Estimation of Seismic Site Response in the Tangshan Area, China, Using Deep Underground Records

Site amplification in the Tangshan area, China, was analyzed using re- cords of small events (ML 2.9-4.4) observed by a strong motion array with a deep (822 m) underground station deployed between 1982 and 1984 following the 1976 Tangshan earthquake. The effect of destructive interference at the underground site due to surface reflections was confirmed to be negligible in this case. The source, propagation path, and site effects were separated simultaneously by linear inversion using the underground reference site. The S-wave quality factor was found to be approximated by the relation 29f 0.9 in the frequency range 1-10 Hz. The conven- tional surface rock reference site was found to have a characteristic frequency- dependent site effect, which increased markedly from the theoretical amplification factor of 2 over the frequency range of interest. A reference-independent technique assuming an x-squared source model was also introduced and produced results that were in good agreement with the results obtained using the underground reference. The reference-independent technique revealed the underground station to have a nearly flat response consistent with the theoretical response, indicating that an un- derground or borehole bedrock site is a better reference than a surface rock for seismic hazard analysis. The reference-independent technique produced accurate ab- solute site amplification factors for the area examined in this study, demonstrating the usefulness of the method for site effect analysis.