Tectonics
Khatoun Pirdadi; Maryam Agh-Atabai; Attieh Eshaghi
Abstract
The 2017 Ezgeleh earthquake (Mw: 7.3), which occurred in the northwest of the Zagros, was followed by many aftershocks. The histogram of the monthly rate of aftershocks shows that, except for the first few months, the aftershock sequence did not follow the Omori law. Therefore, it is necessary to use ...
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The 2017 Ezgeleh earthquake (Mw: 7.3), which occurred in the northwest of the Zagros, was followed by many aftershocks. The histogram of the monthly rate of aftershocks shows that, except for the first few months, the aftershock sequence did not follow the Omori law. Therefore, it is necessary to use more complex methods to investigate the aftershock sequence. In this research, the temporal multifractal method was used. The obtained results show that the temporal pattern of aftershocks has two short and long-scaling ranges. It seems that short and long ranges are related to the distribution of aftershocks within smaller clusters and the pattern of clusters in the aftershock sequence, respectively. These result showed that the pattern in the longer range is more heterogeneous than the shorter one. On the other hand, by removing the smaller aftershocks, the heterogeneity increases. It seems that the occurrence of several more significant aftershocks with a magnitude of more than 5 has caused an increase in the heterogeneity of the temporal pattern of the aftershock sequence. The results also show that the degree of inhomogeneity of the occurrence time of aftershocks is related to the spatial distribution pattern of aftershocks.
F Kamranzad; L Moussavi; M Mojarab; H Memarian
Abstract
In this study,attenuation behavior of moderate to large earthquake aftershock sequences occurred in Iranian plateauhas been investigated according to the empirical Omori Law. Due to proper recordings of instrumental earthquakes from 1990 to 2012, important earthquakes of this period were selected. After ...
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In this study,attenuation behavior of moderate to large earthquake aftershock sequences occurred in Iranian plateauhas been investigated according to the empirical Omori Law. Due to proper recordings of instrumental earthquakes from 1990 to 2012, important earthquakes of this period were selected. After determination of aftershock sequences using temporal-spatial window defined by Gardner &Knopoff (1974), 14 sequenceshaving enough recordings and appropriately distributed over the Iranian plateau were investigated in terms of attenuation behavior curve.Therefore, the Omori curve and parameters (p, c and k)were plotted and calculated for each sequence. Results show that for the Iranian plateau earthquakes, p-values range between 0.39 and 2.7, parameter c values vary from 0.01 to 5, and paremeter k shows values in the range of 10 to 1427.4. This high variability is taken to indicate not only a variety of aftershock occurrence patterns in the Iranian plateau, but also an incomplete and inhomogeneous earthquake catalog.By using the present database, therefore, it is not easily possible to have a zonation based on temporal attenuation behavior of aftershock activitiesover the Iranian plateau. However, the estimations of aftershock attenuation rate for each locality can be used to analyze seismic hazard. Present study showed that the p-values and hence the aftershock attenuation rates in the Alborz and Zagros regions are greater than those in the eastern and central parts of Iran. The higher the rate, the greaterthe energy release, which means a shorter time to gain background seismicity. This result is comparable and consistentwith the amount of energy released in theseismotectonic zones of the Iranian Plateau. Moreover, 7 out of 14 earthquake sequences have secondary aftershocks, which give two values for each Omori parameters. Results demonstrated that with a higher earthquake magnitude, the occurrence of the next big event as well as secondary aftershocks is more likely. Furthermore, for the 7 sequences with secondary aftershocks, a trend of P2 variations is observable. P2 is more than 2.5 for 3 of these sequences that have magnitudes above 7 and occurred along the Iranian plate boundaries. For the other 4 intraplate events, which have magnitudes less than 7, P2 is less than 2. This might be due to a magnitude change or tectonic setting and distance of hypocenter to the main fault nodes. Resultsalso showed that the c and k parameters are highly affected by number of recordings in the catalog. A more complete and homogeneouscatalog would produce well-constrained values for these parameters,which in turnmakes the analysis of the seismicity and physics of the fault zone more accurate.
