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This paper reports results of mathematical modeling applied to the stress and strain in epicentral zones before and after the large earthquakes that occurred on June 22, 2002 in the Qazvin Province, northwestern Iran (Mw = 6.4) and the Gujarat, India earthquake of January 26, 2001 (M = 6.9). The modeling relied on a method for calculating stress and strain in a blocky elastic isotropic heterogeneous medium disturbed by a set of faults that are due to an external tectonic stress field. The boundary conditions were specified based on geological and seismological data. It has been shown that the epicenters of large crustal earthquakes occur in zones of high stress concentration at the ends of tectonic faults. Rupture occurs when the relationship between acting tectonic stresses satisfies the requirement σyy/σxx >3, thus connecting zones of high stress. The evolution of the aftershock process is controlled by the stress drop caused by a new rupture, while the resulting aftershock clusters are in spatial correlation with the stress drop zone. The new rupture propagates in the direction of dominant tectonic fault orientation in the region. We show a relationship to exist between rupture length and the possible retrospectively predicted location and magnitude of the earthquake depending on the elastic energy buildup and its possible release under specified structural tectonic conditions. [ABSTRACT FROM AUTHOR]

In this paper, significant variations in rate of moderate-large earthquake occurrence during the last two centuries at the north of Persia (Alborz, Azarbaijan, and Kopeh Dagh seismotectonic provinces) were chronically investigated. Depths of some historical and instrumental earthquakes were reasonably estimated using macroseismic data. Also, empirical relationships were developed between the maximum intensity and magnitude of the earthquakes in the north of Persia. Two centuries of earthquake data (M ≥5.0) reliably demonstrates that quite periods with durations of about a few years to two decades have happened and have been subsequently followed by active periods with durations of about 10 to 70 years. The time variation of seismic hazards (achieved by the macroseismic maximum intensity, I ≥IV) almost displays a similar behavior at the study area. According to both patterns of the time variations of seismicity and the seismic hazards, it may conclude that we are within a low rate of seismic period, and we may expect a higher rate of seismicity at the Alborz in the next decades. Finally, in both the Azarbaijan and Kopeh Dagh areas, we discovered considerable migrations of the seismicity from the east to the west. [ABSTRACT FROM AUTHOR]