Abstract
Nowadays the seismicl intensity scale has become an important information for real time counter-measures soon after disastrous earthquakes. However, some investigations of damage due to recent earthquakes such as the 2003 Miygiken Hokubu Earthquake revealed problems over how and where to set an instrument for measuring seismic intensity. Especially, the difference in seismic intensity between at the ground surface and in buildings has focused. This paper deals with the variations of seismic intensity using records due to an array observation system for ground and building motions. The array observation system, which was installed at the campus of Tohoku Inst. of Tech. 15 years ago, has obtained some 90 motion records simultaneously at different sites of the free ground surface, the first floor and fourth floor of a 4-story building.
The instrumental intensity scales were obtained from the records to compare their values at the different sites. The results made it statisyically clear that the instensity at the firast floor is smaller by 0.2 than the one at the ground surface. This difference was also found in array observation of microtremors, leading to a conlusive remark so that microtremors are available to predict a difference of seismic intensity depending on installation sites.
