Q for shear-waves in the crust and upper mantle was determined as a function of frequency in the range 1–25 Hz, using band-pass filtered records of about 900 earthquakes occurring in the central Japan area with focal depths from 0 to 150 km. The data were supplied from two stations in the Kanto region, operated by the Earthquake Research Institute, University of Tokyo. The method is taken from Aki and Chouet (1975) and Rautian and Khalturin (1978), and is based on elimination of the source effect from observed spectra of shear-waves, by taking the ratio of their amplitudes to those of coda-waves (measured at a fixed lapse time). The effect of radiation patterns is removed by averaging the ratio over many events. The experimental procedure uses basically a single-station method, and its validity was confirmed by the agreement between results obtained from station Tsukuba and those from Dodaira. We found that Q shows a strong dependence on frequency: Q increases with frequency f proportionally to fn, where n was found to be 0.8 in the northeastern part of Kanto (area A), and 0.6 in the rest of the region (areas B + C). Previously, a value for n of 0.5 had been determined for the Garm area in central Asia by Rautian and Khalturin. In general, our results agree with those of Fedotov and Boldyrev (1969), who obtained values of n around 0.6 for the Kuril-Islands, using a single-station method based on assumptions more restrictive than those adopted here. Q values for the lithosphere have been estimated by various workers using surface-waves with periods greater than 15 s. If we combine the results from surface-waves with those obtained from the S- to coda-amplitude ratio, we find that the frequency-dependence of Q−1 is similar to that obtained from a simple relaxation model, with the peak somewhere around 0.5 Hz.
Initial susceptibility is frequently used as a palaeoclimatic indicator in sediments, but its grain size dependence is not well established. We measured initial magnetic susceptibility χ0 in grown and natural magnetite crystals ranging from 0.09 μm to 6 mm in grain size. Over these five decades of grain diameter, the presented initial susceptibilities are essentially independent of grain size with a mean value of 3.1 SI and a standard deviation of ±0.4 SI. Numerical results of micromagnetic calculations for cylindrical particles in the size range 0.06 μm < d < 0.120 μm agree well with the experimental data. Initial susceptibilities of grown synthetic and natural magnetite crystals larger than 80 μm can be explained with demagnetizing factors and large intrinsic susceptibility (χi > 200) using the relation χ0 = χi(1 + Nχi). The observed number of magnetic domains in magnetite grains between 50 μm and 1000 μm is too low for the required demagnetizing factor of about 0.33. In a lamellar domain model one needs a higher number of domains than those observed, to obtain a demagnetizing factor of 0.33. A simple lamellar stripe domain model without closure domains is therefore not a good approximation for large magnetite grains.Remanent coercive force of grown magnetite grains shows a weak dependence on grain diameter. The remanent coercive force Hcr decreases gradually from about 35 mT to 10 mT between 0.09 μm and 6 mm. A noticeable drop in Hcr occurs at a grain size of about 110 μm, which is interpreted as the transition from pseudo-single-domain to multidomain grains. The remanent coercive force of magnetite grains is not a sensitive indicator of grain size, unlike coercive force or saturation remanent magnetization.
Ferropericlase Mg1−xFexO is believed to be the second most abundant mineral in the Earth’s mantle. Therefore, the electronic and elastic properties of ferropericlase are important for the understanding of the Earth’s interior. Ab initio total energy calculations have been performed for Fe concentrations x≤0.25. The equation of state (EOS) clearly shows a volume expansion as a function of Fe concentration, consistent with experimental data. Magnetic moment calculations as a function of pressure show a high-spin to low-spin transition of Fe2+, and the theoretical transition pressure increases with iron composition. At ambient pressure, we have found that the shear constant C44 reproduces well the experimental data as a function of Fe concentration. The MgO and Mg0.9Fe0.1O minerals show an increasing C44 with pressure, whereas the is slightly negative after 26 GPa for Mg0.8Fe0.2O. The C44 softening could be related to the transition from the cubic to a rhombohedrally distorted phase, recently found by experiment.
A comparison of 15 records of relative geomagnetic paleointensity, spanning the time interval 0.75–1.25 Ma, has been performed after synchronizing the records at the paleointensity minima corresponding to geomagnetic reversals. Filtering and smoothing of the records indicated that, after synchronization, paleointensity features with wavelengths larger than about 20 ky can be matched from one record to another, although age offsets can still be observed in several records. Further synchronization of the records, using intensity minima, permitted construction of a composite stack of these records. This stack provides useful information about the long-term evolution of the geomagnetic field intensity over this period of time, and confirms the existence of a rapid recovery of the field intensity after each reversal. Analysis of the standard deviation and jackknife calculation suggest that improvement in the stack accuracy and resolution could still be achieved with better-constrained age models.
We have examined in detail the stratigraphic variations in magnetic parameters of four cores from the North Atlantic Ocean in areas where the depositional environment has varied with climatic changes. Our objective is to assess whether normalisation of the natural remanent magnetisation (NRM) intensity can cancel the effect of climatically induced variations in magnetic mineral content and grain size and whether a reliable record of relative changes in geomagnetic field intensity can be obtained. After selecting the core sections which meet published criteria for paleointensity normalisation, we have attempted to obtain a record of the geomagnetic field intensity variations over the past 240 kyear, using both ARM and IRM as normalising factors for the NRM. The two methods yield overall similar results, except for the interval 20–30 kyear, where IRM normalisation yields a record more consistent with previous sedimentary and volcanic results, than does the more frequently employed ARM normalisation. The final intensity record docmments a picture of the dipole field moment which presents large similarities with profiles obtained from sediments deposited in different environmental conditions in various regions around the world. We observe a major low at about 42 kyear, which could correspond to the Laschamp event, a broad low in the interval 90–130 kyear, possibly connected to the Blake event, and another low at about 190 kyear, which could reflect the Biwa I event. The processes of acquisition of depositional and postdepositional remanent magnetisation and the physics of the normalising methods are still insufficiently understood. However, these results, obtained from a region characterised by complex environmental dynamics, confirm the potential of sedimentary deep sea cores for relative paleointensity determinations.
Palaeomagnetic secular variation records stacked from five cores ∼ 20 m long extend back through the last glacial cycle to ∼ 100 000 years ago. The time-scale has been constructed from conventional and accelerator radiocarbon ages back to ∼ 45 Ka BP and beyond this by comprehensive pollen studies which identify forest and steppe cycles that have been related to marine oxygen isotopic stages 4 and 5. Spectral analyses of the intensity and directional records using both Fourier and maximum entropy methods reveal strong low-frequency components which fall in the range of the Milankovitch obliquity and precession periodicities (40-20 Ka). It is concluded that although part of this signal may have fed indirectly into the natural remanence via the influence of climatic change on sediment facies, part has been impressed directly by perturbation of the geomagnetic dynamo process at the core-boundary by variations in the Earth's obliquity and precession. These long periods envelop a well-defined pattern, registered also by the independent ‘Mackereth’ core stack back to 50 Ka BP, of irregular secular oscillations of the geodynamo with periods of a few thousands of years.
