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In this paper we investigate the ionospheric turbulence from observations of TEC variations as well as from VLF/LF transmitter signal observations before and during the disastrous seismic activity of August and October 2016 in Central Italy. The Total Electron Content (TEC) data of 8 Global Positioning System (GPS) stations of the EUREF network, which are being provided by IONOLAB (Turkey), were analysed using Discrete Fourier Analysis in order to investigate the TEC variations. The data acquired for VLF/LF signal observations are from the receiver of Thessaloniki (40.59N, 22,78E), Greece, which monitor the VLF/LF transmitters of the International Network for Frontier Research on Earthquake Precursors (INFREP). A method of normalization according to the distance between the receiver and the transmitter is applied on the above data and then they are processed by the Hilbert Huang Transform (HHT) to produce the corresponding spectra for visual analysis. The results of both methods indicate that the High- Frequency limit fo, of the ionospheric turbulence content, increases as the site and the moment of the earthquake occurrence is approaching, pointing to the earthquake locus.

In questo lavoro gli autori studiano la posizione degli epicentri dei terremoti, collegati alla orogenesi in atto nell'Appennino, nell'arco di tempo clic va dal 1900 al 1973. Lo studio eseguito rivela che, da alcuni anni, si verifica uno spostamento dell'attività sismica dalla Marsica alle Marche. Gli autori concludono che è ragionevole aspettarsi che, nel futuro, la principale attività sismica dell'Appennino centrale si manifesterà in quest'ultima zona.

In a long 1970 work on the seismic and geodynamic characteristicsof the Po Valley (to be understood in the broadest sense,that is, as including the Adriatic-Po, trough and all the beds andbasins of the rivers emptying into the upper Adriatic as well as themountains bordering it), Caloi et al observed, in commenting an thediagram of seismic energies in recent centuries up to 1969, with particularreference to the last century, that:"It began with the strong earthquake of Santa Croce in 1875,which suddenly broke the relative seismic quiet that had enduredthroughout the broad zone since 1800. The disastrous seismic crisisof the Lake Santa Croce zone was followed that same year by strongseismic activity on the opposite side of the plain, at Cattolica nearthe sea, along the northern Appennine slopes. It almost seems thatthese violent ruptures of the seismic equilibrium on the two sides of theplain were linked as cause and effect, and vice versa. The same phenomenonoccurred at other times during the past century: the strongAdriatic earthquake of 1934 (Caloi, 1937) ('), for example, was followedtwo years later by the very strong earthquake of Cansiglio (Caloi,1938)

In questa prima parte del lavoro, che ci proponiamodi dedicare alla disastrosa crisi sismica che ha colpito il Friuli nel 1970,ci limitiamo a delineare le caratteristiche presismiche, che hanno precedutola forte scossa del 0 Maggio 1970.Nelle premesse, vengono richiamati gli studi su fenomeni presismici,iniziati in Italia già negli anni Cinquanta.

Results obtained in the study of the arrival times of the Pn phase forthe Friuli earthquake of 6 May 1976 (H = 20h00™15s,5; φ = 46° 16'30" N; λ = 13° 06' 00" E; M = 6.3) with respect to the epicentral distance arereported. Particular emphasis is given to the fact that the travel-timecurves relative to the Pn waves and the corresponding equations are onlyvalid for selected alignment. By plotting the time-distance curves it waspossible to derive the depth of the lower surface of the upper mantle fordifferent zones; the results thus obtained exibit significant differences.

We present a study of a group of long period waves excited by theearthquakes of Friuli (Italy) in 1976 and recorded between longitudinaland transverse body waves by Long-period seismographs at epicentraldistances up to about 3 000 Km. The spectral analysis has permitted usto determine periods of the main phases which make up the wave group.In regard to the longer period phase, which appears first in the recordings,the velocity obtained by the dromochrones equals 8.4 Km/s. The groupvelocity is instead 7.4 Km/s. Such phases have been considered as beinglongitudinal channel waves in the astenosphere. Such hypothesis explainsthe inconsistencies which had appeared in previous studies on channelwaves in the astenosphere.