{{Short description|Unit for measuring earthquakes}} [[File:Hawaii 1998 0p2s 2p50.png|thumb|350px|Ground motion hazard map for Hawaii, based on a 2% probability of exceeding 0.2 second spectral acceleration at 5 Hz in 50 years]] '''Spectral acceleration''' ('''SA''') is a unit measured in '''''g''''' (the acceleration due to Earth's gravity, equivalent to g-force) that describes the maximum acceleration in an earthquake on an object – specifically a damped, harmonic oscillator moving in one physical dimension. This can be measured at (or specified for) different oscillation frequencies and with different degrees of damping, although 5% damping is commonly applied.<ref name=opensha>[http://www.opensha.org/glossary-intensityMeasureType#SA Intensity Measure Type (IMT)] {{Webarchive|url=https://web.archive.org/web/20151004034153/http://www.opensha.org/glossary-intensityMeasureType#SA |date=2015-10-04 }} OpenSHA, accessed 2011-04-14</ref> The SA at different frequencies may be plotted to form a response spectrum.
Spectral acceleration, with a value related to the natural frequency of vibration of the building, is used in earthquake engineering and gives a closer approximation to the motion of a building or other structure in an earthquake than the peak ground acceleration value,<ref name=opensha /><ref name=usgs-sa>[https://earthquake.usgs.gov/learn/faq/?faqID=221 FAQs – What is "spectral acceleration" or SA?] United States Geological Survey, accessed 2011-04-14</ref> although there is normally a correlation between [short period] SA and PGA.<ref name=usgs-sa />
Some seismic hazard maps are also produced using spectral acceleration.
==See also== *Seismic scale
==External links== *[http://www.consrv.ca.gov/cgs/rghm/psha/Pages/sa_01.aspx Spectral Acceleration Hazard Map of California – for 1 sec period] {{Webarchive|url=https://web.archive.org/web/20150511014148/http://www.consrv.ca.gov/cgs/rghm/psha/Pages/sa_01.aspx |date=2015-05-11 }} *[http://earthquakescanada.nrcan.gc.ca/hazard-alea/zoning/NBCC2005maps-eng.php 2005 National Building Code of Canada – Spectral Acceleration Hazard Maps for various periods] *[https://pubs.usgs.gov/of/2007/1043/ Revision of Time-Independent Probabilistic Seismic Hazard Maps for Alaska] *[http://kula.geol.wwu.edu/rjmitch/pga_maps.pdf What is a ground shaking hazard map? – Includes explanations of SA and PGA]
==References== {{reflist}}
Category:Seismology Category:Earthquake engineering
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