bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantitative mapping of precursory seismic quiescence before the 1989, M 7.1off-Sanriku earthquake, Japan
Authors: M. Wyss  , A. Hasegawa  , S. Wiemer  , N. Umino
Published: June 2021

The first main shock of the off-Sanriku earthquake sequence (02/11/89, M 7.1; 18/07/92, M 6.9; 28/12/94, M 7.5) was preceded by a precursory seismic quiescence lasting 2.5 ± 1 year and up to this main shock. The detailed properties of this quiescence were mapped as a function of time and space by a gridding technique using the ZMAP computer code, and the statistical significance was estimated by generating a synthetic catalog based on the microearthquake catalog of Tohoku University, which was the data set used. The statistically most significant expression of this precursory quiescence has a probability of 0.1% to have occurred at random and was located in the eastern part of the 1989 aftershock area, at a point to which the 1994 aftershocks extended also. If we define the dimensions of the quiescence anomaly by a vertical cylinder with the depth of the entire seismogenic layer, centered at the point of most significant quiescence and showing a rate decrease of 75%, then we find its radius is 25 ± 9 km. If we allow other shapes, such as the simplified aftershock volume of 1989, or other simple geometric figures, to define the rate decrease we find dimensions of 80 by 80 km. The characteristics of the quiescence anomaly do not depend strongly on the choice of free parameters within the following ranges: 100 ? number of events ? 400, 2.0 ? Mmin £ 3.0, 1 ? time window ? 3 years. With our method, a thorough analysis of the period before the 1994 main shock is not possible because of the interference of the extended aftershock sequence of 1989. Nevertheless, we identified a quiescence of nearly zero earthquakes located near the center of the 1994 aftershock area that lasted for one year up to that main shock. However, this quiescence period ranked only 46th in significance, behind other quiescences of equal duration and similar dimensions distributed in time and space through the data set. Because of the ubiquitous existence of periods of near zero activity during short periods like one year, we find that quiescences shorter than about 1.5 years cannot be defined with high statistical significance in most earthquake catalogs. In the last two years of the data (1995.3-1997.3) we see no extensive quiescence of high significance off the east coast of Honshu between 36.5° and 42°N in the currently available data.
Fractal dimension confidence interval estimation of epicentral distributions
Authors: L. De Luca  , S. Lasocki  , D. Luzio  , M. Vitale
Published: June 2021

Estimates of the fractal dimension of hypocentral distributions require evaluating the range of independent variables in which fractal parameters exhibit a power law. Systematic and accidental errors are produced mainly by the subjective selection of this range, the insufficiency of data sets as well as by hypocenter mislocations. Therefore it is very important to determine the confidence intervals which are associated with fractal dimension estimates. The effects of various sources of errors are studied using different geometric clusters of epicenters, which have been synthetically generated using a multicluster algorithm with different hierarchical levels, so as to reproduce some characteristics of the patterns typical of real epicenter distributions. Subsequently, groups of differently sized subsets of synthetic epicenters were obtained by randomly sampling each distribution. Confidence intervals of fractal dimensions were thus calculated using all the estimates obtained for the various subsets. This procedure was also tested on real seismic data, consisting of epicentral distributions in three Sicilian areas and five clusters of mining-induced seismic events (Wujek coal mine, Poland). In that analysis both correlation dimensions and their confidence intervals were taken into account.
Assessment of earthquake hazard in Turkey and neighboring
Authors: M. Erdik  , Y. A. Biro  , T. Onur  , K. Sesetyan  , G. Birgoren
Published: June 2021

The aim of this study is to conduct a probabilistic seismic hazard analysis for Turkey and neighboring regions, using the most recently developed attenuation relationships. The seismicity database is compiled from numerous sources, and the tectonic setting of the region has been studied in detail. Utilizing these two major categories of information together with the selected attenuation relationships, the seismic source zones are determined, and PGA contour maps are produced for specific return periods. The study is intended to serve as a reference for more advanced approaches and to stimulate discussion and suggestions on the database, assumptions and the inputs, and to pave the way for the probabilistic assessment of seismic hazard in the site selection and the design of engineering structures.
Seismic hazard mapping in Australia, the Southwest Pacific and Southeast Asia
Authors: K. McCue
Published: June 2021

Where available, regional assessments of earthquake hazard were compiled into the final map and elsewhere a knowledge of the tectonics and site specific studies, often for dams or mines, were cobbled together to produce a regional map. Cornell's method was used with a variety of attenuation relationships chosen for their suitability depending on the tectonics. Various versions of McGuire's program FRISK88 or similar programs including the USGS SEISRISK and the Seismology Research Centre's GMREC were used for the computations.
Compilation of the GSHAP regional seismic hazard for Europe, Africa and the Middle East
Authors: G. Grünthal  , C. Bosse  , S. Sellami  , D. Mayer-Rosa  , D. Giardini
Published: June 2021

