bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectral analysis of geomagnetic data from Kandilli Observatory, Istanbul
Authors: A. H. Bilge  , Y. K. Tulunay
Published: June 2021

The geomagnetic field variation spectra for periods longer than 2 h are analysed using the data obtained at the Kandilli Observatory in Istanbul, Turkey. Among the deterministic components, only the harmonics of the solar daily variation (Si) and the second harmonic of the lunar daily variation (L2) were observed. The seasonal dependence of the D component is analysed; the first harmonic of the solar daily variation (S1) has maximum power in summer, decreasing symmetrically towards the winter; the second harmonic (S2) has equal power in spring, summer and fall, while the third harmonic (S3) has maximal power in spring and fall.
Orographic cyclogenesis in a saturated atmosphere and intense precipitation: baroclinic modal solutions under the joint action of localized mountains and humidity
Authors: R. Benzi  , M. Fantini  , R. Mantovani  , A. Speranza
Published: June 2021

In this paper we analyse the nature of orographic cyclogenesis in a saturated atmosphere by means of a simplified model based on the analysis of linear modal solutions.The space structure of fastest growing modal solutions suggests that three different scales of axtratropical atmospheric motion may simultaneously be activated in a single, growing, unstable mode: the orographic modulation of growing baroclinic modes extending, as we know from the classical modal theory of orographic cyclogenesis, from the scale typical of the primary, extra-tropical cyclone to the scale of the secondary, orographic cyclone, is also characterized by the (smaller) scale associated with strong ascending motion in a saturated atmosphere. Since ascending motion can be associated with intense precipitation, this result is important in view of its potential consequences both on the ability to achieve a good forecast of intense precipitation events in the Mediterranean and on the refinement of the theory of orographic cyclogenesis.
A prototype radonmeter for seismic surveillance
Authors: F. Quattrocchi  , M. Calcara  , B. Porfidia
Published: June 2021

A new 222Rn monitoring prototype has been designed, assembled and tested at the Istituto Nazionale di Geofisica (ING) specifically addressed to seismic surveillance tasks, exploiting environmental monitoring, etc. It operates with an a scintillation technique (photomultiplier + Lucas Cell) coupled with a water input system, that lets continuous dehumidified gas flow, stripped from groundwater under monitoring. Several laboratory tests have been carried out to check the stability and versatility of the system; moreover statistical tests have been accomplished on several data sets obtained with an 241Am radioactive standard source, to check stability of the photomultiplier. A customised water flow system has been developed to perform both the highest efficiency and lowest influence of external noise parameters. This new prototype is very cheap and will be integrated within the new multiparametric geochemical monitoring system GMS II, that is currently being developed at ING, specifically designed for geochemical surveillance of seismic events.
Electrical behaviour of rock samples from Puglia 1 borehole (South Italy): from 20°C to 1000°C
Authors: M. Muschietti
Published: June 2021

During 1992, at the Applied Geophysics Laboratories of Florence (I) and Prague (CS) some electrical measurements were made on deep rock samples to understand the source of low resistivity values measured by logs.The studied samples were sandstones (6380 m depth) and dolomites (6070 m depth) coming from the borehole AGIP-PUGLIA 1 (South Italy). The electrical parameters were the real part of the complex resistivity, the loss tangent and the total harmonic distortion. They were measured as functions of temperature (20 £ T £ 1000°C),confining and internal pore pressure (3 £ p £ 33 MPA), and frequency (0.002 £ f£ 1000 Hz). For the measurements, two different apparatus were used: the Florence apparatus working up to T= 200°C conditioning internal pore fluid, and the Prague apparatus working in temperature (200°÷1000°C). Both the literature and laboratory research confirm that the electrical results at our experimental physical conditions remain unchanged. The obtained results confirmed that deep rocks can show low resistivity valued at special internal fluid pressures and temperatures, according to their mineralogical composition and petrographic structure.
Underground earth strain and seismic radiation measurements with a laser interferometer and a dense small-aperture seismic array
Authors: A. Amoruso  , L. Crescentini  , G. De Luca  , R. Scarpa  , M. Abril  , Al. Cirella
Published: June 2021

This paper describes two geophysical instruments, installed in the underground physics laboratories of Gran Sasso (LNGS-INFN), located in the seismic zone of the Central Apennines, Italy. These instruments monitor strain and seismic radiation with very high sensitivity: one is a 90 m-long laser interferometer, sensitivity 3 x 10-12, frequency response 10-7-10-2 Hz, and has been operating since 1994. The other is a small-aperture seismic array composed of 21 three-component short period (Mark L4C-3D) and 3 broadband (Guralp CMG-3ESP) seismometers. This dense array will be in operation at the beginning of 1998.
Continuous record of the gravity changes at Mt. Vesuvius
Authors: G. Berrino  , G Corrado  , R. Magliulo  , U. Riccardi
Published: June 2021

