Cross-twinning in a natural spinel from Sri Lanka

A modified cross-twinning growth mechanism is put forward to explain the anomalous morphology of a spinel multiple-twin from Sri Lanka, flattened crosswise the twin planes. Cross-twinning in spinel was found also in other specimens from Pegu (Myanmar), and the results were published in a previous paper. This particular type of twinning is derived from the combination of cyclic twinning with lamellar twinning, so that these samples may be thought of as partial fivelings (cubic cyclic {111} twins with five components sharing a common <110> pseudo-fivefold axis). In the present paper, the sample from Sri Lanka has been suitably cut with the aim of focusing the study on the cross-twinning region. The transformation matrices that link the orientation states of each couple of twin components have been determined by means of White Beam Synchrotron Radiation Topography. They showed that the specimen is made up of four twin components (A, B, C and D), with three twin planes: and They also showed that the cross-twinned individuals (B and D) actually are not twinned to each other, and that a simple crystallographic relationship holds between them. X-ray diffraction topography by conventional source allowed to image the crossing-region and to determine that the cross-twinned individuals are in contact through a semi-coherent boundary, with twinning dislocations contributing to relieve the coherency strains. Electron probe microanalyses with wave dispersive spectroscopy showed that the chemical composition is almost homogeneous, at least within the spatial resolution limit of this technique. The similar growth features observed in the spinel sample from Sri Lanka and in those from Myanmar are interpreted as growth marks, indicators of a similar origin: in both cases they are found in impure dolomitic marbles. In particular, the specimen from Sri Lanka results from the interaction of thermal and metasomatic effects due to contact metamorphism. An unusual stepped morphology of the face close to the twin boundary, possibly due to corrosion and re-growth processes acted preferentially at a re-entrant corner by metasomatic fluids, is interpreted as indicator of a metasomatic event that succeeded to the crystal growth, the latter occurred by thermal effect.

Seismic damage accumulation in multiple mainshock–aftershock sequences

Earthquakes are generally clustered, both in time and space. Conventionally, each cluster is made of foreshocks, the mainshock, and aftershocks. Seismic damage can possibly accumulate because of the effects of multiple earthquakes in one cluster and/or because the structure is unrepaired between different clusters. Typically, the performance-based earthquake engineering (PBEE) framework neglects seismic damage accumulation. This is because (i) probabilistic seismic hazard analysis (PSHA) only refers to mainshocks and (ii) classical fragility curves represent the failure probability in one event, of given intensity, only. However, for life cycle assessment, it can be necessary to account for the build-up of seismic losses because of damage in multiple events. It has been already demonstrated that a Markovian model (i.e., a Markov chain), accounting for damage accumulation in multiple mainshocks, can be calibrated by maintaining PSHA from the classical PBEE framework and replacing structural fragility with a set of state-dependent fragility curves. In fact, the Markov chain also works when damage accumulates in multiple aftershocks from a single mainshock of known magnitude and location, if aftershock PSHA replaces classical PSHA. Herein, this model is extended further, developing a Markovian model that accounts, at the same time, for damage accumulation: (i) within any mainshock–aftershock seismic sequence and (ii) among multiple sequences. The model is illustrated through applications to a series of six-story reinforced concrete moment-resisting frame buildings designed for three sites with different seismic hazard levels in Italy. The time-variant reliability assessment results are compared with the classical PBEE approach and the accumulation model that only considers mainshocks, so as to address the relevance of aftershocks for life cycle assessment.     