Mohammad Tatar; M. R. Ebrahimi; F. Yamini Fard
Abstract
Masjed Soleyman reservoir is located in Zagros Mountain of western Iran, which is one of the most seismically active zones of the Alpe-Himalaya 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-Himalaya 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. During impounding of the reservoir regards to national and international catalogues such as International Institute of Earthquake Engineering and Seismology (IIEES) and EHB catalog, an incensement in seismic rate is observed in the Masjed Soleyman region, and 90 days after completing the impounding, an earthquake with magnitude Mw=5.6 is occurred in the close vicinity of 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. Seismic events recorded during a period of 15 month were used in this study. The largest recorded earthquakes during the monitoring of Masjed Soleyman reservoir have magnitudes of ML=3.9 and ML=3.6. Statistical methods such as correlation of the water level changes with variation of the regional seismicity, the foreshocks and aftershocks pattern and decay rate of aftershocks reveal the existing of induced seismicity in the Masjed Soleyman region. In absence of a local network before 2002 September, 25 earthquake (Mw=5.6), our observations indicate this earthquake is truly the first and largest induced earthquake that have been occurred and recognized in Iran.
A. Gholam Zadeh; F. Yamini-Fard
Abstract
A moderate earthquake (MW= 6.0) struck the Qeshm Island in the Hormozgan province on November 27, 2005 (17:22 GMT) and resulted in a severe damage and about 10 casualties. The main-shock was followed by an aftershock with magnitude MW=5.5 with different focal mechanisms from main-shock. A dense local ...
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A moderate earthquake (MW= 6.0) struck the Qeshm Island in the Hormozgan province on November 27, 2005 (17:22 GMT) and resulted in a severe damage and about 10 casualties. The main-shock was followed by an aftershock with magnitude MW=5.5 with different focal mechanisms from main-shock. A dense local network including 17 stations was installed in the region for aftershock study. Analysis of aftershock data shows diffuse distribution of the aftershocks; however, an alignment trending NW consistent with main shock focal mechanism is clear at depth. Two types of focal mechanisms can be observed: strike-slip and reverse. Diffused pattern of aftershock seismicity and focal mechanisms do not allow us to make a distinction between two possible explanations: occurrence of the second event in NW oriented strike-slip fault or partitioning of the deformation in the western border of the Hormoz Straight by reverse and left-lateral strike-slip faults.
M. Tatar; A. M. Farahbod
Abstract
A moderate earthquake (Ms=6.3) struck the coastal region of north of Iran and the central Alborz on 28 May, 2004 was responsible of several damages and about 35 casualties. The mainshock was followed by a large number of aftershocks, the largest ...
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A moderate earthquake (Ms=6.3) struck the coastal region of north of Iran and the central Alborz on 28 May, 2004 was responsible of several damages and about 35 casualties. The mainshock was followed by a large number of aftershocks, the largest one reaching Ml=4.8, based on the analysis of local waveforms. We study the mainshock, first major aftershock, and about 240 aftershocks recorded by Iranian National Seismic Network (INSN), Tehran Telemetry Seismic Network (Institute of Geophysics, Tehran University), and our temporary local seismological stations, which were installed on 30 May, around the epicentral area of this earthquake. Using waveforms of all permanent stations, the coordinates of the mainshock was determined as 36.30 °N for latitude and 51.60 °E for longitude. The analysis of aftershocks indicates that the seismic activity migrate from east where the mainshock occurred toward west close to the location of the largest aftershock (36.36 ° N, 51.45 ° E). 140 selected aftershocks recorded at a minimum of 6 stations, having rms less than 0.15 sec and uncertainties less than 2 km, were used to infer a precise geometry of the fault region. The aftershocks distribution has 30 km long and trends NW-SE parallel to the North Alborz and Khazar faults. The focal depths comprised between 10 and 28 km, unusually deep for Iran. Distribution of aftershocks cluster on cross-section defines a fault plane which dips at 40-50 degree south-westward. Its upward continuation can be related to either North Alborz or Khazar faults. Most of the focal mechanisms are consistent with reverse faulting on NW-SE trending faults, parallel to the main active structures of the region. Well constraint focal mechanisms which dip gently at a rate of 25-40ْ indicate the activity of the second mentioned faults during the Firozabad-Kojour earthquake. Existence of focal depths up to 28 km indicates an unusual brittle lower crystalline crust in this part of central Alborz.
