Although the height of tsunami waves may seem smaller than normal waves, tsunamis can rise higher and possess greater destructive power when they strike the shore due to the movement of water from the ocean floor to the surface.
Tsunamis in Japan Are Unlikely to Affect Vietnam
On September 24, the remote Izu Islands located south of Tokyo, Japan, recorded a minor tsunami following a strong earthquake measuring 5.9 that occurred near an uninhabited island further south in the Pacific Ocean. The Japan Meteorological Agency reported that a tsunami measuring 50 cm hit Hachijo Island, part of the Izu Islands, approximately 40 minutes after the earthquake on September 24. NHK television reported that there have been no reports of damage caused by the tsunami or the earthquake.
Tsunami devastation in Japan.
Dr. Nguyen Hong Phuong, a seismic expert and former deputy director of the Earthquake and Tsunami Warning Center at the Vietnam Institute of Geophysics, stated that Vietnam has never been struck by a tsunami. However, we must remain vigilant and prepared to respond to the threat of tsunamis. I believe this is a task that requires proactivity, alertness, and readiness for future occurrences.
Vietnam is located in the Pacific region, which is assessed to have the highest tsunami risk in the world. However, due to the geographical characteristics of Vietnam’s coastal areas, which are surrounded by many neighboring countries such as China to the north, the Philippines to the east, Thailand to the west, and Indonesia and Malaysia to the south, tsunamis caused by earthquakes in the Pacific do not affect Vietnam. In simple terms, Vietnam’s coastal areas are shielded by the “wall” of these countries. Due to this geographical position, Vietnam is less affected by tsunamis from countries like Japan and Indonesia.
However, Vietnamese seismologists have identified that the threat of tsunamis could still occur for Vietnam in the future, and this threat could originate within the South China Sea. This means that within the South China Sea, areas capable of generating earthquakes that could cause tsunamis have been identified, and tsunamis occurring in the South China Sea could impact Vietnam’s coastal regions.
Tsunamis are one of nature’s most powerful forces and can cause significant destruction. They can create unusually strong currents, leading to rapid flooding and devastation of coastal communities. Low-lying areas such as beaches, bays, lagoons, ports, river mouths, and areas along rivers and streams that flow into the sea are the most vulnerable.
Differences Between Tsunamis and Wind Waves
According to Dr. Nguyen Hong Phuong, it is crucial for the public to clearly distinguish between waves caused by storms and tsunamis, as they are fundamentally different. Tsunamis are caused by gravitational waves resulting from the gravitational pull of planets, as well as powerful movements on the ocean floor due to tectonic shifts.
A tsunami consists of a series of extremely long waves (with wave intervals of tens to hundreds of kilometers from wave peak to peak) caused by significant and sudden displacements of the ocean.
Tsunamis radiate outward in all directions from their point of origin and can travel across entire ocean basins. When they reach the shoreline, they can cause dangerous coastal flooding and strong currents that may last for hours or even days.
According to the National Oceanic and Atmospheric Administration (NOAA), the main difference between tsunamis and wind waves is that tsunamis move through the entire column of water – from the ocean floor to the ocean surface, while other types of ocean waves only affect the surface layer of the ocean.
This is due to how they are generated. Waves are created by the transfer of energy from their source to the ocean. Most types of wind waves are created by wind blowing across the water’s surface. However, tsunamis are generated by significant and sudden movements of the ocean, usually due to earthquakes beneath or near the ocean floor. These sources can generate more energy than wind.
Waves have three basic characteristics: wavelength (the distance between two wave crests), period (the time between two wave crests), and speed.
Wind waves have short wavelengths, typically 90-180 m, while tsunamis have wavelengths ranging from 500-1,000 km, leading to periods of up to 2 hours for each wave compared to wind waves, which last about 5-20 seconds. According to NOAA, the longer the wavelength of a wave, the greater the volume of water it carries. Tsunami speeds in deep water typically range from 800-1,000 km/h, slowing to 30-50 km/h as they approach the shore. Wind wave speeds are between 8-100 km/h.
Even though the height of tsunami waves may appear smaller than normal waves, tsunamis can rise higher and possess greater destructive power upon hitting the shore. As they enter shallower waters near the coast, tsunamis slow down, shorten their wavelength, and increase in height and steepness. Waves that are undulating and lower at their peaks often reach the shore first. When they hit the shore, they create a vacuum effect, pulling water from the shore out to sea and exposing the seabed near the coast.
This phenomenon is a crucial warning sign for tsunamis, as the wave crest and its massive volume of water typically reach the shore about five minutes later. Recognizing this phenomenon can save many lives.
According to Dr. Nguyen Hong Phuong, when a tsunami is about to strike, it is common for the sea water to recede completely, exposing the seabed. This is a sign that a tsunami may hit within minutes. At this moment, instead of going down to the exposed seabed to catch shrimp or fish, one must respond immediately by running away.
When we are at the coast and recognize the signs of an impending tsunami, running deep inland is a viable escape route. However, with very little time, imagine the scenario of having only 15-20 minutes to run from the coast; it would be challenging to escape far and find shelter on higher ground. Therefore, upon seeing signs of a tsunami, you need to determine a direction towards sturdy high-rise buildings and then continue to ascend to higher floors.
Also, because of the long wavelength, the volume of water carried is substantial, and the energy of the tsunami is very high and dangerous. Instead of breaking apart and retreating quickly at the shore, tsunamis often surge inland like a rapidly rising flood, inundating low-lying areas before receding back to the sea. Therefore, even tsunamis that are only a few dozen to several hundred centimeters high can carry strong currents, causing significant destruction and lasting for hours. Large tsunami waves can persist for several days.
