Development of an automatic hypocenter location system for the earthquake early warning in Japan

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  • 緊急地震速報のための即時震源決定手法の開発と今後の課題
  • キンキュウ ジシン ソクホウ ノ タメノ ソクジ シンゲン ケッテイ シュホウ ノ カイハツ ト コンゴ ノ カダイ

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Abstract

We have developed an earthquake early warning system (EEWS) by using a spatially dense and high dynamic range seismic network that consists of 800 stations covering the Japanese Islands. The system is able to determine the hypocenter location and magnitude within a few seconds from the P-wave arrivals at the closest stations and then transmits this information before the arrival of S-waves in areas of potentially serious earthquake damage. Since the available waveform data increases with time, our EEWS is designed to update the earthquake parameters every second. An early warning system should be able to reliably determine earthquake parameters as quickly as possible; it is therefore unreasonable to wait until waveform data from numerous stations have been collected for analysis. Assuming that all stations will observe P-wave arrivals from a large earthquake, we have developed a novel method of determining the hypocentral location that uses the arrival times for only a few stations and also the lack of P-wave arrivals at other stations at a given time (Tnow). The use of Tnow makes it possible not only to reliably determine hypocenter parameters within a few seconds but also to detect spurious arrival times and remove them automatically. By using Tnow in our dense seismic network, our EEWS determines hypocenter parameters for 10 to 20 events per day and 99% of them are determined correctly. The receiving unit of earthquake early warning (EEW) is equipped with a CPU and memory and is on-line via the internet. The addition of an inexpensive seismometer and A/D converter would transform this receiver into a real-time seismic observatory, which we are calling a home seismometer. The spread of home seismometers will bring the construction of very dense seismic network available for EEWS.<br>

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