bluuebunny/crossref_metadata_2025 huggingface dataset:
Autoregressive models as a tool to discriminate chaos from randomness in geoelectrical time series: an application to earthquake prediction
Authors: V. Cuomo  , V. Lapenna  , M. Macchiato  , C. Serio
Published: June 2021

The time dynamics of geoelectrical precursory time series has been investigated and a method to discriminate chaotic behaviour in geoelectrical precursory time series is proposed. It allows us to detect low-dimensional chaos when the only information about the time series comes from the time series themselves. The short-term predictability of these time series is evaluated using two possible forecasting approaches: global autoregressive approximation and local autoregressive approximation. The first views the data as a realization of a linear stochastic process, whereas the second considers the data points as a realization of a deterministic process, supposedly non-linear. The comparison of the predictive skill of the two techniques is a test to discriminate between low-dimensional chaos and random dynamics. The analyzed time series are geoelectrical measurements recorded by an automatic station located in Tito (Southern Italy) in one of the most seismic areas of the Mediterranean region. Our findings are that the global (linear) approach is superior to the local one and the physical system governing the phenomena of electrical nature is characterized by a large number of degrees of freedom. Power spectra of the filtered time series follow a P(f) = F-a scaling law: they exhibit the typical behaviour of a broad class of fractal stochastic processes and they are a signature of the self-organized systems.
Anisotropy of magnetic susceptibility of rocks induced by experimental deformation
Authors: J. Hao  , Z. Gu  , J. Zhou
Published: June 2021

In the present paper, the influence of the rheological process on the Anisotropy of Magnetic Susceptibility (AMS) of rocks is studied experimentally. The cylindrical samples of quartz-magnetite rock undergo a process under the confining stress of 300 MPa, temperature of 500-800 °C and strain rate of 5 ´ 10-5 - 1 ´ 10-4/s. The residual deformation after the above process ranges 9-42%, depending on the experimental condition. It is found that the magnetic susceptibilities and the shapes of magnetic grains in these samples are almost isotropic before deformation. After being deformed, these samples show certain amounts of anisotropy of magnetic susceptibility and the axes of maximum principal susceptibilities deviate from the original ones more or less. Furthermore, the grains become oblate-ellipsoidal and a certain preferred orientation occurs. The grain shape anisotropy seems to be the main reason for AMS formation. It appears that there is a limitation of the piezomagnetic theory in explaining some tectonomagnetic phenomena. The results obtained in this study imply that ductile deformation at high temperature and pressure in depth during a long time-process may result in another kind of response in rock magnetism, which could be a new mechanism of tectonomagnetic variation.
Geomagnetic changes correlated with crustal movement in the north-eastern part of the Izu Peninsula, Japan
Authors: N. Oshiman  , Y. Sasai  , Y. Honkura  , Y. Ishikawa  , Y. Tanaka
Published: June 2021

After the 1989 sea-floor eruption off the east coast of Ito city, no remarkable activities of earthquake swarms were observed in the eastern part of the Izu Peninsula, Central Japan during the period from 1990 to 1992. However, a small swarm activity was again observed in January, 1993 and a remarkable one took place again in May-June 1993. Several months after the subsidence of the swarm activity, abrupt changes in the crustal movement in the inland of the peninsula were observed during the period from September 1993 to February 1994. At some continuous observation sites, well correlated changes in the geomagnetic total intensity were observed almost during the same period when the anomalous changes in the crustal movement were seen in the eastern part of the peninsula. The spatial patterns of negative changes of the total intensity in the northern half and positive ones in the south were seen in the north-eastern edge of the domed distribution of the upheaval. The changes in crustal movement and the geomagnetic field terminated when a small swarm activity occurred at the end of February 1994.
Adaptive regridding in 3D reflection tomography
Authors: G. Böhm  , G. Rossi  , A. Vesnaver
Published: June 2021

3D reflection tomography allows the macro-model of complex geological structures to be reconstructed. In the usual approach, the spatial distribution of the velocity field is discretized by regular grids. This choice simplifies the development of the related software, but introduces two serious drawbacks: various domains of the model may be poorly covered, and a relevant mismatch between the grid and a complex velocity field may occur. So the tomographic inversion becomes unstable, unreliable and necessarily blurred. In this paper we introduce an algorithm to adapt the grid to the available ray paths and to the velocity field in sequence: so we get irregular grids with a locally variable resolution. We can guide the grid fitting procedure interactively, if we are going to introduce some geological a priori information; otherwise, we define a fully automatic approach, which exploits the Delauny triangles and Voronoi polygons.
Non-linear arrival time tomography
Authors: C. B. Papazachos  , G. Nolet
Published: June 2021

The use of 1D or pseudo- 3D ray tracing techniques in linearized tomographic problems leads to solutions for which it is difficult to assess the true resolution and error distribution. For this reason, we employ a revised 3D bending algorithm (Moser et al., 1992) and show that it can be used efficiently for a non-linear inversion in a stepwise scheme. Initial paths are determined from graph theory in order to avoid local minima in bending. The importance of 3D ray tracing in inversion studies and the limitations of the standard 1D approach are demonstrated through synthetic examples. The speed of the ray tracing and the simple scaling scheme allow for an implementation in large-scale tomographic problems.
A tomography image of the Aegean region (Greece) derived from inversion of macroseismic intensity data
Authors: G. N. Stavrakakis  , G. Drakatos  , G. Karantonis  , D. Papanastassiou
Published: June 2021

