bluuebunny/crossref_metadata_2025 huggingface dataset:
Re-evaluation of early instrumental earthquake locations: methodology and examples
Authors: R. D. Adams
Published: June 2021

The difficulties of locating earthquakes in the early instrumental period are not always fully appreciated. The networks were sparse, and the instruments themselves were of low gain, often had inappropriate frequency response and recording resolution, and their timing could be unreliable and inaccurate. Additionally, there was only limited knowledge of earth structure and consequent phase identification and propagation. The primitive Zöppritz tables for P and S, with no allowance for the core, did not come into use until 1907, and remained the main model until the adoption of the Jeffreys-Bullen tables in the mid-1930s. It was not until the early 1920s that studies of Hindu Kush earthquakes revealed that earthquake foci could have significant depth. Although many early locations are creditably accurate, others can be improved by use of more modern techniques. Early earthquakes in unusual places often repay closer investigation. Many events after about 1910 are well enough recorded to be re-located by computer techniques, but earlier locations can still be improved by using more recent knowledge and simpler techniques, such as phase re-identification and graphical re-location. One technique that helps with early events is to locate events using the time of the maximum phase of surface waves, which is often well reported. Macroseismic information is also valuable in giving confirmation of earthquake positions or helping to re-assess them, including giving indications of focal depth. For many events in the early instrumental period macroseismic locations are to be preferred to the poorly-controlled instrumental ones. Macroseismic locations can also make useful trial origins for computer re-location. Even more recent events, which appear to be well located, may be grossly in error due to mis-interpretation of phases and inadequate instrumental coverage. A well converging mathematical solution does not always put the earthquake in the right place, and computer location programs may give unrealistically small estimates of error. Examples are given of improvements in locations of particular earthquakes in various parts of the world and in different time periods.
Crustal structure at the western end of the North Anatolian Fault Zone from deep seismic sounding
Authors: A. E. Karahan  , H. Berckhemer  , B. Baier
Published: June 2021

The first deep seismic sounding experiment in Northwestern Anatolia was carried out in October 1991 as part of the "German - Turkish Project on Earthquake Prediction Research" in the Mudurnu area of the North Anatolian Fault Zone. The experiment was a joint enterprise by the Institute of Meteorology and Geophysics of Frankfurt University, the Earthquake Research Institute (ERI) in Ankara, and the Turkish Oil Company (TPAO). Two orthogonal profiles, each 120 km in length with a crossing point near Akyazi, were covered in succession by 30 short period tape recording seismograph stations with 2 km station spacing. 12 shots, with charge sizes between 100 and 250 kg, were fired and 342 seismograms out of 360 were used for evaluation. By coincidence an M b = 4.5 earthquake located below Imroz Island was also recorded and provided additional information on Moho and the sub-Moho velocity. A ray tracing method orginally developed by Weber (1986) was used for travel time inversion. From a compilation of all data two generalized crustal models were derived, one with velocity gradients within the layers and one with constant layer velocities. The latter consists of a sediment cover of about 2 km with V p » 3.6 km/s, an upper crystalline crust down to 13 km with V p » 5.9 km/s, a middle crust down to 25 km depth with V p » 6.5 km/s, a lower crust down to 39 km Moho depth with V p » 7.0 km/s and V p » 8.05 km/s below the Moho. The structure of the individual profiles differs slightly. The thickest sediment cover is reached in the Izmit-Sapanca-trough and in the Akyazi basin. Of particular interest is a step of about 4 km in the lower crust near Lake Sapanca and probably an even larger one in the Moho (derived from the Imroz earthquake data). After the catastrophic earthquake of Izmit on 17 August 1999 this significant heterogeneity in crustal structure appears in a new light with regard to the possible cause of the Izmit earthquake. Heterogeneities in structure are frequently also heterogeneities in strength and stress that impede or even lock rupture. The Izmit earthquake is discussed in relation to a large stepover or jog at the North Anatolian Fault.
Strain pattern of the Southern Tyrrhenian slab from moment tensors of deep earthquakes: implications on the down-dip velocity
Authors: G. Selvaggi
Published: June 2021

Seismic strain is analysed along the slab face of the Southern Tyrrhenian subduction zone using focal mechanisms of deep earthquakes which occurred in the period 1960-1998. Results show that the slab is mostly affected by down-dip shortening which strongly increases below 250 km depth. Extensional strain is mainly confined to the direction perpendicular to the slab face. The dominance of down-dip shortening and the minor along strike inplane extension implies thickening of the slab below 250 km depth. Assuming constant seismic efficiency along the slab, this strain pattern also implies a decrease of the down-dip velocity below 250 km depth. We also locate lower magnitude intermediate-depth and deep earthquakes using arrival times since 1985 available from the Italian seismic national network. These data show that the slab reaches the deeper part of the upper mantle, as suggested by the occurrence of a few ??600 km depth earthquakes, and that a large portion of the Tyrrhenian slab, between 100 and 250 km depth beneath the offshore of the Calabrian arc, is aseismic. Only a short part of the Tyrrhenian slab is seismically continuous from the top to the bottom. The lack of seismicity may indicate either that aseismic subduction is occurring or that the slab is detached from its upper part. Although the data are still inconclusive, they suggest that an aseismic subduction is the most appropriate interpretation, considering recent tomographic images of the slab and the results of this study, which agree well with the presence of a neutral down-dip stress zone, as also observed worldwide in deep slabs.
Antipersistent dynamics in short time scale variability of self-potential signals
Authors: V. Cuomo  , M. Lanfredi  , V. Lapenna  , M. Macchiato  , M. Ragosta  , L. Telesca
Published: June 2021

