March 2013

Expected Tsunami Heights and Emergency Communication

What Changes Will the Revised Tsunami Warnings/Advisories Bring?

Hidehiko Fukunaga

On March 7, 2013, nearly two years after the Great East Japan Earthquake, the Japan Meteorological Agency (JMA) introduces drastically revised tsunami warnings/advisories. The main purpose of the revision is to avoid the underestimation of expected tsunami heights and prevent people from hesitating to evacuate.

In the first place, were residents in the afflicted area sufficiently informed of expected tsunami heights and the raise of the heights on March 11, 2011? In order to explore this, we conducted a telephone survey of 37 afflicted municipalities in three prefectures. As a result, it is revealed that majority of the municipalities did not inform the residents of the expected heights and the raise of them through the disaster prevention administrative radio system. The reasons for the decision were also surveyed, and many local governments responded that they had felt expected heights were not always accurate and not convincing for the residents.

Since expected tsunami heights have a margin of error in estimation and a certain degree of uncertainty, they have been used purely as a guide. On the other hand, the raised heights are supposed to serve as critical information that tells the increased severity of danger.

In the Great East Japan Earthquake, the expected heights were raised after the sudden elevation of sea level, but the reasons for raise of the expected heights were not immediately informed to the municipalities, broadcasters, and local residents. Actual sudden rise in sea level has a strong impact and gives residents a stark sense of reality. Such information should serve as an easy-to-understand alert for the residents much more than the initial estimation of the heights based purely on seismographic data does.

The revised system stipulates that when the expected height is raised, the tsunami warning/advisory must clarify the reason, and municipalities and the media must sufficiently inform the local residents of it. In emergency communication it is highly important to share the rule on how to interpret information; the raise of expected heights has a significant meaning and the reasons for the raise must be informed to the residents without exception, even during a great tsunami warning.

The NHK Monthly Report on Broadcast Research