G. Della Monica

Very slow-moving crustal strain disturbances

Abstract A model of discrete crustal blocks, separated by a thin weak transition zone whose stress-strain relations contain derivatives of real order implying a memory mechanism in the medium, seems to give a good explanation for the low velocity values (0.03–15 cm/s) of slow moving crustal strain disturbances. Tilt and strain anomalies observed before moderate and large earthquakes are thought to be generated in the focal zone of the forthcoming earthquake, due to the spatial and temporal redistribution of the physical processes that occur during the preparation time of the earthquake. On the basis of experimental results it has been assumed that these anomalies propagate from the hypocentral area. In addition post-seismic deformations show the effects of propagation. The anomalies observed have a duration and a velocity ranging from minutes to months and from O.O5 cm/s to 10 cm/s, respectively. The velocity of migration of post-seismic deformation also lies in this range. The velocity of propagating strain disturbances is obtained as a function of the frequency and of the linear dimension of the oscillating blocks. The velocity values calculated using this model range from 0.1 cm/s to 10 cm/s, for viscosities of 10 9 –10 11 ) g cm −1 s −2+z (0.1≤ z ≤ 0.5) and blocksizes (1–4) · 10 6 cm, at frequencies of 10 −5 -10 −8 s −1 .

Helium content in thermal waters in the Caucasus from 1985 to 1991 and correlations with the seismic activity

Abstract A wide data set of groundwater helium content collected in the Caucasus from 1985 to 1991 has been analyzed. The strongest earthquakes that occurred in this area during the helium measurement period were the: Paravani (May 13th, 1986, M = 5.6), Spitak (December 7th, 1988, M = 6.9) and Georgian (April 29th, 1991, M = 6.9) earthquakes. The analysis of the helium content data revealed statistically significant increases in this parameter on the occasion of the Paravani and Spitak earthquakes and no increase on the occasion of the Georgian earthquake. These results corroborate those obtained by the analysis of scanty helium content data presented in previous papers. With the exception of the anomalous increases, the helium content data were subjected to randomness tests and spectral analysis and cross correlation with meteorological parameters and areal seismicity were also attempted. The main result obtained is that the destructive Spitak earthquake produced some statistical changes in the groundwater helium content fluctuations some years before and after its occurrence.

A digital recording system of electromagnetic emissions

SummaryA new equipment able to reveal the electric component of electromagnetic emission and suitable for underground deployment has been made. The system enables to measure the absolute value of integrated voltage of signals. Data are recorded by a solid-state memory circuit. The system is performed as a plug-in modular design and it can be also used in diverse fields of application.RiassuntoÈ stata realizzata una nuova strumentazione per la rivelazione della componente elettrica delle emissioni elettromagnetiche in laboratori sotterranei. Il sistema è in grado di misurare i valori assoluti della tensione integrata dei segnali. I dati vengono registrati su un circuito di memoria a stato solido. Il sistema è stato realizzato con caratteristiche modulari e può essere utilizzato anche in altri campi di applicazione.РезюмеСконструировано новое оборудование для регистрации электрической компоненты электромагнитных излучений в подземных лабораториях. Система позволяет измерять абсолютную величину суммарного напряжения сигналов. Данные регистрируются в схеме с твердотельной памятью. Предложенная система собрана на специальных сменных модулях и может быть использована для различным прикладных задач.

No preseismic evidence from hydrogeochemical parameters on the occasion of the April 29, 1991, Georgian earthquake, Caucasus

Abstract The behaviour of helium content in thermal waters and of water level in deep wells was investigated on the occasion of the strong (M = 6.9) Georgian earthquake which occurred in the Caucasus on April 29, 1991. Data were collected from a measurement network located in Georgia. Just one significant preseismic variation appeared in the water level which seemed to be connected with the Georgian earthquake. In other wells, only some co-seismic and post-seismic effects are visible. No statistically viable variation was noted in the helium content. In contrast, clear anomalies in these parameters were observed on the occasion of the 1988 Spitak earthquake with the same magnitude which occurred in the Caucasus. We prepose that the location of the epicentre with respect to the measurement network was the reason for this disagreement. We therefore suggest the introduction of a coefficient c into relationships that give the size of the premonitory area as a function of the magnitude of the forthcoming earthquake. This coefficient, ranging from 0 to 1, can take into account the extent to which the focal zone belongs to different tectonic structures and the extent to which the measurement network coincides with these structures.

Tilt measurements and seismicity in Central Italy over a period of approximately three years

Tilt data obtained from July 1983-January 1986 at three sites in the Central Apennines (Italy) have been examined. A filtering of raw recordings indicated the existence of residual anomalies. Attempting a correlation between such anomalies and regional seismicity (3.0 </ magnitudeM </ 5.4) has allowed us to interpret the anomalies as possible preseismic variations. The results seem to confirm the existence of a relation between precursor time T, earthquake magnitude M and epicentral distance R. Finally, the presence of a time delay between the onsets of the anomalies detected at different sites for the same earthquake gives further credit to the possibility of the existence of their propagation.

