Showing papers in "Journal of Geophysical Research in 1979"

A moment magnitude scale

Abstract: The nearly coincident forms of the relations between seismic moment M_0 and the magnitudes M_L, M_S, and M_w imply a moment magnitude scale M = ⅔ log M_0 - 10.7 which is uniformly valid for 3 ≲ M_L ≲ 7, 5 ≲ M_s ≲ 7½, and M_w ≳ 7½.

Modeling of rock friction: 1. Experimental results and constitutive equations

Abstract: Direct shear experiments on ground surfaces of a granodiorite from Raymond, California, at normal stresses of ∼6 MPa demonstrate that competing time, displacement, and velocity effects control rock friction. It is proposed that the strength of the population of points of contacts between sliding surfaces determines frictional strength and that the population of contacts changes continuously with displacements. Previous experiments demonstrate that the strength of the contacts increases with the age of the contacts. The present experiments establish that a characteristic displacement, proportional to surface roughness, is required to change the population of contacts. Hence during slip the average age of the points of contact and therefore frictional strength decrease as slip velocity increases. Displacement weakening and consequently the potential for unstable slip occur whenever displacement reduces the average age of the contacts. In addition to this velocity dependency, which arises from displacement dependency and time dependency, the experiments also show a competing but transient increase in friction whenever slip velocity increases. Creep of the sliding surface at stresses below that for steady state slip is also observed. Constitutive relationships are developed that permit quantitative simulation of the friction versus displacement data as a function of surface roughness and for different time and velocity histories. Unstable slip in experiments is controlled by these constitutive effects and by the stiffness of the experimental system. It is argued that analogous properties control earthquake instability.

Structure under Mount Rainier, Washington, inferred from teleseismic body waves

Abstract: Teleseismic long-period P waves recorded at the World-Wide Standard Seismograph Network station LON (Longmire, Washington) are shown to exhibit strong anomalous particle motion not attributable to instrument miscalibration or malfunction. In particular, a large and azimuthally smoothly varying tangential component is observed after vector rotation of horizontal P waves into the ray direction and after application of a deconvolution technique which equalizes effective source time functions and removes the instrument response. These tangential waves attain amplitudes comparable to the radial component and demonstrate wave form antisymmetry about a NNE azimuth. A model which contains a single high-contrast interface dipping toward the NNE at a depth of 15–20 km can explain most of the characteristics of the long-period P wave data, provided dips are greater than about 10° and only the interference of P and Ps generated at the interface is considered. The model breaks down for later arrivals which are presumably multiples or scattered waves. Examination of long-period S waves from several deep teleseisms shows a prominent Sp arrival 18 s before S. The timing of this phase conversion suggests an interface at about 145-km depth, and its sense of polarity suggests that the velocity contrast is from higher to lower velocities as depth decreases. This interface may correspond to the bottom of the upper mantle low-velocity zone in the area.

Combined wave and current interaction with a rough bottom

Abstract: An analytical theory is presented to describe the combined motion of waves and currents in the vicinity of a rough bottom and the associated boundary shear stress. Characteristic shear velocities are defined for the respective wave and current boundary layer regions by using a combined wave-current friction factor, and turbulent closure is accomplished by employing a time invariant turbulent eddy viscosity model which increases linearly with height above the seabed. The resulting linearized governing equations are solved for the wave and current kinematics both inside and outside the wave boundary layer region. For the current velocity profile above the wave boundary layer, the concept of an apparent bottom roughness is introduced, which depends on the physical bottom roughness as well as the wave characteristics. The net result is that the current above the wave boundary layer feels a larger resistance due to the presence of the wave. The wave-current friction factor and the apparent roughness are found as a function of the velocity of the current relative to the wave orbital velocity, the relative bottom roughness, and the angle between the currents and the waves. In the limiting case of a pure wave motion the predictions of the velocity profile and wave friction factor from the theory have been shown to give good agreement with experimental results. The reasonable nature of the concept of the apparent bottom roughness is demonstrated by comparison with field observations of very large bottom roughnesses by previous investigators. The implications of the behavior predicted by the model on sediment transport and shelf circulation models are discussed.

