A test of the semyenov prediction technique in the central Aleutian Islands

The earthquake-prediction technique based on systematic temporal variations in the ratio of the two body-wave velocities has been tested for selected events in the central Aleutian Islands. The goal was to determine if the phenomenon occurs for earthquakes in an active island arc. Two groups of earthquakes were selected, each in a concentrated source volume and well-placed with respect to the available local seismograph network, one relatively shallow (20 km) and the other along the thrust zone (about 45 km deep).Observational difficulties inherent to an island arc, namely the depth of the hypocenters, the high noise levels, and the complex geology, limit the number of events available for analysis and cause considerable scatter in the data. Nevertheless, a positive result, in which the velocity ratio decreased by about 5%, was obtained for at least one event in the shallow group for which there were sufficient preceding events to establish a definite trend. No premonitory changes were seen for the deeper source volume.A simple extension of the theory of the Wadati diagram permits an estimate of the velocity ratio in the overlying layers as well as in the source medium. The data indicate that under Amchitka Island, the velocity ratio is 1.82 (Poisson's ratio 0.28) in the upper 10 km and 1.71–1.72 (Poisson's ratio 0.24) from 10 km to at least 45 km.