At this point we do not know if the earthquake damaged the containment building enough to undermine its ability to contain the pressure and allow radioactivity to leak out.
According to technical documents translated by Aileen Mioko Smith of Green Action in Japan, if the coolant level dropped to the top of the active fuel rods in the core, damage to the core would begin about 40 minutes later, and damage to the reactor vessel would occur 90 minutes after that.
Concern about a serious accident is high enough that while TEPCO is trying to restore cooling the government has evacuated a 3-km (2-mile) radius area around the reactor. Union of Concerned Scientists