugitive constituents have been transported into the geothermal reservoir from the underlying magma chamber. Probably most of the carbon and sulphur in altered rocks of high‐temperature geothermal systems have been derived from degassing magma bodies.
Gas geochemistry applied to earthquake prediction: An overview
Authors: King Chi‐Yu
Published: February 2008

Terrestrial gases in groundwater and soil air have been extensively studied in recent years in seismically active areas, especially in USSR, China, Japan, and United States, in search of premonitory changes that might be useful for earthquake prediction. Concentrations of radon, helium, hydrogen, mercury, carbon dioxide, and several other volatiles have been found generally to be anomalously high along active faults, suggesting that the faults may be paths of least resistance for the terrestrial gases generated or stored in the earth to escape to the atmosphere. Temporal gas concentration changes with durations of a few hours to many months have been observed before many large earthquakes at a relatively small number of favorably situated stations at epicentral distances of up to several hundreds of kilometers. These “sensitive” stations are generally located along active faults, especially at intersections or bends of faults, or some other structurally weak zones, possibly because of tectonic strain concentration at such places. However, gas concentrations, especially those measured at or near the ground surface, may also be significantly affected by meteorological, hydrological, and other nontectonic changes in the environment. The nontectonically induced variations must be carefully eliminated or recognized in the search of true earthquake‐related anomalies.
Magnetization modeling in the north and equatorial Atlantic Ocean using MAGSAT data
Authors: Hayling K. L. , Harrison C. G. A.
Published: February 2008

We have analysed the shorter wavelength component of the magnetic field observed by Magsat over the North and Equatorial Atlantic Ocean, with the idea of trying to determine the possible sources of this component of the field. In most cases, inversion techniques produce both normally and reversely magnetized sources, and we show that this is not appropriate for the possible sources in the oceanic crust, which at the wavelengths observed by Magsat need to be entirely normally magnetized or with a magnetization of zero. In order to do this, we have applied an annihilator to the solutions produced by our inversion technique. We show how the annihilator that we use is convenient for matching up magnetization solutions for areas that have been inverted separately. One problem with the inversion technique is that the best spacing of the sources can only be found by trial and error. We show that much of the final variation in source spacing is produced by a latitudinal effect, which seems to require sources spaced further apart the lower the latitude. This may be due to contamination of the anomalous field by noise, which might be expected to be worse at lower latitudes. We have also examined the possibility that much of the magnetic source in the oceanic crust is due to induced magnetization. The values of susceptibility necessary are much higher than those measured directly on samples taken from the ocean basins. Remanent magnetization is a more likely candidate, but the values, especially after application of the annihilator, are still large compared with observed values. Only considerable thickening of the crust, combined with remanent magnetization, can explain the high intensities of magnetization that we have modeled. Viscous magnetization is a possible source for the high intensities of magnetization of our model. One problem with an explanation of the field from crustal sources is that in many cases there is little correlation with the geological history of the region. An alternative is that noncrustal sources contribute to the anomalous magnetic field derived from Magsat data. This field, derived by subtracting degrees of harmonic 1 through 13 from a spherical harmonic model, may be contaminated with core field signals, from leakage of high‐amplitude short wavelength anomalies into longer wavelengths, and by external fields.
Geodetic measurement of deformation in the central Mojave Desert, California
Authors: Sauber Jeanne , Thatcher Wayne , Solomon Sean C.
Published: February 2008

