Optical Counterparts of Undetermined Type <Emphasis Type="Italic">γ</Emphasis>-Ray Active Galactic Nuclei with Blazar-Like Spectral Energy Distributions

During its first four years of scientific observations, the Fermi Large Area Telescope (Fermi-LAT) detected 3033 γ-ray sources above a 4 σ significance level. Although most of the extra-galactic sources are active galactic nuclei (AGN) of the blazar class, other families of AGNs are observed too, while a still high fraction of detections (～30%) remains with uncertain association or classification. According to the currently accepted interpretation, the AGN γ-ray emission arises from inverse Compton (IC) scattering of low energy photons by relativistic particles confined in a jet, which, in the case of blazars, is oriented very close to our line-of-sight. Taking advantage of data from radio and X-ray wavelengths, which we expect to be produced together with γ-rays, providing a much better source localization potential, we focused our attention on a sample of γ-ray Blazar Candidates of Undetermined type (BCUs), starting a campaign of optical spectroscopic observations. The main aims of our investigation include a census of the AGN families that contribute to γ-ray emission and a study of their redshift distribution, with the subsequent implications on the intrinsic source power. We furthermore analyze which γ-ray properties can better constrain the nature of the source, thus helping in the study of objects not yet associated with a reliable low frequency counterpart. Here we report on the instruments and techniques used to identify the optical counterparts of γ-ray sources, we give an overview on the status of our work, and we discuss the implications of a large scale study of γ-ray emitting AGNs.     

Influence of substrate tectonic heritage on the evolution of composite volcanoes: Predicting sites of flank eruption, lateral collapse, and erosion

This paper aims to aid understanding of the complicated interplay between construction and destruction of volcanoes, with an emphasis on the role of substrate tectonic heritage in controlling magma conduit geometry, lateral collapse, landslides, and preferential erosion pathways. The influence of basement structure on the development of six composite volcanoes located in different geodynamic/geological environments is described: Stromboli (Italy), in an island arc extensional tectonic setting, Ollagüe (Bolivia–Chile) in a cordilleran extensional setting, Kizimen (Russia) in a transtensional setting, Pinatubo (Philippines) in a transcurrent setting, Planchon (Chile) in a compressional cordilleran setting, and Mt. Etna (Italy) in a complex tectonic boundary setting. Analogue and numerical modelling results are used to enhance understanding of processes exemplified by these volcanic centres. We provide a comprehensive overview of this topic by considering a great deal of relevant, recently published studies and combine these with the presentation of new results, in order to contribute to the discussion on substrate tectonics and its control on volcano evolution. The results show that magma conduits in volcanic rift zones can be geometrically controlled by the regional tectonic stress field. Rift zones produce a lateral magma push that controls the direction of lateral collapse and can also trigger collapse. Once lateral collapse occurs, the resulting debuttressing produces a reorganization of the shallow-level magma migration pathways towards the collapse depression. Subsequent landslides and erosion tend to localize along rift zones. If a zone of weakness underlies a volcano, long-term creep can occur, deforming a large sector of the cone. This deformation can trigger landslides that propagate along the destabilized flank axis. In the absence of a rift zone, normal and transcurrent faults propagating from the substrate through the volcano can induce flank instability in directions respectively perpendicular and oblique to fault strike. This destabilization can evolve to lateral collapse with triggering mechanisms such as seismic activity or magmatic intrusion.     

Syn-orogenic extension in the Peloritani Alpine Thrust Belt (NE Sicily,Italy): Evidence from the Alì Unit

Structural and petrological analyses on the Alì Unit, in the Peloritani Thrust Belt, document the first evidence for Alpine exhumation associated with syn-orogenic extension in this part of the Calabria-Peloritani Arc. The Alì Unit displays ductile structures occurred during three Alpine deformation phases (D_a1, D_a2, D_a3). D_a1 and D_a3 developed in a contractional context, whereas D_a2 was generated in an extensional regime. The present-day tectonic contact between the Alì Unit and the overlying Mandanici Unit is interpreted as a low-angle extensional detachment responsible for the metamorphic break between the two units. Structural overprinting relationships indicate that the development of D_a2 structures and related tectonic exhumation occurred during syn-convergence extension, and were followed by further nappe stacking in the Peloritani Belt. To cite this article: R. Somma et al., C. R. Geoscience 337 (2005).     

Seismic demands on nonstructural components anchored to concrete accounting for structure-fastener-nonstructural interaction (SFNI)

This paper investigates the influence of the hysteretic shear behavior of postinstalled anchors in concrete on the seismic response of nonstructural components (NSCs) using numerical methods. The purpose of the investigation is to evaluate current design requirements for NSC and their anchorage. Current design guidelines and simplified methods, such as floor response spectra (FRS), typically approach the dynamics of the structure-fastener-NSC (SFN) system using simplified empirical formulae. These formulations decouple the structure from the NSC and neglect the behavior of the anchor connection, with the assumption of full rigidity. There is a lack of knowledge on the complex interaction between a host structure, the fastening system, and the NSC, herein referred to as structure-fastener-nonstructural interaction (SFNI). More specifically, it is important to investigate whether and how the actual hysteresis shear behavior that takes place in the anchorage could alter the seismic response of the SFN system and its components. Herein, the results of extensive nonlinear dynamic analyses (NLDA) with different models for the anchorage force-displacement relationship are presented and compared with those obtained with FRS procedures and current code provisions. The anchor models include (a) linear-elastic, (b) bilinear, and (c) a recently developed hysteresis rule. The results of the NLDA showed that the first two approaches are not able to reflect the behavior of an anchor loaded in dynamic shear. Moreover, when using the more refined hysteresis model, it appears that current code provisions might underestimate the component and anchor shear amplification factors for rigid NSC fixed to the host structure through anchors.     

