Residents of a West Seattle neighborhood must have been puzzled one Saturday morning in December 1986. Two men were leaning against the wall of a house and rhythmically pushing against it, and were being urged on by the shouts of another man standing on the roof. A horizontal seismometer was fastened to the top of the roof with a wad of beeswax; a thin cable trailed from the seismometer over the roof edges across the lawn and into a van parked at the curb. A digital seismograph in the van was recording the minute vibrations that were produced by the two men pushing on the wall and that were detected by the seismometer. From those recordings, the men could determine the predominant frequency and damping characteristics of the house when it is forced to oscillate in a horizontal direction as, for example, in an earthquake.
The men were Denver-based U.S Geological Survey (USGS) geophysicists working on the Urban Hazards Field Investigations project. On the previous day they had recorded two events on their seismographs-a distant nuclear explosion in Nevada and a blast at amine near Centralia, Washington. On another day, they used seismic refraction equipment to locate the depth of bedrock and seismic velocity to it at several locations in West Seattle and in the Seward Park-Brighton district of southeast Seattle.
