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MONTAGNE IN EVOLUZIONE E TERREMOTI ALCUNI CASI NELLE PREALPI VENETE, TRENTINE E GARDESANE
Abstract and Figures
The purpose of this paper is to present the state of art about the knowledge on the recent morphotectonic evolution of the Fore-Alps of Veneto, Trentino and Garda Lake area.
The work is divided into three parts. The first part discusses some general aspects of the geological and geomorphological features of the alpine relief.
In the second part different types of geomorphological and sedimentological evidences of recent seismotectonics events are described.
In the third part, are being analyzed, with a morphostratigraphic approach, some morphological and structural units of the Venetian Alps in order to understand, as far as possible, the evolution of
the relief in the course of time intervals comprised between a few millions and several tens of millions
of years.
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The orientation of SHmax is commonly estimated from in-situ borehole breakouts and earthquake focal mechanisms. Borehole measurements are expensive, and therefore sparse, and earthquake measurements can only be made in regions with many well-characterized earthquakes. Here we derive the stress-field orientation using stress-induced anisotropy in nonlinear elasticity. In this method, we measure the strain derivative of velocity as a function of azimuth. We use a natural pump-probe approach which consists of measuring elastic wave speed using empirical Green’s functions (probe) at different points of the earth tidal strain cycle (pump). The approach is validated using a larger data set in the Northern Alpine Foreland region where the orientation of maximum horizontal compressive stress is known from borehole breakouts and drilling-induced fractures. The technique resolves NNW-SSW to N-S directed SHmax which is in good agreement with conventional methods and the recent crustal stress model. We confirm that the natural pump-probe method can be applied to dense large-scale seismic arrays. The technique is then applied to the Southern Alps to understand the contemporary stress pattern associated with the ongoing deformation due to counterclockwise rotation of the Adriatic plate with respect to the European plate. Our results explain why the two major faults in Northeastern Italy, the Giudicarie Fault, and the Periadriatic Line (Pustertal-Gailtal Fault) are currently inactive, while the currently acting stress field allows faults in Slovenia to deform actively. We have demonstrated that the pump-probe method has the potential to fill in the measurement gap left by conventional approaches, both in terms of regional coverage and in depth.
Surface waves extracted from ambient noise cross-correlations can be used to study depth variations of azimuthal anisotropy in the crust and upper mantle, complementing XKS splitting observations. In this work, we propose a novel approach based on beamforming to estimate azimuthal anisotropy of Rayleigh wave phase velocities extracted from ambient noise cross-correlations. This allows us to identify and remove measurements biased by wavefront deformation due to 3D heterogeneities, and to properly estimate uncertainties associated with observed phase velocities. In a second step, phase velocities measured at different periods can be inverted at depth with a transdimensional Bayesian algorithm where the presence or absence of anisotropy at different depths is a free variable. This yields a comprehensive probabilistic solution that can be exploited in different ways, in particular by projecting it onto a lower dimensional space, appropriate for interpretation. For example, we show the probability distribution of the integrated anisotropy over a given depth range (e.g. upper crust, lower crust). We apply this approach to recent data acquired across the AlpArray network and surrounding permanent stations. We show that only the upper crust has a large-scale coherent azimuthal anisotropy at the scale of the Alps with fast axis directions parallel to the Alpine arc, while such large-scale patterns are absent in the lower crust and uppermost mantle. This suggests that the recent Alpine history has only overridden the anisotropic signature in the upper crust, and that the deeper layers carry the imprint of older processes. In the uppermost mantle, fast directions of anisotropy are oriented broadly north-south, which is different from results from XKS-splitting measurements or long-period surface waves. Our results therefore suggest that XKS observations are related to deeper layers, the asthenosphere and/or subduction slabs. The area north-west of the Alps shows strong anisotropy in the lower crust and uppermost mantle with a fast axis in the north-east direction that could be related to Variscan deformation.
An application of the Complete Automatic Seismic Processor (CASP) for seismic monitoring is presented. Its integrated and iterative fully automatic procedure is able to achieve complete data analysis and significantly rapid elaborations. Its performance in real-time seismic monitoring and alerting is tested in the Trentino region (NE Italy) for the period 1st March 2018 – 31st August 2019. CASP precisely and accurately located 386 seismic events, with local magnitudes in the -0.8–3.4 range, and produced a seismic catalogue with a magnitude of completeness around 1.1. Automatic earthquake solutions, with average horizontal and vertical errors of 1.1 and 1.5 km, are very similar to those included in a manually revised reference catalogue. In addition, 146 detected events are located in the area of the local porphyry quarries. CASP alerts are delivered as Short Message Service (SMS), Telegram and e-mail messages within an average time of just over two minutes from the earthquake origin time. These alerts contain earthquake source parameters, ground shaking levels and instrumental intensities. CASP reliability, promptness and robustness permit to civil protection and decision makers to perform a monitoring primarily dedicated to emergency management, in order to evaluate both seismic sources and their effects (peak ground acceleration) at local targets, such as more inhabited territories and critical infrastructures (dams and hydropower plants).
