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Tsunamis & Earthquakes

Tsunami Generation from the 2004 M=9.2 Sumatra-Andaman Earthquake

Comparison with the March 28, 2005 Tsunami

Three months after the devastating 2004 Indian Ocean tsunami, a M=8.6 earthquake occcured offshore northern Sumatra. Although there was intense strong ground shaking and heavy damage associated with this earthquake, the tsunami was much less than expected. We can understand the factors that influence tsunami severity by comparing this event with the 2004 Sumatra-Andaman earthquake and other earthquakes along the Sunda subduction zone.

There are three primary factors influence the variation in local tsunami severity:

Each of these factors are described in more detail below. Animations of both the 2004 and 2005 events show how these factors combine to produce very different tsunamis.

Earthquake Magnitude

Below, we plot local tsunami intensity as a function of earthquake magnitude (M) for a number of tsunamis that have occurred in the past century. Recent tsunamis generated by earthquakes along the Sunda subduction zone are indicated by the yellow stars. In general, tsunami size increases with earthquake magnitude, although there is significant variation in this relationship.

earthquake  magnitude vs. tsunami magnitude
Earthquake magnitude versus local tsunami intensity for subduction zone earthquakes from 1896-2005.

The size of 2004 Indian Ocean local tsunami is consistent with the size of local tsunamis generated by other earthquakes of similar magnitude, for example the 1964 Great Alaska earthquake and tsunami. The 2005 northern Sumatra earthquake and, to some extent, the 2007 southern Sumatra earthquake (M=8.5) are deficient in terms of the local tsunamis produced. In contrast, the 2006 Java earthquake produced a larger than expected tsunami and is among a class of earthquakes called tsunami earthquakes.

Fault Slip Near Trench

If slip during an inter-plate thrust earthquake occurs near the oceanic trench marking where the fault intersects the sea floor, three things happen to increase local tsunami severity: (1) rupture can break through to the trench, increasing slip on the fault; (2) vertical displacement will be greater because of the shallow fault depth below the sea floor; and (3) , there will be greater shoaling amplification of the tsunami (see Panel 3 of Life of a Tsunami) because the vertical displacement is beneath deeper water.

For the 2004 Sumatra-Andaman earthquake, the rupture started at the epicenter and spread throughout the region indicated by the finite fault model of the southern part of the rupture zone derived from global recordings of the earthquake. Analysis of the satellite altimetry data indicates that the southern rupture zone most likely extended to the Sunda Trench and broke through the sea floor as surface faulting. Holding all other parameters of the earthquake constant, a tsunami generated by a sea-floor rupture is greater than one generated by an earthquake that does not rupture the sea floor (i.e., imbedded faulting). The graph below shows a comparison of the vertical displacement profiles for the two cases.

vertical displacement for surface faulting and imbedded faulting
Comparison of vertical displacement profiles for the case of imbedded faulting (solid line) and surface faulting (dashed line).

In addition, if vertical displacement of the sea floor occurs beneath deep water, then the tsunami become greater as it travels toward shore, owing to shoaling amplification.

In the figure below, we show the difference between the initial tsunamis generated from the December 2004 (top) and March 2005 (bottom) caused by differences in earthquake magnitude as well as how close to the trench the fault ruptured.

Comparison of 2004 and 2005 initial tsunamis
Comparison of the 2004 Sumatra-Andaman (top) and 2005 northern Sumatra (bottom) earthquakes in terms of the initial tsunami wavefiel. View to the northwest.

Because most of the rupture area for tsunami earthquakes occurs near oceanic trenches (for example, see the Java 2006 finite fault model), tsunami runup is consistently higher for tsunami earthquakes compared to typical inter-plate thrust earthquakes as shown in the figure above.

Tsunami Beaming

Tsunami beaming refers to the higher tsunami amplitudes in a direction perpendicular to fault orientation during open-ocean propagation. Although tsunami waves are often described as waves spreading out in all directions (like when you throw a pebble into a pond), for long earthquake ruptures, tsunami amplitudes are greater along the azimuth of tsunami beaming. Because the oceanic trench of subduction zones marks the orientation of inter-plate thrust, the tsunami beaming azimuth is also perpendicular to the orientation of the trench. Complexity of the earthquake source and refraction/scattering during propagation will affect the tsunami beaming pattern.

The tsunami beaming plots shown below are constructed by determining the maximum tsunami amplitude over four hours of propagation time. For the 2004 Sumatra-Andaman earthquake, the tsunami beaming pattern is particularly important for explaining high tsunami runup in Sri Lanka and the western coast of the Malay Peninsula.

2004 tsunami beamingTsunami beaming pattern associated with the 2004 Sumatra-Andaman earthquake. Lighter colors represent higher open-ocean tsunami amplitudes.

For the 2005 northern Sumatra earthquake, tsunami beaming is directed south of Sri Lanka and essentially blocked by the island of Sumatra toward the Malay Peninsula.

2005 tsunami beaming
Tsunami beaming pattern associated with the 2005 northernSumatra earthquake. Lighter colors represent higher open-ocean tsunami amplitudes.

Animations

Another way to compare the open-ocean tsunamis between these two earthquakes is to look at the tsunami wavefield for each earthquake, as shown in the snapshots and animations below. In particular, note that the open-ocean wave length for the 2004 tsunami is shorter than for the 2005 tsunami. Because the 2005 tsunami was primarily generated in shallow water, as the wave travels into the deep ocean, it lengthens and decreases in amplitude (the reverse of shoaling amplification).

2004 tsunami wavefield at 1 hour
Tsunami wavefield for the 2004 Sumatra-Andaman earthquake 1 hour after generation. View to the northwest.
See the animation (2.3 MB)

2005 tsunami wavefield at 1 hour
Tsunami wavefield for the 2005 northern Sumatra earthquake 1 hour after generation. View to the northwest.
See the animation (0.7 MB)

Below are other snapshots and animations of the 2004 tsunami from different view points.

2004 tsunami wavefield at 1 hour
Tsunami wavefield for the 2004 Sumatra-Andaman earthquake 1 hour after generation. View to the north.
See the animation (3.7 MB)

2004 tsunami wavefield at 1 hour
Tsunami wavefield for the 2004 Sumatra-Andaman earthquake 1 hour after generation. View to the east.
See the animation (2.2 MB)

Next page, Tsunami Generation ModelingReferences

 


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