Continuous Observations of Crustal Deformation at Nagano City, Central Japan (I)

At Nagano City, Central Japan, ground tilt was continuously observed with a set of water-tube tiltmeter and an Ishimoto type silica tiltmeter (horizontal pendulum) for four years from April 1967 to May 1971.General trend of the tilt shows a secular ascending of the basement to N13°E direction. The movement not only qualitatively but quantitatively corresponds to an unusual upheaval of the crust at northeastern part of Nagano City, which was found by revision of precise levelling. This local upheaval was accompanied with a considerable activity of smaller earthquakes. Our study demonstrates that such a short-span observation of ground tilt can successfully supplement discrete data from levelling, if the observation is carefully planned. Horizontal pendulum tiltmeters are much more convenient to be installed compared with water-tube tiltmeters, but the tilt resulted from the former instruments, θS does not necessarily coincide with that from the latter, θW. In our case, θS is about five times as large as θW though the azimuth of the tilt variation is harmonious one another. A model is offered in order to interpret such amplification of the ground tilt.Tilt variations just after larger Matsushiro earthquakes were investigated, but we could not detect any tilt steps attributed to the dislocation at the origins.