Analyses of over 600 archaeomagnetic data compiled by Burlatskaya and Nachasova (1977) illustrate that our knowledge of the intensity of the Earth's magnetic field is much poorer than generally believed. The data exhibit high scatter and the distribution of sampling localities is extremely limited. Rock magnetic and experimental contributions to the scatter are probably significant, although it is impossible to determine uniquely the sources of scatter without a substantial increase in the data base and without making additional assumptions about the past magnetic field behaviour. Nevertheless, when averaged in 1000 year intervals, the archaeomagnetic intensity data for the past 5000 years can be simply, but non-uniquely, interpreted in terms of a change in the intensity of the dipole field. This interpretation is broadly consistent with independent evidence from radiocarbon data. Because of inconsistencies in radiocarbon data prior to 8000 years B.P. and because of inadequacies in the archaeomagnetic data, the previously alleged sinusoidal variation of the dipole field intensity with a period of 8000–9000 years should be regarded as highly tentative.
We carried out hydrostatic pressure demagnetization experiments up to 1.24 GPa on samples of terrestrial and extraterrestrial rocks and minerals of different lithologies as well as on synthetic samples. The magnetic remanence of samples was measured directly under pressure using a non-magnetic high-pressure cell of piston-cylinder type that was inserted into a high sensitivity SQUID magnetometer. In order to bring light on the pressure demagnetization effect, we investigated 50 samples with different magnetic mineralogies, remanent coercivities (Bcr) and hysteresis parameters. The samples consisted of pyrrhotite-, magnetite- and titanomagnetite-bearing Martian meteorites, taenite-, tetrataenite- and kamacite-bearing ordinary chondrites and pyrrhotite-bearing Rumuruti chondrite; magnetite- and titanomagnetite-bearing basalts, andesites, ignimbrites, obsidians and granites; a variety of pyrrhotite- and hematite-bearing rocks and minerals (jasper, schist, rhyolite, radiolarite); samples of goethite and greigite as well as synthetic samples of dispersed powders of magnetite, hematite, pyrrhotite and native iron set into epoxy resin. Under hydrostatic pressure of 1.24 GPa, applied in a low magnetic field (<5 μT), the samples lost up to 84% of their initial saturation isothermal remanent magnetization (SIRM) without any changes in their intrinsic magnetic properties. We found that the efficiency of the pressure demagnetization is not exclusively controlled by the magnetic hardness of the samples (Bcr), but that it is strongly dependent on their magnetic mineralogy. For a given magnetic mineralogy the resistance to hydrostatic pressure is roughly proportional to ln(Bcr). It was shown that there is no simple equivalence between pressure demagnetization and alternating field demagnetization effects. The pressure demagnetization was shown to be time-independent but repeated application of the same pressure level resulted in further demagnetization.
We present results of a geochemical survey based on 1150 outcrop samples for heat production studies in a E–W oriented band ( km) located approximately between 62 and 63°N in Finland, the central Fennoscandian shield. The results provide representative averages of heat production rate in the studied tectonic units, but also demonstrate the heterogeneity and spatial variation in radiogenic heat production rate of the Precambrian lithosphere.The study area covers formations from the Archaean granite-greenstone terrain in the east via a Palaeoproterozoic mobile belt to a major granitoid complex surrounded by schist belts in central and western Finland. In this study we review the heat production rates as calculated from the U, Th and K total analyses and densities of the samples, and the relations of heat production rate values with major tectonic setting, lithological types, major composition and petrophysical properties of rocks. The data is further correlated with heat flow values measured in boreholes and applied in a lithospheric heat production model in the central part of the shield.Generally, heat production rate increases from Archaean to Proterozoic rocks in E–W direction but this trend is relatively weak and often overrun by lithological variations. The arithmetic means are 1.1±1.3 μW m−3 in the Archaean domain, 1.3±0.6 μW m−3 in the Höytiäinen autochthonous Proterozoic domain, 1.2±0.5 μW m−3 in the Suvasvesi domain characterized by allochthonous Proterozoic cover, 1.0±0.6 μW m−3 in the Proterozoic Raahe–Ladoga Mobile belt, 1.3±1.1 μW m−3 in the Proterozoic Rantasalmi–Haukivuori domain, 1.6±0.8 μW m−3 in the Central Finland Granitoid Complex and 1.4±0.8 μW m−3 in the Bothnian Schist Belt. The standard deviations of the mean values are considerable and reflect the heterogeneity of heat production. All distributions overlap. Heat production rate shows spatial variations which range in scale from kilometres to tens of kilometres.Well-defined systematic variations of heat production rate values with either SiO2 content, density or P-wave velocity could not be found, although the correlations are statistically significant at 1% risk level. The correlations are weak and scattered, and sometimes opposite in sign in the major rock groups (plutonic, metavolcanic and metasedimentary).Heat flow density measured in boreholes increases with increasing heat production rate, but the present data do not support a strict linear dependence between the variables, and the regression line parameters vary depending on the applied heat production values (drill core analyses versus hole site averages from the present study). This indicates that the heat production rate is spatially heterogeneously distributed in the scale of the boreholes and their formations.A lithospheric heat production model was constructed using data from the present study for the upper crust, literature data for lower crustal xenoliths in the study area and a xenolith-derived heat flow density value for the mantle. The total crustal contribution to the surface heat flow density (37 mW m−2) from radiogenic heat sources is calculated as 26 mW m−2, and the contributions from the lower, middle and upper crust are 4, 11 and 11 mW m−2, respectively.
We investigate the stability of hypothetical layered convection in the mantle and the mechanisms how the downwelling structures originating in the lower layer are generated. The stability is studied by means of numerical simulations of the double-diffusive convection in a 2D spherical model with radially dependent viscosity. We demonstrate that the stability of the layering strongly depends not only on the density contrast between the layers but also on the heating mode and the viscosity profile. In the case of the classical Boussinesq model with an internally heated lower layer, the density contrast of about 4% between the compositionally different materials is needed for the layered flow to be maintained. The inclusion of the adiabatic heating/cooling in the model reduces the temperature contrast between the two layers and, thus, enhances the stability of the layering. In this case, a density contrast of 2–3% is sufficient to preserve the layered convection on a time scale of billions of years. The generation of the downwelling structures in the lower layer occurs via mechanical or thermal coupling scenarios. If the viscosity dependent on depth and average temperature at each depth is considered, the low viscosity zone develops at a boundary between the two convecting layers which suppresses mechanical coupling. Then the downwelling structures originating in the lower layer develop beneath upper layer subductions, thus resembling continuous slab-like structures observed by seismic tomography.
