Maps from Tsunami Sources

### Tsunami Travel Time Software

The Tsunami Travel Times (TTT) software was developed by Paul Wessel, Geoware. The TTT software calculates first-arrival travel times on a grid for a tsunami generated at a given earthquake epicenter.

The technique used by the software to compute travel times over an entire grid 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, Y.I., L.B. Chuborav, V.A. Novikov, and A.N. Sudakov, Calculations of tsunami travel time charts in the Pacific Ocean - models algorithms, techniques, results, 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, Department of Fisheries and the Environment, Fisheries, and Marine Service, Ottawa, 336 p.) Long waves are those in which the distance between crests of the wave 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.

There are several situations in which the predicted estimated times of arrival may not match observed arrival times of the tsunami waves, including but not limited to the following:

1. The bathymetry is not accurate in the vicinity of the epicenter.
2. The epicenter is not well located, or its origin time is uncertain.
3. The epicenter is on land and a pseudo-epicenter off the coast must be selected.
4. The bathymetry is not accurate in the vicinity of the reporting station.
5. Nonlinear propagation effects may be important in shallow water.
6. The observed travel times do not represent the first wave but instead are later arrivals.