- Technically, the intensity of the tremor in Cebu City itself is NOT 7.2--it was between 5-6 (the closer to Bohol that part of the city is, the higher the intensity). The area that received the highest intensity rating is none other than the epicenter, which was in Bohol island itself. Just want to clarify. If Cebu City was hit by a 7+ intensity quake, you'd see major damage (i.e. toppling of some skyscrapers, like what happened in Baguio in the 1990's) to large buildings due to the low frequency waves, and not just cracked walls/post and/or destroyed balconies and parapets.
- People are also wondering why this building got damaged and then this one didn't. Or, this part of the building got damaged but this part didn't. The usual layman explanation is that the building that got heavily damaged wasn't built well, and that the ones that didn't have damage or had very little damage was built well. Actually, it's not that simple. The damage inflicted by an earthquake on a building depends on a property oscillation and if applied to earthquakes, the term engineers use is resonance.
Resonance is the up-and-down or back-and-forth motion caused by a seismic wave. During an earthquake, buildings oscillate. Not all buildings respond to an earthquake equally. If the frequency of an oscillation is close to the natural frequency of the building, resonance may cause severe damage. How do we define natural frequency? When a force is introduced on a building it will sway within a set time period. If the building has more mass and/or is stiffer (less flexible), then the natural frequency is lower, because the swaying will be lesser. So as a rule, smaller buildings usually have a higher natural frequency. However, there are many kinds of earthquake waves--there are high frequency ones and there are low frequency waves too. Now, imagine a small boat on the ocean. If there are large but slow waves headed towards it, it isn't affected much--this is analogous to a building experiencing low frequency earthquake waves. However if the same boat is battered by smaller waves coming in fast, then it's bound to capsize--this is analogous to smaller but high frequency earthquake waves. Now if it's a large ship, the same smaller but high frequency waves will not affect it much, but the large, slow waves have the potential to capsize the large ship, because the natural frequency of the larger slow waves are "in synch" with the dimensions of the ship.
Now every earthquake has both high and low frequency waves. So if a high frequency waves passes through an area with relatively small buildings, those buildings will experience damage, but the larger ones will not (however, if the large building has smaller appendages that are not built well, like a balcony etc, that balcony can be damaged because it's natural frequency might match the smaller freq waves). But if the area experiences long slow waves, then larger, high rise buildings will sway and can topple.
This is why damage during an earthquake seems random or without pattern, but if you can trace where the high frequence and low frequency passed, you can actually also pinpoint the areas where damage can be high, depending on what kind of buildings are in the area.
- Can earthquakes be predicted? The obvious answer now is, no we can't. But can it happen? Will the practical prediction of earthquakes occur in the future? This is my say.
Earthquakes occur due to shifts in tectonic plates. These plates are sections of earth's crust, "floating" on a sea of semi-solid mantle. The reason why it's difficult to predict when earthquakes will occur or where they will occur, is because we cannot see the stress building up in faults, which are sections where two plates collide or meet. Now think...if there was just a way to follow the flow of convective currents in the earth's mantle beneath these plates, then we can easily compute stress build up per major plate (and subplates). I think this is the reason why a several billion dollar mission is about to take place, to reach the earth's mantle. This mission will attempt to dig deeper than anyone has dug before, to reach the earth's mantle. Perhaps one of the objectives of the mission is to check mantle convective actions and see if there is a way to measure its convective flows. Perhaps this is the key to finally finding a way to predict earthquakes, but this is just a guess on my part.
Billion-dollar mission to drill into the Earth's mantle planned (Wired UK)
- If not reinforced via modernization methods (i.e. internal steel bracing, steel trusses etc), old churches will be the most affected structures during quakes, simply because when they were built hundreds of years ago, the engineering methods to build them were still not sophisticated, and thus they are unreinforced. I just hope people didn't flee into a church during such quakes--it would be a disaster. even if reinforced, their building materials (stone) are really too heavy and will eventually collapse due to the weight alone, leaving the "skeletal" structure (the internal steel bracing) the only parts standing. I see this sort of damage to churches now in my FB newsfeed, re reinforced ones.
-RODION
Science Facts about the Recent Bohol/Cebu Quake
Originally Posted by woodpusher
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