Evaluation of hard rock spectral models for the Taiwan region on the basis of the 1999 Chi-Chi earthquake data

We analyze the ability of different spectral models to describe the frequency content of ground motion during the 1999 Chi-Chi earthquake (M-W = 7.6, Taiwan) and two large (M-L = 6.8) aftershocks. The spectral models evaluated include the one-corner model of Brune applied with various key parameters (seismic moment and stress drop), and the two-corner-frequency models proposed for eastern North America [Bull. Seismol. Soc. Am. 83 (1993) 17781 and California [Bull. Seismol. Soc. Am. 90 (2000) 255]. The ground-motion spectra predicted by these spectral models for hypothetical very hard rock site were compared with the Chi-Chi earthquake data obtained on rock (class B) and soft rock or very dense soil (class Q sites. The approach also allows us evaluating the generalized empirical amplification function for class B and C sites in the region. It has been found that, the amplitude spectra of recorded ground acceleration (the mainshock and aftershocks) for frequencies larger than 0.3-0.4 Hz agree with the modelled two-corner-frequency spectra calculated using the model proposed for California. The single-corner-frequency model also provides a good agreement with the observations when using so-called 'short-period seismic moment' [Phys. Earth Planet. Interiors 37 (1985) 1081 instead of the reported values obtained from long-period waves. The key parameters used in the single-comer model coincide with parameters of subsources evaluated for the complicated mainshock source. Therefore, it is possible to confirm the suggestion that the short-period seismic waves, at least for the thrust earthquakes, are generated mainly from the fracture of small-scale heterogeneities. The use of two-corner-frequency source model for earthquake spectrum that is based on long-period seismic moment value is equivalent, for frequencies larger than 0.3-0.4 Hz, to the use of single-corner-frequency model that is based on the parameters of major subsource. (C) 2003 Elsevier Ltd. All rights reserved.