A small earthquake struck the northeastern United States on Saturday morning, rattling homes, shaking furniture and waking people from New York to New Hampshire, authorities said. The United States Geological Survey said a moderate earthquake occurred about 15 miles southwest of Plattsburgh, New York, in the northeastern part of the state at 6:50 a.m. and had a preliminary magnitude of 5.1. The USGS said the earthquake, which was at a depth of 3 miles, was felt from Buffalo, New York, to Boston, Massachusetts and Baltimore, Maryland, but there were no initial reports of serious damage or casualties.