bluuebunny/crossref_metadata_2025 huggingface dataset:
Coupling of energy between tectonic processes and earthquakes
Authors: Andrews D. J.
Published: February 2008

The elastic field energy change associated with a faulting process is indeterminate, because work may be done on distant bounding surfaces. Savage (1969) has shown that plastic strain, the source of the earth's self‐stress, must be considered in the resolution of this question (Steketee's paradox). In a self‐stressed earth, faulting will decrease the elastic field energy. Over a period of time following the faulting, distant viscoelastic regions can be expected to respond in such a way as to return stress to a value appropriate to steady flow. In the course of this response, work will be performed on the brittle seismogenic region to increase elastic field energy smoothly throughout the region. Steketee's result for a region bounded by a surface of constant stress is appropriate to the question of tectonic reloading, if account is taken of irregular slip due to past earthquakes. The energy increase due to tectonic reloading is not necessarily equal to energy released in the earthquake and has a different spectral decomposition. Although there is no energy balance in the seismogenic region for a single earthquake, there must be such a balance for a cycle of earthquakes of all sizes.
Ridges, trenches, and polar wander excitation
Authors: Jurdy Donna M.
Published: February 2008

An irregular distribution of mass on the surface of the earth can cause a reorientation of the rotation axis with respect to the earth, i.e., true polar wander. To cause an instability, there must be regions with vertical density contrasts such that there are different contributions per unit area to the inertia tensor of the earth, and also the geometric arrangement of these regions must be such that the off‐diagonal components of the inertia tensor are nonzero. In general, isostatic compensation results in first‐order cancellation of inertia effects, and the smaller second‐order effects will be determined by the specific density distribution. It has been suggested that the rearrangement of the continents and oceans has caused polar wander, but calculations of inertia contributions for density models give near cancellation. The ridges and trenches may have greater potential for causing imbalances. The cold dense slab in the trenches creates a mass excess and thus an inertia excess over normal oceanic lithosphere. A thermal model of an isostatic ridge also gives a positive effect. Excitation tensors are constructed for the ridges and trenches for the present and for three times since the Early Cretaceous to determine the position of the principal axes. Neither the system of ridges nor trenches alone give principal axes near those of the earth for the present. The combination of the ridges and trenches with principal axes nearest the earth's has the ridges weighted equally in magnitude but opposite in sign to the trenches. This weighting is discordant with density models and the sign of gravity and geoid anomalies. A more reasonable weighting, equal and same sign, gives principal axes with no relation to those of the earth.
On the shape of directional data sets
Authors: Engebretson David C. , Beck Myrl E.
Published: February 2008

Existing methods for the analysis of the shape of directional data sets, primarily paleosecular variation data, tend to have serious inconsistencies in their application and in general are difficult to apply. The question of whether or not a given data set is distributed in a circular manner about its mean is important in many types of analysis, particularly so, since circularity is a necessary condition for the application of the statistical techniques of Fisher (1953). A new statistic, called the eccentricity, is developed and shown to offer information as to the shape of a distribution of directions. The eccentricity statistic does not depend on choice of classes or intervals, is invariant to rotations of the distribution about the mean directions, is simple to apply, and is easily understood in terms of a simple physical analogy. The method is applied to 112 Hawaiian lavas (Doell and Cox, 1965). The analysis shows good agreement with a model in which magnetic variations in Hawaii are a consequence of dipole wobble only, with no significant contributions from the nondipole field.
Large thrust earthquakes and tsunamis: Implications for the development of fore arc basins
Authors: Nishenko S. , McCann W.
Published: February 2008

Variations of the subduction regime around the Pacific are reflected in changes of both the lengths of rupture zones and the source areas for tsunamis associated with large shallow earthquakes. Crustal deformation occurring during the earthquake and related tsunamic activity play important roles in the development and maintenance of topographic features in the fore arc region. A regional comparison of the average length of upper slope basins and terraces with the maximum length of earthquake rupture zones shows that longer basins and terraces characteristically occur in regions with larger rupture zones. Examples from Japan, Alaska, and the Aleutians clearly show how these topographic features reflect the size and spatial distribution of seismic‐tsunamic source areas. In many areas this relationship may be explained by the coseismic reactivation of structural units within the fore arc region. The two principal areas of coseismic reactivation are the trench slope break and the frontal arc region. Thus upper slope basins, deep sea terraces, and other bathymetric features may serve as indicators of the tectonic regime and of seismic‐tsunamic risk along convergent margins. Variations of the dimensions of rupture zones also appear to be influenced by the blocklike behavior of the overthrust plate. In many instances the dimensions of upper slope basins and terraces are equivalent to those of the crustal blocks. Regions with basin or terrace lengths greater than 100 km have histories of large ruptures involving adjacent segments. Regions with basin or terrace dimensions less than 100 km tend to rupture independently of adjacent segments.
Deviatoric stresses and earthquake occurrence at the outer rise
Authors: Hanks Thomas C.
Published: February 2008

