Why was Cebu's magnitude 6.9 earthquake more disastrous than Davao Oriental's 7.4?

In a span of less 10 hours, Davao Oriental's Manay town was struck by doublet earthquakes at magnitudes 7.4 and 6.8. These came just over a week after Bogo City in northern Cebu, hundreds of kilometers away, was jolted by a magnitude 6.9 earthquake.

The Bogo earthquake has so far affected 201,245 families and damaged a total of 79,959 houses, while the more powerful Manay earthquake affected 3,519 families with 33 houses reported damaged so far.

At a news forum on Saturday, Philippine Institute of Volcanology and Seismology (PHIVOLCS) Director Teresito Bacolcol explained why the Cebu quake caused more devastation compared to the Davao Oriental quake despite the latter having a stronger magnitude.

"For instance, sa Cebu magnitude-6.9 then sa Davao [Oriental] it's magnitude 7.4 kung titignan natin mas destructive ang nangyari sa Cebu kaysa Davao kasi 'yung sa Cebu, 'yung depth ng lindol is 5 kilometers. 'Yung sa Davao is 23 kilometers," Bacolcol said.

(For instance, the Cebu earthquake was magnitude 6.9 then in Davao Oriental, it was magnitude 7.4. If we assess the impact, the Cebu quake was more destructive than Davao's because in Cebu, the epicenter's depth was 5 km. In Davao, it was 23 km.)

"'Yung seismic energy kasi as it travels from the source sa ilalim ng lupa aakyat 'yan… habang umaakyat 'yan humihina 'yung lakas. So, if it's 23 kilometers, pagdating niya sa ibabaw ay medyo hindi na siya ganoon kalakas [and] if five kilometers lamang 'yung tinakbo niya pataas mas malakas pa rin 'yung seismic energy pagdating sa ibabaw," he said.

(Seismic energy travels from the underground source and goes up... As it goes up, the energy weakens. So, if it's 23 km, when it reaches the top, it's not as powerful. If the seismic energy only traveled five kilometers, it's still powerful when it gets to the top.)

A magnitude 7.4 earthquake rocked Manay, Davao Oriental on Friday morning. It was followed by a magnitude 6.8 in the evening.

PHIVOLCS said the second earthquake in Manay was not an aftershock but the latter half of a doublet—two closely timed earthquakes happening nearby. 

Almost 800 aftershocks were recorded as of 7 a.m. Saturday following the doublet earthquake in Manay.

On September 30, a magnitude 6.9 earthquake struck Bogo City in Cebu, with over 10,000 aftershocks recorded since.

Bacolcol said the Bogo Bay fault, where the magnitude-6.9 earthquake originated, "transcends communities" while the Philippine Trench, where the Manay quake originated, is 48 km away from the town's coast.

Seven people were reported to have died due to the doublet Davao Oriental earthquake. 

The reported deaths from Cebu earthquake, on the other hand, stood at 74. — VDV, GMA Integrated News