  Imaging the structure of major fault zones is essential for our understanding
of crustal deformations and their implications on seismic hazards.
Investigating such complex regions presents several issues, including the
variation of seismic velocity due to the diversity of geological units and the
cumulative damage caused by earthquakes. Conventional migration techniques are
in general strongly sensitive to the available velocity model. Here we apply a
passive matrix imaging approach which is robust to the mismatch between this
model and the real seismic velocity distribution. This method relies on the
cross-correlation of ambient noise recorded by a geophone array. The resulting
set of impulse responses form a reflection matrix that contains all the
information about the subsurface. In particular, the reflected body waves can
be leveraged to: (i) determine the transmission matrix between the Earth's
surface and any point in the subsurface; (ii) build a confocal image of the
subsurface reflectivity with a transverse resolution only limited by
diffraction. As a study case, we consider seismic noise (0.1-0.5 Hz) recorded
by the Dense Array for Northern Anatolia (DANA) that consists of 73 stations
deployed for 18 months in the region of the 1999 Izmit earthquake. Passive
matrix imaging reveals the scattering structure of the crust and upper mantle
around the NAFZ over a depth range of 60 km. The results show that most of the
scattering is associated with the Northern branch that passes throughout the
crust and penetrates into the upper mantle.

Geophysics