{{Short description|Earthquake shaking}}
[[File:ShakeMap for 2001 M6.8 Olympia, WA earthquake.jpg|right|thumb|200px|ShakeMap for the [[2001 Nisqually earthquake]]]]
In [[seismology]], '''strong ground motion''' is the strong [[earthquake]] shaking that occurs close to (less than about 50 km from) a causative [[Geologic fault|fault]]. The strength of the shaking involved in strong ground motion usually overwhelms a [[seismometer]], forcing the use of [[accelerograph]]s (or strong ground motion [[accelerometer]]s) for recording. The science of strong ground motion also deals with the variations of fault rupture, both in total displacement, energy released, and rupture velocity.
As seismic instruments (and accelerometers in particular) become more common, it becomes necessary to correlate expected damage with instrument-readings. The old Modified [[Mercalli intensity scale]] (MM), a relic of the pre-instrument days, remains useful in the sense that each intensity-level provides an observable difference in seismic damage.
After many years of trying every possible manipulation of accelerometer-time histories, it turns out that the extremely simple peak ground velocity (PGV) provides the best correlation with damage.<ref>{{cite web|url=https://pasadena.wr.usgs.gov/shake/pubs/shake/node7.html|title=INSTRUMENTAL SEISMIC INTENSITY MAPS|archive-url=https://web.archive.org/web/20000903180428/http://pasadena.wr.usgs.gov/shake/pubs/shake/node7.html|archive-date=2000-09-03}}</ref><ref>{{cite journal|author3-link=Teng Ta-liang|last1=Wu|first1=Yih-Min|last2=Hsiao|first2=Nai-Chi|last3=Teng|first3=Ta-Liang|date=July 2004|title=Relationships between Strong Ground Motion Peak Values and Seismic Loss during the 1999 Chi-Chi, Taiwan Earthquake|journal=Natural Hazards|volume=32|issue=3|pages=357–373|doi=10.1023/B:NHAZ.0000035550.36929.d0|bibcode=2004NatHa..32..357W |citeseerx=10.1.1.470.4890|s2cid=53479793}}</ref> PGV merely expresses the peak of the first [[integral|integration]] of the acceleration record. Accepted formulae now link PGV with MM Intensity. Note that the effect of soft soils gets built into the process, since one can expect that these foundation conditions will amplify the PGV significantly.
"ShakeMaps" are produced by the [[United States Geological Survey]], provide almost-real-time information about significant earthquake events, and can assist [[emergency management|disaster-relief]] teams and other agencies.<ref>{{cite web|title=ShakeMaps|url=https://earthquake.usgs.gov/earthquakes/shakemap/|access-date=2014-04-01|archive-url=https://web.archive.org/web/20140330233229/http://earthquake.usgs.gov/earthquakes/shakemap/|archive-date=2014-03-30|url-status=dead}}</ref>
==Correlation with the Mercalli scale==
The [[USGS|United States Geological Survey]] created the Instrumental Intensity scale, which maps peak ground velocity on an intensity scale comparable to the felt Mercalli scale. Seismologists all across the world use these values to construct ShakeMaps.
{| class="wikitable" style="font-size: 100%; text-align: left; " |- ! align="center" | Instrumental<br />Intensity ! align="center" | Velocity<br />(cm/s) ! align="center" | Perceived shaking ! align="center" | Potential damage |- |- | align="center" | I | align="center" | < 0.0215 | align="center" | Not felt | align="center" | None |- | align="center" | II–III | align="center" | 0.135 – 1.41 | align="center" | Weak | align="center" | None |- | align="center" | IV | align="center" | 1.41 – 4.65 | align="center" | Light | align="center" | None |- | align="center" | V | align="center" | 4.65 – 9.64 | align="center" | Moderate | align="center" | Very light |- | align="center" | VI | align="center" | '''9.64 – 20''' | align="center" | Strong | align="center" | Light |- | align="center" | VII | align="center" | '''20 – 41.4''' | align="center" | Very strong | align="center" | Moderate |- | align="center" | VIII | align="center" | '''41.4 – 85.8''' | align="center" | Severe | align="center" | Moderate to heavy |- | align="center" | IX | align="center" | '''85.8 – 178''' | align="center" | Violent | align="center" | Heavy |- | align="center" | X+ | align="center" | '''> 178''' | align="center" | Extreme | align="center" | Very heavy |}
