bluuebunny/crossref_metadata_2025 huggingface dataset:
Modelling earth current precursors in earthquake prediction
Authors: D. Patella  , A. Tramacere  , R. Di Maio
Published: June 2021

This paper deals with the theory of earth current precursors of earthquake. A dilatancy-diffusion-polarization model is proposed to explain the anomalies of the electric potential, which are observed on the ground surface prior to some earthquakes. The electric polarization is believed to be the electrokinetic effect due to the invasion of fluids into new pores, which are opened inside a stressed-dilated rock body. The time and space variation of the distribution of the electric potential in a layered earth as well as in a faulted half-space is studied in detail. It results that the surface response depends on the underground conductivity distribution and on the relative disposition of the measuring dipole with respect to the buried bipole source. A field procedure based on the use of an areal layout of the recording sites is proposed, in order to obtain the most complete information on the time and space evolution of the precursory phenomena in any given seismic region.
P-wave crustal tomography of Greece with use of an accurate two-point ray tracer
Authors: G. Drakatos  , G. Karantonis  , G. N. Stavrakakis
Published: June 2021

The three-dimensional velocity structure of the crust in the Aegean sea and the surrounding regions (34.0º-42.OºN, 19.0ºE-29.0ºE) is investigated by inversion of about 10000 residuals of arrival times of P-wave from local events. The resulting velocity structure shows strong horizontal variations due to the complicated crustal structure and the variations of crustal thickness. The northern part of the region generally shows high velocities. In the inner part of the volcanic arc (Southern Aegean area), relatively low velocities are observed, suggesting a large-scale absorption of seismic energy as confirmed by the low seismicity of the region. A low velocity zone was observed along the subduction zone of the region, up to a depth of 4 km. The existence of such a zone could be due to granitic or other intrusions in the crust during the uplift of the region during Alpidic orogenesis.
Crustal and upper mantle velocity structure of Southern Iberia, the sea of Alboran, and the Gibraltar arc determined by local earthquake tomography
Authors: E. Gurria  , J. Mezcua  , M. J. Blanco
Published: June 2021

A "local earthquake tomography" of a large area encompassing the South of Iberia, the sea of Alboran, the Gibraltar arc, and Northern Morrocco, has been performed using first arrival times recorded at various Spanish and Morroccan seismic networks. A total of 52 stations and 639 earthquakes provided over 6300 first P arrivals and 4400 S arrivals. Three features of interest appear in the results: i) a continuous low velocity structure which correlates with the Betics, the Gibraltar arc and the Rif; ii) a high velocity feature which persists to a depth of approximately 30 km, positioned near the coast of Malaga on the northern margin of the Alboran sea; iii) a low velocity feature, extending to a minimum depth of approximately 40 km, which coincides with the Granada basin and a strong negative Bouguer gravity anomaly.
Integrated geophysical survey for the geological structural and hydrogeothermal study of the North-western Gargano promontory (Southern Italy)
Authors: M. Loddo  , R. Quarto  , D. Schiavone
Published: June 2021

A multimethodological geophysical survey was performed in the north-western part of the Gargano promontory to study the geological structural setting and the underground fluid flow characteristics. The area has a complex tectonics with some magmatic outcrops and shallow low-enthalpy waters. Electrical, seismic reflection, gravimetric and magnetic surveys were carried out to reconstruct the geological structures; and in order to delineate the hydrogeothermal characteristics of the area, the self-potential survey was mainly used. Moreover magnetic and self-potential measurements were also performed in the Lesina lake. The joint three-dimensional interpretation of the geophysical data disclosed a large horst and graben structure covering a large part of the area. In the central part of the horst a large ramified volcanic body was modelled. The models show some intrusions rising from it to or near to the surface. The main structures are well deep-seated in the Crust and along them deep warm fluids rise as the SP data interpretation indicates.
Seismo-volcanic sources on Stromboli volcano
Authors: J. Neuberg  , R. Luckett
Published: June 2021

A detailed analysis of broadband seismic recordings leads to models of eruption mechanisms for Strombolian activity. The data used comprise signals from arrays of nine three-component seismometers and video recordings of visual eruptive activity with precise time reference. As a major tool particle motion analysis is used to locate the seismo-volcanic sources. Here, a surface correction is employed to account for the effects of the steep slopes of the volcanic edifice. After careful filtering of the data single seismic phases can be separated and linked to corresponding eruptive features.
On the origin of the long-period tremor recorded at Stromboli volcano (Italy)
Authors: T. Braun  , J. Neuberg  , M. Ripepe
Published: June 2021

