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Tsunamis and Earthquakes

Tsunami Generation from the 2004 M=9.1 Sumatra-Andaman Earthquake

Tsunami Measurements from Space

Most instrumental measurements of tsunamis are from tide gauge stations and bottom pressure recorders in the deep ocean. During the 2004 Indian Ocean tsunami was the first time that satellites collected transects of sea-height data using radar altimetry that clearly showed a tsunami signal during its propagation across the Indian Ocean.

It was by coincidence that the satellites passed over the Indian Ocean at the same time that the first part of the tsunami was propagating from the Sumatra-Andaman source region. Jason-1 yielded the best data to shed light on tsunami generation for this event: this satellite collected almost continuous sea-height data just two hours after the earthquake. Five minutes later, the Topex-Poseidon satellite (a tandem mission with Jason-1) collected intermittant sea-height data of the tsunami. The Envisat and GeoSat Follow-On (GFO) satellites captured sea-height data 3:15 and 7:10 hours after the earthquake, respectively.

Because the ground speed for the Jason-1 satellite is 5.8 km/s, much faster than tsunami propagation, the data can be thought of as a snapshot of tsunami amplitudes along the Jason-1 trackline (red line below). In the computer model of the wavefield shown below, the blue arc represents the wavefront of the distant (far-field) tsunami that emanated directly from the source (i.e., seafloor deformation from movement on the inter-plate thrust). This part of the tsunami wavefield is termed the direct arrival and has almost arrived at India at this time. The smaller amplitude waves that trail behind the blue arc are an effect of dispersion, where shorter wavelength components travel at a slightly slower speed than the wavefront. The green arc represents one of many tsunami refelctions from coastlines and submerged bathymetric features that are also detected and measured by satellite radar altimetry. Jason-1 measured the height of the tsunami wave where its trackline crossed these arcs. To the east, the local tsunami continues to cause major wave action along the Sumatran coast.
jason satellite over Bay of Bengal
Trackline (red line) of the Jason-1 ascending orbit 2 hours after the 2004 Sumatra-Andaman eathquake. The satellite orbits the Earth at an altitude of approximaely 1300 km. Blue arc represents the modeled wavefront (direct arrival) of the outbound tsunami from seafloor displacement caused by the earthquake. Green arc represents one of many tsunami reflections. View to the north.

A more detailed depiction of the different tsunami waves (phases) is shown in the figure and animation below, viewed to the southeast. This simulation has more vertical exaggeration than the previous view to the north in order to reveal individual tsunami phases. Phases d1, d2, and d3 are direct tsunami wave arrivals from the source regions where fault slip occured along the Sumatra-Andaman subduction zone. The phases beginning with the letter r originated as reflections from various coastlines and bathymetric features.

Animation: Simulated view of the tsunami wavefield looking southeast

This animation shows the evolution of tsunami waves caused by the December 26, 2004 earthquake. A detailed depiction of the different tsunami waves (phases) is shown in this animation. Phases d1, d2, and d3 are direct arrivals from source regions located from south to north along the Sumatra-Andaman subduction zone. The phases beginning with the letter r originate as reflections from various coastlines and bathymetric features. The animation ends two hours after the earthquake when the Jason-1 satellite passes through the Bay of Bengal. The Jason-1 trackline is colored pink where the model amplitudes are positive and colored blue where they are negative.

The sea height along the Jason-1 transect is shown below. The high amplitude double peak south of the Equator is primarily the two direct arrivals from the source region (d1 and d2), although reflections from the ninety-east ridge (r1-90e), may also cross the trackline at about the same location as the d2 phase.

Jason sea height profile
Sea height transect from Jason-1 with specific tsunami phases labeled.

A stochastic slip model can be used to represent many different rupture patterns that are consistent with the frequency content observed on seismograms. Shown in the figure below is the vertical displacement of the sea floor for the slip model that best reproduces the double-peak in the Jason-1 satellite altimetry data. Possible source regions that give rise to tsunami phases d1 and d2 are labeled. Other possible sources such as secondary faulting can also explain the double peak.

stochastic vertical displacement map
Map of computed vertical seafloor displacement that best fits the Jason-1 satellite altimetry data. Possible source regions linked to phases d1 and d2 (double peak on satellite altimetry) are labeled. Green/yellow represents uplift. Blue/purple represents subsidence.

Next page, Tsunami Generation ModelingComparison with the March 28, 2005 Tsunami

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