GEOG 1350 Chapter 4: CHAPTER 4

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16 Aug 2016
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GEOG1350 Katy Lemaire
September 29, 2015
CHAPTER 4- TSUNAMIS
4.1 Introduction to Tsunamis
-series of waves caused by a large displacement of water that have extremely long wave lengths
-events that can trigger tsunamis are large earthquakes, landslides, explosive volcanic eruption
or an impact in the ocean (comet, meteor)
Earthquake triggered tsunamis: takes an earthquake of a magnitude of 7.5 or higher to
generate a damaging tsunami, upward or downward movement of the sea floor
displaces the overlying water and initiates a 4 stage process
(1) Displacement of sea floor sets in motion oscillatory waves that transmit energy
outwards and upwards from the source
(2) In the deep ocean, the waves move rapidly and are spaced apart.
(3) As the tsunami nears land, both the water depth and the velocity of the tsunami
decreases, the decrease in velocity is accompanied by a decrease in spacing between
waves
(4) As the first tsunami wave approaches shore, transforms into a turbulent surging mass of
water which rapidly moves in land. Generally, its onrushing surges of debris water. The
run up of a tsunami is the maximum horizontal and vertical distances that the largest
wave reaches at it travels inland. A distant tsunami travels thousands of kilometers
across the open ocean and strikes remote shorelines with little loss of energy. A local
tsunami affects shorelines near the source of an earthquake
Landslide Triggered Tsunamis: the landslide falls from mountains into large bodies of
water, these landslides are sometimes triggered by earthquakes. Most prominent
example is Lituya Bay in Alaska. The rockslide plunged into the bay and displaced sea
water that ran up the opposite valet wall to an elevation of 525m, destroying all forestry
in the way.
Volcanic Triggered Tsunamis: much less common, second most deadly tsunami in
history (Krakatoa) occurred between 2 Indonesian islands, smaller tsunamis can be
triggered by large volcanic mudflow that enter the sea during explosive eruptions.
4.2 Regions at Risk
-coasts in proximity to major subduction zones
-about 85% of tsunamis have been in the Pacific Ocean with the greatest risks in areas such as
Japan, Hawaii, Chile, Peru, Mexico, northeast pacific coast from Alaska to northern California.
-parts of the Mediterranean and Indian ocean as well
-tsunamis generated by underwater landslides are less common but represent a risk to east and
west coasts of the United States and Canada
-research involves excavation and radio carbon dating of tsunami deposits, dating the rings of
trees killed by tsunamis, computer modelling of propagation and study of historical records
-study of the earthquake in Japan in 1700 at the Cascadia subduction zone
4.3 Effects of Tsunamis and Links with Other Natural Hazards
**ways damage can be reduced:
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