Contribution of tsunami data analysis to constrain the seismic source: the case of the 1693 eastern Sicily earthquake

The January 11, 1693 eastern Sicilyearthquake is comparable only with the December 28,1908 Messina Straits event in the Italian seismichistory as regards magnitude (M _L 7), level ofdestruction and number of victims. The shock generateda strong tsunami, which hit the entire eastern coastof Sicily and particularly the town of Augusta. Theproblem of which fault was responsible for theearthquake is still open. Several hypotheses have beenformulated in the literature on the basis of differentkinds of geological, macroseismic and tectonic data,but a general agreement has not been reached yet. Animportant contribution to the discussion may come fromthe analysis of the tsunami data. In two previouspapers (Piatanesi et al., 1996; Piatanesi and Tinti,1998), the hydro-dynamical study of the tsunami basedon finite-element (FE) numerical simulations wascarried out taking into consideration mostlytheoretical faults, i.e. faults selected for theirsignificance in tsunami generation irrespective of thecorresponding geological evidence. This paper has tobe considered the continuation of the mentioned works.We have studied new sources proposed in theliterature, consistent with macroseismic data and/orgeological observations, and simulated thecorresponding tsunamis. We also built a new FE gridintroducing significant improvements in the coastlinerepresentation, and developed and applied a newalgorithm in order to account for the effect of thesea-bottom topography on the tsunami initialcondition. Some of the examined faults are located inthe Scordia-Lentini (SL) graben region and interceptthe coastline, others are placed in correspondencewith the Hyblaean-Malta (HM) escarpment and runparallel or sub-parallel to the coast. The conclusionof our work is that none of these faults respectsfully the available observations on tsunami, and thatthe faults exhibiting the best fit are those placed inthe SL region. It is worth stressing that our resultsare important for the assessment of seismic andtsunami hazard/risk in eastern Sicily, keeping also inmind that the 1693 earthquake has been selected as thereference event for an earthquake-scenario study inCatania and in south-eastern Sicily, called `CataniaProject', funded by CNR/GNDT (Consiglio Nazionaledelle Ricerche/Gruppo Nazionale Difesa dai Terremoti).     

Influence of Site and Source Dependent Ground Motion Scenarios on the Seismic Safety of Long-span Bridges in Cologne, Germany

An overlap in frequency between the 1D resonance frequency of sediments and the vibrational frequencies of long-span bridges might lead to a strongly increased structural response of the latter. Interference of surface waves caused by reflections at dipping interfaces may introduce additional unfavourable amplifications. Therefore, the vulnerability of two bridges crossing the Rhine River in Cologne, Germany, was assessed using ground motion scenarios computed for four profiles crossing the Lower Rhine Embayment. Due to their vibrational frequencies being in the vicinity of resonant peaks in the response spectra, the Severinsbridge showed critical loading and the bridge Cologne-Deutz even exhibited grave failure according to the dynamic FE-simulations.     

Assessment of the stability of H/V spectral ratios from ambient noise and comparison with earthquake data in the Cologne area (Germany)

Situated in an active tectonic region the highly industrialised Cologne area (Germany) suffers from moderate-sized earthquakes. Our contribution to the mitigation of earthquake risk includes a microzonation study using ambient seismic noise and earthquake recordings from two field campaigns. An analysis of these data using the horizontal-to-vertical (H/V) as well as the classical spectral ratio (CSR) technique led to the following observations and conclusions: (1) The spatial variation in the thickness of the sedimentary cover is reliably retrieved using the fundamental resonance frequency estimated from the peak in the (H/V) ratio of ambient noise. (2) This fundamental resonance frequency is stable in time, but the amplitude of the peak (the amplification factor) is not. (3) The relative amplification variation of the H/V ratio in the area should therefore be checked systematically with repeated measurements before drawing conclusions about its significance. (4) The analysis of the H/V ratio of ambient noise provides the lower frequency bound, above which amplification may occur. (5) The shapes of the spectral ratios obtained by the different methods are generally in good agreement. However, the analysis of earthquake data shows that significant amplification of ground motion may also occur at frequencies higher than the fundamental one even when thick sediments are present.     

Southern Ocean deglacial record supports global Younger Dryas

In Northern Hemisphere deglaciation records, the transition from the last glacial to the Holocene indicates a rapid return to near-glacial conditions during the Younger Dryas, whereas their Southern Hemisphere ice core counterparts record two separate cooling events: the Antarctic Cold Reversal and the Oceanic Cold Reversal. Spatial distribution and relative timing of these events in both hemispheres are central for our understanding of causes and mechanisms of abrupt climate change. To date, no marine record from the southern mid-latitudes conclusively demonstrates that the Younger Dryas was a significant event in the Southern Ocean. Here, we present high-resolution oxygen isotope and iron content records of a radiocarbon-dated sedimentary sequence from the Great Australian Bight, which constrains oceanic and atmospheric changes during the last deglaciation. Oxygen isotopes from planktonic foraminifera indicate two rapid cold reversals (between 13.1 and 11.1 kyr BP) separated by a brief warming. The sedimentary iron content, interpreted as a proxy for wind strength, indicates a simultaneous change in atmospheric circulation pattern. Both records demonstrate the existence of cooling events in the Southern Hemisphere, which are synchronous with the Northern Hemisphere Younger Dryas cold reversal (between 12.9 and 11.5 kyr BP). Such evidence for the spatial distribution and timing of abrupt climatic fluctuations is essential data for groundtruthing results derived from global climate models.