MANILA, Sept. 13 (Xinhua) -- A 5.1-magnitude aftershock struck Polillo island off Quezon province in the Philippines' main Luzon island on Friday, the Philippine Institute of Volcanology and Seismology (Phivolcs) said.
The earthquake struck less than an hour after a magnitude 5.5 earthquake struck the island.
The institute said the second earthquake, which struck at 5:18 p.m. local time, hit at a depth of 11 km, about 34 km northeast of Burdeos, a town on Polillo island in Quezon province.
"This is an aftershock of the magnitude 5.5. earthquake (that earlier struck the island)," the institute said in a statement.
An offshore quake struck the island at 4:28 p.m. The first tremor hit at a depth of 10 km, about 40 km northeast of Burdeos town.
The island is in the northeastern region of the Philippine archipelago. It is separated from Luzon island by the Polillo Strait and forms the northern side of Lamon Bay.
Students and office workers in Metro Manila, which also felt the tremor, rushed out of the buildings. Many schools in Metro Manila decided to cancel classes in the wake of the quakes.
The institute said the quake was also felt in Camarines Norte, Quezon City in Metro Manila, Guinayangan town in Quezon province and Tagaytay City.
There was no reported casualties or damages, according to Phivolcs director Renato Solidum.
Solidum also told a local radio it has not received any input yet from Burdeos town.
"We are not issuing a tsunami alert because the magnitude is relatively weak," he added.
However, Solidum said more aftershocks are expected.
