bluuebunny/crossref_metadata_2025 huggingface dataset:
Compressional wave velocity structure of the upper 350 km under the Eastern Snake River Plain near Rexburg, Idaho
Authors: Evans John R.
Published: February 2008

Relative travel time residuals for teleseismic P and PKIKP are used to determine the compressional velocity structure under the eastern Snake River Plain at Rexburg, Idaho. Damped least squares inversion of travel time residuals (modified from the method of Aki) indicates a large body of 3.5±2.5% low relative velocity material centered under the northwest edge of the Snake River Plain. Less well resolved are high relative velocity bodies, one deep under the Idaho‐Wyoming border, the other 300 km below the Idaho batholith. Comparison with similar velocity mappings of the Yellowstone region indicates that the Snake River Plain low‐velocity anomaly is an aged extension of the low‐velocity body under the Yellowstone caldera. The upper half of the body has vanished in the eastern Snake River Plain, and the lower half is less anomalous than at Yellowstone. The strong dependence of mantle velocity structure on position under the Yellowstone‐Snake River Plain volcanic system indicates loss of heat to the surface primarily by advective (heat movement by mass movement) mechanisms, not by conduction. No discrimination between propagating rift and upper mantle heat anomaly models of this northeast progressing volcanic system is possible because velocity structures associated with these hot spot models have not been predicted quantitatively. Upper mantle heat anomaly models (thermal plumes, chemical plumes, gravity anchors) explain the observed structure with fewer ad hoc assumptions, but propagating rift models cannot be eliminated.
A magnetic investigation of Mount Hood, Oregon
Authors: Flanagan Guy , Williams David L.
Published: February 2008

Aeromagnetic data over the Mount Hood area of Oregon are dominated by the topographic expression of highly magnetized Cenozoic volcanics. Three‐dimensional magnetic modeling of the Mount Hood volcanic cone indicates that the bulk of the cone, above approximately 1650 m elevation, is composed of magnetically similar andesites. The andesites are magnetized in a normally polarized direction (I = 80.0°, D = 28.3°) and with a magnetization of 2.9 A/m. The chief exception to this is the possible remains of an old cone which underlies the northwest slope of the Mount Hood cone. Calculations show that this old cone is magnetized with a direction approximately opposite to the earth's present field and with a magnitude of magnetization of 3.9 A/m.
Radio emission associated with rock fracture: Possible application to the Great Chilean Earthquake of May 22, 1960
Authors: Warwick James W. , Stoker Carol , Meyer Thomas R.
Published: February 2008

Stress‐induced piezoelectric fields produce freely propagating electromagnetic radiation when microscopic rock fractures occur in quartz‐bearing rocks. A laboratory study is presented which shows that electromagnetic emissions are produced when microfractures occur in Westerly granite. A detailed model is presented for the emission process which allows an estimate of the total radiated power from an individual microfracture. On the basis of these results a case is presented that an unusual radio emission seen on several widely separated radio astronomy receivers in the northern hemisphere on May 16, 1960, was due to a stress‐induced microfracture along the Chilean fault. This radio event occurred 6 days prior to the great Chilean earthquake of May 22, 1960, and may have been a precursor to one of the largest earthquakes of this century.
Seismicity of the forearc marginal wedge (accretionary prism)
Authors: Chen Allen T. , Frohlich Cliff , Latham Gary V.
Published: February 2008

Three different types of seismic data have been examined for seismic events occurring within the zone called the accreted wedge or forearc marginal wedge that underlies the inner trench wall of some arcs. These types of data are (1) teleseismically recorded earthquakes that have been reported in the literature as occurring in major arc‐trench regions; these events fail to demonstrate that earthquakes occur within the accreted wedge because the uncertainty of focal depth usually exceeds the depth dimension of the accreted wedge; these data include many tsunamigenic earthquakes, (2) local earthquakes located by combined ocean bottom seismograph and land networks in the arc‐trench region in the New Hebrides and the central and eastern Aleutian Trench; none of the more reliable of these hypocenters lies within the accreted wedge; (3) S‐P intervals measured at stations on islands located on the outer ridge or at ocean bottom seismograph stations on the forearc marginal wedge; these data do not show the existence of events occurring within the accreted wedge; e.g., from 18 ocean bottom seismograph stations with a cumulative operation time of about 1 year, the smallest S‐P time is about 2.5 s for events in the New Hebrides and about 4 s for events in the Adak and Kodiak regions. We found no S‐P time smaller than 2 s from 6 years of seismograms recorded at Middleton Island, Alaska, and no S‐P time smaller than 4 s from 25 years of seismograms recorded on Barbados. All of the events could have occurred outside the forearc marginal wedge. Although we have located no events with hypocenters which are situated unambiguously within the accreted wedge, we have located many events which occurred in the vicinity of the accreted wedge. Some occur in the upper portion of the Benioff zone shallower than about 40 km and others occur in the leading edge of the overriding plate. These locations reported in this paper which are within or close to the seismic networks comprise some of the most reliably located shallow events ever reported in convergent margins of island arcs. The absence of seismic activity within the accreted wedge in any island arc that we studied suggests that the deformation evident in this region occurs aseismically.
Absorption band Q model for the Earth
Authors: Anderson Don L. , Given Jeffrey W.
Published: February 2008

