Tsunamis

By: Neha Jain Aditya Sharma

What is a tsunami ?

A tsunami is a very long ocean wave generated by sudden displacement of the sea floor or of the oceanic mass

The displacement of an equivalent volume of water generates the tsunami

Terminology

The term “tsunami” is a Japanese word meaning “harbour wave”

It was so named because the wave is harmless until it enters a harbour

It is frequently called a “tidal wave”, but it has nothing to do with tides

Hazards and risks of tsunamis

Tsunamis can hit with little or no warning

The most prone areas are those associated with earthquakes and volcanoes .

Structure of a wave

Wavelength, , can exceed 200 km

normal ocean waves have wavelengths of about 100 m

trough; peak; wave height, h; amplitude

From Murck et al. (1996)

Velocities in deep water

Tsunamis travel very quickly relative to normal ocean waves

This is particularly the case in open water, where velocities increase with water depth

Velocities can reach 1,000 km/hr in open ocean (normal ocean wave: ~90 km/hr)

Thus, velocities are about 10 times higher for tsunamis

Shallow water

In shallow water, the tsunami waves pile up

As a result, velocities and wavelengths decrease...

…but at the same time, amplitudes can increase enormously...

Causes of tsunamis - all involve displacement of water

Earthquakes

Volcanic activity

Landslides

Meteorite impacts

Tsunami hazards

Extensive flooding

Action of wave on coastal structures, both natural and built

The incredible force of the waves can remobilize huge objects

The event may create drawdown

Effects of tsunami drawdown

Release of dissolved gases (CH4, CO2, H2S)

previously contained in shallow sediments

Potential ignition of gases by their rapid expulsion

As a result, a wave of noxious and burning gases may engulf people BEFORE the wall of water arrives

Mitigation efforts

Warning times

Every ~750 km of travel distance is equal to about 1 hour of warning time

So, as discussed above, there is very little warning time for tsunami generated by local sources, compared to those from distant sources

Response to tsunami

Requires good emergency planning and preparation…

…an educated and trained public…

…which has access to information…

…so the dissemination of this info needs to be efficient and reliable

Personal mitigation

Run (don’t walk) to higher ground

Tell your family and friends

Never go to the beach to watch tsunamis