Time scale properties of self-potential signals are investigated through the analysis of the second order structure function (variogram), a powerful tool to investigate the spatial and temporal variability of observational data. In this work we analyse two sequences of self-potential values measured by means of a geophysical monitoring array located in a seismically active area of Southern Italy. The range of scales investigated goes from a few minutes to several days. It is shown that signal fluctuations are characterised by two time scale ranges in which self-potential variability appears to follow slightly different dynamical behaviours. Results point to the presence of fractal, non stationary features expressing a long term correlation with scaling coefficients which are the clue of stabilising mechanisms. In the scale ranges in which the series show scale invariant behaviour, self-potentials evolve like fractional Brownian motions with anticorrelated increments typical of processes regulated by negative feedback mechanisms (antipersistence). On scales below about 6 h the strength of such an antipersistence appears to be slightly greater than that observed on larger time scales where the fluctuations are less efficiently stabilised.
The influence of active meteorological systems on the ionospheric F-region
Authors: C. R. Martinis  , J. R. Manzano
Published: June 2021

Effects on the F region of two active meteorological systems will be analyzed. These systems are known as Mesoscale Convective Complexes (MCCs). Ionospheric data from a vertical sounder located in Tucumán will be used. By comparing the behaviour of the F region parameters on the days before and after the MCC storm day, we see outstanding differences. These differences occur during night and dawn hours in both cases. The two phenomena show influences on the F region. One case shows an increase in electronic concentration followed by a decrease and the other shows the opposite effect. Gravity wave propagation from the top of clouds could be connected to these MCCs effects. Other possible physical mechanisms are also discussed.
Observations of two special kinds of tremor at Galeras volcano, Colombia(1989-1991)
Authors: F. Gil-Cruz
Published: June 2021

Since the reactivation of Galeras volcano in 1988 its seismic activity has been dominated by a variety of LP waveforms and tremor events. Some of these signals occurred as a response to volcanic activity. Among them, two kinds of tremor deserve special attention, Flute tremor and Spasmodic tremor. Flute tremor has a spectrum of equally spaced peaks and is associated with a quasi-steady degassing process at the top of the lava dome. It is accompanied by a flute-like sound. Its spectral features and the correlation with field observations are consistent with a model generation indicating that a crack or set of cracks are excited to resonance by the release and flow of gas through the lava dome. Spasmodic tremor is composed of several distinct LP-like events joined together by a continuous signal with lower amplitudes. Two types of spasmodic tremor may be distinguished on the basis of their spectral characteristics and field observations. Spasmodic tremor type I is apparently dominated by a mix of P, SH and Rayleigh waves as determined from preliminary polarization analysis. The source appears to be located, in a region west of the active crater. As a first approximation, Spasmodic tremor type I could be associated with magmatic intrusion process occurred in 1989-1991.
Analysis and quasistatic FE modeling of long period impulsive events associated with explosions at Stromboli volcano (Italy)
Authors: M. Kirchdörfer
Published: June 2021

Broadband seismic measurements performed in 1995 and 1996 in the summit region of Stromboli are analyzed. The experiment in 1995 used an array of four Guralp seismometers and one Wielandt-Streckeisen seismometer. The stations were installed around the craters in a semicircle with a radius of about 500 m. This implies that the seismic signals are dominated by near field motions up to frequencies of about 2 Hz. The observed Strombolian explosions are preceded by long-period ground motions occurring between 20 s and 70 s prior to the ejections. They are obviously generated by a slow pressure increase within the magma conduits. The long-period signals are simple compared to the short period wave forms. Four classes of pulse-shaped seismograms can be distinguished. The radiation pattern is radially symmetric with respect to the crater region. Particle motion analysis indicates that the seismic sources are located between 50 and 200 m below the crater terrace. Hydrostatic model sources were studied by means of finite element calculations with different geometries, i.e. ellipsoids, in a solid cone modeling the topography of Stromboli. The results suggest that the explosive events on Stromboli originate from a shallow vertically elongated volume source.
Source process of the October 12, 1992 Cairo earthquake
Authors: H. M. Hussein
Published: June 2021

