جستجوی مقالات مرتبط با کلیدواژه "امواج کدا" در نشریات گروه "زمین شناسی"

In this study, for estimating of coda wave attenuation in the North of Sanandaj-Sirjan Zone the seismograms of earthquakes recorded at the seismic stations which located at the longitude of 48°-50° and latitude of 38°-40° have been used. The data with an epicentral distance less than 200 km and a signal to noise ratio equal to or greater than 3 were used. Due to the volumetric percolation of the coda waves from the medium, the variation in coda waves quality factor was investigated in both lateral and depth, and the results were compared with the values obtained for other regions of Iran and the world. After applying the intermediate filter on waveforms in 10 frequency bands, the quality factor was estimated, and finally, the mean values of the quality factor and frequency dependence factor in the region by using two single-back-scattering and single-isotropic-scattering obtained QC꞊149 ± 9f0.66 ± 0.03 and QC꞊152 ± 12f0.66 ± 0.03 respectively. The comparison of the results with the values have been obtained in Central Iran (Qc꞊94f0.97), Alborz (Qc꞊79f1.07) and Southeast Zagros (Qc꞊72f1.19) indicate more homogeneity in the shallow depth layers and seismicity is lower than Central Iran, Alborz and Zagros. Also, for investigating the depth variations of the quality factor in the region, 11 lapse time window lengths were used in 5 seconds increments. The window lengths are begin from 10 seconds. In the lower window lengths, the low Q0 values represent high heterogeneity in shallow depths of the earth.

As seismic energy propagates through the earth medium, its energy (amplitude) decays due to geometrical spreading, intrinsic attenuation and scattering. Owing to anelastic absorption, intrinsic attenuation converts the seismic energy to heat while scattering redistributes the energy at random heterogeneities. Knowledge of the relative contributions of scattering and intrinsic attenuation is important for appropriate subsurface material identification, tectonic interpretations and quantification of the ground motion. Besides, investigating seismic wave attenuation inside lithosphere allows for a more thorough knowledge as to Earth’s deep structures. The attenuation of short-period S waves, expressed as the inverse of the quality factor (Q−1), helps fathom the physical laws related to the propagation of the elastic energy of an earthquake through the lithosphere. Coda wave attenuation is considered as the combination of scattering and anelastic attenuation. In this study, the quality factor of coda wave was estimated in NW Iran making use of single back scattering method of Aki and Chouet (1975). For this purpose, we analyzed 3720 waveforms recorded by 8 short-period stations of Tabriz network from 1996 to 2013. So as to calculate the frequency relationships for Qc, nine frequency bands with central frequencies of 1.5, 2, 3, 4, 6, 8, 12, 16 and 20 Hz were considered and the lateral and depth variations of Q0 (Qc in 1 Hz) were investigated in the research area. In order to study the lateral variations, we chose coda waves recorded in epicentral distances less than 80 km, in a lapse time window of 30 s. The reason for the selection of such short distance (

The energy of a seismic wave decays while passing througha “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 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 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, 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 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 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 between 2006-2013 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

Summary: Southeastern Iran، in Makran region، due to movement of the Arabian plate toward Eurasia، the oceanic crust is subducted beneath the continental crust forming a subductionzone. This movement and subduction has given a special situation to seismicity and geology of this zone. Covering the lake of seismological studies and the effect of geological structure on seismic wave propagation in this region، we have studied effects of the medium on propagated seismic waves from the source to the receiver using recorded data on stations of International Institute of Earthquake Engineering and Seismology from 2005 to 2011. The coda quality factor، Qc، in addition to engineering applications، could be used and provide remarkable information for seismicity studies and analyzing how the subduction of oceanic crust is treating. Regarding the extent of the studying area، it has been divided into three subareas of Southeastern of Central Iran،Southeastern of Zagros and Makran، then for each، the variation of quality factor with depth is evaluated. The “Single Back-Scattering Method” is used for estimate. The frequency dependence of coda wave quality factor for events up to 100 km epicentral distance at each three subareas is determined. Also، the lateral and depth variation of Qc are computed and discussed. In this study، the Qc values are calculated for 12 lapse times (5 to 60s with a step of 5s) for three regions. The frequency dependent relationships of Qc، for SE Zagros varies from Q  (12 1. 1) f (1. 260. 02) c at 5s to Q  (1251. 1) f (1. 070. 014) c at 60s lapse time windows. Similarly، for SE Central Iran، the relationship varies from Q (17 1. 3) f (1. 130. 051) c at 5s to Q  (147 1. 3) f (0. 890. 038) c at 60s lapse time windows; and for Makran region varies from Q  (12 1. 1) f (1. 30. 015) c at 5s to Q  (137 1. 1) f (1. 030. 018) c at 60s lapse time windows. In all regions، the value of Q is less than 200، which implies، beside a highly tectonically and seismically behavior، a highly heterogeneous medium. The results show an increase in Qc value with increasing lapse time window. Among three studied regions، SE Zagros has the minimum Qc and so is more tectonically active compared with the two others. The North Makran and SE Central Iran are places in thenext ranks، respectively. Estimated Qc at bigger lapse time window indicates less attenuation at bigger depth. In Makran region at a depth of ~97 km the variation rate of Q suddenly increases. Based on available geological cross sections، for the eastern and western Makran، depth of about 100km of oceanic crust is in the northern region of the western Makran، which shows good agreement with our observation that oceanic crust has higher velocity، so attenuation is less than continental crust.