The three-dimensional attenuation structure beneath the Aegean sea and the surrounding regions was determined by inversion of seismic intensity data. A large number of seismic intensity data have been accumulated in a uniform scale in the Aegean region, where the seismic activity is much higher than that of the other Mediterranean regions. Nearly 11000 seismic intensity data from 47 earthquakes that have occurred in these regions were used to determine the seismic attenuation structure. The resulting structure reveals a remarkable contrast of attenuation. In the top layer (depth 0-20 km), low Q is dominant in the central Aegean sea, while high Q is dominant in the surrounding land areas, except for Southwestern Turkey. The low-Q regions correspond to areas of Neogene-Quaternary grabens where the high seismicity of shallow earthquakes appears. In the lower layer (20-40 km) low-Q areas are located in the southeastern part of the Hellenic arc. Some low-Q spots corresponding to the distribution of volcanoes exist along the volcanic arc. The low-Q spots might correspond to diapirs causing subduction volcanism.
Bootstrap inversion for Pn wave velocity in North-Western Italy
Authors: S. Parolai  , D. Spallarossa  , C. Eva
Published: June 2021

An inversion of Pn arrival times from regional distance earthquakes (180-800 km), recorded by 94 seismic stations operating in North-Western Italy and surrounding areas, was carried out to image lateral variations of P-wave velocity at the crust-mantle boundary, and to estimate the static delay time at each station. The reliability of the obtained results was assessed using both synthetic tests and the bootstrap Monte Carlo resampling technique. Numerical simulations demonstrated the existence of a trade-off between cell velocities and estimated station delay times along the edge of the model. Bootstrap inversions were carried out to determine the standard deviation of velocities and time terms. Low Pn velocity anomalies are detected beneath the outer side of the Alps (-6%) and the Western Po plain (-4%) in correspondence with two regions of strong crustal thickening and negative Bouguer anomaly. In contrast, high Pn velocities are imaged beneath the inner side of the Alps (+4%) indicating the presence of high velocity and density lower crust-upper mantle. The Ligurian sea shows high Pn velocities close to the Ligurian coastlines (+3%) and low Pn velocities (-1.5%) in the middle of the basin in agreement with the upper mantle velocity structure revealed by seismic refraction profiles.
Upper mantle velocity structure beneath Italy from direct and secondary P-wave teleseismic tomography
Authors: G. B. Cimini  , P. De Gori
Published: June 2021

High-quality teleseismic data digitally recorded by the National Seismic Network during 1988-1995 have been analysed to tomographically reconstruct the aspherical velocity structure of the upper mantle beneath the Italian region. To improve the quality and the reliability of the tomographic images, both direct (P, PKPdf) and secondary (pP,sP,PcP,PP,PKPbc,PKPab) travel-time data were used in the inversion. Over 7000 relative residuals were computed with respect to the IASP91 Earth velocity model and inverted using a modified version of the ACH technique. Incorporation of data of secondary phases resulted in a significant improvement of the sampling of the target volume and of the spatial resolution of the heterogeneous zones. The tomographic images show that most of the lateral variations in the velocity field are confined in the first ~250 km of depth. Strong low velocity anomalies are found beneath the Po plain, Tuscany and Eastern Sicily in the depth range between 35 and 85 km. High velocity anomalies dominate the upper mantle beneath the Central-Western Alps, Northern-Central Apennines and Southern Tyrrhenian sea at lithospheric depths between 85 and 150 km. At greater depth, positive anomalies are still observed below the northernmost part of the Apenninic chain and Southern Tyrrhenian sea. Deeper anomalies present in the 3D velocity model computed by inverting only the first arrivals dataset, generally appear less pronounced in the new tomographic reconstructions. We interpret this as the result of the ray sampling improvement on the reduction of the vertical smearing effects.
The Siena Graben: combined interpretation of DES and MT soundings
Authors: S. Giammetti  , D. Patella  , A. Siniscalchi  , A. Tramacere
Published: June 2021

