Interesting that most of them are detected in 1997 (4 out of 6 specific ones).

pavel_lishin · on June 17, 2012

Also, by the same thing: Equatorial Pacific Ocean autonomous hydrophone array.

I can't find much about it after ten seconds' worth of googling, but maybe that's when it was constructed?

KC8ZKF · on June 17, 2012

Looks like it:

"In May, 1996 the array was successfully deployed in the eastern equatorial Pacific to begin long-term monitoring of the East Pacific Rise between 20N and 20S."

http://www.pmel.noaa.gov/vents/acoustics/haru_system.html