One such idea is that the earthquake was caused by HAARP, the High Frequency Active Auroral Research Program. According to its web site, HAARP is a "scientific endeavor aimed at studying the properties and behavior of the ionosphere, with particular emphasis on being able to understand and use it to enhance communications and surveillance systems for both civilian and defense purposes." The funding support by the U.S. Navy and Air Force further indicates that there is military interest in the results of this research. But for some reason, HAARP has also been a lightning rod (atmospheric pun semi-intended) for conspiracy theorists. Go ahead, do your own Google search. Or look at Arthur Goldwag's summary and Goldwag's follow-up.
Photo from HAARP photo gallery