bluuebunny/crossref_metadata_2025 huggingface dataset:
The variability in seismic structure of layer 2 near the East Pacific Rise at 12°N
Authors: Purdy G. M.
Published: February 2008

Seismic refraction data collected by using a large air gun source and oceanbottom hydrophone receivers show that within two areas on the flanks of the East Pacific Rise the structure of the uppermost 2 km of the crust is laterally homogeneous. The two areas are 20 and 40 km in extent on 0.5 and 4 m.y. old crust, respectively, and are separated laterally by 110 km. A small but significant difference in structure exists between the two areas, the younger crust exhibiting the higher velocities. Ray tracing through laterally heterogenous models of the shallow crust, the careful calculation of corrections for seafloor topography, and the objective comparison of data sets by using the slant stacking approach suggest that the established apparent variability in shallow crustal structure may in part be an artifact of experimental and interpretative techniques. We propose a reasonable model for shallow crustal structure in which the isovelocity contours parallel the seafloor topography: this permits determination of lateral homogeneity of the velocity gradient in the shallow crust over the subdued topography of the East Pacific Rise, or in the deeper crust in general, where vertical velocity gradients are sufficiently small that topography has negligible effect on the velocity field.
Adjustment, interpolation, and smoothing of GEOS 3 altimeter data
Authors: Horai Ki‐iti
Published: February 2008

A rapid computational scheme has been developed to construct a local or regional murine geoid by adjusting the GEOS 3 altimeter data. The principle of adjustment is to remove the data's inconsistencies by minimizing the difference of measured sea surface height values at the intersection of two crossing satellite tracks. A total of 91,230 crossover points between 1216 ascending and 1199 descending tracks in the latitude range between 60°S and 60°N were computed and listed. The numerical procedure consists of a least squares adjustment of single‐track data that is iterated until the rms crossover residual becomes stationary. The method is, in essence, equivalent to the solution of least squares normal equation, but the corrections' absolute values are indeterminate. The ambiguity was removed by imposing a condition that the errors in the altimeter measurements are mostly random so that their average must be null. The validity of the assumption was confirmed by comparing the local geoid constructed by this method with a standard model such as GEM 10. Numerical examples show that, for an area not larger than 30° longitude by 30° latitude, the crossover rms residual is reduced from more than 5 m to less than 50 cm by making adjustment for the data's erroneous bias and tilt. It enabled us to contour the geoid map with an interval of l m and to locate and delineate geoid anomalies of a few meters in magnitude. In marine geophysical studies it is often necessary to compare the geoid with other marine geophysical data that are usually recorded continually along the ship's track. A formula was derived for projecting the adjusted GEOS 3 data onto a ship's track using the data's autocovariance. The formula's filtering characteristics showed that the use of data distributed within 500 km of the ship's track is effective in profiling a geoid height variation in the wavelength range between 700 and 1000 km, but details with the scale length shorter than 200 km are totally smoothed out. To construct a finer profile, use must be made of the data more localized near the center of projection, provided that the distribution of data is dense enough to warrent a statistical treatment.
Band‐pass filtering of one year of daily mean pressures on Mars
Authors: Niver David S. , Hess Seymour L.
Published: February 2008

The sol‐mean atmospheric pressures measured at the two Viking landers exhibit fluctuations ranging in period from seasonal to a few days. The time series are highly nonstationary, which severely limits standard methods such as power‐spectrum analysis. We take advantage of the rather precise annual periodicity of these data to design sharp numerical filters and use them to split the series at each lander into six time series, each containing oscillations associated with a discrete frequency band. This analysis reveals 2‐ to 4‐sol waves during the cold seasons at both landers, and 8‐sol waves during the spring at Lander 2, confirming previous results from spectral analysis. As previously predicted, high‐frequency waves appear suppressed during the second dust storm. Certain low‐frequency signals, probably associated with the global dust storms, are detected and demonstrated to be significant for the first time.
Ferromagnetic resonance intensity: A rapid method for determining lunar glass bead origin
Authors: Stone Charles D. , Taylor Lawrence A. , McKay David S. , Morris Richard V.
Published: February 2008

Ferromagnetic resonance intensity (IS) was tested as a criterion for use as a nondestructive, reliable, and rapid screening method for distinguishing volcanic from impact glass spherules in the lunar soils. This method relies on the absence or presence of single‐domain Fe formed during impact melting by autoreduction of Fe2+ in the melt. Extensive scanning electron microscope (SEM) inspection of individual glass bead surfaces provided reliable evidence of the mode of glass genesis. Ferromagnetic resonance intensity was tested against the ‘type’ Apollo 15 and Apollo 17 volcanics. Additional volcanics were also found during the course of the study. Ninety‐four percent of the volcanic glasses had IS values less than one, implying the absence of single‐domain Fe. Seventy‐five percent of impact glasses had elevated IS values. The remaining impact beads had IS values less than one. These samples lack single‐domain Fe and could represent impact melts of bedrock with little or no regolith contribution. The ability to discriminate bedrock‐derived impact melts as opposed to impact melts partially or completely derived from regolith, and the primary discrimination of volcanic glasses from impact glasses, suggests the FMR technique can be important in determining the origin of lunar glass spheres.
The variable carbon isotopic composition of type 3 ordinary chondrites
Authors: Grady M. M. , Swart P. K. , Pillinger C. T.
Published: February 2008

