ANNALS OF GEOPHYSICS, VOL. 46, N. 6, December 2003 Ground tilt monitoring at Phlegraean Fields (Italy): a methodological approach Ciro Ricco, Ida Aquino and Carlo Del Gaudio Istituto Nazionale di Geofisica e Vulcanologia – Osservatorio Vesuviano, Napoli, Italy Abstract Among geodetic methods used for monitoring ground deformation in volcanic areas, tiltmetry represents the most rapid technique and therefore it is used by almost all the volcanological observatories in the world. The de- formation of volcanic building is not only the result of endogenous causes (i.e. dykes injection or magma ris- ing), but also non-tectonic environmental factors. Such troubles cannot be removed completely but they can be reduce. This article outlines the main source of errors affecting the signals recorded by Phlegraean tilt, network, such as the dependence of the tilt response on temperature and to the thermoelastic effect on ground deforma- tion. The analytical procedure used to evaluate about such errors and their reduction is explained. An applica- tion to data acquired from the tilt network during two distinct phases of ground uplift and subsidence of the Phle- graean Fields is reported. Key words tilt data – thermoelastic deformation – In order to obtain accurate data, it is necessary thermal correction to use sensors with resolution suitable to the mag- nitude of the expected deformation, trying to in- stall them (if possible) symmetrically in relation 1. Introduction to the crater, to notice anomalies in the azimuthal deformation, or to different heights to estimate Tilt monitoring is a technique for the identifi- the depth of the source (often dykes), remember- cation of some precursors of volcanic eruptions, ing that the maximum slope of the flanks is lo- but it is also a useful tool for the study of the be- cated at a distance half the depth of the same haviour of volcanoes during post-eruptive phases. source (Scandone and Giacomelli, 1998). Ground tilt measurements are very important be- In addition to the Phlegraean Fields, Vesu- cause during an inflation episode the flanks of vol- vius, Etna and Aeolian Islands, tilt arrays cur- canoes deform themselves reaching variations of rently operate in almost all the volcanoes of the inclination in the order of about ten microradians. world such as (to mention only some of them) Such variations happen an almost constant Kilauea (Hawaii), Krafla (Iceland), Piton de la way, but positive gradients of inclination fol- Fournaise (Indian Ocean), Galeras and Nevado lowed by an inversion of the ground tilt record- del Ruiz (Colombia), Tacanà e Fuego (Mexico- ed during the co-eruptive deflations are not rare Guatemala), Mayon and Taal (Filippines), Mer- (Bonaccorso and Gambino, 1997). api and Soputan (Indonesia), Unzen (Japan). The Phlegraean Fields are a restless caldera lo- cated on a NE-SW-trending structure in the graben of the Campanian Plain (fig. 1), formed as a result Mailing address:Dr. Ida Aquino, Istituto Nazionale of the Campanian Ignimbrite eruption, 37 kyr BP, di Geofisica e Vulcanologia – Osservatorio Vesuviano, Via Diocleziano 328, 80124 Napoli, Italy; e-mail: aqui- and the Neapolitan Yellow Tuff eruption, 12 kyr [email protected] BP (Rosi et al., 1995; Orsi et al., 1999). The inter- 1297 Ciro Ricco, Ida Aquino and Carlo Del Gaudio est in this area is due to its intense geodynamic and geo-thermal activity, with strong ground uplifts and subsidence episodes (bradyseismic phases), earthquake swarms, and fumarolic emissions. The last eruption dates back to 1538, but major uplifts occurred in 1969-1972 and 1982-1984 when the ground displacements reached a maximum value of 174 cm and 179 cm respectively; these move- ments refer to the benchmarks (belonging to a Phlegraean levelling line) located in the town of Pozzuoli (fig. 2). The deformation recorded is characterized by an almost bell-shaped form and covers an almost circular area with a radius of 6 km centered in the town of Pozzuoli (Orsi et al., 1999). The subsidence phase, starting at the end of 1995 and interrupted by three small uplifts (fig. 2), is continuously monitored by the tilt network of the Osservatorio Vesuviano (OV). 2. Historical data The first clinometric measurements car- Fig. 1. Location of the volcanic area. ried out in the Italian volcanic areas con- Fig. 2. Vertical displacement of benchmark n. 25 (located in the town of Pozzuoli) over the period 1985-2002 . 