[12] The SVEKALAPKO average traveltime of the converted wave from
the boundary at a depth of 410 km
tps = Ps410 - P = 42.30.1 s
was another observational parameter that was used as a constraint
on the velocity structure beneath RUKSA. The measurements were made
by the technique proposed for the first time by
Vinnik [1977].
The SVEKALAPKO traveltimes
Ps660 - P = 66.2 s and
Ps410 - P = 42.3 s
are smaller than the corresponding values for the standard Earth
by 1.8 s. The traveltime
Ps660 - Ps410 = 23.9 s coincides with
the value of the IASP91 standard Earth. Hence, in the absence
of
Vp/Vs anomalies, seismic velocities in the SVEKALAPKO
area coincide with standard values at depths of 410 km to 660 km
and exceed them at depths of 0 km to 410 km. Most earthquake epicenters
lie east of SVEKALAPKO, implying that the average
Ps410 path
from the boundary to the Earth's surface is shifted toward RUKSA
by about 200 km. If the size of the Fresnel zone is additionally
taken into account, it becomes clear that the region affecting
the propagation of the converted waves includes the RUKSA zone,
but the exact trace corresponding to the 42.3-s traveltime cannot
be determined. Apparently, this time can be considered as
an average for the SVEKALAPKO+RUKSA territory, i.e. for
the SE Baltic Shield.
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