A survey of bulk carbon isotopic compositions has been carried out for a suite of H, L, and LL ordinary chondrites, from the most unequilibrated type 3 to the metamorphosed type 6. The isotopic composition of the higher petrologic types 4–6 is lighter (in the range −25 to −30°‰) and less variable than specimens of type 3 (−11 to −28°‰). There is no simple relationship between bulk carbon and its isotopic composition; several carbon phases of different δ13C may be present in the meteorites in variable proportions. To characterize the variability of type 3 chondrites, a selection has been treated with orthophosphoric acid to test for the presence of terrestrial weathering products of high δ13C. Some contamination and weathering products seem to be present, but such an interpretation is less than straightforward. The heavier isotopic composition and variability of type 3 meteorites may also be due, in part, to the presence of a component of graphite‐magnetite matrix in the H and L group samples.
The effects of C, P, and S on trace element partitioning during solidification in Fe‐Ni alloys
Authors: Willis John , Goldstein Joseph I.
Published: February 2008

The effect of the minor elements P, S and C on the distribution coefficients of Ir, Ge, Ga, Au, Cu, and Cr between the solid and liquid phases in Fe‐Ni alloys was measured experimentally. Distribution coefficients for Ir, Ga, and Au increase with increasing P content while the distribution coefficients for Ir, Ge, and Ni increase with increasing C and S contents. For Ge, the distribution coefficient exceeds 1.0 when the P or C content of the solid exceeds 0.4 or 0.9 wt% respectively, or the S content of the liquid exceeds 7 wt%. The measured distribution coefficients were used in a solidification model of a parent body core. The model assumes fractional crystallization in which the distribution coefficients vary with composition and predicts the trace element distributions in the major iron meteorite groups. The model correctly predicts the observed trends for P vs. Ni and Au vs. Ni in the iron meteorite groups considered. Although the calculated trends for Ir vs. Ni and for Ge vs. Ni are relatively satisfactory for groups IVA and IVB, they fail to produce the observed trends for groups IIAB and IIIAB. Interelement effects of S and C on the distribution coefficients may explain these inconsistencies.
Direct integration of Stokes' Undulation and Vening Meinesz's Deflection Equations for geographically defined gravity anomaly blocks
Authors: Obenson Gabriel
Published: February 2008

We give here a direct solution of this problem by converting, in an indirect way, the relevant integral equations over each block to their equivalent line integrals taken along and around the boundary curves of the block. Results of sample computations given clearly demonstrate the convergence and stability of these line integrals. We also computed geoidal parameters due to a 30°×30° inner zone alone at 71 grid locations in Cameroon, using 1°×1° anomalies and integration steps of 30″ for the four innermost blocks and 180″ for the rest of the blocks and compared these with equivalent values from the midpoint method. This comparison yielded significant differences—rms ΔN = 2.2 m, ΔNmax = 4 m; rms Δξ = 2.9″, Δξmax = 10.5″; rms Δη = 2.0″, Δηmax = 6.7″—that reflect the poor accuracy of the center point method, especially for 1°×1° blocks near the computation point.
Introduction [to Special Section on Anton Hales Symposium]
Authors: Ward R. W. , Mitterer R. M.
Published: February 2008

This volume of the Journal of Geophysical Research is devoted to Anton L. Hales. It contains many of the papers presented at the Anton L. Hales Symposium on ‘Some Recent Advances in Geophysics’ held at The University of Texas at Dallas (UTD) on October 5–6, 1981, to honor Professor Hales on his retirement from UTD at the age of 70. Many former students, colleagues, and friends attended the symposium to honor Anton and to share their experiences with a man who had touched their lives and left his mark on the field of geophysics. Professor Hales' career in geophysics spans 50 years beginning with his first course in geophysics in 1932 at Cambridge University.
Kinematic analysis of strong motion P and SV waves from the Sterling Event
Authors: Langston Charles A.
Published: February 2008

P and SV velocity waveforms from the Sterling explosion of December 3, 1966, are analyzed from a point of view often taken in earthquake source studies. The detonation occurred in the cavity excavated by the Salmon explosion within the Tatum salt dome, Mississippi. Velocity recordings taken from nearby borehole strong motion instruments show the occurrence of significant SV energy (Perret, 1968a, b). Kinematic source models are constructed by considering constraints afforded by the preshot cavity, condition of the surrounding material, and aspects of the P and SV waveforms such as wave polarity and duration. Waveforms for these models are computed and compared directly with the data. Modeling results suggest that the SV waves were radiated primarily by induced normal faulting occurring beneath the cavity rather than by an asymmetric pressure distribution based on the preshot cavity geometry. Secondary arrivals within the P waveforms and SV amplitudes suggest that these normal faults also had a component of tensional displacement which was probably due to forceable injection of material from the blast. A measurement for Qβ of 35 is obtained for salt from the amplitude decay arid pulse width changes observed for SV waves along a profile taken at shot level. This value for Qβ is for the region away from the cracked area induced by the previous Salmon explosion and qualitatively explains the differing frequency content observed at the closest Sterling sensors between P and SV waves. The anomalous SV radiation has little to do with ‘tectonic’ stress release but is a result of the medium's geometry and anisotropy.
Eruption age of a Pleistocene basalt from 40Ar−39Ar analysis of partially degassed xenoliths
Authors: Glllespie A. R. , Huneke J. C. , Wasserburg G. J.
Published: February 2008

