Terms you need to know about earthquakes

With fault lines stretching all throughout the country, Filipinos are no stranger to earthquakes rocking their lives at any given time. According to the Philippine Institute of Volcanology and Seismology (Phivolcs), several quakes happen every day, but their intensities are sometimes not high enough to be felt.

Nonetheless, strong earthquakes have devastated the country in recent times. (MAP: Strongest earthquakes in the Philippines)

The 7.8-magnitude earthquake that struck the northern part of Luzon in July 1990 became one of the most devastating earthquakes in recent history. It killed 2,142, and left around US$369.6 million worth of damage to property. Even so, it only comes 7th among the strongest the country has experienced.

The strongest earthquakes recorded to hit the Philippine archipelago happened in September 1897, when two earthquakes of 8.6 and 8.7 magnitudes struck on two consecutive days in the Celebes sea area, between the islands of Sulu and Basilan.

Dr Arturo Daag of Phivolcs earlier told Rappler that although we cannot predict the next big earthquake, some studies show that “200 years is the return period for another major earthquake in the same area.”

Even without knowing when the “Big One” will strike, it is best to be prepared at any time. Being prepared can reduce risks of injury, since people will know how to react quickly and correctly. (READ: All you need to know about preparing for earthquakes)

Earthquakes are unpredictable and can strike when least expected. Phivolcs reminds the public to be prepared, and also cautions against aftershocks.

It's important to know terms related to quakes. Here are some of the more common ones:

Magnitude

It's used to measure the size of an earthquake. It is most commonly measured with the Richter scale, whose information comes from a device called the seismograph. The seismograph is a device that registers movements once an earthquake occurs. 

Illustration by Ernest Fiestan

Can the magnitude of an earthquake change? In the reporting that followed the two earthquakes that hit Luzon and the Visayas, both magnitudes were raised after initial reports. In the Luzon quake, the earlier report put the magnitude at 5.7, but was later revised to 6.1. In the Visayas quake, the earlier magnitude was reported at 6.1 but later became 6.5. 

This does not mean that the magnitude changed, or that the earthquake grew stronger as it was happening. Rather, it means that seismometers had captured more information about the earthquake, and that the magnitude recorded at the moment of the quake was not yet the complete picture.

Intensity

Represented by Roman numerals I through X, intensity marks how much damage was done on the earth’s surface, structures, and humans. In the Philippines, Philvolcs uses what is called the Phivolcs Earthquake Intensity Scale or PEIS to measure the intensity of an earthquake. 

Focus

Also called the hypocenter, this is where the earthquake originated from – underground or on the fault zone. The Philippine fault zone (PFZ) extends 1,200 km across the archipelago. It cuts across the Philippines from northwestern Luzon to southeastern Mindanao. The PFZ is divided into several segments. Mapping of the entire PFZ is an ongoing project, with 90% of the on-land-stretch mapped. 

Epicenter

The epicenter is the area on the surface of the earth that is directly above the focus or hypocenter. Usually, this epicenter sustains the most damage.

However, in the case of the April 22 earthquake whose epicenter was in Zambales, the hardest hit areas were in Pampanga. Phivolcs explained that this was because the soil in Pampanga was softer and therefore more prone to damage. As such, factors like these figure in the extent of damage caused by an earthquake in various places where it is felt. 

Fault

A fault is a weak point in the tectonic plate where pressure within the crust is released. This pressure can cause an earthquake. An active fault is one that may potentially cause an earthquake in the future. One of the most geologically active fault lines in the Philippines is the Marikina Valley Fault System, consisting of the West Valley Fault and the East Valley Fault. 

Illustration by Ernest Fiestan
Aftershock

An aftershock is a smaller earthquake that may occur after a bigger or main quake. Not all earthquakes have aftershocks, but there is such a thing as an earthquake swarm, which are quakes that happen over months without being followed by a larger quake. – Rappler.com

Michelle Abad

Michelle Abad is a researcher-writer at Rappler. Possessing the heart and soul of a feminist, she is working on specializing in women's issues in Newsbreak, Rappler's investigative arm.

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