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) 1778] 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 C) 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) 108] instead of the reported values obtained from long-period waves. The key parameters used in the single-corner 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.