Body wave, surface wave, and normal mode data are used to place constraints on the frequency dependence of Q in the mantle. With a simple absorption band model it is possible to satisfy the shear sensitive data over a broad frequency range. The quality factor Qs (ω) is proportional to ωα in the band and to ω and ω−1 at higher and lower frequencies, respectively, as appropriate for a relaxation mechanism with a spectrum of relaxation times. The parameters of the band are Q(min) = 80, α = 0.15, and width, 5 decades. The center of the band varies from 101 seconds in the upper mantle, to 1.6×103 seconds in the lower mantle. The shift of the band with depth is consistent with the expected effects of temperature, pressure and stress. High Qs regions of the mantle are attributed to a shift of the absorption band to longer periods. To satisfy the gravest fundamental spheroidal modes and the ScS data, the absorption band must shift back into the short‐period seismic band at the base of the mantle. This may be due to a high temperature gradient or high shear stresses. A preliminary attempt is also made to specify bulk dissipation in the mantle and core. Specific features of the absorption band model are low Q in the body wave band at both the top and the base of the mantle, low Q for long‐period body waves in the outer core, an inner core Qs that increases with period, and low QP/QS at short periods in the middle mantle. The short‐period Qs increases rapidly at 400 km and is relatively constant from this depth to 2400 km. The deformational Q of the earth at a period of 14 months is predicted to be 463.
Shock compression of aragonite and implications for the equation of state of carbonates
Authors: Vizgirda Joana , Ahrens Thomas J.
Published: February 2008

Hugoniot equation of state and release adiabat results are presented for c cut crystals of aragonite, the high‐pressure polymorph of calcite, shocked to pressures of up to 40 GPa. A Hugoniot elastic limit is observed at 2.5±0.8 GPa and is similar to that of calcite, which, depending on orientation, ranges from 1.5 to 2.5 GPa. A phase transition, possibly displacive, occurs between 5.5 and 7.6 GPa. Above shock pressures of ∼10 GPa, the aragonite and calcite Hugoniots are nearly coincident, suggesting transformation of both polymorphs to the same phase. Model calculations, attempting to characterize the high pressure CaCO3 phase are presented. Aragonite release adiabats centered at pressures between 9 and 14 GPa indicate that states with apparent zero‐pressure densities from 2.9 to 3.2 g/cm3 are achieved upon decompression from progressively greater shock pressures. Observed unloading paths from shock pressures above 17 GPa are significantly and consistently shallower (in a density‐pressure plane) than those from lower pressures, and zero‐pressure densities up to 20% below that of the initial aragonite density are achieved upon unloading; these features suggest that vaporization is occurring upon unloading. According to theoretical shock temperature and entropy calculations, however, the minimum shock pressure for vaporization upon release for aragonite is 55 GPa (and 33 GPa for calcite), significantly higher than the observed value.
Application of satellite magnetic anomaly data to Curie isotherm mapping
Authors: Mayhew M. A.
Published: February 2008

A boundary condition on temperature at depth in the continental crust can in principle be obtained by mapping of the Curie isotherm where it forms the base of the magnetic crust. An approach to mapping magnetic crustal thickness using satellite magnetic anomaly data is described. Because magnetic crustal thickness is small with respect to satellite altitude, inversion of the anomaly data yields magnetization models expressing regional variations in the vertical integral of magnetization. This parameter is inversely correlated with regional heat flow in the western United States, suggesting that areas of low apparent magnetization are areas of thin magnetic crust resulting from a shallow Curie isotherm. For a simple model of a uniformly magnetized magnetic crust (or intra‐crustal layer),Curie depth is linearly related to apparent magnetization. If the relation can be calibrated by two or more Curie depth estimates, regional apparent Curie isotherm configuration can be mapped. In an alternative approach, a family of Curie depth models can be computed using assumed magnetization values. In each case, the Curie depth configuration is used as a boundary condition in a thermal model. That model is selected which gives a heat flow distribution best fitting observations. Conductive heat flow models for a traverse across the Rio Grande Rift using a finite element technique and for a map grid using a one‐dimensional thermal model approximation are given as examples.
Moment tensor inversion of long period Rayleigh waves: A new approach
Authors: Romanowicz Barbara A.
Published: February 2008

A modification of the moment tensor inversion method is presented, which significantly improves depth and source mechanism resolution from complex spectra of Rayleigh waves in the period range 30–90 s. The procedure consists of two steps: First, the spectra are separated at each frequency into a depth sensitive and a depth insensitive component. The latter consists of biases and errors and is discarded. The former is then used alone to invert for depth and source mechanism. Each of the five resolvable components of the moment tensor can be monitored separately as a function of depth. The latter is best determined from the residuals curves corresponding to the dominant components, which depend on the source mechanism. Several examples of application are given. It appears that phase velocities determined using one larger event with a well constrained mechanism and depth from body waves can be successfully used as path corrections in a source region around it, 500–1000 km in radius. For other events in the region, depth can now be constrained to within ±5 km and source mechanism, to within ±5° in each of the three angles: strike, dip and slip. On the other hand, if only a rough estimate of depth and source mechanism is needed, regionalized phase velocities can now be used for path corrections. Finally, it is shown that, for certain depth ranges and certain mechanisms, the choice of the periods used in the inversions can be crucial in avoiding biases in depth determination.
Adiabats of quartz, coesite, olivine, and magnesium oxide to 50 kbar and 1000 K, and the adiabatic gradient in the Earth's mantle
Authors: Boehler R.
Published: February 2008

