International Association of Seismology and Physics of the Earth's Interior, XXI Sofia 1988, ESC Proceedings, External Organizations;
http://bib.gfz-potsdam.de/pub/esc/esc-21-ocr.pdf (Any fulltext)
http://bib.gfz-potsdam.de/pub/esc/esc-21.pdf (Any fulltext)
International Association of Seismology and Physics of the Earth's Interior (Ed.)(1989): Activity reports 1986-1988 and proceedings of the XXI General Assembly 23-27 August 1988 Sofia Bulgarien: European Seismological Commission, (ESC-Proceedings ; 21), Sofia, 559 p.