Microearthquakes in the TDP swarm, Turkey: clustering in space and time

Summary. The third occupation (experiment TDP3) of recording sites above a persistent swarm of microearthquakes near the North Anatolian Fault, with a larger seismic network and over a longer period of time, confirms and refines previous observations with greater resolution. The greater resolution in earthquake locations has revealed marked clustering in time and space. Many, perhaps most, of the earthquakes belong to clusters, where successive earthquakes originate in a very small volume and have similar fault mechanisms. Such studies allow the progression of seismic activity of small earthquakes to be followed in some detail, and may reveal features which are hidden in larger and more complex earthquake sequences.     

Simultaneous optical and incoherent scatter observations of two low-latitude auroras

During the last sunspot maximum, major auroras occurred over Boston, MA (L = 3.1) on 23/24 March 1969 and 8/9 March 1970, during which optical and incoherent scatter radar measurements were made simultaneously from the Blue Hill Observatory and Millstone Field Station, respectively. The paper presents the results of these measurements and attempts a self-consistent interpretation of them. It is found that a major increase (5–10 times) in the abundance of molecular species (O₂ and N₂) at F-region heights must have occurred; this was not accompanied by any appreciable increase in the major neutral species, atomic oxygen. From the radar data, the energy input to the F-region can be separated into direct heating by secondary electrons, downward heat flow from the magnetosphere and a low-energy non-ionizing electron flux. The total observed intensity of the predominant optical emission from atomic oxygen at 6300 Å can then be predicted satisfactorily by summing the contributions to this emission from impact excitation by ambient electrons, from the low-energy precipitating electrons and from dissociative recombination.     

Effects of random phase changes on the formation of synthetic aperture radar imagery

The effects of Gaussian random and linear phase change on the response of the matched azimuth processor of a synthetic aperture imaging radar is analyzed numerically.     

The relationship between sedimentary diatom assemblages and lakewater pH in 35 Quebec lakes,Canada

Surface sediment diatoms from 35 soft-water lakes in southern Quebec were studied to examine the relationship with lakewater pH. The lakes ranged in pH from 5.25 to 7.66. The species composition and the relative abundance of diatoms in the study lakes was found to be closely related to pH and/or factors closely associated with pH. Predictive models were developed to infer lakewater pH using simple linear regression equations of index alpha, index B, and multiple regressions using pH preference categories. Among the predictive models examined here, the multiple regression technique provided the highest correlation coefficient (r²=0.88) and the lowest standard error (±0.26 of a pH unit) in computing diatom-inferred pH. This model appears to be the most appropriate to reconstruct lake pH histories in Quebec region.     

On the mass and distance of the Quasi-Stellar Object 3C 273

For the QSO 3C 273 we derive, on the basis of two different theoretical models, expressions for a lower limit to the mass of the QSO, as a function of its distance. We conclude that an appreciable gravitational redshift component is consistent with the observational data only if the QSO mass is at least Galactic in magnitude. The setting of an independent upper limit to the QSO massM10¹⁰ M could indicate that the QSO redshift is predominantly cosmological in nature.     

Growth, linkage, and termination processes of a 10-km-long strike-slip fault in jointed granite: the Gemini fault zone, Sierra Nevada, California

Field-based structural analysis of an exhumed, 10-km-long strike-slip fault zone elucidates processes of growth, linkage, and termination along moderately sized strike-slip fault zones in granitic rocks. The Gemini fault zone is a 9.3-km-long, left-lateral fault system that was active at depths of 8–11 km within the transpressive Late-Cretaceous Sierran magmatic arc. The fault zone cuts four granitic plutons and is composed of three steeply dipping northeast- and southwest-striking noncoplanar segments that nucleated and grew along preexisting cooling joints. The fault core is bounded by subparallel fault planes that separate highly fractured epidote-, chlorite-, and quartz-breccias from undeformed protolith. The slip profile along the Gemini fault zone shows that the fault zone consists of three 2–3-km-long segments separated by two ‘zones’ of local slip minima. Slip is highest (131 m) on the western third of the fault zone and tapers to zero at the eastern termination. Slip vectors plunge shallowly west-southwest and show significant variability along strike and across segment boundaries. Four types of microstructures reflect compositional changes in protolith along strike and show that deformation was concentrated on narrow slip surfaces at, or below, greenschist facies conditions. Taken together, we interpret the fault zone to be a segmented, linked fault zone in which geometrical complexities of the faults and compositional variations of protolith and fault rock resulted in nonuniform slip orientations, complex fault-segment interactions, and asymmetric slip-distance profiles.     

