Lateral variation and frequency dependence of coda- Q in the southern part of Iberia (original) (raw)
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Seismic attenuation in Iberia using the coda- Q method
Geophysical Journal International, 1990
Coda wave analysis is used to obtain frequency-dependent coda-Q values for different seismic zones of the Iberian area. Seventeen source regions around the Geophysical Observatory of Toledo and some four seismic events per region have been considered in this study. We have used an iterative Fourier analysis technique to see the variation of the frequency along the coda, also taking into account the instrument response. We have applied a suitable criterion to select the predominant frequency every 5 s along the coda. The variation of the frequency with time for each region is averaged with a second-degree polynomial, which is compared to master curves obtained directly from the response of the seismograph system, in order to determine the elastic quality factor Q. It has been observed that the frequency-time curves thus obtained are better explained if Q is considered as an exponential function of the peak frequency. The main result of this work is a set of 1Hz Q values with a clear indication that frequency dependence of Q exists, although the bandwidth from which our conclusions are reached is only 0.5-1 Hz. The coda-Q values obtained for the tectonically most stable areas (north Spain) appear somewhat higher, Q > 300, than those corresponding to the seismic active zones (south Spain), Q <250. Thus, a clear relationship is established between Q values and the two major tectonic provinces in Iberia. These results may be helpful for seismic risk and earthquake engineering purposes.
Coda-Q Distribution In the Iberian Peninsula
Geophysical Journal International, 1990
Several attenuation studies have established a frequency dependence law of the anelastic attenuation factor Q in the form Q=Qo(f/fo)v for the approximate 1–10 Hz frequency range. We propose a method that leads to the determination of Qo, which is a function of the reference frequency fo, and the real exponent v with a single station. to carry out the problem we determine a set of master curves as a function of v. We discuss the method, and the different features of the master curves, when it is applied to the complicated regions of the Iberian Peninsula and to several instruments with different responses. Using this new method and the seismographic stations available in the Iberian Peninsula we have mapped iso-Q0 lines, at a reference frequency of 1 Hz, applying inversion methods. the Q0 values determined for Iberia vary between about 100 and about 600. Values close to 100 correspond to the southern part of Iberia. In general, Qo values increase from south to north with values about 600 near the NW part of Iberia. the Pyrenees Mountains and adjacent areas present Q0 values between about 200 and about 350. These results suggest a strong Q0 lateral variation in Iberia. A considerable frequency dependence of coda-Q has also been determined. the v values vary between 0.3 and 0.8. the Q0 values obtained in the Iberian Peninsula show very good agreement with several Q0 values obtained in other regions of the world. Comparison between the iso-Q0 lines and other geophysical parameters, like regional variations of Pn velocities, heat flow, isoseimal intensity distribution and crustal thickness, indicates that lower Q0 values are associated with higher isoseismal intensity attenuation, higher heat flow, lower Pn velocities and thinner crust.
Geophysical Journal International, 1997
Attenuation coefficients of Rayleigh waves propagating across the Iberian Peninsula, and quality factor models were obtained for different two-station great-circle paths. The data used were long-period wave trains contained in the seismograms provided by the ILIHA array installed in the peninsula in the 1980s. More than 300 seismograms were analysed to obtain 14 sets of similar paths, consisting of either paths totally included in one of the four pure tectonic regions or mixed paths crossing several regions. We used the two-station method lo determine attenuation parameters (yR) and then an inversion method to derive Qg' models. The attenuation coefficients of seismic energy in the 10-120 s range are in the range 2.8 x 10p3-0.1 x km-l, and correlate well with both previous results obtained from analogue WWSSN and most seismogeotectonic characteristics of Iberia. The Qa models obtained also agree with a few models for Iberia determined earlier, and they show significant variations from one region to another. As a general feature, the y-values are higher in the active seismotectonic regions (typical maximum of 2.6 x lop3 km-') than in the more stable ones (typical maximum of 1.4 x Moreover, the maximum of Q;' in the active regions (145 x 10-3-120 x In general, both the regionalized attenuation coefficients of Rayleigh waves and the quality factor models exhibit a good agreement with previously published L, wave attenuation and coda Q distribution patterns in Iberia.
