Abstract
We present a study of the duration, Θ, of T wave trains propagating over teleseismic distances in the ocean, and recorded at island stations, in relation to the various magnitudes (mb, Ms, Mw) of the parent earthquake. Theoretical models based on scaling laws predict relations of the form log10 Θ = a+bM, with the slope b equal to 1/3 at low magnitudes, but increasing to 1/2 (and eventually to 1) because of the effect of saturation of the magnitude scales. The investigation of an extensive dataset using records of more than 400 Pacific earthquakes shows a remarkable agreement with these theoretical slopes. In the case of truly large earthquakes (Mw>7), such relations allow an estimation of the earthquake's seismic moment at ultra-long periods within 1 to 2 hr of the origin time, with important applications to efficient tsunami warning.