High precision relative and absolute gravity measurements are periodically carried out at Mt. Vesuvius (Southern Italy) to monitor the changes in the gravity field caused by the internal dynamics of the volcano. Moreover, a recording gravity station is also operating at the Osservatorio Vesuviano (Old Building) aimed at measuring in continuous mode the time variations of the gravity and the tidal parameters, possibly due to changes in the physical state of the volcano. The analysis carried out on an hourly data set spanning the 1994-1996 time interval and a comparison of the results with those previously obtained, exhibit a change in the tidal parameters which occurred between 1991 and 1994. A gravity decrease of about 60 µGal was observed during the same time interval in the same area. Consistent with relative and absolute gravity observations, no significant variations have been detected since 1994 in the tidal parameters and gravity residuals.
Gravity modelling of the lower crust in Sardinia (Italy)
Authors: M. T. Carrozzo  , R. Balia  , M. Loddo  , D. Luzio  , C. Margiotta  , T. Quarta  , R. Trudu
Published: June 2021

In this paper an example is given of an application of statistical techniques to the Bouguer anomalies analysis in order to design a simple crustal model using few a priori assumptions. All gravity measurements carried out in Sardinia have been collected and processed. The Bouguer anomalies have been calculated according to local density estimates. Spectral analysis of the Bouguer anomalies has been carried out along selected profiles in order to estimate the mean depth of the Moho discontinuity and that of an infracrustal discontinuity. The use of this technique inferred the presence of a discontinuity at a mean depth of ~ 28 km, interpreted as Moho and the likely presence of an infracrustal discontinuity at a mean depth of ~18 km, interpreted as the upper-lower crust transition. In order to roughly reconstruct the shape of these interfaces, 2D inversion techniques were applied to the large wavelength components of the Bouguer anomalies, relative to profiles oriented along the E-W direction, extracted from low-pass filtered Bouguer anomaly maps. The density model obtained is compatible with some velocity models achieved from the interpretation of the seismic refraction profiles carried out within the European Geotraverse project.
A 3D gravity model of crustal structure in the Central-Eastern Alpine sector
Authors: R. Cassinis  , F. Federici  , A. Galmozzi  , S. Scarascia
Published: June 2021

Assuming as a starting model the pattern of the Moho boundary as interpreted in a recent study on the basis of the available DSS profiles, a preliminary 3D gravity model of the crustal structures in the Central-Eastern Alpine sector is proposed. The aim of the present work is to confirm the seismic results concerning the Moho and to better shape the main discontinuities in the intermediate and upper crust, where the seismic data are too scattered to allow a reliable interpretation. The gravity field is calculated along twelve cross-sections oriented S-N and crossing the Alpine range from the Padan-Venetian plain to the Bavarian molasse and to the Austrian calcareous Alps. The westernmost section coincides with the European Geotraverse while the easternmost one is positioned at the longitude of about 14ºeast. The assumed density model is very simple (only 6 layers); for each unit the density is maintained constant. The model describes a European mantle dipping southwards underneath an overlapping, uplifted Adriatic mantle. As far as the lower crust is concerned, its top is found at depths between 18 and 28 km, the deepest values being reached in the south-eastern sector; the density appears higher in the Adriatic domain than in the European one and the Adriatic lower crust seems to be deeply indented northwards. The low density surface layers appear very thin in a large area of the northwestern sector, while in the south and southeast their thickness reaches about 10 km. This study must be considered as a complement to the seismic interpretation both as a validation of the model of the deep crust and Moho boundary and as an additional source of information on the upper crust.
A «tidal» magnetic field?
Authors: F. Bocchio
Published: June 2021

t is shown that on the magnetization axis of a uniformly magnetized body of constant density the magnetic field intensity displays a «tidal» structure,i.e. the ratios among the differential magnetic field intensity in three orthogonal directions are the same as the ratios among the gravitational gradient tensor components pertaining to the same directions; it is also seen that the same characteristic ratios occur, both locally and non-locally, among the components of the magnetic field intensity and among the components of the gradient tensors of the two fields.
Agnostic geophysics
Authors: A. Cook
Published: June 2021

The physics of the Earth and the planets itself limits the knowledge that can be obtained of it. Two principle limitations are set in inverse problems and by chaotic dynamics; examples of each are given in this essay and some implications are discussed.
Observations of short period seismic scattered waves by small seismic arrays
Authors: E. Del Pezzo  , M. La Rocca  , R. Maresca  , M. Simini
Published: June 2021

The most recent observations of well correlated seismic phases in the high frequency coda of local earthquakes recorded throughout the world are reported. In particular the main results, obtained on two active volcanoes, Teide and Deception, using small array are described. The ZLC (Zero Lag Cross-correlation) method and polarization analysis have been applied to the data in order to distinguish the main phases in the recorded seismograms and their azimuths and apparent velocities. The results obtained at the Teide volcano demonstrate that the uncorrelated part of the seismograms may be produced by multiple scattering from randomly distributed heterogeneity, while the well correlated part, showing SH type polarization or the possible presence of Rayleigh surface waves, may be generated by single scattering by strong scatterers. At the Deception Volcano strong scattering, strongly focused in a precise direction, is deduced from the data. In that case, all the coda radiation is composed of surface waves.
The effect of arbitrarily small rigidity on the free oscillations of the Earth
Authors: F. Gilbert
Published: June 2021

