Etna discrete geodetic networks
Etna discrete geodetic networks (reference: Alessandro Bonforte)
On Mount Etna, the first trilateration networks, dedicated to monitoring ground deformations by using Electronic Distance Meters (EDM) and theodolites, were installed during the seventies. These early network provided data relevant to eruptions taking place near the summit of the volcano, especially around the North East Crater between 1974 and 1978. Due to their limited extent, these geodetic networks were not able to properly sample any medium-scale deformation induced by changes in the deep internal plumbing system, preventing any clear understanding of the medium-term evolution of the volcanic activity. Moreover, the survey and the maintenance of these networks were difficult, due to the lack of facilities such as good roads or observatories, to the prohibitive environmental conditions in winter time, to the benchmarks being frequently destroyed by volcanic activity and to difficulties in connecting those networks to stable areas. Following this pioneering epoch, and in order to overcome some of the above limitations, three EDM (networks were installed on the intermediate flanks of the volcano, on the northeastern (in 1978), southwestern (in 1980) and southern (in 1983) sides. Those networks aimed to monitor ground deformation patterns related to deformation sources located at medium depth inside the volcano. They were surveyed at least yearly, and more frequently during eruptive periods (e.g. 1978 - 1979). The combination of those efforts ensured that, at the beginning of the 1980’s, Etna was densely covered by geodetic networks. Data from these networks were used to study and interpret the volcanic deformation for more than a decade, in particular during the eruptions of 1981. 1989, and 1991-93. Leveling data collected on recent lava flows in the summit area showed the existence of still ongoing subsidence of these flows, several years after their emplacement. After the 1983 and 1985 eruptions, large horizontal displacements were observed in the upper southeastern part of the volcano, stimulating a debate on the possible risk of slope failure in the eastern part of the volcano.
The GPS network
The first GPS (Global Positioning System) measurement campaign took place in 1988. Based on the experience acquired over the previous 15 years, and in order to improve the existing networks, the 9 GPS sites observed in this first campaign were selected according to the following criteria: (1) To include stable points outside of the volcano edifice. The importance of connecting the local geodetic networks to an external frame had been mentioned in the past but hardly achievable with classical methods, especially on a routine basis. (2) To link the various existing trilateration networks by GPS measurements. The trilateration networks were poorly linked, mostly for logistical reasons due to the shape and size of the volcano. (3) To link the GPS to the leveling and micro-gravity networks. (4) To allow easy re-observation of the GPS points. In contrast with classical geodesy, GPS allowed for the first time to select sites not on inter-visibility or network geometry criteria, but more on accessibility and other logistical considerations. The nine sites were therefore established along roads and trails. This introduction of GPS techniques profoundly changed the operation and application of geodetic surveys on Etna.
Since 1988, GPS measurements have been carried out at least yearly, with sometimes more frequent observations during periods of higher volcanic activity. Since then, the GPS monitoring network has been continuously improved, by installing new benchmarks to improve the density, the geometry and the capability of monitoring particular volcanic and volcano-tectonic features.
The GPS discrete network is currently composed by 80 benchmarks and includes a reference frame and an inner network, composed of various sub-networks.
Mt. Etna GPS reference frame
The reference network includes points located outside and all around the volcano edifice, in order to refer the motion of the measured benchmarks to a stable frame.
Mt. Etna inner network
The inner network is the combination of various local networks, each of them set-up to monitor a particular phenomenon. It includes:
- the main inner network,
- the East-West kinematic profile,
- he North-South kinematic profile,
- the “Ionica” network,
- the Pernicana networks,
and various other local networks.
From 2000, the sub-networks have been progressively merged and connected to the reference network through unique surveys. Due to the different observation strategies on the sub-networks (static and semi-kinematic), the accuracy of positioning is not the same for all points.
Main inner network
The principal inner network covers the volcano from 600 to 2900 m a.s.l.. Its evolution is closely related to the dynamics of the volcano during the last decades and to the different research activities carried out on Etna using geodetic techniques. Following the strategy defined during the first 1988 survey, several benchmarks were installed in order to connect the GPS network to the existing microgravimetric and E.D.M. networks on the volcano. This sub-network is robustly connected to the reference frame and allows the ground deformation events involving the whole volcanic edifice to be monitored.
East-West kinematic profile
The East-West kinematic profile was set-up and surveyed in 1994 to carry out experiments for the application of the Stop-and-Go GPS technique for monitoring purposes. It consisted in 16 nails installed along a linear profile following the path of the SP92 road, surveyed by a small tripod. After the successful exploiting of this profile during the 2001 eruption, it was totally re-configured to improve the quality of the surveying. Now, it is a T-shaped network covering the southern flank of the volcano with 12 self-centering benchmarks, at altitudes ranging from 600 to 2000 m along the “SP92” road, from Serra la Nave, Tarderia and C.da Cassone up to the Rifugio Sapienza area. Eastward, the profile crosses the 1989 fracture field that gave rise to ground deformation events in 2001. It is surveyed by the “Stop and Go” semi-kinematic technique, observing each station twice (fore and back), allowing a good compromise between density of points, surveying time and accuracy.
North-South kinematic profile
The North-South kinematic profile covers a strip of the volcano edifice above 2000 m of altitude, crossing the summit craters area. Several other important active volcanic structures are monitored by this network (e.g. the 1983-85 and 1989-91 fracture zones and the NE Rift zone). It also allows the reduction of gravity data collected in the same area. It is entirely constituted by self-centering benchmarks and it is surveyed using the same strategy adopted for the EW profile. The upper southernmost stations of the profile have been destroyed by the 2001 and 2002 eruptions, so the profile currently develops over the NE rift up to the summit craters. The upper southern part has been replaced by new stations surveyed in static mode.
Ionica network
In 1997, in order to better resolve the deformation field affecting the lower eastern flank of Etna, a GPS network was installed on this sector of the volcano. This is the largest second-order network, after the principal inner network. A short-lived E.D.M. “Ionic Network” existed in the same area from 1982 to 1984, but it was impossible to recover the original benchmarks, most of them having been destroyed by urbanization or covered by dense vegetation. The “Ionica” GPS network consisted of 24 stations and covered the lower eastern flank of the volcano, along the Ionian sea, from Catania to Taormina and from 0 to 1300 m a.s.l.. The network configuration was designed to monitor the dynamics of a number of faults existing in this part of the volcano. Four “Ionica” stations were also belonging to the reference frame or to the principal inner network. One station, also belongs to a smaller GPS network installed to monitor the ground deformation along the lower segment of the Pernicana fault, in the Rocca Campana area. All stations consist of self-centering benchmarks.
Pernicana networks
Two local geodetic networks were installed by “Istituto Internazionale di Vulcanologia” in April 1997 across the eastern portion of the Pernicana fault, on Rocca Pignatello and Rocca Campana areas. GPS technique was adopted to survey the RCN network, while EDM technique was adopted to measure the Rocca Pignatello one (RPN). The two networks are relatively small, covering an area in the order of 1 square km and consisting respectively of 6 and 4 benchmarks. The first measurements of the networks were carried out in early 1997; since then, the networks have beenwere surveyed at least three times per year in order to adequately detected pattern and rate variations of ground deformation. From July 2002, 5 new benchmarks were installed at RPN area and the GPS technique has been adopted to survey them, so in September 2002 the last EDM survey on the old RPN network was carried out.
