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    Progress in Earth and Planetary Science

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    Session convener-recommended article JpGU Meeting 2016

    Earthquake Early Warning: what does “seconds before a strong hit” mean?

    Tajima F, Hayashida T.

    Earthquake early warning, effective preparation for seismic safety at recipients’ sites, short-wavelength ground motion variation

    (top)

    Big-net timeline of EEW alert issuance at tA and gray zones where strong shaking may arrive at tS before tA. The vertical axis is the time relative to an earthquake origin time, and the horizontal axis the epicentral distance (D). Gray zones are illustrated for networks of different station spacing Δx = 10, 20, 30, and 40 km. For example, for a network of Δx = 20 km, the gray zone radius is ~17 km from the epicenter, where the ground shaking may arrive before an alert and is likely to be stronger, causing more damage than outside of it.

    (bottom)

    Big-net station distributions: a. the station spacing in Japan is ~20 km or less over the country. This implies that the average gray zone radius is ~17 km from a hypocenter; b. the station spacing in the West Coast/USA is not homogeneous, i.e., it is ~10 km in the populated metropolitan areas such as around Los Angeles, San Francisco Bay and Seattle whereas it is larger in less populated areas.

    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.