Typical T-wave activity recorded in Polynesia during the volcanic swarm near the Eltanin Fracture Zone in the early 1990s.

Top: Close-up of a 10-second window of T-wave record at Vaihoa (Southern coast of Rangiroa Atoll). Bottom: Fourier spectrum of the record. Note the exceptionally monochromatic character of the wave. (See Talandier and Okal [1996] for details.)