Seismic Reflection - Resonance Correspondence

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(A) Acceleration amplitude spectrum, north-south component, for site KD0 from the 1997 June, M4.9 Bremerton, Washington, mainshock. Borehole data (Shannon & Wilson, 1994) suggest that the strong resonance at about 2.3 Hz is apparently generated by high impedance contrast between loose alluvium and stiffer Quaternary alluvium at about 15-20 m depth. Geotechnical data from a borehole located within a few tens of meters of reflection profile C indicates that the blowcounts/ft increase dramatically from 1 to 88 in the 17- to 20-m-depth range. Reflection profiles B and C above were located about 250 and 300 m east and northeast, respectively, of the earthquake recording station. We assume the reflection annotated in each profile is generated at the fill/alluvium interface. Refraction velocity from the top of the alluvium is approximately 400 m/s as interpreted in C.

Geotechnical data from two boreholes located near seismic profile C at left (after Shannon and Wilson, 1994). Blow counts increase dramatically in the 17-20 m depth range. Downhole shear-wave velocity data above are generally similar to refraction/ reflection velocity structure interpreted in profiles B and C at left above.

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(D) Velocity amplitude spectrum, horizontal component, for site MOR from the M4.9 Bremerton, Washington, mainshock. We interpret in reflection/refraction profile (E), which was located about 30 m from the earthquake recording station, a reflection (highlighted in blue) from a 7-m deep interface, which appears to correspond to the resonance observed in the spectrum.

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(F) Acceleration amplitude spectrum, vertical component, for site BG0 from the 1997 February, M3.7 South Seattle, Washington, mainshock. We interpret in profile (G), which was located about 40 m from the earthquake recording station, a reflection (highlighted in blue) from a 67-m deep interface which appears to correspond to the resonance observed in the spectrum F. This interface is probably located beneath Vashon Till deposits.

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