==Notable earthquakes== {|class="wikitable sortable" ! PGV <br /> (max recorded) ! Mag ! Depth ! Fatalities ! Earthquake |- | 318 cm/s<ref name=IDA/> || 7.7 || 33 km || 2,415 || [[1999 Jiji earthquake]] |- | 215 cm/s<ref>{{Cite web | url=https://earthquake.usgs.gov/earthquakes/eventpage/us6000jllz/shakemap/analysis | title=M 7.8 - Pazarcik earthquake, Kahramanmaras earthquake sequence - ShakeMap | website=earthquake.usgs.gov}}</ref> || 7.8 || 10 km || 62,013 || [[2023 Turkey–Syria earthquakes|2023 Turkey-Syria Earthquakes]] |- | 183 cm/s<ref name="ShakeMap Scientific Background">{{cite web|url=https://earthquake.usgs.gov/earthquakes/shakemap/background.php|title=ShakeMap Scientific Background|access-date=2021-11-03|archive-url=https://web.archive.org/web/20110623092131/http://earthquake.usgs.gov/earthquakes/shakemap/background.php|archive-date=2011-06-23|url-status=dead}}</ref> || 6.7 || 18.2 km || 57 || [[1994 Northridge earthquake]] |- | 170 cm/s<ref name=IDA/> || 6.9 || 17.6 km || 6,434 || [[Great Hanshin earthquake|1995 Great Hanshin earthquake]] |- | 152 cm/s<ref name=IDA/> || 6.6 || 10 km || 11 || [[2007 Chūetsu offshore earthquake]] |- | 147 cm/s<ref name=IDA/> || 7.3 || 1.09 km || 3 || [[1992 Landers earthquake]] |- | 145 cm/s<ref name=IDA/> || 6.6 || 13 km || 68 || [[2004 Chūetsu earthquake]] |- | 138 cm/s<ref name=IDA/> || 7.2 || 10.5 km || 356 injured || [[1992 Cape Mendocino earthquakes]] |- | 117.41 cm/s || 9.1<ref name=USGS0/> || 29 km || 19,747 || [[2011 Tohoku earthquake and tsunami]] |- | 108 cm/s<ref name=NEPAL-EQ-2015/> || 7.8 || 8.2 km || 8,857 || [[April 2015 Nepal earthquake]] |- | 38 cm/s<ref name=usgs/> || 5.5 || 15.5 km || 0 || [[2008 Chino Hills earthquake]] |- | 20 cm/s (est)<ref name=nature/> || 6.4 || 10 km || 115-120 || [[1933 Long Beach earthquake]] |}
==See also== *[[Arias intensity]] *[[Long period ground motion]] *[[Peak ground acceleration]]
== References == {{Reflist|refs= <ref name="IDA">{{cite web | url=https://apps.dtic.mil/sti/pdfs/AD1131695.pdf | archive-url=https://web.archive.org/web/20220601061116/https://apps.dtic.mil/sti/pdfs/AD1131695.pdf | url-status=live | archive-date=June 1, 2022 | title=How Fast Can the Ground Really Move? |publisher=INSTITUTE FOR DEFENSE ANALYSES|access-date=2021-11-03 }}</ref> <ref name="USGS0">{{cite web | url= https://earthquake.usgs.gov/earthquakes/eventpage/official20110311054624120_30/origin/detail | title= M9.1 – Tohoku, Japan |publisher= United States Geological Survey |access-date=2021-11-03 }}</ref> <ref name="NEPAL-EQ-2015">{{cite journal | title=Strong ground motion in the Kathmandu Valley during the 2015 Gorkha, Nepal, earthquake. Earth Planets Space 68, 10 (2016). | journal=Earth, Planets and Space | date=26 January 2016 | volume=68 | issue=1 | page=10 |publisher=Takai, N., Shigefuji, M., Rajaure, S. et al. | doi=10.1186/s40623-016-0383-7 | s2cid=41484836 | last1=Takai | first1=Nobuo | last2=Shigefuji | first2=Michiko | last3=Rajaure | first3=Sudhir | last4=Bijukchhen | first4=Subeg | last5=Ichiyanagi | first5=Masayoshi | last6=Dhital | first6=Megh Raj | last7=Sasatani | first7=Tsutomu | hdl=2115/60622 | hdl-access=free | doi-access=free }}</ref> <ref name="usgs">{{cite web | url=https://earthquake.usgs.gov/earthquakes/eventpage/usp000gddf#general_region | title=M5.5 – Greater Los Angeles area, California | publisher=United States Geological Survey | access-date=2021-11-06 }}</ref> <ref name="nature">{{Cite journal|title=Nature:The 1933 Long Beach Earthquake (California, USA) Ground Motions and Rupture Scenario|journal=Scientific Reports |date=22 June 2020 |volume=10 |issue=1 |page=10017 |doi=10.1038/s41598-020-66299-w |last1=Hough |first1=S. E. |last2=Graves |first2=R. W. |pmid=32572047 |pmc=7308333 |s2cid=219959257 |doi-access=free }}</ref> }} '''Sources''' {{Refbegin}} * {{citation|title=An atlas of ShakeMaps for selected global earthquakes|url=https://pubs.usgs.gov/of/2008/1236/downloads/OF08-1236_508.pdf|first1=T. I.|last1=Allen|first2=D. J.|last2=Wald|first3=A. J.|last3=Hotovec|first4=K.|last4=Lin|first5=P. S.|last5=Earle|first6=K. D.|last6=Marano|year=2008|series=Open-File Report 2008–1236|publisher=United States Geological Survey}} * {{citation|title=An atlas of ShakeMaps and population exposure catalog for earthquake loss modeling|url=https://earthquake.usgs.gov/research/pager/prodandref/AllenEtAlAtlas.pdf|first1=T. I.|last1=Allen|first2=D. J.|last2=Wald|first3=P. S.|last3=Earle|first4=K. D.|last4=Marano|first5=A. J.|last5=Hotovec|first6=K.|last6=Lin|first7=M. G.|last7=Hearne|year=2009|journal=[[Bulletin of Earthquake Engineering]]|volume=7|issue=3|pages=701–718|doi=10.1007/s10518-009-9120-y|bibcode=2009BuEE....7..701A |s2cid=58893137|access-date=2017-09-15|archive-url=https://web.archive.org/web/20120510042722/http://earthquake.usgs.gov/research/pager/prodandref/AllenEtAlAtlas.pdf|archive-date=2012-05-10|url-status=dead}} {{Refend}}
[[Category:Seismology]]