Broadband body waves recorded at 12 digital seismic stations worldwide were used to study the source process of the October 12, 1992 Cairo earthquake. To study the source process of this event the P and SH waveforms from IRIS data center were inverted to double couple source using the method Kikuchi and Kanamori (1991) in which the rupture is presented by discrete subevents with various mechanisms. The best solution consists of only one event with a mechanism 270°/47°/-123° (strike/dip/slip), a normal faulting mechanism with small strike slip component. This solution is almost compatible with the previously suggested mechanisms for the same event. This event took place at a depth of 22 km. This depth explains the lack of surface faulting. The seismic moment is 7.2 ´ 10 17 Nm (Mw = 5.8) with a source duration of 4 s. The estimated fault length is about 11 km, the derived average dislocation (D) is 0.24 m, the stress drop (Ds ) is 1.85 MPa and the Orwan stress drop is 0.425 MPa.
High resolution electromagnetic methods and low frequency dispersion of rock conductivity
Authors: B. S. Svetov  , V. V. Ageev
Published: June 2021

The influence of frequency dispersion of conductivity (induced polarization) of rocks on the results of electromagnetic (EM) sounding was studied on the basis of calculation of electric field of vertical magnetic dipole above horizontally layered polarizable sections. Frequency dispersion was approximated by the Debye formula. Polarizable homogeneous halfspace, two, three and multilayered sections were analyzed in frequency and time domains. The calculations for different values of chargeability and time constants of polarization were performed. In the far zone of a source, the IP of rocks led to quasi-wave phenomena. They produced rapid fluctuations of frequency and transient sounding curves (interference phenomena, multireflections in polarizable layers). In the case of transient sounding in the near zone of a source quasistatic distortions prevailed, caused by the counter electromotive force arising in polarizable layers which may lead to strong changes in transient curves. In some cases quasiwave and quasistatic phenomena made EM sounding curves non-interpretable in the class of quasistationary curves over non-dispersive sections. On the other hand, they could increase the resolution and depth of investigation of EM sounding. This was confirmed by an experience of "high-resolution" electroprospecting in Russia. The problem of interpretation of EM sounding data in polarizable sections is nonunique. To achieve uniqueness it is probably necessary to complement them by soundings of other type.
Seismic quiescence precursor to the 1983 Nihonkai-Chubu (M 7.7) earthquake, Japan
Authors: M. Murru  , R. Console  , C. Montuori
Published: June 2021

We analyzed the seismicity of Northern Honshu-Hokkaido region using the declustered earthquake catalog compiled by the Japan Meteorological Agency (JMA), for the period between January 1970 and December 1994. Making use of the ZMAP software tool, we sought to determine whether the quiescence hypothesis is applicable to 16 main shocks (M ³ 7.0) of the JMA catalog. We found a highly significant seismic quiescence prior to the May 26, 1983 Nihonkai-Chubu, MJ 7.7, earthquake. The quiescence that preceded the event lasted more than 3.5 years, and was located in the Japan Sea, off Akita and Aomori Prefectures. It was characterized by a standard deviate Z = 7.4 (Tw = 3 years), within a volume of approximately 200 by 300 by 40 km around the hypocenter. This volume contained 16 earthquakes (M ? 3.8) during the background period which lasted more than 8 years preceding the quiescence, and none during this one. A high concentration of seismic activity exceeding 15 events per year followed the main shock before the rate returned back to the previous value. No quiescence was observed before another strong event of magnitude 7.8 which occurred on July 12, 1993, 200 km north of the former, while three quiescences with the same Z-value were observed in the same region and in the same time period, not followed by any main shocks (false alarms). The probability that the Nihonkai-Chubu earthquake correlated at random with the quiescence period is estimated as approximately 1%, based on the fraction of space-time covered by alarms. The seismicity rate variations observed before and after the 1983 Nihonkai-Chubu earthquake are similar to those observed in the rupture area of the 1980 Irpinia (Italy), M 6.9 earthquake.
A proposal of regionalization for the application of the CN earthquake prediction algorithm to the Italian territory
Authors: A. Peresan  , G. Costa  , G. F. Panza
Published: June 2021

A regionalization of the Italian territory, strictly based on seismotectonic zoning and the main geodynamic features of the Italian area, is proposed for intermediate-term earthquake prediction with CN algorithm. Three regions, composed of adjacent zones with the same seismogenic behaviour or with transitional properties, are selected for the north, centre and south of Italy, compatibly with the kinematic model. This regionalization allows us an average reduction of the spatial uncertainty of about 35% for the northern and central regions, and of about 70% for the southern region in comparison with previous studies. A general reduction of the percentage of total TIPs, with respect to the results obtained neglecting the seismotectonic zoning, has been observed as well. Therefore, it seems that the seismotectonic model is a useful tool selection of the fault systems involved in the preparation of strong earthquakes. The successful attempt of catalogue upgrading, accomplished using the NEIC Preliminary Determinations of Epicentres, appears to substantiate the robustness of the algorithm against changes in the catalogue.
Seismic hazard assessment of Iran
Authors: B. Tavakoli  , M. Ghafory-Ashtiany
Published: June 2021

The development of the new seismic hazard map of Iran is based on probabilistic seismic hazard computation using the historical earthquakes data, geology, tectonics, fault activity and seismic source models in Iran. These maps have been prepared to indicate the earthquake hazard of Iran in the form of iso-acceleration contour lines, and seismic hazard zoning, by using current probabilistic procedures. They display the probabilistic estimates of Peak Ground Acceleration (PGA) for the return periods of 75 and 475 years. The maps have been divided into intervals of 0.25 degrees in both latitudinal and longitudinal directions to calculate the peak ground acceleration values at each grid point and draw the seismic hazard curves. The results presented in this study will provide the basis for the preparation of seismic risk maps, the estimation of earthquake insurance premiums, and the preliminary site evaluation of critical facilities.
Seismic hazard assessment for Central, North and Northwest Europe: GSHAP Region 3
Authors: G. Grunthal  , The GSHAP Region Working Group
Published: June 2021