The seismic hazard map of the larger Europe-Africa-Middle East region has been generated as part of the global GSHAP hazard map. The hazard, expressing Peak Ground Acceleration (PGA) expected at 10% probability of exceedance in 50 years, is obtained by combining the results of 16 independent regional and national projects; among these is the hazard assessment for Libya and for the wide sub-Saharan Western African region, specifically produced for this regional compilation and here discussed to some length. Features of enhanced seismic hazard are observed along the African rift zone and in the Alpine-Himalayan belt, where there is a general eastward increase in hazard with peak levels in Greece, Turkey, Caucasus and Iran.
The GSHAP Global Seismic Hazard Map
Authors: D. Giardini  , G. Grünthal  , K. M. Shedlock  , P. Zhang
Published: June 2021

The Global Seismic Hazard Assessment Program (GSHAP), a demonstration project of the UN/International Decade of Natural Disaster Reduction, was conducted in the 1992-1998 period with the goal of improving global standards in seismic hazard assessment. The GSHAP Global Seismic Hazard Map has been compiled by joining the regional maps produced for different GSHAP regions and test areas; it depicts the global seismic hazard as Peak Ground Acceleration (PGA) with a 10% chance of exceedance in 50 years, corresponding to a return period of 475 years.
Comparison of different methods of analysis of satellite geomagnetic anomalies over Italy
Authors: B. Duka
Published: June 2021

Different methods of analysis have been applied to satellite geomagnetic data that are claimed to be representative of the crustal geomagnetic field of the Italian area. The methods are compared with each other concluding that the SCHA is the best one. The downward continuation of the field by different methods gives different results, the most realistic are those of SCHA and RHA methods. Some remarks about each method and their results for the Italian area are presented.
Performance prediction methods of HF radio systems
Authors: R. Hanbaba
Published: June 2021

A survey of the historical development of methods for predicting the performance of ionospheric HF radio systems is presented, with special emphasis on modelling. The models considered are ionospheric characteristics and electron-density profile models, a propagation model, transmission loss and gain factors models, characteristics of radio noise models and a reliability model.
Natural catastrophes and point-like processes Data handling and prevision
Authors: G. P. Gregori
Published: June 2021

A frequent approach when attempting to manage a natural catastrophe is in terms of a numerical model, by which we try to forecast its occurrence in space and time. But, sometimes this is difficult or even unrealistic. On more pragmatic grounds we can appeal to a formal analysis of the historical time series of every catastrophe of concern. Only approximately, however, can such series be likened to a point-like process, because the "detector-mankind" experienced substantial changes versus time. Nevertheless, such algorithms can be approximately applied by means of a few suitable assumptions. In the ultimate analysis, four basic viewpoints can be considered: i) either by assuming that phenomena are periodic; ii) or by assuming that an event occurs only whenever some energy threshold is attained (calorimetric criterion); iii) or by assuming that it occurs only whenever the system experiences some abrupt change in its boundary conditions; or iv), whenever no such algorithm is viable due to scanty observational information, just by applying fractal analysis, in terms of the box counting method, or some other more or less related and/or equivalent algorithms. The mutual relations, advantages, and drawbacks of any such approach are briefly discussed, with a few applications. They already lead to an apparently successful long-range forecast of a large flood in Northern Italy which occurred in 1994, and to the prevision of the next explosive eruption of Vesuvius. But the success of every application is closely determined by the quality of the historical database, or by the physical information that is fed into the analysis, rather than by mathematics that per se have only to be concerned with avoiding some arbitrary input being added, based only on the human need for simplicity. The present paper gives a synthesis of several algorithms that were previously independently applied on a simple intuitive basis to different case studies, although with no comparisons or discussion of their similarities and/or differences.
Solar cycle 22 control on daily geomagnetic variation at Terra Nova Bay (Antarctica)
Authors: L. Cafarella  , A. Meloni  , P. Palangio
Published: June 2021

Nine summer geomagnetic observatory data (1986-1995) from Terra Nova Bay Base, Antarctica (Lat.74.690S, Long. 164.120E, 80.040S magnetic latitude) are used to investigate the behaviour of the daily variation of the geomagnetic field at polar latitude. The instrumentation includes a proton precession magnetometer for total intensity |F| digital recordings; DI magnetometers for absolute measuring of the angular elements D and I and a three axis flux-gate system for acquiring H,D Z time variation data. We find that the magnetic time variation amplitude follows the solar cycle evolution and that the ratio between minimum solar median and maximum solar median is between 2-3 for intensive elements (H and Z) and 1.7 for declination(D). The solar cycle effect on geomagnetic daily variation elements amplitude in Antarctica, in comparison with previous studies, is then probably larger than expected. As a consequence, the electric current system that causes the daily magnetic field variation reveals a quite large solar cycle effect at Terra Nova Bay.
Regional ionospheric mapping and modelling over Antarctica
Authors: G. De Franceschi  , B. Zolesi
Published: June 2021

