Long- and middle-term behaviour of the tilt and strain variations in the decade following the 1976 friuli earthquake in NE Italy

Tilt and strainmeter data collected in the Friuli seismic area between 1977 and 1985 have been analyzed to study the stress-strain field and/or the elastic parameter variations in the area after the strong 1976 seismic event.We concentrated on three different frequency bands, respectively linked with the tidal waves, the seasonal effect and the secular one. The results from the first two bands point at an increase of the local rigidity modulus.Relevant stresses induced by the seasonal groundwater level variations have been found, thus supporting the hypothesis that they can act as triggering effects. The secular trend reveals significant tectonic stress modifications. It appears that the post-seismic tectonic stresses faded during 1981–1983, and were followed by a new loading phase.On leave from the Institute of Geodesy and Geophysics of the Chinese Academy of Sciences. Ph. D. student at the Institute of Geodesy and Geophysics, Trieste University.     

An ideal isotropic bidimensional filter and its application in the interpretation of gravity anomalies

Studia Geophysica et Geodaetica - Den Gegenstand der Untersuchung bildete der ideale, zweidimensionale Filter mit der Übertragungsfunktion gleich 1 in einem Kreisgebiet, dessen Mittelpunkt...     

Strain determinations with tiltmeters in cavities

A pair of tiltmeters installed in a cave, at the lower suspension of a pair of large horizontal pendulums, showed similar tilts related to tidal effects but different tilts connected to seiches in the nearby Gulf of Trieste or to underground river floods. Such a difference has been ascribed to the different strain characteristics caused by the phenomena: the former one being mainly a rotation and the latter chiefly a distortion. It is suggested that several tiltmeters installed in a cavity could supply data that would allow one to describe the non-dilational part of the strain.     

Evidence for variations of mechanical properties in the Friuli seismic area

Tidal strains of the earth were extracted from three horizontal strainmeter records (1979–1986) in the Friuli seismic area by the “filter method”. The areal strain factor, ignoring the tidal loading effects, shows that its time variation is significant: their amplitudes in 1986 are about 50% of those in 1979.

The modifications of mechanical properties, estimated in terms of the local shear and bulk modulus variations, were obtained considering the envelope of the tidal strain signals. The seismic wave velocity determined by the simultaneous inversion of the arrival time data of the local seismometric network displayed comparable changes.

A major change both in the seismic velocities and the elastic parameters started in March 1982, which was about 11 months before an earthquake of magnitude 4.1, the largest event from 1979 to 1986, which occurred within the seismic network on Feb. 10, 1983.     

The gravity and isostatic Moho undulations in Qinghai–Tibet plateau

It is our interest to study the Moho depths in the Qinghai–Tibet. An iterative hybrid spectral–classical methodology is applied to invert the gravity data and obtain the 3D variation in Moho depth. The gravity inversion is constrained by results from deep seismic sounding and seismological investigations. The Moho is found between 70 and 75 km depth over most of Tibet. Maximum depths of up to 80 km are found along the margins of the plateau, and shallower depths of 65 km correlate with an important suture running along central Tibet (Bangong Nujiang). At Moho level most of Tibet is isostatically compensated at 90–110%, according to the Airy isostatic model. The Qaidam basin in North-Eastern Tibet and the Tarim basin to the North-West are found to be over-compensated.     

Magneto-telluric recordings from the Friuli seismic zone, northeast Italy

Magneto-telluric measurements were undertaken in the Friuli seismic area of north-east Italy. The purpose was to find possible correlations between magneto-telluric signals and seismic activity. Two stations were operational between July 1987 and June 1988. Apparent resistivity curves were found for September 1987 and January 1988 data, and a magneto-telluric inversion problem was solved. Artificial noise and natural disturbances of the electromagnetic field, especially those related to tectonic processes, are discussed.     

Spectral techniques for the analysis of tidal time series

Summary Three different methods are given for the Fourier analysis of time series leading to the determination of high reliable values of frequencies, amplitudes and phases of the inherent components. For two methods the hypothesis is made that the time series is sufficiently long to separate very close components, while for the third the frequencies are supposed to be given. Two of these methods have been applied to a time series obtained by sampling an artificial tidal function.