MANILA -- Aftershocks from an earthquake on the scale of the one that jolted parts of Luzon this week can persist for "several" days or weeks, the country's chief seismologist said Wednesday, as he urged the public to prepare.

A Magnitude 4.5 aftershock struck Castillejos, Zambales before dawn Wednesday, less than 2 days after a Magnitude 6.1 earthquake hit Central Luzon, killing at least 16 people and shutting Clark Airport for 48 hours.

Some 600 aftershocks have been recorded since Monday's main earthquake but the number is "not unusual," said Phivolcs Executive Director Renato Solidum. Nine of the aftershocks were felt and 92 were strong enough for their locations to be plotted, according to Phivolcs data.

"When a fault moves, it will not totally stop moving. The ground around it will still adjust. That's why we have aftershocks," Solidum told ANC.

"In Zambales, it's not a major earthquake, so we can still record aftershocks several days after, even weeks, but we think in time, the number will decrease and the spacing in between aftershocks will be far in between," he said.

Earthquakes are unlikely to cause significant damage, unless it comes from the West Valley Fault that stretches from Bulacan to Laguna and passes through the capital, he said.

"Please don't panic. Be calm during an event. What is important is understand the scenario," he said.

The 100-kilometer West Valley Fault moves every 400 to 600 years and triggered a magnitude 6.3 to 6.5 quake in 1648, he said.

When the Magnitude 6.1 earthquake struck at rush hour last Monday, office workers in Metro Manila evacuated skyscrapers, train services were halted and penthouse swimming pools burst.

In Porac, a farming town in Pampanga province, the quake flattened a 4-story supermarket to collapse and caused the bell tower of a 17th century church to fall off.

The aftershocks temporarily halted rescue operations at the supermarket rubble where at least 5 people died, with an undetermined number still strapped.

Rescuers earlier drilled holes through piles of collapsed walls to look for several people feared trapped under the rubble, said national disaster council chief Ricardo Jalad.

"Ang mahirap talaga d'yan is yung pagbutas kasi makapal ang semento at maingat din sila sa pagbutas kasi kinakailangan d'yan hindi naman madaganan kung mayroon mang tao doon sa ilalim," he told DZMM.

(Drilling holes there is difficult because the cement is thick and we shouldn't crush the people underneath.)

Soldiers, firemen and personnel from the Metropolitan Manila Development Authority (MMDA) using heavy equipment pulled out 9 survivors and 5 fatalities, said Jalad.

Less than 24 hours after Monday's quake, a Magnitude 6.5 temblor rocked parts of Visayas, and Bicol and Caraga regions on Tuesday afternoon. Authorities said the 2 earthquakes were unrelated.

Sixty-six aftershocks have been recorded following the Visayas quake that was centered in San Julian, Eastern Samar, said Phivolcs.

Five people were injured by the quake that cracked 2 bridges and a church in San Julian and a hospital and school in the nearby coastal town of Tacloban, Leyte, said disaster officials.

-- With reports from Lyza Aquino and Sharon Evite, ABS-CBN News