Draft:PLUM method

Case study of improvement using the PLUM method. These relate to the 2011 Tohoku earthquake. ① shows the area for which a warning was issued at the time, while ② shows a simulation of the warning issuance area using the PLUM method. The rightmost image shows the actually observed seismic intensity. As can be seen within the black circle in the figure, conventional methods failed to account for the extent of the source region, and thus could not predict the strong shaking experienced in the Kanto region. In contrast, the PLUM method predicts shaking based on the spread of the shaking itself; it has enabled the prediction of strong shaking in the Kanto region, even at a far distance from the epicenter.

The PLUM method (Propagation of Local Undamped Motion) is a prediction method introduced by the Japan Meteorological Agency to improve the accuracy of Earthquake Early Warnings, based on lessons learned from the 2011 Tōhoku earthquake and tsunami. It has the characteristic of being able to accurately determine seismic intensity even when a large-scale earthquake occurs, such as megathrust earthquakes.

Background

The magnitude 9.0 earthquake that struck off the Sanriku Coast on March 11, 2011 caused widespread severe shaking, mainly in the Tōhoku and Kantō regions, resulting in extensive damage in these areas. However, the earthquake early warning issued by the Japan Meteorological Agency at the time of the main shock failed to include the Kantō region in the warning area, which was too far from the epicenter, despite it experiencing strong tremors of intensity 5-lower to 6-upper on the Japan Meteorological Agency seismic intensity scale.

This raised the issue that conventional methods could not accurately predict the strong shaking of such massive earthquakes. Therefore, improving the accuracy of the predictions made by the earthquake early warning system became a challenge, for which the PLUM method was developed and then put into action in 2018.

Method

The PLUM method directly estimates seismic intensity from the intensity observed by seismometers, without estimating the earthquake's epicenter or magnitude. It follows the idea that "if strong shaking is detected by a seismometer near the predicted location, that location will experience similarly strong shaking". Although the time difference between the prediction and the actual shaking occurring is short, it allows predicting seismic intensity with high accuracy even for megathrust earthquakes with vast hypocentral regions, such as the 2011 Tohoku earthquake. The details of the technical improvements achieved through this PLUM method are as follows:

Conventional methods (IPF method)

Conventional methods have seen improved accuracy thanks to the IPF method, which has been in use since 2016. When P waves are detected, the epicenter is estimated, the magnitude calculated based on that epicenter, and then the seismic intensity across the entire area is estimated. The IPF method is an advanced version of the conventional methods. When multiple earthquakes occur at the same time, it can separate them much more efficiently than before, avoiding erroneous or excessive warnings.

PLUM method

The PLUM method, which can handle large-scale earthquakes, is fundamentally different than the conventional methods. It predicts seismic intensity from surrounding real-time intensity observations, which instantly calculates the strength of the tremors without predicting the epicenter. During an earthquake early warning, if conventional methods fail to reliably estimate the epicenter details, it is announced as an "hypothetical epicenter element from the PLUM method". Specifically, the epicenter is fixed at the latitude and longitude of the observation point that first detected the earthquake through the PLUM method, the depth is fixed at 10km, and magnitude to 1.0. Therefore, warnings issued using the PLUM method can be distinguished from those employing the conventional methods.

Since 2018

Since the introduction of the PLUM method, the Japan Meteorological Agency's Earthquake Early Warning system compares the estimated seismic intensity using both the conventional methods and the PLUM method (a hybrid of the two methods) and issues the warning based on the higher of the two estimations. This method is called the "hybrid method". It has been in operation since March 22, 2018.

Usage in earthquake early warnings

• 2018 Western Shimane prefecture earthquake around 1:32 a.m. on April 9: The earthquake early warning issued for this earthquake employed the PLUM method for the first time since its introduction.
• 2018 Hokkaido Eastern Iburi earthquake around 3:07 a.m. on September 6: An initial warning and a subsequent warning update using on the PLUM method were issued.
• 2021 Fukushima earthquake around 11:07 p.m. on February 13: Warning updates using the PLUM method have been issued for the Tokyo metropolitan area, Tohoku, and Hokuriku.
• 2022 Fukushima earthquake around 11:36 p.m. on March 16: As the main shock struck just two minutes after the foreshock, which occurred around 11:34 p.m., before the shaking from that initial event had even subsided, a warning based on the PLUM method was issued for the main shock. Subsequently, updated warnings were issued for the Tohoku, Kanto, and Niigata regions as the shaking spread.
• 2023 Noto earthquake around 9:58 p.m. on May 5: In addition to the conventional methods, two warnings based on the PLUM method were issued (following the detection of a small earthquake immediately beforehand).
• 2025 Sanriku earthquake around 5:03 p.m., on November 9: A maximum predicted seismic intensity of 4 had been announced using the conventional methods; however, a warning was issued using on the PLUM method, which had estimated an intensity of 5-lower for northern Iwate prefecture.
• 2026 Kumamoto earthquake around 4:27 p.m., on July 28: The first warning issued used the PLUM method; subsequently, as the shaking spread, updated warnings incorporating the conventional methods were released.

References

Category:Earthquake early warning systems Category:Japan Meteorological Agency