Data from triangulation and trilateration surveys made during 1934–1982 are used to calculate shear strain rates in the central Mojave Desert of California. For the region between the Helendale and Camp Rock faults the shear strain rate was determined to be 0.16 ± 0.03 μstrain/yr, with maximum right‐lateral shear strain occurring on a plane oriented N41°W ± 5°. If we assume that this deformation is due to right‐lateral motion across the northwest trending local faults, the average shear straining corresponds to a relative displacement of 6.7 ± 1.3 mm/yr across this portion of the network, accounting for ∼ 12% of the predicted 56 mm/yr of relative motion between the North American and Pacific plates. From the Camp Rock fault eastward across the network there is a transition from significant to very low strain rates. Examination of nine focal mechanisms and their relation to the local geology and the strain data suggests that most of the long‐term displacement occurs on the major northwest trending faults oriented nearly along the direction of relative motion between the North American and Pacific plates. Secondary faulting, controlled by a Coulomb‐Anderson failure mechanism or by slip on preexisting faults, can account for the occurrence of earthquakes on faults of other orientations.
Single‐stage exposure history of lunar highland breccias 60018, 67435, and 67455
Authors: Eugster O. , Niedermann S.
Published: February 2008

Lunar breccias 60018, 67435, and 67455 experienced a single‐stage exposure to cosmic rays. These materials had been shielded from cosmic rays when 60018 was excavated by the South Ray cratering event about 2 m.y. ago and rocks 67435 and 67455 by the North Ray impact about 50 m.y. ago. After excavation, the shielding depth of the investigated samples was less than 40 g/cm2, in accord with the location at the time of collection. The gas retention ages, as calculated from 136Xe produced by the spontaneous fission of 238U and 244Pu, show that the breccia minerals solidified and cooled 3.6–4.1 b.y. ago. The breccias contain measurable amounts of trapped noble gases, which, however, are two to four orders of magnitude lower than in typical lunar soil.
An experimental investigation of agglutinate melting mechanisms: Shocked mixtures of Apollo 11 and 16 soils
Authors: Simon S. B. , Papike J. J. , Hörz F. , See T. H.
Published: February 2008

Mixtures of chemically contrasting lunar soils (from A‐11 and A‐16) have been shocked at pressures ranging from 18.2–62.0 GPa (182–620 Kb). Other than the generation of impact melts, modal and textural changes caused by shock include destruction of pore space and fused soil clasts and conversion of plagioclase to maskelynite. The loss of the fused soil component in these runs indicates that low agglutinate contents in shocked and/or compacted regolith breccias cannot be considered by themselves to be evidence of formation from immature regolith. From the petrographic and chemical data it appears that the impact glass formed mainly from the fine fraction and the fused soil component in the target, with relatively minor contributions from the other coarse clasts. The impact glasses exhibit the same chemical enrichments and depletions as their corresponding fine fractions and plot on or near a mixing line between the bulk and fine fraction of the soil in which they were formed. From this as well as several other studies it appears that the fusion of the finest fraction (F3) model is valid, and that it accurately predicts the chemical systematics of impact glass formed from lunar soil. In addition, fusion of agglutinates present in the target soil is an important process.
Morphology of large valleys on Hawaii: Evidence for groundwater sapping and comparisons with Martian valleys
Authors: Kochel R. Craig , Piper Jonathan F.
Published: February 2008

Runoff and groundwater sapping processes are important, to varying degrees, in the initiation and evolution of terrestrial and Martian valleys. The resulting channel morphology and valley morphometry appear to be distinctive, depending upon which of the two processes are dominant. Our field reconnaissance and morphometric studies of runoff and sapping valleys on the slopes of Hawaii and Molokai in Hawaii indicate that valley types can be distinguished using remotely sensed data similar to the Viking orbital imagery of Mars. Principal components analysis of morphometric data clearly separate runoff valleys from sapping valleys. Hawaiian sapping valleys, like their possible Martian analogs, are characterized by: (1) steep valley walls and flat floors, (2) amphitheater heads, (3) low drainage density, (4) paucity of downstream tributaries, (5) low frequency of up‐dip tributaries, and (6) strong evidence of structural and stratigraphic control on valley patterns.

