Earthquake damage at a given location depends on properties of the arriving elastic waves, of the ground at the shaken site, and of the structures involved. To investigate effects of the ground on the motion at the surface, five identical seismographs have been operated temporarily at 25 locations within 30 miles from the Seismological Laboratory of the California Institute of Technology, Pasadena, and their records have been compared with those written by an identical instrument recording in routine at the Seismological Laboratory. The ratio of amplitudes at sites on fairly dry alluvium more than 500 feet deep to those at the Seismological Laboratory (on crystalline rock) is frequently 5 : 1 or more for earthquake waves having periods of 1 to 1 1/2 sec. For waves having periods of 0.1 ± sec. and waves having periods exceeding 10 sec. (length more than 10 miles) the corresponding differences in amplitudes are small. Amplitudes of earthquake waves and of the continuous unrest of the ground recorded at sites on water-saturated soft ground may be ten times those recorded at the Laboratory. The period of waves for which the response relative to that at the Seismological Laboratory is greatest usually decreases as the thickness of the alluvium decreases and is about ¼ sec. at stations on alluvium 100 ± feet thick. At stations on crystalline rock the motion of the ground does not differ significantly from that at the Laboratory for waves having periods exceeding ¼ sec.
At sites on alluvium, relatively strong shaking lasts several times as long as at those on crystalline rock; usually this ratio decreases with decreasing thickness of the alluvium. Ground effects may produce appreciable differences in duration and amount of shaking even at localities only a fraction of a mile apart. The importance of selecting crystalline rock or at least dry ground and avoiding water-saturated soft ground for foundations of buildings to reduce potential earthquake damage is stressed.