Estimation of the kinematic source parameters and frequency independent shear wave Quality factor from acceleration records of the Ahar-Varzagan earthquake 2012

Two relatively large earthquakes on the 11th of August of 2012، struck the region of Varzagan and Ahar County، East Azerbaijan Province، NW Iran. The devastation caused by earthquakes in different regions and prediction of strong ground motions from large earthquakes has attracted the attention of seismologists to investigate the attenuation characteristics of the region and source characteristics of earthquakes to better understand the seismic hazards in different regions. Several recent and historical catastrophic earthquakes have destroyed different parts of East Azerbaijan Province. The seismic hazard map of Iran [published by the Building and Housing Research Center (BHRC)] shows that most of the cities in this Province are located within the high or very high relative risk areas. The quality of manmade constructions، especially in small villages and towns، is usually poor. Residences are generally built without considering seismic design regulations، so they are highly vulnerable and will collapse under shaking caused by even moderate earthquakes. A displacement spectrum contains valuable information regarding the source and medium characteristics. The source spectrum of an earthquake can be approximated by the omega-square model (Brune، 1970)، which has ω2 decay of high frequencies above the corner frequency. The source displacement spectrum can be estimated from a displacement record after correcting with diminution function، which accounts for the geometrical spreading and anelastic attenuation. The anelastic attenuation of seismic waves is characterized by a dimensionless quantity called quality factor Q (Knopoff، 1964). So far a few studies have been carried out to understand the attenuation characteristics of the Iranian crust. Examples include the work by Nutlii (1980)، Michell (1995) and Rahimi & Hamzehloo (2008). An analysis scheme for obtaining source parameters and quality factor Q using the generalized inversion has been presented in this paper. The work presented here is approximately based on the technique of Fletcher (1995) and Joshi (2006) that used inversion methods. In this paper، the Brune’s source model (Brune، 1970) is used together with the propagation filter. This study uses the acceleration data of the Ahar-Varzagan main shock recorded by Building and Housing Research Center (BHRC) strong ground motion network. Our main objectives are: (i) to compute the source parameters of these earthquakes using the acceleration data، and (ii) to compute the frequency-independent shear wave quality factor in the recorded stations. In this study to get shear wave quality factor and source parameter in the near field، we used the strong motion data of the two earthquakes. Source parameters are estimated from that are related to two events with moment magnitudes of 6. 5 and 6. 4 in the hypocentral distance range from 22 to 206 Km. In this approach the theoretical S-wave displacement spectra، conditioned by frequency-independent Q، was fitted with the observed displacement spectra. Therefore corner frequency، moment magnitude and frequency-independent Q for each record are estimated simultaneously and the error estimate is given as the root-mean-square over all the frequencies. The source terms estimated here are، 3. 26E+25 (dyn-cm)، ، 0. 17 (Hz)، ، 8. 19 (km)، 86. 74، =32. 02 (bar)، ∆ =110. 4، ∆ =50. 39 (cm)، =9. 13، =5. 51 (sec)) and estimated moment magnitude (، = 6. 2) agree well with values obtained from telesiesmic wave of Harvard University. Estimate of path-average crustal shear –wave quality factors give a range of Q= 71 to 501 for frequency band of 0. 01 to 15 Hz، that for near stations has a low value (high absorption) and for others at the further distance it has high value (low absorption)، which shows good agreement with high-frequency absorption in near field. Independent estimates of Q at various stations give its average value of 276.