Seismicity Parameters and Spatially Smoothed Seismicity Model for Iran

Abstract

Abstract In the present study, seismicity parameters (seismic activity, β ‐value, and maximum regional magnitude) of the Iranian plateau are computed for sites equally distributed all over the country on a grid of 1×1 decimal degree. The most complete available catalog including historical and instrumental earthquakes of the plateau from 734 B.C. to A.D. 2011 is first prepared from numerous resources. Earthquakes within a 200‐km buffer surrounding each grid point are selected, and historical and instrumental parts are more accurately specified, based on the completeness test of Stepp (1972). The instrumental part is also analyzed to identify the various completeness intervals, and the minimum completeness magnitude for each interval is determined by the maximum curvature method. Next, the seismicity parameters are calculated from uncertain and incomplete data by the maximum likelihood procedure of Kijko and Sellevoll (1989, 1992) and Kijko (2004), after removing aftershocks and foreshocks. The spatial variation of seismicity parameters is illustrated in the form of contour maps over the whole Iranian plateau for the first time. The λ ‐value corresponding to the magnitudes M w 4 varies over the plateau from 0.7 to 23.4, and the highest λ ‐values are located in the Zagros and Alborz mountains, confirming that a large portion of the seismic deformation in the plateau is accommodated in these regions. The estimated β ‐values for all points lie between 1.3 and 2.9. This spatial distribution of β ‐values demonstrates the crustal heterogeneity over the plateau. Low β ‐values in northeast and east Iran also indicate high probability of large‐earthquake occurrence. It is observed that the estimated M max is between M w 6.5 and 8, and hence most of that part of the country may experience large earthquakes. The obtained results are important for two aspects: (1) new seismotectonic models could be proposed by combining the presented maps with geological, geodetic, and tectonic data; and (2) these results, together with a detailed geological data describing potential seismogenic sources, can be used directly to evaluate ground‐motion hazard for engineering design and generate seismic‐hazard maps. An approximate and gross estimate of peak ground acceleration (PGA) level at bedrock, for 2% and 10% probabilities of exceedance in a 50‐year period, is also computed using an area‐source model characterized by the presented seismicity parameters. The regional distribution of the estimated PGA sheds a new light on facilitating risk management and allocating national resources for mitigating potential losses at a country‐based level. Online Material: Table of completeness magnitude and seismicity parameters.