Global climatic interpretation of the deuterium‐oxygen 18 relationship for precipitation

Abstract: A theoretical model is derived to account for the deuterium-oxygen 18 relationship measured in meteoric waters. A steady state regime is assumed for the evaporation of water at the ocean surface and the subsequent formation of precipitation. The calculations show that the deuterium and oxygen 18 content in precipitation can be taken as linearly related. From the slope and the intercept (known as the deuterium excess) of the δD-δ18O linear relationship for precipitation we compute the mean values on a global scale of the evaporating ocean surface temperature and the relative humidity of the air masses overlying the oceans. The deuterium excess is primarly dependent on the mean relative humidity of the air masses formed above the ocean surface. Paleoclimatic data may be obtained by this isotopic method from the analysis of old water and ice samples. A moisture deficit of the air over the ocean, equal to only 10%, in comparison to 20% for modern conditions, is deduced from the deuterium-oxygen 18 distribution measured in groundwater samples older than 20,000 years.

Back-arc opening and the mode of subduction

Abstract: Trench-arc systems (subduction zones) can be classified into two types depending on whether or not actively opening back-arc basins are associated with them. This suggests that subduction of an oceanic plate is not a sufficient condition for back-arc opening, though it may be necessary one. Mechanisms that cause the distinction between the two types have been investigated. Earthquake studies suggest that there is a significant difference in the mode of plate motion at interplate boundaries between the two types of trench-arc systems. Extreme cases are Chile, where plate motion is seismic, and the Marianas arc, where it is aseismic. This difference seems to indicate that the stress state in the back-arc area differs between the two types: compression in the Chilean type and tension in the Marianas type. This difference in the stress state is also manifested in other tectonic features, such as topography, gravity, volcanic activity, and crustal movement. Two possible mechanisms for the difference between the two types are suggested: (1) The nature of the contact zone between upper and lower plates changes from tight coupling (Chile) to decoupling (the Marianas) through the evolutionary process of subduction. The decoupling results in an oceanward retreat of the trench and back-arc opening. (2) The downgoing slab is anchored to the mantle, so that the position of a trench is also fixed with respect to the mantle. Since the motion in the mantle is slow compared to that of surface plates, it is the motion of the landward plate which controls the opening and nonopening of back-arcs.

Constant Q-wave propagation and attenuation

Abstract: A linear model for attenuation of waves is presented, with Q, or the portion of energy lost during each cycle or wavelength, exactly independent of frequency. The wave propagation is completely specified by two parameters, e.g., Q and c0, a phase velocity at an arbitrary reference frequency ω0. A simple exact derivation leads to an expression for the phase velocity c as a function of frequency: c/c0 = (ω/ω0)γ, where γ = (1/π) tan−1 (1/Q). Scaling relationships for pulse propagation are derived and it is shown that for a material with a given value of Q, the risetime or the width of the pulse is exactly proportional to travel time. The travel time for a pulse resulting from a delta function source at x = 0 is proportional to xβ, where β = 1/(1 - γ). On the basis of this relation it is suggested that the velocity dispersion associated with anelasticity may be less ambiguously observed in the time domain than in the frequency domain. A steepest descent approximation derived by Strick gives a good time domain representation for the impulse response. The scaling relations are applied to field observations from the Pierre shale formation in Colorado, published by Ricker, who interpreted his data in terms of a Voigt solid with Q inversely proportional to frequency, and McDonal et al., who interpreted their data in terms of nonlinear friction. The constant Q theory fits both sets of data.

Continental delamination and the Colorado Plateau

Abstract: Continental lithosphere is in unstable mechanical equilibrium because its mantle layer is denser than the asthenosphere. If any process such as cracking, slumping, or plume erosion initially provided an elongated conduit connecting the underlying asthenosphere with the base of the continental crust, the dense lithospheric boundary layer could peel away from the crust and sink. An analytic model for sinking velocities at the critical initial time shows that instability occurs if the effective viscosities of the lower continental crust and the rising asthenosphere are no more than 1019 P. Analogies to subduction suggest that the mature instability would grow laterally at plate tectonic velocities; however, it would be almost aseismic. Loss of the cold mantle boundary layer would cause uplift, increased heat flow, reduced seismic velocities, and perhaps emplacement of basalt flows, mantle diatremes, and granodiorite sills. A one-dimensional thermal model of the formation of a new boundary layer predicts a half life of about 3×107 years for this thermal anomaly and uplift. As an example, the geologic and geophysical data from the Colorado Plateau are shown to be consistent with the hypothesis that it was uplifted by a delamination event 30 m.y. ago and perhaps a second event about 5 m.y. ago.