The large Hawaiian sapping valleys are fed by groundwater from high‐level dike‐impounded aquifers breached by channel incision. Flume experiments in weakly cemented sand have produced similar channels by groundwater sapping. In addition, we have mimicked the sequence of valley evolution observed in Hawaii and have shown the importance of groundwater piracy as a major process for widening valley heads. These experiments are useful in testing fundamental ideas concerning how sapping valleys evolve in response to variable structure and stratigraphy. They also provide a model for developing geomorphic criteria useful in the recognition of landforms generated by groundwater sapping.
Lava flows on Mars: Analysis of small surface features and comparisons with terrestrial analogs
Authors: Theilig E. , Greeley R.
Published: February 2008

Small‐scale surface features on lava flows provide information on the eruption parameters and the volcanic history of the flows in which they occur. High‐resolution images of four sites on Mars reveal lava flows with a different type or combination of features including pressure ridges, tumuli, festoon ridges, and ring ridges. Pressure ridges, tumuli, and pressure plateaus are spaced irregularly and tend to be oriented parallel to flow. They are formed by inflation of small flow lobes and are indicative of compound flows emplaced by sporadic eruptions with low effusion rates. Festoon ridges are oriented transverse to flow direction, mimic the flow margin, and have a regular spacing. Using a model by Fink and Fletcher (1978) and Fink (1980 a) interior viscosity values of approximately 108 Pa s were calculated for festoon ridges tens to hundreds of meters apart and a few meters high on two basalt flows in Iceland and a flow on Mars. This viscosity is comparable to results from similar studies of more silicic flows and suggests that high viscosities are necessary to form large festoon ridges. By analogy to the Icelandic flows, Martian flows containing festoon ridges are considered to be large, single unit basalt flows emplaced through flood‐style eruptions. High viscosity of the basaltic flows may result from cooling, a high crystal content, or low effusion temperatures. Disruption of the surface of some of these large sheet flows possibly by lava‐water interaction is suggested by the presence of ring ridges and rugged terrain consisting of mesa‐like mounds and channel‐like troughs.
Strict Bounds on Seismic Velocity in the Spherical Earth
Authors: Stark Philip B. , Parker Robert L. , Masters G. , Orcutt John A.
Published: February 2008

We address the inverse problem of finding the smallest envelope containing all velocity profiles consistent with a finite set of imprecise τ(p) data from a spherical earth. Traditionally, the problem has been attacked after mapping the data relations into relations on an equivalent flat earth. Of the two contemporary direct methods for finding bounds on velocities in the flat earth consistent with uncertain τ(p) data, a nonlinear (NL) approach descended from the Herglotz‐Wiechert inversion and a linear programming (LP) approach, only NL has been used to solve the spherical earth problem. On the basis of the finite collection of τ(p) measurements alone, NL produces an envelope that is too narrow: there are numerous physically acceptable models that satisfy the data and violate the NL bounds, primarily because the NL method requires continuous functions as bounds on τ(p) and thus data must be fabricated between measured values by some sort of interpolation. We use the alternative LP approach, which does not require interpolation, to place optimal bounds on the velocity in the core. The resulting velocity corridor is disappointingly wide, and we therefore seek reasonable physical assumptions about the earth to reduce the range of permissible models. We argue from thermodynamic relations that P wave velocity decreases with distance from the earth's center within the outer core and quite probably within the inner core and lower mantle. We also show that the second derivative of velocity with respect to radius is probably not positive in the core. The first radial derivative constraint is readily incorporated into LP. The second derivative constraint is nonlinear and cannot be implemented exactly with LP; however, geometrical arguments enable us to apply a weak form of the constraint without any additional computation. LP inversions of core τ(p) data using the first radial derivative constraint give new, extremely tight bounds on the P wave velocity in the core. The weak second derivative constraint improves them slightly.
Seismic history and seismotectonics of the Sunda Arc
Authors: Newcomb K. R. , McCann W. R.
Published: February 2008

