Source radiation invariant property of local and near!regional shear wave coda: Application to source scaling for the Mw 5.9 Wells, Nevada sequence

This study documents another unique property of local
and regional shear!wave coda: that scattering in the Earth’s
crust averages out source heterogeneities such as focal
mechanism and directivity. We consider two recent events
which exhibited significant source directivity. The first,
an Mw 4.2 Oakland hills, California strike!slip event on
July 20, 2007, resulted in uncharacteristically large amounts
of damage in the Montclair and south Berkeley neighborhoods,
along strike and north!west of the hypocenter, but almost no
damage to the south!east. The second event, the Mw 5.9 Wells,
Nevada dip!slip event of February 21, 2008, was well!recorded
by the Earthscope Transportable Array (USArray) and
determined to exhibit south!west directivity. We conclude with
an example of corner!frequency ( fc) estimates from the Wells
sequence, which showcase the coda’s low variance, and find
that scaling is non!self!similar, suggesting a change in dynamics
between the mainshock and aftershocks.