Mapping of Coda Attenuation at the Extend of the National Seismological Network of Greece
Bulletin of the Geological Society of Greece, 2004
Coda decay rates of 538 vertical components corresponding to local earthquakes which occurred in Greece during the period 1998 to 1999 were used to deduce the coda quality factor (Qc) characteristics in the Hellenic area. The seismograms have been selected from a broader sample of 776 records obtained at 8 stations of the National Seismographic Network operated by the Institute of Geodynamics of the National Observatory of Athens. Earthquake magnitudes range from 2.5 to 4.0; epicentral distances and depths are smaller than 100 km and 40 km, respectively. Using the Single Back Scattering model, the dependence of Qc on frequencies between 1 and 10 Hz has been investigated at each station and the usual Qc =Qo f relationships have been deduced. The spatial distribution of Qo has been drawn using waves that sample approximately equivalent ellipsoidal volumes with semiminor axis up to 100 km. The corresponding map shows a decreasing trend in SN direction.
Seismic attenuation of coda waves in the eastern region of Cuba
Tectonophysics, 2007
Cuba's seismic attenuation had never been studied in detail. In this paper we present the results of the research on the seismic attenuation of Cuba's eastern zone based upon the information collected by the seismological Cuban network from 1998 to 2003. 581 earthquakes were selected from the Cuban catalogue to make this study. All of them, recorded by at least three seismic stations, had their epicenters located in the eastern Cuban region (19.3-22 N, 79-73 W), epicentral distances between 15 km and 213 km, their coda duration magnitudes ranging from 2 and 4.1 and their focal depths reaching up to 30 km. The seismic wave attenuation was studied using coda waves. The single scattering method proposed by Sato in 1977 was applied, the attenuation and frequency dependency for different paths and the correlation of the results with the geotectonics of the region are presented in this paper. The mean Q c value calculated was Q c = (64 ± 2) f 0.84 ± 0.01. The relatively low Q 0 and the high frequency dependency agree with the values of a region characterized by a high tectonic activity. The Q c values of seven subregions of eastern Cuba were calculated and correlated with the geology and tectonics of the area.
Geophysical Journal International, 1997
The anelastic attenuation in the Almeria Basin (southeastern Iberian Peninsula) is investigated by using seismic data collected during the summer of 1991. A multiple-lapse time-window analysis is applied to high-frequency seismograms corresponding to 20 shallow seismic events with low magnitudes ( m 12.5) and distances less than 71 km, recorded at six short-period seismographic stations. We have constructed corrected geometrical spreading and normalized energy-distance curves for the region over the frequency bands 1-2,2-4,4-8,8-14 and 14-20 Hz. A theoretical model for body-wave energy propagation in a randomly heterogeneous medium has been employed to interpret the observations. Two parameters describe the medium in this model: the scattering attenuation coefficient qs = kQ3 ' and the intrinsic attenuation coefficient yI = kQ, ', where k is the wavenumber and Qr1 and Q, are the intrinsic and scattering attenuation respectively. This model assumes that scattering is isotropic, including all orders of multiple scattering, and predicting the spatial and temporal energy distribution of seismic energy. A least-squares fitting procedure has been used to find the best estimates of the model parameters. The analysis of the spectral amplitude decay of coda waves has provided coda Q,' values at the same frequency bands. The results obtained show that Q, ', Q2 ' and Q, ' decrease with increasing frequency; for frequencies lower than 3 Hz scattering attenuation is stronger than intrinsic absorption and coda Q, ' takes values between intrinsic and total attenuation, being very close to Qy'. Q;' is more frequency-dependent than Q, ; for frequencies greater than 3 Hz intrinsic absorption is the dominant attenuation effect and Q, and Q3 have significant frequency dependence. In order to correlate the results obtained with the major geological and tectonic features of the region, a geotectonic framework for the area is provided and the predominant frequency decay in coda waves is analysed in order to obtain the coda Q frequency dependence following a power law Qc = Qo( f/fo)", where fo is a reference frequency. In this way we have obtained regionalized values of coda Q at 1 Hz (Qo). Finally, a first-order approach has allowed us to obtain intrinsic and scattering quality factors from the obtained QO and v values, leading us to obtain tentative distributions of Q,, Qs and QO at 1 Hz for the area. The derived intrinsic and scattering quality-factor distributions are in good agreement with the tectonic history and the main geological features of the region. Large scattering and intrinsic attenuation (Qs N 80, QI -100) are found in the sedimentary Neogene and Quaternary basin, while scattering is the dominant effect in the old Palaeozoic rocks of the mountains (Qs -200, QI -1000).
