عنوان مقاله [English]
The energy of a seismic wave decays while passing through a “real” medium such as the earth which is not completely elastic. Scattering and attenuation of high-frequency seismic waves are substantial parameters to quantify and to physically characterize the earth medium and from which useful information on medium properties can be inferred. The coda waves in seismograms are one of the most prominent observations supporting the existence random heterogeneities in the earth. Determination of source parameters must take into account the proper attenuation characteristic of the wave path. Moreover, it is essential for seismic risk studies and seismic hazard assessment, and consequently for seismic risk mitigation and engineering seismology. Many researchers used coda waves from small earthquakes to determine local attenuation properties of the crust.
The S-coda has a common amplitude decay curve for lapse time greater than the twice the S-wave travel time. The shape of this decay curve is quantified by using a parameter knows coda attenuation Qc-1. The time domain coda decay method of a single back scattering model is employed to estimate frequency dependence of the quality factor of coda waves modeled using, where is the coda quality factor at frequency of 1 Hz and is the frequency parameter.
The purpose of this study is to determine the coda quality factors from recorded events at 17 stations in the NW of Iranian plateau, using the single backscattering method (Aki and Chouet 1975). Scattering models have been developed in order to infer physical properties of the lithosphere from observations of seismic codas.
In this study, the coda quality factors of seismic waves have been estimated by using local earthquakes with recorded in NW of Iranian plateau. This region includes major faults such North Tabriz Fault and two volcanoes (Sahand and Sabalan) and many thermal units.
The data used in this study consists more than 13000 earthquakes and 26724 high-quality waveform recorded by Iranian National Seismic Network (INSN) and Iranian Seismological Center (IRSC) stations to estimate lateral variations of coda wave quality factor. By using these data set, Qc and its frequency dependency were estimated, in NW of Iranian plateau.
We also investigated lateral and depth variation of Qc in this region. The average frequency relations for NW of Iranian plateau and around North Tabriz Fault (NTF) are , and, respectively. These values show this region is very active region tectonically and seismically. To investigate the attenuation variation with depth, Qc value was calculated for 18 lapse-times (5, 10, 15, 90s) for two data sets comprising epicentral distance range R < 100 km (data set 1) and 100 < R < 200 km (data set 2). As the quality factor is related to the heat flow, as the mechanisms show, it decrease with increasing temperature, because active region greater absorption than stable region. We should note that in this study the results, are taken as mean values of each propagation-path. It is observed that generally with increasing coda wave lapse-time, Q0 (quality factor at 1 Hz) and n (frequency dependence factor) values show increasing and decreasing trend, respectively.
Determinations of and n in the attenuation relationships for different tectonic regions, have been the focus of many studies. Both these parameters appear to represent the level of tectonic activity of a seismic region. According to the results, we observed well correlation between reported lithosphere thickness and trends of and n in longer lapse-times (larger depths). The lateral variation of correlates well with the large scale tectonic units of the studied area. According to the results obtained in this study NTF, Sahand volcano and its surrounding regions are characterized by relatively low and a high gradient of can be observed in the region. Furthermore NW Iran is a region of significant geothermal activity and anomalously high crustal temperatures. These geothermal activities result in smaller values for quality factor and higher attenuation of seismic waves. Such of the and n variations can be attributed to variability in the depth and severity of the crustal velocity gradient.