The system of propagator equations for an elastic solid becomes singular as the shear modulus becomes vanishingly small. In computational applications there is severe loss of precision as the limit of zero shear modulus is approached. The use of perturbation theory to address the effect of very small shear modulus, using the fluid state as a basis, is unsatisfactory because certain phenomena, e.g., Rayleigh waves, cannot be represented. Two approximate methods are presented to account for the singular perturbation. Since most of the Earth is nearly neutrally stratified, in which case the motion is nearly irrotational, one can impose the irrotational constraint and obtain a modified and reduced system of propagator equations. This system does not have the singular perturbation. In the second method the transition zone between a fluid and a solid is represented as an infinitesimally thin, Massive, Elastic Interface (MEI). The boundary conditions across the MEI are dispersive and algebraic. The limit of zero shear modulus is non-singular.
Naturally occurring radionuclides and Earth sciences
Authors: G. Ferrara
Published: June 2021

Naturally occurring radionuclides are used in Earth sciences for two fundamental purposes: age determination of rocks and minerals and studies of variation of the isotopic composition of radiogenic nuclides. The methodologies that are in use today allow us to determine ages spanning from the Earth's age to the late Quaternary. The variations of isotopic composition of radiogenic nuclides can be applied to problems of mantle evolution, magma genesis and characterization with respect to different geodynamic situations and can provide valuable information not obtainable by elemental geochemistry.
Physical modelling of baroclinic development in the lee of the Alps
Authors: A. Longhetto  , L. Briatore  , G. Chabert d'Hieres  , H. Didelle  , E. Ferrero  , C. Giraud
Published: June 2021

When baroclinic development is triggered by an obstacle, like an extended mountain range, the so-called lee, or secondary cyclogenesis can develop. The presence of the obstacle exerts a blocking effect on the lower layers of the impinging airflow, forcing them to go round its borders and reach the lee region with a delay. Blocking and delay are both responsible for the initial pressure decrease downwind of the mountain and for the subsequent proper downstream baroclinic development. According to this rather simple scheme, a cyclogenesis episode in the lee of the Alps was simulated in a hydraulic turntable. The results of these experiments showed a good agreement, both from a qualitative and quantitative point of view, with the analysis of an episode of lee cyclogenesis coupled to a cold outbreak in the Mediterranean, which actually occured in Southern Europe downstream of the Alps.
On the clustering of seismicity in the Southern Tyrrhenian area
Authors: G. Luongo  , A. Mazzarella
Published: June 2021

High-quality shallow, intermediate and deep shocks from the Southern Tyrrhenian area (interval: 1985-1995) are analysed to draw inferences on the complex shape of the underlying Benioff zone. The hypothesis of an active NW-oriented subduction of the lithosphere, generated and stressed by the opening of the Tyrrhenian basin, is confirmed. The fractal behaviour of seismic data in its five-dimensional set (magnitude, time and space) is significantly identified, even if within specific ranges. The light but significant variations of the different fractal dimensions measured during more recent years is explained in terms of a reduction in the fracturing degree of the Southern Tyrrhenian crust.
Stress tensor computation from earthquake fault-plane solutions: an application to seismic swarms at Mt. Etna volcano (Italy)
Authors: S. Gresta  , C. Musumeci
Published: June 2021

Fault-plane solutions of some tens of local earthquakes which occurred at Mt. Etna volcano during 1983-1986 have been inverted for stress tensor parameters by the algorithm of Gephart and Forsyth (1984). Three seismic sequences were focused on which respectively occurred during a flank eruption (June 1983), just after the end of a subterminal eruption (October 1984) and during an inter-eruptive period (May 1986). The application to the three sets of data of both the "approximate" and the "exact" methods evidenced the stability of results, and the stress directions are well defined in spite of the small number of events used for the inversion. The s1 obtained agrees with the regional tectonic framework, nearly horizontal and oriented N-S, only in the shallow crust, and just after the 1984 eruption. This supports the hypothesis of a tectonic control on the end of the eruptive activities at Mt. Etna. Conversely, results concerning the depth range 10-30 km are in apparent disagreement with other investigations (Cocina et al., 1997), as well as with the regional tectonics. The stress was here found homogeneous, but with s1 respectively trending ENE-WSW (June 1983) and E-W (May 1986). We suggest that the stress field could be temporarily modified by a local stress regime driven by the intrusion of uprising magma.
Integer variables estimation problems: the Bayesian approach
Authors: F. Sansò  , G. Venuti
Published: June 2021

