Abstract
We have inverted P- and S-wave travel times from seismograms recorded
by a dense local network to infer the velocity structure in the crustal
volume where the April 6th 2009 main shock nucleated. The goal is
to image local variations of P-wave velocity and Poisson ratio along
the main shock fault zone for interpreting the complexity of the
rupture history. The initial stages of the mainshock rupture are
characterized by an emergent phase (EP) followed by an impulsive
phase (IP) 0.87 s later. The EP phase is located in a very high VP
and relatively low Poisson ratio (v) region. The IP phase marks the
beginning of the large moment release and is located outside the
low v volume. The comparison between the spatial variations of VP
and Poisson ratio within the main shock nucleation volume inferred
in this study with the rupture history imaged by inverting geophysical
data allows us to interpret the delayed along-strike propagation
in terms of heterogeneity of lithology and material properties.
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