Ground Tilt anomalies accompanying the main earthquakes occurred in the central apennines (Italy) during the period 1986–1989

SummaryA four-year set of tilt data (1986–1989) recorded at three sites of the Central Apennines (Italy) and which in the past pointed out quite good raw tilt responses to seismicity, only with moderate thermoelastic effects, have been investigated to search for the possible existence of tilt anomalies accompanying the main earthquakes of the region. It has been observed that, even if fairly good tilt responses to principal evants seem to appear, correlations of similar or better quality are obtained when comparing the same tilts with pressure and mainly with temperature. However, when taking into account earthquakes with magnitudesM≥3.0 and more strictE (strain) andL (source dimension) threshold values (the last two parameters being used to evaluate tilt field at a distanceR), only one event (M=3.5;R=5.1 km) has been selected and tectonic from non-tectonic contributions have been pointed out clearly at one tilt site. The quoted earthquake showed the bestE andR/L values (E=10−6;R/L=2.4) in the whole period and it was preceded by a clear tilt anomaly with amplitude and duration of about 5 μrad and 2 months, respectively. Since the tilt signal in this period is not affected by any non-tectonic effect (maximum correlation coefficient with temperaturer=0.2) the quoted anomaly has been considered as a probable precursor of the above-mentioned earthquake it preceds. Residual, tilt vector oscillations along the direction-including the epicentral area of this event, were also observed. On the contrary, all the remaining anomalous tilts proved to be well justified with pressure and/or temperature variations (r values up to 0.8).

Electromagnetic and seismoacoustic signals revealed in karst caves (Central Italy)

SummarySince 1988–89 equipment for detecting electric, magnetic and seismoacoustic signals has been running inside the Amare cave. The Amare cave is placed on the southern slope of the Gran Sasso chain, that is one of the largest karst areas of the Italian Apennines. In 1992, a similar equipment was installed inside the Cervo cave. This cave is located in another karst area of the Central Apennines, at about 50 km southwestwards of the Amare cave. In both these measurements sites, the signals are recorded every ten minutes in a digital form; the equipment is able to record signals, the frequency of which ranges from some hundred Hz to some hundred kHz. The data collected up to now seem to identify two different states that we call «quiet» and «perturbed» state. In the quiet state only electric and magnetic signals with the highest frequencies appear. These signals are connected with radio broadcastings and with the general lightnings activity of the Earth. A perturbed state is characterized by the sudden appearance of seismoacoustic signals coupled with electric and magnetic ones. This phenomenology is connected with local processes. Rainfall, atmospheric-pressure variations and some thermal effects are responsible for these local processes. A possible model is proposed to justify the observed phenomenology: micromovements of the limestone blocks that constitute the roof of the caves are invoked for the production of seismoacoustic signals. The electrification generated by these movements is invoked for the production of electric and magnetic signals.

Ground tilt variations detected in the central Apennines (Italy) in the period 1986-1989 and their correlation with seismicity

SummaryGround tilt measurements carried out at three tilt sites of the Central Apennines (Italy) revealed a quite good correspondence with seismicity (M≤4.9) which took place within a distance of 100 km from the sites and over a period of four years (1986–1989). The best correlation is obtained when the tilt signals are delayed by about 1.5 months with respect to the seismic one (number of earthquakes per month), suggesting that seismicity is predictable to a certain extent from the local tilt response. Correlations of similar quality showed also that tiltmeters located at different sites seem to react in a different way to events distributed along the two main active fault zones (namely, the Apennine and anti-Apennine ones), indicating an anisotropic response of the medium. To explain how the seismicity, selected along the Apennine and anti-Apennine directions, give rise to different tilt responses at tilt sites located at a relative small distance, the behaviour of the geological structures and of the geodynamic processes of the investigation area have been taken into account.

Abnormal behaviour of some parameters revealed in the Georgian SSR before the Armenia earthquake of December 7, 1988 : a preliminary report

SummaryFor more than ten years several measurements sites able to detect geophysical and geochemical precursory phenomena of earthquakes have been working in Soviet Georgia. On the occasion of the destructive Armenia earthquake of December 7, 1988 (M=6.9) premonitory signals relative to geophysical and geochemical parameters seem to be observed. In this paper anomalous variations of the water level in a well, of the helium content in thermal waters and of the electromagnetic signals in air, obtained in three measurements sites located at about 80 km from the epicentre, are reported. These variations appear beginning from about a month up to some hours before the occurrence of the earthquake and it seems reasonable to consider them as short-time or very short-time precursory phenomena of the earthquake.

GEOPHYSICAL INVESTIGATIONS ABOUT THE CARBONATIC STRUCTURE OF MONTE LA QUEGLIA ( ABRUZZO , CENTRAL ITALY ) .

SummaryMonte La Queglia calcareous relief (Abruzzo, central Italy), an outcrop of the molassic covering, contains a magmatic vein of an ultrabasic type. This is a rare example of its kind in the central Apennines. The structural attitude of the relief poses some interpretative problems concerning its relation with the surrounding carbonatic structures. In order to understand this situation, geophysical investigations were carried out. The results of the investigations made it possible to interpret the structure as a nonrooted one, superimposed upon local molassic facies. Moreover, in the area a probable 1300 m deep magmatic body was found which has no continuity with the ultrabasic outcropping vein.RiassuntoIl rilievo calcareo di Monte La Queglia (Abruzzo, Italia Centrale), emergente dalla coltre molassica, ospita un filone di natura ultrabasica, rara manifestazione del genere nell’Appennino Centrale. L’assetto strutturale del rilievo pone problemi interpretativi circa la sua relazione con le strutture carbonatiche circostanti. Al fine di definire la situazione sono state effettuate indagini geofisiche. I risultati di tali indagini hanno consentito di interpretare la struttura come non radicata e sovrapposta alle locali facies marnosoarenacee. Inoltre nell’area è stata individuata un probabile corpo magmatico a circa 1300 m di profondità che non trova continuità con l’affioramento del filone ultrabasico.