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.
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.