Here is an interesting study about a correlation between elevated total electron content readings preceding earthquakes, particularly March 11, last year.
The gist of the science is this: real-time readings from satellites are available continuously. This data is combined with other data to map global "hot spots" now and are available to view on-line. Earthquakes correlate to these mapped hotspots within days of elevated data.
One theory is that higher than normal radon gas released from faults ionizes the air, which rises to altitudes high enough to be able to segregate it from the background HF interference that masks it at lower altitudes.
Now, there are people who latch onto some of the science to push the HAARP death ray conspiracy, which pollutes the real science. The real stuff is worth looking at.
Very high levels of electron content were tracked off Miyagi and Fukushima, starting on March 8th. The highest level was on the morning of March 11th precisely over the location of the epicenter later in the day.
The main site is here: http://arxiv.org/abs/1105.2841
The direct link to the .pdf is here: http://arxiv.org/ftp/arxiv/papers/1105/1105.2841.pdf
A continuously updated, real-time image of hot spots is here: http://www.ips.gov.au/Images/Satellite/Total%20Electron%20Content/Regional%20Maps/World_tec.gif
Is it truly a precursor? You can decide for yourself. If you see a red hot spot over the fault areas off Tokyo, it might not hurt to make that extra shopping run you've been putting off to stock up on your stash of supplies.