EXPLAINER: What's the difference between the magnitude and intensity of an earthquake?

When an earthquake occurs, authorities give the public an idea of the quake's power by listing its magnitude and intensity. For instance, the strong initial tremor that struck Mindanao on Monday, June 8, was recorded with a Magnitude 7.8 and Intensity VIII in Sarangani, the epicenter. 

What do the numbers mean? And why does one scale use Arabic numbers while the other uses Roman numerals?

According to PHIVOLCS-DOST, magnitude and intensity are the two scales used to measure the power of an earthquake, each describing different aspects of the quake—whether it is tectonic, originating from faults and plate boundaries, or volcanic, induced by rising lava or magma from inside active volcanoes. 

What is an earthquake's magnitude?

The magnitude of an earthquake is a numerical representation of the "size" of seismic waves, or vibrations, generated by the quake's center. It shows the strength of the quake. 

To determine this, scientists use a seismometer, a sensor whose measurement of the energy emitted by seismic waves is recorded on the seismograph.

Any single earthquake has just one magnitude, depicted through Arabic numerals. 

According to the US Geological Survey, the largest earthquake ever recorded was a magnitude 9.5 in 1960 near Valdivia, Chile. The same organization says a magnitude-10 quake or stronger is scientifically impossible since there is no recorded fault long enough to generate such a shakeup. Closer to home, according to Mindanao Weather Update, the Cotabato Trench off the coast of Sultan Kudarat, is capable of generating magnitude 8.3 earthquakes. 

How is magnitude different from intensity?

If the magnitude of a quake measures its strength, its intensity shows how the tremors are perceived by people on the ground, and how the quake impacts objects, structures, and the environment. A quake's intensity is written out in Roman numerals to distinguish it from the magnitude.

PHIVOLCS uses an Earthquake Intensity Scale to determine how an earthquake affects those on the ground. 

At Intensity I, shaking is barely noticeable. Some objects may be "disturbed slightly" while still water in containers may slowly swish. 

An Intensity II earthquake is slightly felt, with hanging objects swinging gently. People indoors may feel the shaking. 

At Intensity III, a quake can generate weak shaking, which can be felt by more people indoors, especially if they are on the upper floors of a building. While the shaking is not that strong, it can still cause dizziness and nausea. 

A quake with Intensity IV is moderately strong, generally felt by individuals indoors and some outdoors. Objects on shelves may rattle, windows and doors may shake, and hanging items may swing noticeably. People feel a vibration and hear creaking of floors and walls, as well as perhaps a deep rumbling. 

An Intensity V quake is strong and felt indoors and outdoors. Sleeping people are woken by the shaking. Light and small objects fall off shelves, and liquids spill out of uncovered containers. Along with buildings, trees and light posts sway.

PHIVOLCS describes an Intensity VI quake as "very strong." It causes fear, panic, and loss of balance. Wall plaster cracks, and old or poorly built structures may be slightly damaged. Boulders and rocks roll off mountainous areas. 

An Intensity VII quake is destructive, according to PHIVOLCS. People indoors on upper floors have a hard time standing and walking upright. Furniture and heavy objects fall. Poorly built structures are damaged, while well-built ones may be slightly damaged. Roads crack. Landslides occur. 

Intensity VIII is "very destructive." Standing is difficult even outdoors. Buildings are noticeably damaged, as well as concrete dikes and bridge foundations. Utility posts and monuments may topple over; water and sewer pipes may bend. Trees and tall buildings violently shake while water in swimming pools, dikes, and rivers will splash and go over. 

At Intensity IX, a quake is "devastating." People are thrown on the ground, and buildings, bridges, and other concrete structures are heavily damaged. Landslides are widespread, as well as cracks on roads. Utility posts, towers, and monuments are either tilted or broken. 

Intensity X is "completely devastating." Most, if not all, man-made structures are destroyed. Massive landslides and liquefaction are seen. River courses change while many trees are uprooted. 

According to PHIVOLCS, the agency has seismic monitoring stations across the Philippines. Each one is equipped with seismometers, whose recordings are sent to the PHIVOLCS Data Receiving Center. With the data, seismologists then determine a quake's magnitude, its depth, and the location of its epicenter. As reports of the quake's intensities from the different affected areas come in, the information is collated and released.