Historic records of the last 300 years reveal two great interplate earthquakes (1833, MW = 8¾ 1861, MW = 8¼–8½), which ruptured major segments of the Sumatra fore arc in western Indonesia; a significant percentage of interplate slip along this portion of the plate boundary appears to occur seismically. The ends of these rupture zones are coincident with clusters of large (Ms ≥ 7) and moderate (6 ≤ Ms ≤ 7) shocks and with heterogeneities in the plate interface as inferred from geologic and geophysical data. The northern extent of the 1833 rupture zone is coincident with a group of moderate and large earthquakes, a structural arch in the fore arc, and a fracture zone on the subducted plate. The southern portion is coincident with a very prominent cluster and the along‐strike extension of a sediment‐filled trough on the subducted plate. Prior to this study, Sumatra was characterized as relatively aseismic as inferred from the lack of great earthquakes in the instrumental record of this century. Java and the Lesser Sunda Islands had major (Ms ≥ 6) earthquakes in the historic record, but none are of the same magnitude as the great events near Sumatra. There is no clear evidence that shocks in the Java fore arc were interplate events, yet geophysical data indicate recent uplift of the trench slope break and a change in strike of the trench axis where the northernmost flank of the Roo Rise, a prominent bathymetrie high, appears to be interacting with the fore arc. This feature was formed at a ridge crest and is more buoyant, and thus more difficult to subduct, than the surrounding seafloor. There is no increase in seismicity associated with this tectonism, however, that may indicate a lack of contact between the crystalline portions of the plates at this time. We infer that a majority of slip on the plate interface near Java occurs aseismically. The variation in occurrence of great interplate earthquakes along the Sunda Arc may be interpreted in terms of a model that attributes variation in interplate coupling to the age of the subducted lithosphere. Great interplate earthquakes occur near Sumatra, where the age of the youngest crust is 46 Ma. The characteristics of the Sunda Arc, and analogies with Pacific arcs, imply that the entire length of Sumatra has the potential to produce great thrust earthquakes; its seismic potential should be considered high (as there has been no recurrence of the great events of 1833 and 1861). In contrast, no such events have been reported off Java and the Lesser Sunda Islands, where the oldest crust is 152 Ma. The plate interface near Java and the Lesser Sunda Islands should be considered to have low seismic potential.
Study of the high‐frequency spectrum of the anomalous gravity potential
Authors: Vassiliou A. A. , Schwarz K. P.
Published: February 2008

The analysis of the high‐frequency spectrum of the anomalous gravity potential is important for a number of current and emerging technologies in geodesy and geophysics. Among them is the precise determination of deflections of the vertical by inertial techniques, the development of data processing methods in airborne gradiometry, and the signal extraction methods for airborne gravimetry. The spectral information required can be obtained by analyzing dense gravity data by fast Fourier transform techniques. An attempt has been made to use available data files in Canada for this purpose. The data consisted of a rather uniform set of (5 arc min × 5 arc min) point anomalies in most of the continental areas of Canada and four sets of point gravity anomalies with an average data spacing of about 1 km in areas of limited extent. The analysis showed that the gravity spectrum has to be resolved to about degree and order 7000, i.e., to a point spacing of about 3 km, to resolve the gravity anomalies to 1 mGal. Thus a similar data spacing would be required in airborne gradiometry and gravimetry in order to get an equivalent resolution. The derived spectrum was used to determine suitable degree variance models and covariance functions for the above applications. The spectrum estimated from the 5 arc min × 5 arc min gravity anomalies covering the nonmountainous areas of Canada indicates that the degree variances of the anomalous potential decay as ℓ−3.6. This means that a number of currently used covariance models do not agree with the results of this analysis. A discussion on the choice of a more adequate model concludes the paper.
Usefulness of weak bands in midinfrared remote sensing of particulate planetary surfaces
Authors: Salisbury John W. , Hapke Bruce , Eastes John W.
Published: February 2008