The Siena Graben study area is located in Tuscany, Central Italy. The local geological structures were extensively studied in the frame of the Italian National Research Council (CNR) and the Commission of the European Communities (CEC) geothermal projects. Axial dipole-dipole geoelectrical soundings (DES) were also performed. Recently we carried out 13 broadband magnetotelluric (MT) soundings, most of which were located very close to the station sites of the mentioned DES. For six of them we made a DES-MT combined interpretation in order to put in evidence resistivity frequency dispersion effects. Indeed, four sites showed DES-MT anomalous responses, in the frequency range 1 ¸ 102 Hz, which can be properly explained as due to dispersion effects in shallow layers. Three of these anomalous sounding sites are located on the neoautochthonous clayey formation, while the fourth is located on the eastern boundary of the graben over an extensive outcrop of the «Macigno» complex. The dispersion-affected soundings are all located in the northern part of the graben, while there is no evidence of such effects in the southern part. This circumstance, together with the estimate of very low time constants of the fitting Cole-Cole dispersion model, can be tentatively explained as due to local and shallow lithological effects (clay-like membrane polarization) rather than to deep geothermal effects (sulphide-like electrode polarization). Moreover, the MT soundings delineated a conductive zone in the upper crust below the resistive geoelectrical basement, located in the northern part of the graben, which appears at present difficult to interpret. Furthermore, the combined analysis of the DES and MT soundings in the same sites has allowed us to resolve one of the most intriguing ambiguities concerning the determination of the depth to the graben basement. The result is a remarkable reduction of the depth to the basement top, especially in the northern part of the graben. Finally, the easternmost MT soundings, located on the resistive outcrops of the massive calcareous formations of the basement and of the «Macigno» complex, permitted us to investigate almost all the lithospheric slab, thanks to the locally greater skin depth penetration. The lithosphere-asthenosphere transition is very well delineated, whereas the crust-mantle transition seems not to have an evident electrical signature.
The spatial amplitude distribution of volcanic tremor at Stromboli volcano (Italy)
Authors: J. U. Mohnen  , R. Schick
Published: June 2021

A portable seismic station consisting of a three-component seismometer in conjunction with a spectral analyzer was deployed in May and June 1994 to record volcanic tremor in a wide area on Stromboli. For the reduction of path effects, tremor spectra were averaged over 164 observation points. They illustrate smooth and broadband spectral lobes in the frequency range between 1-12 Hz. It is suggestive that these overall spectra represent in a first approximation the spectral radiation caused by source processes. Identical and significant maxima at 2.65 Hz and 3.65 Hz are found in all components. No systematic distinction is found in the amplitude values and spectral forms for either horizontal component. The amplitude of the vertical component presents approximately one third that of the horizontal components. A subclassification of the data according to geological strata shows frequency dependent amplitude amplifications. Thick ash and lapilli beds reach a factor of four within frequencies from 4-6 Hz. The influence of these site effects seems minor below 2 Hz. The paper presents maps for each of the three components showing the distribution of the tremor amplitudes averaged over areas of 150 m by 150 m. Model curves derived from fluid-flow acoustics are compared with the tremor spectra.
Low frequency events at Mt. Etna: some problems and open questions
Authors: S. Alparone  , S. Gresta
Published: June 2021

A short period seismic array setting at Mt. Etna symmetrically in regard to the fracture of the 1991-1993 eruption allowed an analysis of low frequency events which occurred in the first phase of the mentioned eruption. We recorded about 50 events, 19 of them belong to a family. They show very low amplitude values and spectral peaks ranging 0.5-4.5 Hz. The evidence of this family of events shows how the process driving the dynamics of the fluid in the volcano is often the same.
High resolution geomagnetic field observations at Terra Nova bay, Antarctica
Authors: S. Lepidi  , U. Villante  , M. Vellante  , P. Palangio  , A. Meloni
Published: June 2021

he preliminary results obtained from the analysis in the micropulsation frequency range of high time resolution magnetic field data recorded at the Antarctic Italian geomagnetic observatory at Terra Nova Bay for 11 consecutive days in February 1994 are reported. The spectral index over the whole Pcl-Pc5 frequency range is of the order of 3.5 and its value significantly increases beyond about 50 mHz. Spectral peaks in the Pc3 frequency range are common, especially during the daytime hours, and are probably due to the direct penetration of upstream waves in the cusp region. From the local time distribution of the micro pulsation power, a signifi - cant activity enhancement around the local magnetic noon emerges, in agreement with previous observations. The analysis of the signal polarisation characteristics in the horizontal plane shows a predominant CW polarisation in the Pcl-Pc3 frequency ranges with the major axis of the polarisation ellipse in the first quadrant.
Vertical electron density profiles from digisonde ionograms. The average representative profile
Authors: X. Huang  , B. W. Reinisch
Published: June 2021

Profile calculations from ionograms using the Huang-Reinisch technique arrive at a set of boundary values and coefficients that describe the profile. From an ensemble of such sets an Average Representative Profile (ARP) is derived which is again expressed in terms of boundary values and coefficients.
The ionospheric storm studies: further development of the mapping technique
Authors: I. Kutiev  , T. Samardjiev  , P. A. Bradley  , M. I. Dick  , L. R. Cander
Published: June 2021

The technique of using instantaneous maps for ionospheric storm studies is further developed. Integral parameters are introduced characterizing the main features of each map. These parameters are the net volumes of ?f0F2, ?M(3000)F2and their gradients. The magnetic storm 1-2 March, 1982 was considered and it was found that before the storm commencement and in recovery phase the Net Gradient (NG) is directed steadily to the East, while in the main phase it turns southward. NG shows where the changes of the F-layer come from. The net volume of ?f0F2 (NF) correlates well with Dst and AE indices.
Do seismologists agree upon epicentre determination from macroseismic data? A survey of ESC Working Group ' Macroseismology'
Authors: I. Cecic  , R. M. W. Musson  , M. Stucchi
Published: June 2021

