Coseismic Three-Dimensional Displacements Determined Using SAR Data: Theory and an Application Test

We present a methodology to construct three-dimensional deformation maps using different parameters that can be extracted using SAR data. We apply the methodology using ENVISAT SAR data before and after the December 26^th, 2003 M_w 6.6 Bam earthquake in Iran to determine spatial quasi-continuous three-dimensional coseismic deformation maps. Two near vertical deformation offset components are computed using Envisat ASAR differential interferometry (DInSAR), while two horizontal deformation offset components are measured using sub-pixel correlation techniques applied to ASAR amplitude images. Using the presented methodology, we combine four unevenly precise independent projections of surface coseismic deformation to obtain the full three-dimensional coseismic deformation field caused by an earthquake. The full 3-D coseismic displacement vector is modeled using elastic deformation models constraining details of the fault geometry and slip distribution at depth. Results from the inversion are interpreted in the framework of the Iranian present-day tectonism. Full exploitation of dense 3-D coseismic surface deformation using SAR data, even for moderate earthquakes, should facilitate distinguishing between different interpretations of the mechanical properties of seismically active areas and within the inherent ambiguity of the geophysical inversion solutions.     

Computing Lippmann Diagrams from Direct Calculation of Mixing Properties of Solid Solutions: Application to the Barite-Celestite System

The Lippmann diagram for the system(Ba, Sr)SO₄-H₂O was computed at 25 °Cby determining the solid-phase activity coefficientsfrom first principles calculations. Directcalculations of the mixing properties of thebarite-celestite series indicate this solid solutionbehaves as non-ideal and non-regular. At 25 °C,the enthalpy of mixing shows a minimum around 50 mole% SrSO₄ due to an ordering tendency. Thefree energy of mixing shows two minima that delimit awide and symmetric miscibility gap (from 2.1 to 97.9 mole% SrSO₄) at this temperature. The excessfree energy of mixing requires a Guggenheim expansionseries of 5 terms to be described, where the termswith odd exponents are null as a consequence of thesymmetric distribution of the mixing properties withcomposition. The Lippmann diagram shows a peritecticpoint that corresponds to the composition of an aqueoussolution which is simultaneously at equilibrium withthe two extremes of the miscibility gap. The largedifference between the solubility products of theendmembers involves a strong preferential partitioningof the less soluble endmember towards the solid phase,which explains the extremely Ba-poor composition ofthe aqueous solution (aqueous activity fraction forBa²⁺ = 0.000446 ) at the peritectic point.     

Stable Atmospheric Boundary-Layer Experiment in Spain (SABLES 98): A Report

This paper describes the Stable AtmosphericBoundary Layer Experiment in Spain (SABLES 98),which took place over the northern Spanish plateaucomprising relatively flat grassland,in September 1998. The main objectives of the campaign were to study the properties of themid-latitude stable boundary layer (SBL).Instrumentation deployed on two meteorologicalmasts (of heights 10 m and 100 m)included five sonic anemometers, 15 thermocouples,five cup anemometers and three propeller anemometers,humidity sensors and radiometers.A Sensitron mini-sodar and a tetheredballoon were also operated continuously. Atriangular array of cup anemometers wasinstalled to allow the detection ofwave events. Two nocturnal periods analysedon 14–15 and 20–21 September are used toillustrate the wide-ranging characteristics of the SBL.     

Long-Period Comets and the Oort Cloud

Long-period (LP) comets, Halley-type (HT) comets, and even some comets of the Jupiter family, probably come from the Oort cloud, a huge reservoir of icy bodies that surrounds the solar system. Therefore, these comets become important probes to learn about the distant Oort cloud population. We review the fundamental dynamical properties of LP comets, and what is our current understanding of the dynamical mechanisms that bring these bodies from the distant Oort cloud region to the inner planetary region. Most new comets have original reciprocal semimajor axes in the range2 × 10^-5 < 1/a_orig < 5 × 10^-5AU^-1. Yet, this cannot be taken to represent the actual space distribution of Oort cloud comets, but only the region in the energy space in which external perturbers have the greatest efficiency in bringing comets to the inner planetary region. The flux of Oort cloud comets in the outer planetary region is found to be at least several tens times greater than the flux in the inner planetary region. The sharp decrease closer to the Sun is due to the powerful gravitational fields of Jupiter and Saturn that prevent most Oort cloud comets from reaching the Earth’s neighborhood (they act as a dynamical barrier). A small fraction of ～10^-2 Oort cloud comets become Halley type (orbital periods P < 200 yr), and some of them can reach short-period orbits with P < 20 yr. We analyze whether we can distinguish the latter, very ‘old” LP comets, from comets of the Jupier family coming from the Edgeworth-Kuiper belt.     

