A COMPARISON OF EARTHQUAKE CODA WAVES AT SURFACE VERSUS SUBSURFACE SEISMOMETERS

The coda waves of 21 Parkfield earthquakes were recorded simultaneously at the surface and at a depth of 198 m. The data reveal significant differences in the early portion of the uphole versus downhole codas. The early portion of the uphole coda experiences a pulse of low-frequency energy not seen in the downhole coda. To explain this observation, we propose a model of the coda that includes body wave to surface wave scattering near the site. By fitting the model to our data, we have found that the fractional conversion factor of body waves to surface waves travelling toward the receiver is 0.03 at the Vineyard Canyon site. The relatively rapid damping of these trapped waves can be modelled by assuming a quality factor of QT = 50. The principles of this model are generally enough to be of use at other locations. -from Authors