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Chapter 3

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Wilfrid Laurier University
Alireza Ghaffari

Risks and Disasters Reading #3 (Module 1) Chapter 3 Catastrophe in the Indian Ocean - December 2004 – countries around Indian ocean o 230 000 people killed, hundreds of thousands injured, millions displaced o Source was from Magnitude 9.1 quake off Sumatra  Third largest in recorded history o Movement of faults produced massive waves that reached Indonesian islands within minutes and other countries later o People were caught by surprise due to lack of tsunami warning o Tsunami took more than 7 hours to cross entire Indian ocean to east African coast o More than of deaths were in Indonesia, which suffered from the earthquake and the tsunami o Sri Lanka, India, Thailand also suffered a great deal - Lesson learned: o Much less common in Indian ocean compared to Pacific ocean o Effective warning systems needed in all countries near ocean basins o Also need emergency protocols that are effective and efficient o Earthquake and tsunami education necessary for people living along coast lines 3.1 Introduction to Tsunami - Tsunami’s are produced by the sudden displacement of ocean, lake or river water - Triggers = large quake, landslide, explosive volcanic eruption, impact in the ocean - Recent damaging tsunami o 1755 – Lisbon – 20 000 people o 1883 – Krakatoa – 36 000 people o 1946 – Hawaiian Islands– 160 people o 1960 – Hawaii – 61 people o 1964 – Alaska/California – 130 people o 1993 – Japan – 120 people o 1998 – Papua New Guinea – 2 100 people o 2004 – Indian Ocean – 230 000 people Earthquake-Triggered Tsunami - Quake can cause tsunami by displacing ground or by triggering landslides o Displacement more common  Block of Earth’s crust moves up or down - Takes M > 7.5 to trigger damaging tsunami - 4 stage Process o Displacement of seafloor transmits outward and upward from source via oscillatory waves  Waves intercept surface and spread outward like ripple o @ Deep ocean, waves move more rapidly and are spaced far apart  Velocity = [(acceleration of gravity)(water depth)]  @ Deep ocean velocity = 720 km/h  Wave crests far apart and only 1 m high – not noticeable o @ Near land, water depth and velocity decrease, space between wave crests decrease, height increases  Speed about 45 km/h o @ Approaching shore  turbulent, surging mass of water – several meters to several tens of meters tall  Destroys everything  Sometimes wave trough arrives first causing ocean to recede - Not one wave  turbulent surges of water - Steep wall can form if one wave overtakes another - Series of waves separated from minutes to hours - Run-up = part of tsunami that = max horizontal/vertical distances that the largest wave reaches traveling to land - Wave returns to open ocean in strong turbulent flow - Tsunami can generate edge waves that travel along shore - Interaction b/w edge waves and later incoming waves can = amplification  2nd wave larger than the first - Distant tsunami: travels thousands of km across open ocean and strikes remote shorelines w/ little loss of energy o Most produced by great subduction - Local tsunami: affects shorelines near source of quake o More deadly b/c arrive fast Landslide-Triggered Tsunami - Landslides @ seafloor OR @ mountain slopes INTO body of water can generate tsunami o Many of the landslides can be triggered by earthquakes (sometimes the quake alone wouldn’t = tsunami) Volcano-Triggered Tsunami - Tsunamis from volcanoes = less common - Second most deadly tsunami in history = eruption of Krakatoa - Smaller tsunamis can be triggered by lahars and pyroclastic flows - Less hazardous as they slow down and decrease in size as distance from volcano increases (unlike quake tsunamis) 3.2 Regions at Risk - All ocean/lake shorelines = at risk - Some coasts at more risk than others b/c of location relative to earthquake, landslides, volcanoes o Coasts close to major subduction zone/directly across the ocean basin from subduction zone capable of generating M9 = greater risk - 85% of tsunamis = @ Pacific Ocean b/c of subduction zones o Areas @ Pacific Basin are at greater risk  Japan, Kamchatka, Hawaii, islands in southern/west Pacific, Chile, Peru, Mexico & northeast Pacific Coast from Alaska to Northern Cali - Japan and Hawaii experience frequent tsunamis 3.3 Effects of Tsunami and Linkages with Other Natural Hazards - Tsunamis have both primary and secondary effects o Primary  related to impact of the onrushing water and its entrained debris, and to the resulting flooding and erosion  Energy can tear up beaches, coastal vegetation, houses, buildings  Effects diminish with distance from coast  Majority of deaths from drowning  deaths also from physical impact o Secondary effects  those that occur in time after event  Fires, polluted water supplies, outbreaks of disease, loss of shelter - Links to natural disasters o Closely linked to offshore earthquakes, landslides, volcanic eruptions, asteroid and comet impacts o Coastline can change through erosion
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