A straightforward comparison between the strain energy (in bending) input rate to the continuously developing outer rise system and the elastic strain energy release rate occurring in the form of normal‐faulting earthquakes in the lithosphere offshore of oceanic trenches requires that extensional strains of ∼10−2 that exist in the upper half of the ‘elastic plate’ indeed be elastic and thus that kilobars of deviatoric stress exist in the outer rise system. This result does not preclude anelastic strains up and beyond ∼10−2 that can and almost certainly do accompany the outer rise deformation; it does, however, presuppose a steady, flexure origin of the outer rise, as is almost always assumed in modeling studies of bathymetry and gravity data. For the purposes to which it is put here, this assumption, at best, is likely to be valid only on a time scale corresponding to several cycles of strain accumulation and release associated with great underthrusting earthquakes onshore of the trench axis; the decade or so of reasonably well‐determined and complete seismic observations that are presently available for the outer rise region represents only a small fraction of this time scale. ‘Lithospheric normal‐faulting earthquakes,’ of which the principal examples are the Sanriku (March 3, 1933) and Java trench (August 19, 1977) earthquakes, cannot arise from tectonic stresses due to bending and thus certainly argue against the flexure origin of the outer rise locally and perhaps globally as well, despite the separate bathymetric, and seismologic evidence which provide substantial support for the flexure origin of the outer rise. At least in the case of the Sanriku earthquake it is unlikely that the entire lithosphere was fractured. In the case of the Java trench earthquake the supposition that the depth of faulting is no more than 30 km implies an exceptionally large stress drop.
Ringing P waves and submarine faulting
Authors: Ward Steven N.
Published: February 2008

Long‐period teleseismic records from shallow focus oceanic events sometimes exhibit ‘ringing’ P waves. The ringing portion of these prolonged wave groups characteristically has a narrow band frequency content, an amplitude which monotonically decreases from values comparable to the primary P pulse, and a high station to station coherence. These traits are suggestive of leaking compressional modes of the oceanic water layer. This layer is an efficient wave guide for elastic body waves once their energy is effectively introduced into the system. Through the modeling of point and extended sources interior to oceanic crust it is found that to account for the large excitation of these modes a particular type of faulting is required. Specifically, the fault must originate in a setting such that a substantial fraction of the dislocation surface is situated above the Moho and be oriented so that a significant amount of compressional energy is propagated upward. These provisions are met by steeply dipping or thrust faults in continental margins and subduction zones where thickened oceanic crustal and sedimentary layers are commonly found. Events of this type are likely to generate perturbations in submarine topography. Hence real‐time observation of ringing P waves from large earthquakes in these geographical provinces could be diagnostic of shallow faulting with high tsunamigenic potential.
Geomagnetic paleointensities by the Thelliers' method from submarine pillow basalts: Effects of seafloor weathering
Authors: Grommé Sherman , Mankinen Edward A. , Marshall Monte , Coe Robert S.
Published: February 2008

Measurements of geomagnetic paleointensity using the Thelliers' double‐heating method in vacuum have been made on 10 specimens of submarine pillow basalt obtained from 7 fragments dredged from localities 700,000 years old or younger. In the magnetic minerals, the titanium/iron ratio parameter x and the cation deficiency (oxidation) parameter x were determined by X‐ray diffraction and Curie temperature measurement. Fresh material (z ≅ 0) provided excellent results: most of the natural remanent magnetization (NRM) could be thermally demagnetized before the magnetic minerals became altered, and the NRM‐TRM lines were straight and well constrained, and geologically reasonable paleointensities were obtained. Somewhat oxidized material (z ≅ 0.2) also provided apparently valid paleointensities: values were similar to those from fresh specimens cut from the same fragments, although only half or less of the NRM could be thermally demagnetized before alteration of the magnetic minerals. More highly oxidized material (z ≅ 0.6) gave a result seriously in error: the paleointensity value is much too low, because of continuous disproportionation of titanomaghemite during the heating experiments and because seafloor weathering had decreased the NRM intensity. From limited published data, the extent of oxidation of titanomagnetite to cation deficient titanomaghemite in pillow basalt exposed on the seafloor appears to be approximately z = 0.3 at 0.2–0.5 m.y., z = 0.6 at 1 m.y., and z = 0.8–1.0 at 10–100 m.y. This implies that valid paleointensities can be obtained from exposed submarine basalt, but only if the basalt is younger than a few hundred thousand years. Equally good paleointensities were obtained from strongly magnetized (L‐type) basalt and moderately magnetized (L‐type) basalt. The degree of low‐temperature oxidation of cubic iron‐titanium oxides in submarine basalts correlates very well with the diminution of amplitude of linear magnetic anomalies when both are compared as a function of crustal age. Systematic radial variation of Curie temperature is a primary feature of submarine basalt pillows, so that estimation of the oxidation parameter z from the Curie temperature alone by assuming a value for x can be in error. Reasonably precise and self‐consistent values of both x and z can be obtained if both the cubic cell dimension and the Curie temperature of the cubic oxide are measured.
Geoid of the Kerguelen Islands area determined from Geos 3 altimeter data
Authors: Balmino Georges , Brossier C. , Cazenave A. , Nouel F. , Dominh Kien , Vales N.
Published: February 2008

