Abstract

A joint inversion method for determining the Lg‐wave attenuation along the propagation path and site response based on seismic spectral ratios was developed and applied to invert the Lg‐wave attenuation γ and site response in the North China region: γ is related to the quality factor Q by γ=πf/UQ, where U is the velocity. Waveform data of 200 earthquakes during 2006–2008 with ML 1.7–5.5 in the area of 38°–41°N, 114°–120°E were selected for this study. More than 1500 three‐component records chosen from 81 stations on the basis of the signal‐to‐noise ratio were used in the inversion. The waveform data were processed in the frequency range of 1–6 Hz with an interval of 0.2 Hz. The results indicate that there is remarkable heterogeneity in Lg‐wave attenuation in the study area. We divided the area into four regions according to the Lg‐wave propagation paths. Lower γ values are found in the tectonic mountain region, higher values occur in the tectonic basin region, and moderate γ values are found for the propagation paths passing through both the mountain and basin regions.

On the vertical component, γ was found to be frequency dependent and in the form forumla and forumla for propagation paths in the mountain and basin regions, respectively, and forumla and forumla for paths passing through both mountain and basin regions with seismic ray direction from the mountain to the basin and vise versa, respectively. The absolute quality factor Q0 ranges from 125 to 306.

The response of rock sites is smooth and flat and less than five at low frequency, while that for soil sites is higher than for rock sites. For soil sites, the amplification of low‐frequency waves is higher than that for high‐frequency waves. For both site response and attenuation, no significant differences were observed between different components.

Online Material: Source parameters of the studied earthquakes.

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