Study on the Relation between Local Earthquakes and Minute Ground Deformation. Part 3. On Effects of Diurnal and Semidiurnal Fluctuations of the Temperature and Atmospheric Pressure on Ground Tilts.

Abstract

The diurnal and semidiurnal effects of atmospheric temperature and pressure upon the ground tilt observed by tiltmeters in Wakayama, Japan are investigated with the use of one year's data. In the first half of the paper, monthly amplitudes and phases of M_2, S_2, O_1 and S_1 constituents have been calculated in respect of ground tilts, oceanic tides. atmospheric temperature and pressure by the Fourier transform method. Interrelation between ground tilts and meteorological disturbances have been examined by the amplitude and phase fluctuations. In the latter half of the paper, coefficients for the effects of temperature, atmospheric pressure and its time gradient upon the ground tilts have been determined from their mean daily variations by the least squares method. Two pairs of tiltmeters installed in the same direction on one concrete base behave in a different manner for diurnal and semidiurnal changes of temperature and pressure, from which it is concluded that these effects have very local characteristics detectable even when the tiltmeters are placed so close to each other. This should be given due consideration in an observation of minute deformations of the ground, and in the evaluation of tidal constituents, especially in that of the S_2 constituent. An estimation of the effects of an atmospheric pressure gradient is attempted for atmospheric tidal waves, the obtained value being 7" per 1 mb/km. but the direction of the ground tilt is opposite to that of the loading influence due to barometric pressure.