A 6.0-magnitude earthquake struck west of Manila on Thursday, the seismology institute reported, causing buildings in the Philippine capital to shake and sending frightened workers out of their offices.
There were no immediate reports of damage or casualties, and no tsunami alert was issued. Buildings in Manila shook for about 30 seconds.
The Philippine Institute of Volcanology and Seismology initially reported the tremor had a magnitude of 6.2, but lowered it to 6.0 after receiving more field data, said its director, Renato Solidum.
"Essentially, this would not trigger significant damage," Solidum said.
Editha Vargaz of the Land Bank of Philippine's risk management group said she and dozens of other colleagues climbed down the stairs to the street from the bank's headquarters on the 31st floor of the 34-story building.
"We were very calm," she said, citing training from regular earthquake and fire evacuation drills.
However, there was panic among some employees in offices at the nearby 14-story Ramon Magsaysay Center, said Ralph Balmaceda, who works for a travel agency on the seventh floor.
While he and other staff hurried down the stairs, "most others were panicking and some even tried to shove others" to reach the street more quickly.
"It was scary because of the previous incidents in other countries," said Balmaceda's office mate Beth Rodriguez. "We thought it would be the same here also."
Solidum said the quake was centered off Lubang Island in Mindoro Occidental province, about 80 miles (130 kilometers) southwest of Manila, six miles (10 kilometers) under the seabed.
The U.S. Geological Survey put the magnitude at 6.1 and depth at 21 miles (33 kilometers).
Lubang Island is near the southern end of the Manila Trench, a fault line about 560 miles (900 kilometers) long on the ocean floor under the South China Sea along the western flank of the Philippines' main island of Luzon.
The Philippine archipelago lies in the so-called Pacific Ring of Fire where earthquakes are common. It is flanked by the Pacific Ocean to the east and the South China Sea to the west with undersea trenches — potential quake triggers — running alongside its coast on both sides.
The last major quake registered a magnitude 7.7 in 1990 and killed nearly 2,000 people on the main northern island of Luzon.
A 7.1-magnitude earthquake set off by a local fault near Lubang whipped up a tsunami that killed 78 people on Mindoro in 1994.