• Earthquakes and volcanic eruptions that cause the sea-floor to move abruptly, resulting in sudden displacement of ocean water in the form of high vertical waves, are called Tsunamis (Harbour Waves) or seismic sea waves.
• It should be noted that only those earthquakes, which have high magnitude and which cause the ocean floor to move up and down, lead to Tsunami events.
• Normally, the seismic waves cause only one instantaneous vertical wave; but, after the initial disturbance, a series of after waves are created in the water, that oscillate between high crest and low trough, in order to restore the water level.
• The speed of wave in the ocean depends upon the depth of water.
• It is more in the shallow water, than in the ocean deep. As a result of this, the impact of Tsunami is less over the ocean and more near the coast, where they cause large-scale devastations.

Therefore, a ship at sea is not much affected by Tsunami and it is difficult to detect a Tsunami in the deeper parts of the sea. It is so, because over deep water, the Tsunami has very long wave-length and limited wave-height.

• Thus, a Tsunami wave raises the ship only a metre or two, and each rise and fall takes several minutes.
• As opposed to this, when a Tsunami enters shallow water, its wave-length gets reduced and the period remains unchanged, which increases the wave height.
• Sometimes, this height can be up to 15 m or more, which causes large-scale destructions along the shores. Thus, these are also called Shallow Water Waves.
• Tsunamis are frequently observed along the Pacific Ring of Fire, particularly along the coast of Alaska, Japan, Philippines, and other islands of south-east Asia, Indonesia, Malaysia, Myanmar, Sri Lanka and India etc.