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What is an earthquake?

An earthquake is a weak to violent shaking of the ground produced by the sudden movement of rock materials below the earth’s surface.

The earthquakes originate in tectonic plate boundary. The focus is point inside the earth where the earthquake started, sometimes called the hypocenter, and the point on the surface of the earth directly above the focus is called the epicenter.

There are two ways by which we can measure the strength of an earthquake: magnitude and intensity. Magnitude is proportional to the energy released by an earthquake at the focus. It is calculated from earthquakes recorded by an instrument called seismograph. It is represented by Arabic Numbers (e.g. 4.8, 9.0). Intensityon the other hand is the strength of an earthquake as perceived and felt by people in a certain locality. It is a numerical rating based on the relative effects to people, objects, environment and structures in the surrounding. The intensity is generally higher near the epicenter. It is represented by Roman Numerals (e.g. II, IV, IX). In the Philippines, the intensity of an earthquake is determined using the PHIVOLCS Earthquake Intensity Scale (PEIS).

Types of earthquake
Earthquake Monitoring System

fault labeled

Image source: U.S. Geological Survey