The appendixes are available with the entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W.,Washington, D.C. 20009. Document B83‐003; $2.50. Payment must accompany order.

We have applied 40Ar−39Ar dating to potassium‐rich granitic xenoliths and host basalt from the Pleistocene Big Pine volcanic field, California. These xenoliths had been partially degassed upon their inclusion in the basaltic lava. Argon released from the xenoliths at extraction temperatures below ∼900°C yielded plateau ages indistinguishable from the total K‐Ar age of the basalt. The best estimate of the age of eruption was 1.18±0.05 (2σ) m.y. 40Ar extracted at higher temperatures included radiogenic argon not degassed from the late Cretaceous xenoliths 1.18 m.y. ago, causing an increase in the apparent age for the high‐temperature fractions. The agreement of the low‐temperature xenolith plateau ages and the basalt K‐Ar ages demonstrates that 40Ar−39Ar analysis of xenoliths may be used to measure the age of eruption of very young lavas. This is significant because in many instances ages cannot be reliably determined by analysis of the lavas themselves.
Determination of surface temperature history from borehole temperature gradients
Authors: Shen P. Y. , Beck A. E.
Published: February 2008

The determination of surface temperature history from borehole temperature gradient profiles is commonly formulated as a linear least squares minimization problem. The surface temperature history is approximated by a series of intervals of constant temperature perturbation superimposed on a mean temperature and the best solution obtained by systematically adjusting the number, duration, and temperature of these intervals so as to minimize the least squares measure between the measured and calculated gradient profiles. However, this inversion problem is improperly posed. The gradient disturbances due to surface temperature perturbations in the series of time intervals are usually “almost linearly dependent.” Thus the accuracy of a numerical solution is dictated by the level of random noise in the data and rounding errors in the computation. Using simulated data, we have systematically examined the effects of different noise levels, sampling ranges (maximum usable borehole depth), and sampling intervals (depth increment) on the resolution and time span of the derived surface temperature history. The method of orthogonal polynomials in curve fitting is also used in an attempt to improve the time span and resolution. The results, however, indicate that under realistic conditions, only about four intervals of temperature perturbation can be used to approximate the history of surface temperature variations. The major implications are (1) large time span and high resolution are mutually exclusive; for example, with a resolution of 100 years, the maximum time span would be about 400 years, whereas if a resolution of 50 years is desired, the maximum time span would be only 200 years; (2) in order to have a reasonable resolution, the epochs at which surface temperature changes took place must be known from independent information because it is impossible to approximate adequately the surface temperature history with four arbitrarily chosen time intervals of constant temperature; and (3) any surface temperature changes which took place outside the time span of interest must be known and their effects removed from the gradient data; otherwise, interference from these temperature changes will lead to a distorted surface temperature history in the time span of interest.
Body wave inversion: Moment tensors and depths of oceanic intraplate bending earthquakes
Authors: Ward Steven N.
Published: February 2008

This paper develops a moment tensor inversion technique suited for analysis of long‐period body waves from distant, shallow earthquakes. In addition to the moment tensor the procedure resolves precise source depths (±5 km) and estimates the duration of faulting. The technique is applied to digital SRO‐DWWSSN recordings of outer rise, intraplate earthquakes. These earthquakes result from bending of oceanic plates prior to subduction. Bending induces horizontal tension near the top of the lithosphere and horizontal compression near its base. A neutral surface separates compressional from tensional regimes. An examination of 11 bending earthquakes in the period January 1981 to July 1982 indicates that the depth to the neutral surface is not geographically uniform. Although the seismicity sample is very small, the depth of the neutral surface appears to correlate with the position of the trench‐arc in the evolutionary sequence of Uyeda and Kanamori (1979). In particular, neutral surfaces at strongly coupled Chilean‐type plate boundaries are elevated as much as 15 km above those at weakly coupled Mariana‐type boundaries. This correlation implies that nonuniformities in neutral surface height are created by variations in regional compressive stress resulting from contrasts in plate‐plate coupling. To account for the observed heights, maximum interplate compressive stresses must be comparable with stresses derived from bending.
Sediment sound velocities from Sonobuoys: Sunda Trench and forearc basins, Nicobar and Central Bengal Fans, and Andaman Sea Basins
Authors: Bachman Richard T. , Hamilton Edwin L. , Curray Joseph R.
Published: February 2008

Supplement is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B83‐007; $2.50. Payment must accompany order.

