Evidenze della rapida variazione di profondità della Moho, in corrispondenza dell'area di Città di Castello (Appennino Settentrionale), dall'analisi di funzioni ricevitore

In questo studio è stata sfruttata l’opportunità di poter analizzare dati provenienti da una densa rete sismica locale temporanea costituita da 30 stazioni a tre componenti, installata nell’ambito di un progetto del Gruppo Nazionale per la Difesa dei Terremoti (GNDT) nel periodo compreso fra l’Ottobre 2000-Maggio 2001, in un’area che si estende per circa 2800 km2 a circa 43° N in Appennino Settentrionale (Piccinini et al., 2003), al fine di ottenere un dettagliato andamento della topografia della Moho, in una zona così complessa, attraverso un’analisi delle Funzioni Ricevitore (Langston, 1979), definendo la struttura di velocità delle onde di taglio (S) al di sotto di ciascuna delle 28 stazioni sismiche. Sono stati analizzati circa 400 eventi telesismici registrati da 28 stazioni con valori di magnitudo M>5 e distanza epicentrale Δ compresi fra 25°-100°. Per il calcolo delle RFs è stato utilizzato il metodo sviluppato da Di Bona (1998), tale metodo consente di ottenere una stima della varianza, permettendo l’utilizzo di forme d’onda generate da eventi di bassa magnitudo (aventi valori di varianza accettabili), con un conseguente ampliamento del data-set. Modellando ampiezze e tempi di arrivo delle fasi Ps in funzione dell’azimuth di provenienza (BAZ) e della relativa distanza epicentrale (Δ), si possono ricostruire le geometrie delle superfici di discontinuità al di sotto delle stazioni sismiche. La fase di modellazione è stata condotta attraverso l’applicazione dell’algoritmo di inversione “neighbourhood” di Sambridge (1999) mediante un approccio monodimensionale. Questo metodo consente di campionare in maniera estensiva lo spazio dei parametri (profondità delle varie interfacce e valori di velocità negli strati compresi fra le interfacce), concentrando la ricerca in quelle regioni dello spazio multiparametrico dove i modelli di velocità trovati hanno un miglior misfit rispetto al dato (la RF) reale. Tale fase di modellazione ha consentito di ricostruire i modelli di velocità delle onde S (Vs) al di sotto di ciascuna stazione. L’analisi comparata dei modelli di velocità delle onde S (Vs) così ottenuti, per ogni singola stazione, mette in luce la natura fortemente eterogenea della porzione più superficiale della crosta dell’area in studio. Nonostante la complessità delle RFs calcolate che si riflette sulla eterogeneità della porzione più superficiale dei profili di Vs ottenuti, è stata individuata con buona continuità l’andamento di una superficie di discontinuità sismica da noi interpretata come transizione crosta-mantello superiore o Moho

Analisi strutturale di crosta e mantello in prossimità dell’alta Val di Chiana (Toscana orientale)

L’Appennino Settentrionale è una catena montuosa NE-vergente ed è il risultato dell’affioramento del prisma di accrezione originato in seguito alla subduzione della litosfera adriatica sotto il mar Tirreno ed ancora in atto (Faccenna et al., 2001). Dall’Oligocene ad oggi, l’Appennino Settentrionale è stato interessato da due fasi deformative: inizialmente compressiva con la formazione di thrusts e più recentemente distensiva (Elter et al., 1975). Attualmente è caratterizzato da un regime crostale distensivo con una velocità stimata circa 2.5 mm/anno (Hunstad et al., 2003). Gli effetti e le conseguenze di questi episodi deformativi sono ben visibili attraverso un’analisi geologica e geofisica. L’area in studio è posta in corrispondenza della transizione tra la successione Toscana ed il settore Tirrenico del dominio Umbro-Marchigiano, quindi, una zona particolarmente dibattuta da un punto di vista geodinamico, a causa della presenza di diverse tipologie crostali, flusso di calore e anomalie gravimetriche

Crustal and Upper Mantle Three-Dimensional Stratification and Anisotropy from Receiver Functions (Northern Apennines-Italy)