This investigation deals with the nature of the long-period seismic signals (>1 s) observed at Stromboli and addresses the question whether they are of volcanic origin or produced by sources such as Ocean Microseisms (OMS). We present results from the analysis of seismic broadband data recorded during 1992 by an array of 9 Guralp CMG-3T seismometers. The determination of the Array Response Function (ARF) shows that array techniques like delay-and-sum beamforming cannot be applied for this purpose, as the extension of the array is limited by the geographical constraint of the island of Stromboli volcano, being simply too small. Spectral analysis reveals three main peaks with periods at 4.8 s, 6 s and 10 s which are not stable in time but vary according to the regional meteorological situation. Whereas 4.8 s and 10 s show up in amplitude spectra calculated during rainy and stormy weather, the 6 s period can be observed during a period of good weather. The signals were first narrowly filtered and then cross correlation, particle motion and amplitudes of the main long periods studied in detail. Relative arrival times as well as seismic amplitudes of the filtered traces do not show any systematic feature but vary with time. Particle motion analysis demonstrates that all long-period signals are recorded by the array as plane waves and that the main propagation direction of the 10 s signal is parallel to the wind direction. No correlation with volcanic activity is obvious. We conclude therefore that the three main long periods are not generated by a close volcanic source. We assume a local cyclone to be the seismic source at 4.8 s and 10 s, which represent the Double Frequency (DF-band) and the Primary Frequency (PF-band), respectively. Concerning the 6 s peak, we speculate a cyclone near the British Isles to act as a seismic source.
A numerical study for convection in a cylindrical model with continuously varying viscosity
Authors: F. Fanucci  , A. Megna  , S. Santini  , F. Vetrano
Published: June 2021

In the framework of a cylindrical symmetry model for convective motions in the asthenosphere, a new profile for the viscosity coefficient depending on depth is suggested here. The numerical elaboration of the above mentioned model leads to interesting results which fit well with experimental observations. In particular these continuously varying viscosity solutions probably describe the convective motions within the Earth better than simple constant viscosity solutions. Consequently the temperature values seem to be a realistic representation of the possible thermal behaviour in the upper mantle.
Electron density profile modeling
Authors: R. G. Ezquer  , M. Mosert de Gonzalez  , T. Heredia
Published: June 2021

The Base Point Model (BPM) is used to model the electron density (N) profile in the ionosphere, This model assumes two Chapman profile expressions one for the bottomside and one for the topside, and requires a characteristic point called "F region base point". The comparison among the modeled and experimental bottom-side N profiles obtained from Tucuman (26,9°S; 65.4°W) ionosonde shows that, in general, there is a very good agreement within 30 km below the height of the maximum N(hm). Cases with a very good agreement for the entire N-profile are observed. The study of the electron content below hm and the Total Electron Content (TEC) measured over Tucuman shows that, the difference among predicted and measured TEC is due to the disagreement in the topside N-profile more than that observed in the bottomside N-profile.
Monthly median f0F2 and M(3000)F2 ionospheric model over Europe
Authors: A. V. Mikhailov  , V. V. Mikhailov  , M. G. Skoblin
Published: June 2021

Monthly median f0F2 and M(3000)F2 ionospheric model, MQMF2, based on the multiquadric (MQ) method of spatial interpolation and a new ionospheric index MF2 describes the monthly median f0F2 and M(3000)F2 over Europe for any UT moment, month and level of solar activity. The multiquadric method allows a surface to be drawn strictly over a given set of points unlike many other currently used ionosphere mapping methods. A non-linear dependence of f0F2 and M(3000)F2 on solar activity level (expressed by MF2 index) is used to establish local models for each ionosonde station. Observations on 28 ionosondes for f0F2 and 19 for M(3000)F2 over 10-30 years were used for model derivation. The MQMF provides better accuracy than the CCIR model in retrospective mode over Europe. Long-term f0F2 prediction with the help of MF2 index for the rising part of solar cycle 22 is shown to provide better prediction accuracy than the CCIR model based on sunspot number R12. MQMF2 is implemented as a code for PC AT-386/387 or compatible, providing tables, plots and maps.
Catalogue of tsunamis generated in Italy and in Côte d'Azur, France: a step towards a unified catalogue of tsunamis in Europe
Authors: S. Tinti  , A. Maramai
Published: June 2021

This work presents a catalogue of the tsunamis generated in the seas watering the Italian coasts, including the neighbouring area of Côte d'Azur (France). Events generated far from Italy and affecting the Italian coasts are not taken into account here. The catalogue, that we will also call the Quick-Look Catalog (QLC), is organised in three main sections that are named the Quick-Look Table, the Quick-Look Accounts File and the References File, having the respective abbreviations of QLT, QLAF and RF. The QLT is a synoptic table containing the relevant information available for each event, one table row corresponding to one event. More details are provided in the QLAF, where each event is dedicated a specific subsection: here the description of the tsunami includes all essential aspects that are suitably referenced and is preceded by a concise report concerning the tsunami cause. Lastly, the RF is the list of all the papers and publications quoted in the QLT and QLAF. Notice that efforts have been made to qualify each event by means of contemporaneous sources, although later sources and indirect sources, such as existing catalogues, have not been disregarded. Besides, specific recent studies on the events have been given special mention. In this work some general review of the past catalogues of tsunamis and of recent trends in the subject are expressed. Particularly, great attention is given to analysing the CFB of the Italian tsunamis due to Caputo and his collaborators (Caputo and Faita, 1984; Bedosti and Caputo, 1986), the acronym being formed by the ordered initials of the authors. Motivations clarifying the need for a new catalogue of the Italian tsunamis are illustrated circumstantially. The very different philosophies that are at the basis of the CFB and of the present QLC lead to quite diverse products and results, that are summarised by a table where the events included in the CFB and in the QLC are compared: the net effect of the rigorous scrutiny applied to the sources and of the coherent analysis applied to the data is that only 67 events are included in the QLC, which is about one third of the events that can be counted in the CFB.
Earthquakes in Rome during the past one hundred years
Authors: A. Tertulliani  , F. Riguzzi
Published: June 2021