A single-heating procedure for the determination of two partially independent values of paleofield intensity for one sample is presented. The procedure combines data for Shaw-type and 'ARM-method' determinations furnishing the ratio thermoremanent magnetization (TRM) to ARM acquisition efficiency corrected for physicochemical alteration to the magnetic carriers. Applicability of the Shaw-method to Fe-bearing samples is demonstrated by simulated paleointensity determinations on synthetic samples containing multidomain grains. The combined Shaw-ARM procedure was applied to a linear basalt sample, but the Thellier-Thellier method could not provide a meaningful determination of the neighboring chip. These conflicting findings may be explained by multiple step-wise heatings causing more damage to carriers than a single heating procedure, and by the natural remanent magnetization in this lunar basalt not being a simple TRM.
Broadband receiver functions analysis is commonly used to evaluate the fine-scale S-velocity structure of the lithosphere. We analyse teleseismic P-waves and their coda from 30 selected teleseismic events recorded at three seismological stations of to the French TGRS network in the Alpes Maritimes. Receiver functions are computed in the time domain using an SVD matrix inversion method. Dipping Moho and lateral heterogeneities beneath the array are inferred from the amplitude, arrival time and polarity of locally-generated PS phases. We propose that the Moho dips 11° towards 25°±10°N below station CALF, in the outer part of the Alpine belt. At this station, we determine a Moho depth of about 20±2 km; the same depth is suggested below SAOF station also located in the fold-trust belt. Beneath station STET located in the inner part of the Alpine belt, the Moho depth increases to 30 km and dips towards the N-NW. Moreover, 1D-modelling of summed receiver function from STET station constrains a crustal structure significantly different from that observed at stations located in the outer part of the Alps. Indeed, beneath CALF and SAOF stations we need a 2 km thick shallow low velocity layer to fit best the observed receiver functions whereas this layer seems not to be present beneath STET station. Because recent P-coda studies have shown that near-receiver scattering can dominate teleseismic P-wave recordings in tectonically complicated areas, we account for effect of scattering energy in our records from array measurements. As the array aperture is wide relative to the heterogeneity scale length in the area, the array analysis produces only smooth imaging of scatterers beneath the stations.
Equilibrium liquidus crystals will float in variety of terrestrial and lunar magmas within a wide range of mantle depths. Density inversions exist for ultrabasic mantle melts and olivine at depths between 245 and 500 km in the Earth. Olivine flotation occurs in high-Ti basalts at ∼400 km in the lunar mantle. Basaltic and komatiitic magmas have density inversions with garnet in the upper mantle transition zone and perovskite in the lower mantle. Peridotitic magma, because of its comparatively large bulk modulus (KT) and pressure derivative of the bulk modulus (K′), is likely to be less dense than liquidus garnet and perovskite at all depths within the Earth.
Single-crystal Brillouin spectroscopy measurements have been carried out to 1073 K and ambient pressure to determine the elastic modulus tensor of a natural orthopyroxene, nearly pure Mg end-member (Mg0.994Fe0.002Al0.004)2(Si0.996Al0.004)2O6 orthoenstatite. Three single-crystal specimens with orthogonal crystallographic orientations were prepared for Brillouin measurements to determine all the single-crystal elastic moduli (Cij). The single-crystal elastic moduli were calculated using the measured velocities of sound and our independent measurement of the volume thermal expansion. These are, to our knowledge, the highest temperatures at which the complete single-crystal elastic modulus tensor of orthoenstatite has been measured. The elastic moduli at ambient conditions obtained in this study are in excellent agreement with previous measurements on Mg2Si2O6 orthoenstatite (OEN). The elastic anisotropy of OEN is approximately the same at all measured temperatures. The velocities in different directions change at different rates, with C33 showing the largest temperature dependence (decreasing with temperature almost twice as fast as C11 or C22). The variation of elastic moduli with temperature is linear up to approximately 673 K, above which some of the moduli decrease with a quadratic dependence on temperature. The non-linear behavior is in accord with recent elastic mode softening observations in orthoenstatite at higher temperatures and could add support to account for a thermally induced solid-state mechanism for the low velocity zone in Earth's upper mantle.
Pressure–volume measurements have been performed for CaSiO3 perovskite to 108 GPa at 300 K using NaCl and argon pressure-transmitting media, and energy dispersive X-ray diffraction (EDXD) in a diamond-anvil cell (DAC). By determining a parameter that is the product of the elastic anisotropy, S, and the uniaxial stress component, t, for each data point, we define the stress condition of the sample. For different points at the same pressure in a temperature-quenched sample, the St value can differ by as much as a factor of 5, indicating heterogeneity in the stress condition. This may be responsible for the large scatter of earlier P–V measurements in the DAC which in general used a large diameter X-ray beam. Also, the St value provides insight into the elastic anisotropy, S, of CaSiO3 perovskite and platinum. The sign of S (positive) for CaSiO3 perovskite agrees with first principles calculations but the magnitude may be inconsistent. A new compression curve at 300 K was obtained for CaSiO3 perovskite by using those data points which represent the most nearly hydrostatic conditions. It is observed that the data points with high St values yield larger volumes than the points with small St values at a given pressure. By selecting the data points having low St values (St≤0.005), combining with lower pressure large volume press (LVP) measurements and fitting to third order Birch–Murnaghan equation of state (EOS), we find that CaSiO3 perovskite is more compressible (V0=45.58±0.05 Å3, KT0=236±4 GPa, and KT0′=3.9±0.2 GPa) than suggested by previous studies. The density and bulk modulus of CaSiO3 perovskite at lower mantle pressures and 300 K are 1–3% greater and 5–15% smaller, respectively, than found in previous studies. This study demonstrates that defining the stress state of the sample is crucial to obtain an accurate 300 K compression curve for unquenchable high-pressure phases.
Transient and steady-state creep of single crystal NaCl was measured in the temperature range 560–736°C and at stresses between 0.027 and 0.188 MPa. Samples were loaded parallel to the 〈100〉 direction and strain was measured using a capacitive transducer having a resolution of 10−7.The non-recoverable portion of the transient response can be represented by a power law in time with an exponent of about 0.55. The activation energy for transient creep in previously deformed crystals is 0.20 MJ mol−1, the same as that for steady-state creep, while the activation energy for transient creep in annealed crystals is 0.16 MJ mol−1 which is approximately equal to the activation energy for core diffusion. Transient strain rates are linearly proportional to stress.Steady-state strain rates are proportional to σ4 for stresses above 0.1 MPa, in agreement with previous studies. Below 0.1 MPa the strain rates have a linear dependence on stress. We interpret this behavior to be analogous to Harper-Dorn creep in metals, although higher dislocation densities and the observations of orthogonal slip bands suggest that a different micromechanism may control Newtonian flow in this non-metallic system.
