Tsunami 101: What you need to know about tsunamis

MANILA, Philippines – Exactly 4 years ago, the whole world witnessed on live television one of the worst disasters in recent times.

At 2:46 pm JST, a magnitude 9 earthquake struck off the coast of Sendai for two minutes. The next thing residents knew, waves as high as 30 meters came rushing to the city, carrying with them houses, cars, trees, and drowning thousands of people.

The quake also destroyed a number of nuclear reactors in the country, causing fear and panic to millions of people due to the threat of radiation.

On that day, the Philippine Institute of Volcanology and Seismology (Phivolcs) was able to monitor the formation of the tsunami through its tide tools – a software that enables experts to view the sea-levels in various parts of the world, including the Pacific Ocean.

“We noticed that (the) Funatu City tide gauge suddenly rose and got destroyed. It leveled up. It just became horizontal, meaning the tide gauge was destroyed,” Phivolcs director Renato Solidum said.

This prompted the agency to raise Tsunami Alert Level 2, warning residents near the coasts about tsunami with waves as high as one meter.

Solidum said that although the tsunami was generated in the Pacific Ocean, it did not heavily damage the Philippines because the Japan trench, the fault that generated the earthquake, is not parallel to the Philippines.

Solidum compares this to dropping an object on water. “If you drop a pebble on the water you will see a circular ripple. But the fault is not a small point. A fault is a line so instead of a pebble, why don’t we drop a long 2×2 piece of wood?”

Dropping is the reverse of uplift, Solidum explained. “So if you drop that wood, you would notice that the highest splash or wave that would generate would be perpendicular to the length of the fault.”

He said that because the Philippine coastline was not facing the Japan Trench, the waves were not high. The highest tsunami that reached the Philippines at the time was less than a meter. This was recorded in Catanduanes.

Due to the strength of the earthquake, experts said that the earth’s axis shifted for about 4 inches, causing days to be shorter by 1.8 microseconds.

The 2011 Sendai earthquake was the strongest to hit Japan since 1900, and the fourth strongest in the world. It killed around 15,800 people and cost $200 billion worth of damage.

If an earthquake of the same magnitude occurred in a location parallel to the Philippines, we could also experience a devastating tsunami. Are you prepared?

Here are some basic information on tsunamis.

WHAT IS A TSUNAMI?

A tsunami is a series of sea waves commonly generated by under-the-sea earthquakes, with heights that could be greater than 5 meters.

In the Philippines, it occurs when the earthquake is at least magnitude 6.5 and is shallow-seated, meaning the epicenter is less than 30 km below the seafloor.

Oftentimes, it is mistakenly associated with storm surges, tall waves that are caused by typhoons passing through seas and oceans. (READ: Storm Surges 101: Are you at risk? Are you prepared for it?)

Volcanic eruptions and landslides may also cause tsunamis.

TYPES OF TSUNAMIS

How vulnerable is the Philippines?

The Philippines’ location makes it vulnerable to tsunamis. Being an archipelago along the Pacific Ocean’s Ring of Fire, it is surrounded by both bodies of water and trenches such as the following:

Location	Ocean/Seas	Trenches
East	Pacific Ocean and Philippine Sea	Philippine Trench
West	West Philippine Sea	Manila Trench
Southwest	Sulu Sea	Negros and Sulu Trenches
South	Celebes Sea	Cotabato Trench
Southeast	Pacific Ocean	Philippine Trench

Metro Manila, the country’s capital is also vulnerable to tsunamis because it faces the Manila Bay. In the past, two tsunamis with heights of 1-2 meters, affected the bay in 1828 and 1863.

Affected areas were Manila, Bataan, Zambales, Cavite, Batangas, and Mindoro which face the Manila Trench, the source of the tsunami in this part of the Philippines, Solidum explained.

On record, at least 3 distant tsunamis have also affected the country in the past. These were the 1960 Great Chilean Earthquake – magnitude 9.5, the 2010 Chile Earthquake – magnitude 8.8, and the 2011 Sendai Earthquake in Japan.

Community-based preparedness

To prepare for the occurence of tsunamis in the country, Phivolcs has initiated various tsunami warning and preparedness projects.

Through funding from the Department of Science and Technology (DOST) in 2006, it was able to develop a tsunami detection system by setting up dry and wet sensors in 10 sites all over the Philippines. It also allowed the agency to produce tsunami hazard maps in coastal areas.

In 2013, the Phivolcs released different comics, based on real stories of Filipino survivors of the 2011 Sendai earthquake. These were written in Filipino.

But while these projects can really be helpful to Filipinos, Solidum has always emphasized the importance of community-based preparedness, especially during locally-generated tsunamis.

The locals, he said, should not only rely on warnings and advice but on the use of natural observances as well.

“In our experiences and observations, 2-5 minutes may mga events na nandiyan na yung tsunami after the earthquake (there are events when the tsunami comes after the earthquake). So in that case, there is very limited time to give warning. The locals should know the safe places to go and evacuate,” Solidum said. – Rappler.com

Sources: Philippine Institute of Volcanology and Seismology, National Police Agency of Japan, National Oceanic and Atmospheric Administration, National Aeronautics and Space Administration, various news websites