A Discussion on the measurement and interpretation of changes of strain in the Earth - Crustal tilt fields and propagation velocities associated with earthquakes

Abstract

Three borehole installation of 15 s horizontal pendulums (of the Lamont lunar type) with capacitance displacement transducers were carried out in 1968-1969 in Central Alaska in Gilmore (GLM near Fairbanks), Patson (PAT) and McKinley (MCK) int he Alaska Range. Data are telemetered over phone lines and v.h.f. radio links and the sensitivity is better than 10~9 rad/m m of chart recording. Tilt steps similar to strain steps have been observed for earthquakes with magnitude from 2 to 8 and distances ranging from 10 to 9000 km. The tilt step propagation velocity from the hypocentre to the station increases from about 1.3 km/s near the epicentral area to 2.6 to 2.8 km/s at 60 to 80 km and to 3.1 km/s at teleseismic distances. Tilt directions, amplitudes and velocities observed at several stations simultaneously for the same earthquake are internally consistent. For local Fairbanks quakes the data from the Alaska long period array (ALPA) also have been used and span a full quadrant from the epicentre. There is an indication that tilt amplitudes depend on the tectonic environment of the station. Examples of tilts from local and regional earthquakes are presented and observations from teleseismisms include the 10 January 1971 ( M = 8.1) New Guinea, the 9 July 1971 ( M = 7.7) Chile and the 14 July 1971 ( M = 8.1) Solomon Island earthquakes.