Two strong earthquakes that struck Davao Oriental on Friday were classified as a rare ‘doublet,’ or two separate but closely timed quakes, a Philippine Institute of Volcanology and Seismology (Phivolcs) official said yesterday.
A magnitude 7.4 earthquake hit waters 44 kilometers northeast of Manay, Davao Oriental, early Friday morning, killing at least two people.
Hours later, past 7 p.m., a separate magnitude 6.8 temblor also struck offshore.
Phivolcs director Teresito Bacolcol said the magnitude 6.8 event was not an aftershock but a distinct earthquake.
‘A doublet earthquake means two closely timed quakes that happened almost in the same area. But they have different epicenters and strengths,’ he said.
‘We will not consider this an aftershock because, when we say aftershock, it would be one magnitude lower than the main shock. So this is a separate event,’ he added.
Science Secretary Renato Solidum Jr. explained that a doublet happens when stress builds up around a stuck fault called an asperity.
‘The initial rupture causes the first large earthquake, but this only partially releases the accumulated stress until it encounters a stuck portion of the fault called an asperity,’ Solidum told The STAR. ‘When the rupture continues, the built-up stress across the asperity is released, producing the second large earthquake.’
Asked if the doublet could lead to a stronger quake, volcanic eruption or tsunami, Bacolcol and Solidum said the likelihood was ‘very small.’
‘If it were to trigger a larger event – say, magnitude 7.5 or 7.6 – then these would have been considered foreshocks. But so far, there’s no indication of that,’ Bacolcol said.
Solidum, however, said that while doublets involve two major quakes, a large rupture could sometimes produce more than two significant events, a phenomenon known as a ‘multiplet’ earthquake.