The performance of regional and global models has been investigated when applied for long term mapping of ionospheric characteristics and modelling the Total Electron Content in the polar cap over an Antarctic region. Comparison results between modelled data and a short period of experimental values available for low solar activity in December 1993 and January 1994 are presented and discussed.
Field lines of gravity, their curvature and torsion, the Lagrange and the Hamilton equations of the plumbline
Authors: E. W. Grafarend
Published: June 2021

The length of the gravitational field lines/of the orthogonal trajectories of a family of gravity equipotential surfaces/of the plumbline between a terrestrial topographic point and a point on a reference equipotential surface like the geoid í also known as the orthometric height í plays a central role in Satellite Geodesy as well as in Physical Geodesy. As soon as we determine the geometry of the Earth pointwise by means of a satellite GPS (Global Positioning System: «global problem solver») we are left with the problem of converting ellipsoidal heights (geometric heights) into orthometric heights (physical heights). For the computation of the plumbline we derive its three differential equations of first order as well as the three geodesic equations of second order. The three differential equations of second order take the form of a Newton differential equation when we introduce the parameter time via the Marussi gauge on a conformally flat three-dimensional Riemann manifold and the generalized force field, the gradient of the superpotential, namely the modulus of gravity squared and taken half. In particular, we compute curvature and torsion of the plumbline and prove their functional relationship to the second and third derivatives of the gravity potential. For a spherically symmetric gravity field, curvature and torsion of the plumbline are zero, the plumbline is straight. Finally we derive the three Lagrangean as well as the six Hamiltonian differential equations of the plumbline, in particular in their star form with respect to Marussi gauge.
Recent deeper geophysical results better account for the tectonics in the Italian area
Authors: C. Morelli
Published: June 2021

Results from extended DSS profiles (1956-1986) in Italy and surrounding land and sea areas offer good constraints for other geophysical and geological data. Integrated interpretations outline the main tectonic features. Collisional tectonics is predominant in the Alps, for which the Adriatic plate acted as hinterland against the European plate foreland. Main results: W-wards, NW- and N-wards oriented overthrusting on the European crust, bending of the lower European crust, European Moho to 70 km depth with the Adriatic mantle indented above, crustal doubling (Adriatic over the European one). In the Apennines, on the contrary, the Adriatic plate acted as a foreland, against the overthrusts generated by the Tuscanian and Tyrrhenian mantellic bodies, heated, elevated and migrated NE-wards and SE-wards, respectively. Also the Adriatic plate bends under this load-centripetally towards the Tyrrhenian sea, so that the Adriatic Moho from 35 km depth is presumed to descend through a flexure till 40-50 km below the Tuscanian and Tyrrhenian land areas. The external peri-Apenninic area is still in compression and includes thick sedimentary basins, from the Po-plain to Sicily. The internal area is in extension, overlapped by thin, stretched crusts of Ligurian and Tyrrhenian origin, whose remnants occupy most of both seas areas, with two areas of oceanic crust in the SE-Tyrrhenian. Rifting and opening is in action also in the Ligurian Sea and Sicily Strait.
Recognizing the Italian zones most prone to next large earthquakes: possible approaches and present chances
Authors: E. Mantovani  , D. Albarello  , C. Tamburelli  , D. Babbucci
Published: June 2021

The defense from earthquakes in Italy would be considerably favoured by knowing where the next strong shocks will most probably occur. However, to obtain this information a well-defined and widely accepted method is not yet available. This work discusses the approaches so far proposed and tries to estimate the relative chances of providing reliable indications on this problem. Particular attention focusses on the methodology based on the concept of «strain migration».
Rheological profiles in the Central- Eastern Mediterranean
Authors: M. Viti  , D. Albarello  , E. Mantovani
Published: June 2021

Seismological investigations have provided an estimate of the gross structnral features of the crust/upper mantle system in the Mediterranean area. However, this information is only representative of the short-term me- chanical behaviour of rocks and cannot help us to understand slow deformations and related tectonic processes on the geological time scale. In this work strength envelopes for several major structural provinces of the Mediterranean area have been tentatively derived from seismological stratification and heat flow data, on the assumption of constant and uniforrn strain rate (10-16 S-1), wet rocks and conductive geotherm. It is also shown how the uncertainties in the reconstruction of thermal profiles can influence the main rheological prop- erties of the lithosphere, as thickness and total strength. The thickest (50-70 km) and strongest mechanical lithospheres correspond to the coldest zones (with heat flow lower than or equal to 50 mW m-2), i.e., the Io- nian and Levantine mesozoic basins, the Adriatic and Eurasian foreland zones and NW Greece. Heat flows larger than 65 mW m-2, generally observed in extensional zones (Tyrrhenian, Sicily Channel, Northern Aegean, Macedonia and Western Turkey), are mostly related to mechanical lithospheres thinner than 20 km. The characteristics of strength envelopes, and in particular the presence of soft layers in the crust, suggest a reasonable interpretation of some large-scale features which characterize the tectonic evolution of the Central- Eastem Mediterranean.
The ionosphere and radio interferometry
Authors: T. A. Th. Spoelstra
Published: June 2021

