Abstract
- We present results from a large-scale study of surface-wave group
velocity dispersion across the Middle East, North Africa, southern
Eurasia and the Mediterranean. Our database for the region is populated
with seismic data from regional events recorded at permanent and
portable broadband, three-component digital stations. We have measured
the group velocity using a multiple narrow-band filter on deconvolved
displacement data. Overall, we have examined more than 13,500 seismograms
and made good quality dispersion measurements for 6817 Rayleigh-
and 3806 Love-wave paths. We use a conjugate gradient method to perform
a group-velocity tomography. Our current results include both Love-
and Rayleigh-wave inversions across the region for periods from 10
to 60 seconds. Our findings indicate that short-period structure
is sensitive to slow velocities associated with large sedimentary
features such as the Mediterranean Sea and Persian Gulf. We find
our long-period Rayleigh-wave inversion is sensitive to crustal thickness,
such as fast velocities under the oceans and slow along the relatively
thick Zagros Mts. and Turkish-Iranian Plateau. We also find slow
upper mantle velocities along known rift systems. Accurate group
velocity maps can be used to construct phase-matched filters along
any given path. The filters can improve weak surface wave signals
by compressing the dispersed signal. The signals can then be used
to calculate regionally determined M<SUB>S</SUB> measurements, which
we hope can be used to extend the threshold of m<SUB>b</SUB>:M<SUB>S</SUB>
discriminants down to lower magnitude levels. Other applications
include using the group velocities in the creation of a suitable
background model for forming station calibration maps, and using
the group velocities to model the velocity structure of the crust
and upper mantle.
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