The activities of the Regional Centre 3 of the Global Seismic Hazard Assessment Program (GSHAP) covering Europe north of 46°N and west of 32°E are summarized starting with the establishment of the GSHAP Centre at the GFZ Potsdam in 1993 and leading finally in the calculation and creation of the GSHAP seismic hazard map in terms of horizontal Peak Ground Acceleration (PGA). Moreover, the activities of separate working groups which contribute with their results for certain parts of the study area to the final product of the Regional Centre are described. Details are given on the development of the homogeneous seismicity working file, the delineation of seismic source zones, the data preprocessing as well as on the chosen PGA-attenuation relations.
Measurements of ionospheric TEC in the direction of GPS satellites and comparison with three ionospheric models
Authors: G. Mirenna  , F. Riguzzi  , E. Zuccheretti
Published: June 2021

The IEN Galileo Ferraris uses GPS for time and frequency synchronization. To obtain high performance it is important to reduce the error due to the ionospheric time-delay in GPS measurements. Evaluations of TEC in the direction of GPS satellites, obtained from three different ionospheric models, have been compared with corresponding measurements by GPS signal.
Palaeomagnetic evidence for non-rotational deformation along the Nea Anchialos Fault System, Central Greece
Authors: D. Kondopoulou  , R. Caputo
Published: June 2021

Four sites in 1.4 Ma old basaltic lavas and two sites in upper Pliocene-lower Quaternary sediments, located both in the hanging-wall and in the footwall blocks of the Nea Anchialos Fault System, have been sampled. This fault system is one of the major E-W trending structures affecting the Thessaly region during Quaternary times.It is possibly connected with the North Aegean Trough to the E and displays recent seismic activity (1980, Volos earthquake). Standard techniques have been used for both field sampling and laboratory analyses. The magnetic carriers were characterised by measuring the thermomagnetic curves, the anisotropy of magnetic susceptibility and the isothermal remanent magnetisation of the samples. Almost all the samples exhibit a reverse polarity. The overall mean paleomagnetic direction is defined by D = 174°, I = 54°, confirming the non-rotational deformation pattern of the Nea Anchialos Fault System as independently inferred from structural investigations.
The magnetic anisotropy of rocks: principles, techniques and geodynamic applications in the Italian peninsula
Authors: A. Winkler  , L. Alfonsi  , F. Florindo  , L. Sagnotti  , F. Speranza
Published: June 2021

Magnetic anisotropy studies have recently come to the forefront as accurate, fast and inexpensive methods in the investigation of the rock fabric. In this paper we summarize the physical principles and the experimental techniques commonly used to resolve the Anisotropy of Magnetic Susceptibility (AMS) and the Anisotropy of Anhysteretic Remanence (AAR) tensors, and we give a description of the parameters which usually describe the magnetic anisotropy properties of a rock. A synthetic review of the magnetic fabric studies carried out on sedimentary rocks of the Italian peninsula is also given, discussing the potentiality of this technique in geodynamic studies.
Retrieval of large volcanomagnetic effects observed during the 1981 eruption of Mt.Etna
Authors: C. Del Negro  , F. Ferrucci  , R. Napoli
Published: June 2021

A large temporal anomaly was retrieved in the total geomagnetic field series recorded in 1981 on Mt. Etna at two continuously recording magnetometers, and associated with the March 17-23 eruption of the volcano. Variations were of such large scale that a 10 nT anomaly was observed at a distance of some 7 km from the eruptive events, calling for a significant extension and depth of the magnetic anomaly source. We discuss here some models which may account for such magnetic changes in relation to the eruption mechanism inferred by other data. The anomaly is thought to be accounted for by the joint effect of piezomagnetism of the country rocks and thermal demagnetisation engendered by a large intrusive dyke.
Magnetic effects due to earthquakes and underground explosions: a review
Authors: V. V. Surkov  , V. A. Pilipenko
Published: June 2021

The physical nature of quasi-static and transient anomalies in the geomagnetic field induced by underground explosions or earthquakes is reviewed. New theoretical results obtained recently and so far little known to general circles of geophysicists are presented. The physical nature of residual magnetic and electrotelluric fields at the explosion point are considered. The seismic waves from explosions or distant earthquakes are suggested to be used as a tool for the preliminary probing of the Earth's crust sensitivity to various seismo-electromagnetic effects. The use of magnetic induction effects for tsunami detection and for crust sounding is outlined. The nature of ULF magnetic impulses related with earthquakes is discussed.
Comparison between geoelectric and electromagnetic sounding responses in volcanic areas
Authors: M. Giudici  , L. Alfano
Published: June 2021