Midinfrared (2.5–25 μm) reflectance spectra of minerals are often used to predict emittance qualitatively. These spectra display weak overtone and combination tone bands, which may be as diagnostic of composition as the strong fundamental molecular vibration bands usually considered for remote sensing applications, but which have been widely ignored. However, unlike the strong bands, the contrast of weak bands relative to the continuum in the midinfrared usually does not decrease with decreasing particle size, but typically increases. To illustrate this behavior, transmittance and reflectance spectra of calcite and quartz are presented for the wavelength range from 4000 to 400 cm−1 (2.5–25 μm). It is the purpose of this paper to point out that these weak bands are potentially useful for compositional remote sensing of particulate planetary surfaces; this will require completion of supporting laboratory studies to document the occurrence of weak bands.
A new covariance model for inertial gravimetry and gradiometry
Authors: Forsberg René
Published: February 2008

A self‐consistent covariance model for the earth's anomalous gravity field is presented within the framework of the planar approximation. The model features simple, closed formulas for autocovariances and cross covariances of geoid undulations, gravity anomalies, deflections of the vertical, and second‐order gradients, both at the reference plane and aloft. Furthermore the main spectral decay of the model gravity power spectral density corresponds closely to Kaula's rule, thus yielding good fits to actual gravity field spectral characteristics. The outlined model may be viewed as the planar equivalent to the spherical Tscherning‐Rapp model. The analytical model is characterized by three free parameters: the gravity anomaly variance, a “shallow” depth parameter, and a “compensating” depth. These parameters act as scale factor, high‐frequency attenuation, and low‐frequency attenuation, respectively. The shallow depth parameter corresponds to twice the Bjerhammer sphere depth of spherical harmonic analysis, while the compensating depth is introduced as an arbitrary mathematical convenience, necessary to obtain finite values for gravity and geoid variance.
Magnitude‐frequency relation for small earthquakes: A clue to the origin of ƒmaxof large earthquakes
Authors: Aki Keiiti
Published: February 2008

In order to test the hyothesis that the ƒmaxof large earthquakes is due to the source effect and is causally related to the corner frequency of small earthquakes when it becomes constant below magnitude about 3, we constructed the frequency‐magnitude relation for earthquakes with magnitude range from −1/3 to 4 recorded at a borehole seismograph station operated by the University of Southern California at Baldwin Hills in the middle of the Newport‐Inglewood fault. A procedure was developed for an accurate estimation of moment magnitude from the measurement of coda amplitude as a function of lapse time. The cumulative number of earthquakes per unit time period and area was estimated by finding the detection limit forts‐pfor each magnitude. We found a clear depature of the observed log frequency‐magnitude relation forM< 3 from the linear relation extrapolated from the data forM> 3. The observed frequency of earthquakes withM≃ 0.5 is almost 10 times less than that expected from the extrapolation of empirical relation for earthquakes with magnitudes larger than 3. We conclude that the observed frequency‐magnitude relation departs from self‐similarity for earthquakes with magnitude smaller than about 3. This departure coincides with the widely observed departure of the moment‐corner frequency relation from self‐similarity. These observations are consistent with the interpretation of ƒmaxof large earthquakes in terms of the slip‐weakening instability model in which the critical slip may be a few tenths of a meter to a few meters and the cohesive zone size may be a few hundred meters to a few kilometers.
Application of an inhomogeneous stress (patch) model to complex subduction zone earthquakes: A discrete interaction matrix approach
Authors: Rundle John B. , Kanamori Hiroo
Published: February 2008