In contrast to the case of instrumental data, the procedures for epicentral parameter determination (coordinates and I0) from macroseismic data are not very well established. Although there are some "rules", upon which most seismologists agree (centre of the isoseismal of largest degree, and so on), the practical application of, such rules displays many problems. Therefore, it is commonly seismologists' practice to find their own pro cedures and solutions; this is particularly evident in the more complicated cases, Such as offshore epicentres or, as in many cases of historical earthquakes, poor sets of data. One of the major consequences is that parametric catalogues are not homogeneous with respect to macroseismic parameters; moreover, merging catalogues compiled according to different criteria can introduce high noise in any catalogue built in such a way. In order to survey the current practice of epicentre determination from macroseismic data in Europe, a set of cases was distributed to the participants of the first meeting of the ESC WG "Macroseismology". A comparison of the 15 sets of results provided by 16 authors, who gave their own solutions and the explanation., of the adopted procedures is given, showing that in some cases the ideas and results are rather distant.
Some geophysical constraints to dynamic processes in the Southwestern Mediterranean
Authors: V. Pasquale  , M. Verdoya  , P. Chiozzi
Published: June 2021

The total tectonic subsidence, thermal state and seismotectonic regime have been analysed to better constrain the dynamic processes which originated the basins of the Southwestern Mediterranean. It is argued that backarc extension and oceanic spreading are the possible and main processes which took place within a compressional framework, driven by the interaction between the African and European plates. As inferred by both subsidence and heat-flux data, in the central part of the Algerian-Balearic basin the crust is oceanic, 20 Ma old on average, originated by a spreading phase, which also affected the Ligurian-Provençal basin. The Alboran basin, which is underlain by stretched continental crust, shows an intermediate seismic activity and a few deep events, explainable by a gravitational collapse of cold lithosphere. After a review of the most recent geodynamical hypotheses, an evolutionary scheme is attempted envisaging the lateral continental escape of the Gibraltar arc. Within a convergent tectonic framework, some lithospheric material could translate almost perpendicular to the convergence direction, and undergo a lateral subduction process, secondary to the main boundary between plates.
Geomagnetic jerks and seismic activity
Authors: F. Florindo  , L. Alfonsi  , A. Piersanti  , G. Spada  , W. Marzocchi
Published: June 2021

By means of a Monte Carlo simulation, we explored the extent to which large earthquakes may induce fluctuations on the secular variation of the Earth's magnetic field. We speculate that the energy released by large earthquakes may perturb the dynamical interaction between the outer core and the lower mantle, and consequently give rise to rapid variations of the main field detectable on the Earth's surface (geomagnetic jerks). In order to test the existence of a possible correlation between the seismic activity and the occurrence of magnetic jerks, we examined both the number of earthquakes and the total moment released in a lapse of time preceding the known geomagnetic jerks of this century. Our results are compatible with the hypothesis of a causal relationship between periods of time characterized by an anomalous global seismic activity and rapid changes of the main magnetic field. Several possible mechanisms accounting for such a correlation are discussed.
From Crimea to Syria. Re-defining the alleged historical earthquake of 63 B.C.
Authors: G. Traina
Published: June 2021

Within the SGA research on the historical seismicity of the Crimean Peninsula (SGA Report, 1990), interest has been focused on the case of the earthquake of 63 B.C. According to regional seismic catalogues as well as to historic and archaeological literature, two late Roman sources. Dio Cassius and Paulus Orosius, allegedly give evidence of an earthquake which happened in the Crimea in this year; the event was linked to the death of Mithridates V1 Eupator, eventually the king of Pontos. Local archaeologists claimed to have found evidence of this event in the excavations of Panticapaeunl (present-day Ker?). In fact. this is the result of a restricted analysis of the written sources. Thence stems a sort of iivulgatan. currently accepted by scholarship, yet not really supported by the evidence. A re-examination of the whole question, including an analysis of all sources avalaible on earthquakes in the Eastern Mediterranean. showed that in that period no seismic event took place in the Crimea. Dio's and Orosius' accounts are instead concerned with another earthquake, already known for Syria from other sources. This historical case gives a proper methodological example of the problems concerned with the analysis of the evidence in historical seismology. not only of Antiquity, but of almost any premodern period.
From historical earthquake records to intensity value: some results of a study
Authors: G. Monachesi  , A. Moroni
Published: June 2021

The use of historical records to assess macroseismic intensity involves problems seldom evidenced. This paper deals with the problems connected with the use of macroseismic scales, with particular attention to MSK 81 and the EMS 92, in the case of two Italian localities (Fabriano in the Marche region and Orciano in North- Western Tuscany) damaged by two earthquakes (1741. April 24 and 1846, August 14) which have a good historical documentation. Nevertheless, it is difficult to obtain the data required by the scales. It follows that intensity estimates are in principle affected by some uncertainties which have been analysed.
A study of local site effects in Benevento(Southern Italy) by the analysis of seismic records of explosions
Authors: G. Iannaccone  , L. Improta  , G. Biella  , M. Castellano  , A. Deschamps  , R. De Franco  , L. Malagnini  , L. Mirabile  , R. Romeo  , A. Zollo
Published: June 2021