Centroid–moment-tensor (CMT) solutions are presented for 1087 earthquakes that occurred during 2003. The solutions are obtained using the method of Dziewonski et al. [Dziewonski, A.M., Chou, T.-A., Woodward, J.H., 1981. Determination of earthquake source parameters from waveform data for studies of global and regional seismicity. J. Geophys. Res. 86, 2825–2852] and applying corrections for aspherical Earth structure as represented by the whole-mantle shear-velocity model SH8/U4L8 of Dziewonski and Woodward [Dziewonski, A.M., Woodward, R.L., 1992. Acoustic imaging at the planetary scale. In: Emert, H., Harjes, H.-P. (Eds.), Acoustical Imaging, vol. 19. Plenum Press, New York, pp. 785–797]. The model of inelastic attenuation of Durek and Ekström [Durek, J.J., Ekström, G., 1996. A radial model of inelasticity consistent with long-period surface wave attenuation. Bull. Seism. Soc. Am. 86, 144–158] is used to predict the decay of the waveforms. The focal mechanisms of the largest, or otherwise significant, earthquakes of 2003 are reviewed.
An investigation of the power law relationship between Nusselt number (Nu) and Rayleigh number (Ra) for Earth’s convecting mantle is presented. The Nu(Ra) relationship was calculated from the results of a model with three dimensional spherical geometry and free slip boundary conditions. Both basally and internally heated convection has been examined. For Nu(Ra)=aRaβ, β was found to be 0.294±0.004 for basally heated systems, which is lower than the value of 1/3 suggested by conventional boundary layer theory. The exponent β=0.337±0.009 for internally heated systems, when the internally heated Ra is converted to a basally heated equivalent for comparison. The influence of the method used to calculate β was also considered, with particular attention paid to high Ra. As an example of the significance of β=0.29 rather than 1/3, a Ra of 109 results in a surface heat flux which is ≈32% lower. Within the range of Ra used in this study, there is no evidence that β changes at high Ra. Therefore, that mechanism cannot be used to moderate mantle temperature when projecting back to early Earth conditions. The differing planform of basally and internally heated models was shown to result in different scaling relationships between root mean square surface velocity and Ra for the two modes of heating, in particular, a much lower surface velocity for internally heated cases relative to equivalent Ra basally heated cases.
The International Association of Geomagnetism and Aeronomy (IAGA) on 12 December 2004 released the 10th generation International Geomagnetic Reference Field (IGRF)—the latest version of a standard mathematical description of the Earth's main magnetic field and used widely in studies of the Earth's deep interior, its crust, ionosphere and magnetosphere. The coefficients were finalised by a task force of IAGA. The IGRF is the product of a large collaborative effort between magnetic field modellers and the institutes involved in collecting and disseminating magnetic field data from satellites and observatories around the world.
A quantitative analysis of some of the magnetic properties of an Apollo 11 lunar dust sample is given. The measurements involved are the acquisition of isothermal and viscous remanence, the thermal demagnetization of isothermal and thermoremanence, and the temperature and frequency dependence of the initial susceptibility. It is shown that the experimental results are in good agreement with those predicted from single domain theory and can be explained on the basis of an assembly of single domain iron grains distributed such that the number of grains within a given volume range is inversely proportional to the square of the volume.
In order to study the role of alkali elements in global mantle circulation, we studied melting phase relations of an alkali basalt JB-1 up to 12.5 GPa under dry conditions. Compatibility of Na and K in clinopyroxene solid solution changes dramatically in the studied pressure range. The liquidus clinopyroxene/meltpartition coefficient for Na2O increases from 0.08 at 1 atm to 2.0 at 10 GPa and that for K2O increases discontinuously at around 7 GPa. Subsolidus clinopyroxene of JB-1 contains over 1 wt.% K2O at pressures above 7 GPa, whereas it contains less than 0.1% at pressures below 5 GPa. Accordingly, subsolidus assemblage of the alkali basalt contains small amount of potassium feldspar up to 6 GPa but contains only clinopyroxene, garnet and coesite at pressures above 7 GPa. There is a huge temperature increase in the JB-1 solidus at pressures between 6 and 7.5 GPa where the subsolidus mineral assemblage changes. The geometry of the solidus curve indicates that partial melting of the alkali basaltic materials must occur at 6–7 GPa in adiabatically ascending mantle flow with broad range of potential mantle temperatures (PMT≥1200°C). Origin of the occurrence of ultrapotassic magmas in continental alkali basalt provinces and that for mildly potassium rich magmas in ocean island provinces are discussed in the light of present experiments.
We conducted detailed rock magnetic investigations on 36 m of drill core collected during Ocean Drilling Program Leg 195 at Hole 1202B (24°48.24′N, 122°30.00′E), in the Southern Okinawa Trough, with the goal of extracting a reliable paleointensity signal with centennial resolution. An age-depth model was established from a chronology obtained by accelerator mass spectromety 14C dating. The sedimentary section spans almost the entire Holocene (0–9.4 kyr) and exhibits sedimentation rates close to 400 cm/kyr. The magnetic properties are dominated by stable, pseudo-single domain magnetite. High-field hysteresis data and the grain-size sensitive ratio of anhysteretic remanent magnetization (ARM) to low field magnetic susceptibility indicate a narrow range of grain sizes and concentrations. Magnetic parameters vary by a factor of 4 thereby fulfilling the usual criteria for a relative paleointensity study. The relative geomagnetic paleointensity was obtained by normalizing the intensity of natural remanent magnetization (NRM) by the ARM and the low field magnetic susceptibility. Both normalizations yield nearly identical results (r = 0.89). Spectral analysis indicates that the record is not significantly affected by local environmental conditions. Comparison of this West Pacific paleointensity curve with other curves suggests a geomagnetic origin for the signal. Millennial-scale features of our record correlate to variations of the archeomagnetic dipole moment. This suggest that the sediments at Hole1202B recorded changes of the geomagnetic field over the studied time interval.
Review papers and specific contributions on the subject of convection in the solid interiors of the terrestrial planets and planetary evolution are presented. Geophysical observations of solid-state convection in the terrestrial planets are reviewed, along with the theory of convection in a layer with a high Prandtl number and numerical approaches to the calculation of convection in planetary interiors. Other papers treat the formation, history and energetics of terrestrial planet cores, the effects of convection on lunar thermal history and the relation between the height of mountains on Venus and the creep properties of Venusian rocks.