One hundred and fifty three Geos 3 altimetry profiles have been obtained in the Indian Ocean around the Kerguelen Islands, during a limited campaign which took place from February to June 1976. A detailed geoid has been computed in this area with a resolution of 0.5°, based on accurate orbits provided by the Defense Mapping Agency (DMA). On this occasion, several algorithms have been developed and compared, which all give the same predicted values (within 1 m) at the no des of a regular grid. Strong correlations with the sea bottom topography are already visible, especially with Crozet, Kerguelen, and Heard Is lands and with the newly discovered seamounts Ob, Lena, and Marion Dufresne. Geophysical interpretation of these correlations are under way.
The analysis of Geos 3 altimeter data in the Tasman and Coral seas
Authors: Mather R. S.
Published: February 2008

A technique has been developed for preprocessing Geos 3 altimetry data to establish a model of the regional sea surface. The algorithms as presently used develop models for 4×107 km2 areas with an internal precision of ± 1 m. This figure is substantially influenced by the data acquisition period and the sea state. There are discrepancies between the sea surface model so obtained and GEM 6‐based geoid profiles with wavelengths of approximately 2500 km and amplitudes of up to 5 m in this region. The amplitudes are smaller when compared with GEM 10‐based geoid determinations. However, the comparison of 14 pairs of overlapping passes in the region indicates altimeter resolution at the ±25‐cm level if the wavelength corresponding to the Nyquist frequency was 30 km. In most cases the spectral analysis of such comparisons shows significant signal strength in the discrepancies after least squares fitting, with wavelengths in excess of 200 km. Regional studies of time‐varying features of the sea surface in the data analysis area are not currently possible owing to inadequate tracking support and the limited time span over which a dense data coverage was available.
Observation of sea surface topography with Geos 3 altimeter data
Authors: Douglas Bruce C. , Gaborski Patricia D.
Published: February 2008

Surface truth data, including an empirical tide model, geoidal undulations, dynamic heights, and infrared imagery in the Geos 3 Atlantic calibration area, have been compared with features in Geos 3 altimeter profiles of sea surface height. The 5′ × 5′ gravimetric geoid computed by J. G. Marsh agrees with altimeter profiles to the level of the sea surface topography (∼50 cm) After removal of a low‐degree polynomial the differences of collinear passes of altimeter data reveal apparent time‐dependent ocean surface features. Some of these can be identified from the surface truth information, but others cannot be correlated with known features, probably owing to limitations of the surface truth data. Independent collinear pairs also indicate that the best of the many orbit computation models used to compute the Geos 3 altitude ephemeris introduces a bias of about 1 m (rms) on a typical pass of altimeter data.
Intercomparison of GEOS 3 Tracking Systems
Authors: Berbert John H. , Carney Devere V.
Published: February 2008

Measurement and time biases of the Geos 3 tracking systems in the Atlantic calibration area were determined in relation to the Goddard lasers. In collocation tests the lasers agreed with each other to about 10 cm. Early RAMLAS laser data contained an apparent time bias of 647 μs. Later, in a laser collocation test, RAMLAS differed from MOBLAS 2 by +11.3 cm and ‐28 μs. An overlooked correction reduced the –28 μs to –8 μs. The geoceivers initially had
biases of 3–6 m/s and T biases of several milliseconds. During the first month of Geos 3 operations the
biases decreased monotonically by 2.5±1.0 m/s. The C band radar biases relative to the lasers were usually within several meters and several tenths of milliseconds. The unified S band system relative biases were usually within ±4 mm/s and ±0.1 ms. The average and scatter of the altimeter biases recovered on 18 laser reference arcs was –5.3±0.6 m.
A note on the comparison of radar altimetry with IR and in situ data for the detection of the Gulf Stream surface boundaries
Authors: Leitao C. D. , Huang N. E. , Parra C. G.
Published: February 2008

A definition of the Gulf Stream boundaries is presented together with its most widely used approximations. Positions of the Gulf Stream boundaries, as obtained by radar altimetry, are compared with those determined by infrared and in situ data such as airborne bathythermographs and airborne radiation thermometers. The data used are derived from the Geos 3 satellite, the Experimental Ocean Frontal Analysis, and other experiments conducted by the U.S. Navy Naval Oceanographic Office during February 1976 and January 1977. Results point out that the radar altimeter can sense the Gulf Stream boundaries with as much precision as the infrared‐derived data analysis. Both techniques provide excellent agreement with the in situ data. The two sensor systems complement each other, since one is an allweather profiler and the other a synoptic two‐dimensional sensor.
Electrical conductivity of pyroxene which contains trivalent cations: Laboratory measurements and the lunar temperature profile
Authors: Huebner J. S. , Duba A. , Wiggins L. B.
Published: February 2008

