Scattering and attenuation of high-frequency seismic waves; development of the theory of coda waves

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 Q,, 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 Y. 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-Qo lines, at a reference frequency of 1 Hz, applying inversion methods. The Q, values determined for Iberia vary between about 100 and about 600. Values close to 100 correspond to the southern part of Iberia. In general, Q, values increase from south to north with values about 600 near the NW part of Iberia. The Pyrenees Mountains and adjacent areas present Q, values between about 200 and about 350. These results suggest a strong Qo 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 Q, values obtained in the Iberian Peninsula show very good agreement with several Q, values obtained in other regions of the world. Comparison betweenthe iso-Q, lines and other geophysical parameters, like regional variations of P,, velocities, heat flow, isoseismal intensity distribution and crustal thickness, indicates that lower Q, values are associated with higher isoseismal intensity attenuation, higher heat flow, lower P,, velocities and thinner crust.

Section: Introductionmentioning

confidence: 99%

Coda-Q Distribution In the Iberian Peninsula

Pujades

Cañas

Egozcue

et al. 1990

“…Generally the coda results from waves that sample the whole region between the source and the seismic array. It contains global information about the heterogeneous structures of the medium, yet neither the different seismic wave propagation mechanisms in heterogeneous media nor the wave type that compose the coda are fully understood [see Herraiz & Espinosa (1986) for a complete review]. The classical studies made to understand the coda are based on statistical analysis (Aki 1969; Aki & Chouet 1975; Kopnichev 1977; Sato 1977a,b; Gao et al 1983).…”

Section: Introductionmentioning

confidence: 99%

Accurate time-frequency-wavenumber analysis to study coda waves

Schisselé

Guilbert

Gaffet

et al. 2004

S U M M A R YAn accurate method is developed to characterize the seismic coda phases recorded by smallaperture arrays. The coda is modelled as a superposition of several interfering wavelets identified by their arrival time, frequency content, backazimuth and apparent velocity of propagation. The wavelets are caused by the diffraction and refraction of the direct wavefield by heterogeneities of the propagation medium. The deterministic modelling is different from the statistical one generally used to retrieve mean parameters of the medium. As the complexity of the medium increases, separation of interfering wavelets needs an accurate timefrequency-wavenumber decomposition method that consists of detection and characterization of the different coherent wavelets propagating through the array. Detection is realized by mean time-frequency decomposition, based on the ridges algorithm. The MUltiple SIgnal Classification (MUSIC) algorithm, allowing a higher separation of simultaneous wavelets in the wavenumber domain, is then used to characterize the propagation parameters of the detected components. An optimal use of the MUSIC algorithm assumes the knowledge of the number of sources that simultaneously propagate through the array. The new iterative technique presented here allows the automatic determination of this number of sources. This methodology is applied to synthetic signals simulated in a heterogeneous medium. Results obtained show that: (i) the diffracted wavefield may be more energetic than the primary direct one and (ii) the relative energy diffracted by each heterogeneity is strongly dependent on the location of the array within the medium. The well-controlled results obtained for the synthetic examples allow interpretations of the observations made during the Annot experiment in the southern French Alps in 1998, where four small-aperture arrays were deployed, with small distances between each array (∼10 km). The time-azimuth-velocity evolutions determined for the earthquakes recorded during this experiment are used to characterize the heterogeneous structures of the medium.

“…Herrmann (1980) proved that the coda, as recorded on broad-band instruments, can also be used to estimate Q. A very complete summary about the state of the art has been written by Herraiz & Espinosa (1986). Q values are related to the attenuation rate of crustal phases (Herrmann 1980;Singh & Herrmann 1983) and to several geophysical parameters such as heat flow, electrical conductivity, seismic velocities and crustal thickness (Der & McElfresh 1977;Lachenbruch 1979;Singh & Herrmann 1983;Pujades et al 1990).…”

Section: Introductionmentioning

confidence: 99%

Seismic attenuation in Iberia using the coda-Qmethod

Payo

Badal

Cañas

et al. 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.