Site-response factors throughout the Los Angeles region demonstrate how shaking varies depending upon the types of geologic materials present. The yellow and orange colors indicate the “softer” materials of sedimentary basins, and the red and green colors represent the harder rocks of hills and mountains. The circles indicate the amplification of earthquake energy in the 2-6 hertz range—the range for which 3- to 5-story buildings are particularly susceptible to the effects of earthquake shaking. Green circles are results from the main shocks of four large earthquakes—the 1971 San Fernando, 1987 Whittier Narrows, 1989 Sierra Madre, and 1994 Northridge events. The blue circles are results from the aftershocks of the Northridge quake. The site-response factor varies as the diameter of the circles, showing that amplifications range up to more than 7 times the reference value of 1.0 for a site on rock. Damage data from the Northridge event (red squares) show the correlation between site-response factors and severe building damage.