We used finite-difference simulations of 10-Hz P-SV and SH waves to estimate the contribution from the deep basin structure to the large ground motion amplification in Santa Monica, California, observed for seismic waves incident from the north. Our simulations of a 17-km deep Northridge aftershock with epicenter 30 km north of Santa Monica show that focusing at the lens-shaped boundary of the sediments can only account for less than half the amplification observed in the area that was heavily damaged during the 1994 M 6.7 Northridge earthquake. The focusing in the simulations caused amplification of up to 1.6 times in a zone 0.65--2.4 km south of the Santa Monica fault and de-amplification at sites just south of the fault where some of the largest amplification is observed in the data. ¿ 1998 American Geophysical Union