The adiabats of olivine, magnesium oxide, and quartz were measured up to 50kbar and 1000 K. An end‐loaded piston‐cylinder apparatus with an in situ pressure gauge and a very fine thermocouple was used to measure (∂T/∂P)s during adiabatic compression. A power law between (∂T/∂P)s and compression yields values of the power n = −∂ ln (∂T/∂P)s/∂ ln ρ that agree with previous results from salts, metals, and fluids. Assuming constant values for n, the adiabtic gradient for an olivine upper mantle and a magnesium oxide lower mantle was calculated. The results agree well with some previous theoretical estimates. The volume dependence of the Grüneisen parameter γ was calculated from the thermodynamic equation ∂ ln γ/∂ ln ρ = ∂B/∂P − n, where B is the isothermal bulk modulus. γ is found to a good approximation to be proportional to volume.

Table 6 is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J82‐002; $1.00. Payment must accompany order.
The Iceland Research Drilling Project in relation to the geology of Iceland
Authors: Fridleifsson Ingvar Birgir
Published: February 2008

The lava pile penetrated by the Iceland Research Drilling Project (IRDP) hole formed in an axial rift zone that remained active for at least the last 13 m.y. The stratigraphic sequence is formed of subaerial volcanics, and it is likely that volcanism was subaerial for several million years or even tens of millions of years prior to the formation of the IRDP sequence. The volcanics in eastern Iceland are tholeiitic, like all Tertiary sequences investigated in Iceland to date, and the lava extrusion rates in eastern Iceland are reported to be similar to those of Tertiary sequences in northern and western Iceland, respectively. The IRDP hole was sited in a dyke swarm extending from the Breiddalur central volcano in the south. The IRDP hole was sited in a regional thermal anomaly with a gradient of about 80°C/km that was found by a series of 100‐m thermal gradient wells in eastern Iceland. Aquifers of 48°C were encountered at about 600‐m depth, below which the thermal gradient lowered considerably. This suggests that the regional thermal anomaly is caused by the flow of warm water at relatively shallow depths.
Compression‐induced nonplanar crack extension with application to splitting, exfoliation, and rockburst
Authors: Nemat‐Nasser S. , Horii H.
Published: February 2008

Uniaxial compression of plates of brittle materials containing pre‐existing planar cracks oriented at certain angles with respect to the direction of overall compression has revealed that the relative frictional sliding of the faces of the pre‐existing cracks may produce, at their tips, tension cracks which deviate at sharp angles from the sliding plane. These tension cracks then continue to grow in a stable manner with increasing axial compression, curving toward an orientation parallel to the direction of axial compression. Within the framework of linear fracture mechanics, the out‐of‐plane extension of a pre‐existing straight crack, induced by overall far‐field compression, is analyzed, and various parameters which characterize the growth process are quantified. It is shown analytically that, for a wide range of pre‐existing crack orientations, the out‐of‐plane crack extension initiates at an angle close to 70° from the direction of the pre‐existing crack; the exact value of this angle, of course, depends on the friction factor and the orientation of the pre‐existing crack. It is found that the growth process is stable initially, but the rate of increase of the length of the extended portion with respect to the increasing axial compression dramatically increases after a certain extension length is attained, and in fact, this length becomes unbounded if a small lateral tension also exists. Various limiting cases are examined and the corresponding analytical estimates are compared with the numerical results, arriving at good correlations. A series of qualitative experiments is performed on thin plates of Columbia Resin CR 39, arriving at excellent agreement with the analytical results. In light of the analysis, the phenomena of axial splitting, exfoliation (or sheet fracture), and rockburst are examined, and it is suggested that they may all be the results of the out‐of‐plane (tensile) extension of pre‐existing cracks, induced by large overall far‐field compressions. This assertion is then supported by a series of experiments which show that the relative frictional sliding of the faces of one or even an array of pre‐existing cracks does not result in coplanar (sliding mode) crack growth, but rather leads to the formation of tension cracks which grow in the direction of maximum compression. Moreover, a pre‐existing crack close to a free boundary grows in a similar manner under compression parallel to the boundary, and shows no tendency to move toward the free surface. Possible lateral buckling which may result, and which may cause further unstable crack extension, is illustrated experimentally, and discussed in an effort to shed light on the phenomena of rockburst and surface spalling.
P wave travel times: Stability and change within the source volume of a M = 7.2 earthquake
Authors: Johnston Arch C. , Wyss M. , Koyanagi R. , Habermann R. E.
Published: February 2008

