عنوان مقاله [English]
The study of induced earthquakes is important in different aspects. One of the most important aspects of reservoir-induced seismicity is the possibility of the triggering of a strong ground motion after reservoir impoundment. This study aims at finding a relationship between the ML5.2 Babolkenar earthquake of 2012 January 11 and the Alborz dam’s impoundment. The Alborz dam is located on the northern flank of the Alborz Mountains. The Alborz dam is an embankment dam and its capacity is 8.6 million cubic meters. Its ridge lies on the tectonized Early Tertiary rocks. The 5.2 ML Babolkenar earthquake of 2012 January 11 is assessed with the use of waveforms provided by different seismic stations. The analyses of waveformes show the centroid depth of 20km and epicenter located at 36.357N, 52.788E. Its thrust fault focal mechanism is in agreement with the fault kinematics inversion implying the reactivation of one of segments of the Khazar fault. Although the event in question could be considered as an ordinary tectonic earthquake, some lines of existing evidence associated with the reservoir impounding data encouraged the authors to consider it as anthropogenic seismicity. Firstly the reservoir is located in the area between two very active thrust fault that contains fractured and permeable rocks. Secondly is the drastic change of seismicity after the onset of reservoir impoundment represents the change of the b-value before, during, and after reservoir impoundment. Thirdly the trigger of the event just after a lowering of the water level. In these cases, earthquake rupture may be interpreted by two different mechanisms: 1) an immediate, undrained, elastic response to the reservoir load and/or an instantaneous pore pressure change in the vicinity of the reservoir due to an undrained response )Skempton, 1954(. The governing equation to explain the undrained response is . In this equation, the change of the undrained compression is related to the Skempton coefficient (B) and the average normal stress at a point located on the fracture ( ). Although the lowering of the reservoir’s water table may change rapidly , the B-coefficient on the other hand, could not change immediately. Therefore the pore pressure is increased even if the water level is decreased leading to satisfying the Coulomb failure criteria. 2) A delayed and/or undrained response due to diffusion of pore pressure. The curve of the accumulative of the seismicity versus the time history of the reservoir impoundment shows two cycles of rising and lowering the water level. Each cycle starts with a gently linear increase of seismicity corresponding to the rising of the water table and ends with a period of increase of the seismicity in response to the water level drawdown. The authors think the diffusion of pore pressure is responsible for the increasing of the seismicity after reservoir impoundment. Given the r=25km as the maximum distance between dam and centroid depth and ∆t=12 months as average time elapsed between the time of that water level increase and the time of the earthquake, the hydraulic diffusivity c=4.95 m^2/sec is estimated by . By the other approach in which the area is affected by the after shakes (here ~100km2) is considered as the r2 in the above-mentioned equation, the c-value takes 0.8 m^2/sec. This c-value discrepancy may be raised by lack of data especially due to lack of local seismic network to the survey of microearthquakes and also unavailable bore-hole to install piezometers measuring the water pore pressure versus time. The c-coefficient may be a useful parameter that is applicable to predict the same reservoir-induced or triggered seismicity in future times. Finally, as mentioned above, the b-value decreased just before the event. This may be in an analogy to the decreasing of the b-value in the other anthropogenic seismic activity i.e. mining tremors, usually called rockburst, and a strong ground motion governed by tectonic activity. The b-value after the event M5.2 increased. Finally, the authors strongly recommend the government plan detailed geological, geophysical and geotechnical studies before and after the impoundment of a new water reservoir to monitor the induced seismicity in the northern flank of Alborz. This may help to mitigate the damages due to the probable triggering of the active faults within this very seismically active belt.