Tsunami Travel Time Maps
NGDC, as the World Data Service for Geophysics (including Tsunamis), and the UNESCO-IOC International Tsunami Information Center, collaborate to provide tsunami travel time maps.
Tsunami Source Maps
Estimated travel times for historical tsunamis based on earthquake epicenters
Coastal Locations Maps
Estimated tsunami travel times from any point in the ocean to selected coastal locations
Maps were generated using Tsunami Travel Times (TTT) software (see below) developed by Paul Wessel, Geoware. TTT software calculates first-arrival travel times on a grid for a tsunami generated at a given earthquake epicenter or coastal location.
- Maps do not provide information on the height or the strength
of the wave, only the arrival times
- There are several situations in which the estimated arrival times may not match observed arrival times of the tsunami waves, including but not limited to the following:
- Bathymetry is not accurate in the vicinity of the epicenter
- Epicenter is not well located, or its origin time is uncertain
- Epicenter is on land and a pseudo-epicenter off the coast must be selected
- Bathymetry is not accurate in the vicinity of the reporting station
- Nonlinear propagation effects may be important in shallow water
- Observed travel times do not represent the first wave but instead are later arrivals
Tsunami Travel Time Software
The technique to compute travel times is an application of Huygen's principle. It states that all points on a wavefront are point sources for secondary spherical waves. Minimum travel times are computed over the grid starting at the point of interest (e.g. earthquake epicenter). From the starting point, times are computed to all surrounding points. The grid point with minimum time is then taken as the next starting point and times are computed from there to all surrounding points. The starting point is continually moved to the point with minimum total travel time until all grid points have been evaluated. This technique is explained in Shokin, et al., 1987, Calculations of tsunami travel time charts in the Pacific Ocean, Science of Tsunami Hazards, vol. 5, p. 85-113.
Tsunamis are categorized as long waves, therefore, tsunami travel times can be computed with water depth as the only variable (Murty, T.S., Seismic Sea Waves Tsunamis, Bulletin 198) Long waves are where the distance between wave crests is much greater than the water depth through which the wave is traveling. Wave speed is computed from the square root of the quantity: water depth times the acceleration of gravity. So, tsunami travel times can be computed without any knowledge of the tsunami's height, wavelength, etc.