The dense seismograph network on the south flank of Kilauea afforded a unique opportunity to study travel time changes as a function of space and time within the source volume of the Hawaii earthquake (Ms = 7.2) of 1975. Careful analysis of more than 600 teleseismic P arrivals from deep Fiji‐Tonga earthquakes revealed the following: out of eight stations studied, six showed constant travel times to within ±0.03 s for up to 10 years with the exception of some small changes of about 0.03 s at the time of the main shock. One station, AHU, showed a highly significant and unique travel time decrease by 0.13 s during 1971/1972. At the beginning of this velocity anomaly a southwest rift intrusion caused closure of surface cracks associated with normal faults located near the station AHU. Also, geodetic measurements revealed that between August and October 1971 compressive strain of 4×10−5 was accumulated perpendicular to the southwest rift in the area of AHU. We conclude that these data show for the first time that in situ velocity increases occur due to the closure of cracks by tectonic forces. The AHU residuals returned to normal approximately at the time of a major earthquake swarm on the fault zone near AHU, during which surface cracks were observed to have opened again. This travel time decrease was corroborated by residuals from Novaya Zemlya explosions, but highfrequency local signals did not show it. Assuming that the stress sensitivity of the velocity was similar to that found by Reasenberg and Aki (1974), we have interpreted the teleseismic residual change at AHU as due to a P velocity increase of about 10% in the top 3.5±1.5‐km of the crust. The only station, WHA, which showed a large (0.2 s) and extended (1972 to 1975) travel time increase was located only 4‐km from the main shock epicenter. We interpret this velocity decrease as a precursor to the 1975 main shock, and we hypothesize that a process reverse from that at AHU caused this anomaly by first opening and then closing cracks in the crust below WHA. Both of these velocity anomalies were extremely localized in crustal volumes of dimensions of about 5±3‐km.
Parental basalts in subduction zones: Implications for continental evolution
Authors: Anderson Alfred T.
Published: February 2008

Basaltic liquid is parental to new crust in subduction zones. It occurs as inclusions of glass in crystals of olivine in andesites and dacites as well as basalts. Volcanoes which have never erupted a basaltic rock nevertheless have basaltic glasses included in extruded crystals. The basaltic liquids are rich in A12O3 and Cl and have higher Cl/K2O than associated andesitic liquids. Because decrease in Cl/K20 with increase in K2O and SiO2 implies vapor saturation, most andesitic liquids develop in vapor‐saturated environments. Estimated concentrations of H2O and CO2 in the liquids indicate that vapor saturation is probably restricted to the crust. Consequently, most andesite is a derivative magma generated from basaltic magma within the crust. New crust which is formed in subduction zones is basaltic in composition, not andesitic like the average upper continental crust. Consequently, continental crust is either basaltic in bulk or becoming less silicic. Alternatively, some other processes transport silica‐poor materials from the crust to the mantle, leaving a silicic crustal residue. The chlorine in the parental basaltic liquids probably is supplied by the complete destruction of amphibole and the extraction of about 10% partial melt in subducted oceanic lithosphere or other mantle. The basaltic composition of parental liquids in subduction zones is probably established in hot mantle overlying the Benioff zone, not the Benioff zone itself.
Paleomagnetism of Late Jurassic rocks in the Northern Canelo Hills, southeastern Arizona
Authors: Kluth Charles F. , Butler Robert F. , Harding Lucy E. , Shafiqullah Muhammad , Damon Paul E.
Published: February 2008

The Canelo Hills volcanics are exposed in the Canelo Hills, a northwest trending range in Santa Cruz County, southeast Arizona. The formation is composed of silicic tuffs and flows as well as volcaniclastic conglomerates and sandstones. Strikes of the rocks are generally to the northwest with moderate dips to the southwest and northeast. Apparent age results from the sequence studied paleomagnetically include two published isotopic dates of 147±6 Ma (K‐Ar, biotite) and 149±11 Ma (whole rock, Rb‐Sr) and a Rb/Sr isochron age, reported here, which indicates an age of 151±2 Ma. Paleomagnetic samples were collected from 17 cooling units in the northern Canelo Hills. Samples from most of these units responded to alternating field (af) demagnetization, and secondary components were generally erased by peak af between 10 and 50 mT. Samples from five sites showed no response to af demagnetization. Thermal demagnetization of samples from these units produced no significant changes in direction of natural remanent magnetization (NRM), although within‐site clustering of NRM directions was improved. Data from two sites were rejected because of failure to isolate a well‐determined characteristic NRM. Of the remaining 15 sites, 10 sites were of normal polarity, while five sites showed reversed polarity. Intensities of the characteristic NRM ranged from 4×10−3 to 3×10−1 A/m. The data from these 15 cooling units yield a formation mean direction of I = 29.9°, D = 334.9° with k = 33.4 and α95 = 6.7°. The resulting paleomagnetic pole is at 62.2°N, 130.3° (dp = 4.1°, dm = 7.4°). This pole is between poles obtained from the Summerville and lower Morrison formations. The Canelo Hills pole is thus consistent both in position and age with the Late Jurassic episode of rapid apparent polar wander originally defined by paleomagnetic data from the Summerville and Morrison formations.
Fault interaction near Hollister, California
Authors: Mavko Gerald M.
Published: February 2008