To calculate phase-velocity dispersion curves, we introduce a method which reflects both structural and dynamic effects of wave propagation and interference. Rayleigh-wave fundamental-mode surface waves from the South Atlantic Ocean earthquake of 19. August, 2016, M = 7.4, observed at the AlpArray network in Europe are strongly influenced by the upper-mantle low-velocity zone under the Cameroon Volcanic Line in Central Africa. Predicting phase-delay times affected by diffraction from this heterogeneity for each station gives phase velocities as they would be determined using the classical two-station method as well as the advanced array-beamforming method. Synthetics from these two methods are thus compared with measurements. We show how the dynamic phase velocity differs from the structural phase velocity, how these differences evolve in space, and how two-station and array measurements are affected. In principle, arrays are affected with the same uncertainty as the two-station measurements. The dynamic effects can be several times larger than the error caused by the unknown arrival angle in case of the two-station method. The non-planarity of the waves and its relation to the arrival angle and dynamic phase-velocity deviations is discussed. Our study is complemented by extensive review of literature related to the surface-wave phase-velocity measurement of the last 120 years.
The Italian Government has decreed a series of progressive restrictions to delay the COVID-19 pandemic diffusion in Italy since March 10, 2020, including limitation in individual mobility and the closure of social, cultural, economic and industrial activities. Here we show the lockdown effect in Northern Italy, the COVID-19 most affected area, as revealed by noise variation at seismic stations. The reaction to lockdown was slow and not homogeneous with spots of negligible noise reduction, especially in the first week. A fresh interpretation of seismic noise variations in terms of socio-economic indicators sheds new light on the lockdown efficacy pointing to the causes of such delay: the noise reduction is significant where non strategic activities prevails, while it is small or negligible where dense population and strategic activities are present. These results are crucial for the a posteriori interpretation of the pandemic diffusion and the efficacy of differently targeted political actions.
We present a new anisotropic seismic tomography model based on a multiscale full seismic waveform inversion for crustal and upper-mantle structure from the western edge of North America across the North Atlantic and into Europe. The gradient-based inversion strategy utilizes the adjoint state method coupled with an L-BFGS quasi-Newton optimization scheme. To improve the handling of large data quantities in the context of full seismic waveform inversions, we developed a workflow framework automating the procedure across all stages, enabling us to confidently invert for waveforms from 72 events recorded at 7,737 unique stations, resulting in a total of 144,693 raypaths, most of them with three-component recordings. The final model after 20 iterations is able to explain complete waveforms including body as well as surface waves of earthquakes that were not used in the inversion down to periods of around 30 s. The model is complemented by a detailed resolution analysis in the form of 3-D distributions of direction-dependent resolution lengths.
Hribovje Monti Berici predstavljajo najjužnejšo kraško morfološko enoto Južnih Alp in obenem svojski tip krasa. Analiza njihove topografije, ugotavljanje morfostratigrafskih ostankov uravnanih površij in različnih rečnih oblik, zaznavanje nekaterih elementov, pomembnih za kronologijo, predstavljenih v tem sestavku, so dovolj, da si je mogoče zamisliti predhodni model geomorfološkega razvoja te gorske skupine. Vodilni morfogenetski elementi so fluvialnega izvora in jih ne določajo le spremembe klime, ampak tudi tektonsko dviganje in/ali spremembe baznega nivoja. Kraške oblike so se razvile predvsem na ostankih fluvialnih oblik ali v skladu z razmeroma neaktivnimi fluvialnimi oblikami. Starost glavnih oblik se razteza čez zelo dolgo obdobje, verjetno v razponu 15 milijonov let. Da so se te zelo stare oblike lahko ohranile, je mogoče razložiti s svojskim geomorfološkim okoljem, ki ga ni zajela poledenitev tekom pleistocena. Primerjava razvojnih modelov večih kraških morfostruktur v Južnih Alpah pomaga razumeti razlike v njihovem geomorfološkem oblikovanju.