The structure of active and inactive volcanoes can be explored with electric and electromagnetic surveys. We test the actual applicability of prospecting methods that employ both stationary and time-varying fields, using mathematical models both for layered and complex structures. The geometry and the resistivities of the geological structures which are considered have been taken from real case studies reported in the literature. In particular we analyse the sensitivity of different methods to conductive and resistant bodies.
The latest aspects of telluric and electromagnetic variations associated with shallow and intermediate depth earthquakes in the South Aegean
Authors: K. Nomikos  , F. Vallianatos  , I. Kaliakatsos  , E. Sideris  , M. Bakatsakis
Published: June 2021

Recent observations of telluric and electromagnetic field variations (radioemissions) assocuated with shallow and intermediate depth earthquakes in the South Aegean are presented. A telemetric system, installed on Crete Island, which continuously records the telluric and electromagnetic field variations is described. In the field stations of the system we measure both the telluric variations (using an appropriate electric dipole configuration) and the horizontal electric component of the electromagnetic field in 3 kHz, 10 hKz, 41 MHz and 53 MHz (using tuned and l/2 antennas respectively. Furthermore, a theoretical model is given on the generation and the form of preseismic telluric variations based on the charge dislocation model.
The iterative signal enhancing method for determining magnetotelluric impedance
Authors: C. Braitenberg  , M. Zadro
Published: June 2021

The methodologies applied by our working group in Trieste for determining magnetotelluric impedance are presented, with special attention to the method used to evaluate the mean impedance over the entire observation interval using a robust procedure. The performance of our method is tested on an example data series, in order to compare it with other published methodologies. We make reference to a recorded time series of the EMSLAB traverse, which has been used to test eight different methods, the results of which have been collected in a published paper.
The Kriging method of ionospheric parameter f0F2 instantaneous mapping
Authors: I. Stanislawska  , G. Juchnikowski  , L. R. Cander
Published: June 2021

This paper discusses the spatial variation of the ionospheric characteristic f0F2 in the PRIME area (35o-55o N,- 10°-20°E) expressed in terms of the variogram and the Kriging mapping method as modelling of an instantaneous experimental situation. It is shown that by applying this method to the real data it is possible to estimate f0F2 at any unsampled location within the restricted area with satisfactory accuracy.
Conception of a macroseismic catalogue for Catalonia (Spain)
Authors: M. T. Susagna  , X. Goula  , A. Roca
Published: June 2021

Since 1989 the Servei Geològic de Catalunya has been collecting macroseismic information of the North East of Spain and the South of France to create a comprehensive and reliable Catalogue which would be of use in seismic hazard assessment. Existing compilations have been submitted to a critical analysis and comparison, bearing in mind the resu1ts of recent historical research. For the seismicity of the present century macroseismic information is being studied by analyzing the original questionnaires. For each earthquake the felt intensities are stored on a data bank system. Due to the geographic location of the area under scrutiny, near the French border, collaboration with French agencies has been necessary to achieve joint information for Pyrenean earthquakes. Ancient instrumental records taken in two observatories (FBR and EBR) working since the beginning of this century have helped to determine focal parameters.
Crustal velocity structure of the Apennines (Italy) from P-wave travel time tomography
Authors: C. Chiarabba  , A. Amato
Published: June 2021

In this paper we provide P-wave velocity images of the crust underneath the Apennines (Italy), focusing on the lower crustal structure and the Moho topography. We inverted P-wave arrival times of earthquakes which occurred from 1986 to 1993 within the Apenninic area. To overcome inversion instabilities due to noisy data (we used bulletin data) we decided to resolve a minimum number of velocity parameters, inverting for only two layers in the crust and one in the uppermost mantle underneath the Moho. A partial inversion of only 55% of the overall dataset yields velocity images similar to those obtained with the whole data set, indicating that the depicted tomograms are stable and fairly insensitive to the number of data used. We find a low-velocity anomaly in the lower crust extending underneath the whole Apenninic belt. This feature is segmented by a relative high-velocity zone in correspondence with the Ortona-Roccamonfina line, that separates the northern from the southern Apenninic arcs. The Moho has a variable depth in the study area, and is deeper (more than 37 km) in the Adriatic side of the Northern Apennines with respect to the Tyrrhenian side, where it is found in the depth interval 22-34 km.
Long-period modelling of MEDNET waveforms for the December 13, 1990 Eastern Sicily earthquake
Authors: D. Giardini  , B. Palombo  , N. A. Pino
Published: June 2021

The availability of broad-band digital data allow "the analysis" of the seismic signal in the low-noise frequency band for structural and seismic source studies, We model complete seismograms -surface and body waves - for the December 13, 1 990, Eastern Sicily earthquake, recorded at regional distances on the MEDNET stations AQU, BNI and KEG, The inversion for the moment tensor is carried out following two approaches: a) a model-independent stralegy to fit complete seismograms in the 100-130 s period range; b) the calibration of phase velocity curves along each path to fit surface waves in the 40-100 s range, Both methodologies yield stable and consistent results: the 1990 Eastern Sicily earthquake had a seismic moment of M/o = 37 x 1024 dyne.cm, corresponding lo magnitude values of mb = 5,5, Ms = 5,7, Mw = ML = 5.8. In a second stage, synthetic seismograms arc generated by full reflectivity in the 5-50 s period range, with a new code based on the "minors" integration. By modelling surface waves in the 30-50 s range and body waves to 5 s periods, we derive average velocity models for the Central and Eastern Mediterranean, and constrain the hypocenter of the 1990 Eastern Sicily earthquake to be in the 13-17 km depth range.
The impact of earthquakes on fluids in the crust
Authors: G. C. P. King  , R. M. Wood
Published: June 2021

