Editing Cascadia subduction zone (section)

== Recent seismicity ==
Subduction zones experience various types of earthquakes (or seismicity); including [[slow earthquake]]s, [[megathrust earthquake]]s, [[interplate earthquake]]s, and [[intraplate earthquake]]s. Unlike other subduction zones on Earth, Cascadia currently experiences low levels of seismicity and has not generated a [[megathrust earthquake]] since January 26, 1700. Despite low levels of seismicity compared to other subduction zones, Cascadia hosts various types of earthquakes that are recorded by seismic and [[Geodesy|geodetic]] instruments, such as [[seismometer]]s and [[GNSS applications|GNSS receivers]].

Tremor, a type of [[Slow earthquake|slow fault slip]], occurs along almost the entire length of Cascadia<ref>{{Cite journal|last1=Brudzinski|first1=Michael R.|last2=Allen|first2=Richard M.|s2cid=6682060|date=2007|title=Segmentation in episodic tremor and slip all along Cascadia|journal=Geology|volume=35|issue=10|pages=907|doi=10.1130/g23740a.1|issn=0091-7613|bibcode=2007Geo....35..907B}}</ref> at regular intervals of 13–16 months.<ref>{{Cite journal|last=Rogers|first=G.|date=2003-06-20|title=Episodic Tremor and Slip on the Cascadia Subduction Zone: The Chatter of Silent Slip|journal=Science|volume=300|issue=5627|pages=1942–1943|doi=10.1126/science.1084783|issn=0036-8075|bibcode=2003Sci...300.1942R|pmid=12738870|s2cid=2672381|doi-access=free}}</ref> Tremor occurs deeper on the subduction interface than the locked area where megathrust earthquakes occur. The depth of tremor along the subduction interface in Cascadia ranges from 28&nbsp;km to 45&nbsp;km,<ref name=":0">{{Cite journal|last1=Bostock|first1=Michael G.|last2=Christensen|first2=Nikolas I.|last3=Peacock|first3=Simon M.|date=2019-05-01|title=Seismicity in Cascadia|journal=Lithos|volume=332–333|pages=55–66|doi=10.1016/j.lithos.2019.02.019|issn=0024-4937|bibcode=2019Litho.332...55B|s2cid=134015941}}</ref> and the motion is so slow that it is not felt at the surface by people or animals, but it can be measured [[Geodesy|geodetically]]. The highest density of tremor activity in Cascadia occurs from northern Washington into southern Vancouver Island, and in northern California.<ref name=":0" /> Tremor in Cascadia is monitored by the [[Pacific Northwest Seismic Network]]'s semi-automatic tremor detection system.<ref name=":0" />

The majority of [[interplate earthquake]]s, or earthquakes that occur near the boundaries of tectonic plates, near the Cascadia subduction zone occur in the [[forearc]] of the overriding [[North American plate]] in Washington, west of the [[Cascade Volcanoes|Cascade Volcanic Arc]] and east of where tremor occurs.<ref name=":0" /> These earthquakes are sometimes referred to as crustal earthquakes, and they are capable of causing significant damage due to their relatively shallow depths. A damaging magnitude 7 interplate earthquake occurred on the [[Seattle Fault]] around 900–930 CE<ref>{{Cite journal|last=Atwater|first=Brian|date=1999-03-01|title=Radiocarbon dating of a Seattle earthquake to A.D. 900–930.|journal=Seismological Research Letters|volume=70|issue=2|pages=190–277|doi=10.1785/gssrl.70.2.190|issn=0895-0695}}</ref> that generated 3 meters of uplift and a 4-5 meter tsunami.<ref>{{Cite journal|last=Arcos|first=M. E. M.|date=2012-06-01|title=The A.D. 900-930 Seattle-Fault-Zone Earthquake with a Wider Coseismic Rupture Patch and Postseismic Submergence: Inferences from New Sedimentary Evidence|url=https://pubs.geoscienceworld.org/bssa/article/102/3/1079-1098/326677|journal=Bulletin of the Seismological Society of America|volume=102|issue=3|pages=1079–1098|doi=10.1785/0120110123|issn=0037-1106|bibcode=2012BuSSA.102.1079A|url-access=subscription}}</ref> A substantial number of [[forearc]] [[interplate earthquake]]s also occur in northern California.<ref name=":0" /> Much less interplate seismicity occurs in Oregon compared to Washington and northern California, although Oregon hosts more [[volcanic activity]] than its neighboring states.<ref>{{Cite journal|last1=Sherrod|first1=David R.|last2=Smith|first2=James G.|date=1990|title=Quaternary extrusion rates of the Cascade Range, northwestern United States and southern British Columbia|journal=Journal of Geophysical Research: Solid Earth|volume=95|issue=B12|pages=19465–19474|doi=10.1029/JB095iB12p19465|issn=2156-2202|bibcode=1990JGR....9519465S}}</ref>

Intraslab earthquakes, frequently associated with stresses within the subducting plate in [[Convergent boundary|convergent margins]], occur most frequently in northern Cascadia along the west coast of Vancouver Island and in Puget Sound, and in southern Cascadia within the subducting [[Gorda plate]], near the [[Mendocino triple junction]] offshore of northern California. The [[1949 Olympia earthquake]] was a damaging magnitude 6.7 intraslab earthquake that occurred at 52&nbsp;km depth and caused eight deaths. Another notable intraslab earthquake in the [[Puget Sound]] region was the magnitude 6.8 [[2001 Nisqually earthquake]]. Intraslab earthquakes in Cascadia occur in areas where the subducting plate has high [[curvature]].<ref name=":0" /> Much of the seismicity that occurs off the coast of northern California is due to [[intraplate deformation]] within the [[Gorda plate]]. Similar to the distribution of interplate earthquakes in Cascadia, intraslab earthquakes are infrequent in Oregon, with its strongest earthquake since statehood being the 5.6 magnitude [[1993 Scotts Mills earthquake]], an [[oblique-slip]] quake.<ref name=":0" /><ref>{{Cite web|last=Strauss|first=Mark|title=Oregon Earthquake History|url=https://oregoninsuranceadvisor.com/earthquake-insurance/oregon-earthquake-history/|url-status=live|access-date=2021-06-21|website=Oregon Insurance Advisor|date=20 October 2014|archive-url=https://web.archive.org/web/20150401022212/http://oregoninsuranceadvisor.com:80/earthquake-insurance/oregon-earthquake-history/ |archive-date=2015-04-01 }}</ref>