Kh. Hessami Azar; Kh. Motaghi; M. Mostafazadeh
Abstract
In this study, we estimated a local seismic hazard map for the Alborz tectonic region located in north of Iran. The technique of mapping local recurrence time, TL, was used to map major asperities of the region and the asperities considered as the area with maximum hazard probability. The analysis was ...
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In this study, we estimated a local seismic hazard map for the Alborz tectonic region located in north of Iran. The technique of mapping local recurrence time, TL, was used to map major asperities of the region and the asperities considered as the area with maximum hazard probability. The analysis was done on the epicenter of more than 5000 events recorded in 19 stations of the Institute of Geophysics, University of Tehran (IGUT), during 1996-2008. Based on the idea that b-value is inversely related to applied stress, we calculated local b-values for each grid and areas with lowest b- and a-values, and as a result shortest TL were interpreted as asperities. Because the ruptures start from the asperities, the mentioned regions have considered as the region with the maximum seismic hazard. The rupture location of MW 6.3 Kojour-Firooz Abad earthquake compared to the recurrence time map and it resulted in the location of this event had showed anomalously short TL based on the background seismicity of region in a decade before it. We computed TL map based on the seismicity before and after Kojour-Firooz Abad earthquake and showed that this large event redistributed the applied stress in the Alborz region. Redistribution by large earthquake led to migration of the applied stress from west of region to east. Based on the microseismicity of the region after Kojour-Firouz Abad earthquake, there are three anomalies in TL map positioned in eastern Alborz. These anomalies are introduced as the regions with maximum seismic hazard for future large earthquake.
M. R. Ebrahimi; Mohammad Tatar
Abstract
Masjed Soleyman reservoir is located in Zagros Mountain of western Iran, which is one of the most seismically active zones of the Alpe-Hymalaya belt. So, it seems to be necessary to carry out widespread studies, especially on the impact of this reservoir with 177 m height and 261 million m3 capacity ...
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Masjed Soleyman reservoir is located in Zagros Mountain of western Iran, which is one of the most seismically active zones of the Alpe-Hymalaya belt. So, it seems to be necessary to carry out widespread studies, especially on the impact of this reservoir with 177 m height and 261 million m3 capacity on occurrence of induced seismicity in the surrounding region. The Gutenberg-Richter relation is one of the well-fitted empirical relations in seismology: it represents the frequency of occurrence of earthquakes as a function of magnitude: , where N is the cumulative number of earthquakes with magnitude larger than M and A and b are constants. In this paper we used b-value to study the heterogeneities in the crust beneath and around the Masjed Soleyman reservoir. In order to better understanding of the impact of this reservoir on seismic activity, a local seismic network of 5 seismological stations was installed in the area on June 2006. About 1924 Seismic events recorded during a period of 15 month were used in this study. We maped both surface and cross-section view of b-value in the region using the computer program ZMAP. The study area was divided into grids with spacing of 0.01o in latitude and longitude. A circle was drawn around each grid point and its radius was increased until it included N=50 earthquakes. The b-value was calculated by using a maximum likelihood method for the selected 50 earthquakes and the grid point was colored corresponding to the b-value. The results show high value of b-value due to reservoir induced earthquakes beneath the Masjed Soleyan lake. The most important factors known responsible for increased heterogeneity in this area, are reservoir loading and increased pore fluid pressure that cause occurrence of swarms and heterogeneous stresses in the area.
