Publication Date:
2017-04-04
Description:
The horizontal pendulums of the Grotta Gigante (Giant Cave) in the Trieste Karst, are long-base tiltmeters with Z¨ollner type
suspension. The instruments have been continuously recording tilt and shear in the Grotta Gigante since the date of their installation
by Prof. Antonio Marussi in 1966. Their setup has been completely overhauled several times since installation, restricting the
interruptions of the measurements though to a minimum. The continuous recordings, apart from some interruptions, cover thus
almost 40 years of measurements, producing a very noticeable long-term tiltmeter record of crustal deformation. The original
recording system, still in function, was photographic with a mechanical timing and paper-advancing system, which has never
given any problems at all, as it is very stable and not vulnerable by external factors as high humidity, problems in power supply,
lightning or similar. In December 2003 a new recording system was installed, based on a solid-state acquisition system intercepting
a laser light reflected from a mirror mounted on the horizontal pendulum beam. The sampling rate is 30 Hz, which turns the longbase
instrument to a very-broad-band tiltmeter, apt to record the tilt signal on a broad-band of frequencies, ranging from secular
deformation rate through the earth tides to seismic waves. Here we describe the acquisition system and present two endline members
of the instrumental observation, the up to date long-term recording, and the observation of the great Sumatra-Andaman Islands
earthquake of December 26, 2004, seismic moment magnitude Mw = 9.1–9.3 [Lay, T., Kanamori, H., Ammon, C.J., Nettles, M.,
Ward, S.N., Aster, R.C., Beck, S.L., Bilek, S.L., Brudzinski, M.L., Butler, R., DeShon, H.R., Ekstr¨om, G., Satake, K., Sipkin, S.,
2005. The Great Sumatra-Andaman Earthquake of 26 December 2004. Science. 308, 1127–1133.]. The secular-term observations
indicate an average tilting over the last four decades towards NW of 23.4 nrad/year. We find evidences that this tilting is regional
and has been going on since at least 125 ka. The recent earthquake of December 26, 2004 was well recorded by the pendulums.We
show that the free oscillation modes were activated, including the lowest modes as e.g. 0T2, 0T3, 0T4, 0T5 and 2S1, 0S3, 0S4, 1S2.
Description:
Published
Description:
164-174
Description:
JCR Journal
Description:
reserved
Keywords:
Geodetic underground measurements;
;
Secular crustal deformation;
;
Free oscillations;
;
Ultra-broad-band tiltmeter;
;
Sumatra-Andaman 2004 earthquake
;
04. Solid Earth::04.03. Geodesy::04.03.06. Measurements and monitoring
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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