Earthquake early warning: what does “seconds before a strong hit” mean?

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 shortwavelength 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.

A Review of the Technical and Socio-Organizational Components of Earthquake Early Warning Systems

“…This desire is shared worldwide, including by over 200 participants (organizations) of a survey that assessed the factors influencing organizational acceptance and use of EEW systems in California (Goltz, 2002). Tajima and Hayashida (2018) conclude that some types of information that can be used to determine the most appropriate actions to take, such as the arrival time, should be also be considered. In particular, the survey respondents in the study by Goltz (2002) indicated that mitigation actions such as "moving away from falling objects" or "shutting off gas lines" are the most appropriate measures in a short warning time (10 s), while "evacuating to an outside area" or "shutting down hazardous materials" are more effective options when a longer warning time is available (50 s).…”

“…Strong organizational links with representatives of official bodies and first responders are also required to make EEW a successful tool for enhancing business continuity in critical infrastructure such as energy, utilities, food supply, communications, and banking (Gasparini et al, 2011). As indicated by Tajima and Hayashida (2018), EEW systems are only effective if the resilience, strength, and operations of the system are ensured before ground shaking occurs.…”

“…The BART system responded successfully to the EEW alert before the arrival of the M 6.0 South Napa Earthquake in 2014. Despite the fact that no trains were running at the time (3: 20 am), the right procedures/protocol were in place (Strauss and Allen, 2016;Tajima and Hayashida, 2018).…”

“…The JMA has provided training to the population of Japan since 2007, when the system was launched publicly (Tajima and Hayashida, 2018). When an estimation is performed at a seismic station, the resulting values of earthquake source parameters and expected ground shaking are transmitted to the JMA on a real-time basis.…”

“…Other states of Mexico that have experienced the effects of earthquakes and do not receive EEW alerts include Estado de México and Tlaxcala. 10 Finally, it has been shown that a denser seismic network in Mexico City could have resulted in a warning time of more than 20 s for the M 7.1 September 19, 2017 earthquake, instead of the 0-6 s lead time that actually occurred (Cuéllar et al, 2018;Tajima and Hayashida, 2018;Santos-Reyes, 2019).…”