10 years after 3.11, the technology behind Japan’s Earthquake Early Warning System continues to improve

TOKYO – After the Great East Japan Earthquake in March 2011 exposed issues of accuracy in Japan’s earthquake early warning system, the system was improved to provide faster and more detailed indicators about the types of earthquakes about to strike.

The system issues warnings before strong tremors from earthquakes. Although launched in 2007, the warnings were not sent to the Kanto region of eastern Japan during the March 2011 earthquake due to the downsizing of earthquakes in the area, thus highlighting the accuracy problems. Ten years later, its accuracy improved, allowing for greater use of the system.

In the event of an earthquake with a projected maximum seismic strength of less than 5 or higher on the Japanese seismic intensity scale of 7 points, the Japan Meteorological Agency (JMA) uses the system to issue warnings via televisions and mobile devices to people in areas likely to score 4 or higher On the scale.

Its primary mechanism calculates the difference in propagation velocities between two types of earthquake waves: the primary wave (P wave), which causes weaker tremors, travels at about 7 kilometers per second, while the secondary wave (S wave), which causes stronger vibrations, propagates at a speed of about 4 km per second. Seismographs near the lower center that detect P-waves quickly warn people and urge people to prepare for subsequent stronger tremors from the S-wave.

In recent years, seismometer monitoring networks have spread to the ocean. They can detect trench-type earthquakes earlier than ground-based seismometers, and warnings can now be issued sooner and with more accuracy than before.

The “S-net” monitoring equipment is lowered into the ocean. (Photo courtesy of the National Institute for Geosciences Research and Resilience to Disasters)

The National Institute for Geosciences Research and Disaster Resilience (NIED), Japan’s disaster prevention research center, in Tsukuba, Ibaraki Prefecture, has installed monitoring systems consisting of seismometers and tsunami meters in water where large earthquakes are estimated, such as earthquakes of a type can occur. The trench.

The S-net monitoring network, which includes 150 systems connected to about 5,500 kilometers of submarine cables, covers the Japan Trench and the Corel Trench off eastern Japan. Donet, which includes 51 monitoring systems, extends over the waters off the Kei Peninsula and Shikoku Island in western Japan, where the Nankai Tru earthquake is expected. JMA began using networks gradually in its early warning system between 2015 and 2020. In theory, S-net enabled the agency to issue warnings of approximately 30 seconds faster than before, while DONET speeded up the process by about 10 seconds.

During the March 2011 earthquake, the early warning system was only triggered in the Tohoku region in northeastern Japan because it reduced the intensity of the earthquakes in the Kanto region to the south. It also issued warnings that were very severe because they considered multiple aftershocks as one major earthquake. To solve these problems, the JMA in 2016 introduced the Iterative Relative Fitting (IPF) method, in which multiple data points are processed simultaneously. This corrected errors in estimating the location and size of the respiratory center.

In 2018, JMA also introduced the publication of the Un-Damped Local Traffic Method (PLUM). It enables him to predict the intensity of earthquakes within a radius of 30 kilometers from each seismometer based on his records. This addition to the traditional method of forecasting the intensity of earthquakes at each observation site after estimating the position and magnitude of the epicenter of the subsidence increases the accuracy.

“The accuracy has improved compared to before the 2011 earthquake, providing us with a time window when earthquakes occur near ocean trenches,” said Naoto Koga, head of information management at the JMA. “We would like people to be prepared routinely so that they can act without panic when issuing warnings.”

Meanwhile, there is a type of tremor that tends to vibrate differently from the intensity of earthquakes: the long-range ground motion, which strongly shakes the upper floors of buildings. Although the intensity of the earthquakes was at level 3 in Osaka, western Japan, in the March 2011 earthquake, the upper floors of the buildings were swaying there for a long time, and the interior materials and fire rated doors were shattered. The JMA plans to add long-range ground motion information to its early warning system early in 2022.

One of the characteristics of the ground motion of long-duration earthquakes is the length of the vibrations. The JMA defines it as the shaking of the Earth for 1.6 seconds or more. Long-range Earth motion is related to volume – not seismic intensity – and tends to propagate away without losing strength. Weak land, such as in the Kanto Plains and Osaka where skyscrapers are concentrated, is vulnerable to damage.

This photo shows the “DONET” monitoring equipment, which consists of a seismometer and a water pressure gauge. (Photo courtesy of the Japan Marine Earth Science and Technology Agency)

In 2013, the JMA classified long-range ground motion into four levels, and began releasing information on it. Level 4, the highest, is described as: “It is impossible to move without crawling; most unsafe furniture moves with some tipping.” This situation was observed in the 2016 Kumamoto earthquakes and the Fukushima earthquake in February this year. Level 3 was in Osaka in March 2011.

NIED has been researching real-time prediction, and in 2015 it created a formula to predict tremors from volume and location of the hypocenter. According to the institute, long-range ground movement warnings will be issued when movement levels are at 3 or 4.

Revisions to the Meteorological Services Act enabled private companies to predict long-term ground movements in September 2020. The National Institute for Environmental Research has conducted demonstration trials in collaboration with more than 10 companies and universities, including an app developer and a construction company.

The data is intended for uses such as generating ground motion information for extended periods of time through an application, and in controlling elevators during earthquakes. RC Solution Co. Tokyo-based information technology company to provide forecasts of earthquake levels on every floor of skyscrapers, so that disaster management centers in buildings can ensure safety.

“It is imperative to discuss how society uses the information,” said Shin Oei, general manager of the Network Earthquake, Tsunami and Volcano Center at the National Institute of Human Development. “For the public to be able to use the information, distributing knowledge about long-range ground motion is also problematic.”

(Japanese origin by Mayumi Nobuta, Science and Environment News section)

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