Some buildings in Santa Cruz, California, particularly those in the downtown section which is built on a flood plain, were severely damaged by the 17 October 1989 Loma Prieta Earthquake. During an aftershock study conducted in the fortnight following the mainshock, an array of seismographs was deployed across downtown to study the site response of the flood plain. Analyses of the records of eight aftershocks reveal that ground motions of the flood plain are amplified reltive to those of crystalline rock sites by factors of 4--8 in the frequency band 2--8 Hz. Both resonance in the whole sedimentary column which overlies the crystalline basement and resonance/broad-band amplification in the low-velocity alluvium of the flood plain contribute to this site response. ¿The American Geophysical Union 1990