Measurements of mean sound velocities in the first, largely unlithified layers in the seafloor were made using the sonobuoy technique in several areas in the northern Indian Ocean. Older measurements were added to new measurements, and regressions for mean and instantaneous velocity versus one‐way travel time of sound are presented for the central Bengal Fan, the central Andaman Sea Basin, the Nicobar Fan, and the Sunda Trench. New data and regression equations are presented for the Mergui‐north Sumatra Basin and for four forearc basins between Sumatra and Java and the Sunda Trench. Minimum velocity gradients were found in those areas where sedimentation rates were high, and sediments have accumulated in thick sections which have not had time to fully consolidate (porosity in the top of the sediment section has not been fully reduced under overburden pressure). These minimum velocity gradients (just under the seafloor) were found in the four forearc basins where they ranged from 0.34 s−1 to 0.84 s−1 with an average of 0.58 s−1. The near‐surface velocity gradient in the Sunda Trench was 1.33 s−1, but was higher in the adjacent, fossil Nicobar Fan (1.62 s−1). In the surface of the Bengal Fan the velocity gradient was low in the upper fan (0.86 s−1), high in the central fan (1.94 s−1), and again lower in the southern fan (1.18 s−1), which may support sedimentation models calling for bypassing of the central fan and higher rates of accumulation on the southern fan.
Shock compression of single‐crystal forsterite
Authors: Wett J. Peter , Ahrens Thomas J.
Published: February 2008

Dynamic compression results are reported for single‐crystal forsterite loaded along the orthorhombic a and c axes to pressures from 130 to 165 GPa. Hugoniot states for the two axes are well described by a single curve offset to densities 0.15–0.20 g/cm3 lower than earlier data for single‐crystal forsterite shocked along the b axis above 100 GPa. Earlier data of Syono et al. [1981a] show marginal support for similar b‐axis behavior in the mixed‐phase region from 50 to 92 GPa. Thus shocked forsterite is most compressible in the b direction for the mixed‐phase and high‐pressure regimes (P > 50 GPa). These data represent the highest pressures for which shock properties have been observed to depend on crystal orientation. Theoretical Hugoniots for mixed‐oxide and perovskite‐structure high‐pressure assemblages of forsterite calculated from recent experimental data are virtually identical and agree with the b‐axis data. The a‐ and c‐axis data are also consistent with both high‐pressure assemblages because uncertainties in equation of state parameters produce a broad range of computed Hugoniots. Our calculated “average” Hugoniot is up to 0.13 g/cm3 less dense than the preferred theoretical Hugoniots, in agreement with earlier measurements on dense polycrystalline forsterite. Interpolation between the single‐crystal forsterite Hugoniots and Hugoniots for fayalite and Fo45 gives Fo88 Hugoniots bracketing Twin Sisters dunite data not previously well fit by systematics. Release paths are steep for the a and b axes but c‐axis release paths are much shallower. Hugoniot elastic limits measured for the a and b axes are in good agreement with previous data of Syono et al.; however, the present data for the a axis reveal a triple wave structure: two deformational shock waves as well as the elastic shock, a feature not previously found. The second shock, with amplitude about 9 GPa and a shock temperature of about 350°K, could perhaps be explained by the forsterite α→β or γ phase transformation.
Physical properties of ordinary chondrites
Authors: Yomogida Kiyoshi , Matsui Takafumi
Published: February 2008

Physical properties (intrinsic and bulk densities, porosity, compressional and shear wave velocities, thermal diffusivity, and conductivity) are measured on 11 ordinary chondrites, one carbonaceous chondrite, and two achondritic clasts of a mesosiderite. With the previously reported results from Antarctic ordinary chondrites, the data are useful in clarifying the difference between H and L chondrites. For example, the intrinsic density of H chondrites (∼3800 kg/m3) is generally higher than that of L chondrites (∼3600 kg/m3). The sample porosity, less than 20%, strongly controls its elastic wave velocities, thermal diffusivity, and conductivity. The variations of elastic wave velocities and thermal properties with porosity show that as with lunar rocks, chondrites contain many cracks. These cracks are thought to be created on their parent bodies by the cumulative effects of many mutual collisions and impacts. Thermal properties are linear functions of elastic wave velocities for both H and L chondrites. The difference between H and L chondrites can be attributed to the content of metallic Fe‐Ni. Correlations between petrologic types, which roughly represent metamorphic temperature, and porosity are less obvious for L chondrites than for H chondrites. Since porosity is not changed significantly by impact events, it appears that the sintering process for L chondrites is independent of the metamorphic events represented by petrologic type.
Precision of geodolite surveys: A Reply to Jackson and Cheng
Authors: Savage J. C. , Prescott W. H.
Published: February 2008

Jackson and Cheng have suggested that the changes in areal dilatation measured in Geodolite surveys by the U.S. Geological Survey may be simply an artifact of the measuring system. Although systematic error could conceivably account for the observed excursions in dilatation, we maintain that the specific criticisms by Jackson and Cheng are incorrect: the excursions in dilatation cannot be attributed to the offset correction nor to proportional error associated with temperature. The absence of both errors is demonstrated by using data that are particularly sensitive to the two effects.
Petrology of the Apollo 11 Highland Component
Authors: Simon S. B. , Papike J. J. , Shearer C. K. , Laul J. C.
Published: February 2008

