NOAA Buoy 52402 NW Kwajalein in 'Event Mode'

That one was going off for a couple of weeks, with no matching quakes in the area, before the recent big Phillipines quake, and aftershocks.

One off the east coast of South America, was also going off for several days, prior to a series of big quakes to the north of it.

My personal theory, a rift is going to open up under the ocean between Papua/New Guinea and Japan.