Three natural orthopyroxene single crystals, measured in the laboratory over the temperature range 850°–1200°C, are more than ½ order of magnitude more electrically conducting than previously measured crystals. Small concentrations (1–2%) of Al2O3 and Cr2O3 present in these crystals may be responsible for their relatively high conductivity. Such pyroxenes, which contain trivalent elements, are more representative of pyroxenes expected to be present in the lunar mantle than those which have been measured by other investigators. The new conductivity values for pyroxene are responsible for a relatively large bulk conductivity calculated for (polymineralic) lunar mantle assemblages. The results permit a somewhat cooler lunar temperature profile than previously proposed. Such lower profiles, several hundred degrees Celsius below the solidus, are quite consistent with low seismic attenuation and deep moonquakes observed in the lunar mantle.
Isothermal compression of TiO2 (Rutile) under hydrostatic pressure to 106 kbar
Authors: Ming Li‐chung , Manghnani Murli H.
Published: February 2008

The pressure‐volume relationship for TiO2 (rutile) was determined under hydrostatic pressure up to 106 kbar at room temperature by means of X ray diffraction, employing the diamond anvil pressure cell and ruby fluorescence pressure calibration technique. Assuming an ultrasonic value of K0′ (6.84) from the Birch‐Murnaghan equation, the zero‐pressure bulk modulus (K0) value derived from the present pressure volume data is 2.11±0.07 Mbar, in good agreement with the ultrasonic isothermal Reuss bulk modulus values (2.076–2.095 Mbar).
Analysis of daily latitude variations
Authors: Graber Michael A.
Published: February 2008

The daily latitude measurements of the International Polar Motion Service are analyzed. The annual oscillation in the data was modeled by separate oscillations in each observatory's latitude data. The separate oscillations varied in amplitude from 0.05″ to 0.15″ with standard deviations of about 0.007″. Within the resolution of the latitude residuals (150 cm), there is no indication of the sharp changes which might be associated with earthquake effects. Then, applying Schuster's test to a periodogram of the residuals indicates that there are probably several processes occurring at amplitudes between 0.007″ and 0.03″ whose solution awaits a more precise measurement technique.
Consequences of an enhanced pole tide
Authors: Dickman S. R.
Published: February 2008

The pole tide is the response of earth's oceans to a free wobble. It has been widely assumed that the response is an equilibrium one; such a response would involve no dissipation of wobble energy and would result in a tide of small amplitude, roughly ½ cm. An equilibrium tide would force the path of the wobbling rotation pole to be slightly elliptical (∼0.01 ellipticity) and would contribute about 1 month to the observed 14‐month Chandler Wobble period. Yet a greatly enhanced pole tide, with amplitude typically 5 times the predicted equilibrium value, has long been observed in the North and Baltic seas. In recent years much evidence has been gathered (through diverse data analysis techniques) which suggests that the global oceanic response is also nonequilibrium. In this paper a theory which accounts for the effects of an arbitrary tide on rotation has been developed and applied to situations of uniform global enhancement and enhancement in the North and Baltic seas only. Pole tide amplitudes up to 10 times equilibrium and phase lags as much as 4½ months behind equilibrium were considered. In the case of North/Baltic seas enhancement, conservation of mass leads to a ‘pumping phenomenon’ whose effect on the oceans worldwide is as significant as the local enhancement itself. The theory only requires an assumption that the time and space dependence of the tide amplitude T can be separated using complex notation, T = X n + X* n*, where X and n represent the spatial and temporal variation of the tide; n is related by a phase lag to the instantaneous rotation pole position. Such separation of variables has been shown to hold in the case of an equilibrium tide. The results support the hypothesis that the oceans are capable of damping the Chandler Wobble; damping times between 2 and 80 years are likely, depending on the degree of enhancement. This removes the need to postulate that the mantle is anomalously anelastic at frequencies below the seismic band. It is also possible that the oceanic contribution to the Chandler period greatly exceeds 1 month; that would imply the fluid core response to wobble is more limited than previously thought, since the contributions of both the core and the oceans must yield a 14‐month Chandler period. This could have serious consequences regarding the outer core's density stratification, which governs the dynamic core response to wobble.
Convection, composition, and the thermal state of the lower mantle
Authors: Jeanloz Raymond , Richter Frank M.
Published: February 2008

We present a thermal model for the lower mantle, which is constructed from petrologically derived estimates of the temperature in the transition zone and from an adiabat based on the thermal properties of MgO and SiO2 measured at high pressures. Superadiabatic contributions to the geotherm through the lower mantle, including those from possible phase transformations but excluding those from thermal boundary layers, are negligibly small. A thermal boundary layer is required at the base of the mantle in order to satisfy our estimate of the lowest possible temperature in the core (about 2800 K); its thickness of about 100 km is well constrained. Even so, our model suggests that we require either a significantly larger heat flux from the core than has been considered reasonable (>50 mW m−2) or the presence of a further thermal boundary layer within the lower mantle in order to arrive at more likely core temperatures (about 3200–3500 K). The latter alternative appears the more plausible, and such a boundary layer requires a barrier to convection. Guided by the present seismological evidence, we suggest that a thermal boundary layer is associated with a chemical discontinuity either at the top of the lower mantle or near its base (D″ region). If seismological data require the D″ layer to be significantly thicker than about 100 km, this is support for a chemically distinct D″ region with multiple thermal boundary layers. Otherwise, the results of this study suggest that the upper and lower mantle may be chemically distinct, hence precluding whole mantle convection.
Tectonomagnetic anomaly during the southern California downwarp
Authors: Johnston M. J. S. , Williams F. J. , McWhirter J. , Williams B. E.
Published: February 2008

