abstract

Three seismic activity maps, the A-value map, the b-value map and the returnperiod map for earthquakes with magnitude 6 and above, have been prepared for the southwest Asia region using the Kaila and Narain (1971) method, and the same are compared with regional tectonics. For the preparation of these maps, a modified relation A = 6.36b - 1.00 has been used instead of the earlier relation where A and b are constants in the cumulative regression curve represented by log N = A − bM. The A-value seismicity map reveals that the Southwest Asia region consists of a number of seismic high zones such as the Caucasus-Abul Samsar high, the Zagros high, the Shahrud-Doruneh high and the Hindukush-West Pakistan high. The Caucasus-Abul Samsar seismic high shows two superimposed trends, one NW-SE which is consistent with the Caucasus tectonic trend and the other NE-SW which is parallel to the Abul Samsar fracture zone. The Zagros seismic high runs in the NW-SE direction almost parallel to the Zagros thrust zone with diversions to the northeast at the two ends. High seismic activity is revealed in the Zagros foothills area rather than the thrust-zone. The Shahrud-Doruneh high shows a NW-SE trend parallel to Kopet Dagh, and, toward the west, it bends down aligning itself almost parallel to the Elburz mountains, thus indicating the possibility of a connection between this high and the Zagros high. The Hindukush-West Pakistan high runs in the NNE-SSW direction consistent with the tectonic trends in this area, indicating the highest seismic activity near the Yasman fracture zone. The b-value seismicity map also reveals the same seismic features as brought out by the A-value map. The b-values obtained by this new method over various regions of southwest Asia agree fairly well with those reported by other workers obtained from earthquake regression curves. The return-period map further brings out the zones of high and low seismic activity which are quite consistent with the A-and the b-value maps, and the regional tectonics.

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