Current ground motion models all assume monotonically increasing spectral amplitude at all periods with increasing magnitude. However, near fault recordings from recent earthquakes confirm that the near fault fault-normal forward rupture directivity velocity pulse is a narrow band pulse whose period increases with magnitude. This magnitude dependence of the period of the near fault pulse is expected from theory, because the period of the pulse is related to source parameters such as the rise time (duration of slip at a point on the fault) and the fault dimensions, which generally increase with magnitude. This magnitude dependence of the pulse period causes the response spectrum to have a peak whose period increases with magnitude, such that the near fault ground motions from smaller earthquakes may exceed those of larger earthquakes at intermediate periods (around 1 s). This paper presents preliminary equations relating the period of the fault-normal component of the forward rupture directivity velocity pulse to the earthquake magnitude, and a preliminary model for the acceleration response spectra of near fault fault-normal ground motions that includes the magnitude dependence of the period of the response spectral peak.
We present results of lattice thermal diffusivity measurements on (Mg0.9Fe0.1)2SiO4 olivine and its high-pressure polymorphs wadsleyite and ringwoodite under mantle conditions. We used the Ångström method on cylindrical samples in multianvil apparatus at pressures up to 20 GPa and temperatures up to 1373 K. Because of the fine polycrystallinity of the specimens (∼30−40 μm or less), there is strong scattering/absorption of light and suppression of radiative transport so that the lattice vibrational component is the dominant heat transfer mode. Lattice thermal conductivities were calculated from the thermal diffusivity results using heat capacity and equation of state data. Olivine thermal conductivities are consistent with previous results obtained at 1 atm [e.g. J. Am. Ceramic Soc. 38 (1954) 107; J. Geophys. Res. 77 (1972) 6966; Science 283 (1999) 1699]. Thermal conductivity increases by approximately 30% at the transition from olivine to wadsleyite (corresponding to the 410 km discontinuity) and a further, but smaller, increase may occur at the transition from wadsleyite to ringwoodite. For each of these phases, lattice conductivity closely follows a T−1/2 dependence on temperature T [Phys. Rev. 119 (1960) 507; J. Geophys. Res. 79 (1974) 703; Brown, 2002]. If such a dependence applies to other silicates and complex crystals, there should be a useful way to estimate conductivities at high temperatures from room temperature measurements.
Investigating the possibility that some earthquakes may significantly differ from the double-couple mechanism is always a subject of concern to seismologists. In this paper we give a detailed study of the 14 November 1986 Taiwan earthquake (Ms = 7.8) using the long-period body-wave waveform inversion. A hybrid global inversion method was used to derive the best-fitting solution and estimate the extreme models of non-double-couple components. The regional P-waveforms provide high resolution to the isotropic component. The inversion result shows that the Taiwan earthquake is an event composed of the reverse dip-slip dislocation source and an isotropic component. A notable source parameter in the result is the relative intensity of the isotropic component, which is defined as , where σ1, σ2 and σ3 are the eigenvalues of the seismic moment tensor with σ1 > σ2 > σ3 and . The best-fitting solution of SP is −0.33, and the extreme values of SP are −0.48 and −0.13, respectively. This result suggests the existence of an obvious compressional isotropic component at the source process of this event.
The length variations and the ultrasonic wave velocities were measured along principal single crystal calcite axes under high pressure.We have observed that six independent elastic constants undergo drastic changes at the calcite I—calcite II transition point at 14.6 kbar. Furthermore, we have determinated the length and volumetric variations in the phases I and II and during the I–II transition.These results allow us to deduce the following important points: influence of the planar CO3 groups on the strong linear compressibility anisotropy in phase I; reciprocal relation between the decrease of elastic constants at the transition point and the anomaly of compressibility of the calcite II; compatibility of our data with the monoclinic structure of calcite II.
The 3-D P-wave velocity structure of the mantle below Europe, the Mediterranean region and a part of Asia Minor is investigated. This study is a considerable extension of an earlier tomographic experiment that was limited to imaging upper-mantle structure only. Here, the Earth's volume under study encompasses the mantle to a depth of 1400 km, and we increase the number of International Seismological Centre (ISC) data for inversion by a factor of four by taking more years of observation, and by including data from teleseismic events. The most important departure from the earlier study is that we do not use the Jeffreys-Bullen model as a reference model, but an improved radially symmetric velocity model, the PM2 model, which is appropriate for the European-Mediterranean mantle.
Rheological properties of mantle minerals are critical for understanding the dynamics of the Earth’s deep interior. Due to limitations in experimental technique, previous quantitative studies of the rheological properties of mantle minerals are limited to either low pressure or low temperature. The present understanding of mantle flow is mostly inferred from the extrapolation of relatively low-pressure data to mantle high-pressure conditions. However, the effect of pressure (represented by activation volume) on the rheological properties of olivine is still controversial. Therefore, deformation experiments, carried out at mantle pressures, are necessary to understand and model mantle flow. Here we report an experimental study of plastic deformation of San Carlos olivine (Mg, Fe)2SiO4 under upper mantle conditions. Macroscopic differential stress and strain rates have been measured in situ in a large-volume high-pressure apparatus using newly developed techniques. The differential stress at high temperature and high pressure that we measured is significantly lower than that estimated by many currently accepted olivine flow laws. We document the first in situ experimental differential stress results in a multi-anvil press. Our results give direct evidence for a relatively small activation volume (less than 10−5 m3 mol−1). This shows that the effect of pressure on dislocation creep is small.
We present shear wave splitting measurements from local slab earthquakes at eight seismic stations of the Japanese F-net array located in the Ryukyu arc. We obtained high-quality splitting measurements for 70 event-station pairs and found that the majority of the measured fast directions were parallel to the strike of the trench and perpendicular to the convergence direction. Splitting times for individual measurements ranged from 0.25 to 2 s; most values were between 0.75 and 1.25 s. Both the fast directions and the split times were similar to results for teleseismic S(K)KS and S wave splitting at the same stations, which suggests that the anisotropy is located in the mantle wedge above the slab. We considered several mantle deformation scenarios that would result in predominantly trench-parallel fast directions, and concluded that for the Ryukyu subduction system the most likely explanation for the observations is corner flow in the mantle wedge combined with B-type olivine fabric. In this model, the flow direction in the wedge is perpendicular to the trench, but the fast axes of olivine crystals tend to align perpendicular to the flow direction, resulting in trench-parallel shear wave splitting.
