STUDY ON METEOROLOGICAL AND TIDAL INFLUENCES UPON GROUND DEFORMATIONS

Abstract

Observational results of the ground deformations obtained by means of tiltmeters and extensometers at two stations in Wakayama City in Kinki District, Japan, are presented and discussed with particular emphasis on the relation to meteorological and oceanic phenomena. The ground at the Oura station inclines gradually to the north-westward and a similar tendency is also discernible at the Akibasan station. It is shown that the annual changes observed are due to the thermal deformation of the ground by the temperature change on the ground surface and that their wave lengths and mode are nearly equal to those of the daily changes, depending mainly on the local topography near the stations. Drift, that might be caused by initial disturbances on the instruments and their surroundings, lasted for several years on a record at Akibasan, which suggests that every discretion is required as to the interpretation of secular changes from conventional tiltmetric or extensometric results. From a concurrent observations by two pairs of tiltmeters of the horizontal pendulum type, it is ascertained that tiltmeters of this type may be used for observations of secular tilting motions with certain qualifications. The diurnal and semidiurnal changes observed by each pair of the two tiltmeters are affected by the atmospheric pressure and air temperature, and each behaves in a different manner, 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 ground deformations and in the evaluation of tidal constituents, especially in that of the Sz constituent. An estimation of the effect of the atmospheric pressure gradient on the ground tilt is attempted as to the atmospheric tidal waves, the obtained value being about 7" per 1 mb/km, but the direction of the ground tilt is oppose to that of the loading influence due to the barometric pressure. From the spectral structures of the tiltgram at Akibasan it is shown that the change in the periodic range from 1 to 33 days in the the ground tilt at the station is scarcely influenced by the changes of the air temperature, atmospheric pressure, its time derivative and loading of the water in the neighboring sea. It is also shown that the effect of precipitation is, on the contrary, very remarkable and that the ground tilt caused by an impulsive precipitation is tentatively represented by at· exp(- f3t) with equal values of f3 for the E-W and N-S components of the tilt. Corrected spectra for the disturbances are given, which suggest predominance of several oscillatory tilting motions. Comparisons of the spectral structures of the tiltmetric and extensometric records with those of the barograms show that the amplitude ratio of the ground tilt or strain to the pressure increases and the phase difference between them also varies slightly with the frequency. This frequency dependence of the spectral ratios can be apparently expressed as the sum of the two parts proportional directly to the change of the pressure and proportional to the time derivative of the pressure change, respectively. A tentative method for elimination of the disturbances due to the oceanic and meteorological changes from the tiltgrams is introduced. Digital filterings of the tiltgrams by several types of filters are also attempted to detect small anomalous ground tilts. There can be seen in these results that peculiar ground tilts of the magnitude of about 0.02" or smaller existed at the two stations shortly before and after the three local earthquakes occurred on Nov. 14, 1960, in this district. Although these phenomena may be related to the occurrence of these earthquakes, further investigations are needed to verify the relation between them.