Kamchatka Earthquake: Why was the tsunami 5 to 6 meters high, even though the magnitude of the earthquake was the same as the Great East Japan Earthquake?

Experts’ Simulations Reveal the “Truth”…

On July 30, a massive earthquake struck near the Kamchatka Peninsula. In Japan, “emergency tsunami warnings” were issued throughout the day in most areas on the Pacific Ocean side, and a 1.3-meter tsunami (Kuji Port, Iwate Prefecture) was observed in Japan.

In the case of the Kamchatka Peninsula earthquake, the seismic waves spread toward Hawaii, and it is thought that part of the waves hit the Emperor Kaiju Mountains, a vast group of seamounts located in the North Pacific Ocean, sending a tsunami toward Japan.”

Professor Fumihiko Imamura, a leading researcher on tsunamis, is currently the Vice President of Tohoku University. When a major earthquake occurs, the seafloor rises and falls as a result of fault displacement. This causes the sea surface to fluctuate, which in turn creates large waves that propagate in all directions, resulting in tsunamis.

The Institute of Oceanography of the Russian Academy of Sciences announced that tsunamis of 5 to 6 meters were visually observed at several locations in Kamchatka and the Kuril Islands.

The earthquake occurred 126 km east-southeast of the Kamchatka Peninsula, with a magnitude of 8.8 and an epicenter depth of 20.7 km. On the other hand, the epicenter of the Great East Japan Earthquake was 130 km east-southeast of the Oshika Peninsula with a magnitude of 9.0 and a depth of 24 km.

Although the distance, depth, and magnitude of the epicenter from the land were almost the same, the Great East Japan Earthquake triggered a tsunami of over 30 meters. Why such a difference?

According to our simulation, a tsunami of over 10 meters would have hit the Kamchatka Peninsula,” said Professor Fumihiko Imamura.

According to Professor Imamura, the size of a tsunami is determined by the magnitude and the depth of the epicenter. If the magnitude and depth of the epicenter are the same as those of the Great East Japan Earthquake, it is natural that the size of the tsunami should also be the same.

In fact, the Sulawesi earthquake in Indonesia in 2006 had a magnitude of 7.5 and an epicenter depth of 10.0 km, and a tsunami of 11.3 m was observed. At that time, a tsunami of over 15 meters was observed.

If an earthquake of magnitude 8 or greater occurs in the seas around the Americas, a tsunami will also hit Japan.

In the case of the Chilean earthquake in South America, a tsunami of up to 6 meters or more surged from Hokkaido to Shikoku more than a day after the quake.

In 1700, a Cascade earthquake, estimated to be around magnitude 9 on the Richter scale, hit the Cascade subduction zone off the coast of Seattle, USA, and sent a tsunami into Japan. Japan was also affected by a tsunami after the 1964 Alaska earthquake, also estimated to be around magnitude 9.

There is a large plate subduction along the coast of the continental United States, and earthquakes are highly likely to occur. If an earthquake of magnitude 8 or greater were to occur there, a tsunami would hit Japan.

A “silent tsunami”… a small intensity earthquake can be followed by a large tsunami.

In the seas around Japan, when the oceanic plate burrows beneath the continental plate and the strain becomes unbearable, the continental plate rises and earthquakes occur. In other words, large tsunamis generally hit areas close to the epicenter, where the intensity of the quake is greater. However, there are exceptions.

The epicenter of the Great East Japan Earthquake was about 130 km east-southeast of the Oshika Peninsula in Miyagi Prefecture, but the most severe tsunamis and damage occurred in Miyako City and Taro-cho in Iwate Prefecture. In Miyako City, the tsunami ran 40 meters up the slope of a mountain. But the intensity of the quake in Miyako City was just under 5 to 5.”

Why did a large tsunami surge into an area that was far from the epicenter and where the tremors were not that great?

At first, I thought it might have been due to the Rias coastline,” he said. When a bay is winding, tsunamis tend to concentrate there,” he said.

However, that was not the only explanation. As a result of continued investigation, the possibility emerged that an undersea landslide had occurred off the northern coast of Iwate Prefecture.

When a landslide occurs on the seafloor, a tsunami is generated, although it is not accompanied by a large tremor. This is known as a “silent tsunami.

A “silent tsunami” is also thought to have occurred off the northern coast of Iwate Prefecture when a relatively soft part of the plate moved slowly and significantly, and this combined with the tsunami triggered by the earthquake off the Oshika Peninsula to create a huge tsunami.

In fact, in the past, there have been large tsunamis even when the shaking was not so great. The Meiji Sanriku Tsunami of 1896 was one such example. The 1896 Meiji Sanriku Tsunami, which was recorded as having an intensity of 3 on the Japanese seismic scale, was followed by a massive tsunami of more than 10 meters.

Even if you think the tremors are small, a huge tsunami can still hit.

You never know when or how a tsunami will strike. Evacuation towers and seawalls are being built, but how should we prepare for a tsunami?

It is important to have the hardware in place, but the most important thing is the software. When the tsunami warning for the Kamchatka Peninsula earthquake was issued, there were traffic jams caused by people evacuating by car. The principle is to evacuate on foot. The use of automobiles is only for people who have difficulty walking, such as the physically disabled and the elderly. We need to establish local rules, such as routes for using automobiles.

The Nankai Trough earthquake is assumed to have an 80% probability of occurring within the next 30 years. The magnitude is expected to be up to 9 on the Richter scale, and a tsunami of 10 meters or more is expected to hit the Pacific coast from Kanto to Kyushu. It is important to be fully prepared.

▼ Professor Fumihiko Imamura, Vice President of Tohoku University. He graduated from Tohoku University in 1984 with a bachelor’s degree in civil engineering. After graduating from Tohoku University in 1984 with a bachelor’s degree in civil engineering, he became an associate professor at Tohoku University and a visiting associate professor at the Disaster Prevention Research Institute of Kyoto University before becoming a professor at the Graduate School of Engineering at Tohoku University in 2000 and president of the Japan Society for Natural Disaster Science in 2008. In April 2002, he became the second director of the International Research Institute of Disaster Science (IIDS) at Tohoku University, which was established in 2012 by bringing together researchers in the fields of humanities, science, engineering, and medicine to apply the lessons learned from the Great East Japan Earthquake. He retired as Director of the same institute at the end of March 2011. Currently, he is leading activities to raise awareness of disaster prevention against natural disasters such as earthquakes, tsunamis, and typhoons. He is the author of many books, including Tsunamis: Threats, Mechanisms, Disaster Prevention and Preparedness (Seizando Shoten).