Frequency 20 : General. The number of times something happens in a certain period of time, such as the ground shaking up and down or back and forth during of a seismic wave. The common unit of frequency is Hertz (Hz)
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}