JAKARTA, Indonesia (CNN) -- A powerful series of earthquakes shook far northeastern Indonesia Thursday, injuring at least 42 people and damaging about 500 homes and other buildings, officials said.
The damage is concentrated on the islands of Karakelong, Kabaruang, Selebahu and Sangir, according to Dr. Arikalang, head of the health ministry in the Talaud region.
Setting off the chain of temblors was a magnitude 7.2 quake at 1:35 a.m. (12:35 p.m. ET Wednesday), the U.S. Geological Survey reported. The major quake was followed by more than 20 aftershocks of magnitude 5.0 or higher. The number is greater than 30 when smaller aftershocks were counted.
The quakes were all clustered off the northeastern tip of Sulawesi Island, about 1,550 miles (2,495 km) east-northeast of Jakarta, where three major tectonic plates come together, according to Indonesia's Meteorological and Geophysics Agency.
The plates -- massive, sometimes continent-sized, slabs of rock floating on the Earth's surface -- collide with each other, causing earthquakes.
Indonesia is located on the "Ring of Fire," an arc of volcanoes and fault lines encircling the Pacific Basin that is prone to frequent earthquakes and volcanic eruptions.
The full extent of damages was not immediately known as an estimated 3,000 people fled coastal areas to surrounding hills for safety in the event of a tidal surge, said Priyadi Kardono, an official with the National Disaster Coordinating Agency.
The Indonesian Meteorological and Geophysics Agency said no tsunamis were generated.
The Ministry of Health said it sent a team to the epicenter to assess damage and provide humanitarian support.
-- CNN's Andy Saputra contributed to this report.