In geodesy as well as in geophysics there are a number of examples where the unknown parameters are partly constrained to be integer numbers, while other parameters have a continuous range of possible values. In all such situations the ordinary least square principle, with integer variates fixed to the most probable integer value, can lead to paradoxical results, due to the strong non-linearity of the manifold of admissible values. On the contrary an overall estimation procedure assigning the posterior distribution to all variables, discrete and continuous, conditional to the observed quantities, like the so-called Bayesian approach, has the advantage of weighting correctly the possible errors in choosing different sets of integer values, thus providing a more realistic and stable estimate even of the continuous parameters. In this paper, after a short recall of the basics of Bayesian theory in section 2, we present the natural Bayesian solution to the problem of assessing the estimable signal from noisy observations in section 3 and the Bayesian solution to cycle slips detection and repair for a stream of GPS measurements in section 4. An elementary synthetic example is discussed in section 3 to illustrate the theory presented and more elaborate, though synthetic, examples are discussed in section 4 where realistic streams of GPS observations, with cycle slips, are simulated and then back processed.
Interannual modulation of extratropical high frequency variability
Authors: R. Caballero
Published: June 2021

A simple explanation is presented for the observed interannual changes in the dominant space and time scales of Northem Hemisphere winter extratropical high frequency variability. It is found that such changes can suc- cessfully be predicted by linearizing a 2-level quasi-geostrophic mode] in spherical geometry around the ob- served zona] mean states. The mechanisms responsible for the selection of the most unstable normal mode are investigated.
Pre-collisional geodynamics of the Mediterranean Sea: the Mediterranean Ridge and the Tyrrhenian Sea
Authors: J. Mascle  , E. Chaumillon
Published: June 2021

Today the Mediterranean Sea consists of a series of small-sized and almost geographically disconnected oceanic or continental crust rooted marine basins. It is also an area almost totally surrounded by mountain ranges, which chiefly belong to the alpine realm. This overall geodynamic setting results from a long term convergence between the two major, African and European, plates. Previous collisions have led to the edification of surrounding chains, while subduction and new-collisional processes tend to create new extensional back-arc basins and wide tectonized accretionary prisms. In this paper we briefly outline the most recent and almost land-locked back-arc basin that has developed in the Mediterranean,i.e., the Tyrrhenian Sea, and the Mediterranean Ridge, which may be regarded as a collisional sedimentary wedge predating a future mountain chain.
Crustal stress regime in Italy
Authors: P. Montone  , A. Amato  , A. Frepoli  , M. T. Mariucci  , M. Cesaro
Published: June 2021

In order to obtain a reliable map of the present-day stress field in Italy, needed to better understand the active tectonic processes and to contribute to the assessment of seismic hazard, in 1992 we started to collect and analyze new data from borehole breakouts in deep oil and geothermal wells and focal mechanisms of earthquakes (2.5 < M <5) occurred in Italy between 1988 and 1995. From about 200 deep wells and 300 focal mechanisms analyzed to date, we infer that: the internal (SW) sector of the Northern Apenninic arc is extending with minimum compressional stress (Shmin) oriented ? ENE, while the external front is thrusting over the Adriatic foreland (Shmin ? NW-SE). The entire Southern Apennine is extending in NE direction (from the Tyrrhenian margin to the Apulian foreland) and compression (in the foredeep) is no longer active at the outer (NE) thrust front. Between these two arcs, an abrupt change in the tectonic regime is detected with directions of horizontal stress changing by as much as 90º in the external front, around latitude 430N. Along the Ionian side of the Calabrian arc the stress directions inferred from breakouts and focal mechanisms are scattered with a hint of rotation from N-S Shmin close to the Southern Apennines, to ~ E-W directions in the Messina Strait. In Sicily, a NW-SE direction of SHmax is evident in the Hyblean foreland, parallel to the direction of plate motion between Africa and Europe. A more complex pattern of stress directions is observed in the thrust belt zone, with rotations from the regional trend (NW í directed SHmax) to NE oriented SHmax. A predominant NW direction of SHmax is also detected in mainland Sicily from earthquake focal mechanisms, but no well data are available in this region. In the northern part of Sicily (Aeolian Islands) a ~N-S direction of SHmax is observed.
Electrical streaming potential precursors to catastrophic earthquakes in China
Authors: Y. Zhao  , B. Zhao  , F. Qian
Published: June 2021

The majority of anomalies in self-potential at 7 stations within 160 km from the epicentre showed a similar pattern of rapid onset and slow decay during and before the M 7.8 Tangshan earthquake of 1976. Considering that some of these anomalies associated with episodical spouting from boreholes or the increase in pore pressure in wells, observed anomalies are streaming potential generated by local events of sudden movements and diffusion process of high-pressure fluid in parallel faults. These transient events triggered by tidal forces exhibited a periodic nature and the statistical phenomenon to migrate towards the epicentre about one month before the earthquake. As a result of events, the pore pressure reached its final equilibrium state and was higher than that in the initial state in a large enough section of the fault region. Consequently, local effective shear strength of the material in the fault zone decreased and finally the catastrophic earthquake was induced. Similar phenomena also occurred one month before the M 7.3 Haichen earthquake of 1975. Therefore, a short term earthquake prediction can be made by electrical measurements, which are the kind of geophysical measurements most closely related to pore fluid behaviors of the deep crust.
The relations between seismically active and electrically conductive zones
Authors: I. G. Kissin  , A. I. Ruzajkin
Published: June 2021

