This map shows that location of a magnitude 5.7 earthquake that struck off Sarangani on April 16. Phivolcs

MANILA — A magnitude 5.7 earthquake struck the waters off Sarangani province on Wednesday morning, the Philippine Institute of Volcanology and Seismology said.

The tremor hit 55 kilometers southwest of Maitum town at 5:42 a.m. It struck at a depth of 1 kilometer. 

Phivolcs initially reported its magnitude at 5.8 and later downgraded it to 5.7. 

While the quake was underwater, it was not strong enough to generate a tsunami, Phivolcs director Teresito Bacolcol said.  

But Phivolcs said the quake could spawn aftershocks and damage to property.

“So far wala pa namang nag-report [na damage] sa amin,” Bacolcol told TeleRadyo Serbisyo. 

The tremor was felt at a "moderately strong" intensity 4 in Kiamba, Sarangani; and T'boli, Banga, Surallah, and Tupi in South Cotabato, Phivolcs said. 

Intensity 4 is "felt generally by people indoors" and could awaken light sleepers. Its vibration is similar to the passing of a heavy truck and could make hanging objects swing "considerably". 

Phivolcs said the quake was also felt at the following intensities in other areas. 

Intensity 3 - Maitum, Glan, Malungon, Malapatan, and Alabel, Sarangani; General Santos City; Norala, Koronadal, Santo Niño, and Polomolok, South Cotabato
Intensity 2 - Maasim, Sarangani; Lake Sebu, Tantangan, and Tampakan, South Cotabato; Davao City 

Local authorities told AFP early indications were that the quake had caused no significant damage.

"It was strong but didn't last long. We have checked but it was negative (for damage or casualties)," said Gilbert Rolifor, a fire department official in Maitum.

Quakes are a daily occurrence in the Philippines, which sits along the Pacific "Ring of Fire", an arc of intense seismic and volcanic activity that stretches from Japan through Southeast Asia and across the Pacific basin.

Most are too weak to be felt by humans, but strong and destructive quakes come at random with no technology available to predict when and where they will happen.