MANILA—A 4.7 earthquake struck off Iba, Zambales while a 5.5 tremor hit Surigao City Saturday, according to the Philippine Institute of Volcanology and Seismology.

In Zambales, the undersea tremor occurred at 8:47 p.m. about 35 kilometers southwest of Iba at a shallow depth of 13 kilometers.

At 1:06 p.m., an offshore tremor hit 121 kilometers northeast of Surigao City at a depth of 70 kilometers. It was followed by four aftershocks, the strongest of which measured at magnitude 5 at 2:11 p.m.

The Philippines sits on the Pacific “Ring of Fire” where continental plates collide causing frequent seismic and volcanic activity.