There’s been a series of non-stop discussion on the recent quake that hit the Visayas these past few days amid the relief and disaster operation conducted at the heavily-stricken areas. I’ve heard facts and non-facts-at-all from radio broadcasters who talked like experts on earthquakes and tsunamis.
I’ve been keeping this manual on “Earthquake and Tsunami” published in 1990 by the Philippine Institute of Volcanology and Seismology (PhiVolcS) that serves as a guide for my news reportage.
Many of the facts in the manual are nowhere can be found in the internet because of the inability of the PhiVolcS to upload it to the cyberspace for some reasons.
One fact that stands out in the manual is the Tsunami map where all of the 27 local tsunamis from 1603 – 1975 are plotted. The map proves the claims of non-experts that tsunamis occur only in open seas are indeed WRONG. The map shows several places in the Philippines with inland seas that were hit by tsunamis. One of which is the coastline of now the Metro Cebu area as indicated in the map. The exact date of this tsunami that hit Cebu once-upon-a-time is not indicated in the map.
Other sources supported my early knowledge that tsunamis could not only occur in open seas but also in inland seas like the Visayas and Cebu in particular.
Tsunamis are disasters that can be generated in all of the world's oceans, inland seas, and in any large body of water. -International Tsunami Information Center
The most destructive local tsunami that hit the Philippines was caused by the Moro Gulf earthquake on August 16, 1976 that killed 3,000 people; at least 1,000 missing; 8,000 injured; and 12,000 families rendered homeless.
Like earthquakes, Tsunamis are also measured by an intensity scale of Intensity 1 to 6:
Intensity Scale 1 : VERY LIGHT– waves not perceptible; recorded on tide gauges.
Intensity Scale 2 : LIGHT– waves noticed only by experienced sea dwellers; may be
noticed on flat shores.
Intensity Scale 3 : RATHER STRONG– Generally noticed; gently sloping coasts are
flooded; light sea vessels carried ashore; slight
reversal of river flows near the coast.
Intensity Scale 4 : STRONG– Significant flooding of the shore; light structures
damaged; sea vessels and light ships displaced inland or to
Intensity Scale 5 : VERY STRONG– General flooding of the shores; solid structures
damaged; light structures destroyed; all vessels
except very large ships drifted inland or to sea;
Intensity Scale 6 : DISASTROUS– Destruction of man-made structures for some
distance inland; trees uprooted; deep coastal
flooding; big ships damaged.
The “tsunami” panic in Cebu City succeeding the main quake on Monday (Feb. 6) afternoon was a result of the lack of appropriate information to the populace especially along the coast lines. The illustration below simply could be understood that it is impossible for a tsunami to reach Cebu City (East side of Cebu) while the epicenter of the quake was somewhere in Tayasan, Negros Oriental (facing West side of Cebu).
The Philippines belongs to the “Pacific Ring of Fire” where it lies on two major tectonic plates of the world. AT LEAST FIVE EARTHQUAKES PER DAY OCCUR IN THE PHILIPPINES.
The fault that caused Monday’s quake is new to Phivolcs and it’s not in the list of earthquake generators known to them. Here are the identified earthquake generators in the Philippines:
1- Philippine Trench
2- East Luzon Trench
3- Manila Trench
4- Collisional Zone between Palawan and Mindoro
5- Negros Trench
6- Collisional Zone between Zamboanga Peninsula and Western Mindanao
7- Sulu Trench
8- Cotabato Trench
9- Davao Trench
10- Philippine Fault Zone and its many branches
11- Many Active faults (e.g. Lubang, Tablas, Casiguran and Mindanao Faults)
PhiVolcS listed the most destructive earthquakes in the Philippines and its impacts:
Epicenter Intensity Magnitude Casualty/Injured
July 2, 1954 : Bacon, Sorsogon VII 8.3 13 101
Apr 1, 1955 : Lanao, Mindanao VII 7.5 291 713
Aug 2,1968 : Casigura, Aurora VII 7.3 270 600
Apr 7, 1970 : Baler, Quezon VII 7.3 15 200
Aug 17,1976 : Moro Gulf,Mindanao VII 7.9 3,739 8,000
July 16, 1990: Cabanatuan City, VIII 7.7 1,283 2,786
Tsunamis can be expected when to occur while earthquake can not be predicted as of this time. However, there are people who tried to predict earthquakes based on its claims as an alleged result of inter-planetary alignment as shown in this video by solarwatcher.net posted on December 16, 2011.
Then there is a conspiracy theory that earthquakes can be triggered by human hands. In this video documentary, America’s H.A.A.R.P (High frequency Active Auroral Research Program) is examined for its possible influence if not manipulation of the earth’s weather.
No one knows exactly when a quake occurs and how long its subsequent aftershocks. As of this post, more than a thousand aftershocks have been recorded since Monday’s quake in the Visayas.