In this paper we give a detailed study of the 16 September 1994 Taiwan Strait earthquake using a broad-band body-waveform inversion. An adaptive hybrid global search algorithm was used to solve the nonlinear problems of inverting the spatial distributions of slip amplitude, rake, slip duration and rupture time on a finite fault. The best-fitting solution reveals that the largest slip of 14 m took place in the hypocentre region, in which the rise time is short. The slip amplitude decreases and the rise time increases from the initiation point outward. A suggested mechanism is that the rupture initiated as a breaking of a strong asperity with low dynamic friction and was arrested by large friction around the asperity. The hypocentre is a region of high frequency radiation. We examined the sensitivity by comparing the inversion solutions utilizing varied parameterizations and constraints. These numerical tests show that the major features of the rupture model are better resolved.
The papers review evidence for recent activity within the moon as manifested by lunar grid system, transient phenomena, moonquakes, and episodic emissions of radiogenic gases. Topics include a survey of lunar transient phenomena, possible causes of such phenomena, evidence that high-frequency seismic events may be shallow moonquakes, lunar seismicity and tectonics, a hypothesis on the nature of sites of lunar gas venting, and a search for sporadic gas emissions from the moon. Other contributions discuss the release of radiogenic argon-40 from the moon, radon-222 emission as an indicator of current activity on the moon, upper limits to gas emission from sites of lunar transient phenomena, physical processes that could produce transient changes on the lunar surface, critical-velocity gas-plasma interaction as a mechanism for lunar transient phenomena, and tidal triggering of moonquakes, transient phenomena, and radiogenic-gas emissions.
The grain size dependence of hysteresis parameters (4.2–294 K) and of the anhysteretic remanent magnetization (294 K) was measured on dispersed spherical Fe3O4 particles in the grain size range 60–160 nm. There are in part marked temperature variations of the hysteresis properties. It is assumed that at least at 130 K (K1 = 0) the magnetic structure of particles in a certain grain size range is characterized by a curling mode magnetic moment configuration which may also form in the remanence state.
The method of molecular dynamics is used to calculate values for the thermal Grüneisen parameter, γ, for a face-centred cubic crystal with several simple central force atomic potential functions at three compressions for comparison with free volume and acoustic γ formulations. Neither is found to agree with the computer experiments. A defect in the free-volume formula for γ is shown to arise from the assumption that motions of neighbouring atoms in a crystal at high temperature are uncorrelated whereas the computer models demonstrate a 25–35% correlation of in-line motions and 5% correlation of transverse motions. The formula can be modified to allow for the empirically observed correlations, but it is concluded that there are still difficulties in analytical approaches to equation-of-state studies of the Earth's deep interior and that the computer modelling method of molecular dynamics has important advantages.
Papers are presented outlining various comparisons of Mercury and the moon. Specific topics include the planetary magnetism and the interiors of the moon and Mercury, the relationship between crustal tectonics and internal evolution, global tectonics, the origin and relative age of lunar and Mercurian intercrater plains, and the relative preservation states of secondary craters. Comparisons are also discussed between lunar and Mercurian rayed craters, large impact structures, isostasy and average crustal viscosity. Attention is given to global seismic effects of basin-forming impacts, interpretations of optical observations of Mercury and the moon, and the production of simple molecules on the surface of Mercury.
Consideration is given to: variations in the Gauss coefficients of the geomagnetic potential over the last several hundred years; eccentric geomagnetic dipole drift; and evidence for geomagnetic jerks from 1931 to 1971. Among other topics discussed are: models of the geomagnetic field in western Europe from 1960 to 1980; the external and internal parts of the geomagnetic jerk of 1970; secular variation of the magnetic mean energy density at the source layer depth; and motions at the core surface in the geostrophic approximation. Consideration is also given to: the unpredictability of geomagnetic secular variation; the use of short-term core-mantle coupling and geomagnetic variation impulses as Lagrangian tracers of core motions; and a comparison of two sequential geomagnetic polarity transitions in the upper Olduvai and the lower Jarmillio in the Southern Hemisphere.
In aseismic regions the lack of large earthquakes in modern times makes the study of historical earthquakes essential as such events often serve as the yardstick for estimating seismic hazards. Since the largest earthquakes with few exceptions took place hundreds of years ago we may claim that their recurrence times are very large or alternatively their magnitudes are biased upwards. Scandinavia is no exception in this regard and one interesting event in particular is the Kattegat earthquake of 22 December 1759. It is unique in the sense that a macroseismic questionnaire was circulated in Zealand within days of its occurrence. The puzzling feature here is that intensity ranges from VII to I across Zealand but then given I=III for Hamburg and other North German cities. We have chosen two avenues in reassessing the magnitude of the Kattegat earthquake, namely (1) 2D finite-difference (FD) waveform modelling to evaluate possible amplitude amplifications due to sub-surface basin structures and (2) re-assessing the intensity reports of the 1759 Kattegat earthquake in light of the new European Macroseismic Scale (EMS-98).Our 2D FD results show that the presence of the Danish and the North German basins led to shear wave amplitude amplification by factors of 10 and 5, respectively. This is equivalent to a significant increase or overestimation of the area of perception (tied to I=III). The re-assessment of the macroseismic observations in light of the above results and the EMS-98 downgrade the Kattegat earthquake MS magnitude from 5.6 to 5.1.
The phase relations in hydrous and anhydrous mid-ocean ridge basalt were determined at pressures of 18–28 GPa. Liquidus phase relations in hydrous and anhydrous MORB are different. Garnet is the liquidus phase at pressures below 21 GPa, Ca–Al (CAS) phase and stishovite are the liquidus phases at pressures of 22–27 GPa, and stishovite and Ca-perovskite are the liquidus phases above 27 GPa, whereas Ca-perovskite is a liquidus phase of anhydrous MORB at pressures above 23 GPa. Under subsolidus conditions, we have found that in the hydrous MORB system the stability fields of Al-bearing perovskite and Na–Al (NAL) phase might shift to lower pressure by about 1.5 GPa compared to the dry MORB system. This shift could be explained by oxidation of a garnet-bearing assemblage by hydrous fluid and formation of Fe3+-bearing aluminous perovskite at lower pressures relative to the anhydrous system and/or differences in water solubility of the phases existing in perovskite-bearing assemblages. Our data indicate that hydrous basaltic crust remains denser than peridotite along the geotherm of a subducting slab, i.e. there is no density crossover between peridotite and basalt. Therefore, in slabs going through the 660 km discontinuity, basalt would gravitationally sink into the lower mantle under relatively hydrous conditions. The delamination of former basaltic crust near the 660 km discontinuity might be possible under relatively dry conditions of subduction. There are no stable highly hydrous phases in MORB above 10 GPa even at lower temperatures corresponding to subducting slabs. Therefore, MORB cannot be an important carrier of water to the deep Earth interior. However, it can be constantly supplied by water-bearing fluid from the underlying peridotite part of the descending slab. Thus, it is plausible that water can control subduction of the oceanic crust into the lower mantle.
