The Mw=9.0 Tohoku Earthquake resulted in failures of the two dams impounding Fujinuma Lake, in Fukushima Prefecture. After the event, several mechanisms were postulated regarding the failure modes of the dams. The authors have performed numerical dynamic analyses whose principal aim was to determine the dams’ likely failure mechanism. Our numerical analyses of the Fujinuma Main Dam predicted several meters of lateral displacement of the downstream face due to seismic shaking, i.e., sliding, and a corresponding large drop in the crest elevation. This drop rendered the dam vulnerable to overtopping which ultimately breached the dam. Additional numerical stability analyses also found that the Fujinuma Saddle Dam had a static factor of safety below unity under rapid drawdown conditions, and was therefore vulnerable to a rapid release of the reservoir water when the main dam was breached.