 |
| Titel |
Lapse time dependence of coda Q: Anisotropic multiple-scattering models and application to the Pyrenees |
| VerfasserIn |
Ludovic Margerin, Marie Calvet |
| Konferenz |
EGU General Assembly 2013
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250074302
|
|
|
|
|
|
| Zusammenfassung |
| The attenuation properties of the crust have been widely explored from measurements of the
quality factor of coda waves (Qc) throughout the world. A number of studies have reported an
increase of Qc with the lapse time in the coda of shear waves excited by local earthquakes.
Based on a single-scattering interpretation, this observation is generally ascribed to
depth-dependent attenuation properties in the crust. In recent years a number of observations
-in particular seismic wave equipartition- have put forward the importance of multiple
scattering in the coda. The main purpose of this study is therefore to clarify the role of
multiple scattering in the lapse time dependence of Qc using numerical simulations and
observations from the Pyrenees. Thanks to the European project Interreg SISPYR, we
collected all available waveform data from various institutions that operate seismological
networks along the range. We selected around 5000 waveforms from 159 local
earthquakes (with a magnitude larger than 3) which occured between 2001 and 2010. The
coda quality factor of short-period S-waves has been measured as a function of the
length of the coda window (Lw) for different choices of the onset time of the coda.
In the 2-16 Hz frequency band, we observe a transient regime characterized by
an increase of Qc with Lw, followed by a stabilization around a plateau whose
value depends on the central frequency of the signal and on the location along the
range. Using Monte Carlo simulations of wave transport in a variety of random
media (exponential, Von-Karman or gaussian heterogeneity power spectra), we
demonstrate that the lapse-time dependence of Qc in the Pyrenees may be modeled by
anisotropic multiple scattering of seismic waves, without invoking any depth dependence
of the attenuation properties in the crust. In our model, anisotropic scattering is
quantified by the ratio between the transport and scattering mean path (l*-l). At 6
Hz, we show that pyrenean data require an anisotropy factor l*-l greater than 5, a
transport mean free path l* greater than 100 km, an intrinsic quality factor Qi around
900 ± 300. From the frequency-dependent plateau of Qc at large lapse time, we infer an
intrinsic quality factor of the form Qi ~ 400f0.4 in the Pyrenees. We also show
how the rapid increase of the lapse-time dependence of Qc with frequency may
be exploited to put constraints on the power spectrum of heterogeneities in the
crust. A preliminary model of heterogeneity consistent with the observed lapse time
dependence of Qc suggests that the Pyreneean crust is relatively poor in short-wavelength
structure. |
|
|
|
|
|
|