Chapter 3 – Tsunami
Catastrophe in the Indian Ocean
• Close to 230,000 people killed on December 26, 2004
• Moment magnitude 9.1 quake just off the island of Sumatra along a fault
o The fault ruptured a distance over 1200 km.
o Seafloor slipped as much as 5m vertically and 15m horizontally.
o No warning system in bordering countries.
o Waves took over 7 hours to cross the entire Indian Ocean
• Tsunamis are much more common in the Pacific Ocean than the Indian Ocean
3.1 Introduction to Tsunami
• Several types of events can trigger a Tsunami, including large earthquakes, landslides, or
explosive volcanic eruptions.
• Quakes can cause tsunami by displacing the seafloor or by triggering a large landslide.
• Displacing the seafloor occurs when a block of the Earth’s crust moves rapidly up or
down during an earthquake.
• In general, it takes a quake with a magnitude of 7.5 or over to generate damaging tsunami
• 4 Stages:
o Displacement of seafloor sets in motion oscillatory waves that transmit energy
outward and upward from the source (much like ripples in a pond).
o In the deep ocean, waves are rapid (up to 500km/h) and spaced far part. In the
deep ocean, the spacing between crests of waves can be more than 100 km, and
the height of waves is generally less than 1 m, meaning you would never know a
tsunami is coming in the deep ocean
o As the waves approach land, velocity decreases, and the forward speed may be
about 45 km/h. Decrease in spacing also occurs
o As the first wave approaches water, it forms into a turbulent, surging mass of
water that move inland. Several metres to several tens of metres high.
• Common misconception: Tsunami is only one wave.
• The runup of the tsunami is the maximum horizontal and vertical distances that the
largest wave reaches.
• Edge waves that travel alonndthe rdore interact with tsunami wasts to create a complex
amplification, causing the 2 or 3 waves to be larger than the 1 .
• Distanttsunami or teletsunami travels thousands of kms and strikes remote shorelines. A
local tsunami affects shoreline from the source of the earthquake over 100 km away.
• Landslides on the seafloor or landslides falling from a mountain into a body of water. In
many cases, earthquakes trigger these.
• Landslide tsunami loses energy over distances of thousands of kilometres, and wave run
ups on the west coast of North America would not be catastrophic. 2
• Much less common
• However, second most deadly tsunami in history was caused by huge eruption of
Krakatoa, an active volcano in the Sundra Strait (Indonesia).
o Produced nearly 21km of fragmented rock and ash and destroyed 2/3 of the island
3.2 Regions at Risk
• Some coasts are much more at risk than others
• Coasts in proximity to a major subduction zone, or directly across the ocean basin from a
subduction zone capable of generating M9 earthquakes are at major risk.
• 85% of tsunami have been in Pacific Ocean
o Areas of greatest risk: Japan, Hawaii, Chile, Peru, Mexico, and northeast Pacific
Coast from Alaska to northern California.
o Other areas of risk include eastern Indian Ocean and parts of Mediterranean
• Alaska earthquake on March 27, 1964 was the 3 largest in 20 century and responsible
for tsunami that killed 130 people as far away as California.
3.3 Effects of Tsunami and Linkages with Other Natural Hazards
• Tsunami have both Primary and Secondary effects
o Primary – related to the impact of the onrushing water and its entrained debris,
and to the resulting flooding and erosion.
o Most of the damage to both the landscape and human structures r