The Northern Apennines (NA) were predominantly formed by a Meso-Cenozoic sedimentary sequence thrust northeast and stacked over the Adriatic foreland during Late Miocene-Pleistocene. Extension on the Tyrrhenian margin is synchronous with thrust emplacements along the external Apenninic chain and is associated by crustal thinning, normal faulting, ductile deformation, volcanic activity and high heat flow. Both, the extensional and the compressional fronts migrated towards the Adriatic foreland during the Plio-Pleistocene. Crustal extension everywere disrupted structural architectures formed during the preceding compressional phase leading to the development of thinned, uplifted and extended crust in the Tuscany mainland. Several models have been proposed to explain the evolution of the NA that are acknowledged to be tectonically complex. We present results from a Receiver Functions (RFs) analysis of teleseismic events recorded at Arezzo seismic station (Tuscany). A broad-band station (ARZ) is installed on the north-east margin of the “Val di Chiana” extensional syntectonic basin. We selected and grouped in “bins” high signal/noise teleseismic events of four years of recording to compute a data-set of RFs. We applied a classical inversion scheme (a Neighbourhood Algorithm) and we carried out a three-dimensional modelling. As a criterion to identify and to distinguish the effects of azimuthal anisotropy from those of lateral heterogeneity, we included the harmonic angular analysis performed by stacking Radial (R) and Transverse (T) components with weights depending on the backazimuth. The results of these analysis provide a detailed three-dimensional image of the S-velocity lithosphere structure

Moho-depth and subglacial sedimentary layer thickness in the Wilkes Basin from Receiver Function Analysis

Wilkes Basin lies to the east of the Transantarctic Mountains. The origin of this sub-glacial basin is still controversial. Flexural uplift of the Transantarctic Mountains has been suggested as the geophysical process which generated the basin (Stern & ien Brink, 1989). Other studies proposed a continental rift structure for this region (Ferraccioli et al., 2001). The two models differ mainly in the crustal structure predicted beneath the basin. In the former, crustal thickning is expected to be originated from the high rigidity of the East Antarctic Craton lithosphere. Otherwise, the rift structure hypothesis is consistent with a broad crustal thinning. During the WIBEM 2003 campaign, we deployed five broadband seismic stations across the basin. We selected high signal/noise teleseismic recording to compute a data-set of receiver functions. We applied a classical inversion scheme, the Neighbourhood Algorithm, to our data-set. Here, two different and complementary studies are presented. We constrain the Moho geometry beneath the Wilkes Basin from the analysis of low-frequency P-to-S conversion at the base of the crust. Also, we investigate the nature of the basin mapping the presence of subglacial sediments using the P-to-S conversion at the ice-bedrock interface

Crustal Structure Across Northern Victoria Land, Antarctica, From Receiver Function Analysis

Global crustal model, from gravity studies, imaged a thick crust (>40 km) under Eastern Antartic craton (EAC). This global trend ends abruptly west of the Transantarctic Mountains (TAM), which border EAC along its western margins. There, the crust raises up to about 20 km. While this model points out the difference between EAC and the Ross sea crustal structures, its intrinsic spatial resolution gives little help to solve some regional geophysical issues, like the TAM orogenesis and the formation and nature of the Wilkes Basin. In this study, teleseismic Receiver Functions (RFs) are used to image the S-velocity crustal structure in finer details. We computed RFs from teleseismic events recorded during three different austral summer compaigns: BackTAM, WIBEM and WISE. Broadband seismic stations were deployed along a transect which spans from the coast of Northern Victoria land (NVL) to the far interior of the EAC plateau. The transect, almost perpendicular to the regional TAM axis, came across four different geological/geophysical settings: the alloctonhous terranes of the NVL, the TAM sector, the Wilkes Basin and the EAC plateau. Each area shows peculiar crustal structures and we propose both finer local S-velocity models and a regional crustal model

Crustal structure and Moho depth profile crossing the central Apennines (Italy) along the N42 degree parallel.

We present results from a teleseismic receiver-function study of the crustal structure in the central Apennines (Italy). Data from fifteen stations deployed in a linear transect running along the N42 degree parallel were used for the analysis. A total number of 364 receiver functions were analyzed. The crustal structure has been investigated using the neighborhood algorithm inversion scheme proposed by Sambridge [1999a], obtaining crustal thicknesses, bulk crustal VP/VS ratio and velocity-depth models. In each inversion, the degree of constraint of the different parameters has been appraised by the Bayesian inference algorithm by Sambridge [1999b]. The study region is characterized by crustal complexities and intense tectonic activity (recent volcanism, orogenesis, active extensional processes), and these complexities are reflected in the receiver functions. However, the relatively close spacing among the seismometers (about 20 km) helped us in the reconstruction of the crustal structure and Moho geometry along the transect. Crossing the Apennines from west to east, the Moho depth varies by more than 20 km, going from a relatively shallow depth (around 20 km) on the Tyrrhenian side, deepening down to about 45 km depth beneath the external front of the Apenninic orogen, and rising up again to about 30 km depth in correspondence of the Adriatic foreland. Despite the strong variability of the crustal thickness, the average crustal VS values show little variation along the transect, fluctuating around 3 km/s. The average VP values obtained from the VS and VP /VS are generally lower than 6 km/s

