lecture 18: tsunamis

how are tsunamis generated

the sudden displacement of large volumes of water

how does a tsunami travel

a series of waves are generated

initial wave heights - earthquakes

15m

open ocean heights

1-2 m

how fast does a tsunami travel

velocity of a tsunami is directly linked to water depth - because tsunami waves drag on the sea bottom across the entire ocean

speeds in deep ocean

900 km/h

speeds on continental shelf

100-200 km/h

time between tsunami waves

~15-25 minutes

as tsunamis approach the shore

water depths decrease and so drag against the sea bottom increases (lows down the wave)

increase of initial ocean wave height

by 6 times in shallow water and funnelling of waves in bays can have exactly the same effect

precursory trough

commonly seen as a rapid withdrawal of the sea

what can a trough cause

grounding of boats, exposure of fish on the beach, etc, and may carry people and debris out to sea

run up height

maximum vertical height above sea level reached by a tsunami on shore -> can be much higher than initial wave height

inundation

maximum horizontal distance travelled inland by the tsunami from the normal shoreline -> can be up to several km

what do run up and inundation depend on

1. initial wave height

2. local bathymetry and topography

3. wave interference

Chile 1960 (earthquake-triggered tsunami)

• M 9.5

• 3 main tsunami waves strike

• successive heights of 4.5m, 8m and 10.7m.

• 1,655 killed

• 3,000 injured

• 2 million homeless

• $550 million damage in southern Chile

Hawaii - Chile 1960

• 10,000 km from epicentre

• 3rd wave at Hilo Harbour was 4m high

• killed 61 people and badly injured 282

• $75 million damage in Hawaii.

Japan - Chile 1960

• main tsunami waves preceded by large troughs

• 5 waves over 6 hours - last was over 5 m high

• 138 deaths and $50 million damage in Japan

• also 32 dead/missing in the Philippines and $500,000 damage to the west coast of the US

 

Sumatra 2004

• M9.15 earthquake

• offset of the sea bed for 1,200 km

• 5m high tsunami moving at 700km/hr hit Sumatran coast

• then hit provincial capital of Banda Aceh - up to 8 km inland

• wiped out 25 km² of the city of Banda Aceh

• 185,000-230,000 people killed and 1.74 Million displaced in 14 countries. > US$14 billion (2004) in humanitarian aid donated

Sumatra 2004 - coastal towns and villages

• Lhoknga subsided by 1.5m during earthquake

• only the best-constructed buildings, e.g. mosques, survived

Krakatau 1883 - volcano triggered tsunami - volcano

eruption dominated by andesite, but may also have involved some basalt and dacite

explosion that was heard up to 2,250 miles away. The climactic 1883 explosion created an atmospheric shockwave that circled the globe 4 times

Krakatau tsunami

• with run-ups up to 40m high

• killing ~30,000

• many ships in the region were transported and stranded several kilometres inland

• Krakatau tsunami was recorded in tidal gauges in UK

submarine landslide-triggered tsunamis

seismic reflection mapping of the subsurface of the North Sea highlighted the presence of a submarine landslide with a jumble of chaotic blocks

Norway submarine landslide - Storegga slide

• moved in 3 events: 30000BP, 6000BP and 7950BP

• constituting 5,580 km³ of debris

• internal slide blocks can be as large as 10 km x 30 km x 200 m

• trigger may have been offshore earthquake, pore pressures or gas hydrates

the Canary islands - la Palma

suspected to be unstable and prone to collapse à Future Giant Landslide + Tsunami?

asteroid impact triggered tsunamis

Asteroid impact that generated the 200 km wide Chicxulub crater, Mexico

Implicated in the K-T extinction event (‘Dinosaur Killer’)

Tohoku Japan 2011

• M 9.1

• GPS shows up to 35 m (80 m?) eastward movement in ~ 3 mins

• tsunami run-up height of 39 m

• inundation of 10 km in Sendai total inundation area 561 km²

• Confirmed fatalities: 15,894 (still missing: >2,500)

• 120,000 buildings destroyed, > 1 Million partly destroyed

• Total economic cost $235 billion

tsunami risk mitigation

1. Reducing the hazard, value or vulnerability

2. Increasing preparedness

Fukushima Daiichi nuclear accident 11 March 2011

14-metre high tsunami disabled the cooling system of three Fukushima Daiichi reactors.

All three cores largely melted in the first three days

Sea wall built only for 5 m high

reason for the Fukushima accident

tsunami flooded and disabled back-up power generators for pumps

radioactive contamination

3 of the Fukushima Daiichi reactors exploded

deaths

no deaths or cases of radiation sickness, but >100,000 people evacuated