abstract

The isoseismal maps for twelve earthquakes in different parts of Iran were analyzed to study the attenuation of intensities with distance. The attenuation is quite sensitive to the selection of epicentral intensities, I0, for the earthquakes considered. A graphical method presented in this paper provides a consistent basis for the estimation of epicentral intensities, for different earthquakes, from the intensity-distance plot. The attenuation relations were derived by using an iterative least squares fit procedure, wherein an initial approximate estimate of epicentral intensity for each earthquake is successively improved. The isoseismal maps for a number of earthquakes are elongated in the direction of local structural trend/causative faults. Therefore, three different attenuation relations were derived.

 
I(R)=I0+6.4530.00121R4.960log(R+20)R<120kmaverageattenuationI(R)=I0+4.8240.00548R3.708log(R+20)R<160kmparalleltotheisoseismalsI(R)=I0+8.7290.01158R6.709log(R+20)R<110kmtransversetotheisoseismals

where I(R) is the intensity at a distance R (km) from the epicenter.

The average attenuation of intensities in Iran is slightly more rapid than the San Andreas province attenuation.

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