What is a T-phase?

A "T-phase" or "T-wave" is an acoustic phase from an earthquake that travels through the ocean. The "T" stands for "tertiary", as in: P-waves are "primary", S-waves are "secondary", and T-waves are "tertiary", because they travel the slowest and so arrive 3rd. Basically, when an earthquake occurs in the earth's crust under the ocean, the usual crustal phases are generated (P and S waves), but in addition part of the energy goes into the ocean as acoustic energy, and that is the T-wave. It's basically the seafloor rumbling. Not all earthquakes generate T-waves (since they need to be near water). T-waves are typically recorded by hydrophones, but on some islands seismometers sometimes record T-wave signals that have been converted to crustal phases when they hit the island. Similarly, we sometimes "hear" big earthquakes located in Hawaii, because enough of the earthquake energy gets converted into T-waves on the island flanks.

The great thing about T-waves is that they let you detect much smaller events than can be recorded by seismometers on land. Almost all the world's seismometers are on land, after all, and so it is hard to detect anything small out in the oceans far from any instruments. The key is that sound propagates incredibly efficiently in water, whereas the crustal phases get attenuated in the earth. So we can detect much smaller earthquakes by "listening" for them, than we can by "feeling" for them. This is why the Navy has the SOSUS system. They could listen for tiny sounds made by Soviet subs (when they were still around) just like in Hunt for Red October. Of course the Navy doesn't care too much about earthquakes, but we convinced them to let us tap into their system so that we could listen for them and use this information to learn about the mid-ocean ridge.