55 巻 (2002-2003) 2 号 p. 167-180
The JMA seismic intensity scale has been playing an important role in earthquake engineering such as seismic risk analyses and planning of disaster response. Since 1996, the seismic intensity has been measured automatically with seismic intensity meters for its objectivity and rapid announcement to the public and officials. The seismic intensity meters have been densely installed. However, it has been pointed out that the scatter of the instrumental seismic intensity at neighboring sites is not small. Therefore, the spatial stochastic properties of the instrumental seismic intensity should be examined for proper utilization of the seismic intensity scale as the regional intensity of seismic ground motions. In this study, seismic intensity difference (the difference between instrumental seismic intensity observed at the two sites during each eathquake) is introduced and investigated to examine the statistical characteristics of the instrumental seismic intensity. First, the probability distribution of the seismic intensity differences is analyzed, and then the probability density functions, averages, standard deviations and percentiles are derived. Second, above-mentioned statistics are estimated using seismometer arrays of the Chiba and SMART-1 databases. Then, the relationships between these statistics and the station separations are analyzed. We have found that the means and standard deviations monotonously increase with increasing logarithm of the station separation distances ranging from about several meters up to six kilometers. The 50th, 80th and 95th percentiles are calculated to examine the seismic intensity difference. Finally, the probability of occurrence of the same seismic intensity scale at two sites is considered. This consideration reveals that the probability can be given by the analytical expression using the probability density function of the seismic intensity differences.