Ilocos Sur offshore quakes may decrease or lead to tsunami —PHIVOLCS

A series of earthquakes occurred offshore of Ilocos Sur in the past few days, which may possibly decrease or generate a tsunami, the Philippine Institute of Volcanology and Seismology (PHIVOLCS) said on Friday night.

The earthquakes, totaling 49, occurred since 4:02 a.m. on December 17, according to the Department of Science and Technology-PHIVOLCS Philippine Seismic Network, PHIVOLCS said in a Facebook post.

These had preliminary epicenters clustering offshore in the West Philippine Sea, approximately 100 km west northwest of the municipality of Santa Catalina.

The earthquakes' magnitude ranged from 1.8 to 5.0, PHIVOLCS added.

"The largest earthquake (denoted as yellow star) in the sequence occurred on 19 December 2024 at 09:09 AM (PST) with magnitude M5.0 and focal depth of 27 km, and was reportedly felt in Santa Catalina, Ilocos Sur at Intensity II using the PHIVOLCS Earthquake Intensity Scale," the agency said.

PHIVOLCS said the earthquakes are associated with movements along the Manila Trench —an ocean trench west of the country that runs as deep as 5,400 meters.

"Records of broadband waveforms were processed using the SWIFT system and indicated a predominantly normal mechanism as shown in Figure 2. Based on this mechanism and the epicentral location of these events, this earthquake sequence is associated with the active subduction along the Manila Trench," PHIVOLCS said.

READ: Serye ng lindol, nararanasan sa West Philippine Sea sa kanluran ng Sta. Catalina, Ilocos Sur. | via @phivolcs_dost pic.twitter.com/GzgwWhQj0m

— DZBB Super Radyo (@dzbb) December 20, 2024

"As of kagabi, mayroon nang naitalang mga aftershock na recorded na 68... Ang located ay 27," PHIVOLCS Science Research Assistant Dandy Camero said in an interview on Dobol B TV on Saturday.

(As of last night, there were 68 aftershocks recorded. Those located numbered 27.)

Camero said the earthquakes were tectonic in origin.

The agency said there are two possible scenarios based on the current earthquake activity: "(a) it may continue with its present activity and eventually decrease in the next few days, or (b) it may intensify and lead to a stronger earthquake that may generate a tsunami."

PHIVOLCS said it is continuing to monitor the progress of the earthquake activity.

It reminded the public to do the Duck, Cover and Hold pose in case a strongly felt earthquake occurs.

READ: INFOGRAPHIC: How to prepare for an earthquake

READ: BE IN THE KNOW: What to do before, during, and after an earthquake

"People in the coastal communities should stay alert and be aware of and monitor the natural signs of an approaching local tsunami - strong ground shaking (SHAKE), sudden and unusual changes in the sea level (DROP), and unusual sound (ROAR)," PHIVOLCS added.

When such occur, the public should "immediately move to high grounds or away from the shoreline," it said.

"Lilinawin lang natin, hindi natin sinasabi na may paparating na tsunami pero patuloy pa ring magbabantay ang PHIVOLCS," Camero said.

(We just want to clarify: We are not saying that there is a tsunami coming, but PHIVOLCS is continually monitoring.) —KG, GMA Integrated News