EARTHQUAKES are usually caused when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves that make the ground shake. When two blocks of rock or two plates are rubbing against each other, they stick a little. They don't just slide smoothly; the rocks catch on each other. The rocks are still pushing against each other, but not moving. After a while, the rocks break because of all the pressure that's built up. When the rocks break, the earthquake occurs. During the earthquake and afterward, the plates or blocks of rock start moving, and they continue to move until they get stuck again. The spot underground where the rock breaks is called the focus of the earthquake. The place right above the focus (on top of the ground) is called the epicenter of the earthquake.
The rocks that make up the crust of the Earth are full of fractures. On some of these fractures—known as faults—rocks slip past each other as the crust rearranges itself in the process known as plate tectonics. But this slipping doesn’t happen easily—rocks are stiff, rough, and under a lot of pressure from rocks around and above them. Because of this, rocks can pull at or push on each other on either side of a fault for long periods of time without moving much at all, which builds up a lot of stress in the rocks. When rocks do finally slip past each other, they move suddenly, releasing the built-up stress all at once and sending shockwaves through the surrounding rocks. The shaking that results is what we call an earthquake.
The largest recorded earthquake in the world was a magnitude 9.5 (Mw) in Chile on May 22, 1960.
Here are some of the strongest earthquakes in the Philippines that caused major destruction and casualties:
A magnitude 8.0 earthquake took place near Mindanao and Sulu a little past midnight of Aug. 17, 1976 that was felt as far as Visayas. It was then followed by a massive four to five meters high tsunami covering 700 kilometers of coastline bordering the island. Because it was dark, the people were caught by the raging water which claimed 8,000 lives, injuring 10,000, and leaving 90,000 more, homeless; two. A total of 2,412 people died and at least P10-billion worth of damages to public and private properties was reported after a magnitude 7.8 earthquake struck Northern and Central Luzon at around 4:00 p.m. of July 16, 1990. Hyatt Terraces Plaza, Nevada Hotel, Baguio Hilltop Hotel, Baguio Park Hotel, and FRB Hotel, all in Baguio collapsed trapping and burying people alive. Although the epicenter was recorded in Nueva Ecija, it caused more damage in the City of Pines. And the quake that just lasted for about a minute was one of the tragedies in the country that would never be forgotten;
The magnitude 7.5 earthquake that crushed Luzon on Nov. 30, 1645 at about 8:00 pm was called the “most terrible earthquake” in Philippines history. The Epicenter of the said quake was in Nueva Ecija caused by the San Manuel and Gabaldon Faults. The extent of the tremor was felt as far as Cagayan Valley. It has caused many landslides which buried many people alive and destroyed many buildings and churches including Manila Cathedral. That time, only Spanish are counted so the recorded number of casualties was only 600 while the injured was 3,000.
Most of the people in Casiguran, Aurora was still fast asleep when a magnitude 7.3 earthquake struck at 4:19 a.m. of Aug. 2, 1968. It was another deadly and shocking seismic activity in the country. And the City of Manila got the most severe damage. Many buildings were either damaged or destroyed totally. The said event was also called the Ruby Tower earthquake after the said six-story building located in Binondo collapsed and caused the death of 260 people. A total of 268 people died that day and 261 more were injured.
It was around 8:12 a.m. When a strong earthquake on Oct. 15, 2013 jolted most of Central Visayas, particularly Bohol and Cebu. It was felt in the whole area of Visayas and reached as far as Masbate Island in the north and Cotabato in Southern Mindanao. According to the National Disaster Risk Reduction and Management Council (NDRRMC), a total of 222 people died, eight went missing and 976 others were injured. An estimated 73,000 structures were damaged wherein more than 14,500 of which were destroyed totally.
“Once you have been in an earthquake you know, even if you survive without a scratch, that like a stroke in the heart, it remains in the earth's breast, horribly potential, always promising to return, to hit you again, with an even more devastating force.” ~ Salman Rushdie, “The Ground Beneath Her Feet.”
An earthquake achieves what the law promises but does not in practice maintain - the equality of all men.” ~ Ignazio Silone