How are tsunami generated?
Why is this important for insurers?
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Associated damaging phenomena: earthquakes
(subsidence), landslides, volcanic eruptions, impacts
Spatial distributions vary between tsunami sources
Frequency - magnitude distributions vary between
different tsunami sources
Hence:
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Different regions have different tsunami frequency magnitude distributions
Generation of tsunami by
earthquakes
Earthquakes cause tsunami directly by sudden
uplift and/or subsidence of the ocean or sea floor
above the fault rupture
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Some earthquakes are much more efficient at
generating tsunami than others
surface - rupturing earthquakes are often
tsunamigenic: deep earthquakes are not
rare “tsunami earthquakes” are thought to be near
surface events with low rupture velocity
Earthquake fault geometry and
tsunami generation potential
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Strike - slip earthquakes (horizontal movements)
generate almost no tsunami (except indirectly)
Normal and reverse fault earthquakes produce
moderate tsunami
Most large tsunami generated directly by
earthquakes are produced at subduction zone plate
boundaries (e.gs Alaska, Peru-Chile, Japan, Lesser
Antilles)
Tsunami generation at subduction
zone plate boundaries
Why is it so important?
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Very large earthquakes
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offshore uplift produces large waves
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impact of tsunami exacerbated by semi - permanent
onshore subsidence (72 hour rule problems!)
Have we seen the largest directly
earthquake - generated tsunami?
The size of tsunami DIRECTLY caused by earthquakes is
related to earthquake magnitude:
The largest possible tsunami generated in this way is not
much larger than the largest observed tsunami (e.g. Chile
1960).
Landslide - generated tsunami
Ocean trench slope failures
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associated with subduction zones
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earthquake - triggered
Fjord - side rockfalls and landslides
Continental slope and river delta front sediment
failures
Volcano landslides
Trench slope failures
Especially at continental margins where
sediments accumulate rapidly
Volumes up to 1000 - 2000 cubic kilometres
Historical examples:
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< 100 cubic kilometres
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Sanriku coast, 1933
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Sissano, 1998 (waves up to 15 m high)
Fjord landslide tsunami
Common in Alaska, Norway, Chile, New Zealand
Large runups due to confined space
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Valdez inlet, Alaska, 28/03/1964: 67 m (triggered by
1964 earthquake)
Lituya Bay, Alaska, 09/07/1958: 450 m (partly caused
by glacier collapse)
Of local importance except for high value
facilities.
Continental slope sediment failures
Passive continental margins (e.g. Atlantic)
slope angles 0.5 - 5 degrees: inefficient
volumes up to 20 000 cubic kilometres
Examples:
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Grand Banks 1929
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Storegga (7000 - 8000 years ago)
Volcano landslides
second major source of tsunami deaths
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around 25% of total, including Krakatoa deaths
Large blocks sliding on steep slopes are efficient
tsunami sources
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velocities up to 100 metres / second
avalanche deposits up to hundreds of kilometres
offshore
Usually (NOT always) triggered by eruptions
Frequent small collapses of island
stratovolcanoes
0.1 - 10 cubic kilometres volume
concentrated around Pacific Rim and in
Caribbean
historic examples:
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Unzen (Japan) 1792 (old lava dome collapsed into
Kagoshima Bay)
Ritter Island (New Guinea) 1888 (15 m tsunami 100
km away; 4 m tsunami 600 km away)
Rare giant collapses of oceanic
island volcanoes
occur in all major oceans
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Hawaii, Canary Islands, Reunion, Cape Verde
Islands
Volumes 100 to 2000 cubic kilometres
Giant transoceanic tsunami
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more than 50 m high on distant coastlines
Major source of tsunami hazard in Atlantic
Tsunami propagation I
Propagate across entire ocean basins without loss
of energy
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only hazard (except global catastrophes) to operate
at distances of 10 000 km + from source
Cause simultaneous losses on opposite sides of
the same ocean
Tsunami propagation from the 1960
Chile tsunami (Satake 1964)
How do tsunami propagate so
efficiently across oceans?
