What is a tsunami run-up and why is it important for hazard mapping?

• A. The maximum vertical extent of the tsunami onshore; used to estimate inundation extent and evacuation need.
• B. The maximum horizontal extent of the tsunami along the coast; used to estimate arrival times.
• C. The speed of tsunami waves at offshore location.
• D. The height of waves offshore.

Answer: (A)

Tsunami run-up is about how far the water reaches on land and how high it rises above normal sea level as the wave comes ashore. This vertical onshore extent is the key measure used in hazard mapping because it translates the tsunami’s impact into real land-area flooding and potential damage. By knowing the maximum run-up for a given coastal setting, scientists can delineate inundation zones, estimate how much area could be flooded, and plan evacuation needs and routes accordingly. It also helps assess how local topography and barriers will influence the inland extent, which is essential for designing safer infrastructure and land-use plans.

The other ideas don’t describe run-up: the maximum horizontal extent along the coast doesn’t capture how far inland the water goes, which is crucial for safety planning; the speed of offshore waves doesn't tell us how far inland they will push or how high they’ll rise on shore; and offshore wave height isn’t the measure that determines inland inundation once the wave interacts with coastlines and topography.