MANILA, Philippines (4th UPDATE) - Three major earthquakes struck the southern part of the Philippines on Saturday morning, the Philippine Institute of Volcanology and Seismology (Phivolcs) reported.

The quakes happened in succession, with the first striking at 6:08 a.m.. Phivolcs said the next 2 tremors happened at 6:51 a.m. and 7:15 a.m.

Phivolcs chief Renato Solidum reported that the first earthquake, which was tectonic in origin, had a magnitude of 6.9. He said the quake's depth was 599 kilometers.

Solidum said the first earthquake generated a magnitude 5.8 aftershock, which took place at 6:19 a.m.

In an interview over radio dzMM, Solidum said the quake's epicenter was traced at 94 kilometers southwest off Cotabato City.

It was felt at Intensity 2 in Surigao and General Santos cities.

Earthquake bulletins released by the Phivolcs said that the 2nd and 3rd earthquakes' measurements were magnitude 6.7 and 7.1, respectively.

The 2nd earthquake was felt at Intensity 2 in Butuan and Surigao cities.

The last earthquake, meanwhile, was felt at Intensity 4 in Tanjay and San Jose cities in Negros Oriental province, and Intensity 3 in General Santos City; Lingin town in Surigao del Sur province; San Jose and Hamtic towns in Antique province; and Irosin and Sorsogon City in Sorsogon province.

It registered Intensity 2, meanwhile, in Cotabato, Kidapawan, Dipolog, Tacloban, Davao, Legaspi, Cagayan de Oro, Cebu, and Dumaguete cities.

In Manila and Antipolo, the earthquake was also felt at Intensity 2.

Solidum assured the public during a separate interview on ANC's "Headlines" that the earthquakes will not generate a tsunami because of their depths.

He said the 3 earthquakes' depths range from roughly 570 kilometers to 630 kilometers.

He said they do not expect to receive reports of damages.

"Essentially, that large magnitude earthquake, which would come from a very deep source, the shaking it would generate at the surface will be much less because [of] the thick pile of rock that the energy will encounter as it moves into the surface. So it will not be damaging," he explained. "There will be no tsunami generated by these events.”

Benito Ramos, executive officer of the National Disaster Coordinating Council (NDCC), said that the earthquakes generated "a little shake."

Ramos added that NDCC people "did not even see cracks" during initial inspections on structures in the affected areas.

Higher USGS readings

The US Geological Survey (USGS), meanwhile, measured the first earthquake at 7.3. It said that the 2nd earthquake had a magnitude of 7.6, also traced off Cotabato City, and the 3rd quake at magnitude 7.4, striking 120 kilometers off Pagadian City.

Asked why the USGS's measurements were much higher than that of Phivolcs, Solidum explained that both agencies use different types of measuring earthquake magnitudes.

He said that the "seismic network of Phivolcs is the better network to locate the earthquakes" because the tremors happened within its jurisdiction.

Solidum also clarified that in measuring earthquakes, seismologists focus more on the intensity rather than the magnitude.

"Intensity is very important; it's not the magnitude," he said. -- with reports from Trinidad Velasco, ABS-CBN Northern Mindanao and ANC