Laterally varying scattering properties in the North Anatolian Fault Zone from ambient noise cross-correlations

Chantal van Dinther (1) , Ludovic Margerin (2) , Michel Campillo (1)

1 ISTerre - Institut des Sciences de la Terre
2 IRAP - Institut de recherche en astrophysique et planétologie

Intrinsic absorption and scattering properties provide us with information about the physical state and heterogeneity of the Earth's crust. These properties are usually obtained by observing the energy decay of naturally occurring earthquakes, leading to sparse spatial sampling and therefore average scattering values over a large region. The present study uses ambient noise cross-correlations to analyse the energy decay and scattering properties over a part of the North Anatolian Fault (NAF; Turkey) from the continuous records of the 73 stations of the DANA temporary array in the frequency band 0.1-0.5 Hz. The region covered by the stations has rapidly varying geological characteristics and is highly faulted around the northern strand of the NAF. We measured in the noise correlations the space-time evolution of the energy of the coda waves. We first perform measurements in separate sub-regions. The local scattering and attenuation properties are obtained by global optimization of a 2-D solution of the radiative transfer equation for surface waves. We found that the mean free path and attenuation coefficient are considerably varying laterally with strong scattering observed in the region lying along the northern strand of NAF. The optimization provides well-constrained values for the scattering mean free path (l) on the order of 10km in the fault region. The mean free path is much larger (>100km) in the neighboring regions. We compare our global observations with simulations of scattered energy in a laterally variable scattering model using 2-D radiative transfer. These simulations confirm the large contrast of heterogeneity between NAF and the surrounding crust and provide further constraints on the lateral extent of NAF. When sources are located inside the fault zone, we find that energy leakage controls the spatio-temporal distribution of coda wave energy in the medium. This in turn suggests that lateral variations of scattering properties should be taken into account in future monitoring studies.

Mots clés: Wave scattering and diffraction; Seismic noise; Coda waves; Seismic atten- uation; Fault zone rheology; Numerical modelling

Domaines: Géophysique [physics.geo-ph]; Physique [physics]; Planète et Univers [physics]; Sciences de la Terre

Chantal van Dinther, Ludovic Margerin, Michel Campillo. Laterally varying scattering properties in the North Anatolian Fault Zone from ambient noise cross-correlations. Geophysical Journal International, 2021, 225 (1), pp.589-607. ⟨10.1093/gji/ggaa606⟩. ⟨hal-02929364v2⟩