By JES B. TIROL
On6 February 2012at11:49 A.M.a magnitude 6.9 earthquake occurred that shook the Central Visayas Region and parts of the Western Visayas Region. The epicenter of the earthquake was located at latitude 9.82 N and longitude 123.31E. The initial determination of the depth of focus of the earthquake was 10 km but officially published as 20 km below the surface. The nearest town at this location is Tayasan, Negros Oriental.
That earthquake was relatively strong. Since the earthquake was classified as a shallow earthquake, extensive damage was expected. The8 February 1990eathquake inBoholwas also rated as magnitude 6.9 but later on upgraded to magnitude 7.
As a certified Structural Engineering Specialist with special training in earthquake engineering, it behooves upon me to share my knowledge.
There are two basic scales used in measuring the strength of an earthquake. One is the magnitude scale or the open ended Richter Scale and the other is the Intensity Scale.
The Magnitude Scale is based on the energy released by the earthquake at the source and rated from 1 to any number. So far the strongest earthquake ever recorded in the magnitude scale is 9.5. Bear in mind that the increase of the scale is logarithmic and not a straight line. In the magnitude scale, the next whole number is 32 times the previous number. In other words, the energy released by a magnitude 5 earthquake is 32 times stronger than a magnitude 4 earthquake.
The Intensity Scale is based on the damage at the particular site. The Intensity Scale used in thePhilippinesis the Rossi-Forel Scale. It used to be from 1 to 9 but now modified by PHIVOLCS to 1 to 10.
The structural engineers prefer the magnitude scale because the energy released can be computed to give the probable force exerted by the earthquake on buildings and structures. The engineers will utilize the acceleration of the earthquake waves measured by the Strong Motion Accelerograph (SMA). We can compute the force byNewton’s Second Law that states, Force equals Mass times Acceleration (F=ma). The force is utilized to design buildings and structures.
The Social Services Agencies prefer the Intensity Scale because it indicates the damage done and they can react accordingly. Intensity 1 is hardly perceptible. Intensity 5 already causes buildings to shake, hanging objects will sway, and free standing objects topple. Intensity 6 has already some damages and Intensity 7 has already poorly constructed building collapsing.
At the town ofTayasanthe Intensity Scale was evaluated as 6 while atDumagueteCityit was evaluated as Intensity 7 because a building has collapsed. InTagbilaranCityandCebuCityit was evaluated as Intensity 5. Intensity 10 is already total devastation.
It is possible to have a big magnitude rating with small intensity rating. This happens when the earthquake is so deep below the ground say from 150 km to 750 km.
It is also possible to have a small magnitude rating with a big intensity rating. This happens when the earthquake is shallow say 5 km to 50 km down.
Four Kinds of Waves
The earthquake will propagate four (4) kinds of waves. First is the Primary or P-wave, second is the Secondary or S-wave, third is the Love Wave and fourth is the Rayleigh wave. These waves will travel in a concentric direction from the focus or source of the earthquake.
The P-wave is a compression-dilation wave that travels forward and it travels the fastest. The S-wave is a transverse or side-to-side motion of the waves as it travels forward. As the earthquake waves reaches the ground surface, the primary waves behaves like a sound wave as it make contact with the air. Sometimes it causes a rumbling sound.
However some of the waves travel along the land surface. The surface waves are the Love Wave and Rayleigh Wave. The Love Wave has a side-to-side motion and travels along the land surface like a river. It is dangerous to tall buildings because if the river-like flow hits a tall building it causes the building to twist. The Love Wave is also the reason why in some places the damage will be heavy while a nearby place not hit by the Love Wave has little damage.
The Rayleigh Wave is a retrograde and up and down motion of the wave on the ground. It is comparable to a summersault of an acrobat. It it hits a building at the same time with a secondary wave, it can cause the building to topple over.
In a shallow focus and large magnitude earthquake like what happened last Monday, the four kinds of waves will mix together. In this situation it usually happens that the up and down shaking of the ground and structures will be stronger than the lateral or side to side shaking of the building. As I look at the pictures of damages shown in television, I could observe that most of the damages were caused by up and down movements of the ground.
Previous to my 1998 lecture during an international conference of earthquake engineers at Taipei, Taiwan, nobody would design for the up and down movement of the ground. Since the seismic engineering literature did not have a term for it, I introduced the name “witík”. Eventually the witík was recognized and by year 2000 the Earthquake Engineering Codes in the United States already included the analysis for up and down forces and not only the side sway forces.
Tsunami is a Japanese word that means harbor wave in English. It is now undertood to mean huge waves at sea caused by earthquakes.
Earthquakes from magnitude 6.9 and above can produce tsunamis. However, tsunamis can only occur at beaches that are sloping downward to the sea.
In lakes and confined bodies of water, the proper term is “seiche” (pronounced sāsh). The effect is similar to water in a basin. If you shake the basin, the water will sway from side to side or sometimes spill over the basin.
TheTañonStraitor the body of water betweenNegrosIslandandCebuIslandis very narrow. It could not produce a tsunami. However, it can produce a seiche. So the warning should not have been for a tsunami but for a seiche. However, since the popular word is tsunami, we can forgive the PHIVOLCS for issuing a Tsunami 2 warning even though it is not the correct term.
Do you know that Sugboanon Bisaya has terms for tsunami and seiche? They are sag-ulî = tsunami and súlyà = seiche.
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