We have studied the seismic response of the city of Rome using the available macroseismic data of local earthquakes which occurred during the past one hundred years. These earthquakes were generated by three dislinct seismogenic sources falling within the present extent of Rome. The comparison with the effect produced in Rome by a large Apennine earthquake (January 13. 1915) suggests that the damage patterns are similar and that they are mainly controlled by the local geology and morphology. The analysis shows that most of the damage was concentrated in buildings located on alluvial deposits of the Tiber River rather than in buildings underlain by different lithologies. In addition, the largest concentration of heavy darnage occurred in buildings located on the alluvial deposits of the right-hand side of the Tiber River valley, and particularly where the buried interface between Holocene and Pliocene deposits is steepest. This close relationship between damage pattern on the one hand, and geology and geometry of the shallowest deposits on the other hand, supports the results of ground motion modeling studies of the same area and similar observations collected in different regions of the world during large earthquakes.
Macroseismic intensity evaluation with the <>
Authors: G. Ferrari  , P. Gasperini  , E. Guidoboni
Published: June 2021

The use of a macroseismic scale often requires subjective choices and judgments which may produce inhomogeneities and biases in the resulting intensities. To get over this problem it would be necessary to formalize the decision process leading to the estimation of the macroseismic intensity but, on historical records, this is often hindered by the poorness and incompleteness of the aLailable information and by the intrinsical ambiguity of the common language. Moreover. all the intensity scales have always been created and updated to be used <<in the field. on contemporary earthquakes and then it may happen tkdt even detailed historical descriptions often do not correspond to the descriptive frameworks of any grade. I11 order to face these problems. we propose in this work a computer method for the evaluation of the macroseismic intensity which makes use of the <>T. his approach reproduces the taslts performed by the hurnan brain which. taking advantage of the tolerance of imprecision. is able to handle with information bearing only an approximate relation to the data. This allows to understand and make explicit some passes of the evaluation process that are unconsciously followed by the macroseismic exper
Seismic response of the geologic structure underlying the Roman Colosseum and a 2-D resonance of a sediment valley
Authors: P. Mozco  , A. Rovelli  , P. Labak  , L. Malagnini
Published: June 2021

The seismic response of the geologic structure beneath the Colosseum is investigated using a two-dimensional modeling for a vertically incident plane SH wave. Computations indicate that the southern part of the Colosseum may be exposed to a seismic ground motion with significantly larger amplitudes, differential motion and longer duration than the northern part. because the southern part of the Colosseum is underlain by a sedimentfilled valley created by sedimentary filling of the former tributary of the River Tiber. A 2-D resonance may develop in the valley. Unlike the previous theoretical studies on 2-D resonance in sediment-filled valleys, an effect of heterogeneous valley surroundings on the resonance is partly investigated. A very small sensitivity of the maximum spectral amplifications connected with the fundamental and first higher modes to the presence of a horizontal surface layer (with an intermediate velocity) in the valley surroundings is observed in the studied models.
Archaeologists and earthquakes: the case of 365 A.D.
Authors: A. Di Vita
Published: June 2021

Archaeological evidence of an earthquake in the second half of the 7th century A.D. on Crete is described and the 365 A.D. cruniversaln earthquake problem is resumed. Some general probleins regarding the study of seismic effects on archaeology in the Mediten-anean area are presented. A few imbalances are indicated due to the lack of an effective collaboration between technical experts and historians.
Marine geophysical methods for archaeological investigation of volcanic and bradyseismic areas
Authors: F. Giordano
Published: June 2021

The aim of this work was to test several geophysical methods for the identification and study of submerged prehistorical coastlines and archaeological sites. This research program was carried out in collaboration with the <> in the Gulf of Pozzuoli and dealt in particular with the bathymetric strips extending from - 5 m to - 50 m Within these strips we identified the coastline dating from the Rornan period. former beach boundaries associated with the vertical movements of the earth's surface caused by seismic-volcanic activity, and the variations in sea level following the climactic changes throughout the last 15000 years. The UNIBOOM system was used for this part of the programme, perinittiilg the identification of several coastlines and submerged beaches lying at different levels. The use of a modern Side Scan Sonar - for the morphological invesdgation of the sea bed - in a zone which had been the object of numerous archaeological surveys in the past, permitted previously unknown structures near the Lacuus Baianus to be identified. Other features worth pointing out include the operating speed of the system (15000 m'lfirst minute approx) and its observation capacity in cloudy waters compared to visible radiation, as well as its ability to penetrate thin layers of mud which generally impede direct underwater observation.
Historical evidence of faulting in Eastern Anatolia and Northern Syria
Authors: N. N. Ambraseys  , C. P. Melville
Published: June 2021

