Immediate and high-precision hypocentral determination for earthquake early warning applications using distributed acoustic sensing

Earthquake early warning (EEW) systems in Japan use data from as little as a single observation point, even at the stage in which only the P wave is detected at one point, to estimate the epicentral distance and magnitude to rapidly issue a warning. This approach aims to maximize lead time but has the drawback of significant estimation errors (epicentral distance: half to double, back azimuth: + or -30 degrees ). Using distributed acoustic sensing (DAS) with a dense network of observation points allows for rapid and precise earthquake hypocenter determination, significantly reducing estimation errors compared to single-station methods in EEW system. However, no prior studies have investigated the immediacy of using DAS data-which is indispensable for early earthquake warnings. Therefore, we examined the use of DAS for rapid and precise hypocenter determination with the goal of integrating it into the EEW system. We used fiber-optic cables along the Kyushu Shinkansen to analyze strain-rate waveforms, detect P waves, and determine earthquake hypocenter locations. We detected P waves using short-term average /long-term average simultaneously across multiple channels. We then determined the epicenter using Hypomh. For rapid determination of the hypocenter location after P-wave detection, we assessed the hypocenter location each time the number of P-wave detection points increased by 20. For earthquakes with an epicentral distance of less than 50 km, we successfully determined the hypocenter within 12 km of the Japan Meteorological Agency-reported location. This was achieved by identifying the point where the sum of the maximum-likelihood estimation errors was minimized within 2 s of the initial P-wave detection in any channel. The findings suggest that an EEW system using DAS holds potential as a powerful tool to provide rapid and accurate hypocenter information, especially for near-field earthquakes that occur directly beneath populated areas, where lead times are short, and the potential damage is substantial.