The series of offshore earthquakes in Ilocos Sur has significantly decreased following a peak on Dec. 19 to 20, said the Philippine Institute of Volcanology and Seismology (Phivolcs) on Sunday, Dec. 22.
Phivolcs said it began monitoring a series of earthquakes in the Manila Trench offshore of Santa Catalina, Ilocos Sur, on Dec. 17. Since then, the frequency of tremors has decreased over the past two days.
As of 8 p.m. on Sunday, Phivolcs reported that the sequence has resulted in 178 recorded earthquakes, with 62 of them being plotted—meaning they were located by at least three seismic stations.
Only two of these earthquakes were felt by residents.
Phivolcs added that the magnitudes ranged from 1.8 to 5.0, and the depths varied between 1 and 80 kilometers.
Worst-case scenario
In the computational modeling of the Manila Trench, Phivolcs divided the trench into four segments, with the earthquakes occurring in Segment 2 (MT2) offshore of Santa Catalina.
The agency said that, based on the model, the worst-case scenario would involve an 8.4-magnitude earthquake if the entire MT2 were to move.
If this generated a tsunami, the first wave could reach Palauig, Zambales within two minutes, while the highest wave could reach 14.7 meters in Vigan City, arriving within four minutes.
However, Phivolcs emphasized that it is impossible to predict the exact timing of an earthquake or tsunami.
Residents are advised to remain alert to signs of a local tsunami: strong earthquakes (shake), sudden changes in sea level (drop), and unusual sounds from the sea (roar).
If any of these signs are observed, Phivolcs said immediate evacuation to higher ground or away from the sea is crucial.