Resurveys of the local geomagnetic field in southern California, using magnetometers in a differential mode, indicate the development of an anomalous field of more than 10 γ that appears to correspond to the partial collapse of the southern California uplift. The 10‐γ field increase occurred within a 30‐km fault segment near the junction of the San Andreas and San Jacinto faults. It took place episodically in time, 80% accumulated between late 1973 and the end of 1974 and the remaining 20% between 1974 and 1976. Changes of smaller maximum amplitude and opposite sense occurred to the southeast along the San Jacinto fault. Swarm seismicity started at the northwest end of the zone of maximum anomaly in late 1976 but ceased in late 1977. The anomaly amplitude has slowly decreased with time since its peak in 1976. Tectonomagnetic models imply localized stress changes at depth of at least 10 bars in the vicinity of the magnetometer sites near the various faults, although details concerning the geometry and dimensions of the region responsible for the magnetic field changes are poorly resolved.
Predictability of ground displacement and velocity near an earthquake fault: An example: The Parkfield Earthquake of 1966
Authors: Bouchon Michel
Published: February 2008

Using the discrete wave number representation method (Bouchon and Aki, 1977; Bouchon, 1979), we model the Parkfield earthquake of 1966 as a Haskell‐type dislocation source embedded in a layered medium. We show that the displacement and velocity recorded near the source (station 2 of the Cholame‐Shandon array) can be fully accounted for by the propagation of the rupture on the branch of the fault closest to the station. The slip and slip velocity inferred are about 40 cm and 130 cm/s, respectively. The ground motion at the accelerograph site and in the region close to the source was very strongly amplified by the sedimentary layer. The rupture, after being initiated at depth, stayed buried well under the sediments on the main segment of the fault. The dislocation then jumped across Cholame Valley and became quite shallow. On this southeast branch of the fault the rupture most likely penetrated the sediments. The results obtained constitute an encouraging step toward predicting the strong ground motion at a given site for a potential earthquake fault.
Gravitational potential energy of the Earth: A spherical harmonic approach
Authors: Rubincam David Parry
Published: February 2008

A spherical harmonic equation for the gravitational potential energy of the earth is derived for an arbitrary density distribution by conceptually bringing in mass‐elements from infinity and building up the earth shell upon spherical shell. The zeroth degree term in the spherical harmonic expansion agrees with the usual expression for the energy of a radial density distribution. The second degree terms give a maximum nonhydrostatic energy in the crust and mantle of −2.77×1029 ergs, an order of magnitude below McKenzie's (1966) estimate. McKenzie's result stems from mathematical error. Our figure is almost identical with Kaula's (1963) estimate of the minimum shear strain energy in the mantle, a not unexpected result on the basis of the virial theorem. If the earth is assumed to be a homogeneous viscous oblate spheroid relaxing to an equilibrium shape, then a lower limit to the mantle viscosity of 1.3×1020 P is found by assuming that the total geothermal flux is due to viscous dissipation of energy. This number is almost six orders of magnitude below MacDonald's (1966) estimate of the viscosity and removes his objection to convection. We have not determined where MacDonald's treatment diverges from our own. If the nonequilibrium figure is dynamically maintained by the earth acting as a heat engine at 1% efficiency, then the viscosity is 1022P, a number preferred by Cathles (1975) and Peltier and Andrew (1976) as the viscosity of the mantle.
Comments on the gravity and topography of Mars
Authors: Lambeck Kurt
Published: February 2008

The long‐wavelength variations in the Martian and terrestrial potential and topography have been compared in an attempt to understand the origin of the Mars gravity field, and to estimate the average lithospheric thickness of Mars. The geopotential and topographic power spectra of Mars contain more absolute power in the low degree harmonics (l < 7) than the corresponding terrestrial spectra and, unlike the earth, the two quantities correlate positively for low degrees. The two planets also differ from each other in that density anomalies in the Mars crust and lithosphere contribute more to the long wavelength gravity field than they do in the earth. The maximum shear stresses set up by a fully compensated Martian topography, including the Tharsis plateau, are somewhat less than those occurring on earth but the stresses set up by the observed gravity, assuming that the gravity originates from density anomalies wholly within the lithosphere, appear to be excessive. Excluding the second and third degree harmonics, the part of gravity that correlates with topography and that is assumed to be a consequence of isostatically compensated density anomalies, results in stresses that are comparable to those found on earth if the thickness of the layer is of the order 100–200 km. The low degree harmonics l = 2, 3 in the gravity and topography describe essentially the Tharsis complex and they require a layer of about 400 km thickness with a finite strength of about 500 bars, if this plateau is supported statically. If these values are excessive, the Tharsis region requires an explanation that differs from the rest of the planet. Partial isostatic compensation of this region is improbable as the stresses become excessive. Without further information, the contributions to gravity density anomalies in the lithosphere and mantle cannot be separated but it does appear that the Martian lithosphere is much thicker than the earth's and that its mantle may be relatively less heterogeneous than the terrestrial mantle.
Oblique anhysteretic remanent magnetization
Authors: Denham Charles R.
Published: February 2008