The higher electrical conductivity of rocks in the middle and lower parts of the Earth's crust is generally related to the presence of fluids in rocks. The metamorphic processes of dehydration contribute to release of fluids, above all, water; these processes are also responsible for an increase in rock porosity and fracturing. These processes influence the stressed-strained state of the medium under specific conditions. A probable mechanism of earthquake source formation on the contact of blocks with different rates of dehydration and, consequently, different electrical conductivity is discussed. The spatial positions of electrically conductive and seismically active zones are correlated and definite relations between them are found with special reference to the vast area of the Northern Tien Shan within Kirgizstan and some other regions. The greatest concentration of earthquake sources is observed mainly near the contacts between blocks with contrastingly different electrical conductivity values and on sites with a sharp drop in conductive-layer depths.
Electrotelluric noise minimization for seismoelectric effect detection
Authors: B. Shirman  , A. Shapira
Published: June 2021

Three electrotelluric stations were installed in Israel in order to examine electric field changes associated with earthquake occurrence at the Bar-Giora observatory, at the Ein Gedi field school and on Mt. Tur'an. During the observation period 1994-1995, no earthquake was shown to cause any changes in the electric field at the measurement sites. This study focused on signal processing of electric field continuous measurements. The main purpose was to identify, and consequently filter out, signals caused by local and extra-terrestrial sources. New procedures for data processing were developed mainly using data from the Bar-Giora station, which has been instrumental in the experiments.
A search for the volcanomagnetic signal at Deception volcano (South Shetland I., Antarctica)
Authors: A. Garcìa  , I. Blanco  , J. M. Torta  , M. M. Astiz  , J. M. Ibáñez  , R. Ortiz
Published: June 2021

After the increase in seismic activity detected during the 1991-1992 summer survey at Deception Island, the continuous measurement of total magnetic intensity was included among the different techniques used to monitor this active volcano. The Polish geomagnetic observatory Arctowski, located on King George Island, served as a reference station, and changes in the differences between the daily mean values at both stations were interpreted as indicators of volcanomagnetic effects at Deception. A magnetic station in continuous recording mode was also installed during the 1993-1994 and 1994-1995 surveys. During the latter, a second magnetometer was deployed on Deception Island, and a third one in the vicinity of the Spanish Antarctic Station on Livingston Island (at a distance of 35 km) and was used as a reference station. The results from the first survey suggest that a small magma injection, responsible for the seismic re-activation, could produce a volcanomagnetic effect, detected as a slight change in the difference between Deception and Arctowski. On the other hand, a long term variation starting at that moment seems to indicate a thermomagnetic effect. However the short register period of only two stations do not allow the sources to be modelled. The future deployment of a magnetic array during the austral summer surveys, throughout the volcano, and of a permanent geomagnetic observatory at Livingston I. is aimed at further observations of magnetic transients of volcanic origin at Deception Island.
Rigorous time domain responses of polarizable media
Authors: M. Caputo
Published: June 2021

The scope of this note is to study a model of induced polarization which fits the usually accepted frequency dependent formula of Cole and Cole, but is more general and allows the time domain observations to retrieve the parameters describing the induced polarization phenomena of the medium. By introducing the memory mechanisms, represented by derivatives of fractional order, in the relation between the electric flux density and the electric field and considering the fractional order differential equation which follows, I solve it with mathematically rigorous and closed formulae and compute the responses to a step function, a box, a set of positive boxes and a set of alternating positive and negative boxes. I also introduce a method which retrieves the parameters describing the medium when comparing the theoretical curves with the observed ones. The responses to these signals also allow to estimate the temporary alteration of the medium when repeated positive (negative) signals are input; the response increases (decreases) in amplitude when the signals are all positive (negative), it decreases when the signals are alternatively positive and negative in agreement with the known attitude of the medium to induced polarization.
Standard geomagnetic observatory data in tectonomagnetism: case study related to the M 5.7 Timisoara, Romania, earthquake
Authors: M. Popeskov
Published: June 2021

There has recently been much discussion of large-scale interactions of fault zones and the influence of large-scale processes in the preparation and triggering of earthquakes. As a consequence, an official recommendation was issued to set up observational networks at regional scale. In this context, the existing network of standard geomagnetic observatories might play a more important role in future tectonomagnetic studies. The data from standard geomagnetic observatories are basically not appropriate for the detection of small-magnitude and, in most cases, spatially very localized geomagnetic field changes. However, their advantage is a continuity in a long-time period which enables the study of regional tectonomagnetic features and long-term precursory changes. As the first step of a more extensive study aimed at examining the features of observatory data for this purpose, a three-year data set from five European observatories has been analyzed. Some common statistical procedures have been applied along with a simple difference technique and multivariate linear regression to define local geomagnetic field changes. The distribution of M ³ 4.5 earthquakes in Europe, in a corresponding period, was also taken into account. No pronounced field variation, related in time to the M 5.7 Timisoara (Romania) earthquake on July 12, 1991, was found at Grocka observatory at about 80 km from the earthquake epicenter. However, an offset in level of the differences in declination which include Grocka observatory, not seen in the case of differences between other observatories, could be associated with a possible regional effect of the M 4.8 earthquake which occurred in September 1991 at about 70 km SE from Grocka.
Magnetic variation analysis for the June 1993 seismic events in Central Italy
Authors: T. Ernst  , R. Teisseyre  , A. Meloni  , P. Palangio  , M. Marchetti
Published: June 2021

