TsunamisTsunamis

BANDA ACEH, INDONESIA: June 23, 2004 BANDA ACEH, INDONESIA: June 23, 2004 A satellite image of the waterfront area of A satellite image of the waterfront area of

Aceh province's capital city before the Aceh province's capital city before the tsunami.tsunami.

BANDA ACEH, INDONESIA: December 28, BANDA ACEH, INDONESIA: December 28, 2004 2004

An image taken after the tsunami shows An image taken after the tsunami shows destroyed housing and the shoreline nearly destroyed housing and the shoreline nearly

wiped out.wiped out.

What is a Tsunami?What is a Tsunami?

When mass movement, such as an When mass movement, such as an earthquake or landslide, suddenly earthquake or landslide, suddenly displaces a large amount of water from its displaces a large amount of water from its equilibrium state a disastrous wave called equilibrium state a disastrous wave called a a tsunamitsunami can form. can form.

Tsunami literally translates from Japanese Tsunami literally translates from Japanese to “harbor wave” but are often call tidal to “harbor wave” but are often call tidal waves because small, distant-source waves because small, distant-source tsunamis resemble tidal surges.tsunamis resemble tidal surges.

Tsunami SourcesTsunami Sources

Earthquakes Earthquakes ((e.g.e.g. Sumatra, 2004: >200,000 Sumatra, 2004: >200,000 people killed; Papa New Guinea, 1998: ~3,000 people killed; Papa New Guinea, 1998: ~3,000 people killed)people killed)

Volcanic eruptionsVolcanic eruptions ( (e.g.e.g. Krakatoa, 1883: Krakatoa, 1883: tsunamis killed 30,000 people; Santorini, 2002).tsunamis killed 30,000 people; Santorini, 2002).

Mass MovementMass Movement ( (e.g. e.g. Alaska, 1958: waves up to Alaska, 1958: waves up to 518 m high formed in Lituya Bay).518 m high formed in Lituya Bay).

Extraterrestrial ImpactsExtraterrestrial Impacts - large impacts have the - large impacts have the potential to create enormous tsunamis.potential to create enormous tsunamis.

Tsunami Earthquake SourcesTsunami Earthquake Sources

Earthquakes that suddenly uplift or down-drop Earthquakes that suddenly uplift or down-drop the sea floor generate tsunamis. the sea floor generate tsunamis.

Generally such surface deformation is largest for Generally such surface deformation is largest for reverse and normal faulting earthquakes, and reverse and normal faulting earthquakes, and small for transform faulting events thus the small for transform faulting events thus the potential for tsunamis is lower for strike slip potential for tsunamis is lower for strike slip faults faults (e.g(e.g. the Balleny earthquake 1998 did not . the Balleny earthquake 1998 did not generate a tsunami). In general tsunami are generate a tsunami). In general tsunami are generated by reversal faults.generated by reversal faults.

Tsunami GenesisTsunami Genesis

Tsunamis are Tsunamis are caused by events caused by events that drastically and that drastically and suddenly shift a suddenly shift a large volume of large volume of water.water.

From Plummer McGeary Carlson

Tsunami EarthquakesTsunami Earthquakes

Some earthquakes have generated very Some earthquakes have generated very large tsunamis for their “size”. These large tsunamis for their “size”. These events are called events are called tsunami earthquakestsunami earthquakes.. Analysis of seismograms from these events Analysis of seismograms from these events

suggest that they are the result of low-suggest that they are the result of low-frequency seismic energy.frequency seismic energy.

These earthquakes present a problem for These earthquakes present a problem for tsunami warning systemstsunami warning systems

Tsunami EarthquakesTsunami Earthquakes

One way to identify these events is to One way to identify these events is to compare Ms to Mwcompare Ms to Mw Ms ~ 20 seconds periodMs ~ 20 seconds period Mw ~ 100-200 seconds periodMw ~ 100-200 seconds period

Since the signals are enriched in long Since the signals are enriched in long periods the magnitude is unusually larger periods the magnitude is unusually larger than the Ms estimate. than the Ms estimate.

Standard Earthquake

M~7.0

Slow-source Tsunami Earthquakemb ~5.8, MS ~7.2, MW~7.7

An earthquake with a big An earthquake with a big vertical component is more vertical component is more “tsunamogenic” than a purely “tsunamogenic” than a purely horizontal event.horizontal event.

“Slow” events with a long “Slow” events with a long duration are also sources of duration are also sources of larger tsunamislarger tsunamis

From E. Okal

Describing Ocean WavesDescribing Ocean Waves Ocean wavesOcean waves are deformations of the sea surface. are deformations of the sea surface. Wavelength: Wavelength: distance between crests (distance between crests ()) Wave height: Wave height: vertical distance between crest and vertical distance between crest and

troughtrough Period: Period: time between 2 successive crests to pass (T)time between 2 successive crests to pass (T)

Describing Ocean WavesDescribing Ocean Waves The deformation propagates with the wave speed while on average The deformation propagates with the wave speed while on average

water remains in the same position (the water does not pile up on the water remains in the same position (the water does not pile up on the beach).beach).

Water moves in the propagation direction at the crest while moving in Water moves in the propagation direction at the crest while moving in the opposite direction at the through.the opposite direction at the through.

Water of a deep-water wave moves in a circular orbit on a circle Water of a deep-water wave moves in a circular orbit on a circle which diameter is decreasing downward. The motion become which diameter is decreasing downward. The motion become negligible at a depth of ~ half wavelength.negligible at a depth of ~ half wavelength.

