Real-time calculation of seismic intensity used in Japan

Since 1996, the seismic intensity redefined by Japan Meteorological Agency (JMA) has been used in Japan. This seismic intensity (JMA seismic intensity) can be calculated instrumentally from acceleration records and never be determined from human perception or damage investigation. Consequently, the seismic intensity used in Japan completely became an instrumental index. Nowadays the JMA seismic intensities are calculated by thousands of officially approved seismic intensity meters deployed entire region in Japan and the intensities are announced up to about two minutes when a large earthquake occurred. Nevertheless, calculation of the JMA seismic intensity has a real-time delay of several tens seconds, since it needs a filtering procedure for the acceleration records with 60 seconds duration. It was used to not be issue at the beginning, when the seismic intensity was redefined. However, with requirements of earthquake early warning (EEW) system, Prompt calculation of the intensity is necessary, because the predicted seismic intensity has to be announced before arrival of seismic wave, although the seismic intensity can not immediately be calculated just after the arrival due to above processing. In order to solve above problem, we would like to recommend a new calculation scheme for a real-time seismic intensity index (Ir), which is a similar index as the JMA seismic intensity (Ijma). This index, Ir, can be calculated from each time series with a moving time window on real-time and its maximum value, Ia corresponds approximately to the Ijma. Actually, relationship between Ijma and Ia was inspected statistically by quantitative strong motion records, and Ia consists with Ijma in wide intensity range. Even for a small processor installed in strong-motion observations, Calculation of Ir is much faster than the processing Ijma. Therefore, Ir is suitable for using in an EEW system consistent with the JMA seismic intensity.