A large-scale numerical model of sea ice

Abstract: The described large-scale sea ice model, which is capable of coupling with atmospheric and oceanic models of comparable resolution, simulates the yearly cycle of ice in both the Northern and Southern Hemispheres. Model results for the yearly cycle of sea ice thickness and extent in both the Arctic and the Antarctic are presented. Horizontally the model resolution is approximately 200 km, while vertically four layers - ice, snow, ocean, and atmosphere - are considered. Thermodynamic processes based on energy balances at the various interfaces and dynamic processes based on wind stress, water stress, Coriolis force, internal ice resistance, and the stress from the tilt of the sea surface are incorporated. It is assumed that the ice within a given grid square is of uniform thickness, although each square has a variable percentage of its area ice free.

Active faulting and cenozoic tectonics of the Tien Shan, Mongolia, and Baykal Regions

Abstract: We present a study of the active tectonics of central Asia based on an interpretation of Landsat imagery and supplemented with published field observations and seismic data. Reverse faulting dominates the tectonics of the Tien Shan but is associated with prominent northwest trending right lateral strike slip fault systems. Both types of faulting imply approximately north-south maximum compressive stress. The active tectonics of the Altai and of southern Mongolia are controlled by largescale conjugate strike slip faulting; left lateral on east-west planes and right lateral on north-northwest planes. This implies that the maximum compressive stress is oriented approximately northeast-southwest. Farther north, strike slip faulting gives way to predominantly normal faulting in the Baykal rift system. We interpret all of the active faulting to be a consequence of lateral displacements of the crust caused by the penetration of the Indian subcontinent of Eurasia. We also interpret the gradual change from thrust faulting and high altitudes in the south and west to normal faulting and lower mean elevations in the north and east to reflect a smooth change in the average state of stress. This suggests that the details of the complex intracontinental deformation in Asia are better described by the deformation of a continuum than by the relative motion of a small number of rigid blocks. Intracontinental rifting in the northeast, in particular, may result from a state of stress analogous to the secondary tension that commonly arises within bounded plastic materials indented by a rigid die.

Dayside merging and cusp geometry

Abstract: Geometrical considerations are presented to show that dayside magnetic merging when constrained to act only where the fields are antiparallel results in lines of merging that converge at the polar cusps. An important consequence of this geometry is that no accelerated flows are predicted across the dayside magnetopause. Acceleration owing to merging acts in opposition to the magnetosheath flow at the merging point and produces the variably directed, slower-than-magnetosheath flows observed in the entry layer. Another consequence of the merging geometry is that much of the time closed field lines constitute the subsolar region of the magnetopause. The manner in which the polar cap convection patterns predicted by the proposed geometry change as the interplanetary field is rotated through 360 deg provides a unifying description of how the observed single circular vortex and the crescent-shaped double vortex patterns mutually evolve under the influence of a single operating principle.

A model for the evolution of the Indian Ocean and the breakup of Gondwanaland

Abstract: Magnetic anomaly and fracture zone information is used to develop a self-consistent tectonic history of the Indian and South Atlantic oceans. Working backward in time we have made three reasonably well constrained (39, 53, and 65 Ma) and two speculative (80 and 115 Ma) reconstructions of the positions of the Gondwana continents (Ma is m.y.B.P.). Our final fit, which is constrained by the recognition of Mesozoic anomalies off Antarctica and in the Mozambique Basin, places Dronning Maud Land against southern Mozambique and Madagascar in the northern position against Kenya. We suggest that after the initial rifting, Antarctica moved away from Africa in a southerly direction relative to present-day Africa. This started the formation of the Southwest Indian Ridge. Most of the present length and geometry of the ridge result from migration of triple junctions so do not reflect predrift continental outlines. India and Madagascar moved with Antarctica until India separated from first Antarctica then Madagascar, when it started moving north toward Asia. In our reconstructions we find no necessity for significant relative motion between the Antarctic Peninsula and South America from the early Cretaceous to the Oligocene. From the breakup of Gondwanaland to the present we identify seven significant events. These are (1) first break in the late Triassic/early Jurassic between East and West Gondwanaland with initial motion along long transform faults parallel to the present African east coast, (2) early Cretaceous separation of Africa and South America and possibly simultaneous separation between India and Australia-Antarctica, (3) cessation of motion between Africa and Madagascar, (4) break between India and Madagascar in the late Cretaceous, (5) Paleocene reorganization in the northwest Indian Ocean when the Seychelles left India, (6) Eocene separation between Australia and Antarctica with Australia joining the Indian plate, and (7) India's collision with Asia and subsequent commencement of spreading on the Central Indian Ridge, and later opening of Drake Passage.