New data for 38 highland fragments hand‐picked from Apollo 11 coarse fines are reported. Petrographic, mineralogic, and bulk chemical data show that (1) the Apollo 11 highland component is non‐KREEPy and like that of Apollo 16; (2) poikilitic rocks, granulitic breccias, and the anorthosite‐norite‐troctolite suite are the most abundant rock types, followed by polymict breccias and glasses; (3) both the ferroan anorthosite and Mg‐rich plutonic suites are represented in the Apollo 11 highland component; (4) except for one sample, the intermediate‐K Fra Mauro and high‐K Fra Mauro groups are not represented. The data and observations are consistent with local derivation of the highland material from beneath relatively thin basalt flows and addition to the regolith via vertical mixing.
Chryse Basin channels: Low‐gradients and ponded flows
Authors: Lucchitta B. K. , Ferguson H. M.
Published: February 2008

Gradients on the floors of the Martian outflow channels that are derived from radar‐elevation profiles across Lunae Planum and the Chryse Basin have much lower values than those obtained from the U.S. Geological Survey's topographic map. To arrive at this conclusion, 26 radar‐elevation profiles clustered at four latitudinal channel crossings were analyzed and approximate longitudinal gradients of the channel floors were constructed. Whereas the gradients of Maja and Ares Valles are similar to those of the catastrophic flood channels in the Scablands of Washington State and water or ice could have moved through them without difficulty, the gradients of Simud and Tiu Valles are essentially level, and the movement of fluids to the north poses problems. It is proposed that ponding from local runoff occurred before channel formation along the course of Simud and Tiu Valles. The ponding may have formed lakes in depressions associated with the Valles Marineris grabens, ancient craters in the chaotic terrain area, and possibly even the regional low where most chaotic terrains occur. It is envisioned that lakes eventually overflowed, drainages became integrated, and headward erosion breached interfluves, thus forming the present channel configurations. When dams broke, floods, mudflows, or mixtures of water, sediments, and ice were released catastrophically, with a final gigantic flood from the Valles Marineris system of troughs. This flood would have had sufficient head to move fluids across nearly level gradients through the Simud and Tiu channels. The ponding and flooding probably occurred during a time when the climate was different from that at present, permitting surface runoff of fluids and standing bodies of water.
Magnetic dipole moment estimates for an ancient lunar dynamo
Authors: Anderson K. A.
Published: February 2008

The four measured planetary magnetic moments combined with a recent theoretical prediction for dynamo magnetic fields suggests that no dynamo exists in the moon's interior today. For the moon to have had a magnetic moment in the past of sufficient strength to account for at least some of the lunar rock magnetism, the rotation would have been about twenty times faster than it is today and the radius of the fluid, conducting core must have been about 750 km. The argument depends on the validity of the Busse solution to the validity of the MHD problem of planetary dynamos.
Radiation history of lunar microbreccias and lithic chrondrules from Weston Meteorite by track data
Authors: Kashkarov L. L. , Genaeva L. I. , Tarasov L. S. , Baryshnikova G. V. , Lavrukhina A. K.
Published: February 2008

Tracks generated in the lithic chondrules of the brecciated Weston meteorite as well as in the lunar microbreccias by iron group nuclei of solar cosmic rays are studied. The results for about 2000 olivine and pyroxene grains contained in 67 lithic chondrules indicate that a relict irradiation of the material occurred at an early stage in solar system formation, i.e., before these chondrules were assembled. The results for about 1000 olivine and pyroxene grains in 13 microbreccias strengthen our earlier conclusion that part of the chondrules from the Weston meteorite may have formed in processes analogous to brecciation. The newly obtained data confirm that the formation processes of the Weston lithic chondrules, containing solar flare irradiated crystals, were similar to the impact formation of microbreccias on the lunar regolith without heating compacted material up to temperatures of 600°–700°C.
Acoustic fluidization and the scale dependence of impact crater morphology
Authors: Melosh H. J. , Gaffney E. S.
Published: February 2008

A phenomenological Bingham plastic model has previously been shown to provide an adequate description of the collapse of impact craters. This paper demonstrates that the Bingham parameters may be derived from a model in which acoustic energy generated during excavation fluidizes the rock debris surrounding the crater. Experimental support for the theoretical flow law is presented. Although the Bingham yield stress cannot be computed without detailed knowledge of the initial acoustic field, the Bingham viscosity is derived from a simple argument which shows that it increases as the 3/2 power of crater diameter, consistent with observation. Crater collapse may occur in material with internal dissipation Q as low as 100, comparable to laboratory observations of dissipation in granular materials. Crater collapse thus does not require that the acoustic field be regenerated during flow.
Linear moment tensor inversion for shallow thrust earthquakes combining first‐motion and surface wave data
Authors: Michael Andrew J. , Geller Robert J.
Published: February 2008