This paper reviews the effects of the ionosphere on radio astronomjcal observations, what we can learn about the ionosphere from radio interferometry, and a procedure to correct for these effects. This study analyzes the results obtained from observations of celestial point soUl.ces with the Westerbork Synthesis Radio Telescope, WSRT, in the Netherlands from the period 1970-1991. The main conc1usions are: 1) A1though seasona1 effects are c1ear, the occurrence and "strength" of ionospheric irregu1arities show no dependence on solar activity. 2) Assuming that the frequency of occurrence of ionospheric disturbances in Spring and Autumn are similar, Ihe "ionospheric" Winter starts on day 348 ± 3 and ali seasons last for three months. 3) Travelling ionospheric disturbances, TIDs, occur most frequently during daytime in Winter periods. 4) The propagation parameters of these travelling ionospheric irregularities and their periods indicate that these belong main1y to the c1ass of medium sca]e TIDs. 5) Radio interferometry is a powerful tool to locate irregularities causing scintillation and to determine their dimensions. 6) The occurrence of non-periodic irregu1arities is, however, not a function of time of day. 7) The daily variation in the amplitude and frequency of occurrence of the TIDs suggest that the generation of gravity waves may be caused by winds and tides in the lower thermosphere/mesosphere. On the basis of the availab1e data, a definition of a "disturbance measure" indicating to what extent the ionosphere is "quiet" is proposed. Procedures to correct for ionospheric effects and an eva1uation of the different methods to obtain information on the ionospheric e1ectron content are reviewed in sections 8 and 9, respectively.
Tentative list of major deformation events in the Central-Eastern Mediterranean region since the middle Miocene
Authors: D. Babbucci  , C. Tamburelli  , E. Mantovani  , D. Albarello
Published: June 2021

A list of major constraints to impose on evolutionary reconstructions of the Central-Eastern Mediterranean region for the middle Miocene period to the Present is proposed. Each constraint is constituted by a tentative description of the deformation and related tectonic regime which affected major active zones during each evolutionary phase. This information has been derived from the analysis of the available observations in the various branches of Earth Sciences, trying to select clearly recognized deformations, possibly supported by independent observations. The list here reported has been used to constrain the evolutionary reconstruction proposed by Mantovani et al.(1997).
Electrical structure of Plaine des Sables caldera, Piton de la Fournaise volcano (Reunion Island)
Authors: P. A. Schnegg
Published: June 2021

An Audio Magnetotelluric (AMT) profile has been carried out across the Plaine des Sables, a former caldera of the active Piton de la Fournaise volcano, Reunion Island. Located in the Western Indian Ocean, between the Mascarene and Madagascar basins, this basaltic shield volcano originates from the activity of a hot spot. Our aim was to determine the internal structure of the volcano, in particular the shallow electrical properties of an area extending between the old and the new caldera rims. Although several teams had already conducted AMT work in this region a few years ago, there was a need for more a detailed, in depth survey. Our final model displays a noticeable slope of the Plaine des Sables basement oriented toward the present Fournaise summit. This slope is interpreted as resulting from successive landslides toward the ocean. We conclude that this dipping, electrically good conducting layer, probably belongs to the flat layering of an older caldera.
Monitoring of the geomagnetic and geoelectric field in two regions of Greece for the detection of earthquake precursors
Authors: G. Vargemezis  , J. Zlotnicki  , G. Tsokas  , B. C. Papazachos  , E. E. Papadimitriou
Published: June 2021

Two magnetotelluric stations have been installed in the South-Eastern Thessaly basin (Central Greece), which have recorded the geomagnetic and geoelectric fields since 1993. The aim is to detect long lasting abnormal changes of the geoelectric field which may be due to impending earthquakes. The geoelectric recordings were checked against the climatic changes such as temperature changes and precipitation and no correlation was observed. Ten anomalies were observed with characteristics similar to seismoelectric signals which have been reported in the literature and thus we can assume that these changes constitute precursory phenomena. The duration of these signals varies from several days to a few weeks. Some of them keep on developing until the occurrence of an earthquake, and others appear like transient changes several days before. The high seismicity of the area where the stations are located creates difficulties in the correlation of the signals with certain shocks.
Modelling of anisotropic resistivity changes caused by stresses
Authors: K. P. Teisseyre
Published: June 2021

This paper examines the existence of oriented crack systems in the rock medium under stress load and its influence on the resistivity of the medium. Electric resistivity in the presented model depends on the rock frame's resistivity and on the influences of all the systems of cracks. In the assumed model, in the medium two or more crack classes can coexist, for example filled with gas and with aqueous solution. Examples of numerical simulations are compared to results from measurements in the laboratory and in the field í during monitoring of earthquakes and mining shocks. The relevance of this model to rock state surveying is discussed
The fractality of marine measurement networks and of the Earth's sampled magnetic field
Authors: F. Giordano  , I. Ortosecco  , L. Tramontana
Published: June 2021

