Attenuation relationships for the peak horizontal ground acceleration (PHA) and for the response spectra of the horizontal ground acceleration (RSHA) for the main seismic zones of the Iranian plateau (i.e., Alborz, Zagros, and Eastern Iran) have been developed. The region bounds from 24 to 40 degrees north latitude, and from 44 to 62 degrees east longitude. The selected data consist of 447 ground horizontal acceleration records from 233 earthquakes with M = 4.0 to M = 7.4 recorded by the Iranian strong-motion network. The recording distances range from 3 to 450 km. The method of present analysis follows the two-step procedure used by Joyner and Boore (1981) with the following general form: log(A) = a + bM- log(r) – cr r = (d 2 + h2) 1/2 4.0 < M< 7.4, where A is either PHA or RSHA in cm/sec/sec, M is the moment magnitude, d is the closest distance to the surface projection of the fault rupture in kilometers, and a, b, c and h are the parameters to be evaluated. The table below gives the parameters for the aforementioned zones. Standard deviation of each regression is also given. RSHA2 and RSHA3 are the pseudo responses of horizontal acceleration at 0.2 and 0.3 seconds with 5% of critical damping, respectively. Zone A a b c h sd Albotz PHA 1.028 0.402 0.00170 14.20 0.253 RSHA2 1.241 0.439 0.00234 21.90 0.257 RSHA3 0.598 0.542 0.00260 28.22 0.276 Eastern Iran PHA 0.899 0.392 0.00056 06.63 0.212 RSHA2 0.659 0.492 0.00068 06.63 0.242 RSHA3 0.373 0.521 0.00030 09.50 0.233 Zagros PHA 0.721 0.436 0.00300 02.67 0.318 RSHA2 0.277 0.484 0.00316 03.55 0.371 RSHA3 -0.644 0.721 -0.00169 03.04 0.365

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