Dipartimento di Geologia e Geofisica, Università di Bari, Bari, Italia

A multimethodological geophysical survey was performed in the north-western part of the Gargano promontory to study the geological structural setting and the underground fluid flow characteristics. The area has a complex tectonics with some magmatic outcrops and shallow low-enthalpy waters. Electrical, seismic reflection, gravimetric and magnetic surveys were carried out to reconstruct the geological structures; and in order to delineate the hydrogeothermal characteristics of the area, the self-potential survey was mainly used. Moreover magnetic and self-potential measurements were also performed in the Lesina lake. The joint three-dimensional interpretation of the geophysical data disclosed a large horst and graben structure covering a large part of the area. In the central part of the horst a large ramified volcanic body was modelled. The models show some intrusions rising from it to or near to the surface. The main structures are well deep-seated in the Crust and along them deep warm fluids rise as the SP data interpretation indicates.

The estimation of the earth transfer functions in MT prospecting method poses the greatest difficulty. As in the seismic prospecting method this task requires the development of advanced processing techniques. In order to assess the performance of each technique, controlled synthetic data and different noise types, which simulate the observed signals, are required. This paper presents a procedure to generate a wide-band noise-free electromagnetic field to be used both for magnetotelluric and audio-magnetotelluric studies. Furthermore, an effort was made to extend the simulation procedures to generally stratified and simple inhomogeneous earth structures. The discrete-time magnetic field values are generated through the inverse Fourier transform of a continuous amplitude spectrum and a sampling procedure. The electric field time series are obtained by the convolution of the magnetic field time series, calculated in the interested frequency band, with a non-causal impedance impulse response. Polarized fields, which are important when inhomogeneous media are considered, are also generated.

On 23 July 1930, the Irpinia region in southern Italy experienced a destructive (M 6.7) earthquake that struck the eastern sector of the southern Apennines moutain belt. Previous studies suggest that this earthquake was caused by a seismogenic source having oblique right-lateral kinematics and striking at an angle between the general trend of NE-verging large dip-slip faults in the southern Apennines (~ NW-SE) and the E-W near-vertical, strike-slip right lateral faults that have been recently discovered in the foreland, east of the main extensional axis. Also, the ~14 km hypocentral depth of the 1930 earthquake that has been calculated in previous studies is likely located within the basement below the Apula carbonate platform succession. This puts the source of the 1930 earthquake not only in an intermediate region between pure normal (NW-SE) and strike-slip right-lateral (E-W) large seismogenic faults in the southern Apennines, but also at an hypocentral depth between the 12-13 km depth of the earthquakes caused by normal faulting (like the Irpinia 23 Nov. 1980, M 6.9 one) and the 15-20 km depth of the earthquakes caused by strike-slip faulting in the foreland (like the 31 Oct.-1 Nov. 2002, M 5.8 Molise ones). In this framework, we performed a magnetotelluric (MT) study to investigate the evidence of preferential direction in resistivity anisotropy and to compare it with the strike of the 1930 seismogenic fault.

L’area del terremoto dell’Irpinia del 1930 corrisponde in profondità all'avampaese Apulo, inflesso al di sotto delle unità alloctone accavallate nella porzione frontale del cuneo orogenico, ma non coinvolto nelle strutture dei cosiddetti duplex dell'Apula sensu Patacca and Scandone (2004). In particolare, le profondità ipocentrali del terremoto del 1930 corrispondono al basamento sottostante la successione della Piattaforma Carbonatica Apula. Il campo macrosismico e i dati strumentali disponibili (si veda la sorgente in DISS, 2007, con relativa bibliografia, e Pino et al., sottomesso) suggeriscono una sorgente sismogenetica con un’orientazione e una cinematica che rappresentano una sorta di transizione tra la direzione W-E a cinematica trascorrente destra, che caratterizza la sismicità propria delle aree di avampaese sia affiorante che sepolto, e la direzione NW-SE a cinematica normale, che caratterizza la sismicità connessa all'estensione lungo l'asse della catena Appenninica (si veda il terremoto del 1980). In questo quadro, l’obiettivo dello studio magnetotellurico è stato quello di investigare i volumi di crosta al di sotto della successione Apula per valutare l'eventuale presenza di direzioni preferenziali dell'anisotropia di resistività che fossero confrontabili con la direzione della sorgente del terremoto del 1930. Il verificarsi di tale evenienza avrebbe potuto essere infatti interpretato come indizio di una zona di debolezza regionale, che avrebbe condizionato le caratteristiche geometriche e cinematiche della sorgente del terremoto stesso. Partendo dall’area sismogenetica segnalata nel DISS per questo terremoto, sono stati effettuati in un’area di circa 1000 km2 sondaggi magnetotellurici in 15 siti, nell’intervallo di 0.009- 4000 s. Per ciascun sito si è proceduto alla misura delle tre componenti ortogonali del campo magnetico e di tre componenti del campo elettrico, di cui due lungo la stessa linea e ortogonali alla terza. Ciò ha consentito la stima dei parametri magnetotellurici per due sondaggi adiacenti, al fine di meglio controllare possibili problemi di rumore antropico o strumentale. Le stazioni, fino ad un massimo di tre, hanno operato in contemporanea fungendo l’una per l’altra da remote reference (Gamble et al., 1979). Va sottolineata la buona qualità dei dati acquisiti sia in termini di stime stabili con diverse tecniche di analisi, che per basso scattering delle curve di resistività apparente e fase. Le risposte sperimentali sono state poi comparate con i dati di pozzo disponibili, verificando un ottimo accordo. È stata inoltre eseguita un’analisi sulle proprietà fisiche e geometriche del tensore impedenza, adottando lo schema di decomposizione di Weaver et al. (2000) dal quale è derivato poi lo studio degli invarianti magnetotellurici per la definizione della dimensionalità delle strutture elettriche investigate ai vari periodi (ovvero alle varie profondità). Circa il 75% dei dati analizzati implica strutture assimilabili necessariamente a modelli tridimensionali e le quattro componenti del tensore impedenza sono significativamente diverse da zero. Per questo tipo di strutture, seguendo Weaver et al. (2000), è comunque possibile definire una direzione di eterogeneità elettrica. Ciò è stato fatto per ciascun sondaggio e per ciascun periodo di stima. Mediante la trasformazione di Niblett–Bostick è stato poi ottenuto lo strike elettrico in funzione della profondità stimata. Viene riportata la direzione di strike per i vari sondaggi alla profondità stimata nell’intervallo 8 - 16 km, riferibile quindi a una porzione di crosta al di sotto del resistivo che identifica le successioni della Piattaforma Apula.