A scientific collaboration between the Istituto Nazionale di Geofisica (Italy) and the Warsaw Academy of Science (Poland) gave rise to the installation of a few stations for the long-term measurement of magnetotelluric fields in Central Italy. The investigation sites were determined following the individual seismic interest of each location. For this project, the magnetic observatory in L'Aquila was also equipped with electric lines, for simultaneous measurements of the telluric field. After a few years of experience some of the installed stations had to be removed for their high noise level that made this study almost impossible. A first time interval was considered from January 1992 to February 1993 and showed the existence of significant changes in magnetotelluric parameters related to earthquake occurrence time, an extension of that analysis was made to include the event of June 1993 using the magnetic field time variation.
Seismic anisotropy in tomographic studies of the upper mantle beneath Southern Europe
Authors: J. Plomerovà
Published: June 2021

Regional seismic tomography of Iberia, Italy, the South Balkans and the Aegean region down to about 400 km are discussed along with results of studies on the anisotropic structure of the lithosphere based on an analysis of spatial variations of P-residuals. The P-residual spheres, showing azimuth-incidence angle dependent terms of relative residuals, map lateral changes of the anisotropic structure of the subcrustal lithosphere related to large tectonic units. Isotropic velocity perturbation models correlate, in general, with models of the lithosphere thickness but in some provinces they are affected by neglecting the anisotropic propagation within the lithosphere.
Tomographic image of the crust and uppermost mantle of the Ionian and Aegean regions
Authors: B. Alessandrini  , L. Beranzoli  , G. Drakatos  , C. Falcone  , G. Karantonis  , F. M. Mele  , G. N. Stavrakakis
Published: June 2021

We present a tomographic view of the crust and uppermost mantle beneath the Central Mediterranean area obtained from P-wave arrival times of regional earthquakes selected from the ISC bulletin. The P-wave velocity anomalies are obtained using Thurber's algorithm that jointly relocates earthquakes and computes velocity adjustments with respect to a starting model. A specific algorithm has been applied to achieve a distribution of epicentres as even as possible. A data set of 1009 events and 49072 Pg and Pn phases was selected. We find a low velocity belt in the crust, evident in the map view at 25 km of depth, beneath the Hellenic arc. A low velocity anomaly extends at 40 km of depth under the Aegean back arc basin. High velocities are present at Moho depth beneath the Ionian sea close to the Calabrian and Aegean arcs. The tomographic images suggest a close relationship between P-wave velocity pattern and the subduction systems of the studied area.
Compilation of a recent seismicity data base of the greater Alpine region from several seismological networks and preliminary 3D tomographic results
Authors: S. Solarino  , E. Kissling  , S. Sellami  , G. Smriglio  , F. Thouvenot  , M. Granet  , K. P. Bonjer  , D. Slejko
Published: June 2021

Local earthquake data collected by seven national and regional seismic networks have been compiled into a travel time catalog of 32341 earthquakes for the period 1980 to 1995 in South-Central Europe. As a prerequisite, a complete and corrected station list (master station list) has been prepared according to updated information provided by every network. By simultaneous inversion of some 600 well-locatable events we obtained one-dimensional (1D) velocity propagation models for each network. Consequently, these velocity models with appropriate station corrections have been used to obtain high-quality hypocenter locations for events inside and among the station networks. For better control, merging of phase data from several networks was performed as an iterative process where at each iteration two data sets of neighbouring networks or groups of networks were merged. Particular care was taken to detect and correctly identify phase data from events common to data sets from two different networks. In case of reports of the same phase data from more than one network, the phase data from the network owning and servicing the station were used according to the master station list. The merging yielded a data set of 278007 P and 191074 S-wave travel time observations from 32341 events in the greater Alpine region. Restrictive selection (number of P-wave observations >7; gap <160 degrees) yielded a data set of about 10000 events with a total of more than 128000 P and 87000 S-wave observations well suited for local earthquake seismic tomography study. Preliminary tomographic results for South-Central Europe clearly show the topography of the crust-mantle boundary in the greater Alpine region and outline the 3D structure of the seismic Ivrea body.
A geophysical study of a coastal plane in Sardinia
Authors: G. Ranieri  , L. Ferrero  , A. Godio
Published: June 2021

