Peak accelerations of the Loma Prieta main shock have been tabulated from instruments maintained by a number of organizations. We have analyzed a subset of 86 records from nominally free-field sites, which have been subdivided into rock, alluvium, and bay-mud categories according to data available in various reports. After correction for attenuation, the peak accelerations on rock, alluvium, and bay-mud sites are factors of 1.6, 1.8, and 4.5 larger, on the average, than Joyner and Boore’s (1988) predicted values for a M= 6.9 earthquake. The mean motions for the rock and alluvium sites are somewhat greater than one standard deviation away from the predicted value, but the mean acceleration from the bay-mud sites is well outside the range expected from analyses of data from previous earthquakes from rock and alluvium sites. Large amplitudes of motions on bay-mud sites relative to rock sites (a factor of 2.8 for the average of the recordings of the Loma Prieta main shock) has been found previously from recordings of distant earthquakes and explosions, but the Loma Prieta earthquake provided the first opportunity to study the relative amplitudes from strong-motion recordings.

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