A series of moderate earthquakes rattled the Tokyo area Saturday, with one temblor registering a preliminary magnitude of 5.1, Japan's weather agency said. No tsunami warnings were issued.

The magnitude 5.1 temblor struck at 4:55 p.m. (0755 GMT) just off the eastern coast of neighboring Chiba prefecture (state) with its epicenter about 12 miles below sea level, the Meteorological Agency said.

There were no immediate reports of injuries or damage from the quake. A magnitude-4.8 temblor Saturday morning startled a 67-year old Chiba woman, who fell and broke her arm as she scrambled to get out bed and to a safe place, media reports said.

The Chiba area has been plagued by a series of temblors since Thursday, several of which have been strong enough to be felt to its southwest in Tokyo.

Japan, which sits atop four tectonic plates, is one of the world's most earthquake-prone countries. Last month, a magnitude 6.8-quake struck northern Japan, killing 11 people and causing a fire and small radiation leak at a nuclear power plant.