[1] The real-time earthquake information system (REIS) of the Japanese seismic network is developed for automatically determining earthquake parameters within a few seconds after the P-waves arrive at the closest stations using both the P-wave arrival times and the timing data that P-waves have not yet arrived at other stations. REIS results play a fundamental role in the real-time information for earthquake early warning in Japan. We show the rapidity and accuracy of REIS from the analysis of 4,050 earthquakes in three years since 2005; 44 percent of the first reports are issued within 5 seconds after the first P-wave arrival and 80 percent of the events have a difference in epicenter distance less than 20 km relative to manually determined locations. We compared the formal catalog to the estimated magnitude from the real-time analysis and found that 94 percent of the events had a magnitude difference of ±1.0 unit.

The Japan Meteorological Agency (JMA) has started the practical service of Earthquake Early Warning (EEW) and a very dense deployment of receiving units is expected in the near future. The receiving/alarm unit of an EEW system is equipped with a CPU and memory and is on‐line via the internet. By adding an inexpensive seismometer and A/D converter, this unit is transformed into a real‐time seismic observatory, which we are calling a home seismometer. If the home seismometer is incorporated in the standard receiving unit of EEW, then the number of seismic observatories will be drastically increased. Since the background noise inside a house caused by human activity may be very large, we have developed specialized software for on‐site warning using the home seismometer. We tested our software and found that our algorithm can correctly distinguish between noise and earthquakes for nearly all the events.

We propose a new approach to improve the accuracy of the back-azimuth estimation in real-time by using a single station record. Compared with the conventional approach in which the length of the time window for the analysis is fixed for all data, the accuracy and speed of the estimation are drastically improved by introducing a variable-length time window which is determined by the first half cycle of the wavelength of the initial P-wave, because this window tends to reduce the influences of trailing scattered waves. The analysis, using the K-NET dataset, shows that the estimation, using this new approach, is improved by 28% and 0.25 s in accuracy and speed, respectively.

Masahiro

The precise calculation of anticipated seismic intensity is an important component of Earthquake Early Warning (EEW) procedures. The EEW method adopted by the Japan Meteorological Agency (JMA) uses event magnitude, hypocentral distance, and site amplification factor for this calculation, in which the site amplification factor is represented by a single scalar without consideration of spectrum contents. Even when two earthquakes occur at the same location with the same magnitude, their observed distributions of seismic intensity are not always the same. And even at adjacent measurement stations, the interstation difference in seismic intensity of one earthquake is not always the same as that of another earthquake. To evaluate these expected uncertainties in the current JMA EEW method, we analyzed the distribution of recorded seismic intensities from adjacent earthquakes and also compared the intensities at adjacent observation sites. The uncertainties are 0.29 JMA intensity units when the JMA magnitude is used as an index of source factor and 0.22 when the average of the observed seismic intensities is used. The uncertainties are 0.21 when site amplification factor is represented by single scalar value. These results may indicate the intrinsic precision limits of anticipated seismic intensities in the current JMA EEW method.