A numerical model is used to study fault stress and slip near Hollister, California. The geometrically complex system of interacting faults, including the San Andreas, Calaveras, Sargent, and Busch faults, is approximated with a two‐dimensional distribution of short planar fault segments in an elastic medium. The steady stress and slip rate are simulated by specifying frictional strength and stepping the remote stress ahead in time. The resulting computed fault stress is roughly proportional to the observed spatial density of small earthquakes, suggesting that the distinction between segments characterized by earthquakes and those with aseismic creep results, in part, from geometry. A nosteady simulation is made by introducing, in addition, stress drops for individual moderate earthquakes. A close fit of observed creep with calculated slip on the Calaveras and San Andreas faults suggests that many changes in creep rate (averaged over several months) are caused by local moderate earthquakes. In particular, a 3‐year creep lag preceding the August 6, 1979, Coyote Lake earthquake on the Calaveras fault seems to have been a direct result of the November 28, 1974, Thanksgiving Day earthquake on the Busch fault. Computed lags in slip rate preceding some other moderate earthquakes in the area are also due to earlier earthquakes. Although the response of the upper 1 km of the fault zone may cause some individual creep events and introduce delays in others, the long‐term rate appears to reflect deep slip.
Pressure dependence to 100 kilobars of the Phonons of MgO at 90 and 295 K
Authors: Chopelas Anastasia , Nicol Malcolm
Published: February 2008

Vibronic spectra of V2+ and Cr3+ impurities in MgO have been used to determine the pressure dependences of the energies of MgO phonons at several critical points of the Brillouin zone to more than 100 kbar at 90 and 295 K. From these data, approximate densities of states for MgO at 100 and 500 kbar and vibrational partition functions were constructed, and contributions to the constant‐volume heat capacity, entropy, internal and free energies, and the Debye temperature were calculated. Argon was successfully used as a nearly hydrostatic pressure medium for the measurements at 90 K to 102 kbar.
Attenuation in partially saturated rocks
Authors: Mochizuki Satoshi
Published: February 2008

Based on limited experimental data, the attenuation of acoustic waves in partially saturated rocks has been suspected to result from the effect of fluid flow. Global flow is accompanied by a viscous energy loss, which results in the attenuation of acoustic waves. In this theoretical study, we explore the effects of global flow and gas saturation level on acoustic wave attenuation. The attenuation predicted by theory is much smaller than that actually supported by experimental data for sandstones at low frequencies (≦1000 Hz). The technique involves the use of Biot's equations of two interacting continua that have been modified to accommodate confined gas and liquid by introducing effective and average values of fluid compressibility, fluid mass, and viscosity.
Dispersion of very long‐period rayleigh waves along the East Pacific Rise: Evidence for S wave velocity anomalies to 450 km depth
Authors: Wielandt E. , Knopoff L.
Published: February 2008

The phase dispersion of fundamental‐mode Rayleigh waves from the second Gazli earthquake has been measured at periods between 40 and 400 s between Los Angeles and the South Pole. Over most of its length, the path between the two stations is parallel and close to the East Pacific Rise. The phase velocities we have observed are significantly lower than the global averages. The inversion for the oceanic part of the path gives a shear‐wave velocity model with (1) unusually low velocities down to a depth of about 200 km, with a minimum of 4.1 km/s around 100 km, (2) normal or slightly less than normal velocities between depths of 200 km and about 400 km, and (3) a depression of the olivine‐spinel phase boundary by about 50 km, or an equivalent velocity reduction around a depth of 450 km in a model without mantle discontinuities. We can make no definitive assertions regarding deeper anomalies in the mantle. The velocity anomalies below a depth of 200 km are consistent with the assumption that the temperature of the upper mantle between the low velocity zone and the spineloxide transition at 671 km in model 1066B, is elevated by 140° K relative to the average earth. Corrections to customary data processing procedures are offered that are designed to eliminate bias in estimates of dispersion at periods greater than 240 s.
On the compensation of geoid anomalies due to subducting slabs
Authors: McAdoo David C.
Published: February 2008

Candidate models of the forces that oppose the sinking of slabs are all constrained to produce results consistent with the following observation: relative geoid highs, which one assumes are due to slabs, characteristically occur over subduction zones. However, certain models of subduction‐induced flow, such as one based on a Newtonian half‐space mantle, yield surface depressions and concomitant geoid lows that more than fully compensate the geoid highs due to the slab alone. This study has extended a published model of viscous corner flow in subduction zones in order to demonstrate that it can—in certain cases—produce the requisite net geoid highs. Specifically, the relative geoid highs are produced if mantle flow is distinctly non‐Newtonian (stress exponent n > 2). Results in the form of geoid slope or horizontal gravity disturbance profiles are computed for typical values of the slab parameters; they are compared with a representative profile of geoid slopes derived from SEASAT altimeter data in order to show qualitative similarities. It is concluded that the effect of non‐Newtonian flow as opposed to Newtonian is to spread out the induced surface deformation, thereby stretching out the regional compensation to wavelengths (transverse to the trench) of many thousand kilometers.
Recomputation of vertical crustal motions near Palmdale, California, 1959–1975
Authors: Holdahl Sandford R.
Published: February 2008

