General view of Grotta Gigante from the exit stairs

The Grotta Gigante horizontal pendulums are a pair of

tiltmeter A tiltmeter is a sensitive inclinometer designed to measure very small changes from the vertical level, either on the ground or in structures. Tiltmeters are used extensively for monitoring volcanoes, the response of dams to filling, the small ...

s used for monitoring Earth movements, mounted in the

Grotta Gigante Grotta Gigante ("Giant Cave", sl, Briška jama or ''Jama pri Briščikih''), also known as ''Riesengrotte'' or as ''Grotta di Brisciachi'', is a giant cave on the Italian side of the Trieste Karst (''Carso''), close to the village of Borgo Grot ...

in Italy. The horizontal pendulums, installed by the geodesist Antonio Marussi in 1959, are sensitive to deviations of the vertical, to rotations and shearing of the cave. The pendulum beam is suspended horizontally by two steel wires, an upper one fixed to the roof of the cave, the lower one fixed to the floor. The pendulum beam rotates in the horizontal plane around a virtual near-to-vertical axis that passes through the upper and lower mounting points.

Measurements

Some movements are aperiodic, others regular, as the Earth tides caused by the lunar and solar gravitational field. The pendulums measure a distance of 95 m between the upper and lower mountings, which contributes to the fact that the instruments detect tectonic movements with high precision and are relatively immune to some of the noise which affects smaller instruments. The Earth's crust is the outer brittle layer of our planet, on average thick in continental areas. This moves up and down by ten centimeters during the time of 12 and 24 hours due to the attraction of Moon and Sun, and is accompanied by a local tilting of some parts in a billion of radians. The movements of the pendulums has contributed to identify the free oscillations of the Earth, and to show that the secular term tilting of the cave is towards the northwest. The marine tides of the Adriatic have a loading effect in the cave, as have also the underground karstic floods of the river Timavo that disappears at the

Škocjan Caves Škocjan Caves (; sl, Škocjanske jame, it, Grotte di San Canziano) is a cave system in Slovenia. Due to its exceptional significance, Škocjan Caves was entered on UNESCO’s list of natural and cultural World Heritage Sites in 1986. Internati ...

in Slovenia. The Grotta Gigante tiltmeters are the only existing instruments to have recorded four out of five greatest earthquakes in the recent 50 years, which are the earthquakes of Chile 1960 (the greatest earthquake ever instrumentally recorded) and Chile 2010 (the fifth in the scale of recorded worldwide mega-events) the tsunami-generating event of the Sumatra-Andaman islands of 2004 (third greatest event) and the event of Japan 2011 (fourth greatest event), allowing an absolute-amplitude comparison between these events.

References

* Braitenberg C., Nagy I. (2014). Illustrating the superposition of signals recorded by the Grotta Gigante pendulums with musical analogues. Acta Carsologica, 43/1, 139-147 {{coord, 45, 42, 33.18, N, 13, 45, 49.79, E, format=dms, display=title Pendulums