The character of the hydrological changes that follow major earthquakes has been investigated and found to be critically dependent on the style of fault displacement. In areas where fracture-flow in the crystalline crust communicates uninterrupted with the surface the most significant response is found to accompany major normal fault earthquakes. Increases in spring and river discharges peak a few days after the earthquake and typically excess flow is sustained for a period of 4 12 months. Rainfall equivalent discharges, have been found to ceed 100 mm close to the fault and remain above 10 mm at distances greater than 50 km. The total volume of water released in two M 7 normal fault earthquakes in the Western U.S.A. was 0.3-0.5 km3. In contrast, hydroIogical changes accompanying reverse fault earthquakes are either undetected or else involve falls in well-levels and spring-flows. The magnitude and distribution of the water-discharge for these events is compared with deformation models calibrated from seismic and geodetic information, and found to correlate with the crustal volume strain down to a depth of at least 5 km. Such relatively rapid drainage is only possible if the fluid was formerly contained in high aspect ratio fissures interconnected throughout much of the seismogenic upper crust. The rise and decay times of the discharge are shown to be critically dependent on crack widths, for which the «characteristic» or dominant cracks cannot be wider than 0.03 mm. These results suggest that fluid-filled cracks are ubiquitous throughout the brittle continental crust, and that these cracks open and close through the earthquake cycle. Seismohydraulic fluid flows have major implications for our understanding of the mechanical and chemical behaviour of crustal rocks, of the tectonic controls of fluid flow associated with petroleum migration, hydrothermal mineralisation and a significant hazard for underground waste disposal.
Application of seismogram synthesis to the study of earthquake source from strong motion records
Authors: F. Cotton  , M. Campiello
Published: June 2021

We present the results of an analysis of the Michoacan and Landers earthquakes to constrain the kinematic description of the rupture process. The example of the Michoacan earthquake shows that a crack type model is better a dislocation model to describe the displacement in the vicinity of the fault. We also show that this in contradiction with the fact that the faulting appears to be a complex process. We attribute this complexity to instabilities in crack growth. The June 28, Landers earthquake offers an exceptional opportunity to apply a new inversion technique to a major strike slip event. We model the rupture evolution including local differences in slip durations and variations in rupture velocity. The slip distribution shows that this event consists of a series of regions of high slip (sub-events) separated by regions of relative low slip. Our inversion favors the hypothesis that the duration of the slip at each point is of the order of the duration of the rupture of each sub-event and is consistent with a crack type process occurring during each sub-event. For such a large earthquake, the slip duration is however smaller than the total rupture duration.
Source characteristics of moderate size events using empirical Green funclions: an application to some Guerrero (Mexico) subduction zone earthquakes
Authors: P. Capuano  , A. Zollo  , S. K. Singh
Published: June 2021

The records of an aftershock (M ~ 4) of a moderate size event (M = 5.9) which occurred along the subduction zone of Guerrero (Mexico), are used as empirical Green functions (EGF) to determine the source characteristics of the mainshock and of its smaller size (M = 5.5) foreshock. The data consist of accelerograms recorded by the Guerrero Accelerograph Array, a high dynamic range strong motion array. The three events appear to be located close to each other at distances much smaller than the source to receiver distances. The fault mechanism of the mainshock is computed by non-linear inversion of P polarity readings and S wave polarizations determined at two near source stations. The foreshock and aftershock fault mechanisms are similar to that of the mainshock as inferred from long period data and shear wave polarization analysis. The maximum likelihood solution is well constrained, indicating a typical shallow dipping thrust fault mechanism, with the P-axis approximately oriented in a SSW direction. The source time functions (STFs) of the mainshock and foreshock events are determined using a new method of deconvolution of the EGF records at three strong motion sites. In this method the STF of the large event is approximated by a superposition of pseudo triangular pulses whose parameters are determined by a non-linear inversion in frequency domain. The source time function of the mainshock shows the presence of two separate pulses, which can be related to multiple rupture episodes. The relative location of mainshock sub-events is done by using plots of isochrones computed from measurementes of the time delay between pulses on the STF records at each station. The first sub-event is located no more than 2.5-3 km away from the other along the fault strike. The STF retrieved from foreshock records shows single pulse waveforms. The computed STFs are used to estimate seismic moments, source radii and stress release of the events assuming a circular fault model and constant rupture velocity. The final rupture model for the mainshock indicates the successive breaking of two nearby large slip patches having sizes of the order of a few kilometers and with the rupture propagating at an average speed close to the shear wave velocity.
A study of rupture characteristics of the 40 s subevent of the 1980 Irpinia earthquake
Authors: A. Mendez  , F. Pacor
Published: June 2021

