Seismic attenuation in Iberia using the coda- Q method (original) (raw)
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Lateral variation and frequency dependence of coda-Q in the southern part of Iberia
Geophysical Journal International, 1991
bA method based on the coda attenuation law: Q=Q0(f/f0)v leads to the determination of the lateral variation of coda-Q in the southern part of the Iberian Peninsula using seismograms belonging to the seismological network of the Cartuja Observatory, located in Granada. The lateral variation of Q0 (Q value corresponding to a reference frequency f0 of 1 Hz) and its frequency dependence for the 1 to 5 Hz frequency range are, in general, in agreement with coda-Q values for frequencies less than about 1 Hz, previously determined in the region under study.To determine the coda-Q values analytical functions have been used to fit the magnification curves of the vertical component short-period seismographs belonging to the Cartuja network. The problem is solved by using least-squares techniques and non-linear inversion. The determined coda-Q0 values and its frequency dependence correlate well with several known geophysical parameters in the southern part of the Iberian Peninsula.
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.
Characteristics of the seismic attenuation in two tectonically active zones of Southern Europe
Pure and Applied Geophysics PAGEOPH, 1991
The seismic energy attenuation in the frequency range of 1-18 Hz was studied in the two tectonically active zones of Irno Valley (Southern Italy) and Granada Basin (South-East Spain). Data were recorded by short period vertical components seismographs for low-magnitude local earthquakes. The method of coda waves, assuming single S to S scattering approximation, was used to calculate the quality factor Q from the two data set. Results show a quality factor increasing with frequency, following the empirical law Q = Qoff. Qo and n are lower for the Irno Valley than for Granada. This result is interpreted in terms of different scattering environments present in the two investigated areas.
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).
On 24 th February 2004 a significant earthquake (Md = 6.4) occurred in the north of Morocco causing great damage in the vicinity of Al Hoceima region. This area is characterized by a complex faulting system as a result of compressional tectonic forces. Three short period stations are set in this area of interest and recordings from these stations were used in this study. In order to complete our knowledge of attenuation, 60 local earthquakes are recorded a few days after the great earthquake with magnitude Ml 2.6-5.0 to estimate seismic attenuation. For this purpose, we applied the single backscattering model of Aki & Chouet 1975 in the frequency range for 1 to 4 Hz. The study of coda waves was limited to a relatively short lapse time (10 Seconds) in order to sample the earth's crust only .The values of Q c estimated for all the three stations show a strong frequency dependent relationship of the form Q c =Q 0 f n , where Q 0 is Q c at 1Hz , and n represents the degree of freque...
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.
Seismic Activity Rates in the Iberian Península
Evaluating the seismic hazard requires establishing a distribution of the seismic activity rate, irrespective of the methodology used in the evaluation. In practice, how that activity rate is established tends to be the main difference between the various evaluation methods. The traditional procedure relies on a seismogenic zonation and the Gutenberg-Richter (GR) hypothesis. Competing zonations are often compared looking only at the geometry of the zones, but the resulting activity rate is affected by both geometry and the values assigned to the GR parameters. Contour plots can be used for conducting more meaningful comparisons, providing the GR parameters are suitably normalised. More recent approaches for establishing the seismic activity rate forego the use of zones and GR statistics and special attention is paid here to such procedures. The paper presents comparisons between the local activity rates that result for the complete Iberian Peninsula using kernel estimators as well as two seismogenic zonations. It is concluded that the smooth variation of the seismic activity rate produced by zoneless methods is more realistic than the stepwise changes associated with zoned approaches; moreover, the choice of zonation often has a stronger influence on the results than its fairly subjective origin would warrant. It is also observed that the activity rate derived from the kernel approach, related with the GR parameter "a", is qualitatively consistent with the epicentres in the catalogue. Finally, when comparing alternative zonations it is not just their geometry but the distribution of activity rate that should be compared.
Seismic S-wave coda attenuation in the Dominican Republic as a tool for seismic hazard mitigation
Natural Hazards, 2020
High-quality waveforms of the vertical and horizontal components of 1356 seismic events recorded from 2013 to 2016 by the Dominican Republic's seismic network, were analyzed to study the local spatial distribution of the coda frequency-dependent attenuation Q c (f) −1. Q c (f) was estimated at central frequencies of 1.5 (± 0.5), 3.0 (± 1.0), 6.0 (± 2.0), and 12.0 (± 4.0) Hz and interpreted by considering the single backscattering model. Our results of the attenuation of coda waves may explain why some regions of the Dominican Republic (DR) are more prone to suffer damage due to earthquakes than other areas. The majority of the cities and towns in the DR that were severely damaged by earthquakes in the past are located in zones of low and very low coda wave attenuation. In contrast, cities located in zones of high Q c (f) −1 tend to suffer less damage. Our findings identify regions with low seismic attenuation that reflect zones with soft soils that could be impacted by future large events in the DR. These results can be used as a tool for planning seismic hazard mitigation and emergency response as well as for land use regulations.
Pure and Applied Geophysics, 1988
A technique to detect spectrum variations versus time along seismic signals is applied to coda waves of local earthquakes (Friuli, Northern Italy). The technique consists of an autoregressive modeling and utilizes nonlinear spectral analysis where the spectrum of stochastic processes is estimated as the transfer function of the filter that whitens the process under analysis. This approach appears to be particularly well suited to those investigations where automatic measurements of the instantaneous frequency have to be carried out on digital data. The detection of variations of the instantaneous frequency along the coda allows computation of seismic-Q in the lithosphere and its frequency dependence: the result obtained is Q = 100f^{0.4} which appears to be strongly consistent with that, based on the estimate of the coda amplitude decay in the band including the most significant frequencies of the signals under analysis.
On 24th February 2004 a significant earthquake (M d = 6.4) occurred in the north of Morocco causing great damage in the vicinity of Al Hoceima region. This area is characterized by a complex faulting system as a result of compressional tectonic forces. Three short period seismic stations are set in this area of interest and recordings from these stations were used in this study. In order to complete our knowledge of attenuation, 60 local earthquakes are recorded a few days after the great earthquake with magnitude Ml 2.6 - 5.0 to estimate seismic attenuation. For this purpose, we applied the single backscattering model of Aki & Chouet 1975 in the frequency range for 1 to 8 Hz. The study of coda waves was limited to a relatively short lapse time (20 Seconds) in order to sample the earth’s crust only. The values of Q c estimated for all the three stations show a strong frequency dependent relationship of the form Q c = Q 0 f n , where Q 0 is Q c at 1 Hz , and n represents the degree of frequency dependence, and reflects the level of crustal heterogeneities to varying degrees. The average frequency dependent attenuation relationship has been obtained which indicates that the attenuation is high in this region. Finally to conclude our work, the values of Q 0 suggest that Al Hoceima area is highly heterogeneous and the n parameter indicates a meaning frequency dependence of Q c .