23 Novembre 2016
November 23, 2016
When a strong quake struck off the coast of Fukushima Prefecture in the early hours of Nov. 22, a tsunami warning was immediately issued for Fukushima Prefecture's coastal areas. However, there was a delay in issuing a tsunami warning for the coast of Miyagi Prefecture, directly north of Fukushima Prefecture, causing confusion among residents over whether they needed to evacuate.
When an earthquake measures lower than magnitude 8, and tsunami are expected to reach a height of more than 1 meter up to 3 meters, tsunami warnings are issued. Tsunami advisories, meanwhile, are issued when tsunami heights are expected to be at a maximum between 0.2 meters and 1 meter.
The magnitude-7.4 quake struck at 5:59 a.m. on Nov. 22. But the tsunami advisory issued for Miyagi Prefecture was not switched to a tsunami warning until 8:09 a.m., approximately six minutes after a 1.4-meter tsunami -- the tallest tsunami recorded across the country following the latest quake -- had been observed at Sendai Port. It had already been two hours and 10 minutes since the quake had hit.
"We don't know why the waves were so high at Sendai Port," a befuddled official at the Japan Meteorological Agency (JMA) said. "It's extremely difficult to make accurate predictions."
At 6:02 a.m., about three minutes after the earthquake occurred, the JMA issued a tsunami warning for Fukushima Prefecture, and tsunami advisories for the Pacific Coast from Aomori Prefecture in the north all the way down to Chiba Prefecture near Tokyo.
The height and arrival times of tsunami are calculated using a database with some 100,000 possibilities that take into consideration such factors as the magnitude, location and depth of a quake. To increase the accuracy of these calculations, data taken from the surface of the ocean and the ocean floor by GPS wave gauges installed about 20 kilometers offshore by the Ministry of Land, Infrastructure, Transport and Tourism is also used. Despite such data, the JMA was unable to predict the 1.4-meter tsunami, ultimately issuing a tsunami warning after the wave had already hit.
According to the JMA, not only is seabed topography more complicated closer to land than offshore, waves can grow irregularly in height after crashing into the coast. At a press conference held at JMA headquarters, senior coordinator for seismological information Koji Nakamura said, "We will analyze the causes for the delay, and consider making changes to our tsunami prediction database if necessary."
The delay in issuing a tsunami warning affected residents' evacuation.
The Miyagi Prefecture city of Higashimatsushima responded to the tsunami warning by cancelling classes at 11 municipal elementary and junior high schools. Yet, a total of 78 students attending two elementary schools arrived at their respective schools, because the school cancellation notification emails failed to reach their parents in time. Looking back on what happened, an official with the Higashimatsushima Municipal Board of Education said, "It was difficult to determine when we should make the decision to cancel classes."
In the city center of Ishinomaki, residents trying to evacuate to high ground by car surged from around 8 a.m., causing traffic jams. Some 150 cars gathered at Ishinomaki Junior High School and Kadonowaki Junior High School -- both municipal schools on high ground -- forcing school officials to open up their school yards to the public. "As soon as the tsunami advisory was switched to a tsunami warning, people began arriving in droves," the vice principal at Kadonowaki Junior High said. "It drove home the fact that residents remember the 2011 disaster and take tsunami warnings seriously."
Experts have pointed out that the traffic jams caused by people fleeing by car in the 2011 Great East Japan Earthquake, tsunami and nuclear disaster were an issue that needed to be resolved. Indeed, the traffic jams that occurred following the latest quake were not limited to Miyagi Prefecture.
On a prefectural road running inland from Onahama, a coastal area of the city of Iwaki, Fukushima Prefecture, traffic was backed up by almost 1 kilometer for about an hour starting at 6:30 a.m. The city's disaster plan drafted after the March 2011 disasters stipulates that evacuation should generally take place on foot; at a disaster drill carried out on Nov. 5, participants walked to designated evacuation sites. An official at the Iwaki Municipal Government's Onahama outpost speculated on the reasons why many residents chose to evacuate by car, saying, "It allows people to stay warm, and to escape together with their families."
No major damage was reported from the quake or tsunami, but there is much improvement needed in tsunami prediction techniques and evacuation methods. Shinji Toda, a professor at Tohoku University's International Research Institute of Disaster Science (IRIDeS) said, "Coastal areas in the Tohoku region must be vigilant toward massive earthquakes and tsunami for another 10 to 20 years."