1298 Ground tilt monitoring at Phlegraean Fields (Italy): a methodological approach cerned Mount Vesuvius and date back to 1935 (Imbò, 1939). The spirit levels placed by Im- bò in 1935 on the seismic pillar of the OV his- torical building (by recording on smoked pa- per) and oriented N70W-S70E and N20E- S70W measured the apparent deviation of the vertical from October 1935 to January 1939. In 1939, Imbò observed five eruptive intervals (Strombolian activity on the crater bottom) characterized by a fracturing of the intracra- teric cone and by lava flows. He noticed an in- verse parallelism between the direction of la- va flows and the deviation of the vertical and evidenced a direct correlation between the SSW apparent tilt of the volcano and a lower- ing of the eruptive column, and NNE tilt and Fig. 3. Levelling line and tilt stations belonging to the uplifting of the column (Imbò, 1939). OV Phlegraean surveillance array. In 1968, two Verbaandert-Melchior hori- zontal pendulums were installed in the Earth Physics Institute of Naples University (Lo Bas- cio and Quagliariello, 1968). These were used two stations were set up in October 1987 in to compare the clinometric data obtained by Pozzuoli (DMA and DMC) (fig. 3). other stations located at the base of the Vesu- The analog signals were digitized through an vius cone (Imbò, 1959). A/D converter on a Canon X-07 computer (fig. 4b). In April 1970, the Institute of Geodesy and The stations DMB and BAI initially worked Geophysics of the University of Trieste set up a with an 8 bit acquisition program allowing data network of four clinometric stations (equipped accumulation in the RAM memory with a sam- with two optical tilt sensors with recording on pling rate between 1 and 60 cph. The data were paper, oriented NS and EW) to study the recorded in both graphic and numerical form. ground deformation caused by the 1970-1972 Later, a 12 bit acquisition program was used Phlegraean Fields bradyseismic crisis (Man- to control the operation of the whole system, zoni, 1972) (fig. 2). i.e. switch the bulb on and off, reception ana- logical signal, digital conversion and data accu- mulation. The fundamental difference from the 3. Tilt network OV – Institute de Physique first 8 bit acquisition program was that the lat- du Globe (IPG) ter program turned on the tilt station only in the time period of sampling and therefore the elec- In 1987, a tiltmetric network was set up at tric absorption of the installation was reduced. the Phlegraean Fields (fig. 3) in cooperation Data in memory were periodically trans- with the IPG, which provided the instruments. ferred onto a cartridge directly in the field, Each station was equipped with two tiltmeters through the same program and later were sent sensitive along two orthogonal directions, using to a PC through an RS232 Serial interface (Lu- Zöllner bifilar suspension model, made by P.A. ongo et al., 1988). Blum at Paris IPG (fig. 4a,b) (Aste et al., 1986; Briole, 1987). The first station of this type, called DMB, was installed at the end of 1985 in 4. Automatic tiltmeter network Pozzuoli, and located in a gallery aligned NS, at about 15 m below the surface (figs. 3 and 5). Between 1991 and 1992, the stations The second station (BAI) was set up in April equipped with the pendulums were replaced 1980 in a cistern of Baia Castle; later another by stations with electronic sensors, easier to 1299 Ciro Ricco, Ida Aquino and Carlo Del Gaudio handle and to install. These sensors were bi- axial, bubble-type, short baselength platform tiltmeters, manufactured by Applied Geome- chanics (AGI)-Mod. 702 A (AGI, 1997). These instruments are made of steel and contain two transducers (one for each axis) formed by a glass case closed at the ends by three elec- trodes and containing an electrolytic liquid. The case is enclosed in a bridged electric cir- cuit; the inclination of the ground produces a voltage proportional to the amount of resist- ance imbalance. The tiltmeter is also equipped with a temperature sensor. The signals record- ed by each sensor are A/D converted and recorded automatically at a sampling rate be- tween 6 and 60 cph. The station also acquires the supply voltage before the telematic con- nection with the Monitoring Center at OV in a Naples. In 1993, another station (OVO) of this type was lodged on a pillar of armed cement inside a gallery at 25 m depth from the sur- face, near the OV historical building at Er- colano (Mount Vesuvius). OVO is 2.5 km WNW from the crater of the volcano. In 1996, another two stations were set up at Trecase, in the Forestry Barrack (TRC) and at Torre del Greco (CMD). TRC and CMD are located 2.5 km SSE and to 5 km SSW from the crater, re- spectively. Today, the whole network is formed by the b Vesuvian (fig. 6) and Phlegrean arrays (fig. 3). Fig. 4a,b. a) Section, view from the top of Blum’s op- 5. Tiltmeter applications and instrumental tic clinometer. All the pendulum mobile parts E, which characteristics are made of silica (with a coefficient of thermal dilata- tion β = 0.54 ppm/°C), are contained in a pyrex bell B The long-period tilt monitoring of a vol- under vacuum.