Neglected basement ductile deformation in balanced-section restoration: An example from the Central Southern Alps (Northern Italy)

In the construction of balanced sections through thick-skinned belts, basement bodies are frequently assumed to be rigid and internally undeformed (with the exception of regional scale fault-bend-folding) and only backstripping of thrust faults is performed during their retrodeformation, possibly leading to underestimations of the regional shortening. This simplifying assumption is generally made because reliable information on internal strain in basement is lacking and the sedimentary cover of the internal parts of thrust belts has been removed by erosion.In a basement exposure in the eastern Orobic Alps, Alpine-age ductile structures (mainly chevron to sub-isoclinal folds rarely associated with cleavage formation) were recognized on the basis of overprinting criteria, folding style and fabric orientation and their areal distribution was determined by foliation trace mapping. Although unevenly distributed in different lithologies, ductile deformation is considerable (average shortening is 47%). Neglecting such internal strain during construction of balanced sections leads, for this area, to an underestimation of at least 10%, but could be more than 40% if the observed basement shortening is extrapolated to other basement-involved thrust bodies across the entire width of the deformed belt.It is concluded that detailed structural studies of the basement, for example those using a foliation trace mapping technique, are necessary to define the internal strain of basement bodies so that the shortening of thick-skinned belts can be more accurately calculated.     

Quartz <Emphasis Type="Italic">c</Emphasis>-axis texture mapping of mylonitic metapelite with rod structures (Calabria,southern Italy): Clues for hidden shear flow direction

This quantitative microstructural study deals with textures of quartz domains within a mylonitized metapelite collected near a thrust surface corresponding to the tectonic contact between two metamorphic units, which crop out in the Aspromonte Massif, southern Calabria (Italy). The sample investigated lacks a mesoscopic stretching lineation. Therefore, quartz c-axis fabrics were investigated in two mutually orthogonal thin sections (a) parallel to the quartz rod lineation and perpendicular to the foliation (YZ plane) and (b) perpendicular to the quartz rods and perpendicular to the foliation (XZ plane); the data were generated using classical (manual measurements of quartz c-axis using U-stage) and modern methods (Computer Integrated Polarization microscopy). Both these sections show oblique foliations at ca. 40° from the main shear plane, implying that the actual X direction (stretching lineation that is absent on the mesoscopic scale) must lie between these two sections. Quartz c-axis data from the YZ section when rotated by 90° are similar with those from the XZ section. Hence, the data from the two sections are merged. These data when rotated by an angle of 50° from the direction of quartz rod lineation, gives an asymmetrical pattern indicating top-to-the-North sense of shear. This was confirmed by investigating quartz c-axis patterns in a section striking NS and perpendicular to the foliation. Based on the study it is thus concluded that this method can be used to do kinematic analysis in rocks that are devoid of stretching lineations. Apart from the above, the advantages and disadvantages of the classical and modern methods of quartz c-axis analysis are discussed.     

Rainfall stochastic modeling for runoff forecasting

Rainfall fields estimation over a catchment area is an important stage in many hydrological applications. In this context, weather radars have several advantages because a single-site can scan a vast area with very high temporal and spatial resolution. The construction of weather radar systems with dual polarization capability allowed progress on radar rainfall estimation and its hydro-meteorological applications. For these applications of radar data it is necessary to remove the ground clutter contamination with an algorithm based on the backscattering signal variance of the differential reflectivity. The calibration of the GDSTM model (Gaussian Displacements Spatial-Temporal Model), a cluster stochastic generation model in continuous space and time, is herewith presented. In this model, storms arrive in a Poisson process in time with cells occurring in each storm that cluster in space and time. The model is calibrated, using data collected by the weather radar Polar 55C located in Rome, inside a square area of 132 × 132 km², with the radar at the centre. The GDSTM is fitted to sequences of radar images with a time interval between the PPIs scans of 5 min. A generalized method of moment procedure is used for parameter estimation. For the validation of the ability of the model to reproduce internal structure of rain event, a geo-morphological rainfall-runoff model, based on width function (WFIUH), was calibrated using simulated and observed data. Several rainfall fields are generated with the stochastic model and later they are used as input of the WFIUH model so that the forecast discharges can be compared to the observed ones.     