In this paper we evalllate the seismic amplification effects in the town of Benevento llsing records of an explo- sion of 500 kg fired at abollt 7 km. Seismic records were obtained at 43 selected sites in the city by digital three-component stations. A data selection performed on the signal-to-noise ratio reduced the available data to 26 stations. We used the spectral ratio techniques to evaluate the amplification effects of each recording site compared both to the average spectrum calculated over 26 stations and to a reference seismic station. The shapes of the spectral ratios were classified according to the geological characteristics of the site. A characteristic shape of the spectral ratio was observed to be related to the specific structure of the subsoil. In particular, the sites on basin sediments (Lagonegro Unit) and Middle Pleistocene conglomerates failed to show amplification effects; the sites on thick alluvial deposits showed amplification in the 5-9 Hz range; finally, sites on alluvial-lacustrine deposits amplified the seismic signal at frequencies depending on the characteristics and the thickness of the deposit. In addition, damage distribution caused by the 1688 earthquake in Benevento was related to the thickness of the surface layers in the ancient built-up area of the town. The study of the spectral ra- tios showed that these deposits amplify ground motion at frequencies between 9 and 12 Hz, i.e. frequencies close to the natural period of the most widespread buildings at that time in Benevento. Geological and seismic data were jointly used to carry out a zonation of the urban area of Benevento on the basis of homogeneous seismic responses. The validity of this analysis is limited to the main frequency band and amplitude of ground motion produced by the explosions.
Enhanced variability in the topside ionosphere
Authors: T. Gulyaeva  , P. Spalla
Published: June 2021

Variability of total electron content (TEC) observed by the Faraday rotation method at Florence has been stud- ied with the same technique applied independently to the ionospheric parameters foF2 and M(3000)F2 of the ground-based vertical-incidence sounding database (VID). Results of daily and monthly TEC disturbance indices at sub-ionospheric point are compared with variability of the ionosphere at Rome and Gibilmanna (de- duced from VID) for a period of 1976 to 1991. During moderate and high solar activity the variability of TEC is greater than the variability of VID, whereas during solar minimum the situation is opposite. In this context joint TEC and VID observations distinguish either the F region peak or the topside ionosphere heights where the dynamic processes dominate at different times.
Strong earthquakes and geomagnetic jerks: a cause-effect relationship?
Authors: F. Florindo  , L. Alfonsi
Published: June 2021

Secular variation of the geomagnetic field observed at the Earth's surface has been found to undergo impulsive accelerations Gerks) lasting less than a few years. In this paper the relations between jerks and the occurrence of strong earthquakes (Ms ? 7.0) is analysed for this century , disclosing a positive correlation between the maximum number of recorded strong earthquakes and jerk occurrence. Analysing only very strong earth- quakes (Ms ? ; 8.0) the jerks seem to take pIace with a time delay of about 2-5 years with respect to earthquake occurrence. Reliable processes that could justify this intriguing correspondence are suggested.
Focal mechanism of seismic events with a dipolar component
Authors: F. Batini  , M. Caputo  , R. Console
Published: June 2021

In this paper we model the geometry of a seismic source as a dislocation occurring on an elemental flat fault in an arbitrary direction with respect to the fault plane. This implies the use of a fourth parameter in addition to the three usual ones describing a simple double couple mechanism. We applied the radiation pattern obtained from the theory to a computer code written for the inversion of the observation data (amplitudes and polarities of the first onsets recorded by a network of stations). It allows the determination of the fault mechanism gener- alized in the above mentioned way. The computer code was verified on synthetic data and then applied to real data recorded by the seismic network operated by the Ente Nazionale per l'Energia Elettrica (ENEL), monitoring the geothermal field of Larderello. The experimental data show that for some events the source mechanism exhibits a significant dipolar component. However, due to the high standard deviation of the amplitude data, F-test applied to the results of the analysis shows that only for two events the confidence level for the general- ized model exceeds 90%.
Paleomagnetism of the Plio-Pleistocene continental sediments from the north -eastern edge of the Fucino basin (Central Italy)
Authors: L. Alfonsi  , L. Sagnotti  , F. Galadini
Published: June 2021

The paleomagnetism of the Plio-Pleistocene continental sediments cropping out at the north-eastern edge of the Fucino extensionaI basin (Italy, Central Apennines) was investigated. The area is characterized by strong neo-tectonic activity and the original purpose was to investigate possibIe verticaI axis rotations in Plio-Pleisto- cene sediments, in order to improve the understanding of the recent geodynamic processes. Scarcity of suit- able outcrops limited sampling at 8 sites (83 specimens) from the north-eastern edge of the basin, in clay-rich intervals beIonging to two different sedimentary cycIes. The paleomagnetic resuIts pointed out a peculiar mag- netic behaviour common to the whole set of studied samples. The Natural Remanent Magnetization (NRM) is dominated by a vigcous normal component acquired under the influence of the present geomagnetic field, stable only below 200°C. Another (reverse) very weak component, stable at higher temperatures (up to 400°C), is present in most of the samples. This component can be precisely isolated for only 7 specimens from 3 different sites and therefore the information gained is not statistically sufficient for any tectonic reconstruction. Rock magnetism analyses showed a variable magnetic mineralogy j but the NRM carriers are not well represented in the artificial remanences produced in the laboratory. Results suggest that the natural viscous remanence is most likeIy carried by coarse multi-domain magnetite.
The ING Seismic Network Databank (ISND) : a friendly parameters and waveform database
Authors: S. Barba  , R. Di Giovambattista  , G. Smriglio
Published: June 2021

