This is strong evidence of something happening just before an earthquake. It could be happening all the time, but they just looked carefully after the fact. Still it is interesting to explore the proposed physics.
They were expecting 'every day' M8 earthquakes. This was in their living memory, and they did not consider a rare earthquake. In this way, they were evil twins to every other nuclear person in the world. The big problem with large earthquakes is that they act as stacked dominoes, ready to fall. You might get a signal for an M8, but not know that everything is set up for an M9.
All earthquakes are a stick-slip response to earth movement. As such, they need a critical displacement, dc, to make the transformation from static to dynamic friction. Think of standing in a slippery, wet bathtub, where the critical displacement is near zero. As your foot starts to slip, it has no idea of the general balance of the rest of your body. You can have a little slip, or rip down the shower curtains!
For this radon signal, it is assumed that a section of the fault is starting a rapid approach to dc. It is most likely a small zone of 100 km. The pictures show a couple of zones. This movement supposedly releases radon. I am having difficulty thinking of the fate of this radon through the deep ocean. Is it coming up in bubbles? Can it diffuse so rapidly? Anyway, then this radon screws up the weather. Thick ocean sediments also come to mind, as in how does the radon get through clay?
Maybe the little micro-tremors shake up the sediments and release methane, but I still think you need bubbling to achieve such a rapid result. Off to the next M9 candidate!