Orientation of the weak biasing field obliquely, rather than parallel, to the strong alternating field causes an oblique anhysteretic remanent magnetization (OARM) which is systematically misaligned with respect to the bias. Experiments on a deep‐sea sediment also showed that with increasing bias angle the intensity of OARM diminishes greatly, the coercive force is slightly lower, and the remanence is monodirectional over all coercive force bands. An adaptation of the Stoner‐Wohlfarth mechanism described here may help to explain some fundamental aspects of OARM behavior.
High‐accuracy magnetic field difference measurements and improved noise reduction techniques for use in tectonomagnetic studies
Authors: Ware R. H.
Published: February 2008

Previous geomagnetic field measurements in Colorado have been extended using three self‐calibrating rubidium magnetometers accurate to about 0.01 γ. The magnetometers were deployed either in an L‐shaped array or in a collinear array and provided simultaneous measurements of the scalar field at sites separated by 12–16 km. Previous noise reduction work using two‐station magnetic field difference data from Colorado removed variations from the east‐west scalar difference which were correlated with variations in the field component D. The earlier analysis work has been extended to remove also noise correlated with the field components H and Z. Noise due to susceptibility and magnetization differences between sites or induced currents in conductivity anomalies is reduced by this technique. However, the main purpose of this paper is to demonstrate a new noise reduction technique that removes noise due to large‐scale gradients in geomagnetic variations. In effect, the method removes variations in the magnetic field difference between two sites which are correlated with variations in the difference between two other sites in a collinear array. This approach, which does not require vector data, reduced gradiometer noise to 0.03 γ rms for 37 days of hourly averages.
Earthquakes and bending of plates at trenches
Authors: Chapple William M. , Forsyth Donald W.
Published: February 2008

The mechanisms, distribution, and total moment of earthquakes within the bending oceanic plate seaward of trenches constrain possible mechanical models of the lithosphere. The average annual horizontal slip in normal fault earthquakes, as estimated from the cumulative seismic moment, exceeds the total extension predicted by an elastic plate model. Thus bending is not predominantly an elastic process but must include a large amount of permanent deformation. Normal‐faulting earthquakes are associated with every major trench system in the Pacific and Indian oceans, but events indicating horizontal compression are rare. The predominance of normal faults over thrust faults, normal events as deep as 25 km within the lithosphere (well constrained by pP), and the existence of large (Ms > 7.5) normal fault earthquakes suggest that the average oceanic lithosphere is not under regional horizontal compressive stress which is a large fraction of the bending stresses. The location of thrust fault events as deep as 40–50 km within the bending plate requires the lithosphere to have significant strength at least as deep as 50 km. A two‐layer plate model with elastic‐perfectly plastic rheology can satisfy these seismological constraints and fit the observed shape of the trench and outer rise. Large regional stresses are not required, and variations in thickness and strength are not needed to fit individual profiles. In our preferred average model the mechanical plate is 50 km thick, with a weak (1‐kbar yield strength) upper layer and a strong (6 kbar) lower layer. The mechanism for plastic strain at yield is assumed to be slip accompanying earthquakes. Normal fault earthquakes are consistent with Coulomb criterion failure, but some other failure mechanism must be invoked for the thrust fault earthquakes. The bottom of the plate is thought to be defined by a transition to steady state creep, induced either by increasing temperature with depth or by a change from dry to wet rheology. Variations in regional stresses should produce corresponding changes in the shapes of the topographic profiles. An examination of the tectonic settings of a number of profiles suggests that increasing horizontal compression should decrease the curvature of the profile in the vicinity of the zero‐crossing point on the seaward wall of the trench.
Theory and experiments on centrifuge cratering
Authors: Schmidt R. M. , Holsapple K. A.
Published: February 2008

