Probabilistic Seismic Hazard Analysis and Strong Ground Motion Assessment in Southwest Alborz, Iran

Introduction: 

A significance of seismic studies is that the correct seismic analysis of any type (seismic hazard analysis, seismic risk analysis, ground seismic response analysis, seismic site effects, and structural dynamic analysis) can offer useful economic parameters and avoid conservative design and implementation, which lead to an irrational increase in project costs and poor implementation, which in turn causes increased risk and possibility of destruction. According to the seismotectonic map of Iran (Berberian, 1976), earthquakes in Alborz are shallow. There are also some intermediate earthquakes, and overall, the eastern Alborz is more earthquake prone than the western Alborz [1].

Materials and Methods:

The maximum magnitude (Mmax) is usually estimated based on the general characteristics of seismic activity and geological similarities. In applied studies, Mmax is often estimated based on correlation of seismic magnitude and different fault parameters such as rupture, fracture surface area, maximum surface displacement, and seismic moment release rate. Multiple correlations have been proposed to relate these parameters and the earthquakes magnitude. (Table 1) shows some correlations by different scholars. Correlations in (Table 1) were used to calculate the maximum empirical magnitude [3].Table 1. Correlations between the earthquake magnitude and different fault parametersCorrelationProposed byNo.Ms=5.4+LogLRMohajer and Nowroozi (1978)1Mw=3.66+0.91LnLRZare (1995)2Ms: Surface wave magnitude                                                          Lf: Fault length (km)Mw: Moment magnitude                                                                  LR: Rupture length (km) Seismicity Parameter EstimationThe K-S method was used to achieve seismicity parameters within the scope of this study [2].

Discussion and Results:

The results of probabilistic seismic hazard analysis were calculated using attenuation relationships Zare 1999 [4], Ambraseys 1995, Boore, Joyner and Fumal 1981in studying region. These results are presented in (Table 2) and (Table 3).Table 2. strong vertical ground motionReturn periodH.PGA(g)4750.2524750.44Table 3. strong Horizontal ground motionReturn periodH.PGA(g)4750.4224750.70 ConclusionThe recent Malard Earthquake with a magnitude of 5.2 on the Richter scale and multiple earthquakes with magnitudes above 4 have increased the importance of seismic studies in the region. Seismic hazard studies are among the key preliminary urban development studies for preventing seismic vulnerability. The identification of seismic source zones is closely related to development infrastructure in any region. The results of these studies are widely used in vital projects such as water, gas, oil transmission lines, dam and airport construction, and residential development, and overlooking them may cause great damages. The earthquake hazard analysis based on the accurate location of seismic zones will provide more reliable results. The investigation of the region under study, its history of seismicity, and the recent earthquakes indicate the existence of seismic activity in the region. Considering the shallow depth of earthquakes, the intensity of earthquakes occurred in the region is high. Moreover, the calculation of β and λ parameters (ranging from 6.2 to 7.6) shows the seismicity of the region, indicating the need for observing safety measures in the constructions in the region. As mentioned earlier, the recent seismic activities and earthquakes in the region have doubled the importance of seismic studies and measures for strengthening seismic stations in the region. Moreover, the review of seismic catalogs show that the study area has been inactive over the past few decades and hence its sudden activity is quite significant. Also according to the calculations, Alborz Province is located in highly active seismic zone.