he Istituto Nazionale di Geofisica (ING) Seismic Network Database (ISND) includes over 300000 arrivaI times of Italian, Mediterranean and teleseismic earthquakes from 1983 to date. This database is a useful tool for Italian and foreign seismologists ( over 1000 data requests in the first 6 months of this year). Recently (1994) the ING began storing in the ISND, the digital waveforms associated with arri,Tal times and experimen- tally allowed users to retrieve waveforms recorded by the ING acquisition system. In this paper we describe the types of data stored and the interactive and batch procedures available to obtain arrivaI times and/or asso- ciated waveforms. The ISND is reachable via telephone line, P.S.I., Internet and DecNet. Users can read and send to their E-mail address alI selected earthquakes locations, parameters, arrivaI times and associated digital waveforms (in SAC, SUDS or ASCII format). For r;aedium or large amounts of data users can ask to receive data by means of magnetic media (DAT, Video 8, floppy disk).
Tectonic interpretation of large scale geodetic measurements (VLBI, SLR) in the Central Mediterranean region: constraints and uncertainties
Authors: E. Mantovani  , D. Albarello  , C. Tarnburelli  , M. Viti
Published: June 2021

Some considerations are reported on the tectonic setting and microplate mosaic in the Mediterranean zones where VLBI and SLR stations are located. In particular, the possible sources of ambiguity in the determination of the Africa-Eurasia and Adriatic-Eurasia relative motions from geodetic data are discussed. Possible alternative kinematic interpretations, with respect to those reported in the literature, are then proposed.
Dynamic fault rupture processes of moderate-size earthquakes inferred from the results of kinematic waveform inversion
Authors: T. Mikumo
Published: June 2021

Several attempts have been made recently to infer the dynamic rupture processes of moderate-size earthquakes from kinematic waveform inversion and dynamic crack inversion. These studies have revealed a quite heterogeneous distribution of dynamic stress drop and relative fault strength over the fault for most earthquakes. In two strike-slip California earthquakes, negative stress drop has been identified in a shallow section of the fault, suggesting the possible existence of a zone of velocity-strengthening frictions. The dynamic models yielded quite short rise times comparable to those inferred from kinematic modelling of observed waveforms. The short slip durations for these earthquakes may probably be attributed to shorter length scale of fault segmentation due to the heterogeneities of shear stress and fault strength.
Stress triggering of earthquakes: evidence for the 1994 M = 6.7 Northridge, California, shock
Authors: R. S. Stein  , J. Lin  , G. C. P. King
Published: June 2021

A model of stress transfer implies that earthquakes in 1933 and 1952 increased the Conlomb stress at the site of the 1971 San Fernando earthquake. The 1971 earthquake in turn raised stress and produced aftershocks at the site of the 1987 Whittier Narrows and 1994 Northridge ruptures. The Northridge main shock raised stress in areas where its aftershocks and surface faulting occurred. Together, M ? 6 earthquakes near Los Angeles since 1933 have stressed parts of the Oak Ridge, Sierra Madre, Santa Monica Mountains, Elysian Park, and Newport-Inglewood faults by > 1 bar. While too small to cause earthquakes, these stress changes can trigger events if the crust is already near failure, or advance future earthquake occurrence if it is not.
An experiment of spectral induced polarization
Authors: B. Balia  , G. P. Deidda  , A. Godio  , G. Ranieri  , L. Sambuelli  , G. Santarato
Published: June 2021

A Spectral Induced Polarization (SIP) survey was carried out in a mining test site in Sardinia (Italy). Measurements were developed along a profile by using an axial dipole-dipole array with 10 AB positions and 6 MN positions for cach AB. The amplitude and phase spectra of the apparent resistivity were acquired in the 0.25-4096 Hz frequeney range. The results obtained through the processing and inversion step seem to confirm that, with respect to the classical TD/FD Induced Polarization, SIP allows better discrimination of some important characteristics of mineral deposits such as mineral content and grain size.
Determination of Moho depth and dip beneath MEDNET station AQU by analysis of broadband receiver functions
Authors: A. Megna  , A. Morelli
Published: June 2021

We applied the receiver function technique to retrieve Moho depth and dip beneath the MEDNET very-broadband seismographic station at l'Aquila, in the Central Apennines. Broadband data available for teleseismic events recorded in two years of operation were sufficient to delineate a rather simple structure consisting of a 32-34 km thick crust, in agreement with previous studies based on refraction seismics. In addition, the data show relatively large variation in the amplitude of the converted P-to-S phase generated at the crust-mantle interface as a function of azimuth. These variations are consistent with synthetic receiver functions generated for an incident P wave interacting with an interface dipping ~ 8° to the north. Observations of amplitude ratios of converted phases, polarity of first-motion in the SH directíon, and relative travel time delay are all consistent with a model assuming a Moho discontinuity about 33 km deep gently dipping towards north. The receiver function technique has shown to be an efficient tool for investigating deep crustal structure, giving localized but reliable information.
Automatic processing of seismic events recorded on a mini-array Signal analysis combined with neural networks
Authors: A. Bottero  , Y. Cansi  , B. Massinon
Published: June 2021