The two classes of earthquakes which preceded the explosive eruption of Mount St. Helens, Washington, on 18 May 1980, can be related to two different forms of energy release. This is manifest both in the predominant periods of the two classes of seismic energy release at 1.0 and 0.55 s, and in the ratio of their source dimensions, in the range 2 : 1 to 6 : 1, inferred from characteristic magnitudes of the two possible classes of earthquakes apparent in the discrete frequency-magnitude distribution.The observed increase in the ratio of long to short period amplitudes of surface waves recorded on MSO, a local WWSSN station, can be numerically related to the growth of a magma chamber or conduit within which are generated the transient oscillations thought to be responsible for volcanic tremor. These calculations predict an increase in the chamber's diameter at a rate of 2–7 m day−1 at depth in the 2 months preceding the eruption, and this is consistent with an observed growth rate of 2 m day−1 of the precursory bulge which appeared at the surface on the mountain's north flank at the time.The frequency-magnitude distribution is analysed in two parts corresponding to a power law at low magnitudes and a Gaussian distribution of typical fault lengths at high magnitudes—an extension of the characteristic earthquake model first developed for individual fault zones. The power law can in general have a non-integer exponent, which can be equated to a fractal self-similarity dimension D. Temporal changes in D, which is proportional to the seismic b-value, are proposed as an alternative and complementary description of the way seismic energy is released to the more commonly used viewpoints of changes in stress and heterogeneity.Despite a reassessment of the completeness of the magnitude catalogue, no clear seismic precursor to the explosive eruption is observed in the b-value, although an earlier phase of phreatic eruptions was strongly associated with a b-value anomaly.
The nature of short-term mantle rheology for timescales between the seismic frequency band and a few hundred years remains relatively unknown. We have made use of two pieces of information, which have emerged as a consequence of the recent acquisition of data from the LAGEOS satellite, to place some contraints on the rheological parameters of short-term mantle rheology. The first is the secular variation of the gravitational harmonic . The second is the amount of dispersion of the tidal Love number for the 18.6 y tide, which has been inferred to be ∼ 20%. Two types of rheology have been considered. The first is a frequency-dependent Q rheology. An analytical expression for the transformed shear modulus has been developed from the truncated retardation spectrum explicitly for this purpose. The second anelastic rheology we have used is a standard linear solid, where the parameters of importance are the relaxation strength Δ and the short-term viscosity ν2. Rheological parameters of the frequency-dependent Q model previously proposed by Lambeck and Nakiboglu to explain the dispersion of the tidal Love number for a 18.6 y period are found to produce too much dispersion for the Chandler wobble and to yield seismic Q's between O(103) and O(104) for fundamental toroidal modes, which are not compatible with the data. We have used instead a three-layer model consisting of an elastic lithosphere, a mantle with a standard-linear solid rheology whose Debye peak lies beyond the long time cutoff of the retardation spectrum, and an inviscid core. For relaxation strength of Δ around 1 and short-term viscosity ν2 < 1021 P unacceptably large rates of would result from earthquake excitation. A sharp decrease of the tidal dispersion takes place for ν2 ≅ 5 × 1020 P and Δ = 1.0. For smaller values of relaxation strength, Δ = 0.15 the maximum tidal dispersion is only 9% and decreases gradually as ν2 is increased beyond 5 × 1020 P. We have found that the maximum dispersion caused by the Maxwell rheology is small, around 5%, even for asthenospheric viscosities of O(1018P). These results suggest that the datum of 18.6 y tidal dispersion can be utilized eventually to place constraints on potential candidates of short term mantle rheology.
The main purpose of this paper is to describe in some detail an experiment which allows the measurement of elastic constants of mantle minerals into the temperature range exceeding , where θ is the Debye temperature. We need these experimental data to establish firmly the thermodynamic and mechanical behavior of solids well into the classical range of T, which is of especial interest to lower mantle geophysics.The second purpose is to describe two applications of the experiment, of special importance to geophysics: (1) the determination of υpandυs seismic velocities up to upper mantle temperatures, and the method of extrapolation to lower mantle temperatures; and (2) the calculation of critical parameters for use in mantle thermodynamics. The experiment is called rectangular parallelepiped resonance (RPR).
We study the possible seismic gap in the Concepción–Constitución region of south-central Chile and the nature of the M=7.8 earthquake of January 1939. From 1 March to 31 May 1996 a seismic network of 26 short period digital instruments was deployed in this area. We located 379 hypocenters with rms travel time residuals of less than 0.50 s using an approximate velocity distribution. Using the VELEST program, we improved the velocity model and located 240 high precision hypocenters with residuals less than 0.2 s. The large majority of earthquakes occurred along the Wadati–Benioff zone along the upper part of the downgoing slab under central Chile. A few shallow events were recorded near the chain of active volcanos on the Andes; these events are similar to those of Las Melozas near Santiago. A few events took place at the boundary between the coastal ranges and the central valley. Well constrained fault plane solutions could be computed for 32 of the 240 well located events. Most of the earthquakes located on the Wadati–Benioff zone had “slab-pull” fault mechanism due to tensional stresses sub-parallel to the downgoing slab. This “slab-pull” mechanism is the same as that of eight earthquakes of magnitude around 6 that are listed in the CMT catalog of Harvard University for the period 1980–1998. This is also the mechanism inferred for the large 1939 Chilean earthquake. A very small number of events in the Benioff zone had “slab-push” mechanisms, that is events whose pressure axis is aligned with the slab. These events are found in double layered Wadati–Benioff zones, such as in northern Chile or Japan. Our spatial resolution is not good enough to detect the presence of a double layer, but we suspect there may be one.
In this short contribution, geomagnetic measurements in Central Africa made by Capelo and Ivens – two Portuguese explorers – during the years 1877 and 1885 are provided. We show the scarce number of geomagnetic observation in Africa compiled until now. These Portuguese explorers performed a considerable amount of measurements of geomagnetic declination (44 measurements), inclination (50) and horizontal component (50) of the geomagnetic field. We compared the results attained by these keen observers with those derived from the global geomagnetic model by Jackson et al. [Jackson, A., Jonkers, A.,Walker, M., 2000. Four centuries of geomagnetic secular variation from historical records. Philos. Trans. R. Soc. Lond. 358, 957–990].