The Monti Berici constitute the most southerly karst morpho-unit of the Southern Alps and a peculiar type of karst. The analysis of their topography, the identification of a »morpho-stratigraphy« of the relicts of surface planation and of the different types of fluvial forms, and the recognition of the few elements of chronological significance present in this context, allow the delineation of a preliminary model of the geomorphological evolution of this mountain group. The main morphogenetical elements are of fluvial origin and have been determined, not only by the climatic changes, but also by the tectonic uplifting and/or by changes of the base level. The karst landforms have mostly evolved on the relict fluvial forms or in the context of relatively inactive fluvial forms. The age of the main forms extends over a very long time span, probably in the order of 15 millionyears. The preservation of very old forms can be explained by the peculiar geomorphological environment which was not affected by glacial erosion during the Pleistocene. The comparison of the evolution models of several karst morpho-units in the Southern Alps helps to understand the differences in their geomorphological styles.
In the Southern Alps the Montello ridge is one of the most singular sub-alpine groups, a kind of low plateau comparable to a turtleback. This morphostructure, constituted by a conglomerate rock unit of messinian age formed mostly by limestone pebbles bound by a carbonatic cement, is the expression of the up-arching by compressional shortening of a tectonic wedge. The gradual upsurge of the morphostructure above the plain has exposed it to the erosional processes first of all by the rivers and secondarily by karst denudation. On the base of a computer aided analysis the main morphological sub-units are distinguished and analysed. Some of these sub-units present characters of wide rock cut terraces. The characters of the dolines, consisting in a large population of more than 2000 forms, allow to evaluate the relative age of the surfaces of the correspondent sub-units. The sub-unites of the hill can be divided into two groups: the north-west group and the south-east group. The dolines of the NE group are more numerous and smaller than those of the SE group. In the NW group a step of seven rock cut terraces is easily recognisable where the volumetric developments of the dolines are well correlated with the relative altitudes of the same terraces. In the morphostructure the following types of dolines have been distinguished: a) plateau dolines, b) terrace-scarp dolines, c) terrace-slope dolines. The plateau dolines are those of the summit plateau of the hill; the terrace-scarp dolines are influenced by the scarps of the terraces; the terrace-slope dolines are similar to the plateau dolines but are settled on the 'slopes' of the terraces determined by the anticline bending of the morphostructure. In the morphodinamical context of the Montello hill the dolines begun to develop as large and shallow forms and their development in the vertical dimension (depth) is controlled by the time, by the character of the basin (the terrace-scarp dolines are in general deeper than the plateau and terrace-slope dolines), and by the geological structure (the dolines along the anticline axis are deeper than those to the north and to the south of it). With reference with the general evolution of the morphostructure it is possible to recognise the older age of the central and eastern surfaces. This could be related with a former 'emersion' from the alluvial plain of this part of the morphostructure and perhaps with a westward migration of the up-bulging of the hill.
The interdisciplinary study of the different karstic landforms, slope deposits, and relative paleosols revealed the close links existing between local lithologico-structural situations, old and recent tectonic activity, and geomorphic and pedogenetic processes. This karstic landscape is the result of phases of intense karstic morphogenesis (accompanied by subtropical pedogenesis) alternating with periglacial phases, during which the karst was modified. Neotectonic uplifts of a few hundred metres are easily recognizable in several places. -after Author
This study investigates the seismotectonic characteristics of the central Alps, and mainly the Lombard sector. The analysis of the historical and recent instrumental seismicity can identify the areas where earthquakes with magnitude around 5 occurred: the Chur and Verona areas, the region around Lake Garda, and the Lombard Po plain, slightly south of the foothills. Minor seismicity interests Engadin, northern Valtellina, Müstair, and the Venosta valleys. Most of the earthquakes show a dip-slip focal mechanism, generally related to reverse faults, but a few strike-slip motions were observed as well. Six vertical cross-sections have been constructed by collecting information about the surficial geology, deep crustal structures, and seismicity. A direct link between the hypocentral distribution and the tectonic structures is sometimes difficult, but the analysis has given some seismotectonic suggestions. Three main seismotectonic domains remain identified: the Lombard Po plain, the Garda area, and the Alps s.s. Among these domains, the Garda area shows the highest seismicity and the earthquakes are associated to the Giu-dicarie system and its possible interference with the Schio-Vicenza system. The general pattern of the seismogenesis is the key to defining the seismogenic zones to be used for seismic hazard assessment at regional and national scales.
