{{Short description|Study of earthquakes that happened in the past}} thumb|Sketch of trench wall [[File:Cascadia 1700 tsunami layer Oregon.jpg|right|thumb|Sandsheet thought to have resulted from the tsunami caused by an earthquake on January 26, 1700, river bank Oregon]] [[File:Seismite Ordovician Kentucky Close.jpg|thumb|Seismite formed by liquefaction of sediments during a Late Ordovician earthquake (northern Kentucky, USA)]] '''Paleoseismology''' is the study of ancient earthquakes using geologic evidence, such as geologic sediments and rocks.<ref name=":02">{{Citation |last=McCalpin |first=James P. |title=Chapter 1 Introduction to Paleoseismology |date=2009-01-01 |work=International Geophysics |volume=95 |pages=1–27 |url=https://linkinghub.elsevier.com/retrieve/pii/S007461420995001X |access-date=2025-03-25 |series=Paleoseismology |publisher=Academic Press |doi=10.1016/s0074-6142(09)95001-x |last2=Nelson |first2=Alan R.|url-access=subscription }}</ref> It is used to supplement seismic monitoring to calculate seismic hazard. Paleoseismology is usually restricted to geologic regimes that have undergone continuous sediment creation for the last few thousand years, such as swamps, lakes, river beds and shorelines.
== Methodology ==
=== Procedure === Paleoseismology studies start with finding an active fault.<ref name=":1">{{Cite book |url=https://link.springer.com/referencework/10.1007/978-3-030-58631-7 |title=Encyclopedia of Solid Earth Geophysics |date=2021 |publisher=Springer International Publishing |isbn=978-3-030-58630-0 |editor-last=Gupta |editor-first=Harsh K. |series=Encyclopedia of Earth Sciences Series |location=Cham |language=en |doi=10.1007/978-3-030-58631-7}}</ref> The definition of an active fault can vary, but it is usually based on having tectonically deformed quaternary-age materials that can potentially cause earthquakes.<ref name=":1" /> Satellite imaging with high resolution is often used to find such faults, but because of its resolution limitations, there are also other methods such as ground-penetrating radar (GPR), aeromagnetic surveys, and seismic reflection surveys.<ref name=":1" /> Faults can be dated in absolute terms using radiocarbon dating.<ref>{{Cite journal |last=Nishikawa |first=Osamu |last2=Furuhashi |first2=Kyoko |last3=Masuyama |first3=Masashi |last4=Ogata |first4=Takeyuki |last5=Shiraishi |first5=Tateo |last6=Shen |first6=Chuan-Chou |date=2012-01-01 |title=Radiocarbon dating of residual organic matter in travertine formed along the Yumoto Fault in Oga Peninsula, northeast Japan: Implications for long-term hot spring activity under the influence of earthquakes |url=https://linkinghub.elsevier.com/retrieve/pii/S0037073811002661 |journal=Sedimentary Geology |volume=243-244 |pages=181–190 |doi=10.1016/j.sedgeo.2011.11.001 |issn=0037-0738|url-access=subscription }}</ref> thumb|Trenching is done across or parallel to a fault zone. It can help determine properties of paleoearthquakes such as time.
=== Paleoseismic evidence === Evidence of paleoearthquakes is classified into three levels: primary or secondary, location, and timing.<ref name=":0">{{Citation |last=McCalpin |first=James P. |title=Chapter 1 Introduction to Paleoseismology |date=2009-01-01 |work=International Geophysics |volume=95 |pages=1–27 |url=https://linkinghub.elsevier.com/retrieve/pii/S007461420995001X |access-date=2025-03-25 |series=Paleoseismology |publisher=Academic Press |doi=10.1016/s0074-6142(09)95001-x |last2=Nelson |first2=Alan R.|url-access=subscription }}</ref> For level 1, primary evidence is tectonic deformation, while secondary evidence is effects of this such as sediment deposition and elevation changes.<ref name=":0" /> However, distinguishing these is not always clear. Level 2 is the location, which is on or off fault.<ref name=":0" /> Level 3 is timing, which is either instantaneous (coseismic, or the same time as the earthquake) or delayed (postseismic, or after the earthquake).<ref name=":0" /> Types of evidence to identify paleoearthquakes were usually first identified as evidence after historical earthquakes.<ref name=":0" />
Paleoseismic investigations are commonly performed through trenching studies. A trench is dug, and geologists record the attributes of the rock layers.<ref name=":1" /> On-fault evidence includes warping and disconformity, angular unconformity, fracturing, fissures, and colluvial wedges.<ref name=":1" /> Off-fault evidence includes liquefaction of sand, tsunami deposits, turbidite, and marine terrace uplift.<ref name=":1" />
== Studies and findings == Using paleoseismology, it is now known that nearly all movement of the fault takes place with large earthquakes.<ref>{{cite web |title=Caltech Press Release, 7/21/2004, Dr. Kerry Sieh |url=http://pr.caltech.edu/media/Press_Releases/PR12558.html |url-status=dead |archive-url=https://web.archive.org/web/20041030201800/http://pr.caltech.edu/media/Press_Releases/PR12558.html |archive-date=2004-10-30 |website=pr.caltech.edu}}</ref> All seismic events with a moment magnitude of over 8 leave some trace in the sedimentation record.
Another example involves the megathrust earthquakes of the Pacific Northwest. It was thought that seismic hazard in the region was low because relatively few modern earthquakes have been recorded. However, paleoseismology studies showed evidence of extremely large earthquakes (the most recent being in 1700),<ref name=":2">{{cite web |title=Geological Society of America Penrose Conference on "Great Cascadia Earthquake Tricentennial" |url=https://earthquake.usgs.gov/regional/pacnw/paleo/greateq/index.html |url-status=dead |archive-url=https://web.archive.org/web/20080511022454/http://earthquake.usgs.gov/regional/pacnw/paleo/greateq/index.html |archive-date=2008-05-11 |accessdate=2017-08-29 |publisher=United States Geological Survey}}</ref> along with historical tsunami records. In effect, paleoseismology found that the subduction zone under British Columbia, Washington, Oregon, and far northern California, is hazardous in the long term and can generating coastal tsunamis of several hundred feet in height at the coast.<ref name=":2" /> Periodically, a slip will occur, which causes the coastal portion to reduce in elevation and thrust toward the west, leading to tsunamis in the central and eastern north Pacific Ocean (with several hours of warning) and a reflux of water toward the coastal shore.<ref name=":2" />thumb|600px|left|{{center|Multiple image view from the platform.<br />The fault has been marked with cordage and various features labeled.}}
thumb|600px|left|{{center|Image with enhanced annotation}} == See also ==
* Archaeoseismology * Earthquake * Earthquake magnitude * Fault * Historical earthquakes * Paleotempestology * Paleotsunami * Seismite * Seismology * Tectonics * Tsunami * Unconformity
== References == {{Reflist}}
==Sources== *James P. McCalpin (2009) Paleoseismology (2nd Edition), Academic Press, {{ISBN|0-12-373576-9}}, {{ISBN|978-0-12-373576-8}} *James P. McCalpin (1996) Paleoseismology, Elsevier, {{ISBN|0-12-481826-9}}
==External links== * [http://www.paleoseismicity.org/ Paleoseismicity.org – Online platform for paleoseismologists] * [https://web.archive.org/web/20130207063339/http://tierra.rediris.es/aequa/paleoinqua.html INQUA Paleoseismology/ web site of the International Focus Group on Paleoseismology and Active tectonics]. TERPRO Commission, International Union for Quaternary Research
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Category:Historical geology Category:Fields of seismology