Tsunamis

BANDA ACEH, INDONESIA: June 23, 2004 A satellite image of the waterfront area of Aceh province's capital city before the tsunami.

BANDA ACEH, INDONESIA: December 28, 2004 An image taken after the tsunami shows destroyed housing and the shoreline nearly wiped out.

What is a Tsunami?  When mass movement, such as an earthquake or landslide, suddenly displaces a large amount of water from its equilibrium state a disastrous wave called a tsunami can form.  Tsunami literally translates from Japanese to “harbor wave” but are often call tidal waves because small, distant-source tsunamis resemble tidal surges.

Tsunami Sources  Earthquakes (e.g. Sumatra, 2004: >200,000 people killed; Papa New Guinea, 1998: ~3,000 people killed)  Volcanic eruptions (e.g. Krakatoa, 1883: tsunamis killed 30,000 people; Santorini, 2002).  Mass Movement (e.g. Alaska, 1958: waves up to 518 m high formed in Lituya Bay).  Extraterrestrial Impacts - large impacts have the potential to create enormous tsunamis.

Tsunami Earthquake Sources  Earthquakes that suddenly uplift or down-drop the sea floor generate tsunamis.  Generally such surface deformation is largest for reverse and normal faulting earthquakes, and small for transform faulting events thus the potential for tsunamis is lower for strike slip faults (e.g. the Balleny earthquake 1998 did not generate a tsunami). In general tsunami are generated by reversal faults.

Tsunami Genesis  Tsunamis are caused by events that drastically and suddenly shift a large volume of water. From Plummer McGeary Carlson

Tsunami Earthquakes  Some earthquakes have generated very large tsunamis for their “size”. These events are called tsunami earthquakes. Analysis of seismograms from these events suggest that they are the result of low- frequency seismic energy. Analysis of seismograms from these events suggest that they are the result of low- frequency seismic energy. These earthquakes present a problem for tsunami warning systems These earthquakes present a problem for tsunami warning systems

Tsunami Earthquakes  One way to identify these events is to compare Ms to Mw Ms ~ 20 seconds period Ms ~ 20 seconds period Mw ~ seconds period Mw ~ seconds period  Since the signals are enriched in long periods the magnitude is unusually larger than the Ms estimate.

Standard Earthquake M~7.0 Slow-source Tsunami Earthquake m b ~5.8, M S ~7.2, M W ~7.7 An earthquake with a big vertical component is more “tsunamogenic” than a purely horizontal event. “Slow” events with a long duration are also sources of larger tsunamis From E. Okal

Describing Ocean Waves  Ocean waves are deformations of the sea surface.  Wavelength: distance between crests ( )  Wave height: vertical distance between crest and trough  Period: time between 2 successive crests to pass (T)

Describing Ocean Waves  The deformation propagates with the wave speed while on average water remains in the same position (the water does not pile up on the beach).  Water moves in the propagation direction at the crest while moving in the opposite direction at the through.  Water of a deep-water wave moves in a circular orbit on a circle which diameter is decreasing downward. The motion become negligible at a depth of ~ half wavelength.

Describing Ocean Waves  Energy moves in the propagation direction.  Most ocean waves are produced by wind bringing the energy from the wind offshore toward the coast.  The rate at which a wave loses its energy is inversely related to its wavelength. Long-wavelength waves can travel further.

Describing Ocean Waves  Deep water waves are surface waves.  Deep Water: the water depth where a wave passing overhead is not discernable at the sea bed.  Deep Water Waves: the wavelength is < 1/2 Water depth (D)

Describing Ocean Waves  Wind Waves: T~ 10-20s ~10-600m  Deep Water Velocity: v= /T (v~1-30m/s)  The speed of deep water waves depends on wavelength, deep water waves are dispersive.  Shallow Water Velocity:

Describing Ocean Waves  Shallow Water Velocity:  The shallow water velocity does not depend on wavelength. Shallow water waves do not show dispersion.  As the wave approaches shallow water the shape of the motion becomes more elliptical and the velocity slows down. To conserve energy the wave rises higher.

Describing Ocean Waves  Tsunami Wave: T~3600 s ~800 km  Since the ocean has an average depth of 5 km it is always a shallow water wave, the velocity is increasing with ocean depth. (friction with the bottom lower)  Typical tsunami wave velocity (water depth 5000m) v~220 m/s = 792 km/hr (cruise velocity Jumbo 747 ~800km/hr)

Describing Ocean Waves  Tsunami Wave: T~3600 s ~800 km  Since the long-wavelength waves lose less energy a tsunami can travel transoceanic distances with only limited energy loss.  In the deep ocean the amplitude of a tsunami is a few cm to few dm on a very long wavelength: it is not felt aboard a ship or seen from air in open ocean (but can be measured by buoy or satellite altimeter).  When a tsunami approaches the shoreline the velocity decreases (D diminish) and in order to conserve energy (proportional to v and H) the amplitude increases.

From UNESCO/PTWC tsunami booklet

An Example  Tsunami Wave Example: Sumatra 2004  How long does it take to get to Sri Lanka? Distance ~1600 km Water Depth ~4000 m T= 2000/713=2.2 hr

An Example  Tsunami Wave Example: Sumatra 2004  How long to get to Thailand? Distance ~500 km Water Depth ~1500 m T= 500/430=1.1 hr

An Example  Tsunami Wave Example: Sumatra 2004  “Correct” numerical model using observed source and high definition bathymetry of the front propagation Courtesy: K. Satake, unpublished

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

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