Thermodynamics and crystal chemistry of the hematite–corundum solid solution and the FeAlO3 phase

High-temperature oxide-melt calorimetry and Rietveld refinement of powder X-ray diffraction patterns were used to investigate the energetics and structure of the hematite–corundum solid solution and ternary phase FeAlO₃ (with FeGaO₃ structure). The mixing enthalpies in the solid solution can be described by a polynomial ΔH_mix=WX _hem(1?X _hem) with W=116 ± 10 kJ mol^?1. The excess mixing enthalpies are too positive to reproduce the experimental phase diagram, and excess entropies in the solid solution should be considered. The hematite–corundum solvus can be approximately reproduced by a symmetric, regular-like solution model with ΔG _excess=(W _H ?TW _S )X _hem X _cor, where W _H= 116 ± 10 kJ mol^?1 and W _S =32 ± 4 J mol^?1 K^?1. In this model, short-range order (SRO) of Fe/Al is neglected because SRO probably becomes important only at intermediate compositions close to Fe:Al=1:1 but these compositions cannot be synthesized. The volume of mixing is positive for Al-hematite but almost ideal for Fe-corundum. Moreover, the degree of deviation from Vegard's law for Al-hematite depends on the history of the samples. Introduction of Al into the hematite structure causes varying distortion of the hexagonal network of oxygen ions while the position of the metal ions remains intact. Distortion of the hexagonal network of oxygen ions attains a minimum at the composition (Fe_0.95Al_0.05)₂O₃. The enthalpy of formation of FeAlO₃ from oxides at 298 K is 27.9 ± 1.8 kJ mol^?1. Its estimated standard entropy (including configurational entropy due to disorder of Fe/Al) is 98.9 J mol^?1 K^?1, giving the standard free energy of formation at 298 K from oxides and elements as +19.1 ± 1.8 and ?1144.2 ± 2.0 kJ mol^?1, respectively. The heat capacity of FeAlO₃ is approximated as C _p (T in K)= 175.8 ? 0.002472T ? (1.958 × 10⁶)/T ²? 917.3/T ^0.5+(7.546 × 10^?6) T ² between 298 and 1550 K, based on differential scanning calorimetric measurements. No ferrous iron was detected in FeAlO₃ by Mössbauer spectroscopy. The ternary phase is entropy stabilized and is predicted to be stable above about 1730 ± 70 K, in good agreement with the experiment. Static lattice calculations show that the LiNbO₃-, FeGaO₃-, FeTiO₃-, and disordered corundum-like FeAlO₃ structures are less stable (in the order in which they are listed) than a mechanical mixture of corundum and hematite. At high temperatures, the FeGaO₃-like structure is favored by its entropy, and its stability field appears on the phase diagram.     

Rodinia connections between Australia and Laurentia: no SWEAT,no AUSWUS?

Although geological comparisons between Australia and North America have provided a basis for various Neoproterozoic Rodinia reconstructions, quantitative support from precisely dated palaeomagnetic poles has so far been lacking. We report U–Pb ages and palaeomagnetic results for two suites of mafic sills within the intracratonic Bangemall Basin of Western Australia, one of which is dated at 1070 ± 6 Ma and carries a high‐stability palaeomagnetic remanence. Comparison of the Bangemall palaeopole with Laurentian data suggests that previous reconstructions of eastern Australia against either western Canada (SWEAT) or the western United States (AUSWUS) are not viable at 1070 Ma. This implies that the Pacific Ocean did not form by separation of Australia–Antarctica from Laurentia, and that up to 10 000 km of late Neoproterozoic passive margins need to be matched with other continental blocks within any proposed Rodinia supercontinent. Our results permit a reconstruction (AUSMEX) that closely aligns late Mesoproterozoic orogenic belts in north‐east Australia and southernmost Laurentia.     

Loess in the Czech Republic: Magnetic properties and paleoclimate

Summary We have studied 454 oriented samples from seven loess outcrops in the Czech Republic for comparison and correlation of the magnetic properties with those of the loess profiles in China and Central Asia. Three sections at Sedlec (Prague), Zeměchy and Dolní Věstonice cover the time span of the last glacial-interglacial cycle. Loess from the middle Pleistocene period including pedocomplexes V and VI was studied at Sedlešovice (Znojmo) and Karlštejn. Loess of uncertain — possibly lower to middle Pleistocene age — was sampled at Sedlec near Mikulov. At Červeny Kopec (Red Hill, Brno) we investigated the oldest loess near the Matuyama/Brunhes (M/B) boundary. The characteristic remanence (ChRM) determined after magnetic clearning is of normal polarity throughout all sections except at Červeny Kopec. There, two polarity changes from reversed to normal were found within two sections initially thought to represent two successive stratigraphic intervals. However, the marked similarities between lithological, susceptibility, declination and inclination profiles lead us to suggest that the two polarity changes are in fact one and the same. The two sections are therefore laterally equivalent and both have recorded the M/B field reversal. The susceptibility variations were used to mutually correlate the Czech sections and to construct a composite profile across the present day loess outcrops in the Czech Republic. They also serve as a paleoclimate proxy which can be compared and correlated with the paleoclimatic records observed in the Chinese and Central Asian loess. The origin of the low field susceptibility variations was studied by measuring the frequency dependence of susceptibility which revealed that the enhancement within the pedocomplexes is controlled by the amount of fine-grained ferromagnetic minerals present.