By analysis of leveling data, a large area in southern California was identified as having uplifted between 1960 and 1974; the pattern and magnitude of the apparent movement were first discussed by Castle et al. (1976). The uplift zone included 12,000 km2 with the maximum vertical movement of 25 cm centered near Palmdale. Castle (1978) later increased the estimate of uplift at Palmdale to 35 cm. The pattern of uplift was constructed by reviewing profiles of repeated levelings. After field experiments confirmed that the southern California leveling data were contaminated by refraction errors, Holdahl (1981) developed a temperature stratification model to enable retroapplication of Kukkamaki's refraction correction to the affected data. Vertical motion in the Palmdale region has now been reassessed by simultaneous least squares adjustment of the networks formed by the corrected observations. The new results show 7.5±4.0 cm of apparent upward movement at Palmdale. Part or all of the 7.5 cm might be attributable to residual systematic error. The great concern associated with Palmdale, if based on motion calculated from leveling data, does not seem warranted. Castle's original result was strongly influenced by a single level route to Palmdale. Refraction error accumulates rapidly on this route because of the long gentle slope and the location of sightings over railroad ballast where vertical temperature gradients are unusually large. Perhaps more important than the existence of the Palmdale Bulge is the question of whether large aseismic vertical motions of entire regions can actually occur in a short time span. The Palmdale Uplift is often referred to as a good example of this type of motion. The rigorous analysis of refraction‐corrected leveling data described in this paper supports the approximate computations of Strange (1981) that indicated that what was thought to be uplift at Palmdale was only the appearance of uplift created by different refraction error accumulations in successive surveys. The primary cause of this difference was reduced sight lengths starting in 1964.
The structure and evolution of ancient impact basins on Mars
Authors: Schultz Peter H. , Schultz Richard A. , Rogers John
Published: February 2008

The number of Martian impact basins, once thought deficient owing to a low number of impactors, has been significantly increased through the identification of structural imprints that remain even after nearly complete erasure of basin massifs. Five ancient multiringed impact basins were selected for detailed study and exhibit a systematic pattern of rejuvenation of the multiring plan through endogenic modification. Concentric zones of unstable terrain are believed to reflect deep‐seated, impact‐related fractures that have localized regional igneous processes in a manner analogous with the basin‐controlled distribution of basalts on the moon. Most Martian outflow channels were found to originate along the unstable ring zones. Hydrothermal melting and temporary confinement of channel source materials at sites analogous to lunar sinuous rille source regions are proposed to play an important role in the distribution of Martian outflow channels. Ancient basins also can be delineated by the concentric pattern of topographically and structurally controlled narrow valley networks that are particularly abundant along the outer rings where basin ejecta deposits occur. Remnant massifs and scarps commonly exhibit extensive furrowing by narrow valley networks, perhaps due to a different paleoclimate or sapping during a period of geothermal melting of trapped ices. The systematic patterns of valley networks, stable and unstable annuli, scarps, and massifs provide bases for interpreting impact structure in general and for recognizing multiringed structures elsewhere. The Chryse basin, in particular, is proposed to represent a major multiringed structure that controlled the formation and distribution of eastern Valles Marineris and the chaotic terrains of Margaritifer Sinus in response to volcanism/tectonism related to Tharsis. We conclude that (1) Mars is probably not deficient in large‐body impactors; (2) the multiring patterns of basins represent deep‐seated fracture zones that can be reexposed through subsequent volcanism, hydrothermal activity, sapping, and differential erosion; (3) many Martian terrains and features (e.g., chaotic terrains and outflow channels) are controlled by ancient basins; and (4) the resurrection and destruction of impact basins reveal an important third dimension (depth) for interpreting impact basin formation.
Spectral characteristics of the iron oxides with application to the Martian bright region mineralogy
Authors: Sherman David M. , Burns Roger G. , Burns Virginia Mee
Published: February 2008

In an effort to determine the nature and significance of the iron oxide mineralogy on Mars, reflectance spectra of eight polymorphs of FeOOH and Fe2O3 are presented together with additional data on these materials. The Kebulka‐Munk theory is used to qualitatively constrain the effect of other components that might interfere with iron oxide absorption features. Although the wavelength range 0.4–1.0 μm is potentially the most diagnostic spectral region for delineating the iron oxide phases, the effect of temperature complicates the identification of a given Fe3+ phase based on the position of the 6A1→4T1 absorption feature. The Fe3+ crystal field transitions are spin forbidden, but most of the iron oxide polymorphs exhibit anomalously intense crystal field absorption features resulting from magnetic coupling between adjacent FeO6 octahedra. The resulting deviations from observed remote sensed reflectance spectra of Mars may provide a basis for the exclusion of many iron oxide phases as significant components of the Martian Fe3+ mineralogy. Comparison with the visible region spectra of Martian bright regions suggest that the predominant Fe3+‐bearing phase may be a magnetically disordered material. Possible candidates include amorphous gels, some ferric sulphates, and other minerals in which Fe3+ ions in the crystal structure are not magnetically coupled.
Daily temperature variations on Mars
Authors: Ditteon Richard
Published: February 2008