We highlight the fractal behaviour of marine measurement networks when determining the Earth's total magnetic field and the spatial trend of the field itself. This approach is a convenient alternative method of assessing the coverage of an area by a set of measurements whenever the environmental situations do not permit a regular distribution of the measurement points. The Earth's magnetic field is sampled in marine areas when the measuring apparatus is moving, even at low speeds, whilst attempts are made to respect the spatial planning which has been pre-determined on the basis of the resolution sought after. However, the real distribution of the measurements presents numerous disturbances which are mainly due to environmental factors. In the case of distributions containing vast areas with no measurement points it is no longer possible to apply Shannon's theorem in 1-D and 2-D. In our paper we apply the fractal theory to certain 1-D and 2-D measurement distributions order to obtain a coverage estimate of the area and the capacity of reconstructing the field. We also examine the trend of the power spectra S of numerous magnetic profiles noting that almost all of them illustrate the dependency with the frequency f in the form S » f-b which is characteristic (necessary condition) of self-similar or self affine fractals.
Geophysical studies applied to near-surface karst structures: the dolines
Authors: E. Bozzo  , S. Lombardo  , F. Merlanti
Published: June 2021

The formation of dolines is a very common near-surface karst phenomenon which is widespread and evident throughout most of Italy. In this study different geophysical methods have been applied to doline-like structures in the volcaniclastic section of the Alps in order to define their main morpho-structural characteristics. An integrated analysis of the geophysical data was also carried out to ascertain the quality and the resolution obtained through these geophysical methods and more in general, to evaluate their applicability to areas displaying features which are similar to those currently investigated. The fundamental differences found in different dolines are related to the geometries of the contact between the sublayer and the infill and to the mineralogy of the latter. From a geophysical standpoint the contact represents a discontinuity between materials characterized by different geophysical properties, whereas the infill shows properties which vary according to their composition and texture.
Application of the Wiener filter to magnetic profiling in the volcanic environment of Mt. Etna (Italy)
Authors: C. Del Negro
Published: June 2021

The frequency-domain Wiener filtering was applied to magnetic anomalies in the volcanic area of Mt. Etna. This filter, under suitable conditions (additive noise, linear processing and mean-square error criterion), can furnish an effective tool for discriminating the geologic feature of interest (the signal) from the noise. The filter was first tested with synthetic data. Afterwards it was applied to a magnetic profile carried out across the principal fault system of the Mt. Etna volcano, that hosted the dykes feeding both the 1989 and the 1991-93 eruptions. The magnetic anomalies linked to the volcanic section and those linked to the contact between the clay basement and the lava coverage show significant spectral overlap. Thus by estimating the power spectrum of the signal, obtained resolving the forward problem, a least-squares Wiener filter has been designed. In such context, it was possible to verify the effectiveness of Wiener filters, whereas traditional band-pass filtering proved inadequate. In fact, analysis of the noise showed that all the meaningful components of the observed magnetic field were resolved. The results put further constraints on location and geometry of the shallow plumbing system of Mt. Etna.
Soil volumetric water content measurements using TDR technique
Authors: M. Menziani  , M. R. Rivasi  , S. Pugnaghi  , R. Santangelo  , S. Vincenzi
Published: June 2021

A physical model to measure some hydrological and thermal parameters in soils will to be set up. The vertical profiles of: volumetric water content, matric potential and temperature will be monitored in different soils. The volumetric soil water content is measured by means of the Time Domain Reflectometry (TDR) technique. The result of a test to determine experimentally the reproducibility of the volumetric water content measurements is reported together with the methodology and the results of the analysis of the TDR wave forms. The analysis is based on the calculation of the travel time of the TDR signal in the wave guide embedded in the soil.
Some considerations on the use of the geoelectric «square» array
Authors: F. Merlanti  , M. Pavan
Published: June 2021

The «square array» is what we may consider to be an unconventional geoelectric configuration since it is not widely used and therefore there are few examples of practical application. The purpose of this research was to verify the operating effectiveness of this configuration in terms of profile and sounding, and the significance of the set of possible measurements and derived parameters. This was also obtained by comparing the relative measurements with the most common linear arrays (Wenner, Schlumberger, tripotential). The experiment was carried out in two different zones. In the first area, corresponding to the archaeological site of Marzabotto (Bologna), the target was represented by wall remnants inserted in a substantially homogeneous medium, from an electrical point of view, and at depths that are less than those of the dimensions of the device used. At the second site, located in the valley of Landrazza (Savona), the situation was very different, with a valley section on a calcareous bedrock filled with poorly classified residual sediments. An overall analysis of the results showed that the square technique is more exhaustive than the classical linear arrangements when performing soundings. Instead, with regard to profile development, it is not as preferred since it involves a greater amount of work without generating improved information. From analysis of the experimental results, considerable doubts arose about the meaning and the use of the anisotropy coefficients and the error term as defined theoretically. These parameters turned out to be of little use with regard to the characterization of the ground anisotropy and for checking the reliability of the measurements.
Self-potential anomalies in some Italian volcanic areas
Authors: R. Di Maio  , V. Di Sevo  , S. Giammetti  , D. Patella  , S. Piscitelli  , C. Silenziario
Published: June 2021

