Geophysics
Mojtaba Naghavi; Habib Rahimi; Ali Moradi
Abstract
In this study, we have used recorded local earthquakes by 17 permanet seismic stations to separate intrinsic and scattering attenuation in North-West of Iranian pleateau. Intrinsic and scattering attenuation can be applied as useful tools to study the geodynamic and tectonic characteristics of a region. ...
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In this study, we have used recorded local earthquakes by 17 permanet seismic stations to separate intrinsic and scattering attenuation in North-West of Iranian pleateau. Intrinsic and scattering attenuation can be applied as useful tools to study the geodynamic and tectonic characteristics of a region. They also represent thermal, compositional and deformational characteristics of the crust and upper mantle. The wave attenuation has strong correlation with seismicity and heterogeneity of medium and is regularly used in the study of tectonically active regions of the world. Single backscattering and coda normalized methods are used to estimate the coda Q (Qc) and Qs respectively, using 14,969 earthquakes which are recorded by the stations. The results show this region is very active region tectonically and seismically. Due to low values of Quality factor and thus high attenuation values of body and shear waves in North West part of Iran, amplitude of the propagated waves are decreased severely in the interested area.The intrinsic attenuation and the Coda wave attenuations curves around the North Tabriz fault are closer in comparison with entire northwestern Iran region and Tabriz city, indicating a strong attenuation of the earthquake waves around this fault system. Similarly, these curves are closer in Tabriz city than those calculated for the northwestern region of Iran which expresses the overriding intrinsic attenuation from the effect of dispersion. The attenuation effect of seismic waves reduces the damages caused by earthquakes at appropriate distances of faults at the time of earthquake occurrence.
M Naghavi; H Rahimi; A Moradi
Abstract
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 ...
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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.
M Nemati; A.R Ghods; M.R Ghassemi
Abstract
Due to absence of an appropriate scale for estimation of ML for the earthquakes in eastern Alborz Range, we calculated 1113 synthetic Wood-Anderson peak amplitudes from waveforms of 215 earthquakes recorded by 23 stations at local hypocentral distances. The events were recorded by two local temporary ...
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Due to absence of an appropriate scale for estimation of ML for the earthquakes in eastern Alborz Range, we calculated 1113 synthetic Wood-Anderson peak amplitudes from waveforms of 215 earthquakes recorded by 23 stations at local hypocentral distances. The events were recorded by two local temporary seismological networks installed during 2007 and 2008 by the Geological Survey of Iran (GSI) and the stations of the permanent network of the Institute of Geophysics of University of Tehran (IGUT). Both temporary networks were installed for two discontinuous periods of nine months in the eastern- middle Alborz. In order to estimate an empirical attenuation curve for ML amplitudes, A, read from the stations at very short hypocentral distances, we fit a parametric relationship to the peak amplitude readings while considering geometrical spreading, intrinsic attenuation and stations corrections. We obtained the following empirical attenuation relationship:
Log Aij=-1.986log (Rij/100)-0.00452(Rij-100)-3+Sj
Where Rij is hypocentral distance in km between the jth station pair and ith earthquake and Sj is value of station correction for the jth stationThe realtionship clearly indicates a larger attenuation for shear waves in short hypocentral distances below 20 km. Our new ML relationship implies that using ML relationship derived for hypocentral distances larger than 50 km would overestimate ML magnitude of events recorded by our local networks by about half of unit magnitude. Thus we suggest that for local networks in other regions lacking any local ML relationship, ML relationship derived in this study to be used.
F Kamranzad; L Moussavi; M Mojarab; H Memarian
Abstract
In this study,attenuation behavior of moderate to large earthquake aftershock sequences occurred in Iranian plateauhas been investigated according to the empirical Omori Law. Due to proper recordings of instrumental earthquakes from 1990 to 2012, important earthquakes of this period were selected. After ...
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In this study,attenuation behavior of moderate to large earthquake aftershock sequences occurred in Iranian plateauhas been investigated according to the empirical Omori Law. Due to proper recordings of instrumental earthquakes from 1990 to 2012, important earthquakes of this period were selected. After determination of aftershock sequences using temporal-spatial window defined by Gardner &Knopoff (1974), 14 sequenceshaving enough recordings and appropriately distributed over the Iranian plateau were investigated in terms of attenuation behavior curve.Therefore, the Omori curve and parameters (p, c and k)were plotted and calculated for each sequence. Results show that for the Iranian plateau earthquakes, p-values range between 0.39 and 2.7, parameter c values vary from 0.01 to 5, and paremeter k shows values in the range of 10 to 1427.4. This high variability is taken to indicate not only a variety of aftershock occurrence patterns in the Iranian plateau, but also an incomplete and inhomogeneous earthquake catalog.By using the present database, therefore, it is not easily possible to have a zonation based on temporal attenuation behavior of aftershock activitiesover the Iranian plateau. However, the estimations of aftershock attenuation rate for each locality can be used to analyze seismic hazard. Present study showed that the p-values and hence the aftershock attenuation rates in the Alborz and Zagros regions are greater than those in the eastern and central parts of Iran. The higher the rate, the greaterthe energy release, which means a shorter time to gain background seismicity. This result is comparable and consistentwith the amount of energy released in theseismotectonic zones of the Iranian Plateau. Moreover, 7 out of 14 earthquake sequences have secondary aftershocks, which give two values for each Omori parameters. Results demonstrated that with a higher earthquake magnitude, the occurrence of the next big event as well as secondary aftershocks is more likely. Furthermore, for the 7 sequences with secondary aftershocks, a trend of P2 variations is observable. P2 is more than 2.5 for 3 of these sequences that have magnitudes above 7 and occurred along the Iranian plate boundaries. For the other 4 intraplate events, which have magnitudes less than 7, P2 is less than 2. This might be due to a magnitude change or tectonic setting and distance of hypocenter to the main fault nodes. Resultsalso showed that the c and k parameters are highly affected by number of recordings in the catalog. A more complete and homogeneouscatalog would produce well-constrained values for these parameters,which in turnmakes the analysis of the seismicity and physics of the fault zone more accurate.
