1. Method for detecting huge earthquakes using multiple ocean bottom seismometer data

We have developed a method to detect huge earthquakes that utilizes acceleration data observed by multiple ocean bottom seismometers. This method aims to issue warnings more quickly to a wide area on land when a huge earthquake occurs in the ocean (Figure 1).
Conventionally, when the observed value of a single ocean bottom seismometer exceeds a specified threshold, an alarm is output to the land area near the ocean bottom seismometer, considering the promptness and robustness of the warnings. In the proposed method, we assume a huge earthquake (for example, M7.5 or greater) has occurred in the ocean and calculate the seismic ground motion at each ocean bottom seismometer near the hypothetical hypocenter. These values are then used as new thresholds for detecting huge earthquakes. When an earthquake actually occurs, if the observed values of the multiple seismometers exceed the thresholds, the system determines that it is likely to be a huge earthquake and issues an earthquake early warning to a wide area on land. The proposed method, which relies on exceedances of thresholds by multiple ocean bottom seismometers, not only ensures that huge earthquakes are not missed but also helps prevent unnecessary detections of minor to moderate earthquakes, making it a highly reliable method. Additionally, since the method relies only on comparing predefined thresholds with observed values, it has a low calculation load and can be easily integrated into earthquake early warning system for railways.

In setting the thresholds, we estimated attenuation relations (the relationship between hypocentral distance and maximum amplitude) using data from ocean bottom seismometers, along with the site amplification characteristics at each ocean bottom seismometer's location. In addition to the attenuation relations based on land-based seismometer data that have been conventionally proposed, using these estimated results allows for setting more reliable thresholds for each seismometer individually, specifically for earthquakes occurring in the ocean (Figure 2).