The isoseismal maps for 22 earthquakes in different regions of Iran were analyzed to study the attenuation of seismic intensity with the distance from the epicentre under a certain surface wave magnitude (Ms). The attenuation relationships were derived by using an iterative least squares fit procedure. These equations were derived from more basic concepts assuming that the intensity is proportional to the logarithm of seismic energy density at any location empirically. The isoseismal maps are elongated in the direction of local structural trend of causative faults. Therefore, attenuation relationships for the main direction of fault, transverse to it and average attenuation were derived.
Ia =11.564 + 0.943M — 2.508Ln(Ra + 33) a = 0.79 Ra <200km along the main direction of fault
Ib= 9.469 + 0.717M —2.121Ln(Rb + 13) a = 0.49 Rb
I = 11.926 + 0.831M — 2.7Ln(R + 22) a = 0.49 R<167km average attenuation Where M is the surface wave magnitude and I is the intensity at a distance R(km) from the epicenter.
The attenuation of seismic intensity in Iran is faster than the East and West of China and Sicily and Calabria in Italy, apart from different tectonic characteristics of these regions.