Observations of Crustal Movements by Newly-Designed Horizontal Pendulum and Water-Tube Tiltmeters with Electromagnetic Transducers (2) ―Variations in the Amplitude and Phase of Tidal Tilts Observed with a Water-Tube Tiltmeter at Kamitakara―

Abstract

Using the data collected over nearly one year obtained with a water-tube tiltmeter installed at the Kamitakara Station (36°17'N, 137°20'E, H=800m) in the northwestern Chubu region, Japan, temporal variations in the amplitude and phase of the M_2 and O_1 constituents have been investigated by means of the least-squares method. The results show that variations in the amplitude and phase of tidal tilts observed in three of four directions are ±2～4% and ±1～2° for the M_2 constituent, and ±3～6% and ±3.5° for the O_1 constituent, respectively, during the observation period. Considering the error limit in sensitivity measurements, it may be concluded that both the M_2 and O_1 tidal constants have not changed significantly during this period, suggesting no essential change of elastic properties in the shallow crust. This conclusion may be consistent with the fact that seismic activity around the region has not been so high. Successive analyses with 30-day data have also been performed by shifting the central epoch by a 2-day step, and it has become clear that there is a common periodic oscillation in the amplitudes and phases of both M_2 and O_1, but this can be easily removed by a moving-average method. It might be possible to find out more detailed variations in the tidal constants from the averaged results.