Describing Ocean WavesDescribing Ocean Waves Energy moves in the propagation direction.Energy moves in the propagation direction. Most ocean waves are produced by wind bringing the Most ocean waves are produced by wind bringing the

energy from the wind offshore toward the coast.energy from the wind offshore toward the coast. The rate at which a wave loses its energy is inversely The rate at which a wave loses its energy is inversely

related to its wavelength. Long-wavelength waves related to its wavelength. Long-wavelength waves can travel further.can travel further.

Describing Ocean WavesDescribing Ocean Waves Deep water waves Deep water waves are surface waves.are surface waves. Deep Water:Deep Water: the water depth where a wave passing the water depth where a wave passing

overhead is not discernable at the sea bed.overhead is not discernable at the sea bed. Deep Water Waves: Deep Water Waves: the wavelength is < 1/2 Water the wavelength is < 1/2 Water

depth (D)depth (D)

Describing Ocean WavesDescribing Ocean Waves Wind Waves:Wind Waves: T~ 10-20s T~ 10-20s ~10-600m~10-600m Deep Water Velocity:Deep Water Velocity: v= v=/T (v~1-30m/s)/T (v~1-30m/s) The speed of deep water waves depends on The speed of deep water waves depends on

wavelength, deep water waves are wavelength, deep water waves are dispersivedispersive.. Shallow Water VelocityShallow Water Velocity: :

v g2

tanh2D

dL

20cgD

Describing Ocean WavesDescribing Ocean Waves Shallow Water Velocity:Shallow Water Velocity:

The shallow water velocity does not depend on wavelength. The shallow water velocity does not depend on wavelength. Shallow water wavesShallow water waves do not show dispersion. do not show dispersion.

As the wave approaches shallow water the shape of the As the wave approaches shallow water the shape of the motion becomes more elliptical and the velocity slows down. motion becomes more elliptical and the velocity slows down. To conserve energy the wave rises higher. To conserve energy the wave rises higher.

v gD

Describing Ocean WavesDescribing Ocean Waves Tsunami Wave: Tsunami Wave: T~3600 s T~3600 s ~800 km ~800 km Since the ocean has an average depth of 5 km it is Since the ocean has an average depth of 5 km it is

always a shallow water wave, the velocity is increasing always a shallow water wave, the velocity is increasing with ocean depth. (friction with the bottom lower) with ocean depth. (friction with the bottom lower)

Typical tsunami wave velocity (water depth 5000m) Typical tsunami wave velocity (water depth 5000m) v~220 m/s = 792 km/hr (cruise velocity Jumbo 747 v~220 m/s = 792 km/hr (cruise velocity Jumbo 747 ~800km/hr) ~800km/hr)

v gD

Describing Ocean WavesDescribing Ocean Waves Tsunami Wave: Tsunami Wave: T~3600 s T~3600 s ~800 km ~800 km Since the long-wavelength waves lose less energy a tsunami Since the long-wavelength waves lose less energy a tsunami

can travel transoceanic distances with only limited energy loss.can travel transoceanic distances with only limited energy loss. In the deep ocean the amplitude of a tsunami is a few cm to few In the deep ocean the amplitude of a tsunami is a few cm to few

dm on a very long wavelength:dm on a very long wavelength: it is not felt aboard a ship or it is not felt aboard a ship or seen from air in open ocean seen from air in open ocean (but can be measured by buoy or (but can be measured by buoy or satellite altimeter).satellite altimeter).

When a tsunami approaches the shoreline the velocity When a tsunami approaches the shoreline the velocity decreases (D diminish) and in order to conserve energy decreases (D diminish) and in order to conserve energy (proportional to v and H) the amplitude increases.(proportional to v and H) the amplitude increases.

HD12

HD22

vD2

vD1

gD2

gD1

From UNESCO/PTWC tsunami booklet

An ExampleAn Example Tsunami Wave Tsunami Wave Example: Sumatra 2004 Example: Sumatra 2004 How long does it take to get to Sri Lanka?How long does it take to get to Sri Lanka?

Distance ~1600 kmWater Depth ~4000 m

T= 2000/713=2.2 hr

v gD 9.8* 4000 198m

s713

km

hr

An ExampleAn Example Tsunami Wave Tsunami Wave Example: Sumatra 2004 Example: Sumatra 2004 How long to get to Thailand?How long to get to Thailand?

Distance ~500 kmWater Depth ~1500 m

T= 500/430=1.1 hr

v gD 9.8*1500 120m

s430

km

hr

An ExampleAn Example Tsunami Wave Tsunami Wave Example: Sumatra 2004 Example: Sumatra 2004 ““Correct” numerical model using observed source and Correct” numerical model using observed source and

high definition bathymetry of the front propagationhigh definition bathymetry of the front propagation

Courtesy: K. Satake,

unpublished

An ExampleAn Example Tsunami Wave Tsunami Wave Example: Sumatra 2004 Example: Sumatra 2004 How high is the wave?How high is the wave?

HD12

HD22

vD2

vD1

gD2

gD1

1

1

2

HD12

HD22

0.62

HD22

gD2

gD1

9.8 *10

9.8 * 4000 HD2 7.6m

NOAA

Recognize fraudulent images - Recognize fraudulent images - Don’t be fooledDon’t be fooled