Traveltime of the Converted Wave From the 410-km Boundary

[12]  The SVEKALAPKO average traveltime of the converted wave from the boundary at a depth of 410 km tps = Ps410 - P = 42.3pm0.1 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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