An investigation of the relation between source characteristics and T phases in the north Pacific area

Abstract

Earthquake body waves refracted into the ocean are called T waves. They propagate as acoustic waves in the deep ocean sound channel at frequencies near 10 Hz. Arrival times of T waves, as recorded on widely spaced hydrophones in the sound channel off Eniwetok, Midway, Wake and Oahu Islands in the northern Pacific, are used to locate the T-phase source area. Sources thus located are correlated with published earthquake epicenters. The amplitude, onset rate, duration, frequency content and number of peaks of T-phase signals differs with earthquake magnitude as well as epicentral location. A study of T phases from earthquake epicenters in a localized source area, the western Aleutian Islands, was undertaken to investigate the cause of variation in T-phase signals. The earthquakes studied occurred in a series of aftershocks following the 7 3/4 magnitude earthquake south of the Rat Islands in the Aleutian Islands on February 4, 1965. The aftershocks occurred in a zone, about 300 miles long and 150 miles wide, roughly parallel to the WNW-ESE trend of the Aleutian arc. T-phase signals were different for earthquakes on the Aleutian arc, and behind the arc, from earthquakes on the Aleutian insular slope and Aleutian Trench. The T-phase strength, for earthquakes of a given magnitude, was greater for earthquakes on the arc, least for earthquakes in the Trench and outer ridge, and intermediate for earthquakes on the insular slope. The onset rate of T-phase signals varied with earthquake magnitude, but in general was most rapid for epicenters on the Aleutian arc, intermediate for epicenters on the Aleutian slope and gradual for epicenters in the Trench and outer ridge. Decay rates varied with earthquake magnitude because of reverberation and topographic reflections. Onset and decay rates were symmetrical for earthquakes in the Trench and on the outer ridge. Earthquakes on the Aleutian arc produced multiple peaked T phases due to radiation of T waves from more than one slope in the epicentral area. Earthquakes far out on the Aleutian slope. bench, Trench and outer ridge sometimes produced multiple peaked T phases because of both reflections from the slope behind the epicenter and radiation of T waves from that slope as well as from the epicenter. The accuracy of source location. as compared with epicentral location using the arrival times of earthquake body waves, was best for earthquakes on the Aleutian arc and in the Trench, and poorest for slope and bench earthquakes. Source solutions for signals from underwater explosions at known locations in the area indicate a source location accuracy of ±10 miles. The accuracy of earthquake epicentral location by T phase is limited by the relatively broad signal peak of the T phase, and the fact that the point, or points, of radiation of the T phase is usually at an ocean bottom slope near the epicenter instead of at the epicenter itself.