In previous research, trace amplitudes of surface wave maxima recorded by undamped Milne seismographs were used to determine the surface-wave magnitudes Ms of large shallow earthquakes which occurred prior to 1912. For this purpose, the effective gain of these instruments was calibrated by using the surface-wave magnitudes Ms(GR) which were calculated from the unpublished worksheets for Seismicity of the Earth of Gutenberg and Richter. In this paper, the real quality of Ms(GR) is critically re-evaluated by using independent sets of data. It is found that Ms(GR) for the period 1904–1909 is considerably overestimated. The average excess from the real Ms is 0.5 units for 1904–1906, 0.4 for 1907, 0.3 for 1908–1909 and 0.0 for 1910–1912. This overestimation is so systematic and large that the previous results are all redetermined. The average effective gain of Milne instruments is revised to be 21.9; previously, the gain depended on Ms. This revision results in systematic reduction in the previously assigned magnitudes. The revised values of Ms for 264 shallow earthquakes, with Ms=6.8 and over in the period 1897–1912 inclusive, are listed. The present revision is large enough to preclude the possibility of the high activity of large shallow earthquakes around the turn of the century. The present results have a direct effect on all the magnitude catalogues of shallow earthquakes which occurred prior to 1909.
In order to arrive at a uniform basis for a study of the global seismicity, the surface-wave magnitude Ms of shallow earthquakes for 1898–1917 is newly determined by using maximum amplitudes of surface waves recorded by undamped Milne and Omori seismographs. The effective gain of these instruments needs to be calibrated with great care, because of the lack of a damping device. The calibration of Milne instruments is based on a comparison of amplitudes measured by Milne instruments with those by more modern, damped instruments. The effective gain varies from 7 to 20 in the range of Ms from 6 to 8.5. The reported amplitudes for the years 1910–1912 are twice as large as the standard, and need to be corrected. By applying these results to a wealth of the Milne data, Ms is determined for a large number of earthquakes between 1898 and 1912. To supplement the data for 1913–1917, records of undamped Omori seismographs are used. For the entire period from 1898 to 1917, 308 shallow earthquakes with are listed here. Both this list and the previous catalogue of Abe (1981) for 1918–1980 provide a catalogue of large shallow earthquakes much more uniform than any previously established. Between 1899 and 1910 the annual frequency of shocks with is 20, which is twice as large as the annual average of 10 during 1911 and 1980.
AbstractA new telemetered seismological network has been recently installed in northern Algeria. At present, this new seismic network includes 30 stations, with a final goal of 32 stations connected by radio and dedicated telephone lines to a centre located at the CRAAG headquarters in Alger-Bouzare´ah. The Algerian Telemetered Seismological Network (ATSN) stations are all equipped with short-period seismometers. Monitoring by this network will allow a more precise knowledge of the seismicity in Algeria and southern Europe. As an example, the sequence of the Rouina earthquake (M = 5.2) of 19 January 1992 recorded by the ATSN is described.
Paleomagnetic and sedimentological studies carried out on four cores, about 11 m long, from the bottom sediments of Escondido Lake (southwestern Argentina) are described. This lake is geologically related to the set of lakes of glacial origin located in western Patagonia.Using several magnetic studies, the predominance of pseudo-single/single domain magnetite as main carrier, as well as an increase of concentration and grain size with depth was determined.The pairs of magnetic susceptibility and intensity of remanent magnetization profiles show a good correlation with the sedimentological macroscopic descriptions, and also with those of shorter cores previously studied. The stability of the natural remanent magnetization was investigated by alternating-field demagnetization.Within-lake correlation was based on lithology, magnetic susceptibility and intensity of remanent magnetization, because the patterns of these parameters showed similarity for every core.Radiocarbon dating suggests higher deposition rates during the early stage of the lacustrine basin (Late Pleistocene). The transfer function depth–age also allowed constructing a secular variation record from south Argentina for the past 19,000 years.The declination and inclination logs show some anomalous directions at about 18,000 calibrated years.
In order to obtain a uniform magnitude catalogue, surface-wave magnitudes Ms and broad-band body-wave magnitudes mB have been determined for large shallow earthquakes from 1904 to 1980. In making the catalogue homogeneous, the author consistently adheres to the original definitions of Ms and mB given by Gutenberg (1945) and Gutenberg and Richter (1956). The determinations of Ms and mB are all based on the amplitude and period data listed in Gutenberg and Richter's unpublished notes, bulletins from stations worldwide, and other basic information. mB is measured on broad-band instruments in periods of ∼8 s. Consistency of the magnitude determinations from these different sources is carefully checked in detail. More than 900 shallow shocks of magnitude 7 and over are catalogued. The meaning of the magnitude scales in various catalogues is examined in terms of Ms and mB. Most of the magnitudes listed by Gutenberg and Richter (1954) in their “Seismicity of the Earth” are basically Ms for large shocks shallower than 40 km, but are basically mB for large shocks at depths of 40–60 km. The surface-wave magnitudes given by “Earthquake Data Reports” are higher than Ms by 0.2 unit whether the combined horizontal amplitude or the vertical amplitude is used. mB and the currently used 1 s body-wave magnitude are measured at different periods and should not be directly compared.
Old seismograms of the Tungus event of June 30, 1908, are analysed and compared with contemporary records of air explosions from Novaya Zemlya and Lop-Nor.Observed arrival times and absolute amplitudes of Rayleigh waves, infrasonic-coupled ground-motion and SH crustal shear waves at Jena, Tiflis, Tashkent and Irkutsk, support a source model which consists of a combined action of an atmospheric-explosion equivalent to a vertical point impulse of magnitude 7 · 1018 dyn sec and a ballistic-wave equivalent to a horizontal point impulse of magnitude 1.4 · 1018 dyn sec striking S65°E toward the northwest.All the accumulated knowledge in earthquake and explosion seismology since the beginning of the century is harnessed to the effort of interpreting the observed seismograms. The agreement between the observations in the epicentral area and the ensuing signals in the far seismic and acoustic field support the hypothesis that the Siberian UFO explosion had the effects of an Extraterrestrial Nuclear Missile of yield 12.5 ± 2.5 Mt.
Examination of annual mean values of the geomagnetic elements from magnetic observatories worldwide has shown that a sudden change in secular acceleration, which has been termed a jerk, took place at about 1970. Malin et al. used the set of secular variation models compiled by Hodder to provide a global description of the jerk, and attempted to quantify it by computing the mean square value of the jerk field over the Earth's surface. To determine the ability of the method of Malin et al. to distinguish the 1970 jerk from noise their analysis has been repeated at 2-y intervals from 1931.5 to 1971.5, a period during which no other similar jerk is thought to have occurred. It is shown that the mean square value of the jerk field is not a reliable indicator of the existence of a jerk when taken alone as noise contributes to it additively giving rise to “apparent” jerks. This point is illustrated by examples of contour maps of the jerk field for 1949.5 which show how large mean square values relate to spurious foci in regions where there are no magnetic field observations. These maps contrast with those for the 1970 jerk which relate well to the observations and it is shown that the 1970 jerk is well-resolved from noise and is unique within the time-span considered.