Importance of Bathymetry from Space in Tsunami Hazard Forecasts


On Sunday evening June 9 1996 at 8:03 pm Alaska Daylight Time (0403 UTC on June 10) a magnitude 7.9 earthquake struck the Andreanov Islands of Alaska.  The earthquake generated a tsunami which was more than 1 meter high at Adak Island, Alaska, and more than 0.5 meter high at Kahului, Hawaii.  While this tsunami posed little danger to humans, it was large enough to be well-recorded by tide gauges and bottom pressure gauges, permitting scientists to model its travel across the Pacific Ocean.  A computer model of the tsunami's travel is visualized in this movie.


tsunami movie


Tsunamis travel across oceans as "shallow water waves", meaning that the tsunami feels the effect of the bottom shape in even the deepest parts of the ocean.  Seamounts on the ocean floor thus have a great influence in directing where the tsunami energy goes, and when it will arrive at a coast.  As the movie makes clear, ocean floor topography has three effects that must be understood and modeled to make more effective hazard assessments and warnings:

(1) the tsunami energy doesn't take a direct path from the earthquake to coastal areas;

(2) the first arriving wave may not be the most energetic one;

(3) two nearby coastal communities can receive very different amounts of tsunami energy.

Further research has shown that detailed seafloor topography from satellite altimetry (Smith and Sandwell, Science, 277 (5334), 1956-1962, 1997) is necessary for calculating the scattering of tsunami energy by seamounts (Mofjeld et al, Geophys Res Lett, 28 (2), 335-337, 2001; Mofjeld et al, Oceanography, 17 (1), 38-46, 2004).  A "tsunami scattering index" based on bathymetry from space is part of the SIFT (Short-term Inundation Forecast for Tsunamis) system now being implemented in the Pacific warning centers.  This tsunami scattering index could be more reliable with a new bathymetry from space mission, which would locate many more seamounts and give more reliable estimates of seamount summit depths, a key parameter in the tsunami scattering index.