We present a new method for automatic processing of mini-array records of regional events. It is based on a comprehensive analysis of the cross-correlation functions. This leads to a set of time-delays used to compute the azimuth and velocity of the travelling wave only in case of consisteney of the time-delay set. The second step takes into account the time-frequency representations of these wave parameters to identify each regional wave using a neural network. The resulting standard error on azimuth is 3° and the relative error on distance is less than 20%.
The-GEOSCOPE Program
Authors: G. Roult  , J. P. Montagner
Published: June 2021

he purpose of the GEOSCOPE Program was the installation of 25 stations in the standard configuration defined by the FDSN (VBB 24 bit, continuous recording at 20 samples/s). The installation is almost complete. The effort this year focussed on the accessibility of data, either through the IRIS/GOPHER system for large earthquakes, through CD-ROM production, and on line using the Juke-box JUMBO in the GEOSCOPE Data Center (Paris). This aspect will be stressed as the cooperation between IRIS and GEOSCOPE intensifies.
Canadian plans for participation in GSETT 3
Authors: R. N. North
Published: June 2021

The Geological Survey of Canada (GSC) is making preparations for Canadian participation in GSETT 3 but will be unable to make a formal commitment until the necessary resources have been secured. As Canada is expected to provide at least four alpha stations, and a significant number of beta stations, the financial resources that will be needed are substantial, even though in many respccts the GSC is, with the recent modernization of the Yellowknife array and the ongoing installation of the Canadian National Seismograph Network (CNSN), well positioned to make a significant contribution to GSETT 3. The CNSN currently (October 1993) consists of 17 broad band stations and will grow to 23 and 33 such stations by December 1993 and December 1994 respectively. Some 40 50 short period stations will complete the network. Data from all sites are continuously telemetered in real time to network acquisition centres in Ottawa and Sidney, British Columbia, archived to optical disk, and kept on line in a 72 h ring buffer. Most of the broadband sites could serve as either alpha or beta stations once the necessary software for continuous data transfer, or on request provision, of data from the selected sites has been completed. This software wili be configured so that changes in station selection are easy to implement, and this will provide considerable flexibility to the GSETT 3 planning and operations working groups in selecting the optimum network. Backup stations can be designated in the case of station failures, and the network centre in British Columbia will serve, at least for beta stations, as a backup NDC to that in Ottawa. Data from. the Yellowknife array are collected in Yellowknife and forwarded in ten minute files to Ottawa, where processing is completed and the results archived. This arrangement would not meet the deadlines for receipt of alpha station data at the IDC and new hardware and software will be needed to forward the data more immediately from Yellowknife to Ottawa. Although the procedures and formats for both alpha and beta station data have not yet been agreed upon, or even discussed, by the GSE, it is apparent that new facilities will be re quired in Ottawa to multiplex and reformat data for transmission to the IDC. We anticipate that a dedicated 56 kbaud link will be needed between Ottawa. and Washington.
Results from Swedish oblique soundings campaigns
Authors: M. Broms  , B. Lundhorg
Published: June 2021

In the context of the COST 238, PRIME project, two campaigns of oblique soundings with the Chirpsounder receiver at Linkoping, Sweden were made in parallel with vertical sounding campaigns. One campaign was made in June, 1992, when transmissions from Southern Spain were monitored, the other in December, 1992 when a transmitter at Chelveston, U.K, was monitored. The scaled values of F2MOF, 2-hop F2MOF and LOF give information on the variation of these parameters on short time scales and from day to day. High correlations between 2-hop F2MOF and F2MOF are found. Good agreement was found between the 2-hop MOF and MUF(1400)F2 calculated from vertical soundings at St. Peter Ording, Germany.
Correlations in daily deviations from the median of f0F2, M(3000)F2 and VF
Authors: S. S. Kouris  , K. Barbatsi  , T. D. Xenos  , B. Zolesi
Published: June 2021

The statistical analysis of the hourly daily deviations from the corresponding monthly-median values of the factor M(3000)F2 and of the critical frequency of the F2-layer, f0F2, reveals that the correlation between these parameters is poor. A similar analysis between the hourly daily deviations from the corresponding monthly-median of the height hF and the f0F2 leads to similar results. These results indicate that the factor M(3000)F2, the height hF and the critical frequency f0F2 may have a different daily variability.
Interplanetary magnetic field and its possible effects on the mid-latitude ionosphere II
Authors: Y. Tulunay
Published: June 2021

