Bohol already recorded at least seven aftershocks of tectonic origin in the first 14 days of the year, even more than three years since the magnitude 7.2 earthquake in October 2013.
The latest as of press time was recorded at 2:35 p.m. yesterday with theÂ ground shaking at magnitude 4.3 had depth of 19 kilometers with epicenter located four kilometers south from the east of Cortes town, the first municipality north of Tagbilaran City.
It was still considered as an aftershock of the magnitude 7.2 earthquake on October 15, 2013.
The Philippine Institute of Volcanology and Seismology (Phivolcs) had explained that if the position of the ground shaking is within the rupture zone of the 2013 earthquake it is considered an aftershock except if the magnitude goes more than magnitude 7.2.
A preceding aftershock was recorded at 4:32 a.m. on January 13, a magnitude 2.2 ground shaking with epicenter traced to five kilometers north of east of Catigbian and had a depth of 21 kilometers.
Phivolcs monitored an earlier aftershock at 8:07 a.m. on January 9.
It was a magnitude 2 quake of tectonic origin with depth of 31 kilometers and its epicenter traced to seven kilometers south of east of Buenavista.
The first aftershock in 2017 was recorded at 7:21 p.m. on January 2 at magnitude 2.1 and with depth of 21 kilometers.
Its epicenter was traced 20 kilometers south of east of San Miguel.
Another one followed, a magnitude 2 quake, at around 11:21 a.m. on January 3.
It had a depth of 33 kilometers and which epicenter was traced five kilometers north of east of Catigbian.
Another milder aftershock at magnitude 1.8 was recorded at around 11:39 p.m. on the same day.
Phivolcs traced its epicenter to 26 kilometers north of east of Mabini and its depth of 32 kilometers.
A magnitude 2.6 aftershock followed at 1:03 p.m. on January 4 which had a depth of 24 kilometers and its epicenter traced to five kilometers south of west of Alicia.
Seismologists had described earthquakes with magnitude 2.5 or less as those â€œusually not felt, but can be recorded by seismographâ€; magnitude 2.5 to 5.4 as those â€œoften felt, but only causes minor damageâ€; magnitude 5.5 to 6.0 as those â€œslight damage to buildings and other structuresâ€; magnitude 6.1 to 6.9 as those that â€œmay cause a lot of damage in very populated areasâ€; magnitude 7.0 to 7.9 as those in â€œmajor earthquakes that can cause serious damage; magnitude 8.0 or greater as those â€œgreat earthquakes that can totally destroy communities near the epicenterâ€.
As to their categories, a magnitude 3-3.9 quake is minor; magnitude 4-4.9 is light; magnitude 5-5.9 is moderate; magnitude 6-6.9 is strong; magnitude 7-7.9 is major and magnitude 8 or more is great.
As to its intensity, Phivolcs described intensity I as â€œscarcely perceptibleâ€ which means it the ground shaking isÂ â€œperceptible to people under favorable circumstances. Delicately balanced objects are disturbed slightly. Still Water in containers oscillates slowlyâ€.
Intensity II is â€œslightly feltâ€ which means it is â€œfelt by few individuals at rest indoors. Hanging objects swing slightly. Still Water in containers oscillates noticeablyâ€.
Intensity III is â€œweak- – felt by many people indoors especially in upper floors of buildings. Vibration is felt like one passing of a light truck. Dizziness and nausea are experienced by some people. Hanging objects swing moderately. Still water in containers oscillates moderatelyâ€.
Intensity IV is â€œmoderately strong- – felt generally by people indoors and by some people outdoors. Light sleepers are awakened. Vibration is felt like a passing of heavy truck. Hanging objectsswing considerably. Dinner, plates, glasses, windows and doors rattle. Floors and walls of wood framed buildings creak. Standing motor cars may rock slightly. Liquids in containers are slightly disturbed. Water in containers oscillate strongly. Rumbling sound may sometimes be heardâ€.
Intensity V is â€œstrong- – generally felt by most people indoors and outdoors. Many sleeping people are awakened. Some are frightened, some run outdoors. Strong shaking and rocking felt throughout 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 are noticeableâ€.
Intensity VI is â€œvery strong- -many people are frightened; many run outdoors. Some people lose their balance. Motorists feel like driving in flat tires. Heavy objects or 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 shakenâ€.
Intensity VII is â€œdestructive- -most people are frightened and run outdoors. People find it difficult to stand in upper floors. Heavy objects and furniture overturn or topple. Big church bells may ring. Old or poorly-built structures suffer considerably damage. Some well-built structures are slightly damaged. Some cracks may appear on dikes, fish ponds, road surface, or concrete hollow block walls. Limited liquefaction, lateral spreading and landslides are observed. Trees are shaken strongly. (Liquefaction is a process by which loose saturated sand lose strength during an earthquake and behave like liquid)â€.
Intensity VIII is â€œvery destructive- -people panicky. People find it difficult to stand even outdoors. Many well-built buildings are considerably damaged. Concrete dikes and foundation 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 mat tilt or topple. Water and sewer pipes may be bent, twisted or broken. Liquefaction and lateral spreading cause man- made structure 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 faults rapture may be observed. Trees are violently shaken. Water splash or stop over dikes or banks of riversâ€.
Intensity IX is â€œdevastating- -people are forcibly thrown to 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 monument are tilted, toppled or broken. Water sewer pipes are bent, twisted or broken. Landslides and liquefaction with lateral spreadings 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 on slops over dikes and banksâ€.
Intensity X is â€œcompletely devastating- -practically all man-made structures are destroyed. Massive landslides and liquefaction, large scale subsidence and uplifting of land forms and many ground fissures are observed. Changes in river courses and destructive seiches in large lakes occur. Many trees are toppled, broken and uprootedâ€.