TIME VARIATIONS OF M2 EARTH TIDE AMPLITUDES AS A REFLECTION OF THE PROCESS GOING ON DURING A SILENT EARTHQUAKE

Abstract

The dependence of earth tide parameters on the elastic properties of rocks suggests the use of these parameters for earthquake prediction. Observations conducted in the epicentral zones of some large earthquakes revealed preseismic variations in the amplitudes of major waves, amounting to 10%. Continuous observations with horizontal quartz strainmeters carried out during 26 years at a station in an aseismic zone were used in this study. That area is a karst region with surface and buried karst expressions. Construction work had been in progress in a vicinity of the station some time during its operation, including underground work. The surface karst was found to be activated during that work with accompanying changes in the depth to the main aquifer and, as appeared on closer examination, with changes in M2 earth tide amplitudes. The M2 wave, as the wave least affected by meteorological factors, had the maximum amplitude along the azimuth considered, which was determined with the highest accuracy. The amplitude decreased with time and in 1986 was the lowest for the period of study (28% below the theoretical value). The amount of decrease is above the measurement error and can be regarded as reliably determined. That period was marked by the formation of new karst sinkholes in the adjacent area and by the lowest level of the main aquifer. Later the M2 amplitude was increasing and reached the 1970-1971 value by 1996. The whole phenomenon can be viewed as a natural-man-induced model of a silent earthquake.