The Irpinia project, as carried out by ISMES under a commission from ENEL, had as objectives the developement of a general methodology to simulate broad-band seismic ground motion at near-source and regional distances, and the application of this methodology to the 1980 Irpinia earthquake. Within this general framework, one goal was the comparison of four previously published models for this earthquake in order to arrive at a plausible description of the source process. The comparative study was cast as an inverse problem: that of inferring the spatial extent and temporal behaviour of the rupture process, from geodetic measurements of surface deformation and near-source recordings of ground velocity. This study was complicated by the fact th the Irpinia earthquake was a complex event, involving at least three distinct rupture episodes in a time span of 40 s. However, this same complexity offers the opportunity of illustrating the use of inversion methodologies to 1) infer the spatial slip distribution on a multiple fault system; 2) address the problem of determining the accuracy of the inferred slip models, and 3) use information describing the static characteristics of an earthquakes as an aid in understanding the kinematics of the rupture. This last point is illustrated for the 40 s subevent through the results of a forward modeling study of high-frequency acceleration waveforms using a rupture model based on the inversion results.
Slip heterogeneity, body-wave spectra, and directivity of earthquake ruptures
Authors: P. Bernard  , A. Herrero
Published: June 2021

We present a broadband kinematic model based on a self-similar k-square distribution of the coseismic slip, with an instantaneous rise-time and a constant rupture velocity. The phase of the slip spectrum at high wave number is random. This model generates an ?-squared body-wave radiation, and a particular directivity factor C2d scaling the amplitude of the body-wave spectra, where Cd is the standard directivity factor. Considering the source models with a propagating pulse and a finite rise-time, we assume that within the slipping band, the rupture has some random character, with small scale rupture in various directions. With such a model, the pulse cannot be resolved, and the directivity factor is still C2d at low frequency; at periods shorter than the rise-time, however, the directivity effect drops to much smaller rms values. This frequency dependent directivity effect, which is expected to be the strongest for sites located in the direction of rupture, was evidenced for the Landers 1992 earthquake, leading to a 2 to 3 s rise-time of the slip pulse. This kinematic model can be used with more refined theoretical Green's functions, including near-field terms and surface waves, or with empirical Green's functions, for generating realistic broadband records in the vicinity of moderate to large earthquakes, in a frequency range relevant for engineering applications (0 Hz to about 20 Hz).
Magnetotelluric measurements in the Monte Amiata region
Authors: A. Manzella  , S. Bellani  , L. Brogi  , Q. Jong  , E. Pinna  , A. Rossi
Published: June 2021

A preliminary magnetotelluric (MT) survey was carried out in Southern Tuscany, Italy, to delineate the resistivity structure in an area which does not belong to classic geothermal zones but is still characterized by anomalously high heat flow. The one-dimensional (1D) resistivity inversion shows a low resistivity zone characterized by high heat flow. Based on 1D resistivity information, detailed two-dimensional (2D) resistivity modelling was carried out. A reasonable 2D resistivity model was proposed for shallow depths, and was found to be in good agreement with the available geological and geophysical information on the area. The present results show a low resistivity anomaly, characterized by high heat flow. The anomaly seems to extend beyond the classic Mt. Amiata geothermal region.
Technical procedures for aeromagnetic surveys in Antarctica during the Italian expeditions (1988-1992)
Authors: E. Bozzo  , A. Colla  , G. Caneva  , A. Meloni  , A. Caramelli  , G. Romeo  , D. Damaske  , D. Moeller
Published: June 2021

For most of Antarctica, the geophysical data now available are those of aeromagnetic surveys performed there from 1950 to 1960. Until 1984, the inaccurate positioning and insufficient monitoring of geomagnetic time variations allowed the investigation of the geomagnetic residual field only along profiles. The Ganovex IV aeromagnetic survey, performed by BGR-USGS over the Ross Sea and the Northern Victoria Land, and the geophysical investigations of BAS on the Southern Antarctic peninsula and the Ronne ice shelf region corresponds to the recent advancement of these techniques in Antarctica. The first experiments of aeromagnetic measurements, during the Italian expeditions in Antarctica were made during the 1988-1989 field season. Some geomagnetic helicopter borne profiles were accomplished with a Proton Precession Magnetometer (PPM) in the Terra Nova Bay-Gerlache Inlet area. In the 1989-1990 ItaliAntartide expedition some profiles were flown over the suture between the Wilson and Bower terranes, in Northern Victoria Land. During the 1991-1992 expedition, in cooperation with researchers of BGR (Bundesanstalt fur Geowissenschaften und Rohstoffe) of Hannover, the GITARA I (German ITalian Aeromagnetic Rescarch Antarctica) program, as part of the LIRA (Litospheric Investigation in the Ross Sea Area) project, was carried out. The investigated area lies between the latitudes 74°18' S and 75°18' S and the longitudes 160°30' E and 164°30'E and it corresponds to a portion of the North Victoria Land, located between the Eisenhower Range and the, DrygaIski Ice Tongue. The survey was made with a Cesium vapour magnetometer. The positioning system was of the Range-Range type, it consisted of three transmitters (beacons), installed inside the investigated area and located with GPS measurements. The line spacing was 4.4 km, with tie lines every 22 km. The survey covered an area of 6500 km2 . Four PPM base stations for the determination of the time variation corrections were installed. The aeromagnetic anomalies wilI allow the comparison with the ground geomagnetic surveys performed during the previous Antarctic expeditions, also in connection with the aeromagnetic maps of Ganovex IV, located at the northern and eastern boundaries of the GITARA area.
Tripotential data processing for HES interpretation
Authors: P. Cosentino  , D. Luzio
Published: June 2021

