The Friuli (NE-Italy) tilt/strain gauges

In summary

The tilt/strainmeter network managed by the Tectonophysics and Geodynamics Research Group of the Dept. Mathematics and Geosciences of the University of Trieste (DMG UNITS), has by now a long history of records - the Trieste Grotta Gigante horizontal pendulum station having been set up in 1959 and the Friuli tilt/strainmeter stations in 1977. Since then the stations have been continuously recording the strain-rate in one of the most seismic areas of the Alpine arc, giving invaluable information on crustal deformation in a tectonically active area. Although maintaining from the time of installation essentially the same mechanical features, the instrumentation has undergone modernization, in order to apply recent technical developments to the network. This regards mainly data acquisition, which now, except for one station, is digital. The data are all available and are stored in a Deformation-Database. At first a description of the essential technical and mechanical properties of the instrumentation constituting the network is given. The mean power spectrum of all instruments covering five decades is presented, which is a powerful means to compare the quality of different stations. Following theoretical considerations of the expected pre- or coseismic deformation accompanying local events, the observations of those events is presented, which ought to give the greatest signals. The coseismic steps are modeled for those events for which a fault plane solution was available.

Introduction

The tilt/strainmeter network of the Dept. of Earth Sciences, University of Trieste is installed in the seismic zone of Friuli and South to this area, near Trieste, where the observation of deformation of the crust is of extreme interest with regard to the research of preseismic phenomena and in general of the actual movements in seismic zones (Zadro, 1978; 1980, Ebblin and Zadro, 1979).

The instrumentation which monitors deformation in the testfield includes at best 10 tiltmeters, 3 strainmeters and 2 horizontal pendulums sited in Trieste. Since 1998 the tilt/strainmeter network operates a reduced number of instruments (4 tiltmeters, 3 strainmeters, 2 horizontal pendulums) due to budget shortages. The seismicity is recorded by the microseismicity network of Osservatorio Geofisico Sperimentale (OGS) since 1977, complete to magnitude 2.5 in Friuli and by the Istituto Nazionale di Geofisica (ING) national seismologic se2rvice. Recently (1995) the Department of Earth Sciences has set up a network of strong motion seismographs in the same area. A vast amount of meteorological stations and groundwater measuring stations are kept up by the Regione Friuli Venezia Giulia and the Ufficio Idrografico e Mareografico di Venezia. During years 1995-1999 in one of the stations (Villanova) also Radon concentration in soil has been measured continuously, (Braitenberg et al., 1997, Garavaglia et al., 1998)

The first instruments of the network to be operative were the Trieste horizontal pendulums, which recorded starting from 1973 episodes of long period oscillation (4-10 min) resembling surface waves (Zadro, 1978). The episodes lasted up to several hours, with an increase of amplitude and occurrence frequency up to the catastrophic 1976 Friuli earthquake (epicentral distance about 100 km). Observation of earth tides with all the instrumentation allows the study of the crustal elasticity structure and its variation with time. This is particularly interesting in connection with the 1976 earthquake, which preceded the installation of the Friuli stations by only a few months, and thus gave the possibility to observe postseismic crustal movements (Zadro, 1980) in these stations. Furthermore it was possible to detect variations in the mechanical properties of the crust, connected to the healing of the ruptured material (Mao et al., 1989; Braitenberg et al., 1995). The more than decennial records acquired by the present time have allowed the study of long term deformation of the zone, which has given very interesting results regarding the geodynamics involved (Zadro and Rossi, 1991; Rossi and Zadro, 1996). Another problem under study is the interpretation of short term anomalies on the instruments of the tilt/strainmeter network in the light of seismic events and the stress-conditions connected to them (Ebblin et al., 1980; Mao et al., 1990; Zadro, 1992; Dal Moro and Zadro, 1999; Rossi et al., 1999). A further topic of interest is that concerning the influence of hydrologic factors on seismicity and on the deformational records, which has been treated in Zadro et al., (1987), Brussa Toi and Ebblin, (1988), Braitenberg et al., (1993; 1996), Dal Moro and Zadro (1998) and Braitenberg (1999).

Geologic and tectonic setting

The Friuli is located on the NE border of the Adriatic plate, in the eastern part of the Southern Alps, where the overlap with Dinaric structures takes place (Barbano et al., 1985). The Southern Alps are interpreted, in global tectonics, as the result of the continuation of relative motion between the European plate and the Adriatic microplate. These movements are still active, as demonstrated by neotectonic structures and by seismic activity. The seismicity is presently limited to the upper 15km of depth. The northward movement of the Adriatic plate in the Neogene and Quaternary involved increasing areas of the Southern Alps. This should have caused intense shortening, mainly in the post-Hercynian cover, with a build up of over-thrusts verging towards the South and its probable detachment from the Paleozoic basement in the more Southern sector. In the pre-alpine area especially, Dinaric overthrusts have been split up and incorporated in the South-Alpine overthrusts and partly re-utilized in the South-Alpine tectonogenesis, causing very complicated structural relationships. The most important regional features are the E-W striking South-Alpine overthrusts (mainly in the NW) and the SE-NW striking Dinaric overthrusts (mainly in the SE and buried in the Friuli plane). These structures are intersected by subvertical faults, striking about N-S and often showing strike-slip motion. Both structures (overthrusts and strike-slip faults) are compatible with the actual stress field, with NNW-SSE to N-S maximum compressive stress (Zanferrari et al., 1982).

The observational sites of the tilt/strainmeter network

The Deformation Database regards data collected in 5 tiltmeter stations (Fig.1) each equipped with two Marussi tiltmeters, located in different parts throughout the seismic Friuli zone. In one of these stations (Villanova) three Cambridge-wire strainmeters are installed. The Trieste station, 80 km to the South of the seismic zone is set in the Grotta Gigante of the Trieste Karst, unique for its high vault, is equipped with two long period Zöllner type horizontal pendulums and two Marussi tiltmeters.

The aim of the tilt/strainmeter network sets a number of definite requirements on the selection of the observation sites and on the characteristics of the instrumentation. Regarding the selection of the sites, special attention had to be set to the thermal stability and isolation to human noise. This was obtained by the installation of the stations in natural caves or old military fortifications from the 30’s. The description of the stations and their geographic coordinates are given in Tab. Ia) and Ib).

The instruments

In the following the relevant instrumental constants are given. The data acquisition has been transformed from analogical on paper to digital in all Friuli-stations by the year 1990. The electronic system is switched on every hour for 4 minutes, during which data acquisition is performed, giving hourly data values after application of a software filtering process.

Marussi Tiltmeters

The Marussi tiltmeters are traditional horizontal pendulums with Zöllner type suspension, constructed at the Istituto di Geodesia e Geofisica, (now merged into the Dept. of earth Sciences) University of Trieste. In Fig. 2 a schematic view of the instrument is seen.

Cambridge wire-strainmeters

The three strainmeters installed in the Villanova cave are of the type Cambridge Wire Strainmeter. A wire strainmeter of this type is described in detail in King and Bilham, (1976). It consists of an invar wire held in tension by a frictionless balance. Strain in the ground appears as rotations of the balance which are detected using an inductive displacement transducer. In table IV the technical details of the instrument are given.

References

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