High frequency fall-off of source spectra using Q -free spectra estimation |
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| Authors: | P MOZAFFARI ZHONG-LIANG WU YUN-TAI CHEN MING WANG |
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| Institution: | 1. Institute of Geophysics, China Seismological Bureau, Beijing 100081, China;Institute of Geophysics, Tehran University, Tehran 14394, Iran
2. Institute of Geophysics, China Seismological Bureau, Beijing 100081, China |
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| Abstract: | A broadband seismogram may be represented by a convolution of source time function, the propagation operator and the scattering/attenuation
operator. As the propagation operator is frequency-independent, and the scattering/attenuation effect may be described by
a Q-factor depending on frequency via Q
−1=Q
1
−1
+(Q
2
ω)−1, considering a combination of the displacement spectra, the source spectra may directly be estimated, in which the contribution
of the Q-value may be eliminated automatically. Using this algorithm, the near-source broadband seismograms of five aftershocks of
the 1988 Lancang-Gengma, Yunnan Province, China earthquake were processed. The results for the M
L=3.0 and M
L=3.5 aftershocks show that for the same earthquake, similar high-frequency fall-off may be obtained from different recording
stations. The result for the M
S=6.7 aftershock shows that consistent source parameters may be gotten from different seismic stations. The parameters estimated
also agree with the results obtained by empirical Green’s function approach. The high-frequency spectra of the aftershocks
exhibit a typical f
−γ fall-off. For the M
L=3.0 aftershock, γ≈3; for the M
S=6.7 and M
L=4.0 aftershocks, γ≈2; and for the M
L=3.5 and M
L=3.0 aftershocks, γ≈2.5. The corner frequency of the M
S=6.7 aftershock indicates that it has a small source dimension, implying that it may come from the rupture of a small but
strong barrier.
Contribution No. 99FC2003, Institute of Geophysics, China Seismological Bureau, China.
This work is supported by the Joint Seismological Sciences Foundation (95-07-411). Also partly supported by NSFDYS49725410. |
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| Keywords: | source spectra Q-value Lancang-Gengma earthquake |
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