In an isotropic medium the physical properties, e.g., the propagation velocity of seismic waves or the hardness of rock, are independent on the direction considered
Christian Nerger and Johannes Schweitzer (Assisting Editor in Compiling), Glossary of Terms Frequently Used in Seismology and Observatory Practice, US Geological Survey (http://www.usgs.gov/), the Montana Bureau of Mines and Geology (http://www.mbmg.mtech.edu/) and the International Data Center of the CTBTO (http://www.ctbto.org), USA 2004. http://gfzpublic.gfz-potsdam.de/pubman/item/escido c:4141:2/component/escidoc:4142/Glossary_rev1.pdf% 3Bjsessionid=216960F299E7DC5292AAE080C9BACFB3, (ND/ Erthq - Amer) {25}