We derive a linear moment tensor inversion method (which we call second nodal plane inversion) for obtaining a double couple focal mechanism when only one nodal plane is known. This typically is the case for shallow thrust events, for which only the steeply dipping nodal plane can be determined from teleseismic first‐motions; conventional moment tensor inversion methods, are known to fail for these events because the portion of the moment tensor corresponding to vertical dip‐slip faulting is extremely inefficient in exciting surface waves. In this paper we invert long‐period (256 s) fundamental mode Rayleigh waves from the IDA network; the inversion technique can also be applied to body wave amplitude data or to entire records (as a sum of normal modes), given the appropriate normal equations. We study several shallow (h<35 km) thrust earthquakes for which the steeply dipping nodal plane can easily be found from body wave first‐motions. In contrast to standard surface wave moment tensor inversion techniques, which have serious stability problems for such shallow events, our technique successfully finds the scalar moment and second nodal plane.
A modified theory of bottom crevasses used as a means for measuring the buttressing effect of ice shelves on inland ice sheets
Authors: Jezek Kenneth C.
Published: February 2008

Bottom crevasses are fractures that extend upward into floating ice shelves. They form when seawater penetrates the base of the ice shelf and ruptures the ice up to the level at which englacial stresses equal the stress of the seawater. For a freely floating ice shelf, the penetrating level of closely spaced crevasses is estimated at about half the ice thickness h; for an isolated crevasse the level is about πh/4. However, an analysis of the heights and locations of bottom crevasses in the Ross Ice Shelf shows that none of the crevasses approach the predicted limits, perhaps because the existing theory does not include the back stress σb which is present in bounded ice shelves. By reformulating the theory to include a back stress term, σb can be evaluated experimentally from radar measurements of crevasse height and ice thickness. The magnitude of σb (2 bars in the grid northwest corner of the ice shelf) suggests the ice shelf is playing an important role in buttressing the inland ice sheet.
Geodetic studies in Baja California, Mexico, and the evaluation of short‐range data from 1974 to 1982
Authors: Darby Desmond , Gonzalez J. Javier , Lesage Philippe
Published: February 2008

The short‐range geodetic data from northern Baja California, Mexico, for the period 1974–1982 are carefully analyzed. These data contribute to an understanding of the complex pattern of faulting associated with the Pacific‐North American plate boundary in this region. Survey precisions are evaluated and significant systematic errors are found to exist. A technique of studying a scale‐free displacement solution is developed as an aid to interpreting the data. We conclude that (1) the motion on the San Miguel‐Vallecitos fault system is presently in a right‐lateral sense and at a level that warrants trilateration surveys at least annually, (2) present geodetic data permit no statement about movement on the Agua Bianca fault, and (3) the mesa, whose flank delineates the Cerro Prieto fault in the Valle de Mexicali, lies in a zone which has undergone significant horizontal areal compression, at a rate of 14±5 ppm/yr, in addition to a right‐lateral tensor shear at a rate of 4±1 ppm/yr, oriented N(;33°±9°)W, between 1980 and 1982.
Eruption forecasting at Kilauea Volcano, Hawaii
Authors: Klein Fred W.
Published: February 2008

Data from a network of seismometers, a tiltmeter and the pattern of summit inflation between eruptions have made qualitative eruption forecasting possible on Kilauea volcano for over 20 years. This paper formulates quantitative forecasting by calculating the probability of an eruption based on current levels of tilt, tilt rate, seismicity, and fortnightly tide. Plots of eruption probability as a function of various parameters are derived using a set of 29 eruptions during 1959–1979. The method tests the precursory significance of various parameters over different time scales and determines probabilities by comparing data before eruptions with data values generally. Tilt level is an eruption precursor significant to better than 99.9% when averaged over any interval from 1 to 20 days. Tilt rate is a precursor significant to better than 99.999%, but noise in the data requires that it be averaged over 30 days or more. Earthquakes are a short‐term precursor significant to better than 99% when averaged over 1–10 days for larger earthquakes near the summit caldera and over 5–20 days for very small earthquakes within the caldera. The fortnightly modulation of tides influences eruption probability and has a precursory significance of 97%. Probabilities are independent of the time elapsed since the last eruption. The eruption probability, when tested against the eruption record from which it was derived, is significant to the 99.98% level when compared with random guessing. Ongoing 1‐day, 7‐day, and 30‐day forecasts are calculated by computer at the Hawaiian Volcano Observatory and supplement the qualitative interpretation of geophysical data.
Seismicity and tectonics of the Rivera Plate and implications for the 1932 Jalisco, Mexico, earthquake
Authors: Eissler Holly K. , McNally Karen C.
Published: February 2008

