Hundreds of low-level and medium-sized earthquakes have struck central Idaho since last month, puzzling geologists who wonder whether the ruptures portend a much larger temblor to come or are merely the rumblings of a seismic fault previously thought to be dormant. The recent earthquake swarm, beginning on March 24 and climaxed by a 4.9 magnitude tremor on Saturday, has produced no reports of injuries or severe damage but has rattled nerves in a region where Idahos most powerful known quake, measured at 6.9, killed two children in 1983.