Attenuation of Intensity with Epicentral Distance in the Iberian Peninsula
Bulletin of the Seismological Society of America, 2000
We have classified the attenuation of the Medveded, Sponheuer, and Karnik (MSK) intensity into five types (each as a function of the epicentral intensity I o) based on the mean radii of 254 isoseismal maps, mainly historical earthquakes in the Iberian Peninsula. Geographically representing each earthquake with its corresponding attenuation tendency, it can be seen that those with low attenuation lie west of the Peninsula and those with high attenuation in the south and east. This regionalization seems to be due as much to the seismotectonic characteristics (different crustal types and size of the earthquakes) as to the different construction types in each region. These attenuation values are similar to those of southern Europe, but much higher than those found in the United States. From the point of view of seismichazard evaluation, these laws represent an improvement with regard to those used so far. We have extended previous attenuation studies to the whole of the Iberian Peninsula, and, in some points, differences of attenuation assignment of almost two degrees of intensity have been corrected.
Lateral variations of coda Q and attenuation of seismic waves in Southwest Anatolia
Journal of Seismology, 2008
The seismic quality factor (Q c) and the attenuation coefficient (δ) in the earth's crust in southwest (SW) Anatolia are estimated by using the coda wave method based on the decrease of coda wave amplitude by time on the seismogram. The quality factor Q o , the value of Q c at 1 Hz, and its frequency dependency η are determined from this method depending on the attenuation properties of scattered coda waves. δ is determined from the observations of amplitude variations of seismic waves. In applying the coda wave method, firstly, a type curve representing the average pattern of the individual coda decay curves for 0.75, 1.5, 3.0, 6.0, 12.0, and 24.0 Hz values was estimated. Secondly, lateral variation of coda Q and the attenuation coefficients for three main tectonic patterns are estimated. The shape of the type curve is controlled by the scattering and attenuation in the crustal volume sampled by the coda waves. The Q o and η values vary from 30 to 180 and from 0.55 to 1.25, respectively for SW Anatolia. In SW Anatolia, coda Q-f relation is described by Q ¼ ð102 AE 8Þ Ã f 0:82AE0:07 and δ = 0.008 km −1. These results are expected to help in understanding the degree of tectonic complexity of the crust in SW Anatolia.
Estimation of Coda Wave Attenuation in Northern Morocco
Pure and Applied Geophysics, 2017
We studied the attenuation of coda waves and its frequency and lapse-time dependence in northern Morocco. We analysed coda waves of 66 earthquakes recorded in this region during 2008 for four lapse time windows of length 30, 40, 50, and 60 s, and at five frequency bands with central frequency in the range of 0.75-12 Hz. We determined the frequency dependent Q c relation for the horizontal (NS and EW) and vertical (Z) component seismograms. We analyzed three-component broadband seismograms of 66 local earthquakes for determining coda-Q based on the single back-scattering model. The Q c values show strong frequency dependence in 1.5-12 Hz that is related to high degree of heterogeneity of the medium. The lapse time dependence of Q c shows that Q 0 (Q c at 1 Hz) significantly increases with lapse time that is related to the depth dependence of attenuation and hence of the level of heterogeneity of the medium. The average frequency-dependent Q c (f) values are Q c ¼ ð143:75 AE 1:09Þf ð0:864AE0:006Þ ; Q c ¼ ð149:12 AE 1:08Þf ð0:85AE0:005Þ and Q c ¼ ð140:42 AE 1:81Þf ð0:902AE0:004Þ for the vertical, north-south and east-west components of motion, respectively. The frequency-dependent Q c (f) relations are useful for evaluating source parameters (Singh et al. 2001), which are the key inputs for seismic hazard assessment of the region.
Geophysical Journal International, 2013
The coda normalization method is one of the most used methods in the inference of attenuation parameters Q α and Q β. Since, in this method, the geometrical spreading exponent γ is an unknown model parameter, the most part of studies assumes a fixed γ , generally equal to 1. However γ and Q could be also jointly inferred from the non-linear inversion of codanormalized logarithms of amplitudes, but the trade-off between γ and Q could give rise to unreasonable values of these parameters. To minimize the trade-off between γ and Q, an inversion method based on a parabolic expression of the coda-normalization equation has been developed. The method has been applied to the waveforms recorded during the 1997 Umbria-Marche seismic crisis. The Akaike criterion has been used to compare results of the parabolic model with those of the linear model, corresponding to γ = 1. A small deviation from the spherical geometrical spreading has been inferred, but this is accompanied by a significant variation of Q α and Q β values. For almost all the considered stations, Q α smaller than Q β has been inferred, confirming that seismic attenuation, in the Umbria-Marche region, is controlled by crustal pore fluids.