June 2013

The Imminence of Giant Tsunami Hazard
and the Revision of Tsunami Warnings / Advisories

How Information Distribution by Municipalities and the Media will Change

Hidehiko Fukunaga

After the Great East Japan Earthquake, the expected impact of giant earthquake and tsunami has been intensified, and the content of tsunami warnings/advisories was revised on March 2013. The revision includes the usage of qualitative expressions of expected tsunami heights; for example, “giant” is used in the first alert when there is a possibility of giant earthquake. In this report, the author explores how municipalities and broadcasting media intend to convey the imminence of mega tsunami to the public.

The word “giant” used for expected tsunami heights has a strong impact. Sendai City and Susaki City decided to employ this expression in their notifications using the disaster prevention administrative radio system to evacuate the residents: the former from a bitter experience that many of the residents did not evacuate immediately after the 3/11 earthquake, the latter for the preparation for the predicted Nankai Trough Quake which may cause high tsunami waves in a short amount of time. Meanwhile, Kamaishi, a coastal city of Iwate Prefecture, where tsunami warnings/advisories are issued on a relatively frequent basis, decided to use “very high tsunami” instead of “giant” in their notifications to prevent the ”Cry Wolf” syndrome.

Other than these expressions, imperative tones such as “Do so and so” or “Order to do so and so” will be used in Sendai and Kamaishi. The timings for introducing the imperative tones differ: Sendai use them from the first alert, and Kamaishi from when the situation becomes imminent with the tsunami waves about to surpass the breakwater.

Regarding the expressions used by broadcasting media, NHK shows large captions on the screen such as “Evacuate now!” in the flash report and uses imperatives including “Order to do” and strong tones including “Please recall the Great East Japan Earthquake.” Meanwhile, Iwate Broadcasting Company (IBC) does not use imperatives or strong tones, but use “Please recall the Great East Japan Earthquake” as an exception.

The styles of informing the imminence differ depending on each entity’s experience and perception of earthquake disasters as well as on its prediction of and reaction to mega tsunami disasters.

The NHK Monthly Report on Broadcast Research