Near‐source seismic hazard and design scenarios

Earthquakes damage engineering structures near, relatively to the rupture's size, to the source. In this region, the fault's dynamics affect ground motion propagation differently from site to site, resulting in systematic spatial variability known as directivity. Although a number of researches recommend that records with directivity‐related velocity pulses should be explicitly taken into account when defining design seismic action on structures, probabilistic seismic hazard analysis (PSHA), in its standard version, seems inadequate for the scope. In the study, it is critically reviewed why, from the structural engineering point of view, hazard assessment should account for near‐source effects (i.e., pulse‐like ground motions), and how this can be carried out adjusting PSHA analytically via introduction of specific terms and empirically calibrated models. Disaggregation analysis and design scenarios for near‐source PSHA are also formulated. The analytical procedures are then applied to develop examples of hazard estimates for sites close to strike–slip or dip–slip faults and to address differences with respect to the ordinary case, that is, when pulse‐like effects are not explicitly accounted for. Significant increase of hazard for selected spectral ordinates is found in all investigated cases; increments depend on the fault‐site configuration. Moreover, to address design scenarios for seismic actions on structures, disaggregation results are also discussed, along with limitations of current design spectra to highlight the pulse‐like effects of structural interest. Finally, an attempt to overcome these, by means of disaggregation‐based scenarios specific for the pulse occurrence case, is presented. Copyright © 2012 John Wiley & Sons, Ltd.     

Unusual magma storage conditions at Mt. Etna (Southern Italy) as evidenced by plagioclase megacryst-bearing lavas: implications for the plumbing system geometry and summit caldera collapse

At Mt. Etna volcano, the emission of plagioclase megacryst-bearing lavas, known locally as “cicirara”, has occurred rarely and generally in association with unusual volcanological phenomena. In this work, we interpret the magma chamber processes and the structural features of the plumbing system that led to the production of these peculiar volcanic rocks, based on a detailed study of plagioclase megacrysts, including their oscillatory zoning, sieve textures, and fluid inclusions. Patchy zoning suggests limited ascent in the deep levels of the plumbing system, based on the plagioclase nucleation threshold and the volatile saturation depth. At intermediate, water-undersaturated levels of the plumbing system ascent is faster, as indicated by crystals with coarse sieve textures. Storage at shallow, water-saturated levels (less than 6 km deep) is associated with oscillatory zoning with very small changes in An. Slightly larger An variations coupled with different wavelengths provide evidence of convection of crystals across distinct zones of the chamber. Stripes of melt inclusions formed at steps of magma ascent and volatile loss, whereas layers of fluid inclusions may be related to episodes of volatile flushing into the magma chamber. In contrast, strongly sieve-textured envelopes with An increase and constant FeO may be related to mixing with more volatile-rich magmas of similar composition. We interpret the repeated occurrence of “cicirara” lavas as evidence that the shallow portion of the plumbing system underwent a progressive coalescence of a complex network of dykes and sills in response to increasing rates of magma supply from depth. Major magma withdrawals from this larger reservoir may be linked to episodes of summit instability associated with major caldera collapses.     

Engineering design earthquakes from multimodal hazard disaggregation

To define reference structural actions, engineers practicing earthquake resistant design are required by codes to account for ground motion likely to threaten the site of interest and also for pertinent seismic source features. In most of the cases, while the former issue is addressed assigning a mandatory design response spectrum, the latter is left unsolved. However, in the case that the design spectrum is derived from probabilistic seismic hazard analysis, disaggregation may be helpful, allowing to identify the earthquakes having the largest contribution to the hazard for the spectral ordinates of interest. Such information may also be useful to engineers in better defining the design scenario for the structure, e.g., in record selection for nonlinear seismic structural analysis. On the other hand, disaggregation results change with the spectral ordinate and return period, and more than a single event may dominate the hazard, especially if multiple sources affect the hazard at the site. This work discusses identification of engineering design earthquakes referring, as an example, to the Italian case. The considered hazard refers to the exceedance of peak ground acceleration and 1s spectral acceleration with four return periods between 50 and 2475 year. It is discussed how, for most of the Italian sites, more than a design earthquake exists, because of the modeling of seismic sources. Furthermore, it is explained how and why these change with the limit state and the dynamic properties of the structure. Finally, it is illustrated how these concepts may be easily included in engineering practice complementing design hazard maps and effectively enhancing definition of design seismic actions with relatively small effort.