The Viking infrared thermal mapper (IRTM) has measured the infrared brightness temperature of Mars for more than a full Martian year. The daily variation of the 20‐μm brightness temperature can be predicted to within ±5 K for most of the Martian surface during any season if the radiometric albedo and the thermal inertia of the model are allowed to increase with increasing atmospheric dust opacity. However, for approximately 32% of the Martian surface area, no values of thermal inertia or albedo can fit the thermal observations. These temperature anomalies isolate one large region on Mars which connects the low thermal inertia regions of Arabia, Tharsis, and Elysium. The anomalies do not correlate with elevation, geologic units, morphology, or atmospheric dust content. All regions with Lambert albedo less than 0.18 can be well fit with the standard thermal model, but all areas with albedo greater than 0.28 are anomalous. There is a strong inverse correlation between the magnitude of the anomaly and the thermal inertia. This correlation indicates that some surface property is responsible for the anomaly. In the anomalous region the observed temperatures are warmer in the morning, cooler late in the afternoon, and decrease more slowly during the night than the Viking model temperatures. Of all the physical processes likely to occur on Mars, but not included in the Viking thermal model, only a layered soil can explain the observations. A low thermal inertia layer covering a high thermal inertia material will heat up quickly in the morning, and cool down quickly in the afternoon. At night the high thermal inertia material will moderate the temperature. A possible interpretation for the layering deduced from the IRTM observations is a layer of aeolian deposited dust about one thermal skin depth thick (1 to 4 cm), covering a duricrust. Because dust accumulates in some areas after every global dust storm, the duricrust must be actively forming. A one thermal skin depth dust layer covering a duricrust is compatible with Viking lander observations, IRTM spectral observations, and Viking orbiter imaging.
Infrared observations of Phobos and Deimos from Viking
Authors: Lunine Jonathan I. , Neugebauer Gerry , Jakosky Bruce M.
Published: February 2008

The surface thermal properties of Phobos and Deimos have been determined from observations made with the Viking Orbiter Infrared Thermal Mapper (IRTM), at wavelengths ranging from 6 to 20 μm. The data, composed of both global and high‐resolution infrared photometry of the satellite surfaces as well as eclipse observations, indicate surface material of low thermal conductivity comparable to that of the earth's moon. Values of the thermal inertia I consistent with the data for Phobos are 0.9 ≲ I ≲ 1.6 × 10−3 cal cm−2 s−1/2 K−1, and 0.6 ≲ I ≲ 2.0 × 10−3 cal cm−2 s−1/2 K−1 for Deimos. It is concluded that both satellites are covered with a vertically uniform layer of finely divided material at least several centimeters thick. Observed differences between brightness temperatures at different wavelengths on Phobos are due mainly to topographic slopes and to the presence of ∼5% by area high inertia or blocky material.
On the recurrence time of great earthquakes on a long transform fault
Authors: Bonafede Maurizio , Boschi Enzo , Dragoni Michele
Published: February 2008

A Maxwell viscoelastic model is proposed for the lithosphere in the proximity of a long, vertical strike slip fault. In this framework, formulas for the recurrence time of great earthquakes are derived by imposing boundary conditions of (1) uniform strain rate in the fault region or (2) steady, aseismic sliding at depth. In both cases a minimum effective viscosity of the fault region is found in order that a critical stress for earthquake occurrence can be reached; at higher viscosities the recurrence time approaches a minimum, limiting value. Application of the model to the northern ‘locked’ segment of the San Andreas fault would indicate a recurrence time significantly longer than the often quoted 100‐year period. These results differ considerably from those obtained from similar models employing boundary conditions of assigned stress at large distance from the fault.
Pn velocity and cooling of the continental lithosphere
Authors: Black Paul R. , Braile Lawrence W.
Published: February 2008

Average upper mantle Pn velocities and heat flow were computed within continental physiographic provinces in North America from published data. Pn velocity and heat flow data display an inverse relationship and were found to correlate with a statistically significant correlation coefficient. Temperatures at the crust‐mantle boundary were estimated from the heat flow values, and these were used to demonstrate a correlation between Pn velocity and temperature of upper‐mantle material. The value of (∂Pn/∂T)P thus obtained (−4.4×10−4 to −8.1×10−4 km s−1 °C−1) is within the range of temperature derivatives determined from laboratory studies of ultramafic rocks. This dependency of Pn velocity on temperature implies that one possible explanation for the observed geographical distribution in uppermantle seismic velocity is that the variation in Pn velocity is primarily a temperature effect. Combined with the relationship between heat flow and crustal age for continents, the Pn versus heat flow correlation suggests a relationship between Pn velocity and crustal age, probably due to progressive cooling of the continental lithosphere after a thermotectonic event.
Waveforms and spectra of preshocks and aftershocks of the 1979 Imperial Valley, California, earthquake: Evidence for fault heterogeneity?
Authors: Pechmann James C. , Kanamori Hiroo
Published: February 2008

