Given the relevance of this subduction system, I would expect that a wide range of detectors (temperature, vibration, seismometers, whatever) are deployed in the depth of these trenches. What would be the canonical source at which one could access the readings of these detectors?
The idea of seafloor observatories making their data available freely and online is a vision and there are observatories being proposed by several of the major oceanographic institutions. Search for the term "seafloor observatory."
You might also take a look here, or contact the researchers:
Realtime Data from the Deep Sea Floor Observatory (off Kushiro-tokachi, off Muroto, off Hatsushima)