The Mesozoic South Atlantic Ocean and evolution of its continental margins

Abstract: Gravity and magnetic anomalies bordering the continental margins of the southern South Atlantic Ocean are compared, in detail, on conjugate sides of the ridge crest, and a model for the boundary between oceanic and continental basement is given. The area of study includes the predominantly sheared margins of the Agulhas-Falkland fracture zone and the rifted margins of Argentina and southern Africa south of the Rio Grande Rise and Walvis Ridge, respectely. These margins have associated with them, for the most part, linear magnetic anomalies that can be modeled as edge effect anomalies separating oceanic from continental basement. Coincident with the magnetic anomalies are gradients in the isostatic gravity anomaly. We have taken the location of these geophysical lineaments on the African margin and rotated them clockwise to fit the anomalies on the Argentine margin. This fit, which gives us a new pole of total closing for the South Atlantic Ocean, obviates, for the most part, the gaps and overlaps observed in other reconstructions. The improved fit thereby suggests rigid plate behavior and minimum stretching of continental crust during the early opening of the southern South Atlantic Ocean.

Effect of surface roughness on the microwave emission from soils

Abstract: The effect of surface roughness on the brightness temperature of a moist terrain was studied through the modification of Fresnel reflection coefficient and using the radiative transfer equation. The modification involves introduction of a single parameter to characterize the roughness. It is shown that this parameter depends on both the surface height variance and the horizontal scale of the roughness. Model calculations are in good quantitative agreement with the observed dependence of the brightness temperature on the moisture content in the surface layer. Data from truck mounted and airborne radiometers are presented for comparison. The results indicate that the roughness effects are greatest for wet soils where the difference between smooth and rough surfaces can be as great as 50K.

Properties and effects of dust particles suspended in the Martian atmosphere

Abstract: Direct measurements of the optical depth above the two Viking landers are presented for a period covering the summer, fall, and winter seasons in the Northern Hemisphere, a time period during which two global dust storms occurred. The data are used to define the properties of suspended dust particles in the Martian atmosphere and to assess their role in a number of meteorological and geological processes. Major conclusions are that (1) both the radiative effects of dust particles and the thermodynamical effects of large-scale atmospheric motions have a significant impact on the vertical temperature structure; (2) Pertinent feedback effects play an important part in the generation of some local dust storms, in the expansion of local dust storms to global proportions, and in the subsequent decay of global dust storms; (3) An important mechanism for the removal of dust particles from the atmosphere is the CO2 condensation-sedimentation process; and (4) that the polar laminae are constructed from atmospheric dust and water ice is hypothesized.

Characterization of barriers on an earthquake fault

Abstract: Recently, Bouchon (1979a) reinterpreted strong motion seismograms obtained during the Parkfield earthquake of 1966 using a new method applicable to a finite propagating dislocation source in a layered medium. His results and other pertinent data, interpreted in terms of the barrier model of Das and Aki (1977), suggest that the rupture may be stopped by a barrier with the specific fracture energy of about 109 erg cm−2. Using the formulas of Ida (1973a), we estimated parameters of the barrier as follows: breaking slip of about 20 cm, cohesive stress of about 100 bars, and length of end zone (nonelastic zone) of a few hundred meters. The barrier parameters for the great 1857 earthquake were also obtained from the description of surface fault breaks by Wallace (1968). The result led to the estimation of maximum acceleration of about 1.5g near the fault, under the assumption that the end zone length is proportional to the diameter of individual crack of the barrier model. Barriers for other earthquakes are discussed, and they are classified into geometrical barriers such as fault bend and corner and inhomogeneous barriers such as the high-velocity anomaly straddling the San Andreas fault near San Juan Bautista. The barriers act not only as a stopper of rupture but also as an initiator of rupture, as well as a stress concentrator, causing twin earthquakes and migration or progression of major earthquakes along the plate boundary.

