On their popular Nova series, PBS recently broadcasted the NHK-produced episode of “Surviving the Tsunami.” The documentary follows the stories of a few of the lucky survivors, and explains some of the research being done to explain both the origin of such a terrible tsunami and the manner in which some people survived.

The documentary opens with the stories of survivors from Kamaishi City, Iwate Prefecture. Some people failed to evacuate because in the past, they would evacuate, but the tsunami never came. Others did not evacuate because they simply did not hear the warning announcement while they were traveling in their cars.

One man was in his car as it was being pulled out towards the sea during the large backwash. He got out of his car because he thought it was the most likely way that people would find his corpse, but this ultimately provided him the opportunity to jump onto a bridge just before his car was pulled into the open ocean.

The 2011 Tōhoku earthquake created a tsunami unlike any other observed in history. Yoshihiro Ito, a geophysicist of the Tōhoku University, is studying what created the bizarrely sharp “sheer-wall” tsunami wave that hit northern Japan. Unlike most tsunami in which the snapping of tension between tectonic plates causes a wave, he believes there is another element at play.

Surveys of the ocean floor have shown a hard level of sediment near the fault line. Dr. Ito believes that secondary faults within the sediment allowed a sharp upward transfer of energy in the ocean after the original “snap” of the plates, creating the spike which became the sheer-wall tsunami wave.

In addition to studying how the tsunami formed, there are also those who are studying supercritical flow and how some buildings managed to survive it.

In supercritical flow, the velocity of the flow is greater than the velocity of the wave. For example, a pebble will flow faster through a supercritical flow than a leaf floating on the top of the wave. Very large forces result from supercritical flow, and video evidence shows such types of waves hitting towns in the line of the tsunami. During one study discussed in the documentary, a 2.5 m wave at supercritical flow was capable of a force of 14 tons per square meter, more than enough to destroy a concrete wall.

The documentary also discusses the difference between the parts of the wave that hit Iwate prefecture (further north) and Miyagi prefecture (between Tokyo and Iwate).

Due to the location of the points at which the tsunami originated, the sheer-wall tsunami that hit Iwate prefecture was behind (and separate from) the wider, flatter tsunami wave that hit the Sendai region. The overall affect was that the wide wave was reinforced by a slightly weaker sheer-wall wave, and the water penetrated Sendai far more inland than was predicted by the Tsunami hazard warning.

All of the evacuation sites in Sendai city were flooded. Though there were survivors at these locations, the lack of information threatened and destroyed many lives. Most people fled to the expressway, which is 8 meters above ground.

The most important message of the documentary is that one must always evacuate when there is a tsunami warning and stay in the evacuation area until the warning is lifted.

The source of the above images and the full documentary (in English, and only available in USA):

http://video.pbs.org/video/2145229025#

One of the stories covered in the documentary (in Japanese):

http://www.youtube.com/watch?v=88nPQlMAr9M

-emily