A small earthquake struck just off the coast of Maine Monday, causing slight tremors but no damage, the US Geological Survey said. The agency said the 2.4-magnitude earthquake was centered 4 miles southeast of Boothbay Harbor. Police departments in Boothbay Harbor and Bath said they began receiving calls after the quake struck at 1:19 p.m., but there were no reports of damage or injury. The geological agency received reports of the quake from as far away as Plymouth, N.H., 171 miles from the epicenter.