Magnetosphere-ionosphere coupling

Abstract: A simple two-dimensional model of the magnetosphere-ionosphere system is discussed in which a localized electromotive force applied across a magnetic field line at t=0 is shown to propagate along the magnetic field with the Alfven velocity. The perpendicular electric field is assumed to reverse direction across the field line. Since the perpendicular electric field is limited in space, the propagation involves parallel electric fields whose magnitude depends on the characteristic scale length of the applied emf and the local plasma parameters. The electric field pulse associated with the ‘shock’ front is reflected at the ionosphere and propagates back to the source region. The finite Pedersen conductivity in the ionosphere damps the wave, and a steady state current system is established in the order of several hours. The parallel electric field can accelerate ions and electrons.

Revised geomagnetic polarity time scale for the interval 0–5 m.y. B.P.

Abstract: A change in the constants used in K-Ar dating and a significant increase in new data have made a recompilation and recomputation of data used to define the Late Cenozoic K-Ar polarity time scale highly desirable at this time. All available data in the range 0–5 m.y. have been recalculated using the refined constants, with 354 data points in this time interval now meeting the minimum criteria for acceptability. Recalculation of the major polarity epoch boundaries has yielded ages of 0.73 m.y. for the Brunhes-Matuyama, 2.48 m.y. for the Matuyama-Gauss, and 3.40 m.y. for the Gauss-Gilbert boundaries. A revised polarity time scale has been constructed based on available K-Ar data and information obtained from marine magnetic anomalies and deep-sea sedimentary cores.

A water mass model of the World Ocean

Abstract: A numerical model of the world ocean is developed to investigate the role of the ocean in the earth's heat balance. Climatological wind stress, temperature, and salinity are imposed as upper boundary conditions. An equilibrium solution is obtained based on an extended numerical integration over the equivalent of 1000 years. Seasonal variations are included. A series of numerical integrations over shorter periods indicate that quantitative aspects, such as the scale depth of the thermocline, are very sensitive to the closure parameterization representing the effect of unresolved scales of motion. The mean depth of the thermocline is found to be in proportion to the global available potential energy. Larger wind driving increases the scale depth of the thermocline, while larger lateral friction or diffusion leads to a shallower thermocline. The model predicts three major meridional cells in the upper thermocline in each hemisphere, corresponding to the three meridional cells of the atmosphere. The tropical and mid-latitude cells are largely wind driven. Thermohaline effects are dominant in the polar meridional cells. Seasonal changes in winds have a profound effect on the meridional circulation in the tropics and cause a flux of surface water from the summer to the winter hemisphere. It is suggested that this mechanism is an important factor in moderating climate by transferring excess heat from the summer hemisphere into the winter hemisphere.

Inertial limit on corotation

Abstract: Corotation of a planetary magnetosphere with the rotation frequency of the planet is maintained by the viscous torque exerted by ion-neutral collisions in the planetary atmosphere, this torque being transmitted to the magnetosphere by Birkeland currents. In a steady state this torque balances the inertial drag associated with the production and/or outward transport of magnetospheric plasma. The viscous torque in the atmosphere requires some departure from rigid corotation, i.e., some difference between the average rotation velocities of the ionospheric plasma and of the un-ionized atmosphere. In this paper we calculate the inertial corotation lag as a function of radial distance in the magnetosphere, the solution being parameterized in terms of the Pedersen conductivity of the atmosphere and the rate at which plasma mass is produced and transported outward in the magnetosphere. Although insignificant in the case of earth's magnetosphere, the calculated inertial corotation lag is significant in the case of Jupiter’s magnetosphere, where the rotation frequency may decrease by a factor of the order of 2 between the planetary surface and the magnetopause. One interesting consequence is that the active sector of Jupiter's magnetosphere (which is associated with a longitudinally restricted sector of enhanced ionospheric conductivity) should rotate faster, at a given distance, than adjacent longitude sectors and should therefore sweep up plasma from adjacent longitudes, thus amplifying the preexisting enhancement of plasma concentration in the active longitude sector.

Acoustic fluidization: A new geologic process?

