Abstract
An earthquake early warning (EEW) system is designed to detect an event, determine its parameters (hypocenter, magnitude, and origin time), and issue an alert to sites/areas where necessary actions should be taken before destructive seismic energy arrivals. At present, large-scale EEW systems are operational in several countries around the world. The most extensive nationwide EEW system has been operating in Japan since 2007, and was able to issue alerts broadly when the moment magnitude (Mw) 9 Tohoku-Oki earthquake hit in 2011. The casualties caused by this event were far less than those caused by other deadly earthquakes (Mw > 6.6) in this century. Many other countries attributed the fewer death victims to the advanced large-scale EEW system, and plan to install systems similar to Japan’s model. However, the historical and environmental background in Japan, both in terms of earthquake hazards and safety preparation, differs considerably from other countries. In addition, EEW systems that use data from a large-scale network (i.e., “a big-net” hereafter) still have limitations. There are thus numerous factors that other countries should consider to benefit from installing a Japan-styled EEW. In this article, we review how research and development associated with EEW have been carried out, and how EEW systems presently function. We then show short-wavelength variation of ground motions within the typical station interval of a big-net using data recorded by a dense local seismic network in Japan. However, it is not particularly meaningful to attempt detailed modeling of varieties of ground motion within the station interval for a big-net EEW operation, because the possible combinations of earthquake sources, paths of wave propagation, and recipient sites are infinite. We emphasize that in all circumstances, for recipients to benefit from EEW, seismic safety preparations must be implemented. Necessary preparations at sites do not diminish in importance after incremental improvements in station coverage and/or algorithms in a big-net operation. Further, scientists and engineers involved in EEW projects should strive to publically disseminate how big-net EEW systems work, and also why, to achieve maximum benefit, these systems should always be supplemented by preparations at recipients’ sites.
Highlights
Earthquakes have caused the worst natural disasters repeatedly
It turned out that the March 9th event was a mere foreshock of the Mw9 main event that was the 4th largest event recorded instrumentally since 1900, and one of the seven deadliest events with more than 10,000 casualties in the twenty-first century. It was a rare experiment of the nationwide earthquake early warning (EEW) system to detect and estimate the magnitude of a large earthquake that has evolved to a great Mw9 while issuing alerts with updated information, so the public can take response actions in a very limited time frame
The big-net EEW system may not issue an alert before strong shaking to affected areas in the gray zone for this earthquake while an Mw6-class earthquake can cause railway damage in areas up to ~ 30 km from the epicenter
Summary
Earthquakes have caused the worst natural disasters repeatedly. In the twenty-first century alone, there were a number of catastrophic earthquakes that destroyed human lives and environments. Once an earthquake takes place, a large-scale real-time monitoring network installed to monitor seismic activity in a broad region can detect it, and its earthquake early warning (EEW) system (referred to as a big-net EEW system hereafter) can issue an alert to affected areas through various media seconds before strong ground shaking
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