Regression of 842 vertical S-wave seismograms from 45 microearthquakes sampling the Wadati-Benioff source zone beneath western Washington suggests a regional control of ground amplitude at 7 to 10 Hz frequency. Mean station amplitude residuals fall in two distinct populations that are geographically coincident with the tectonic regions of the Coast Range-Puget Lowlands to the west (yielding smaller amplitudes) and the Cascade range to the east (yielding larger amplitudes). The mean 10 Hz response spectral velocity recorded by the eastern stations is twice that of the western stations. The amplitudes suggest attenuation properties, averaged over the uppermost 60 km, are different for the two regions. ¿ American Geophysical Union 1991