There is not a standard methodology for tsunami modeling. The lack of historical tsunami information is partially due to the lack of human population in affected areas to witness and report tsunamis and because of changes in land use over the years. Earthquakes are felt over a wider area, so there is more available information about their effects.

The tsunami modeling for the map was not based on recurrence interval because there is not enough historical information on tsunamis in California. It is not probabilistic but is based on potential earthquake sources and hypothetical extreme landslide sources. Much has been assumed or simulated to obtain a western shore inundation line for this tsunami map. Maximum run-up to a specific contour* (see above) was determined to be reasonable and the map denotes land at an elevation below that contour, but above sea level in a potential run-up area. The run-up areas for Northern California are considered more conservative than those for Southern California because there is more historical information for the southern coastline. The San Francisco grid for the map was done by Dr. Utku Kanoglu of USC.

This model is similar to the one used by Japanese tsunami researchers but is considered slightly more accurate for complex coastlines. Projections used in Oregon identify three inundation lines - low, medium, and high; this degree of accuracy is generally considered unreliable. Historical data is based on the density of buildings, structures, and anything else that might impact the effects of run-up at the time of a particular tsunami. There are substantial changes in these conditions over time.

The tsunami inundation map is intended as a "worst-case" scenario based on limited historic records. The map is for evacuation planning uses only; it is not an "official" State map to be used for land use planning and real estate disclosure requirements.

2. Has this projected inundation map been confirmed by any field survey or detailed topographic review?

Model results were field verified for accuracy by land survey. There is no information from the high water line to a depth of approximately 30 feet due to the limitations of bathymetry measurements. Researchers used GPS and took pictures in an attempt to fill in the blanks. They drove the entire map survey area and performed measurements at random transects using eye level surveying gear. They measured the sum total of the vertical distance of water marks from tsunami run-up and the inland penetration to the extent feasible.

3. What is the degree of accuracy/reliability of this inundation projection? On what level of detail of topography and bathymetry is it based?

The bathymetry is based on 50-100 meter grid spacing. It is now possible to obtain much more accurate measurements with triangulated sonar - to one meter. However, the San Francisco Bay is too shallow to be measured with this sonar method.

4. What assumptions regarding tsunami origin, mechanics and other critical factors serve as the basis of preparation this inundation projection? Are there any major professional disagreements about these assumptions we should be aware of?

The basis for this projection was published in a 1998 article by Titov and Synolakis: Numerical modeling of tidal wave run-up, Journal of Waterways, Port, Coastal and Ocean Engineering, ASCE, pp. 157-171. There are no major professional disagreements about these assumptions.

5. What differences can we expect between local-source tsunamis and distant-source tsunamis? Are there near-fault effects on tsunami we should be considering?

There is little or no time for warning prior to local-source tsunamis and there are shorter intervals between waves. Distant-source tsunamis have produced waves at long intervals. It is recommended that people stay out of an inundation area for at least 3 hours after the initial wave.