Comparison of Seismic Approaches for Strong Ground Motion Prediction using Weak Motion Data

Three component velocity seismograms from a Northridge earthquake aftershock recorded by 62 stations in the San Fernando Valley, Santa Monica Mountains, and Santa Monica, California are used to quantify the relations between amplification factors derived from different methods, including P- and S-wave peak amplitudes, P- and S-wave spectral ratios, and coda wave spectral ratios. Some main conclusions include: 1). Site amplification factors derived from S-wave peak amplitudes are correlated with the damage pattern; 2). Coda wave amplification factors obtained from the vertical and the horizontal components are correlated with a mean correlation coefficient of 0.73 ± 0.23, justifying the use of S-coda waves recorded on vertical seismograms as an estimate of coda amplification factors. 3). In areas where the damage was mainly caused by geological structure effects such as mid-Santa Monica, coda wave amplification factors are not correlated with the damage pattern caused by the main shock. 4). The good correlation between S-wave peak amplitudes and the damage pattern may imply that damage to 1-2 story low buildings is more likely to be caused by impulsive strong pulses in a very short time (1-2 seconds) than by the continuous oscillation of lower amplitude waves operating over a few seconds or tens of seconds. 5). Future microzonation for a given area should take into account more than one method to estimate hazard including assessment of the effect of local geological structure and source location