More than 200 aftershocks have been recorded as of Wednesday morning, a day after a strong magnitude 6.6 earthquake struck the province of Masbate and other parts of Luzon and Visayas, the Philippine Institute of Volcanology and Seismology (Phivolcs) said.
In the primer released by Phivolcs, it said that a total of 218 aftershocks ranging from magnitude 1.6 to 4.4 have so far been recorded by the bureau as of 4 a.m., Wednesday, 108 of which were plotted, while five were felt.
The Masbate quake, which occurred at 8:03 a.m., Tuesday, was felt with a maximum ground shaking intensity of VII, which was recorded in the town of Cataingan, Masbate, the epicenter of the tremor.
Under the Phivolcs Earthquake Intensity Scale (PEIS), Intensity VII was described by the agency as "destructive" wherein "old or poorly-built structures suffer considerable damage."
What can we expect?
The state seismology bureau said minor to moderate aftershocks are still expected to jolt in the epicentral area, but "occurrences of strong aftershocks cannot be discounted."
"These may continue for several days to weeks, some of which may be felt," it advised.
Aside from strong ground shaking, Phivolcs said that landslides, rock falls, and other types of mass movements may occur in mountainous or hilly areas, while liquefaction, manifested by subsidence, sand boils or lateral spreads may affect low-lying, water-saturated, and sandy areas near river banks and shorelines
"In particular, coastal areas are vulnerable to inundation of seawater due to the combination of liquefaction-induced subsidence and changes in sea level during high tide," Phivolcs explained.
The agency also pointed out that although the epicenter of Tuesday's quake is offshore, no destructive tsunami waves were generated as there was no significant vertical displacement of the seafloor.
"Tsunamis are produced by earthquakes with significant vertical movements and from earthquakes generated by active trenches. Localized sea-level disturbances may occur though as a result of submarine landslides," it explained.
Past earthquakes in Masbate
According to Phivolcs, Masbate is one of the "seismically active regions" in the country due to the presence of active faults including the Masbate segment of the Philippine Fault and potentially active faults such as Uson Fault and the Southern Masbate Fault.
Phivolcs said that at least 11 quakes with magnitudes ranging from 6.0 to 7.0 have struck Masbate province and nearby areas between 1869 and 2003.
The last strong tremor recorded in the province was a magnitude-6.2 event in February 2003, which was felt as far as Kalibo, Aklan -- at least 200 kilometers away from the epicenter.
The maximum ground shaking intensity then was felt at PEIS VIII or "very destructive," and generated an approximately 23-kilometer long surface rupture.
Meanwhile, Phivolcs reminded the public to be cautious of structures visibly weakened, having signs of damage, or are partially collapsed due to the recent earthquake as the expected aftershocks may cause further damage to them.
Slopes should also be checked for tension or incipient cracks that may have resulted from the strong ground shaking as it is more susceptible to landslides, the bureau added.
"The best course of action is preparedness. Immediate life safety is the priority when evacuation after an earthquake is necessary. It is important for the public to understand that an earthquake evacuation takes priority over a COVID-19 stay-at-home order," Phivolcs emphasized.
"It is also important that the risks of COVID-19 spread among the public during evacuations are managed. At all times, people affected by the earthquake are also strongly advised to wear face masks and to strictly observe physical distancing before, during, and after the evacuation," it added.