Abstract
The paper reports long-term tiltmeter observations at the Matsuyama Crustal Movement Observatory, Shikoku, Japan, since 1949, with special reference to the postseismic land movement following the Nankaido earthquake of 1946. The land tilting there has accumulated in a general trend of the NNE-down sense, amounting to 3 sec (in arc) or more in the past twenty-five years. More precisely, the tilt accumulation was almost monotonous in the N-S component, whereas in the E-W component, its sense was reversed from E-down to W-down about 1956-7. These modes of tilting seem to harmonize with the regional land movement as derived from levelling data in the Shikoku district. The sea-level changes at three mareographic stations, Matsuyama, Uwajima and Tosa-Shimizu, all in west Shikoku, suggest slow postseismic movement in the Uwajima-Tosa-Shimizu area. Thus, aseismic local fault slip following the 1946 quake is hypothesized in order to interpret this remarkable effect. Tiltmeters have recorded short-term disturbances of unknown causes, occasionally. Comparison of these events with the sequence of local earthquakes of considerable magnitude (M=3.3-6.6) sometimes indicates chronological coincidence of the two sorts of events. But further investigation must be developed to draw a conclusion. Brief discussion is added on the stability of a float-type water-tube tiltmeter, in which a test observation has been made since 1972 successfully.
