EXPLAINER: Why Phivolcs revises magnitude level after quakes happen

Kristine Sabillo, ABS-CBN News

Posted at Apr 23 2019 08:35 PM | Updated as of Apr 23 2019 08:36 PM

Workers clean up debris that fell from the Pampanga-Bataan boundary arch on Tuesday in the wake of the 6.1-magnitude quake that shook parts of Luzon. Pampanga suffered from severe damage as its ground had been softened by lahar following the June 1991 Mt. Pinatubo eruption, said Phivolcs director Renato Solidum. Jonathan Cellona, ABS-CBN News

Minutes after an earthquake was felt in different parts of Luzon on Monday afternoon, Phivolcs issued a bulletin on the 5.7-magnitude quake. 

Later, however, the agency updated it to a magnitude level of 6.1.
Early afternoon on Tuesday, another tremblor was felt in the Visayas area, even reaching parts of Mindanao and Bicol. Phivolcs initially said it was a 6.2-magnitude quake but in an hour it was upgraded to a 6.5-magnitude earthquake.

Phivolcs Director Renato Solidum said the revisions are due to different equations and data used.

“We can wait longer to ensure a more exact magnitude level that is final. But because of news requirements and the need to assess if there is an incoming tsunami, we have to estimate the magnitude. During or right after an earthquake, we have a formula that we use while not all data is available,” Solidum said during an interview over dzMM’s SRO program on Monday evening.

He pointed out that it takes a while to gather data needed for a more precise reading. 

Phivolcs science research specialist Lucille Rose Sanico said the magnitude of an earthquake is triangulated using data from three of the nearest monitoring stations. Phivolcs currently has 101 seismic monitoring stations. Those that are not manned are considered satellite of remote stations that have equipment that can send data to Phivolcs. 

After the initial earthquake bulletin, Phivolcs is able to factor in more data and can use another equation for a more precise number. 

Sanico said the second report usually contains the intensity levels of the affected areas. 


According to Solidum, knowing the intensity of the earthquake in different areas is important to understand how it has affected houses or buildings in a certain area.

He compared an earthquake to a punch where the energy of the punch is the magnitude and the impact or damage of the punch as the intensity. 

“Kung ikaw ay boksingero yung bigat at bilis ng suntok energy yun, magnitude. Pero yung epekto sa kalaban kung magblack eye sya, matutumba o ma-knock out o mamamatay iyon ang intensity,” he said.

(If you are a boxer, the weight and the speed of the punch that’s energy, magnitude. But the effect of that on your opponent, if he will get a black eye, if he gets knocked down or knocked out or dies, that’s intensity.)

In their bulletin, Phivolcs have two sets of intensities for strong earthquakes. Instrumental intensities are based on the measurements received or read from intensity meters distributed in different areas in the country, according to Sanico. 

Meanwhile, reported intensities are based on eyewitness accounts of their partners from the LGU or even from the general public. Sanico said LGU partners usually determine the intensity level based on the Phivolcs Earthquake Intensity Scale (PEIS). If they are not that familiar with the scale, they can simply describe what happened – if objects started moving or if walls cracked.

These reports are gathered and summarized to determine which towns and cities should be included in the updated earthquake bulletin.

Origin and depth

Sanico said an earthquake can either be tectonic or volcanic in origin. Volcanic earthquakes are linked to active volcanoes like the Mayon or Kanlaon volcano.

Tectonic earthquakes, on the other hand, are caused by the movement of tectonic plates, which are large sections of the Earth’s crust.

The depth of the earthquake also plays a part in the intensity of the earthquake since shallow ones are felt more.

Final arbiter

Phivolcs reminded the public that while Phivolcs has 101 monitoring stations, USGS only has two stations in the Philippines. That is why Phivolcs’ measurements are supposed to be more accurate. 

However, Solidum explained that the final magnitude measurement is determined by the International Seismological Center in the United Kingdom. The Center receives reports worldwide and reviews them before coming up with a definitive record of an earthquake’s magnitude a couple of years after.