Abstract: A number of geologic processes, particularly seismic faulting, impact crater slumping, and long runout landslides, require the failure of geologic materials under differential stresses much smaller than expected on the basis of conventional rock mechanics. This paper proposes that the low strengths apparent in these phenomena are due to a state of 'acoustic fluidization' induced by a transient strong acoustic wave field. The strain rates possible in such a field are evaluated, and it is shown that acoustically fluidized debris behaves as a newtonian fluid with a viscosity in the range 100,000 to 10,000,000 P for plausible conditions. Energy gains and losses in the acoustic field are discussed, and the mechanism is shown to be effective if internal dissipation in the field gives a Q approximately greater than 100. Whether such values for Q are realized is not known at present. However, acoustic fluidization provides a qualitatively correct description of the failure of rock debris under low differential stresses in the processes of faulting, crater slumping, and long runout landslides. Acoustic fluidization thus deserves serious consideration as a possible explanation of these phenomena.

Formation of Martian flood features by release of water from confined aquifers

Abstract: It is proposed that the rapid release of water under great pressure from deeply buried aquifers is responsible for the formation of the Martian channels suggestive of catastrophic flooding (outflow channels). Fine channels in the Martian surface suggest the presence of surface water early in the history of the planet, which would have entered the ground water system through the porous near-surface rocks. Subsequent global cooling would have trapped the ground water under a thick permafrost layer and formed a system of confined aquifers. High pore pressures within the aquifers are considered to have triggered the breakout of water from the aquifers at rates of from 10 to the 5th to 10 to the 7th cu m/sec, which would be prevented from reentering the ground water system by the layer of permafrost. Outflow from the aquifer is also considered to have caused the undermining of adjacent areas and the collapse of the surface to form areas of chaos, often associated with channels.

Simultaneous measurement of atmospheric CH2O, O3, and NO2 by differential optical absorption

Abstract: Formaldehyde mixing ratios are reported for rural areas around Julich and for maritime air at the north coast of Germany. The measurements were made using a ground-based UV-optical absorption technique which allowed the simultaneous determination of CH2O, NO2, and O3 with detection limits of 0.1, 0.1, and 1 ppb, respectively. In Julich, which may be regarded as typical for central European background atmosphere, mixing ratios varied from 0.1 to 6.5 ppb from May through October 1978. In maritime air under conditions when photochemical equilibrium was expected, formaldehyde concentrations of 0.2 ppb were observed, which can be accounted for by photochemical oxidation of methane alone. However, the high concentrations of formaldehyde found at Julich indicate other sources of formaldehyde.

Soil Transport by Winds on Mars

Abstract: Results of low-pressure wind tunnel testing and theoretical considerations are used to estimate the eolian transport of surface material on Mars Saltation on Mars, equations of particle motion, computational results, and analytical determination of surface material movement are considered A semiempirical formula is developed for estimating the total amount of surface material moving in eolian saltation, surface traction, and suspension Numerical solutions of the equations of motion for particle trajectories on the surface of Mars are presented The ratio of final particle speed to the particle threshold friction speed is found to be several times that of saltation on earth

Geochemical modeling of mantle differentiation and crustal growth

Abstract: A simple two-reservoir. model with time-dependent transport coefficients between the reservoirs has been used to model the abundances of K, Ar, Rb, Sr, Sm, Nd, U, Th, and Pb and the isotopic compositions of Ar, Sr, Nd, and Pb in the earth's mantle and continental crust. The transport coefficients, like heat production, are considered to decay exponentially with time. Models which involve the whole mantle in generating the continental crust yield 87Sr/144 and 143Nd/144Nd ratios for the residual mantle which are higher and lower than midocean ridge basalts, respectively. A model which involves only half of the mantle in the production of continental crust produces a residual mantle with isotope ratios similar to those of midocean ridge basalt. The 40Ar/36Ar ratios in the atmosphere and suboceanic mantle are reproduced by this model without any inequality in the upward transport coefficients of K and Ar but with a smaller downward transport coefficient for Ar than for K. The results imply that the earth's crust may have developed by extraction of material from only half of the mantle and constrain the possible convective regimes that have existed in the mantle.