Historical data show that like the North Anatolian fault zone, which was delineated by a series of earthquakes during this century from east to west, so was the conjugate Eastern Anatolian fault zone delineated from the northeast to the southwest by a succession of large earthquakes in earlier times, with a major event at its junction with the Dead Sea fault system. This event was associated with surface faulting and occurred in a region seismically quiescent for nearly two centuries.
Main shock and aftershocks of the December 13, 1990, Eastern Sicily earthquake
Authors: A. Amato  , R. Azzara  , A. Basili  , C. Chiarabba  , M. Cocco  , M. Di Bona  , G. Selvaggi
Published: June 2021

n this paper we describe the location and the fault plane solution of the December 13, 1990, Eastern Sicily earthquake (ML = 5.4), and of its aftershock sequence. Because the main shock location is not well constrained due to the geometry of the permanent National Seismic Network in this area, we used a "master event" algorithm to locate it in relation to a well located aftershock. The revised location is slightly offshore Eastern Sicily, 4.8 km north of the largest aftershock (ML = 4.6) that occurred on December 16, 1990. The main shock has a strike-slip mechanism, indicating SE-NW compression with either left lateral motion on a NS plane, or right lateral on an EW plane. Two days after the main event we deployed a local network of eight digital stations, that provided accurate locations of the aftershocks, and the estimate of source parameters for the strongest earthquake. We observed an unusual quiescence after the ML = 5.4 event, that lasted until December 16, when a ML = 4.6 earthquake occurred. The fault plane solution of this aftershock shows normal faulting on E-W trending planes. Between December 16 and January 6, 1991, a sequence of at least 300 aftershock" was recorded by the local network. The well located earthquakes define a small source region of approximately 5 x 2 x 5 km3, with hypocentral depths ranging between 15 and 20 km. The paucity of large aftershocks, the time gap between the main shock occurrence and the beginning of the aftershock sequence (3.5 days), their different focal mechanisms (strike-slip vs. normal), and the different stress drop between main shock and after- shock suggest that the ML = 5.4 earthquake is an isolated event. The sequence of aftershocks began with the ML = 4.6 event, which is probably linked to the main shock with a complex mechanism of stress redistribution after the main faulting episode.
Geochemical evolution of groundwaterof the Iblean Foreland (Southeastern Sicily) after the December 13, 1990 earthquake (M = 5.4)
Authors: M. Dall'Aglio  , F. Quattrocchi  , S. Tersigni
Published: June 2021

Geochemical surveys were performed by the Istituto Nazionale di Geofisica (ING), between December 1990 and July 1991, in the framework of an interdisciplinary task study throughout the Siracusa epicentral area: these studies were aimed at col1ecting specific information on the geochemical patterns of fluids, in relation to the geodynamic and seismic evolution of the nol1hern Iblean Foreland area, stal1ing from the December, 13, 1990 Syracuse earthquake (M = 5,4). The results of the hydrogeochemical surveys, discussed in this paper, were in part unexpected. In particular, a steady decrease of the PCO2 va1ues, after the eal1hquake, in ground- waters of the epicentral area, along a NNW-SSE fault bordering the Augusta Graben (Brucoli Sulphureous Spring), was observed. This observation enabled us to reconstruct the geochemical processes triggered by the earthquake: a sudden and strong release of CO2 of deep origin, probably related to a pore pressure uprising and/or to a water/rock interaction changes in the vicinity of the seismogenic structure. The existence of deep- fluid uprising (CO2, 222Rn, NHj, H2S), as well as the variation in time of geochemica1 flows accompanying seismic activity along this NNW-SSE anomalous-sites 1ine, within the whole Iblean Foreland, witnes., the activity (as concern as fluidodynamic and geochemistry) of the NNW-SSE striking Ibleo-Maltese Escarpment fault system. This fact can be taken into account in locating the seismogenic structure responsible for the 1990 earthquake, like a contribution of the geochemical methods applied to seismotectonics. During June 1992, a more complete ana1ysis of the Iblean Foreland groundwaters was performed, co1lecting data on the geochemi- ca1 feature, of the different aquifers in aseismic period. Mu1tivariable statistics, chemical equilibria studies and mapping with our geochemical data, were also performed.
Inference of a seismic gap from geological data: Thessaly ( Central Greece) as a case study
Authors: R. Caputo
Published: June 2021

As a result of neotectonic, morphotectonic and seismotectonic research it is now possible to draw detailed maps of the major active faults affecting Thessaly, a large region of continental Greece. For many of these faults, where specific studies have been carried out, the degree of fault activity (i.e. the long-term slip-rate) has been also assigned ranging from 0.05 to 4 mm/yr-l. In the present work, the main morphotectonic features and seismotectonic characteristics of the more important faults are recalled from previous works, while the recent tectonic activity is compared with the seismic activity of the area. The occuuence of both large (M ?x2264; 6.0) and moderate (M ?x2265; 6.0) earthquakes during the present century is concentrated in the southern sector. Earthquakes have been virtually absent in the northern part of the region during the same period. In contrast, according to geological and geomorphological criteria, the recent (Late Quaternary) tectonic evolution of the region and the overall extensional rate do not seem to djffer significantly in the two sectors. Although palaeosejsmological trenches, geodetic surveying and the record of the microseismic activity may enhance our knowledge of this problem, on the basis of available geological (structural and morphological) data, it is likely that the northern sector of Thessaly represents a large seismic gap. The implications on seismic hazard in one of the more populated regions of Greece are also discussed.
Remanent and induced magnetization in the volcanites of Lipari and Vulcano (Aeolian Islands)
Authors: E. Zanella  , R. Lanza
Published: June 2021

