A magnitude 5.1 earthquake struck the Bicol region Saturday evening, followed by an aftershock Sunday morning, reports from the Philippine Institute of Volcanology and Seismology (Phivolcs) showed.

Both were still aftershocks to the tectonic quake that shook Bicol last Aug. 15, Alex Baloloy, senior science research analyst of the Phivolcs Mayon Volcano observatory at Ligñon Hill here, said.

The epicenter of Saturday evening's earthquake was plotted at 36 kilometers northeast of Sorsogon province, with a depth of 11 km.

Intensity 4 was felt in Sorsogon City and Legazpi City in Albay province, Intensity 3 in Irosin, also in Sorsogon, and Intensity 2 in Catbalogan, Samar.

Another quake was felt at about 8:30 a.m. Sunday in Albay province.

Initial data from the US Geological Survey website plotted the Sunday quake's epicenter at 65 km east-southeast of Legazpi City, with a magnitude of 4.9.

No immediate casualties or injuries were reported in these recent aftershocks.

In an earlier interview, senior geologist Arlene Dayao of the Bicol Mines and Geosciences Bureau, advised that aftershocks to the Aug. 15 quake could continue at least two weeks later.

Phivolcs officials assure Albayanos that the quakes were not related to any activity in Mayon Volcano.

The magnitude of an earthquake measures the seismic energy it releases. Intensity describes its strength based on the quake's observable effects on the ground. Intensity 2 means people would feel the movement when they are rest. Intensity 3 means people indoors would feel the movement. Intensity 4 means most of the people indoors would feel the shaking, which is strong enough to cause hanging objects to swing and dishes to rattle.