West Coast/Alaska Tsunami Warning Center

Operations Manual

NOAA/NWS/WCATWC

910 South Felton Street

Palmer, Alaska 99645

http://wcatwc.arh.noaa.gov

Section 4.3.1: Modeling Results

Last Updated: 5/2007

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Far-field tsunami amplitude forecasts can be made using the Model/Results option in the EarthVu program. These forecasts are based on pre-computed tsunami models scaled by observed sea level data. Over 300 pre-computed models are available. Maximum amplitudes (zero-to-peak in meters) are saved at 99 places along the coasts of Alaska, British Columbia, Washington, Oregon, California, Hawaii, and at all DART buoys. Models are run with grid increments of 5' over the deep ocean, 1' over the shelf, and, where necessary to describe local shoreline configuration, 12" near-shore. Non-linear, shallow water equations are used with friction effects over the finer grids included. Inundation is not taken into account. The basic technique is described by Kowalik and Whitmore (1991, Science of Tsunami Hazards). The methodology of utilizing pre-computed models during tsunami warnings is described by Whitmore and Sokolowski (1996, Science of Tsunami Hazards).

To display results from the pre-computed tsunami models, click on the Models/Results menu option. Locations of the pre-computed models are shown on the screen. Green circles show the location of models with Mw 7.5, red boxes Mw 8.2, blue boxes Mw 9.0, and yellow boxes Mw 9.5. Fault parameters were assigned based on historic earthquakes in the area where possible, and trench geometry/plate motions otherwise. A dialog window with instructions will appear after clicking on model results. Click on the desired location and another dialog window will appear which allows magnitude specification. Choose a magnitude and model results will be shown on the screen. If there is no model of the desired magnitude within 7.5 degrees from the mouse click, no results will be shown.

On the model display window expected wave heights are shown by the length of a line next to each place name. The line lengths represent maximum zero-to-peak wave height in meters at that location. The display window can be scrolled with the mouse roller-ball, or by clicking on the scroll bar. DART buoy wave heights are shown at the bottom. Results, along with source parameters and scaling factors, can be printed with the Print menu option if a printer is connected to the PC. A summary of the source parameters is shown in the title bar. By default, the scaling factor is 1.0. During a warning this factor will be changed as the wave is recorded at tide gages outside the source zone. Scaling the model helps account for source parameters which are different from those used in the model. It is important to only use results from tide gages outside the immediate source zone in case local effects such as landslides have added to the wave height recorded there. To scale the model, choose the Scale With Observed menu option. Select the tide station from the list on the model display window. The models can only be scaled by results from tide gages which were included in the model. Below the tide station name, enter the zero-to-peak amplitude recorded at that gage in meters. The results shown on the screen will be linearly scaled by your entry. As more data is recorded on different tide gages, continue to add these through the Scale With Observed option, and the new tide gage data will be averaged into the scale factor.