OBSERVED SPECTRAL CHARACTERISTICS OF VERTICAL GROUND MOTION RECORDED DURING WORLDWIDE EARTHQUAKES FROM 1957 TO 1995

A comprehensive near-source strong motion database was compiled. The database includes over 2,800 free-field uncorrected peak ground acceleration (PGA) values from 48 worldwide earthquakes and more than 1,300 free-field response spectra from 33 worldwide earthquakes. The database includes the data recorded within 60 km of the causative fault from earthquakes ranging from 4.7 to 7.7 in magnitude. Attenuation models of PGA and response spectra for both the vertical and horizontal components were developed as functions of magnitude, source-to-site distance, type of faulting, and local soil conditions. The study clearly demonstrates the strong dependence of vertical-to-horizontal (V/H) spectral ratio on oscillator period, source-to-site distance, and local soil conditions. V/H shows a weaker and more limited dependence on magnitude and type of faulting. The largest short-period V/H ratios are observed to occur on Holocene Soil at short periods and short distances where they can reach values in excess of 1.5 at 0.1-sec period. The largest long-period V/H ratios are observed to occur on Hard Rock where they can reach values as high as 0.7. We conclude that the standard engineering practice of assigning V/H a value of two-thirds is unconservative at short periods, especially for unconsolidated soil, but conservative at long periods, and should be modified.