In recent years it has been recognized that the level of shear and normal stress along a fault can vary; thus the stress is spatially and temporally inhomogeneous. Moreover, it has also been suspected that faults might interact in some way, with the result that a variety of earthquake magnitudes might be produced along a given length of fault at varying times. In order to explore these ideas we have developed a quantitative formalism, which we call the interaction matrix method, to express the influence of one fault upon another. This matrix is calculated by use of the energy change for a system of interacting cracks or faults and therefore gives energy‐consistent results. Specifically, the interaction matrix relates the area‐averaged stress on the fault segment to the area‐averaged slip state on all the other fault segments in the system. Since any fault can be subdivided into an arbitrary number of fault segments, the interaction matrix can have arbitrary dimension; in fact, the continuum limit is recovered as the dimension of the matrix approaches infinity. We combine this matrix method with a segmentation, or “patch,” model for earthquakes, in which each discrete segment of a fault has the same coseismic stress change (defined as the difference between the driving stress at which healing occurs minus the driving stress at which sliding starts) each time it slips. We show that slip on a patch during an earthquake can vary substantially, depending on how it interacts with other nearby patches. In this model it is quite possible for the spatial distribution of stress on the fault following an event to be again in a spatially inhomogeneous state, rather than in a uniform state, as is often assumed. Hence the seismic moment produced by an earthquake on a given set of patches can vary substantially, depending on the sequence of sliding and healing on the different patches. To apply these ideas, we devised a means to calculate the interaction matrix elements and used them to quantitatively examine earthquake sequences off the Colombia‐Ecuador coast and in the Nankai Trough near Japan.
Geodetic measurements near Parkfield, California, 1959–1984
Authors: King N. E. , Segall P. , Prescott W.
Published: February 2008

Trilateration measurements have been made by various agencies since 1959 near Parkfield, California, in the transition zone between the central creeping section and the southern locked section of the San Andreas fault. Within the transition zone there are several creep meters and alignment arrays, four small‐aperture trilateration networks, and a broad trilateration network extending 70 km southwest from the San Andreas fault to the Pacific coast. Line length changes observed after the 1966 ML = 5.6 Parkfield earthquake indicate a geodetic moment of 4.5 ± 1.0 × 1025 dyn cm. The data are consistent with either 91 ± 8 cm slip on a rupture zone which is 30 km long and extends from 3 to 8 km below the surface or 59 ± 5 cm on a 30‐km‐long zone 2–10 km deep. This moment estimate includes some postseismic slip and is a factor of 2–4 greater than the moment determined from seismic surface waves. The post‐1966 data set is well suited to discriminate between the effects of surface and deep slip. Surface slip rates determined by creep meters, alinement arrays, and small‐aperture trilateration networks decrease from 29 mm/yr at Slack Canyon to zero south of Highway 46; a gradient of roughly −0.48 ± 0.01 mm/yr/km. Deformation of a subset of the trilateration network between Cholame and San Luis Obispo is consistent with shallow slip of 2.9 ± 1.2 mm/yr from the surface to a depth of 16 km and 33.4 ± 5.5 mm/yr below 16 km. The trilateration data alone do not uniquely constrain both the transition depth from shallow to deep slip and the deep slip rate. A transition depth near 16 km is preferred because it provides the best fit to the data and because the predicted slip rate is consistent with independent estimates of the total slip rate across the San Andreas fault. This simple two‐dimensional model neglects along‐strike variations in the deformation. The data show no evidence for significant strike‐slip deformation across faults between the San Andreas and the Pacific coast.
The Tangshan aftershock sequence
Authors: Shedlock Kaye M. , Baranowski Jean , Weiwen Xiao , Liang Hu Xin
Published: February 2008