Complete compensation of criss-cross deflection in a negative ion accelerator by magnetic technique

During 2016, a joint experimental campaign was carried out by QST and Consorzio RFX on the Negative Ion Test Stand (NITS) at the QST Naka Fusion Institute, Japan, with the purpose of validating some design solutions adopted in MITICA, which is the full-scale prototype of the ITER NBI, presently under construction at Consorzio RFX, Padova, Italy. The main purpose of the campaign was to test a novel technique, for suppressing the beamlet criss-cross magnetic deflection. This new technique, involving a set of permanent magnets embedded in the Extraction Grid, named Asymmetric Deflection Compensation Magnets (ADCM), is potentially more performing and robust than the traditional electrostatic compensation methods. The results of this first campaign confirmed the effectiveness of the new magnetic configuration in reducing the criss-cross magnetic deflection. Nonetheless, contrary to expectations, a complete deflection correction was not achieved. By analyzing in detail the results, we found indications that a physical process, taking place just upstream of the plasma grid, was giving an important contribution to the final deflection of the negative ion beam. This process appears to be related to the drift of negative ions inside the plasma source, in the presence of a magnetic field transverse to the extraction direction, and results in a non-uniform ion current density extracted at the meniscus. Therefore, the numerical models adopted in the design were improved by including this previously disregarded effect, so as to obtain a much better matching with the experimental results. Based on the results of the first campaign, new permanent magnets were designed and installed on the Extraction Grid of NITS. A second QST-Consorzio RFX joint experimental campaign was then carried out in 2017, demonstrating the complete correction of the criss-cross deflection and confirming the validity of the novel magnetic configuration and of the hypothesis behind the new models. This contribution presents the results of the second joint experimental campaign on NITS along with the overall data analysis of both campaigns, and the description of the improved models. A general picture is given of the relation among magnetic field, beam energy, meniscus non-uniformity and beamlet deflection, constituting a useful database for the design of future machines

A snapshot of the Northern Apennines (Italy) seismicity, merging catalogue and new seismic data

In this paper we present the seismicity analysis of a small sector of the Northern Apennines in 27 terms of spatio-temporal distribution, merging data from the Italian seismic bulletin with new 28 data collected by temporal seismic networks. Our attention is focused on the region enclosed 29 between Toscana, Umbria, Marche and Emilia-Romagna. This area is mainly characterized by a 30 diffuse seismicity, partly clustered in small sequences (Mw < 4.7). Improved seismicity locations, 31 together with stress field analysis allows to characterize the manner of seismogenic stress release 32 in the area. Two regions of significantly different seismic release behavior could be 33 distinguished: (i) the inner/western part (Tuscan side) of the study area, where seismicity is 34 clustered at shallow depths (<18 km) and where strong earthquakes occurred in the past, (ii) the 35 outer(eastern) part (Marche side), where the seismicity is diffuse across all of the crustal volume, 36 reaching depths of down to 30 km. 37 Along the Apenninic chain, seismicity is nearly absent inside well defined zones. In our opinion, 38 these peculiarities of seismicity release could be related to the heterogeneity of crustal volume 39 and to the transition between Tyrrhenian and Adriatic domains

Discovering geothermal supercritical fluids: a new frontier for seismic exploration

Exploiting supercritical geothermal resources represents a frontier for the next generation of geothermal electrical power plant, as the heat capacity of supercritical fluids (SCF),which directly impacts on energy production, is much higher than that of fluids at subcritical conditions. Reconnaissance and location of intensively permeable and productive horizons at depth is the present limit for the development of SCF geothermal plants. We use, for the first time, teleseismic converted waves (i.e. receiver function) for discovering those horizons in the crust. Thanks to the capability of receiver function to map buried anisotropic materials, the SCF-bearing horizon is seen as the 4km-depth abrupt termination of a shallow, thick, ultra-high (&gt;30%) anisotropic rock volume, in the center of the Larderello geothermal field. The SCF-bearing horizon develops within the granites of the geothermal field, bounding at depth the vapor-filled heavily-fractured rock matrix that hosts the shallow steam-dominated geothermal reservoirs. The sharp termination at depth of the anisotropic behavior of granites, coinciding with a 2 km-thick stripe of seismicity and diffuse fracturing, points out the sudden change in compressibility of the fluid filling the fractures and is a key-evidence of deep fluids that locally traversed the supercritical conditions. The presence of SCF and fracture permeability in nominally ductile granitic rocks open new scenarios for the understanding of magmatic systems and for geothermal exploitation