We have compared digitally‐recorded waveforms of ML 2.0–2.8 earthquakes that occurred in two small areas along the Imperial fault before and after it broke in the ML 6.6 Imperial Valley earthquake on October 15, 1979. Eight preshocks (1977–1979) from a 4½ by 1½ km area centered 4 km SE of the mainshock epicenter have strikingly similar waveforms over the entire record length (∼30 s), with an average peak cross correlation between seismograms of 0.74. The seismograms are well correlated at frequencies up to at least 4 Hz. This implies similar source mechanisms and hypocenters within ¼ of the 4‐Hz wavelengths, i.e., <200–400 m. Five aftershocks from the same area show an average peak cross correlation between seismograms of only 0.23. Any associated changes in mechanism must be small because they are not reflected in the first motion data. Analysis of frequency content of these events using bandpass‐filtering techniques showed no systematic temporal changes in spectral shape. Ten preshocks and 24 aftershocks from a 1½ by 2 km source area centered along the fault 16 km NW of the mainshock epicenter were also studied. First motion data suggest that all of the aftershocks and a swarm of six preshocks on December 7–9, 1978, were associated with the main fault but that four earlier preshocks were not. The six preshocks on December 7–9, 1978, were tightly clustered, as evidenced by the strong similarity of the waveforms (most peak cross correlations ≥0.6). During this swarm the 8‐ to 16‐Hz spectral amplitude increased relative to the 1‐ to 2‐Hz spectral amplitude over the whole record length by about a factor of 3, suggesting a systematic increase in stress drop. Groups of like events are also present among the aftershocks in this data set. The average peak correlation for pairs of aftershocks, 0.43, is almost the same as that for pairs of preshocks, 0.45, if all 10 preshocks are included. However, several sources appear to have been active simultaneously during the aftershock period so that no more than two to three consecutive aftershocks have maximum cross correlations ≥0.6. The highly localized sources characterized by waveform similarity may represent fault asperities or clusters of asperities. Our observations are consistent with a decrease in the number of these asperities as the weaker ones fail under increasing stress during the intervals between large earthquakes.
Continental rifting and the implications for plate tectonic reconstructions
Authors: Vink Gregory E.
Published: February 2008

Previous plate tectonic reconstructions have tried to recreate the pre‐rifting (Pangea) configuration of the continents by matching contours or lineaments that are thought to represent the continental boundaries. Such reconstructions have the inherent assumptions that no extension occurs within the continent during rifting, that the continental boundaries are isochrons, and that the continents rift without distortion. This paper proposes a model for continental breakup with distortion, where ocean basins are formed by propagating rifts. This model challenges the assumptions made by previous reconstructions and presents a new method for representing the prerift geometry of the continents. As rifts propagate through a continent, the region in front of the rift extends by continental faulting and crustal thinning while the region behind the rift expands by seafloor spreading. The propagating rift model implies that large amounts of continental extension (up to 150 km) occur and that the continental boundaries are not isochrons. This extension due to rifting results in ‘apparent’ overlap when the continents are returned to their original configuration. Because of the gradational nature of this extension, the best representation of the pre‐rift configuration is obtained by matching the initial rifting point and having overlap increase in the direction of rift propagation. The overlap is equivalent to the amount of extension that has taken place on both continental edges. Reconstructions based on this model are presented for the Gulf of California, the Gulf of Aden, the Norwegian‐Greenland Sea, and the South Atlantic Ocean. They are able to resolve the discrepancies that occur when rigid plate tectonics are applied. This model has implications that can be used to predict the age and structure of the continental shelves and the location and orientation of the oldest magnetic isochrons.
Further efforts to limit lunar internal temperatures from electrical conductivity determinations
Authors: Hood L. L. , Herbert F. , Sonett C. P.
Published: February 2008

A Monte Carlo procedure is used to generate a representative set of eighteen electrical conductivity profiles that are consistent with previously published lunar transfer function data in the frequency range 10−5 to 10−3 Hz. The electrical conductivity is most strongly constrained by these data in the approximate depth range 450 to 1350 km. Published laboratory electrical conductivity versus temperature data (extrapolated to higher and lower temperatures) are applied to convert the eighteen electrical conductivity profiles into temperature profiles for a series of radially homogeneous olivine‐pyroxene mixtures ranging from 100% olivine [(Mg/(Mg + Fe) = 0.91] to 100% aluminous orthopyroxene (6.8 wt% Al2O3). The 100% olivine composition yields selenotherms that approach the Ringwood‐Essene solidus at depths near 500 km while the addition of aluminous orthopyroxene (either 1.9 or 6.8 wt% Al2O3) in concentrations exceeding 15–30 vol% leads to cooler selenotherms that approach the solidus only at depths greater than about 1000 km. The shape of the temperature‐depth profiles is not found to be strongly sensitive to the olivine/pyroxene ratio if the assumption of radial homogeneity is valid at depths greater than 450–500 km.

On the basis of independent geophysical constraints (low seismic shear wave attenuation in the upper mantle, maintenance of mascon anisostasy over 3–4 b.y., locations of the deep moonquake foci), the profile envelopes for compositions containing more than 15–30 vol% aluminous orthopyroxene are considered to be most probable. A comparison of the latter with present day selenotherms calculated according to thermal history models indicates the greatest consistency with the model of Toksöz et al. [1978] which assumed initial partial melting and differentiation to a depth of 500 km and significant radial heat transport by subsolidus convection only during the first few b.y. of lunar history.