A map of coda Q at 1 Hz (Q0) was constructed for China by analyzing more than 1000 seismograms from 80 seismic stations distributed throughout China. The resultant contour map of Q0 shows a strong correlation with the epicenters of historic great earthquakes in China. The high Q0 regions are devoid of the epicenters, and the low Q0 regions are full of them. A remarkably linear relation was found between the logarithm of coda Q0 and the maximum earthquake within the area sampled by coda waves (radius of about 150 km from each station) when we consider earthquakes which occurred from 1700 to present. Roughly speaking, if Q0 is 1000, Mmax is about 4 and if Q0 is 100, Mmax is about 8. This relation will be very useful for siting critical facilities such as nuclear power plants and waste disposal sites.

We also found evidence in support of the migration of low Q0 region coincident with the well-known migration of high seismicity region over the distance of several hundred kilometers during the period of several hundred years. If the migration is due to a diffusion process, the diffusion constant is of the order of 100 km2/yr, which agrees very well with those associated with the aftershock migration, fluid injection, and Vp/Vs anomaly.

Comparison of coda Q0 with Q1 estimated from the intensity data shows that they agree very well for Q1 > 150, supporting the validity of the S to S single backscattering model for coda waves. For Q1 < 150, however, coda Q0 becomes constant independent of Q1. This behavior is qualitatively consistent with numerical experiments of Frankel and Clayton, and suggests that coda Q may be measuring the intrinsic Q of about 150 for these regions where Q1 < 150. This limiting coda Q value may depend on regions because coda Q lower than 100 has been observed in some active regions in this study, and also coda Q as low as 30 to 40 has been reported in California, Mexico, and Italy.

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