Determination of Lg Coda Q from local earthquakes in the Central Alborz, Iran



In the present study, the Q factor is obtained for the Central Alborz in Iran using the Lg coda method. The energy of a seismic wave decays while passing through a ‘real’ medium such as the earth which is not completely elastic. The decay in energy is due to non-elastic phenomena and is called intrinsic attenuation and is characterized with the Q parameter, the large values of which represent small values of attenuation and as Q approaches zero the pertaining attenuation will become quite strong. Therefore Q could be considered as a measure of elasticity of the media
The objective of this study is to determine the Lg coda Q from ground motion recorded at 32 short-period stations in the central Alborz, north central Iran. Lateral variations of Q and its frequency dependence were estimated using 1020 high quality Lg waveforms in the frequency range between 0.3-7.0 Hz. In the studied area, the factor resulted from this research, on average, is given by: . The lateral variation of correlates well with the large scale tectonic units of the studied area. Damavand volcano and its surrounding region is also characterized with a significant gradient in values, going from anomalously low values in the western side to the average values in the eastern side. The current seismicity in the Damavand area is mostly confined to the south-western part, which is characterized with anomalously low values and sharp gradient of .
In Damavand region, has a relatively higher gradient than that of the surrounding region. It sharply declines moving from east to west. The map correlates well with the large-scale tectonic units of the studied area and also several clear trends corresponding to different characteristics of seismic activity and attenuation field. Most of Q factor variations can be attributed to the lateral heterogeneity as well as the severity of the crustal velocity gradient. Since the seismicity in the area is quite shallow (earthquake depths mostly are less than 30 km), the results can be attributed to the average of upper part of the 30 km of the crust of the study area.