The study of Self-Potential (SP) space and time variations in volcanic areas may provide useful information on both the geometrical structure of the volcanic apparatuses and the dynamical behaviour of the feeding and uprising systems. In this paper, the results obtained on the islands of Vulcano (Eolian arc) and Ponza (Pontine archipelago) and on the Mt. Somma-Vesuvius complex are shown. On the island of Vulcano and on the Mt. Somma-Vesuvius apparatus areal SP surveys were performed with the aim of evidencing anomalies closely associated to the zones of major volcanic activity. On the island of Vulcano a profile across the fumaroles along the crater rim of the Fossa Cone was also carried out in order to have a direct relationship between fumarolic fracture migration and flow rate and SP anomaly space and time variations. The areal survey on the island of Ponza, which is considered an inactive area, is assumed as a reference test with which to compare the amplitude and pattern of the anomalies in the active areas. A tentative interpretation of the SP anomalies in volcanic areas is suggested in terms of electrokinetic phenomena, related to the movement of fluids of both volcanic and non-volcanic origin.
Geophysical monitoring of the Purace volcano, Colombia
Authors: M. Arcila
Published: June 2021

Located in the extreme northwestern part of the Los Coconucos volcanic chain in the Central Cordillera, the Purace is one of Colombia's most active volcanoes. Recent geological studies indicate an eruptive history of mainly explosive behavior which was marked most recently by a minor ash eruption in 1977. Techniques used to forecast the renewal of activity of volcanoes after a long period of quiescence include the monitoring of seismicity and ground deformation near the volcano. As a first approach toward the monitoring of the Purace volcano, Southwest Seismological Observatory (OSSO), located in the city of Cali, set up one seismic station in 1986. Beginning in June 1991, the seismic signals have also been transmitted to the Colombian Geological Survey (INGEOMINAS) at the Volcanological and Seismological Observatory (OVS-UOP), located in the city of Popayan. Two more seismic stations were installed early in 1994 forming a minimum seismic network and a geodetic monitoring program for ground deformation studies was established and conducted by INGEOMINAS.
Characteristics of volcanic tremor accompanying the September 24th, 1986 explosive eruption of Mt. Etna (Italy)
Authors: S. Gresta  , G. Lombardo  , R. Cristofolini
Published: June 2021

Features of the volcanic tremor recorded before, during and after the eruptive event which occurred at Mt. Etna on September 24th 1986, are described. The whole eruption was particularly short in time (about eight hours) and characterized by an extremely violent explosive activity with lava fountains a few hundred meters high. As the complete record of the seismic signals generated during the whole eruptive episode was available, a detailed spectral analysis of the volcanic tremor recorded at four stations, located at increasing distance from the summit of the volcano, was carried out. Fourier analysis, that was performed using temporal windows of about 11 min in duration, pointed to some large fluctuations of the overall spectral amplitude, as well as some frequency variations of the dominant spectral peaks. The ratio of the overall spectral amplitude recorded at the highest station and at the peripheral ones, was calculated in the two spectral bands 1.0-2.5 and 2.6-6.0 Hz, respectively. The significant contribution of energy at low frequency values supports the hypothesis of a subvertical planar source, which was active during the paroxysmal stage of the eruption. Such results are also supported by the analysis of the attenuation function of the spectral amplitude.
Analysis of the intermittent volcanic tremor observed at Mt. Etna, Sicily during March-May 1987
Authors: S. Gresta  , E. Privitera  , A. Leotta  , P. Gasperini
Published: June 2021

Volcanic activity at the summit craters of Mt. Etna includes phreatic eruptions which jeopardize tourists and other people who sporadically approach the top of the volcano. To date no appreciable precursors of this kind of activity are known. Solely intermittent volcanic tremor might be considered a short-term seismic forerunner. The pattern of volcanic tremor amplitude at Mt. Etna during springtime 1987 was investigated and revealed significant periodicities before both phreatic explosions occurring at the summit craters during that time span. Results are in agreement with a boiling mechanism as the source of intermittent volcanic tremor, even if the lack of data on the ground water table in the upper part of the volcano did not allow the formulation of quantitative models.
Relationship between seismicity and eruptive activity at Mt. Etna volcano (Italy) as inferred from historical record analysis: the 1883 and 1971 case histories
Authors: R. Azzaro  , M. S. Barbano
Published: June 2021

