TOKYO -- Following the March 2011 Great East Japan Earthquake's exposure of accuracy issues in Japan's Earthquake Early Warning system, the system has been improved to provide faster, more detailed indications of what kinds of earthquake are about to strike.
The system issues warnings ahead of strong tremors from earthquakes. Though launched in 2007, warnings were not sent to the east Japan Kanto region during the March 2011 earthquake because seismic intensity in the region was underestimated, thereby highlighting accuracy problems. Ten years on, its precision has improved, allowing for greater use of the system.
In the event of an earthquake with a maximum seismic intensity predicted at a lower-5 or higher on the 7-point Japanese seismic intensity scale, the Japan Meteorological Agency (JMA) uses the system to issue warnings via TVs and mobile devices to people in areas likely to register a 4 or higher on the scale.
Its basic mechanism calculates the difference of spreading speeds 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 shaking, spreads at around 4 kilometers per second. Seismometers near the hypocenter that detect P waves quickly issue warnings and urge people to prepare for the subsequent stronger shakes from the S wave.
In recent years, seismometer observation networks have spread to the ocean. They can detect trench-type earthquakes earlier than land-based seismometers can, and it is now possible to issue warnings sooner and more accurately than before.
The National Research Institute for Earth Science and Disaster Resilience (NIED), the center of Japan's disaster prevention research, in the Ibaraki Prefecture city of Tsukuba, has installed observation systems consisting of seismometers and tsunami gauges in waters where it is estimated big earthquakes, like trench-type temblors, can happen.
The observational "S-net," comprising 150 systems connected by some 5,500-kilometers of undersea cables, covers the Japan trench and Kuril trench areas off east Japan. "DONET," which has 51 observation systems, spans waters off the Kii Peninsula and Shikoku Island in west Japan, where the Nankai Trough earthquake is predicted to strike. The JMA began to gradually utilize the networks in the early warning system between 2015 and 2020. In theory, S-net has enabled the agency to issue warnings up to roughly 30 seconds quicker than before, while the DONET has sped up the process by as much as about 10 seconds.
During the March 2011 earthquake, the early warnings system triggered only in the northeast Japan Tohoku region because it underestimated seismic intensity in the Kanto region to its south. It also issued warnings that were too severe because it read multiple aftershocks as a single, big quake. To solve these problems, in 2016 the JMA introduced the iterative proportional fitting (IPF) method, in which multiple data points are processed simultaneously. This has corrected inaccuracies when estimating the hypocenter's location and its magnitude.
In 2018, the JMA also introduced the propagation of local undamped motion (PLUM) method. It enables it to predicts seismic intensities within a 30-kilometer radius of each seismometer based on its recordings. This addition to the conventional method for predicting seismic intensity at each observation location after the hypocenter's location and magnitude are estimated, further increases precision.
"Accuracy has improved compared to before the 2011 earthquake, offering us a window of time when quakes occur near ocean trenches," said Naoto Koja, head of information management at the JMA. "We'd like people to be routinely prepared so that they can act without panicking when warnings are issued."
Meanwhile, there is a kind of tremor that tends to shake differently to seismic intensity: long-period ground motion, which wildly shakes buildings' upper floors. Although seismic intensity was at level 3 in the west Japan city of Osaka in the March 2011 earthquake, buildings' upper floors swayed there for a long time, and interior materials and fireproof doors broke. The JMA plans to add long-period ground motion information to its early warnings system as early as 2022.
One characteristic of long-period earthquake ground motion is the length of vibrations. The JMA defines it as shaking the ground for a period of 1.6 seconds or more. Long-period ground motion is correlated with magnitude -- not seismic intensity -- and tends to spread far without losing strength. Weak ground, such as in the Kanto and Osaka plains where skyscrapers are concentrated, are prone to damage.
In 2013, the JMA ranked long-period ground motion into four levels, and started issuing information about it. Level 4, the highest, is described as: "It is impossible to move without crawling; and most unsecured furniture moves with some toppling over." This situation was observed in the 2016 Kumamoto earthquakes in and the Fukushima earthquakes in February this year. It was level 3 in the city of Osaka in March 2011.
The NIED has been researching real-time prediction, and in 2015 created a formula to predict tremor levels from the magnitude and the hypocenter location. According to the institute, long-period ground motion warnings will be issued when motion levels are at 3 or 4.
Revisions to the Meteorological Service Act enabled private companies to forecast long-period ground motions in September 2020. The NIED has conducted demonstration experiments in cooperation with over 10 companies and universities, including an app developer and a construction firm.
The data is intended for uses such as issuing long-period ground motion information via an app, and in controlling elevators during earthquakes. Tokyo-based IT firm RC Solution Co. plans to provide predictions of tremor levels on each floor of skyscrapers, so that buildings' disaster management centers can ensure safety.
"It's necessary to discuss how society will use the information," said Shin Aoi, director-general of the Network Center for Earthquake, Tsunami and Volcano at the NIED. "In order for the public to use the information, distributing knowledge on long-period ground motion is also an issue."
(Japanese original by Mayumi Nobuta, Science & Environment News Department)