Centrifuge experimental techniques provide possibilities for laboratory simulation of ground motion and cratering effects due to explosive loadings. The results of a similarity analysis for the thermomechanical response of a continuum show that increased gravity is a necessary condition for subscale testing when identical materials for both model and prototype are being used. The general similarity requirements for this type of subscale testing are examined both theoretically and experimentally. The similarity analysis is used to derive the necessary and sufficient requirements due to the general balance and jump equations and gives relations among all the scale factors for size, density, stress, body forces, internal energy, heat supply, heat conduction, heat of detonation, and time. Additional constraints due to specific choices of material constitutive equations are evaluated separately. The class of constitutive equations that add no further requirements is identified. For this class of materials, direct simulation of large‐scale cratering events at small scale on the centrifuge is possible and independent of the actual constitutive equations. For a rate‐independent soil it is shown that a small experiment at gravity g and energy E is similar to a large event at 1 G but with energy equal to g3E. Consequently, experiments at 500 G with 8 grams of explosives can be used to simulate a kiloton in the field. A series of centrifuge experiments was performed to validate the derived similarity requirements and to determine the practicality of applying the technique to dry granular soils having little or no cohesion. Ten shots using Ottawa sand at various gravities confirmed reproducibility of results in the centrifuge environment, provided information on particle size effects, and demonstrated the applicability of the derived similitude requirements. These experiments used 0.5–4 grams of pentaerythritol‐tetranitrate (PETN) and 1.7 grams of lead‐azide explosives. They were placed at zero depth of burial and were detonated at gravities up to 450 G. These results provide rules for scaling crater dimensions in Ottawa sand over a range of more than 10 orders of magnitude in total energy release.
Long‐term premonitory seismicity patterns in Tibet and the Himalayas
Authors: Keilis‐Borok V. , Knopoff L. , Allen C. R.
Published: February 2008

An attempt is made to identify seismicity patterns precursory to great earthquakes in most of Tibet as well as the central and eastern Himalayas. The region has considerable tectonic homogeneity and encompasses parts of China, India, Nepal, Bhutan, Bangladesh, and Burma. Two seismicity patterns previously described were used: (1) pattern Σ is a peak in the sum of earthquake energies raised to the power of about 2/3, taken over a sliding time window and within a magnitude range less than that of events we are trying to predict; and (2) pattern S (swarms) consists of the spatial clustering of earthquakes during a time interval when the seismicity is above average. Within the test region, distinct peaks in pattern Σ have occurred twice during the 78‐year‐long test period: in 1948–49, prior to the great 1950 Assam‐Tibet earthquake (M = 8.6), and in 1976. Peaks in pattern S have occurred three times; in 1932–1933, prior to the great 1934 Bihar‐Nepal earthquake (M = 8.3), in 1946, and in 1978. The 1934 and 1950 earthquakes were the only events in the region that exceeded M = 8.0 during the test period. On the basis of experience here and elsewhere, the current peaks in both Σ and S suggest the likelihood of an M = 8.0 event within 6 years or an M = 8.5 event within 14 years. Such a prognostication should be viewed more as an experimental long‐term enhancement of the probability that a large earthquake will occur than as an actual prediction, in view of the exceedingly large area encompassed and the very lengthy time window. Furthermore, the chances of a randomly occurring event as large as M = 8.0 in the region are perhaps 21% within the next 6 years, and the present state of the art is such that we can place only limited confidence in such forecasts. The primary impact of the study, in our opinion, should be to stimulate the search for medium‐ and short‐term precursors in the region and to search for similar long‐term precursors elsewhere.
Burg's Maximum Entropy Algorithm versus the Discrete Fourier Transform as a frequency estimator for truncated real sinusoids
Authors: Swingler D. N.
Published: February 2008

The frequency estimation errors due to both techniques are compared, and it is shown that there are some interesting similarities and differences. A major result is that for even relatively short data lengths the maximum entropy approach should not automatically be regarded as the superior technique for the frequency estimation of truncated real sinusoids.
Thermal transfer between the continental asthenosphere and the oceanic subducting lithosphere: Its effect on subcontinental convection
Authors: Rabinowicz Michel , Lago B. , Froidevaux Claude
Published: February 2008

Convection under certain continents should be strongly affected by the presence of a neighboring subduction zone. The cold subducting oceanic lithosphere is removing heat from the continental mantle. Model cavities with lateral cooling are investigated to quantify the physics of such situations. Convection cells with a large aspect ratio are generated, and a sizable portion, typically 50% or more, of the heat produced under continents is transferred to the oceanic lithosphere in the models. This leads to geophysical speculations concerning the global heat balance in the earth and the capability of large subcontinental cells to drive the continental plates towards the trenches. Other points are discussed, particularly the influence of the chosen rheology (constant viscosity, Herring‐Nabarro or Weertman creep) and the reduced heat flow profiles.
Observations of a direct current concentration on the Eastern Sierran Front: Evidence for shallow crustal conductors on the Eastern Sierran Front and beneath the Coso Range
Authors: Towle J. N.
Published: February 2008

Observations of the dc magnetic field resulting from a temporary single‐pole operation of the Pacific Northwest‐Southwest DC Power Intertie indicate an unexpectedly large concentration of current on the eastern Sierran Front and the possibility of a secondary concentration of current associated with the Coso Range. High electrical conductivity in central and eastern Oregon, in the Basin and Range province, and on the eastern Sierran Front would account for these observations.
In‐plane propagation of shear microcracks in brittle rocks under triaxial compression
Authors: Janach Walter , Guex Lan Huong
Published: February 2008