The paper deals with some aspects of the planning and the interpretation of the IP and resistivity measurements in the San Priamo coastal plane (Sardinia, Italy). The plane presents some hydrogeological problems related to previous flowages and salt water intrusion from the sea. The geophysical results verified the good resolution of the joint elaboration of the IP and the resistivity soundings, in order to delineate some hydrogeological aspects of the San Priamo coastal plane. In particular, the technique was suitable for detecting salt water intrusion in a fresh acquifer, with good reliability.
Central Italy magnetotelluric investigation. Structures and relations to seismic events: analysis of initial data
Authors: A. Meloni  , P. Palangio  , M. Marchetti  , E. Teisseyre  , J. Marianiuk  , T. Ernst
Published: June 2021

A scientific collaboration between the Warsaw Academy of Science, (Poland) and the National Institute of Geophysics (Italy), gave rise to the installation of few stations for the long term measurement of magnetotelluric fields in central Italy. The selection of investigation sites was determined by the individual seismic interest of each location. The project began in the summer of 1991, with the installation of 2 magnetotelluric stations in the province of Isernia, (Collemeluccio and Montedimezzo). In 1992, 2 more stations became operative, one in the province of Rieti, (Fassinoro), the other in the province of L'Aquila, (S. Vittoria). For the purpose of this project, the magnetic observatory in L'Aquila was also equipped with electric lines, for the measurement of the telluric field. The aim of the analysis here presented, is to show that is possible to follow the temporal evolution of magnetotelluric characteristic parameters. At Collemeluccio this evolution was compared with the seismic released energy for events recorded within the study area.
On the persistency of crater assignment criteria for Stromboli explosion-quakes
Authors: R. Carniel  , F. Iacop
Published: June 2021

On the basis of the results of previous works by our group, this paper aims to investigate the correlation between features of a kind of seismic events recorded at Stromboli (the so called explosion-quakes) and the source of such explosions, i.e. the different craters. The purpose is that of finding parameters in order to try an automatic assignment of new events to their crater of origin. These parameters must be searched for both in time and in frequency domain. Afterwards the stability in time of the discrimination is discussed. An important « side effect» of this analysis is the discovery that the percentage of summital explosions not producing seismic signals recordable by a 2 s seismometer is not negligible.
A preliminary survey of the broadband seismic wavefield at Puu Oo, the active vent of Kilauea volcano, Hawaii
Authors: D. Seidl  , M. Hellweg  , P. Okubo  , H. Rademacher
Published: June 2021

The seismic wavefield near an active volcanic vent consists of superimposed signals in a wide range of frequency bands from sources inside and outside the volcano. To characterize the broadband wavefield near Puu Oo, we deployed a profile of three three-component broadband sensors in a 200 m long line about 1.5 km WSW of the active vent. During this period, Puu Oo maintained a constant, but very low level of activity. The digital data logger recorded the wavefield continuously in the frequency band between 0.01 and 40 Hz between June 25 and July 9, 1994. At the same time, local wind conditions along with air temperature and pressure were monitored by a portable digital weather station. On the basis of characteristic elements, such as waveform, spatial coherence between stations, particle motion and power spectra, the wavefield can be divided into three bands. The dominant signals in the frequency band between 0.01 and 0.1 Hz are not coherent among the stations. Their ground velocities correlate with the wind speed. The signals in the 0.1 to 0.5 Hz band are coherent across the profile and most probably represent a superposition of volcanic tremor and microseisms from the Pacific Ocean. Much of the energy above 0.5 Hz can be attributed to activity at the vent. Power spectra from recordings of the transverse components show complex peaks between 0.5 and 3 Hz which vary in amplitude due to site effects and distance. On the other hand, power spectra calculated from the radial components show a clearly periodic pattern of peaks at 1 Hz intervals for some time segments. A further remarkable feature of the power spectra is that they are highly stationary.
Features of seismic events and volcanic tremor during the preliminary stages of the 1991-1993 eruption of Mt. Etna
Authors: G. Lombardo  , G Coco  , M. Corrao  , S. Gresta
Published: June 2021

The study of the spectral features of volcanic tremor and low frequency events (l.f.e.) recorded before and during the preliminary phases of the powerful 1991-1993 eruption of Mt. Etna is briefly described. Significant modifications were observed in the spectral signature of l.f.e. before the onset of the eruptive event, as well as in the temporal distribution of the volcanic tremor dominant frequencies. We interpret both l.f.e. and tremor changes in terms of a spatial modification of the source, as the paroxysmal eruptive activity is approaching. Such findings also appear quite interesting for the identification of markers of the modifications which some seismic events of the volcano undergo in the early stages heading the occurrence of an eruption.
Unusual seismic signals associated with the activity at Galeras volcano, Colombia, from July 1992 to September 1994
Authors: R. A. Torres C.  , D. M. Gómez M.  , L. Narvàez M.
Published: June 2021

