| The JMA (Japan Meteorological Agency) seismic intensity scale has been playing an important role in earthquake engineering such as seismic risk analyses and planning for disaster response. Since 1996, seismic intensity meters have been densely installed by JMA and many local governments for automatic and rapid announcement of seismic intensities in the region. However, it has been pointed out that the variation 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 (SID), the difference between instrumental seismic intensity observed at two sites during each earthquake, was introduced and investigated to examine the statistical characteristics of the instrumental seismic intensity. First, the probability distribution of the SIDs was analyzed using mathematical expressions for probability density functions, averages, standard deviations and percentiles that were derived based on the assumption that the instrumental seismic intensities are Gaussian random variables.Second, the instrumental seismic intensities were calculated using the database of the Chiba and SMART-1 seismometer arrays. The intensities were corrected by the site amplification indices estimated from station-wise average of the intensities. In order to eliminate the deterministic differences, the SIDs were calculated from these corrected intensities. Then, the relationships between above-mentioned statistics of the SIDs and the station separations were analyzed. We have found that the averages and standard deviation monotonously increase with increasing logarithm of the station separation distances ranging from about several meters up to six kilometers throughout the two different arrays. The 50th, 80th, and 95th percentiles were also calculated from the standard deviations of the SIDs.Finally, the probability of occurrence of the same/different seismic intensity scale at two sites was considered by using both observed seismic intensity scale and theoretical ones estimated from the standard deviation of the SIDs. This consideration revealed the following: 1) the same intensity scales can be observed with 95 per cent probability at two stations separated by several ten meters, 2) with 75-80 per cent probability for stations separated by several kilometers, and 3) there is less than 0.5 per cent probability to observe intensity scale difference greater than or equal to two for the site pairs with separation distances discussed in this study. It was also confirmed that the theoretical probability estimated from the standard deviation of the SIDs is close to those probabilities. |
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