Vertical ground movements on the coast of the kamchatka Gulf: their specific features in the epicentral zone of the August 17, 1983, earthquake M = 6.9, before and after

Abstract

Class 1 levelling commenced at the end of June 1983 in the presumed vicinity of the epicentre of the imminent earthquake along the levelling line established in 1969. The purpose was to reveal vertical displacements which precede an earthquake. The previous results (1969–1970, 1977–1980) show that the coastline of the Kamchatka Gulf subsided by 30–40 mm in 7 years relative to the territory 15–20 km away from the coastline. Comparison of average values of measurements in 1983 with the 1977 data has shown that, during that period, the sign of vertical movements changed, some bench marks or the coastline rose by 6–8 mm relative to the ones inland. Before the August 17, 1983, earthquake, oscillatory motions of bench marks were observed. Inclinations along the sections of the levelling line of the same azimuth occurred in various directions; that is the vertical ground surface deformations had a “corrugated” character. Fifteen days before the earthquake, an increase in the direct and reverse differences between sections was noted in the course of levelling. As the date of the earthquake drew closer, the absolute value of the direct and reverse differences increased. The velocity of the vertical deformations 2 days before the earthquake was calculated at 2.4 mm/day over 6 km. The data obtained indicate that re-levelling can be used to forecast major earthquakes in island arcs and similar structures.