The first quake also set off landslides and prompted evacuations of coastal areas nearby because of a brief tsunami scare.
A second quake with a 6.8 magnitude on Friday also sparked a local tsunami warning.
Both were caused by movement in the same fault line, the Philippine Trench, Philippine Institute of Seismology and Volcanology chief Teresito Bacolcol said.
"The second one is a separate earthquake, which we call a doublet quake," Bacolcol told The Associated Press.
"Both happened in the same area but have different strengths and epicentres."
Bacolcol and other authorities expressed fears that the second nighttime earthquake could further weaken or collapse structures already undermined by the first.
President Ferdinand Marcos Jr, facing his latest natural disaster after a recent deadly quake and back-to-back storms, said the damage was being assessed and rescue teams and relief operations were being prepared and would be deployed when it was safe to do so.
The first quake was centred at sea about 43 kilometres east of Manay town.
At least seven people were killed, including two patients who died of heart attacks at a hospital and a resident who was hit by debris in Mati city in Davao Oriental, Ednar Dayanghirang, regional director of the government's Office of Civil Defense, said.
Three villagers died and several others were rescued with injuries by army troops and civilian volunteers in a landslide set off by the first quake in a remote gold-mining village in Pantukan town in Davao de Oro province.
Office of Civil Defense deputy administrator Bernardo Rafaelito Alejandro said that several buildings sustained cracks in their walls, including an international airport in Davao city, but it remained operational without any flights cancelled.
"I was driving my car when it suddenly swayed and I saw power lines swaying wildly. People darted out of houses and buildings as the ground shook and electricity came off," Jun Saavedra, a disaster-mitigation officer said.
"We've had earthquakes in the past, but this was the strongest."
The intense ground swaying caused cracks in several buildings, including a high school, where about 50 students were brought to a hospital by ambulance after sustaining bruises, fainting or becoming dizzy.
The Pacific Tsunami Warning Centre in Honolulu said that small waves were detected on the coasts of the Philippines and Indonesia before the threat passed about two hours after the first quake.
A tsunami warning that set off evacuations in six coastal provinces near Davao Oriental was later lifted without any major waves being detected.
The Philippines is still recovering from a September 30 earthquake with a 6.9 magnitude that left at least 74 people dead and displaced thousands of people in the central province of Cebu, particularly in Bogo city and outlying towns.
The archipelago also is lashed by about 20 typhoons and storms each year, making disaster response a major task of the government and volunteer groups.
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