The role of remanent and induced magnetization as sources of magnetic anomalies in the Lipari and Vulcano islands has been studied by systematic sampling. Remanent magnetization is higher than induced magnetization in almost all lithotypes. Its polarity is normal, and the mean directions are close to the present magnetic field. A slight thermal enhancement of the magnetic susceptibility occurs up to 450-500 °C, followed by a fall up to the Curie point, which is comprised in the range 550 ± 30 °C. This points to titanomagnetite as the main carrier of magnetization. The blocking temperature spectrum of the remanence ranges between the Curie point and 400 °C in most lithotypes, and falIs to 150-200 °C in the pyroclastic deposits. The results as a whole yield an outline of the areal distribution of the total magnetization intensity within the two islands.
Volcanomagnetic anomalies: a review and the computation of the piezomagnetic field expected at Vulcano (Aeolian Islands, Italy)
Authors: C. Del Negro  , G. Budetta  , F. Ferricci
Published: June 2021

he volcanic area of Vulcano experienced major unrest, which brought the fumarolic field temperatures from slightly less than 300 °C to ca. 700 °C between 1988-1993. The structure underlying the crater, investigated by drillings and by different geophysical techniques, is relatively well-known. This led us to attempt modelling the magnetic anomaly which could be generated by sudden pressure variations in the magma chamber at shallow depth. The rocks embedding the intrusive rock penetrated by drill-holes to a depth of ca. 2000 m are characterized by high susceptibility, which points to the possibility of obtaining significant magnetic anomalies with acceptably weak pressure pulses. The model for straightforward computing of the anomalous field was drawn accounting for (1) the inferred geometry of the Curie isotherrn, (2) presence of a spherical magma reservoir, 2 km wide and centred at a depth of 3.5 km, overlain by (3) a 0.5 km wide and 1.5 km high cylinder simulating the intrusion first revealed by drillings. The model elements (2) and (3) behave as a single source zone and are assumed to lie beyond the Curie point, the contribution to the piezomagnetic effect being provided by the surrounding medium. Under such conditions, a 10 MPa pressure pulse applied within the sourcezone provides a 4 nT piezomagnetic anomaly, compatible with the amplitude of the anomalies observed at those volcanoes of the world where magnetic surveillance is routinely carried out. The analytical method used for computation of the magnetic field generated by mechanical stress is extensively discussed, and the contribution of piezomagnetism to rapid variations of the magnetic field is compared to other types of magnetic anomalies likely to occur at active volcanoes.
Large teleseismic P-wave residuals observed at the Alban Hills volcano, Central Italy
Authors: G. B. Cimini  , C. Chiarabba  , A. Amato  , H. Mahadeva Iyer
Published: June 2021

We collected teleseismic waveforms from a digital microseismic network deployed by the Istituto Nazionale di Geofisica (ING) in collaboration with the U.S. Geological Survey (USGS), on the Alban Hills Quaternary volcano during the 1989-1990 seismic swann. About 50 events were recorded by the network, 30 of them by at least 4 stations. We analysed the data in order to image crustal heterogeneities beneath the volcano. The results show large delay time residuals up to - 1 second for stations located on the volcano with respect to station CP9 of the National Seismic Network located about 20 km to the east, on the Apennines. This suggests that the whole area overlies a broad low-velocity region. Although the ray coverage is not very dense, we model the gross seismic structure beneath the volcano by inverting the teleseismic relative residuals with the ACH technique. The main features detected by tbc inversion are a low-velocity zone beneath the southwestern fiank of tbc volcano, and a high-velocity region beneath the center. The depth extension of these anomalous zones ranges between 5 and 16 km. The correspondence between the low-velocity region and the most recent activity of the volcano (- 0.027 Ma) leads us to infer the presence of a still hot magmatic body in the crust beneath the southwestern side of the volcano, whereas the central part overlies the older and colder high-velocity volcanic roots related to the previous central activity (0.7 to 0.3 Ma).
The FDSN: history and objectives
Authors: A. M. DzIewonski
Published: June 2021

The origins of the Federation of Digital Seismograph Networks (FDSN) can be traced to the summer of 1984. At that time, GEOSCOPE - the French global network of broadband instruments - was already well under way, and in the United States, the Incorporated Research Institutions for Seismology (IRIS) had just published its Science Plan for Global Seismographic Network (GSN). There was clearly an opportunity and the need to involve scientists from other countries in planning for the future of global seismology. An ad hoc meeting of some ten West European seismologists had been arranged in August during the annual meeting of the European Geophysical Society in Louvain. This may be considered to signify the beginning of widescale international cooperation, even though this particular group eventually became the nucleus of ORFEUS (Observatories and Research Facilities for EUropean Seismology). Rather than taking an active role in deployment of new stations, it chose to focus on the issue of providing the service for data collection and exchange, with an important mission of developing the requisite software.
The China Digital Seismograph Network
Authors: Y. T. Chen  , Q. D. Mu  , G. W. Zhou
Published: June 2021

