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Bulletin of the SeismologicalSocietyof America,Vol. 87, No. 3, pp. 731-744, June 1997 Near-Surface Attenuation and Site Effects from Comparison of Surface
 

Summary: Bulletin of the SeismologicalSocietyof America,Vol. 87, No. 3, pp. 731-744, June 1997
Near-Surface Attenuation and Site Effects from Comparison of Surface
and Deep Borehole Recordings
by Rachel E. Abercrombie*
Abstract Near-surface attenuation and site effects are investigated using the seis-
mograms of 17 local earthquakes recorded at depths of 0, 0.3, 1.5, 2.5, and 2.9 km
in the Cajon Pass borehole, southern California. The borehole penetrates 500 m of
Miocene sandstone and then crystalline, granitic basement rock. Previous estimates
of site response have been limited to shallower holes, where the surface reflection
interferes with the upgoing direct wave, and the deepest sensor is not below the
severe near-surface effects, in bedrock. Spectral ratios of the direct P and S waves
for each earthquake between the different recording levels are well modeled with
frequency-independent Q and amplification. At the borehole, Qe - 27 + 8, and Qs
- 21 __+7 in the upper 2.9 km, increasing from Qp -26 and Qs -15 in the upper
300 m to Qp -133 and Qs -47 between 1.5 and 3 km. One event was also recorded
at a surface granite site, less than 1.5 km from the wellhead. Comparison of the 2.9-
km recording with that at this granite site gives Qe -50 and Qs -23. The similarity
of these values with those of previous studies at a wide range of sites suggests that
Q is very low in the near surface, independent of rock type. Near-surface amplifi-
cation appears considerably more site dependent. At the wellhead, the amplifications

  

Source: Abercrombie, Rachel E. - Department of Earth Sciences, Boston University

 

Collections: Geosciences