Ice in the lunar polar regions

Abstract: The idea that ice and other trapped volatiles exist in permanently shadowed regions near the lunar poles was proposed by Watson, Murray, and Brown (1961). It is reexamined in the present paper, in the light of the vast increase of lunar knowledge. The stability of the traps and the trapping mechanism are verified. Four potential sources of lunar H2O, namely (1) solar wind reduction of Fe in the regolith, (2) H2O-containing meteoroids, (3) cometary impact, and (4) (the least certain) degassing of the interior, can supply amounts of trapped H2O estimated in the range of 10 to the 16th to 10 to the 17th g. Two important destructive mechanisms have been identified: photodissociation of H2O molecules adsorbed on the sunlit surface and sputtering or decomposition of trapped H2O by solar wind particles. The effect of impact gardening is mainly protective. The question of the presence of H2O in the traps remains open; it can be settled by experiment.

The mean age of mantle and crustal reservoirs

Abstract: Mantle and crust evolution is discussed in terms of two simple transport models. In model I, continents (j = 3) are derived by melt extraction over the history of the earth from undepleted mantle (j = 1), and the residue forms a depleted mantle (j = 2), which today is the source of mid-ocean ridge basalts. In model II, new additions to continents are derived from a mantle reservoir 2, which becomes more depleted through time by repeated extraction of melts. Transport equations were solved for stable s, radioactive r, and daughter d isotopes for arbitrary mass growth curves M_j(τ). For both models the isotopic composition and concentrations of trace elements are shown to reduce to simple mathematical expressions which readily permit calculations of basic evolutionary parameters from the data. For longlived isotopes (λ^(−1) ≫ 4.5 aeons) for model I the deviations in parts in 10^4 of the ratio of a daughter isotope to a stable reference isotope of the same element in reservoirs j = 2, 3 from that of 1 is given by E_(dj)^*=Q_d^*t_(Mj)ƒ_j^(r/s). Here t_(Mj) is the mean age of the mass of j,ƒ_j^(r/s) is the enrichment factor of the ratio of a radioactive isotope to a stable isotope relative to that in 1, and Q_d^* ≃ const. Thus for long-lived isotopes such as ^(147)Sm and ^(87)Rb the only time information that can be obtained from model I from measurement of the relative chemical enrichment factors and isotopic ratios at a single time is the mean age of the mass of the continental crust and the complementary depleted mantle reservoir. This mean age is independent of the long-lived parent-daughter system. An analogous result is obtained for model II, where E_(d.2)^*= Q_d^*(ƒ_2^(r/s))t. Here (ƒ_2^(r/s)) is the weighted time average of the enrichment factor, and τ is the time measured from the origin of the earth. The mean age of the mass of the crust (t_(M.3)) and the time parameter t_(r/s)=t(ƒ_2^(r/s))/ƒ_2^(r/s) for the crust in model II will for long-lived parent-daughter systems be different depending on the element fractionation during partial melting. Decay systems with small parent-daughter fractionations during partial melting may, however, be used to estimate the mean age of the continental crust. Sm-Nd and Rb-Sr isotopic data for continental crust, depleted and undepleted mantle, have been used to evaluate both models and yield young mean ages for the mass of the continental crust of 1.8 and 1.5 aeons for model I and model II, respectively. Both models also suggest that the rate of growth of the continents for the last 0.5 aeon is much less than the average growth rate. The young mean age of the continents implies either rapid refluxing of crustal materials to the mantle in the period from 4.5 to 3.6 aeons or that very little early crust ever formed. Mass balance calculations for both models show that the continents were only formed from ∼30% of the total mantle, leaving 70% of the mantle as undepleted. The major difference in the two models lies in the difference in the compositions of newly derived crust. For model I the trace element concentrations in new additions to the crust is constant, and the isotopic values are those of the undepleted mantle reservoir, in agreement with recent Nd isotopic studies. The correlation line between e_(Nd) and e_(Sr) for young basalts can be explained with model I by mixing depleted and undepleted mantle, but it cannot in any simple way be explained by model II. Model II implies that new additions to the continents have the isotopic characteristics of depleted mantle and that the concentration of Rb, U, Ba, and other highly incompatible trace elements in newly added material have changed by a factor of ∼10 through time, for which there is no evidence. For both models the simple analytical expressions derived herein permit calculations of earth models with great facility without requiring a computer calculation.

