Seismicity of the Iranian Plateau and bordering regions during the past 22½ yr from 1 July 1957 through December 1979 is investigated in four successive time intervals by analyzing close to 4000 earthquakes in the region. This tectonically active part of the Alp-Himalayan belt is defined by a broadband of diffuse seismicity and contains several mobile belts surrounding small, relatively stable blocks. The major zones of mobility in decreasing order of activity are, Hmdukush, Zagros, Elborz, Chaman fault system, east-central Iran, and the Caucasus and eastern Turkey. The most conspicuous aseismic block is that of western Afghanistan, but smaller blocks in central Iran, Azarbayejan, and the southern Caspian Sea also show noticeable stability. The a and b parameters of the frequency-magnitude recurrence curve during successive time intervals in three of the mobile zones which are located in Iran show an increasing trend to stabilized levels at the end of the period. The annual average of seismic moment rates in Zagros during the past 22½ yr requires nearly a two order of magnitude increase to account for the relative plate motion between the Arabian and central Iran plates.

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