FIGURE 2.18 Three component records of acceleration, velocity, and displacement from the Pacoima Dam record of the San Fernando earthquake. The horizontal instruments are approximately parallel and perpendicular to the horizontal component of the rupture (N14E and S76E, respectively) and show accelerations exceeding 1 g. SOURCE: D.M. Boore and M.D. Zoback, Two dimensional kinematic fault modeling of the Pacoima Dam strong-motion recordings of February 9, 1971, San Fernando Earthquake, Bull. Seis. Soc. Am.,64, 555-570, 1974. Copyright Seismological Society of America.

engendered much discussion regarding the utility of PGA as a measure of seismic hazard. This pulse did not make a significant contribution to the overall response spectra values, except at the shortest periods (173), and when the data from all available earthquakes were considered, PGA was only weakly correlated with the size of the earthquake (174). From these and subsequent studies, it became clear that the PGA was not necessarily the best determinant of seismic hazard to structures; other characteristics—such as the response spectrum ordinates, anisotropic motions, and the occurrence of intense, low-frequency velocity pulses—were found to be more important.

After the 1971 San Fernando earthquake, policy makers tried to update building codes in light of the large amount of data on ground motion