Aftershocks of the Tangshan, China, earthquake sequence occurred on strike‐slip, normal, and thrust faults in an intensely faulted region. The generally northeast trending aftershock zone outlines an area 150 km long and 30 km wide, except for a northwest‐southeast trend of aftershocks which is about 60 km wide. The main shock fault plane inferred from the locations of the aftershocks varies in strike from N30°E at the southern end to N50°E at the northeastern end, with the change in strike occurring near the epicenter of the main shock. Fault plane solutions of the aftershocks indicate that strike‐slip faulting occurred along the Tangshan fault and thrust faulting occurred on smaller, transverse faults south and west of the Tangshan fault. Although faulting during the main shock included right‐lateral, strike‐slip, and thrust movement, perturbations to the prevailing stress field due to slip during the main shock and the configuration of existing faults encouraged normal faulting in the northeast part of the zone. The largest aftershock occurred about 45 km northeast of the main shock and was associated with normal faulting at shallow depths. Two other large aftershocks, both associated with large strike‐slip components, occurred southwest of the main shock. Fault plane solutions and P wave and S wave forms of these two aftershocks indicate primarily left‐lateral motion along a northwest striking fault plane. The aftershocks range in depth from near the surface to about 20 km. Fault plane solutions of 61 of the aftershocks indicate that the overall deformation consists of north‐south extension accommodated by both normal and strike‐slip faults such that some east‐west crustal shortening and possibly some crustal thinning took place.
Siderophile elements in planetary mantles and the origin of the Moon
Authors: Drake Michael J.
Published: February 2008

The origin of the moon is examined in the context of theories of planetary accretion and of siderophile element abundances inferred for the upper mantles of the earth, moon, and shergottite parent body (SPB=Mars?). The lunar origin hypotheses examined are collisional ejection in a giant impact, and coaccretion from a circumterrestrial disk of metal‐depleted material.

If the moon originated in a giant impact, the impactor must have contained free metal and was most probably differentiated with siderophile elements segregated into a core, in order to account for the low absolute and generally nonchondritic relative abundances of moderately and noble siderophile elements in the lunar mantle. If impactor metal was accreted to the earth's upper mantle during the impact and if it perfectly scavenged siderophile elements from the upper mantle into the core, then it is necessary to postulate a late‐stage chondritic “veneer” in order to account for the high absolute and chondritic relative abundances of noble siderophile elements in the present‐day upper mantle of the earth. These elements are less abundant in the mantles of the moon and SPB than in the earth, and they may not be present in chondritic ratios. If they are present in chondritic ratios (the scatter in noble siderophile element data permits this possibility), they are present at approximately 10−3 and 10−4 × CI concentrations in the SPB and moon, respectively. In either case, the noble siderophile elements present a problem for the chondritic “veneer” version of the collisional ejection hypothesis because they are not present in any reservoir in the moon at the required abundance levels.

If the impactor metal that was accreted to the earth imperfectly scavenged siderophile elements from the earth's mantle, it is not necessary to postulate a late stage “veneer.” However, the giant impact would substantially melt or vaporize the earth's mantle, yet there is no evidence in samples derived from the upper mantle for large‐scale vertical compositional zoning that might be expected to result from crystallization in a gravity field, by analogy with the moon and with terrestrial layered intrusions. Mantle nodules from all over the world are homogeneous with respect to compatible siderophile elements such as Ni, Co, and Ir at the hand specimen scale.

Coaccretion hypotheses of lunar origin are inherently less testable using siderophile elements than collisional ejection hypotheses because abundances of these elements in the mantles of the earth and SPB are less useful as constraints. At present, none of the hypotheses of lunar origin satisfactorily accounts for siderophile element concentrations in the mantles of the earth, moon, and SPB.
Effects of elevation and ridged plains thicknesses on Martian crater ejecta morphology
Authors: Horner V. M. , Greeley R.
Published: February 2008

