Residence time of diffuse waves in the crust as a physical interpretation of coda Q: application to seismograms recorded in Mexico

Details Residence time of diffuse waves in the crust as a physical interpretation of coda Q: application to seismograms recorded in Mexico Residence time of diffuse waves in the crust as a physical interpretation of coda Q: application to seismograms recorded in Mexico

Aug 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999geoji.138..343m&link_type=abstract

Geophysical Journal International, Volume 138, Issue 2, pp. 343-352.

Astronomy and Astrophysics

Astronomy

33

Coda, Diffusion, Multiple Scattering, Q, Radiative Transfer

Scientific paper

We consider a simple model for elastic wave propagation in the Earth's litho-sphere consisting of a heterogeneous and scattering crust overlying a homogeneous mantle; that is, we neglect backscattering from the upper mantle. This rough condition is not strictly valid everywhere, but this model allows us to investigate the effects of variations of diffracting properties between crust and mantle on the coda decay. The free parameters describing the wave propagation are the mean free path of the waves and the intrinsic absorption in the crust. In the framework of the radiative transfer theory, we treat, accurately, jumps in S-wave speed and the mean free path at the Moho. Without intrinsic absorption, this configuration yields a synthetic coda decay 1 /t exp (- 2pif t/Q_c^*)a form similar to that proposed by Aki & Chouet (1975) to parametrize the observations. The exponential decay in our purely elastic model is due to the partial leakage of diffuse energy into the mantle. In order to test the applicability of our model, we have determined the coda Q parameter (Q_c) in the 1-15 Hz frequency band from data recorded at four stations in Mexico. In the low-frequency regime (around 1 Hz), we suggest that the energy leakage significantly affects coda Q, whilst at higher frequencies (around 10 Hz), the anelastic attenuation dominates. A weak intrinsic absorption expressed by a frequency-independent parameter, Q_i, of about 1000 accounts satisfactorily for the observed Q_c values in the whole frequency range. Our very simple model has only two free parameters and offers an attractive alternative physical interpretation of coda Q. In Mexico, this model is sufficient to interpret the coda observations. According to our model, in regions where the mean free path is much larger than the crustal thickness (weakly heterogeneous crust), the leakage effect should be small, and the observed Q_c will mainly be due to the intrinsic absorption. On the other hand, when the mean free path is of the order of the crustal thickness (strongly heterogeneous crust), we predict a strong sensitivity of the decay rate of the coda to the crustal thickness. This dependence may be important for the interpretation of the regional variations of coda Q. We propose introducing a new parameter, tau_d, the residence time of diffuse energy in the crust, which is physically more meaningful than a quality factor to quantify the leakage effect.

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