Seismicity on and near the boundaries of the Rivera plate is examined in order to determine the subduction regime of the Rivera plate and its influence on the earthquake behavior of the northernmost section of the mid‐America trench. In particular, we address the question of a Rivera‐Cocos boundary.There have been several large historic earthquakes in the coastal areas of the Mexican states Colima and Jalisco, but the last large event was in June 1932 (the 1932 Jalisco earthquake, Ms = 8.1). The present quiescence has lasted longer than the average recurrence interval for the Mexican subduction zone, and the Jalisco area is now categorized as a seismic gap. However, if subduction in this area is dominated by the motion of the Rivera plate, which is subducting at half the rate of the Cocos plate, the deficiency of earthquake activity may not be unusual. In an attempt to better delineate the Rivera‐Cocos boundary,intermediate‐sized earthquakes along and near the Rivera fracture zone and the mid‐America trench were relocated with the joint epicenter determination method. We found that in general, the catalog locations are fairly accurate, with the average change in epicenter upon relocation being 12 km. Seismicity along the Rivera fracture zone follows the physiographic trend of the fracture zone, but east of its intersection with the East Pacific Rise, seismicity is more diffuse. Two intermediate‐sized earthquakes in this diffuse area have focal mechanisms suggesting that they represent Rivera‐Cocos boundary motion. Reexamination of arrival time data for the 1932 Jalisco earthquake yields an epicenter at 19.57°N, 104.42°W, close to the boundary zone inferred from these two earthquakes. We conclude that the possibility that the 1932 Jalisco earthquake broke the northernmost section of the Cocos‐North American plate interface, as opposed to the Rivera‐North American plate interface, cannot be ruled out.
Tectonic implications of the microearthquake seismicity and fault plane solutions in southern Peru
Authors: Grange F. , Hatzfeld D. , Cunningham P. , Molnar P. , Roecker S. W. , Suárez G. , Rodrígues A. , Ocola L.
Published: February 2008

Using recordings from 15 portable instruments and six permanent stations operated during two field investigations of microearthquakes in southern Peru, we determined locations of 888 shallow and intermediate depth earthquakes and 56 fault plane solutions. A series of tests evaluating the effects of elevation corrections, erroneous velocity structure, and random errors in arrival times on calculated locations suggest that the locations of most of the events are uncertain by less than 7 km. At shallow depths (<60 km) the seismicity is concentrated in three belts: one offshore, one beneath the coast, and one beneath the Western Cordillera. There is also scattered but infrequent shallow activity near the Altiplano. The belts offshore and beneath the coast are apparently associated with slip along the Nazca‐South American plate boundary. Although the style of deformation is complicated, one can infer that beneath the coast the active boundary between the Nazca and South American plates is within the mantle rather than at the base of the South American crust. The belt beneath the Western Cordillera is about 30 km in width and dips southwest from near the surface to a depth of 60 km. Fault plane solutions suggest that crustal shortening is taking place in a northwest‐southeast direction in this region. The significance of this activity is difficult to explain, but these earthquakes provide no clear evidence of “tectonic erosion” in this area. A composite fault plane solution from the few events near the Altiplano shows normal faulting, which is typical of the active tectonics of that region. The results of the investigation of intermediate depth earthquakes shows that the downgoing slab in southern Peru is warped and not torn in the region where the dip changes from nearly horizontal to a dip of about 30°. They also suggest that the axis of the contortion is not perpendicular to the coast but parallel to the direction of plate motion. Moreover, the volcanic line ends at the contortion, which suggests that a relatively thick wedge of aesthenospheric material is necessary for the formation of subduction related volcanism. Fault plane solutions for events between 60‐and 100‐km depth show a large scatter, but some suggest a detachment of a sliver of the downgoing slab. Solutions for deeper events are more consistent with one another with the T axes for most events being nearly horizontal and generally normal to the projections of the seismic zone.
Hypocentral trend surface analysis: Probing the geometry of Benioff Zones
Authors: Bevis Michael , Isacks Bryan L.
Published: February 2008

A hypocentral trend surface is a continuous function of latitude and longtitude fitted by least squares to a set of hypocenters so that it predicts depth to the “middle” of a Benioff Zone. In this paper we take a relatively simple approach to hypocentral trend surface analysis. The hypocentral trend surface is constructed from a spherical surface harmonic expansion whose coefficients are selected so as to minimize the standard vertical deviation between hypocenters and trend surface. The vertical deviation of a hypocenter from the hypocentral trend surface is called its residual. Consideration of a hypocentral trend surface cannot be divorced from consideration of the associated residuals and their spatial distribution. Any analysis of Benioff Zone geometry trades‐off presumed thickness of the zone with its presumed shape. This trade‐off can be investigated by examining suites of candidate hypocentral trend surfaces (together with their associated residual distributions) generated by varying the number of degrees of freedom available to the trend surface. Hypocentral trend surfaces are generated for three high quality sets of hypocenters obtained by local seismic networks in Honshu (Japan), Cook Inlet (Alaska) and South Peru. Hypocenters beneath Honshu generate a bimodal distribution of residuals about their trend surface. The vertical separation of the upper and lower sheets of this Double Benioff Zone averages about 33 km. No Double Benioff Zone configuration is discovered below Cook Inlet or South Peru. Regional trend surfaces are established for intermediate‐depth teleseismic data from South and Middle America. In South America (0°–40°S) teleseismic data suggest that along‐strike transitions between “flat” and moderately steeply dipping sections of the Benioff Zone are achieved by flexure of a coherent slab rather than by fragmentation of the slab into tear‐bound flaps or separate tongues of lithosphere. Hypocentral trend surface analysis of teleseismic data from South Peru suggests a geometry similar to that indicated by hypocentral trend surface analysis of the local data of Hasegawa and Sacks. An eastward shallowing Benioff Zone beneath southwest Mexico is closely associated with the geometry of the trans‐Mexican volcanic arc. The Benioff Zone beneath Central America is more steeply dipping. The flat slab‐steep slab transition near 96°W may relate to the difference in age of oceanic lithosphere on either side of the Tehuantepec Ridge. While the strong flexure of the subducted Nazca plate beneath Ecuador is closely associated with the Carnegie Ridge, the Nazca ridge extropolates slightly north of the flexure in subducted lithosphere beneath South Peru. The flexure appears instead more closely associated with the pronounced inflection in the map view shape of the leading edge of the overriding South American plate.
Q structure of the basin and range from surface waves
Authors: Patton Howard J. , Taylor Steven R.
Published: February 2008