Relationships between the distribution of Martian crater ejecta morphologies and both elevation and ridged plains thicknesses have been investigated. We observed a slight correlation between elevation and the distribution of ejecta types for the ridged plains, but not for regions of the dissected cratered plateau unit. A stronger correlation was identified between the thicknesses of Lunae and Hesperia Plana—two major occurrences of the ridged plains—and the complexity of ejecta morphologies. For a given crater size range, the complexity of the ejecta increases with decreasing thicknesses of the ridged plains. Because more of the lower unit is excavated where the upper layer is thin, we conclude that some property of the lower layer increases ejecta complexity. One possible explanation is that the lower layer may fracture to smaller ejecta sizes during excavation; thus relatively more ejecta can be entrained by the Martian atmosphere in areas where the ridged plains are thin. Another possibility is that a greater volatile content within the lower layer may result in increased fluidization both by vapor expansion during impact, resulting in increased brecciation within the lower layer, and by the incorporation of volatiles within the ejecta. The slight relationship between elevation and the distribution of ejecta types probably results from a slight correlation between elevation and ridged plains thicknesses within Lunae Planum. This effect appears significant within our data for the ridged plains because almost half of our data were obtained in this region.
On The spectral reflectance properties of materials exposed at the Viking landing sites
Authors: Guinness Edward A. , Arvidson Raymond E. , Dale‐Bannister Mary A. , Singer Robert B. , Bruckenthal Eileen A.
Published: February 2008

Bidirectional reflectances, calibrated to within 10% uncertainties, for blue (0.40–0.52 μm), green (0.50–0.59 μm), and red (0.60–0.74 μm) channels were determined for 31 block and soil exposures imaged by the Viking Lander Cameras. Reflectances were corrected for atmospheric attenuation by using optical depth measurements obtained by the Lander Cameras and for skylight illumination by subtracting the brightness values of adjacent shadowed areas. Reflectances for two facets on each of two large blocks were computed for a common geometry by determining facet orientations from high resolution Lander stereo images and by fitting the Hapke‐Irvine photometric function to reflectances estimated for several geometries. Results indicate that individual facets on the same block have different optical properties. The darkest, grayest block facets have properties similar to laboratory reflectance spectra of mafic rocks that are thinly coated with ferric iron‐rich palagonite. Most block surfaces have reflectances consistent with thin to optically thick covers of palagonitic material. Limited exposures of soil have spectra that are also similar to palagonitic material. The majority of the soils sampled and a few block surfaces have spectra with steeper slopes and greater spectral curvatures than the palagonite analog, consistent with greater degree of ferric iron crystallinity. The soils could be an intimate mixture of palagonitic material and other materials not seen in end member form, but, if so, these other materials would need an even greater slope and curvature than observed soils. The differences in reflectance properties for common block surfaces compared to common soils imply that local weathering of blocks has not contributed significantly to the soil exposed at the landing sites.
Barred olivine chondrules in ordinary chondrites
Authors: Weisberg Michael K.
Published: February 2008

Barred olivine chondrules were studied in H‐L‐LL3‐6 chondrites. No important differences were found between barred olivine chondrules in H3, L3, and LL3 chondrites. However, comparison of type 3 barred olivine chondrules with all chondrule types in H‐L‐LL3 ordinary chondrites show that barred olivine chondrules are enriched in FeO, Al2O3, Na2O, and K2O and are depleted in SiO2 and MgO. Mineral abundances of barred olivine chondrules show that they are enriched in the feldspathic component compared to most chondrules. Pyroxene is rarely absent and is sometimes a major component. Pyroxene‐rich barred olivine chondrules in type 3 chondrites correlate negatively with feldspathic glass‐rich barred olivine chondrules, indicating that pyroxene formed at the expense of olivine bars. Barred olivine chondrules in ordinary chondrites decrease systematically in Cr2O3 and TiO2 with increasing petrologic type. FeO, however, shows a systematic decrease with petrologic type in H chondrites, no change in L chondrites, and a systematic increase in LL chondrites. The conclusions of this study are: (1) Barred olivine chondrules are mainly intermediate in major element composition between the abundant Fe‐Mg chondrules and the rare Al‐rich chondrules. (2) Systematic compositional changes in barred olivine chondrules from type 3–6 in H‐L‐LL chondrites may be the result of primary differences. (3) Precursor components that melted to form barred olivine chondrules were enriched in olivine‐rich and volatile (feldspar and feldspathoid‐rich) components. The alkalies may have been condensed where alumina was already available.