Analysis of Observed and Predicted Tsunami Travel Times for the Pacific and Indian Oceans

• Published in: Pure and applied geophysics 2009-02, Vol.166 (1-2), p.301-324
• Main Author: Wessel, Paul
• Format: Article
• Language: English
• Subjects: Approximation; Bathymetry; Deviation; Earth and Environmental Science; Earth Sciences; Earthquakes; Errors; Geophysics; Geophysics/Geodesy; Indian Ocean; Mathematical analysis; Oceans; Plate tectonics; Seismic activity; Seismology; Statistics; Travel time; Tsunamis
• ISSN: 0033-4553; 1420-9136
• DOI: 10.1007/s00024-008-0437-2

I have examined over 1500 historical tsunami travel-time records for 127 tsunamigenic earthquakes that occurred in the Pacific and Indian Oceans. After subjecting the observations to simple tests to rule out gross errors I compare the remaining reports to simple travel-time predictions using Huygens method and the long-wave approximation, thus simulating the calculations that typically take place in a tsunami warning situation. In general, I find a high correspondence between predicted and reported travel times however, significant departures exist. Some outliers imply significantly slower propagation speeds than predicted; many of these are clearly the consequences of observers not being able to detect the (possibly weak?) first arrivals. Other outliers imply excessively long predicted travel times. These outliers reflect peculiar geometric and bathymetric conditions that are poorly represented in global bathymetric grids, leading to longer propagation paths and consequently increased travel times. Analysis of Δ t , the difference between observed and predicted travel time, yields a mean Δ t of 19 minutes with a standard deviation of 131 minutes. Robust statistics, being less sensitive to outliers, yield a median Δ t of just 18 seconds and a median absolute deviation of 33 minutes. Care is needed to process bathymetry to avoid excessive travel-time delays in shallow areas. I also show that a 2×2 arc minute grid yields better results that a 5×5 arc minute grid; the latter in general yielding slightly slower propagation predictions. The largest remaining source of error appears to be the inadequacy of the point-source approximation to the finite tsunami-generating area.