In this paper historical and recent seismological data are analysed in order to investigate the relationship between seismicity and eruptive phenomena at Mt. Etna volcano. The 1883 and 1971 case histories have been proposed because they are significant events in the recent history of the volcano regarding volcanic hazard and show very different evolutions of associated seismic activity and eruption dynamics. The first (1883) represents flank eruptions characterised by high seismic release, short duration and moderate effusion rate whereas the second (1971) can be ascribed to eruptions starting as summit or subterminal events and thereafter developing on the flanks with a minor level of seismicity, higher effusion rate and prolonged duration. The pattern of seismic activity during 1883 and 1971, as inferred from historical record analysis, and the different associated type of eruption may be a result of diverse stress conditions acting on the volcanic system. The interpretation of the seismic behaviour by considering historical eruptions in a systematic fashion will contribute to a clearer understanding of volcanic phenomena at Mt. Etna.
Investigating seismogenic faults in Central and Southern Apennines (Italy): modeling of fault-related landscape features
Authors: L. Cucci  , G. D' Addezio  , G. Valensise  , P. Burrato
Published: June 2021

The direct identification of seismogenic structures in Southern Italy is often a difficult task, since finding seismogenic faults with clear surface expression associated is unlikely, even if they are the source of large magnitude earthquakes. A possible way to solve this problem is the investigation of a series of features of the surface landscape that could be attributed to repeated movements along a fault. In this paper we investigated the drainage, ancient coastlines, marine terraces and recent deposits of three different active tectonic basins in central and Southern Italy. The combination of the modeling of these fault-related landscape features with historical and seismological data better constrained the geometry of the seismogenic faults responsible for three large magnitude historical events in the studied areas. The results show that in this section of the peninsula seismogenesis is often connected with blind almost pure dip-slip normal faults; the long-term activity of these faults produces intermontane basins or coastal plains that are sinking relative to the surrounding areas; the application of this methodology emphasises the importance of geological and geomorphological studies for the recognition of seismogenic faults and for a complete assessment of the seismic hazard in Italy.
Deformation along an apparent seismic barrier: a palaeoseismological study along the North Anatolian Fault
Authors: G. W. Michel  , C. Janssen
Published: June 2021

An overstep in the North Anatolian Fault, possibly acting as a seismic barrier, was investigated for its structural and palaeoseismological characteristics. Study interests were: i) to find overstep related spatial changes in deformation which would help assess the structure as a long term singularity in the fault; ii) to identify short term, event-triggered changes in structures where the major fault enters the overstep i.e., in an area where seismogenic slip is impeded or even arrested; iii) to investigate whether or not the surface overstep is related to a seismic barrier, and iv) to discuss structures that might possibly be characteristic of barriers. In order to achieve this: a) largescale faults were mapped in the area, b) fault-slip data were measured in 56 outcrops along and within the overstep, and c) trenches were dug at the eastern rim of the overstep where recent earthquake structures had been reported. Derived long term stretching directions and ratios change significantly over the fault step and structures of recent major earthquakes suggest different deformation mechanisms for different events at the same Location e.g. strike-slip, thrust and normal faulting.
Late Holocene relative sea level changes in SW Crete: evidence of an unusual earthquake cycle
Authors: S. C. Stiros
Published: June 2021

Coastal challenges ill West Crete ill the last 4000 years can be described as a series of 11 relatively small (25 cm on the average) land subsidences alternating with short (150-250 year long) relatively still stands of the sea level. At 1500 B.P. an up to 9 m episodic relative land uplift and tilting of this part of the island occurred, but since then no significant coastal changes have been identified. There is strong evidence that these Late Holocene coastal changes are not a product of fluctuations of sea level, but reflect palaeoseismic events. The sequence of the latter is at variance with models of seismic deformation deduced from a wide range of observations in different tectonic environments, including coastal uplifts near major trenches: according to these models, strain buildup and release through earthquakes is described as a cyclic and rather uniform process, the earthquake cycle. In this process, the permanent seismic deformation accumulates after each earthquake to produce geological features, while the long-term deformation rate is approximately equal to the short term one. Obviously this is not the case with West Crete. The unusual pattern of seismic deformation in this island has been observed in other cases as well, but its explanation is not easy. The juxtaposition of different earthquake cycles, variations in the source and rate of stress or internal deformation of the uplifted hanging wall of a thrust in the pre-seismic period are some possible explanations for this unusual pattern of earthquake cycle in Greece.
Interplanetary magnetic field and its possible effects on the mid-latitude ionosphere III
Authors: Y. Tulunay
Published: June 2021

Using critical frequencies, f0F2 from the Lannion, Slough, Poitiers, Garchy, Dourbes, Rome, Juliusrud, Gibilmanna, Pruhonice, Uppsala, Kaliningrad, Miedzeszyn, Sofia, Athens and Kiev ionosonde stations, the possible effects of the orientation of the Interplanetary Magnetic Field (IMF) on mid-latitude ionosphere are further investigated. This time, only the southward polarity changes in IMF Bz with seasonal effects were considered. The same method of analysis was employed to facilitate a comparison between the recent results presented here with those which appeared in the preceding papers in the series. That is, the regular diurnal, seasonal and solar cycle variations in the f0F2 data were removed by subtracting the mean of the f0F2 for the same UT on all magnetically quite days (Ap < 6) within 15 days around the IMF Bz turnings (Tulunay, 1994). This last paper also includes the seasonal effects on the ionospheric data. The results confirm that much of the day-to-day variability of the mid-latitude ionosphere may be related to the orientation of the southward IMF Bz , characterized by the ionospheric winter anomaly. Day-to-day ionospheric variability becomes more significant towards higher latitudes.
The F1 ledge: density, height and slope
Authors: B. W. Reinisch  , X. Huang
Published: June 2021

