Disturbances in thermogravitational Atmospheric Tides during the strong earthquake of Volvi, North Greece in 1978
Atmospheric tides, excited by the periodically varying luni-solar gravitational attraction and heating by the Sun due to the earth revolution, subject the influence of parameters affecting the thermal exciter (atmospheric temperature, water vapour content, ozone content, molecu-lar viscosity and conductivity, ion content and ion drag of the ionosphere) as well as of pa-rameters affecting the gravitational exciter (atmospheric density, topography and near surface crustal inhomogenities) and are sufficiently well studied the last two centuries. Chapman and Linzen (1970) give an extensive presentation of the state of the art up to the recent years. Atmospheric tides in the area of Thessaloniki were studied by analysing an 11-year sample of hourly barometric measurements (Arabelos et al. 1997). In a recent paper Biagi et al. (2002) reported the presence of tidal harmonics in the power spectrum content of three LF radio sig-nals indicating that atmospheric tides are one of the factors affecting the propagation of LF radio waves. In addition they reported anomalous increase in the intensity of the radio signals, which was propagated over the epicentre of a seismic sequence in Slovenia during the last month before the beginning of the sequence. They suggested that preseismic exalting of the atmospheric tides, produced by a gravity decrease due to fluid diffusion in a broad area around the epicentre, was responsible for the radio signal exalting. This fact renders the hope that observables related to atmospheric tides may probably be used as earthquake precur-sory phenomena. Thessaloniki, being 30 km southeast from the very active seismic area of the lakes Langada and Volvi, lives under the threat of a potentially large earthquake. So a program for the investigation of a possible correlation of the atmospheric tides parameters with seismic activity in the area of Thessaloniki, was initiated. In this paper we present the result of this investigation!, which concern the thermo-gravitational tidal disturbances in! the are a of Thessaloniki during the strong earthquake (M=6.5) which occurred on June 20, 1978 in the area of the lakes Langada and Volvi (Mygdonia basin). In the present analysis the data sets used were the following: (a) Normal Tidal Potential model of Tamura (1987), which com-prises 1200 waves, (b) Barometric pressure and atmospheric temperature data from the De-partment of Meteorology and Climatology of the Aristotle University of Thessaloniki (accuracy 1 hPa and 0.1oC respectively), and (c) Earthquake catalogue of the Geodynamic Insti-tute/National Observatory of Athens with magnitude completeness threshold ML =3.2.
Proceedings of the 5th International Symposium on Eastern Mediterranean Geology Thessaloniki
- Pub Date:
- April 2004
- Barometric variations;