Tsunami: What it is, warning signs, how to stay safe

Following the magnitude 7.8 earthquake that struck Mindanao on Monday, many residents witnessed unusual changes along the coastline, from powerful waves in General Santos City to the sudden retreat of seawater in parts of Zamboanga del Sur and Davao de Oro.

According to Kuya Kim’s report on “24 Oras” on Tuesday, Rea Dela Cruz Pacheca and her family were enjoying the beach in General Santos City when the quake happened.

In her video, the once-calm waters can be seen becoming treacherous.

"Malakas 'yung lindol, malakas din 'yung alon tapos biglang nahati 'yung tubig. Nagsabi po sila na tatakbo daw sa highway kasi medyo mataas 'yung kalsada," she shared.

Meanwhile, videos captured by netizens in Pagadian, Zamboanga del Sur and Pantukan, Davao de Oro, showed the waters receding.

Per Kuya Kim, these are all warning signs of a tsunami.

But what exactly is a tsunami, and what should people watch out for to stay safe?

According to the Department of Science and Technology's Philippine Institute of Volcanology and Seismology (DOST-PHIVOLCS), a tsunami is caused by a large earthquake.

“A tsunami is a hazard caused by a large earthquake. The keyword is large earthquake. Hindi naman lahat ng lindol ay may tsunami. 'Pag maliit lang naman 'yung shaking, wala naman 'yung tsunami,” explained DOST-PHIVOLCS Supervising Science Research Specialist Engr. Angelito Lanuza.

Contrary to popular belief, a tsunami does not always happen in big waves.

“‘Yung conception ng karamihan, akala nila tsunami should be big waves, 'no? Basta ito lang po 'yung pinakasimple. 'Pag ang pagbabago po sa pagtaas ng karagatan ay gawa ng lindol ito ay tinatawag na tsunami. Whether it is big or not,” he said.

“Nagiging malawakan po ang tsunami kasi naganap ito doon sa displacement ng seabed. Kumbaga, 'yung isang palanggana ng tubig, alugin mo. So talagang 'yung buong tubig gagalaw,” he added.

One of the most important natural warning signs is the sudden withdrawal of seawater from the shoreline.

“Naalog po ang ilalim ng karagatan. Ang mangyayari po niyan, naiipon ang tubig muna sa kalagitnaan ng karagatan. It's a sign pagka nawala po 'yung dagat. Babalik at babalik po 'yan,” Lanuza said.

In the event of a tsunami threat, Lanuza urged residents to pay attention to natural warning signs and act immediately.

Per Lanuza, communities are taught the "Shake, Drop, and Roar" principle, which highlights the importance of recognizing strong ground shaking, unusual sea movement, and roaring sounds from the ocean as possible indicators of a tsunami.

He also noted that local government units have already been provided with hazard visualizations and are expected to have evacuation plans in place.

“Tapos lahat po ng munisipyo, binigyan natin sila ng mga visualization. Dapat ngayon tapos na 'yung kanilang mga evacuation plans para pagka ganiton level nakaramdam sila ng intensity 6, hindi 'yun doon pa sila magtatanungan saan sila pupunta,” he said.

Lanuza likewise said that they canceled the tsunami warning on Monday afternoon.

"We cancelled the advisory at around 3 p.m. Ganun kasi 'yung procedure natin, after the last waves of the tsunami, at least two hours," he said.

On Monday morning, a 7.8-magnitude earthquake struck offshore Sarangani in Mindanao. On Tuesday, the Office of Civil Defense said that the reported death toll from the quake has increased to 37.

For those who wish to extend help to the victims of the Mindanao earthquake, several organizations are accepting monetary and in-kind donations.

—Jade Veronique Yap/CDC, GMA News