U–Pb evidence for a polyorogenic evolution of the HP–HT units of the NW Iberian Massif

. A isotope dilution thermal ionisation mass spectrometry U-Pb geochronological study was carried out on the high-pressure and high-temperature units (HP-HT units) overlying the oceanic suture in the Allochthonous Complexes of the NW Iberian Variscan Belt. The rocks investigated are seven granulite- to eclogite-facies paragneisses and one leucosome within mafic high-pressure granulites in the Ordenes and Cabo Ortegal Complexes of NW Spain. U-Pb dating of zircon, monazite, titanite and rutile reveal the presence of a pervasive Early Ordovician metamorphic event at ca. 500-480 Ma and a later Early Devonian event at ca. 400-380 Ma. The U-Pb ages, in conjunction with petrological and structural data, indicate that the high-pressure event recorded by these rocks is Early Ordovician in age. Monazite ages in the paragneisses suggest that peak metamorphic conditions were reached at ca. 500-485 Ma. Subsequently, the rock ensemble underwent exhumation accompanied by partial melting and zircon growth at ca. 485-470 Ma. Melting of mafic granulites was coeval with this latter episode as indicated by zircon crystallisation age in the leucosomes dated at ca. 486 Ma. Based on these data and on the general features of magmatism and metamorphic evolution, it is proposed that this process took place at a convergent plate boundary within a peri-Gondwanan oceanic domain. Monazite, titanite and rutile data in some of the samples studied show evidence of a second metamorphic episode that took place between ca. 400 and 380 Ma (with a peak at ca. 390-385 Ma). This Early Devonian event, at variance with the previous one, was not pervasive, but, rather, was localised in areas of intense Variscan tectonothermal reworking. It is claimed that this later metamorphic event was recorded by the U-Pb system in areas where monazite and titanite growth was enhanced by fluid circulation in highly strained rocks (Variscan shear zones). According to previous structural studies and Ar-Ar dating of fabrics, this Early Devonian episode took place as the HP-HT units were deformed and thrusted upon the ophiolitic units in the early stages of the Variscan collision.     

Some Insights into Topographic,Elastic and Self-gravitation Interaction in Modelling Ground Deformation and Gravity Changes in Active Volcanic Areas

Surface displacements and gravity changes due to volcanic sources are influenced by medium properties. We investigate topographic, elastic and self-gravitation interaction in order to outline the major factors that are significant in data modelling. While elastic-gravitational models can provide a suitable approximation to problems of volcanic loading in areas where topographic relief is negligible, for prominent volcanoes the rough topography could affect deformation and gravity changes to a greater extent than self-gravitation. This fact requires the selection, depending on local relief, of a suitable model for use in the interpretation of surface precursors of volcanic activity. We use the three-dimensional Indirect Boundary Element Method to examine the effects of topography on deformation and gravity changes in models of magma chamber inflation/deflation. Topography has a significant effect on predicted surface deformation and gravity changes. Both the magnitude and pattern of the geodetic signals are significantly different compared to half-space solutions. Thus, failure to account for topographic effects in areas of prominent relief can bias the estimate of volcanic source parameters, since the magnitude and pattern of deformation and gravity changes depend on such effects.     

Surface Synoptic Circulation and Daily Precipitation in Catalonia

Summary Catalonian daily precipitation patterns were obtained by means of S-mode Principal Component Analysis (PCA) applied to subsets built up according to synoptic surface airflow. Daily data from November to April 1990–1994 from 45 raingauge sites in Catalonia without missing data were used. Eight circulation classes for days in this period were considered. A Precipitation Activity Index (PAI) was calculated for each station and class, leading to the determination of the highest activity regions for each subset. In addition, we determined which circulation type produced most precipitation at each station of the network. Separate PCAs were performed for each of the eight classes considered and the first four Varimax rotated solutions were interpreted for each circulation type. The physical meaning of the rest of the factors were omitted as they were associated with local behaviour. Finally, some concluding remarks on the cyclogenic properties of the Western Mediterranean Basin and the diversity of the extracted patterns are presented and supported by the authors experience in forecasting in Catalonia. Received February, 11, 1997 Revised May 23, 1997     

Dynamical capture and physical decay of short-period comets

The brightness evolution of short-period comets is discussed in connection with their physical lifetimes. It is shown that changes in the fraction of the free-subliming area of the nuclear surface may be more important than mass decrease in determining brightness variations. The decrease in the activity of short-period comets caused by the buildup of a dust mantle may be interrupted—and partially reversed—by dust blowoffs that leave exposed areas of fresh ices. Short-period comets may thus be subject to random brightness fluctuations that make quite uncertain any derivation of their physical lifetime based on comparisons of their absolute brightness at different apparitions. As an alternate procedure, the numerical integration of the whole sample of short-period comet orbits carried out by A. Carusi, L.Kresák, E. Perozzi and G. B. Valsecchi (1984, Long-Term Evolution of Short-Period Comets. Istituto Astrofisica Spaziale Internal Report 12, Rome) is used to draw conclusions about the transfer rate of their perihelia from Jupiter's region to the region of the terrestrial planets (heliocentric distances<1.5 AU). It is found that about one short-period comet per century reaches the region of the terrestrial planets. From this result and under the assumption of a steady-state comet population, an average lifetime of the order of 6 × 10³ years (～10³ revolutions) is derived for a typical kilometer-sized short-period comet of perihelion distance q ～ 1 AU. Such a rather long comet lifetime, as compared to some previous derivations, is consistent with the survival of some periodic comets on small-q orbits of long dynamical time scales.     

Variation of the geomagnetic field in Costa Rica during the total solar eclipse of July 11, 1991

Ground measurements of the geomagnetic field, during the total solar eclipse of July 11, 1991, were carried out at three sites in Costa Rica located within the eclipse's path of totality. Near totality, there was a significant change in the total intensity,F. Its normal decrease in the afternoon hours was interrupted during a period of about 45 min, in whichF varied very little. This was due mainly to a small variation in horizontal intensity,H, during such a period. The departure ofF from normal lasted about 100 min; its maximum departure was near 10 nT and occurred about 38 min after totality, The declination,D, also experienced a small change (about 2.3 min) during the total period of the eclipse. The vertical intensity,Z, and the inclination,I, did not show significant changes.