In this paper some methods are proposed and compared to correct the experimental measurements for preliminary processing of tripotential data which are acquired for HES prospecting. However, the use of those methods should be based upon an accurate analysis of all experimental data. Such an analysis ought to involve: 1) an estimate of the averaged measurement errors with their variance and distribution in both the space and the three apparent-resistivities domains; 2) the choice of a resistivity model capable of describing the actual volume under study. The differences among the three values of apparent resistivity measured on a point are generally influenced both by the resistivity distribution below ground as well as by the eventual measurement errors. The proper choice of the method of correction which may be useful to merge the resistivity values and minimize the measurement errors is also linked to the separation of modelling effects. Consequently, the model chosen should be selected in relation to the above mentioned analyses. It thus becomes useful to know the general relations among the three apparent resistivity values for some simple structures, e.g. the two-layer model with a slowly changing first layer thickness. This theoretical model is presented and discussed using two new composed apparent resistivities, namely p? and p?, which seem to be useful tools in HES interpretation. The behaviour of the calculated responses can be useful also for a fast data inversion.
A simple P-wave polarization analysis: its application to earthquake location
Authors: B. Alessandrini  , M. Cattaneo  , M. Demartin  , M. Gasperini  , V. Lanza
Published: June 2021

We present a method for hypocentral location which takes into account all three components of ground motion and not only the vertical one, as it is usually done by standard least-square techniques applied to arrival times. Assuming that P-wave particle motion direction corresponds to the propagation direction of the seismic wave, we carried out a simple statistical analysis of ground motion amplitudes, carefully using three-component records. We obtained the azimuth and the emersion angle of the seismic ray, which, added to Pg and Sg arrival times, allowed us to find reliable hypocentral coordinates of some local events by means of a ray-tracing technique. We compared our locations to those obtained using a least-square technique: our polarization method's dependence on the accuracy of the model used (on the contrary, the least-square technique proved to be quite stable with respect to changes in the model's velocity parameters) led us to conclude that polarization data provide coherent information on the true ray-path and can be successfully used for both location procedures and seismic wave propagation studizs in strongly heterogeneous media.
The ORFEUS Data Center
Authors: B. Dost
Published: June 2021

1993 the ORFEUS Data Center (ODC; Dost, 1991) changed hosting organisation. It moved within the Netherlands from the University of Utrecht to the Royal Netherlands Meteorological Institute (KNM1) in de Bilt. This change in hosting organisation was necessary to ensure a longer term stability in the operation of the ODC. Key issues for the ODC are the rapid on-line data access and quality controlled, complete and efficient off-line data access. During 1992 the ODC became the European node in the international SPYDER system which provides near real-time access to digital broadband data from selected high quality stations. Electronic messages trigger soveral centers well distributed over the globe. These centers then collect the data by modem from selected stations in their region. Finally, data are distributed between data centers over internet.
Preliminary results on correlation in daily f0F2 and M(3000)F2 variations
Authors: S. S. Kouris  , T. D. Xenos  , B. Zolesi  , K. Barbatsi  , Y Nissopoulos
Published: June 2021

A statistical analysis of the hourly daily values of the factor M(3000)F2 and the F2-layer critical frequency f0F2 is carried out first for each hour of the day through-out a given month of a given year at a given station (hour-to-hour variation) and then for each day of a given month and year at a given station (day-to-day variation). This analysis shows that these ionospheric characteristics are poorly correlated. It is concluded that the daily values of the first characteristic cannot be estimated from the second by using a simple linear equation.
A tectonomagnetic effect detected in Central Italy
Authors: G. Mele  , A. Meloni  , P. Palangio
Published: June 2021

Significant variations in the absolute value of the geomagnetic field intensity related to tectonic events, as earthquakes and volcanic eruptions, have been observed in several cases. To detect such a tectonomagnetic effect related to seismic activity, a seismomagnetic network was installed by the Istituto Nazionale di Geofisica (ING) in the Abruzzi region (CentraI Italy), in July 1989. This area is being uplifting since the Pliocene. A logistic compromise between geophysical requirements and the electrified railway system tracks distribution led to the installation of five total magnetic field intensity data acquisition sites. From July 1989 to September 1992 geomagnetic intensity data were simultaneously recorded at all stations and compared to that recorded at the L'Aquila Observatory, located in the same area. A variation of about 10 nT in the absolute level of the geomagnetic field was measured at two stations located on the eastern side of the network. We suggest that the detected magnetic anomaly could resuIt from aseismic-changes in crustal stress during this time.