Gravity changes during the Alaska earthquake

Abstract

A gravity traverse made in 1962 along the Richardson Highway between Valdez and Tonsina, Alaska, with a LaCoste and Romberg geodetic meter having a reading sensitivity of 0.01 mgal, was repeated in September 1964 with the same instrument. The observed gravity changes were used in computing elevation changes produced by the March 27, 1964, earthquake. Elevation changes determined by this method are in general agreement with changes determined by post-earthquake releveling of a 1923 Coast and Geodetic Survey first-order geodetic line coincident with the traverse. The conversion factor could not be precisely determined because the maximum elevation change at any station amounted to only about 0.6 meters. Gravity changes observed in other parts of the earthquake area, where greater elevation changes occurred (up to 3 meters), indicate that the conversion factor is closer to the normal Bouguer gravity gradient (0.2 mgal/m) than to a free-air gradient (0.3 mgal/m). This suggests that the elevation changes were accompanied by a net change of the total mass affecting the gravity readings, rather than by purely chemical or elastic changes. The data are preliminary and indicate the need for more pre- and post-earthquake comparisons.