The China Digital Seismograph Network (CDSN) program was initiated in May 1983. On October 1, 1986, the CDSN began to distribute the network-day tapes to the research community. On October 22, 1987, the CDSN began full operation. The CDSN are supported by the State Seismological Bureau, People's Republic of China and the United States Geological Survey. The operation and maintenance of the network are taken by the staffs at the 10th Division of the Institute of Geophysics, State Seismological Bureau, QGSSB). At present the CDSN includes ten field stations, Le Beijing (BR), Lanzhou (LZH), Enshi (ENH), Kunming (KMI), Qiongzhong (QIZ), Shanghai (SSE), Urumqi (WMQ), Hailar (HIA), Mudanjiang (MDJ), and Lhasa (LSA), two national centers, i.e. the Network Maintenance Center (NMC) and the Data Management Center (DMC), both at the Institute of Geophysics, State Seismological Bureau, Beijing.
Preliminary calibration of candidate alpha stations in the GSETT-3 network
Authors: H. P. Harjes  , M. Jost  , J. Schweitzer
Published: June 2021

The technical concept of a future global seismic monitoring system includes 50 to 60 core stations, mostly arrays, which provide the primary detection and location capability. Due to the average station distance, these core (alpha) stations form a teleseismic network. Many of the proposed stations are to be newly installed and before the network can be regarded as fully operational, the stations have to be calibrated. As for traditional seismic networks, the station residuals - compared to a standard earth model - have to be determined. The standard earth model is defined in terms of travel-time tables and amplitude-distance curves. After recording a representative set of events, station residuals with respect to travel-time and magnitude can be calculated. In case of arrays, the determination of mislocation vectors (azimuth and slowness residuaIs) are of ulmost importance if array slowness vectors are used as starting solutions in a location procedure. Finally, in a monitoring context it is very important to estimate the station sensitivity for varying background noise conditíons and - in case of arrays - to know the frequency dependent improvement by beamforming. This paper uses the newly installed high-frequency GERESS array in Germany to demonstrate the calibration procedure.
Discrimination of teleseismic events in Central Asia with a local network of short period stations
Authors: T. Tiira  , M. Tarvainen
Published: June 2021

The difficult problem of distinguishing underground nuclear explosions from earthquakes at teleseismie distances was approached using short period seismic data from 6 stations in South and Central Finland. The events were nuclear tests mostly from the Semipalatinsk and Lop Nor test sites and earthquakes from adjacent areas. The magnitude range of the events was from 4.1 to 6.6. The features of the two classes of events were examined by computing spectral ratio, third moment of frequency (TMF) and complexity from P wave signals. The spectral discrimination parameters were extracted from spectra computed in 5 different ways in order to obtain all possible information even from weak events. The standard FFT spectra were computed from. raw data, after noise adaption and data adaption, from correlograms and using combinations of adaption and correlation: methods. This was done to employ not only the spectral differences of the events but also the temporal variation of energy and lack of it as a function of frequency. The optimum frequeney windows for spectral ratio and TMF were defined using stacked spectra of about 10 events from both classes. No single discriminant could classify all the events. Their performance varied significanfly for different stations, but on average the spectral discriminants had slightly higher discrimination capability than complexity. The distributions of all discriminants were studied and a group separation function was formed using an optimum set of discriminants. Instead of discriminant values their relative positions in the corresponding distributions of nuelear tests and earthquakes were used as inputs to the function. A weight for each discriminant was derived from the amount of overlap in the distributions of earthquakes and nuelear tests. All 75 events in the data set were correctly classified with the method. The testing was performed with a jack knife method to create an independent test data base
PASHA: regional long-term predictions of ionospheric parameters by ASHA
Authors: G. De Franceschi  , A. De Santis
Published: June 2021

In this paper it is shown that the adjusted spherical harmoníc analysis (ASFIA), previously used for the regional mapping of the monthly medians of the critical frequency of the F2 ionospheric layer (f0F2), can be adapted to obtain a regional Iong-term prediction model of the ionospheric parameters. By assuming that, at a fixed month, the monthly median value of f0F2 is a function of the geographic longitude lambda and colatitude teta and of the 12-month running mean value R12 of the Zurich sunspot number, i.e. f0F2 =f(lambda,teta, R12), PASHA (regional Iong-term predictions of ionospheric parameters by ASHA) is applied for mapping and modelling of this ionospheric parameter over Europe. Some examples are shown concerning the PASHA model results obtained for different solar activity conditions, seasons and geographic latitudes. The PC version of the FORTRAN 77 program is also available enabling the easy use of the PASHA model to obtain, as a final output, both a grid (2 x 2 degrees) of the predicted monthly medians of f0F2 in the European area and the 24 predicted monthly medians of the parameter at a given point in the region of interest.
Long-period P waveform modeling of upper mantle phases in the West Mediterranean basin
Authors: N. A. Pino
Published: June 2021

