MANILA – Seismometers in different parts of the country record about a thousand ground movements daily, and classify each event either as a main shock, a foreshock or an aftershock, state seismologists said.
"If there is a series of earthquake events occurring in the same area, the first large earthquake is called the main shock," Philippine Institute of Volcanology and Seismology chief Renato Solidum told ABS-CBN News.
Seismic events before the main shock are identified as foreshocks, while those that were recorded after the most significant tremor in the area are labeled as aftershocks.
"Aftershocks and foreshocks are all earthquakes, but all these must be related to a major one or a larger one," he said.
The largest aftershock that can happen is "typically one level of magnitude lower than the main shock," Solidum said. Seismologists adjust nomenclatures given to seismic activities in the event that a stronger quake succeeds a main shock.
"In the 2011 earthquake in Japan, the magnitude 9 earthquake, 2 days before it there was a magnitude 7.2. That was big enough. If that happens, the nomenclature of the earthquake changes. The 7.2 quake becomes the foreshock and the magnitude 9 is the main shock," Solidum said.
Seismologists estimate the magnitude a fault system can generate by computing its length and depth.
Once a fault releases energy and triggers a tremor, it would take centuries before the same area could store enough energy to generate a temblor with similar or greater magnitude, Solidum said.
"Although we do not discount the possibility of similar or larger earthquakes, the probability will be low. Typically within the day, the number of aftershocks would be maximized and with time, it would get smaller and smaller," the Phivolcs chief said.
Aftershocks could take place years after the main shock jolted the area, he said.
"Even in the Bohol (2013) earthquake, we would see that even after a year, there are smaller events in the area because it will take some time for the rocks to stabilize," he said.
The 6.1-magnitude quake that centered in Castillejos town in Zambales, generated at least 600 after shocks two days after the tremor jolted parts of Metro Manila and Central Luzon.
Aftershocks from the April 22 Zambales quake continue to be recorded, but its strength points to a downward trend, Phivolcs earlier said.