A new attenuation relation for peak horizontal acceleration applicable to the near source region in Japan is developed. The data base consists of 1372 horizontal components of peak ground acceleration from 28 earthquakes in Japan and 15 earthquakes in the United States and other countries. Coefficients describing the decrease in acceleration with increasing distance found by most previous studies of Japanese data are significantly smaller than those found by analyzing individual earthquakes. This phenomenon is examined and found to result from use of general nonstratified multiple regression analyses. The present analysis uses a two-step stratified regression procedure and an attenuation model that accounts for geometrical spreading and anelastic attenuation but has magnitude-independent shape at very short distances. The resulting relation in Japan is
where A is the mean of the peak acceleration from two horizontal components at each site (cm/sec2), R the shortest distance between site and fault rupture (km), and M the surface-wave magnitude. The median estimate of peak horizontal acceleration at the source region is 620 cm/sec2, independent of earthquake magnitude. Effects of four different ground condition (rock, hard-, medium- and soft-soils) on the attenuation relation are also examined. Average peak horizontal accelerations for the rock and the soft-soil sites are 60 and 140 per cent respectively of the value predicted from the equation.