MANILA - State seismologists in the Philippines and in the United States gave different locations of the Luzon quake's epicenter earlier this week shortly after the 6.1-magnitude tremor was felt in parts of Central Luzon and Metro Manila.

Phivolcs plotted the epicenter near the town of Castillejos in Zambales, while US-run United States Geological Survey (USGS) said the quake was centered off Floridablanca town in Pampanga.

Discrepancies in data between the two seismology agencies are common as their sources of information differ, Phivolcs Research Assistant Bhenz Rodriguez told reporters.

Seismologists need ground readings from at least 3 seismic monitoring systems before it could plot the magnitude and location of an earthquake.

Phivolcs collects its data from 101 local monitoring stations, while USGS sources ground readings from Phivolcs' Davao hub and collates it with information from seismometers mounted in neighboring countries, he said.

"Kung Pilipinas lang ang concerned, mas maganda 'yung data natin kasi mas localized. 'Yung stations natin mas malapit sa isa't isa kaya mas accurate," he said.

(If we're only talking about the Philippines, then our data is better because it is more localized. The stations where we source our data are closer to one another so the data is more accurate.)

Collating data from foreign monitoring systems in other countries, however, allows the USGS to deliver results faster, Rodriguez said.

"We can deliver seismic data within 5 minutes [pero] sa ibang bansa kaya sila mabilis, automatically napi-pick na nila 'yung phases ng lindol," he said.

(But other countries tend to get it faster because they can automatically pick the phase of an earthquake.)

"Sa atin, automatic din naman siya kaso minsan nagkakaron ng system glitch kaya may tendency minsan na mauuna sila."

(Here, it can also be done automatically but we encounter some system glitches so there is a tendency for them to disseminate the information first.)

The government has been adding seismometers annually to improve ground readings as the country is situated near the Pacific Ring of Fire and the Philippine trench, two underwater systems that could trigger tremors, seismologists said.