At 9:43 AM on October 8, a magnitude 7.4 earthquake occurred in Manay, Davao Oriental, located roughly 129 kilometers east-northeast of Davao City. On the same day, at 11:32 AM, another earthquake with a magnitude of 5.8 was recorded in the same area. Before the day concluded, yet another tremor, measuring 6.7 in magnitude, affected the same place.
According to Britannica Encyclopedia, the magnitude of an earthquake serves as an indicator of the strength or size of the seismic waves produced by the earthquake. It measures the quantity of seismic energy released and is generally assessed using the moment magnitude scale, which operates logarithmically—indicating that each whole number increase corresponds to a tenfold rise in measured amplitude and approximately 31.6 times greater energy release.
The intensity scale, on the other hand, is separate from the magnitude of an earthquake. In the context of earthquakes, intensity pertains to the level of shaking felt at a particular site as a result of the earthquake. This is assessed based on the impact of seismic waves on individuals, buildings, and the surrounding environment. The intensity scale measures these impacts, with greater values signifying more severe shaking.
Recently, the Philippine Institute of Volcanology and Seismology (PhiVolcs) came up with 10 intensity scales of earthquakes. It replaces the Rossi-Forel scale which the institute used in the past. Let’s take a closer look at them so you become familiar if such intensity happens in your place:
I. Scarcely perceptible: It is perceptible to people under favorable circumstances. Delicately-balanced objects are disturbed slightly. Still water in containers oscillates slightly.
II. Slight felt: It is felt by few individuals at rest indoors. Hanging objects swing slightly. Still water in containers oscillates slightly.
III. Weak: It is felt by people indoors especially in the upper floors of buildings. Vibration is felt like the passing of a light truck. Dizziness and nausea are experienced by some people. Hanging objects swing moderately. Still water in containers oscillates moderately.
IV. Moderately strong: It is felt generally by people indoors and some people outdoors. Light sleepers are awakened. Vibration is felt like the passing of a heavy truck. Hanging objects swing considerably. Dinner plates, glasses, windows and doors rattle. Floors and walls of wood-framed buildings creak. Standing motor cars may rock slightly.
V. Strong: Generally felt by most people indoors and outdoors. Many sleeping people awakened. Some are frightened; some run outdoors. Strong shaking and rocking are felt throughout the building. Hanging objects swing violently. Dining utensils clatter and clink; some are broken. Small, light and unstable objects may fall or overturn. Liquids spill from filled open containers. Standing vehicles rock noticeably. Shaking of leaves and twigs of trees is noticeable.
VI. Very strong: Many people are frightened, many run outdoors. Some people lose their balance. Motorists feel like driving with flat tires. Heavy objects and furniture move or may be shifted. Small church bells may ring. Wall plaster may crack. Very old or poorly built houses and man-made structures are slightly damaged, though well-built structures are not affected. Limited rockfalls and rolling boulders occur in hilly to mountainous areas and escarpments. Trees are noticeably shaken
VII. Destructive: Most people are frightened and run outdoors. People find it difficult to stand on the upper floors. Heavy objects and furniture overturn or topple. Big church bells may ring. Old or poorly built structures suffer considerable damage. Some well-built structures are slightly damaged. Some cracks may appear on dikes, fish ponds, road surfaces or concrete hollow block walls. Limited liquefaction, lateral spreading and landslides are observed. Trees are shaken strongly.
VIII. Very destructive: People are panicky. People find it difficult to stand even outdoors. Many well-built buildings are considerably damaged. Concrete dikes and foundations of bridges are destroyed by ground settling or toppling. Railway tracks are bent or broken. Tombstones may be displaced, twisted or overturned. Utility posts, towers, and monuments may tilt or topple. Water and sewer pipes may be bent, twisted or broken.
Liquefaction and lateral spreading cause man-made structures to sink, tilt or topple. Numerous landslides and rockfalls occur in mountainous and hilly areas. Boulders are thrown out from their positions particularly near the epicenter. Fissures and fault rupture may be observed. Trees are violently shaken. Water splashes or slops over dikes or banks of rivers.
IX. Devastating: People are forcibly thrown to the ground. Many cry and shake with fear. Most buildings are totally damaged. Bridges and elevated concrete structures are toppled or destroyed. Numerous utility posts, towers and monuments are tilted, toppled or broken. Water and sewer pipes are bent, twisted or broken.
Landslides and liquefaction with lateral spreading and sandboils are widespread. The ground is distorted into undulations. Trees are shaken very violently with some toppled or broken. Boulders are commonly thrown out. River water splashes violently or slops over dikes and banks.
X. Completely devastating: Practically all man-made structures are destroyed. Massive landslides and liquefaction, large scale subsidence and uplifting of landforms, and many ground fissures are observed. Changes in river courses and destructive seiches in lakes occur. Many trees are toppled, broken or uprooted.
Just a word of reminder: After earthquakes, be prepared for the aftershocks. These are usually weaker earthquakes that follow the main shock of an earthquake sequence.–###
