Abstract: In this paper, we calculate seismic apparent stresses for 117 earthquakes occurred from September 2003 to December 2007 in the capital region of China, with the minimum magnitude limit of M_L2.5. The spectral technique was used to estimate the low frequency flat level (Ω₀) and the corner frequency (f_c) of Brune ω² model and radiated energy (E_s). We first use the multi-window FFT to obtain the observed spectrum, and then after extracting the instrument response it is corrected for geometry spreading and attenuation. For geometry spreading correction, Gail's three section geometry spreading model was used. The frequency-dependent Q value was used for inelastic attenuation correction. The genetic algorithm was applied to determine a pair of f_c and Ω₀ of Brune ω² model, which yields the least deviation between theoretical ω² model and corrected spectra. Seismic moment (M₀) was calculated from Ω₀, and E_s was obtained by integrating corrected spectral with limited bandwidth compensation. Apparent stress can be calculated from M₀ and E_s. The results turn out to fit well with some previous work, the apparent stress does not vary with earthquake size; f_c decreases with M₀ following the scaling law M₀∝f^-3_c; M₀ and M_L follows the scaling low lg(M₀)-1.20M_L+10.0. In the end, we give the spatial distribution of the calculated apparent stress. The result indicates that the value of earthquake apparent stress concentrated in three zones of the capital region and their spatial distribution characteristics are different. There is a high value in 2004 northeast of Wen'an region where M_S5.1 earthquake occurred in Jul 2006, and apparent stress in the Tangshan region on the whole increases from southwest to northeast.

Key words: Apparent stress, Earthquake moment, Corner frequency, Radiated energy, Capital region