Regionalized Rayleigh and Love wave attenuation coefficients have been measured across the Basin and Range province of western United States in the frequency range 0.02–0.2 Hz. The measurements were made by using the methods of Tsai and Aki (1969) and Yacoub and Mitchell (1977) adapted to work on any number of events simultaneously. Rayleigh wave Q values at low frequency approach values near 40, which are significantly lower than previous measurements in the western United States. Love wave Q values, on the other hand, are quite high at low frequencies. We suggest that interference between fundamental and higher modes may explain the Love wave observations. The Rayleigh and Love wave attenuation measurements have been inverted simultaneously for shear‐wave attenuation as a function of depth. A frequency‐independent Q model is consistent with both Rayleigh wave and short‐period Love wave attenuation data. The shear‐wave Q (Qβ) model is characterized by low Qβ in the lower crust (Qβ∼100) and Qβ decreasing in the upper mantle with lowest values (Qβ∼30) beneath 60 km depth. Forward modeling shows that a high‐Q lower crust or upper mantle lid is inconsistent with the data. Our interpretation of these results is that the lithosphere is poorly developed beneath the Basin and Range and that the partially molten asthenosphere may reach very shallow depths, possibly to the base of the crust. This interpretation of the Q model in conjunction with a number of geological and geophysical evidences suggests that attenuation mechanisms involving partial melt predominate in the lower crust and upper mantle of the Basin and Range.
Evidence for magma intrusion during the Mammoth Lakes Earthquakes of May 1980 and implications of the absence of volcanic (harmonic) tremor
Authors: Aki Keiiti
Published: February 2008

The compensated linear vector dipole (CLVD) mechanism (P wave radiation with conical nodal surfaces) found for the Mammoth Lakes earthquakes of May 1980 has been attributed to dike intrusion by Julian and his colleagues. There is a concern about this interpretation because magma intrusion through a crack should radiate compressional first motion everywhere. Chouet's calculation on a magma intrusion model, however, showed that the main dynamic motion as well as the final static displacement have patterns similar to the CLVD mechanism. We found an example of observed radiation pattern similar to the CLVD mechanism from eruption earthquakes at Mount Aso obtained by Sassa, and we interpret the observation as misidentification of the main motion as the initial motion. In searching for a similar misidentification for the Mammoth Lakes earthquakes we found that there are numerous reports of compression for short‐period P waves at stations where long‐period P waves show unequivocal dilatation. Furthermore, we found that the compressional short‐period first motion is systematically earlier than the dilatational one. These observations can be explained if the source of magma intrusion generated compressional first motions which were very weak and of high‐frequency contents in the direction where long‐period P waves are dilatational. The seismic moment of the Mammoth Lakes earthquakes suggests an involvement of magma volume of the order of 10−2 km3. However, we have not yet observed long‐period events or volcanic (harmonic) tremors which are often observed at active volcanos. We review recent advances in theories and observations on volcanic tremor and conclude that for the volume of magma reservoir suggested for the Mammoth Lakes area a fluctuation of pressure of the order of 10 −2 bar would generate measurable tremor. This conclusion is obtained under the assumption that both the viscosity of magma and the radiation loss of the magma reservoir are low enough for the excitation of a harmonic tremor. We found that the concept of “reduced displacement” introduced by Aki and Koyanagi for body waves and by Fehler for surface waves is very useful for estimating the physical parameters of a volcanic tremor source.
Annealing and stability of multidomain magnetite
Authors: Smith Guy , Merrill Ronald T.
Published: February 2008

Standard rock magnetic measurements and domain wall resonance measurements for magnetite are combined in an attempt to define better how domain walls are pinned and how such pinning is related to magnetic stability. Using these two different measures of stability, before and after annealing at 650°C, we find that domain walls are primarily pinned by dislocation arrays. Tightly pinned walls can exist in grains that show relatively low values of coercivity, indicating that magnetic screening can play a critical role in determining the properties of remanence.