Over the last decade electromagnetic (EM) measurements have provided new constraints on the upper-crustal structure of the major fault zones in the world, both when they act as conduit and as a barrier, due to strong sensitivity of resistivity to fluids circulation and mineralization. On the track of a high impact magnetotelluric (MT) study performed across the San Andreas Fault, high resolution EM data were collected in the Colfiorito epicentral area along profiles crossing some main fault lineaments. Being the study focussed both on shallow that on intermediate resistivity distribution in the brittle upper-crust, a MT profile was integrated by several electrical resistivity tomographies (ERT). The latter were successful in locating faults even where the structures are buried by a wide covering of Quaternary deposits and in the recognition of different electrical signatures of the faults. MT resistivity model crossing Mt. Prefoglio normal fault clearly imaged the typical thrust structures of the area and a high conductive zone spatially related to the fault. Seismicity seems to be located outside such conductive area, whose behaviour suggests a fluidised and altered zone incapable of supporting significant stress internally.

Using a multidisciplinary dataset based on gravimetric, seismic, geodetic and geological observations,we provide an improved picture of the shallow structure and dynamics of the southern edge of the Tyrrhenian subduction zone.With a local earthquake tomographywe clearly identify twomain crustal domains in the upper 15 kmcharacterized by different P-wave velocity values: a high-velocity domain comprising southeasternmost Tyrrhenian Sea, NE Sicily and Messina Straits, and a low-velocity domain comprising Mt. Etna and eastern Sicily. The transition between the two domains shows a good spatial correspondence with a wider set of faults including the Taormina Fault System (TFS) and the Aeolian–Tindari–Letojanni Fault System (ATLFS), two nearly SE-striking fault systems crossing northeastern Sicily and ending on the Ionian shoreline of Sicily according to many investigators. Within this set of faults, most of the deformation/seismicity occurs along the northern and central segments of ATLFS, compared to lowactivity along TFS. A lack of seismicity (both recent and historical) is observed in the southern sector of ATLFS where, however, geodetic data reveal significant deformation. Ourmultidisciplinary dataset including offshore observations suggests the southeastward continuation of the ATLFS into the Ionian Sea until joiningwith the faults cutting the Ionian accretionarywedge described in the recent literature. Our findings imply the existence of a highly segmented crustal shear zone extending from the Aeolian Islands to the Ionian Abyssal plain, that we believe plays the role of accommodating differential motion between the Southern Tyrrhenian unit and the western compressional domain of Sicily. The ATLFS, which is a main part of the inferred shear zone, behaves similarly to what often observed at the edges of retreating subduction

This paper presents a magnetotelluric (MT) survey of the unstable eastern flank of Mt. Etna. We take thirty soundings along two profiles oriented in the N-S and NW-SE directions, and from these data recover two 2D resistivity models of the subsurface. Both models reveal three major layers in a resistive-conductive-resistive sequence, the deepest extending to 14 km bsl. The shallow layer corresponds to the volcanic cover, and the intermediate conductive layer corresponds to underlying sediments segmented by faults. These two electrical units are cut by E-W-striking faults. The third layer (basement) is interpreted as mainly pertinent to the Apennine-Maghrebian Chain associated with SW-NE-striking regional faults. The detailed shapes of the resistivity profiles clearly show that the NE Rift is shallow-rooted ( 0–1 km bsl), thus presumably fed by lateral dikes from the central volcano conduit. The NW-SE profile suggests by a series of listric faults reaching up to 3 km bsl, then becoming almost horizontal. Toward the SE, the resistive basement dramatically dips (from 3 km to 10 km bsl), in correspondence with the Timpe Fault System. Several high-conductivity zones close to the main faults suggest the presence of hydrothermal activity and fluid circulation that could enhance flank instability. Our results provide new findings about the geometry of the unstable Etna flank and its relation to faults and subsurface structures.