On February 13, just before the 10th anniversary of the Great East Japan Earthquake and Tsunami, I felt a strong tremor at my home in Sendai. The Initial microtremor duration was so long that I thought the epicenter must be far away, but the strength of the tremor made me think that it might have been an expected large earthquake in the Tokyo area. I immediately went to my children's bedroom, held them in my arms, and waited for the shaking to stop, but when it reached its peak, I was worried that the 53-year-old university housing would collapse. When the earthquake alert sounded after the tremors started, I thought the epicenter was near, but I knew it was not a direct earthquake occurred right beneath Sendai. After the shaking finally stopped, the power did not go out, so I switched on the TV and tried to grasp information such as the epicenter and the strength of the earthquake, but at the same time I was worried that the violent shaking would generate a tsunami. A few minutes later, the Japan Meteorological Agency announced that the epicenter of the earthquake was off the coast of Fukushima, but it was 55 kilometers deep and there was no need to worry about a tsunami.
I was relieved to hear that, but I was also left wondering whether such a large earthquake would really cause a tsunami or not. The reason was that the maximum seismic intensity was just under 6- in the Japanese scale in central Miyagi Prefecture and 6+ in southern Miyagi Prefecture and coastal areas of Hamadori in Fukushima Prefecture. As a result, there was no tsunami with inundation, so the JMA's decision not to issue a tsunami warning/advisory was accurate. By the way, the JMA issued a tsunami forecast with a slight sea level change about 7 minutes after the earthquake, but I don't think this information was heard through the media. First of all, what is the vague expression of slight sea level change? Most people will not be able to answer exactly, but this is actually a small tsunami. In fact, a tsunami of up to 20 cm was observed at Ishinomaki Port in Miyagi Prefecture 107 minutes after the earthquake, and 10 cm tsunami was also observed at Sendai Port and Soma Port in Fukushima Prefecture. In other words, to be precise, "the 13 February 2021 Earthquake off the coast of Fukushima did not inundate the land, but generated a small tsunami in the coastal areas.
Now, what should the people in the coastal areas have done? Most of them experienced the tremors and tsunami of the Great East Japan Earthquake and Tsunami in 2011. The duration of an earthquake is about two minutes, and the Japan Meteorological Agency (JMA) can immediately determine if a tsunami is generated within three minutes based on the magnitude of the earthquake and the location of the epicenter. The initial magnitude estimate was 7.0, but was later raised to 7.3. In the case of the Great East Japan Earthquake and Tsunami, the duration of the earthquake was as long as three minutes after it occurred, so at 14:49, three minutes after the 14:46 earthquake, the initial magnitude estimate was 7.9, but at 16:00 the magnitude was raised to 8.4, at 17:00 the moment magnitude was raised to 8.8, and two days later it was announced that the magnitude was 9.0. In the process, the estimated tsunami height was also raised, but it is well known that power outages and other factors prevented residents from receiving the latest information, resulting in confusion.
On November 22, 2016, an earthquake measuring 7.4 on the Richter scale occurred off the coast of Fukushima Prefecture, causing tremors with a maximum intensity of 5- on the Japanese seismic intensity scale. The Japan Meteorological Agency (JMA) issued a 3-meter tsunami warning for Fukushima Prefecture and a 1-meter tsunami advisory for Miyagi Prefecture. However, about two hours later, a 1.4-meter tsunami was observed at Sendai Port, and the tsunami advisory for Miyagi Prefecture was immediately raised to a tsunami warning.
In conclusion, I believe that people living in coastal areas that are at risk of tsunamis should have considered the risk of tsunamis when the earthquake occurred on February 13. I later learned from news reports that some people actually did so, but not that many. Let's try to remember that again." When a major earthquake strikes, people in coastal areas have to evacuate for fear of tsunami. The mechanisms of earthquakes and tsunamis are still not completely understood. It takes time to get accurate information on earthquakes from the authority. The Japan Meteorological Agency, local governments, and researchers are not perfect. Warnings, advisories, and evacuation information issued by the Japan Meteorological Agency and local governments are very important to save lives, but we should not rely on them entirely. One of the bitter lessons we learned from the Great East Japan Earthquake and Tsunami was to "think and act for your own safety.”
Now that we are approaching the tenth anniversary of the Great East Japan Earthquake and Tsunami, the earthquake that occurred on February 13, 2021 is a warning to us all to stay alert. There are still things we don't know about the mechanism of tsunami generation. Why don't we think for ourselves and consider evacuating just in case?