Long-period P waveforms of some Italian crustal earthquakes recorded at the WWSSN stations located in the Iberian Peninsula have been modeled to derive a 1D upper matle compressional velocity model. A technique based on the Cagniard-de Hoop method has been used to compute synthetic seismograms. Waveforms have first been computed for published velocity models referred to different tectonic provinces and compared with the data. A model that strongly improves the fits to the data is then presented. The proposed model, called WMP, is characterized by a 100 km thick lid overlaying a low velocity zone, a 1% velocity discontinuity located at 313 km depth, that is required to fit a lower amplitude phase, and an abrupt increase in the velocity gradient starting from 370 km. This latter is preferred to the sharp discontinuity located at about 400 km that is present in various models obtained for upper mantle structure with analogous techniques. Within the lid and the low-velocity zone, WMP displays features that are typical of old ocean structures like the Northwest Atlantic Ocean.
Interpretation of VLF-EM anomalies of 3D structures by using linear filtering techniques
Authors: M. Djeddi  , H. A. Baker  , A. Tabbagh
Published: June 2021

For almost four decades now, the VLF-EM method has been used to locate different conducting structures. The interpretation is usually carried out on a profile anomaly where a possible estimation of the depth and the dip can be obtained. The maps are only used, after applying the Fraser filter, to best position the anomaly and no further quantitative interpretation is given. In this paper a linear filtering technique is developed based on the X and Y derivatives of the VLF-EM anomaly. This technique helps in determining the exact dimensions of the causative structures as well as their depths regardless of their conductivities. This work also shows that the known Fraser filter that is usually applied on a profile data can be adapted, for the first time, in studying quantitatively 3D structures if it is applied to raw data in the X and Y directions. It can now be used to estimate the dimensions of the targets.
Applicability of the dipole-dipole method for structural studies: examples from the Northern Apennines
Authors: M. Giudici  , L. Alfano
Published: June 2021

We present some results of a geoelectrical investigation program conducted in the Northern Apennines, namely in the Val d'Aveto and Bobbio window and surrounding areas. Field activity included the execution of more than 50 vertical electrical soundings with continuous polar dipole-dipole spread. We image the geometries of some deep geological structures; in particular we found a resistive background, whose resistivity is different along the geoelectrical profiles. In our interpretation the resistive background consists of subligurid and tuscan units underlying the alloctone Ligurid units in the area surrounding the Val d'Aveto and Bobbio window. The resistive background was not found, at least at the same depths, toward north-east. Therefore, the geoelectrical survey revealed the position of the front of the subligurid and Tuscan nappes toward the plain for a depth of about one kilometer.
Integrated geoelectrical and magnetotelluric exploration at Gran Sasso d'Italia range (Central Apennines)
Authors: L. Alfano  , M. Giudici  , A. Zaja
Published: June 2021

We carried out three electrical soundings, with continuous polar dipole-dipole array, and three magnetotelluric stations at Gran Sasso d'Italia, in the area of Campo Imperatore. Geoelectrical data show the existence of a resistive background with exceptionally high values of resistivity (greater than 25000 ?  m) at a depth of about 250 m. The magnetotelluric data were used to constrain the thickness of this resistive layer, below which we find an intermediate conductive structure and a deeper resistive background. The geological interpretation of this result is still open to discussion, but we can exclude that the shallow resistive layer consists of Cretaceous limestones, since their resistivity has been estimated to be about 5000 ?  m.
Buried archeological remains connected to the Greek-Roman harbor at Tindari (north-east Sicily): results from geomorphological and geophysical investigations
Authors: Carla Bottari  , Stefano Urbini  , Marcello Bianca  , Maria D'Amico  , Marco Marchetti  , Francesco Pizzolo
Published: June 2021

In recent years, detailed geoarcheological investigations have been carried out to search for traces of the ancient Tindari harbor (north-east Sicily, Italy). A digital terrain model supported the hypothesis that 2,000 yr ago the Oliveri Basin was a suitable landing place that was protected from prevailing winds. This model was generated from uplift data, sea level changes, historical cartographic data and three-dimensional reconstruction of the sedimentary succession of the cover. The present position of some historical buildings represent an archeological marker of the shoreline progression. Recent excavations during the construction of the Messina-to-Palermo motorway brought to light some portions of an ancient archeological complex. The thickness of the walls and the volume of the collected archeological material suggests dating between the 1st century BC and the 4th century AD. After that time, heavy environmental changes due to human activities in the area led to inaccurate underestimation of the role of Tindari harbor in the past. A geophysical investigation was carried out in the area surrounding the archeological complex to search for new buried structures related to the ancient settlement, and to be open to any results of the paleotopographic reconstruction of the area. The applied geophysical techniques included passive seismic and ground-penetrating radar. This survey indicates the presence of buried structures, such as walls and floors, that probably belong to a Roman villa. Furthermore, it defines the depth of the Holocene sedimentary cover of the Oliveri coastal plain, which strengthens the hypothesis formulated for its morphological evolution.
The velocity of P-waves below the " 20" discontinuity "
Authors: B. FEDERICO
Published: June 2021

