International Association of Seismology and Physics of the Earth's Interior. European Seismological Commission, X Leningrad 1968, ESC Proceedings, External Organizations;
http://bib.gfz-potsdam.de/pub/esc/esc-10-2-ocr.pdf (beliebiger Volltext)
http://bib.gfz-potsdam.de/pub/esc/esc-10-2.pdf (beliebiger Volltext)
International Association of Seismology and Physics of the Earth's Interior. European Seismological Commission (Ed.)(1970): Proceedings of the X Assembly of the European Seismological Commission Leningrad, 3-11 September 1968 (Volume 2), (ESC-Proceedings ; 10.2), Moscow : Academy of Sciences of the USSR, 492 p.