Diurnal variations of the F1 region ionization at 170 km altitude and the slope ?170 = dN/dh at this height are analyzed for mid-latitude and equatorial stations: Millstone Hill, Ramey, Puerto Madryn and Jicamarca. Both the density N(t) and the slope ?(t)at 170 km show well defined diurnal variations with day-to-day variabilities of less than 10%. The heights of the F1 ledge, hmF1, are spread over ± 20 km and are therefore of limited value for modeling purposes.
Paleoseismology related to deformed archaeological remains in the Fucino Plain. Implications for subrecent seismicity in Central Italy
Authors: F. Galadini  , P. Galli
Published: June 2021

During paleoseismological investigations on the seismogenic structure responsible for the 1915 earthquake in the Fucino Plain (Central Italy), some trenches were excavated at the intersection between Roman-age channels and a fault characterized by Holocene activity. Channel displacement observed within the trenches has been related to an event which occurred approximately between the 6th and 9th century A.D. Written records describing damage caused in Rome indicate that two strong events occurred during this period in Central Italy, although their epicentral areas are Undefined. The first event occurred immediately before 508 A.D. while the second happened in 801 A.D. Two other earthquakes during this period (618 A.D. and 847 A.D.) are reported in catalogues, but without( corresponding information regarding damage in Rome. Available information is not conclusive about the age of the earthquake responsible for the displacement of the channels although geological, historical and archaeological data indicate it is most likely related to the 508 A.D. event. Should the hypothesis regarding the age of the earthquake be correct, a subrecent , incompletely-documented earthquake may be related to a specific seismogenetic area. Taking into account that the paleoseismological analysis has highlighted a close similarity between the surface faulting pattern of this event and the one that occurred in 1915, the former may be a "twin" of the latter.
The weight of pseudo-objectivity
Authors: J. Vogt
Published: June 2021

For nearly twenty years, numerous methodological problems of historical seismology, sometimes set in a wider frame, have been discussed by a continuous sequence of papers. One or another paper emphasized the role of pscudo-objectivity without, however, pronouncing these ominous words. Ultimately the need was I'clt to deal with the weight of pseudo-objectivity jn a sll.aightforward way, although scientists, as one enlightened reviewer of this paper wrote, often do not dare to rajse problems of a more or less psychological and episte- mological kind. Besides generaI statements, a random set of surprising examples is given, from the author's own experience or tì.om his readings. The major problem of computerized catalogues is emphasized. Even modern macroseismic enquiries, sometimes considered a routine work, don't escape the pitfalls of pseudo-ob-jectivity. While subjectivity is despised, with sometimes extreme statements, condemning historical seismology on the whole, a kind of "constructive subjectivity" with an ability to master complex problems, seem, preferable to the dangers of a growing pseudo-objectivity.
Determination of parameters for historicaI British earthquakes
Authors: R. M. W. Musson
Published: June 2021

Examination of 20th century data from the U.K. shows a good correlation between felt area and instrumental magnitude, producing good magnitude values for historical earthquakes where felt area is known; this is a more reliable method than using I0. The equations: ML = 1.03 log A3:; -0.19; ML = 0.92 log A4 + 0.71 give magnitude from areas within isoseismals 3 and 4 MSK. Dcpths for historical earthquakes have also been obtained using a modification of the well-known Sponheuer method, using the program MACDEP; the regional value for (á is 0.003 and depths for events with magnitude above 4 ML range from 3 to 25 km.
Attenuating intensities
Authors: L. Peruzza
Published: June 2021

The study presents a methodology in which fractile distances for a given macroseismic intensity are used as a point measure on which to do attenuation characterization. The use of statistically derived distances expected not to be exceeded at a given probability avoids ambiguous definition of isoseismal radii, and consequent mis- interpretation of attenuation parameters with a physical content. The utility and definiteness of such a fractile distance is evident when treating many earthquakes, of different magnitudes and for which the macroseismic data-sets are of different sizes. The methodology is applied to 55 Italian earthquakes of the last four centuries, with epicentral intensity ranging from VII to XI MCS. Propagation properties in volcanic districts are peculiar, with a rapid decay of observed intensity from the epicentre. The attenuation properties of the other crustal en- vironments are not clcarly correlated with the geodynamic domain; the earthquakes show a mean behaviour that appears to depend on relatively homogeneous conditions of propagation. Source depth, tectonic style and finiteness of the source are not addressed because only macroseismic surveying had been applied to most of the earthquakes, and they cannot be linked to surficial fault ruptures. The study is intended to provide mean attenuations that can be used in probabilistic seismic hazard studies; the variabiljty of individual earthquake attenuations for a region introduces the need to use different parameters in future deterministic earthquake scenarios.