In deep oceans tsunami waves have:
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long wavelength (50 - 1000 km)
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high velocity (200 - 300 m/s: 500 - 750 mph)
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small amplitude (< 1 m even for very large tsunami)
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minimal energy losses
In shallow water they slow down and:
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wavelength decreases
amplitude increases (height X water depth constant
for any one tsunami)
Tsunami propagation II
Long - distance vs. local tsunami hazards.
Example: Hawaii
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< 20% of tsunami recorded in the Hawaiian islands
are generated locally
Only 2 locally - generated tsunami have caused major
damage in the past 150 years
Earthquake - generated tsunami hazard in Hawaii
correlates with earthquake hazard around the Pacific
rim
Tsunami propagation III
Impact - area effects: interaction of tsunami with
submarine slopes
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Refraction: bending of waves passing over
submarine topography
Interference: collision of waves reflected from
coastlines
Resonances: in harbours and enclosed bays
Patterns of tsunami refraction
around bays and headlands
Refraction over a canyon
and into a bay
shoreline
wave ray path
Refraction at a headland
shoreline
submarine contour
Impact - area effects cause wide
local variations in tsunami hazard
Places kilometres to tens of kilometres apart have
very different wave heights and runups in the same
event:
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Flores, Indonesia: refraction leading to wave collision
and large runups on REAR of circular island (1992)
Hilo bay, Hawaii: constructive interference (1946,
1960)
Alberni inlet, British Columbia: resonance (1964)
Distribution of runups around Hawaii in tsunami
of 1st April 1946
Maximum runups:
16 m (Hawaii);
11 m (Maui)
variation due to:
resonance (Hilo)
refraction (Maui,
Molokai)
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Direction of tsunami arrival influences impact area effects
How far does tsunami inundation
extend inland? I
Runup: maximum height above water line
attained by tsunami inundation on steep coasts:
strongly dependent upon local topography
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Local tsunami magnitude: log2 of maximum runup
maximum runup typically a few times wave height in
shallow water
How far do inundation zones extend
inland? II
“Run-in”: Maximum distance of inundation
inland from water line on flat coastal areas
For any one initial wave height h0
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strongly dependent on local topography
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strongly dependent on surface roughness
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trees, buildings, rough lava flow tops all greatly
reduce inland extent of inundation.
Run-in typically 20 - 100 times runup on adjacent
steep coasts
How far do inundation zones extend
inland? III
m axim um inundation
distance (m )
Run-in as a function of h0 and roughness
100000
10000
n=0.015
1000
n=0.035
100
n=0.07
10
1
5
10
20
30
w ave height (runup) (m )
50
Damaging effects of tsunami:
Hydrodynamic forces
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velocities of breaking waves 10 - 30 m/s (up to 100 m/s in giant
tsunami?): erosion of surfaces beneath (scouring)
Floating
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of buoyant objects (wooden houses float in as little as 1 - 2 m of
water)
Impacts
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of transported objects (small ships dumped up to 2 - 3 km
inland by major tsunami)
Tsunami damage mechanisms
Impacts
Scouring
Types of property especially
vulnerable to tsunami
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Port facilities (piers, oil tanks, warehouses, railyards)
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Ships and boats in harbour (NOT in the open sea)
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Beachfront housing and hotels
Concentrations of value-at-risk themselves increase
damage due to tsunami
Global size - frequency distribution
of tsunami events
Tsunami frequency magnitude distribution
for different tsunami
sources
(very schematic: order
of magnitude uncertainty
exists at frequencies
< 1 / 1000 years)
Tsunami magnitude: log scale
Estimated risk associated with tsunami
compared to other natural hazards
Multiply deaths per event by $10000 to $100 million to obtain typical economic
losses
Tsunami fatality rates by region (C / Med = Caribbean, Mediterranean)
6
All natural disasters
Transportation of terrestrial origin
accidents
5
4
Fatality rate
(deaths /
year: log 10 3
scale), worldwide except 2
as indicated
Pacific
1 / SE Asia
C / Med
0 Atlantic
-1
0
1
Climate - controlled
collapses after global
warming?
Impacts
Volcanic
winter ?
2
3
6
4
5
7
Deaths per event
(logarithmic-10 scale)
8
9
World population
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10