Size of great earthquakes of 1837–1974 inferred from tsunami data

Abstract: A new magnitude scale Mt is defined by using the logarithm of the maximum amplitude of far-field tsunami waves measured by tide gauges or their substitutes. The Mt scale is experimentally adjusted to the Mw scale introduced by Kanamori (1977), so that the Mt scale measures the seismic moment of a tsunamigenic earthquake as well as the overall size of tsunami at the source. Mt and the conventional tsunami magnitude m are distinct scales. By using many amplitude data of tsunami waves now available the values of Mt are assigned to 65 tsunamigenic earthquakes that occurred in the Pacific area during the period from 1837 to 1974. The 1960 Chilean shock has the largest Mt, 9.4. The 1946 Aleutian (Mt = 9.3), the 1837 Chilean (Mt = 9¼), and the 1964 Alaskan (Mt = 9.1) events follow. Nine great events having Mt = 9 or over occurred during this period, and their occurrence is clustered in the years around 1840, 1870, and 1960. Of all the 65 events listed, at least six unusual earthquakes having tsunamis with an amplitude disproportionately large for their surface-wave magnitude Ms are identified from the Mt-Ms relation.

An analysis of the charge structure of lightning discharges to ground

Abstract: Sources of charge for the individual strokes of four multiple-stroke flashes to ground have been determined, using measurements of the electrostatic field change obtained at eight locations on the ground beneath the storm. The resulting charge locations have been compared to 3-cm radar measurements of precipitation structure in the storm. The field changes of individual strokes were found to be reasonably consistent with the lowering to ground of a localized or spherically symmetric charge in the cloud. The centers of charge for successive strokes of each flash developed over large horizontal distances within the cloud, up to 8 km, at more or less constant elevation between the −9° and −17°C environmental (clear air) temperature levels. Comparison with the radar measurements has shown that the discharges developed through the full horizontal extent of the precipitating region of the storm and appeared to be bounded within this extent. In one instance where cellular structure of the storm was apparent, the strokes selectively discharged regions where the precipitation echo was the strongest. Vertical extent of the stroke charge locations was small in comparison with the vertical extent of the storm. The field changes in the intervals between strokes have been found to exhibit many of the features which Malan and Schonland used to infer that ground flashes discharge a nearly vertical column of charge in the cloud. This and other evidence is used to show that their observations, which were made at a single station, could instead have been of horizontally developing discharges. The interstroke field changes have been analyzed using a point dipole model and found to correspond to predominantly horizontal charge motion that was closely associated with the ground stroke sources for the flashes. The interstroke activity served effectively to transport negative charge in the direction of earlier stroke volumes and often persisted in the vicinity of an earlier stroke volume, while subsequent strokes discharged more distant regions of the cloud. Long-duration field changes that sometimes preceded the first stroke of a flash have been analyzed and found to correspond to a series of vertical and horizontal breakdown events within the cloud, prior to development of a leader to ground. These events were associated in part with the negative charge region that became the source of the first stroke and effectively transported negative charge away from the first stroke charge volume and from the charge volumes of subsequent strokes. Several continuing current discharges were found also to progress horizontally within the cloud and sustained currents in the range of 580 A to less than 50 A. The continuing current field changes were consistently better fitted by the monopole charge model than the field changes of discrete strokes within the same flash.

Dynamic recrystallization during creep of single-crystalline halite: An experimental study

Abstract: Single crystals of pure and impure halite have been dynamically recrystallized during compression creep at temperatures between 250° and 790°C and stresses between 1.5 and 120 bars. Recrystallization was found to occur by two different mechanisms: at lower temperatures and stresses the new grains result from the rotation of subgrains without grain boundary migration (rotation recrystallization), and at higher temperatures and stresses the final texture results from the migration of the high-angle grain boundaries of the rotated subgrains. Migration recrystallization was shown to occur for critical stress and temperature conditions, allowing rapid grain boundary migration. A curve separates the two domains in the σ, T plane and moves to higher temperatures and stresses for crystals of higher impurity content; for natural crystals, only rotation recrystallization can occur. In each recrystallization regime the recrystallized grain size is uniquely related to the applied stress, thus yielding two different geopiezometers, which should not be applied indiscriminately to natural tectonites to determine lithospheric or mantle deviatoric stresses. The experimental results are interpreted by the Lucke et Stuwe theory for impurity-controlled grain boundary migration.