After the emplacement of a lava dome at Galeras volcano in 1991, seven eruptions occurred from July 16, 1992, to September 23, 1994, six of which were preceded by quasi-monochromatic, long-duration seismic events with slowly decaying coda named «tornillos» (screws). The dominant frequencies of these unusual seismic signals are related to source characteristics and show temporal changes, diminishing and then tending to stabilize before an eruption. At the same time, the accumulated number and the duration of these signals increase several days prior to the eruption. The increase in the duration of the tornillo events and the decline of the dominant frequencies both suggest an increasing impedance contrast between the surrounding solid material and the fluid. These characteristics may be associated with an increase in the free gas phase in the magma produced by saturation of volatiles due to cooling, crystallization and partial solidification of the column of magma plugging the conduits. The solidified magma can contribute to sealing the conduits and preventing free gas escape, with consequent generation of overpressure. An eruption is initiated when the overpressure exceeds the resistance strength of the solid material.
Spectral precursors of paroxysmal phases of Stromboli
Authors: R. Carniel  , F. Iacop
Published: June 2021

In this work we investigate the characteristics of the seismicity at Stromboli volcano during more than two years, i.e. from 11 May 1992 to 21 August 1994. The three paroxysmal phases of 1993 mark significant changes in the Strombolian activity; nevertheless, these are not the only ones observed. In fact, the energy content, both in terms of volcanic tremor and of number of events drops to very low values after the periods of intense activity, accompanied by a change in the spectral content of the tremor. However, equally abrupt changes in the frequency content, not accompanied by evident intensity variations, can be observed some weeks after the end of the crises. The volcano seems therefore to behave like a dynamical system with many «quite stable » states with abrupt transitions between them. An interesting observation is the appearance of an energy concentration in the spectral sectors below 3 Hz before more violent eruptive episodes; although the duration of such a phenomenon is variable, it has to be investigated as a possible precursor of potentially dangerous activity of the volcano. A continuous monitoring of the spectral content of volcanic tremor on Stromboli is confirmed to be an essential tool in order to understand the behaviour of Stromboli volcano and to try to forecast its paroxysmal phases.
Shear-wave polarization alignment on the eastern flank of Mt. Etna volcano (Sicily, Italy)
Authors: F. Bianco  , M. Castellano  , G. Milano  , G. Vilardo
Published: June 2021

Recently, with the improvement of three-component seismic networks, studies revealing anisotropic characteristics in different regions have assumed great interest. In a complex volcanic area like Mt. Etna (Sicily, Italy), the existence of both iso-oriented fault systems and intrusive bodies consisting of olivine and clinopyroxene suggest the presence of anisotropic structures. In order to investigate this we analyzed the physical phenomenon of shear-wave splitting since under certain constraints, shear waves are less sensitive to local heterogeneity. The aims of this paper are: 1) to evaluate if in a structural complex situation like that at Mt. Etna the signal crossing an anisotropic volume could be enhanced in spite of effects due to undirectional properties along the source-receiver path; 2) to investigate the correlations, if any, between polarization direction of the leading shear wave and the patterns of compressive stress acting on the investigated area. Therefore we measured time-delays between the S-onsets on the horizontal components of 3D seismograms to reveal the possible seismic anisotropy in the Etnean region; moreover, we analyzed the polarization vector of shear-waves seismic data recorded during a survey carried out in the spring-summer 1988. We found clear evidence of splitting that we attributed to the presence of an anisotropic volume not homogeneously distributed on the eastern slope of Mt. Etna volcano.
2D seismic tomography of Somma- Vesuvius. Description of the experiment and preliminary results.
Authors: A. Zollo  , P. Gasparini  , G. Biella  , R. de Franco  , B. Buonocore  , L. Mirabile  , G. De Natale  , G. Milano  , F. Pingue  , G. Vilardo
Published: June 2021

A multidisciplinary project for the investigation of Mt. Vesuvius Structure was started in 1993. The core of the project is represented by a high resolution seismic tomography study by using controlled and natura1 sources. The main research objective is to investigate the feeding system of the vo1cano and to retrieve details of the upper crustal structure in the area. A first 2D using seismic experiment was performed in May 1994, with the aim of studing the feasibility of lIsing tomographic techniques for exploring the vo1cano interiors. Particularly, this experiment was designed to obtain information on the optimal sources-receivers configuration and on the depth extension of the volume sampled by shot-generated seismic waves. 66 three-component seismic stations and 16 single-component analogue instruments were installed by several Italian and French groups to record signals generated by three on-land, underground explosions. Sources and geophones were deployed along a 30-km NW-SE profile passing through the volcano crater. Receivers were placed at an average spacing of 250 m in the middle of the recording line and at 500 m outside. The arrival time data base was complemented by first P and S readings of micro earthquakes which occurred in the recent past within the volcano. The first arrival data set was preliminary used to determine the shallow structure of the volcano by applying Thurber's (1983) tomographic inversion technique. This analysis shows evidence for a high-velocity body which extends vertically from about 400 m below the crater down to at least 3000 m and for a shallow 300-500 m thick low-velocity cover which borders the edifice. Data from the distant shot show evidence for arrivals of deep reflected/converted phases and provide information on the deeper structure under the volcano. The results from the interpretation of 2D data are used for planning a 3D tomographic survey which will be cauied out in 1996.