Using criticaI frequencies, f0F2 from the Uppsala, Lannion and Dourbes ionosonde stations, the possible effects of the orientation of the IMF on mid-latitude ionosphere are further investigated. For this purpose, the regular diurnal, seasonal and solar cycle variations in the f0F2 data were removed by subtracting the mean of f0F2 for the same UT on all the magnetically quiet days (Ap< 6) within 15 days around the IMF Bz turnings. This yields the deviation from the average quiet-time value ?f0F2. The data are sorted according to the polarity of the IMF Bz and the effects of the southward turnings are discussed. Hapgood et al. (1991), Tulunay et al. (1991), Tulunay and Rahman (1992) investigated the possible effects of the IMF on mid-latitude ionosphere by employing the Slough and Argentine Islands f0F2 data. In order to facilitate a comparison the same method of analysis is being adopted again. However, in the present work the southward polarity changes in IMF Bz with no consideration of the IMF sector structure were considered only.
Active seismic deformation in the Italian peninsula and Sicily
Authors: A. A. Kiratzi
Published: June 2021

Recent and historical seismicity as well as reliable fault plane solutions are used in order to perform a moment tensor analysis and estimate the active crustal and sub-crustal deformation of the Italian peninsula and Sicily. The results show that in Northern Italy, along the Alps, the deformation is taken up by compression at N162°E and a rate of 1 mm/yr. The thickening of the seismogenic layer is taking place at a rate of 0.1 mm/yr. In Central Italy, along the Apennines, extension is prevailing at N28°E and a rate of 3 mm/yr which causes thinning of the seismogenic layer at a rate 0.5 mm/yr. In Southern Italy, at Calabria, the deformation is taken up as extension at N40°E and a rate of 11 mm/yr. At the island of Sicily, compression is occurring at N25°E and a rate of 1 mm/yr. These results are in agreement with plate motion models for the area. The analysis of the deep seismicity of the Tyrrhenian Sea showed that the descending slab is in a state of down dip compression at N146°E and a rate of 2 mm/yr.
National borders earthquakes: an attempt at intensity maps unification
Authors: V. De Rubeis  , A. Maramai  , A. Tertulliani
Published: June 2021

In this paper the problem of unification of macroseismic maps across national borders is discussed. Up to now some attempts in this way have been carried out, evidencing the difficulties present in such analysis. A filter technique working on intensity data is applied on these particular kinds of events, with the goal of homogenizing different macroseismic fields coming from neighboring countries. The filter, aIready tested on Italian earthquakes, has been used on five European earthquakes of this century, for which different data sets and partial interpretations exist in literature. This approach works well with data sets limited to a single country. Moreover it could be successfully utilized for border events too, wherever a unification of intensity assessment methods has been carried out among neighboring countries.
Seismic deformation at the Alban Hills volcano during the 1989-1990 seismic sequence
Authors: G. Selvaggi  , F. D'Ajello Caracciolo
Published: June 2021

We analysed the one-year-long seismic swarm at the Alban Hills volcano which occurred during 1989-1990. We portray spatial distribution of seismic moment release, better delineating the activated volume during the swarm. The seismic structure is imaged as a 7-km long, 3-km wide, and 3-km thick volume, located between 2 and 5 km depth, and NW-SE striking. Fault plane solutions and scalar seismic moments for the largest earthquakes provide the description of the average strain rate tensor. The principal strain rate axes show a dominant extension in NE-SW direction, a SE-NW direction of compression and a negligible thickening rate. P and T axes direction of the smaller earthquakes suggests that the same mode of deformation is distributed all over the activated volume. These results are discussed in terms of seismic deforming processes active at the Alban Hills volcano, in the frame of magmatic inflation recently invoked to explain the rapid vertical uplift affecting part of the volcano. The observed average deformation is consistent with shear failures occurring on faults connecting stress-oriented dykes in response to an increasing fluid pressure.
Rigorous time domain responses of polarizable media II
Authors: M. Caputo  , W. Plastino
Published: June 2021

We present and test in detail with synthetic data a method which may be used to retrieve the parameters describing the induced polarization properties of media which fit the generally accepted frequency dependent formula of Cole and Cole (1941) (CC model). We use time domain data and rigorous formulae obtained from the exact solution of the problem found in a previous note (Caputo, 1996). The observed data considered here are the theoretical responses of the medium to box inputs of given duration in media defined with different parameters; however, as is usually done, only the discharge data are used (Patella >F2F1<, 1987). The curve at the beginning of the discharge is studied in some detail. The method is successful in identifying the parameters when the data fit the CC model; if the medium is not exactly of the CC type the method may also help identify how the medium departs from the CC model. The Laplace Transform of the discharge for a box type input data is also given.
A geoelectrical survey above an Antarctic ice shelf
Authors: F. Merlanti  , M. Pavan
Published: June 2021

A geoelectrical survey was performed on the Hells Gate ice shelf (Victoria Land-Antarctic) within the framework of an integrated geophysical and glaciological research program. The resistivity profiles show a similar trend, with resistivity values ranging from about 25000 W · m to 500000 W · m. These results have been interpreted as the effect of a sharp transition from "marine ice" to "continental" ice an interpretation that is consistent with the results of surface mapping. Interpreting the Vertical Electrical Soundings (VES) is a complex process. In fact, the alternating layers of ice with different compositions and salt content generate great uncertainty relative to the corresponding electric stratigraphies. To solve these problems of equivalency, all the available constraints were used including the drilling thickness, seismic reflection profiles as well as radar profiles. The results were used to provide what is mainly a qualitative overview that is coherent with the glaciological hypotheses relative to the evolution and structure proposed by some researchers for this ice shelf.