Correlation of 1-to 10-Hz earthquake resonances with surface measurements of S-wave reflections and refractions in the upper 50 m

Resonances observed in earthquake seismograms recorded in Seattle, Washingt on, the central United States and Sherman Oaks, California, are correlated with each site's respective near-surface seismic velocity profile and refle ctivity determined from shallow seismic-reflection/refraction surveys. In a ll of these cases the resonance accounts for the highest amplitude shaking at the site above 1 Hz. These results show that imaging near-surface reflec tions from the ground surface can locate impedance structures that are impo rtant contributors to earthquake ground shaking. A high-amplitude S-wave re flection, recorded 250-m northeast and 300-m east of the Seattle Kingdome e arthquake-recording station, with a two-way travel time of about 0.23 to 0. 27 sec (about 18- to 22-m depth) marks the boundary between overlying alluv ium (V-s < 180 m/sec) and a higher velocity material (V-s about 400 m/sec). This reflector probably causes a strong 2-Hz resonance that is observed in the earthquake data for the site near the Kingdome. In the central United States, S-wave reflections from a high-impedance boundary tan S-wave veloci ty increase from about 200 m/ sec to 2000 m/sec) at about 40-m depth corres ponds to a strong fundamental resonance at about 1.5 Hz. In Sherman Oaks, s trong resonances at about 1.0 and 4 Hz are consistently observed on earthqu ake seismograms. A strong S-wave reflector at about 40-m depth may cause th e 1.0 Hz resonance. The 4.0-Hz resonance is possibly explained by construct ive interference between the first overtone of the 1.0-Hz resonance and a 3 .25- to 3.9-Hz resonance calculated from an areally consistent impedance bo undary at about 10-m depth as determined by S-wave refraction data.