Tsunamis are Japanese vocabularies consisting of 'tsu' and 'nami'. 'Tsu' is defined as a port and 'nami' is defined as a wave. Thus, a tsunami can be interpreted as a harbor wave or a wave that only occurs around the harbor.
The word 'tsunami' itself first appeared after Japanese fishermen returned from sea to port and saw damage that occurred around the port without knowing the cause. According to the fishermen, when they were at sea, they did not see or feel the arrival of a big wave that was able to devastate the port and its surroundings.
From this fact they then came to the conclusion that the tsunami only occurred around the port. Thus, what is a tsunami? According to Bakornas PB (2007), the tsunami was sea waves with a long period caused by impulsive interference from the seabed. These impulsive disturbances can be in the form of earthquakes under the sea, volcanic eruptions or landslides.
Other definitions of tsunamis were raised by the Department of Energy and Mineral Resources (2008) which defines tsunamis as a series of ocean waves. capable of spreading at speeds of more than 900 km per hour, mainly caused by earthquakes that occur on the seabed.
Meanwhile, BNPB (2011) defines tsunamis as a series of giant sea waves that arise due to shifts in the seabed due to earthquakes . BMKG (2012) defines tsunamis as large waves that occur when parts of the ocean floor change due to volcanic eruptions, underwater landslides, or underwater earthquakes.
From some of the above tsunami definitions it can be concluded that the tsunami is a series of sea waves caused by disruption in the seabed due to earthquake tectonics, volcanic eruptions, or landslides. Among these disturbances, underwater earthquakes are the main cause of tsunamis.
Tsunami Occurrence Process
Tsunamis are series of waves that occur as a result of underwater earthquakes, volcanic eruptions at sea or landslides under the sea. Tsunamis do not occur as simply as turning a palm, but rather through a series of processes. The process of the tsunami can be divided into four stages, namely the initial stage, the phase of wave separation, the amplification stage, and the crawling stage.
1. Early Phase
Most tsunamis are triggered by earthquakes centered under the sea with an epicenter depth less than 60 km from the seabed. The earthquake strength reached more than 7 SR and occurred in the tectonic plate collision area.
As a result of this collision, one plate will be pushed down and one other plate will rise to the top. Typically, tectonic plates that are pushed downward are oceanic plates at sea and the plates above them are thinner continental plates. Ocean plates that are pushed down then rub against the continental plates that are above it.
The vertical collision of these plates then creates faults or faults whose spans reach 5 to 10 meters. This fault movement forces the water column above the seabed to fluctuate up and down. The energy produced by this impulse of water is then turned into kinetic energy or tsunami waves that propagate straight.
2. Wave Separation Phase
After a tsunami wave is formed which is accompanied by gathering energy, the tsunami wave then moves to spread in all directions and away from the epicenter. Some tsunami waves are propagated to the ocean and others are propagated to the nearest beach. Looking at its direction, these two tsunami waves propagate in different or opposite directions and at different speeds.
The velocity of the propagation of the second wave of the tsunami is very dependent on the depth of the ocean and the gravitational force of the earth. The deeper the ocean, the speed of tsunami wave propagation that spreads to the ocean will be faster than the speed of tsunami waves traveling towards the coast.
The velocity of wave propagation in the sea can reach 500-1000 km / hour. However, the velocity of the tsunami waves leading to the coast is only a few km / hour. The slowing of the speed of the tsunami waves leading to the coast is caused by the shallower depth of water.
3. Amplification Stage
The reduced velocity of wave propagation actually increases the height of the tsunami waves that hit the coast. Tsunami wave height that leads to the coast is very dependent on the energy contained in it.
On the coast, energy converges vertically because of the shallower depth of water and horizontally because the wave velocity decreases.
Thus, there are two things which occurs when tsunami waves propagate towards the coast. The first is an increase in the amplitude of tsunami waves or the rise and fall of tsunami waves. And second, the reduction in tsunami wavelengths. Both of these events produced the first tsunami waves to hit the coast.
4. Phantom Phase
Tsunamis are a series of waves. Therefore, tsunami waves that first enter the mainland are usually not the largest waves. In a sense, a tsunami wave that is larger and has a high destructive force is a tsunami wave that enters next to the mainland.
The magnitude of the second or subsequent tsunami wave has greater energy and speed so that it is able to creep into the land as far as 500 meters . In addition to the coming tsunami waves, the reverse tsunami waves also have the same amount of damage due to being able to drag whatever they pass through.
Impact of the Tsunami
Various tsunami events occurring in Indonesia and throughout the world have their own impacts, among which are as follows.
1. Causing fatalities
Various tsunami events that occur throughout the world often cause fatalities. One of the tsunami events that caused the biggest casualties was the tsunami caused by the earthquake in Acek in 2004.
Death occurred not only in Aceh but also in countries in the Indian Ocean affected by the Aceh tsunami. According to the data, the total number of fatalities caused by the Aceh tsunami earthquake reached nearly 300,000.
2. Death of livestock
In addition to causing human casualties, the tsunami also caused the death of livestock owned by the community. This is due to many livestock that can not escape when the tsunami waves hit.
3. Cause damage
In addition to causing casualties, tsunamis that hit land can also cause damage even at low speeds. Damage caused includes damage to road infrastructure, bridges, hotels, housing or settlements, electricity poles, and buildings with strong construction.
4. Damaging and destroying ecosystems
Tsunamis that come with great height and strength can damage and destroy ecosystems. An example is the Lituya Bay Alaska tsunami in 1958. The height of the tsunami triggered by an avalanche to the Lituya Bay in Alaska reached more than 500 meters. The impact is the destruction of ecosystems and vegetation found on the slopes of the mountains of St. Elias.
5. Loss of property
Another impact of the tsunami was the loss of property owned by the community. Previously, they owned a house. cars, motorbikes or other property, after the tsunami hit they lost everything they owned. This is due to the sweeping waves of tsunamis that go very far inland or are dragged back to the ocean.
This is our discussion about the understanding of tsunamis, the process of tsunamis, and the impact of tsunamis. May be useful. Thank you.