This is an article from MIT stating the ionosphere (50 to 600 miles up) heated up prior to the 2011 quake (and subsequent tsunami) in Japan. Unusual data was also collected regarding the 2010 Haiti quake.

A hypothesis is then put forward about radon gas released from the earth being able to heat the ionosphere (minimum 50 miles to 600 miles above the Earth's surface).

Not a scientist here, but how would it concentrate at that high an altitude with winds scattering it and also, in this case, coming up out of a deep ocean?

One thing that CAN heat the ionosphere, and which was DESIGNED to heat the ionosphere, is HAARP.

"Last year, we looked at some fascinating data from the DEMETER spacecraft showing a significant increase in ultra-low frequency radio signals before the magnitude 7 Haiti earthquake in January 2010

Today, Dimitar Ouzounov at the NASA Goddard Space Flight Centre in Maryland and a few buddies present the data from the Great Tohoku earthquake which devastated Japan on 11 March. Their results, although preliminary, are eye-opening.

They say that before the M9 earthquake, the total electron content of the ionosphere increased dramatically over the epicentre, reaching a maximum three days before the quake struck.

At the same time, satellite observations showed a big increase in infrared emissions from above the epicentre, which peaked in the hours before the quake. In other words, the atmosphere was heating up."