Velocity values for P-waves below the " 2 0 ° discontinuit y " are calculated by applying the Wadati-Masuda method, and by using separately calculated travel-time curves for the P-waves and the PdPwaves of each of the five earthquakes examined. The velocities calculated have a mean value of 10.21 ± 0.24 km/sec, and are without exception greater than those obtained in a previous piece of research (9.57 km/sec). As regards the value of the velocity of the P-waves above the "20° discontinuity" (8.96 kin/sec), the percentage difference between the velocity above and the velocity below the "20° discontinuity" is 14%. Application of the Herglotz-Wiechert procedure in the zone of the mantle below the "20° discontinuity" does not give significant variations in the correlation between velocity and depth, in comparison with the results obtained by other authors.
The aftershock sequence of the north-west Kashmir earthquake of September 3, 1972
Authors: V. K. SRIVASTAVA  , R. K. S. CHOUHAN  , R. NIGAM
Published: June 2021

This paper is an attempt to study the aftershock sequence of the Northwest Kashmir earthquake of September 3, 1972. b value of the sequence is 1.59. The area of active strain zone is approximately 2.9 • ]013 sq. cm. The total average strain, average elastic energy and average stress of the rock before slip are 3.3 • 10~5, 3.2 ergs/cm3 and 19.8 kg/cm2. The strain rebound curve of the sequence has been constructed which shows a dual type of recovery where the compressional elastic creep is followed by the shear creep recovery. The relaxation time of the sequence is about 0.7 day, showing the Kelvin body like behaviour of the upper mantle.
The ancient and contemporary geophysical stress field in a calcareous mountain-range
Authors: A. E. SCHEIDEGGER
Published: June 2021

The stress systems wliich are and have been active in a well-defined calcareous mountain massif (the Iloclikonig massif in Austria) have been determined: (I) f r om joint orientations in the Paleozoic basement, (II) from joint orientations in the calcareous superstructure and (III) from in situ stress measurements in a mine beneath the massif. It was found (I) t h a t t h e stresses which produced the Paleozoic joint system is quite distinct f r om t h e other systems active in the area, (II) t h a t ' t h e joints in the supers t r u c t u r e correspond to a stress system t h a t is in conformity with the general north-south thrust thought to have created the Alps and (III) that t h e stresses measured in situ represent nothing but the effect of the presentday overburden topography.
Some characteristics of the seismicity of the Tyrrhenian Sea Region
Authors: S. MESZAROS  , P. HEDERVARI
Published: June 2021

In the first p a r t of the paper the seismic strain release of the T y r r h e n i a n Sea Region (including Italy), as the function of time, is examined on the basis of t h e d a t a of the e a r t h q u a k e s t h a t took place f r om 1901.01.01 to 1970.12.31, between the northern l a t i t u d e s of 34° and 44° and between the eastern longitudes of 8° and 18.5°, respectively. All registered shocks with a R i c h t e r - m a g n i t u d e of 5.5 or over it were considered, i n d e p e n d e n t l y f r om t h e focal d e p t h . Three periods were recognized in the a c t i v i t y ; t h e lengths of which are not t h e same, however. I n the second p a r t the elastic strain release in accordance with the focal d e p t h of t h e same e a r t h q u a k e s is t r e a t e d briefly. It was found t h at t h e t o t a l strain-release had a maximum value in t h e depth between 0 and 74 kms and there was a minimum between the depth of 300 and 524 kins with an interval between 375 and 449 kms within which no earthquakes occurred at all. The general p a t t e r n of the d i s t r i b u t i o n of seismicity as t h e f u n c t i o n of hypocentral d e p t h reminds to the well-known picture, one can experience in other regions where i n t e r m e d i a t e and deep shocks occur. This s t a t e m e n t is consistent w i t h t h e idea, according to which t h e seismicity of t h e Tyrrhenian Sea Region can be discussed and explained in t h e light of t h e theory of new global tectonics. F i n a l l y , in the t h i r d p a r t of the study, the authors have s t a t e d t h at in some cases multiple events occurred b e n e a t h t h e Tyrrhenian Sea Region. Such multiple seismic events were detected in the case of other areas, such as the Fiji-Tonga-Kermadec Region, the seismic belt of South America etc., — but, according to the knowledge of t h e authors, this is t h e first occasion when multiple seismic events are demonstrated in the Tyrrhenian Sea Region.
Fabry–Perot interferometer as a solar background noise suppressor: application to daytime lidar
Authors: Karnam Raghunath  , Karnam Ramesh  , Sanama Narayana Reddy
Published: June 2021

Continuous atmospheric probing by a lidar is a requirement for many applications. However, due to high solar background noise during the daytime, lidar operations are mostly restricted to night-time. While many techniques are in practice, like reducing the receiver field of view, changing the view angle, introducing a narrow band Interference Filter (IF), these are applied to circumvent problems, rather than to suppress the noise. Using a Fabry-Perot interferometer as a narrow passband filter for solar background noise suppression is a known technique, and its potential is exploited in our system. An optical-fiber-coupled lidar system with its transmitter injection seeded was developed and has been operated during the daytime at Gadanki (13.6˚N, 79.2˚ E). The signal-to-noise ratio of the return signal is used as the performance indicator, to evaluate the improvements. Signal-to-noise ratios with and without the Fabry-Perot interferometer are measured with near identical test set-ups. The signal-to-noise ratio enhancement factor is ca. 4, in agreement with the theoretical value. The performance is compared when the receiver fields of view are changed.