The localized separation of the two crack faces near the tip of a shear microcrack, which is otherwise kept closed by a normal pressure, is suggested as a possible mechanism for the propagation of microcracks in rocks loaded in triaxial compression. Finite element calculations show that when a shear crack runs along a surface of elastic discontinuity (interface between different minerals or between differently oriented domains of an anisotropic mineral), it can remain open at its tip while a normal pressure acts across that part of the crack which has reclosed. Such a separation bubble allows the shear crack to propagate in plane without frictional sliding taking place. It is speculated that dilatancy could be the result of a residual separation of the reclosed crack faces. On the basis of this mechanism a heuristic failure model is derived, which can correlate the published triaxial failure data of Westerly granite up to a confining pressure of 2000 MPa (20 kbar).
Paleomagnetism of the Triassic Hound Island Volcanics, Alexander Terrane, southeastern Alaska
Authors: Hillhouse J. W. , Grommé C. S.
Published: February 2008

The Hound Island Volcanics, a Late Triassic sequence of submarine basalts, has been investigated to determine possible large‐scale movements of crust in southeastern Alaska. Data from oriented cores at 12 sites bordering Keku Strait yielded a mean paleomagnetic pole at latitude 23.0°N, longitude 188.8°E (α95 = 11.0°). Reliability of the pole is affirmed by a fold test and by the consistent antipolarity of normal and reversed magnetic directions. The anomalous position of this pole compared to that derived from coeval paleomagnetic data in central North America indicates that the region surrounding Keku Strait was rotated approximately 100° in a counterclockwise sense. The paleolatitude determined from the basalts is not significantly different from that predicted by data from the craton. Therefore the apparent northward drift of the Alexander terrane indicated by Paleozoic paleomagnetic results occurred before Late Triassic time.
Gravimetric tidal loading computed from integrated Green's functions
Authors: Goad Clyde C.
Published: February 2008

The M2 gravity load tide at a worldwide selection of observation points has been evaluated for the ocean tide model developed by Schwiderski. The usual spatial convolution technique was employed for these calculations. However, because ocean tide representations are usually expressed as areas or cells of constant amplitude and phase, it has been found that the integrals of the impulse response functions (commonly referred to as Green's functions) are more desirable. These integrals are most efficient when the convolution is performed using areas bounded by constant geocentric and azimuthal angles (known to geodesists as the template method) rather than the usual technique of using areas bounded by latitude and longitude. It is found that the Schwiderski M2 global ocean tide model predicts gravity load tides to an accuracy of ±0.5 μgal. Souriau's calculations for loading effects on tidal gravity data available for western Europe are confirmed to an accuracy of ±0.2 μgal for most sites. Other results are available from California, Australia, and Japan.
Watkins Seamount: Preliminary paleomagnetic results
Authors: Keating B. , Sager W.
Published: February 2008

Magnetic modeling studies have been conducted on the newly named Watkins seamount in the central Pacific. The results, while preliminary, give a paleomagnetic pole position of 69.8°N and 13.3°E. The pole position lies near pole positions of other Cretaceous seamounts from the central Pacific, indicating that the seamount may be Cretaceous in age. The paleoinclination suggests that the seamount formed south of the magnetic equator, a hypothesis consistent with overall Pacific plate motion since the Late Cretaceous.
Terrestrial heat flow in the Brazilian highlands
Authors: Vitorello Icaro , Hamza V. M. , Pollack Henry N.
Published: February 2008

We report heat flow measurements from 84 boreholes and one underground mine at 19 widely spaced sites in eastern and central parts of Brazil. Three sites in the stable São Francisco Craton comprising rocks with Transamazonic ages (2600–1800 Ma) or older present an average heat flow of 42±5 (sem(standard error of the mean)) mW m−2, eight sites located in the Late Precambrian Braziliane metamorphic belt have an average heat flow of 55±4 (sem) m W m−2, and three sites in the Paraná basin, locus of Late Jurassic‐Early Cretaceous basaltic volcanicity, have a mean heat flow of 68±6 (sem) mW m−2. Heat flow results from the Late Cretaceous‐Early Tertiary alkalic intrusion of Poços de Caldas have yielded a site mean of 55 mW m−2, comparable to results obtained from the Braziliane belt into which the alkalic plug has intruded. Four other sites yielded results of lesser quality and are not included in the above means. The age‐grouped means show a systematic decrease of heat flow with increasing time since the last tectonothermal event, in agreement with global data for terrains of similar age elsewhere. Heat production measurements of aggregate samples reflect the variability of the rock types, ranging from the ultramafics with characteristic low heat generation (<0.1 μW m−3) to the alkalic rocks with very high heat generation (>20 μW m−3). Radio isotope concentrations of the surface rocks at several sites are thought not to be representative of the crustal distribution. A best fitting line to four acceptable heat flow‐heat production pairs from metamorphic terrains of the Brazilian Coastal Shield tentatively defines a heat flow province with a reduced heat flow of 28±7 mW m−2 and a slope (depth distribution parameter) of 13±2 km, both within the range found in other shield areas, although the slope of 13 km is among the largest reported for heat flow provinces. Thus the Brazilian shield is characterized by surface and reduced heat flow not unlike other shields, but its heat production appears to be less concentrated near the surface and distributed over a greater depth.