Amplitudes, polarity and time of peaks of strong ground motion during the 1994 Northridge, California, earthquake

Plots of smoothed contours of peak amplitudes and of areas with the same peak sign are presented for the radial, transverse and vertical components of acceleration, velocity and displacement. These were drawn by hand based on strong motion recordings, and represent direct observational evidence of the nature of the attenuation of strong motion with distance at high, intermediate and low frequencies. The contours of peak amplitudes indicate that, close to the source, those are affected by the source radiation pattern, and away from the source, by the geological structure. Slower attenuation of peak amplitudes with distance is observed for waves travelling through the sediments of the Los Angeles basin. Large areas with consistent peak polarity are observed, often tens of kilometers in size, indicating that the sign of the peak is not random. The time of the peak amplitude, relative to first arrivals of S-waves, was also calculated; the areas where this time was greater than 7 s were contoured and shaded, indicating peak occurrence later than the direct arrivals from the source. These plots show that, at distances larger than about 20–30 km, for acceleration, the largest peak occurs mostly before, and, for displacement, mostly after the arrival of surface waves. This indicates that the attenuation of strong ground motion is governed by body waves at short periods and by surface waves at long periods. The presented plots will be useful in refinement of attenuation laws for ground motion peak amplitudes, and for frequency dependent response spectrum ordinates.