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An earthquake is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in size from those that are so weak that they cannot be felt to those violent enough to propel objects and people into the air, and wreak destruction across entire cities. The seismicity, or seismic activity, of an area is the frequency, type, and size of earthquakes experienced over a period of time. The word tremor is also used for non-earthquake seismic rumbling.
In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within the Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as subduction zones or transform faults. Energy release associated with rapid movement on active faults is the cause of most earthquakes. Faults may also displace slowly, by aseismic creep.
The Seattle Fault is a zone of multiple shallow east–west thrust faults that cross the Puget Sound Lowland and through Seattle in the vicinity of Interstate Highway 90. The Seattle Fault was first recognized as a significant seismic hazard in 1992, when a set of reports showed that about 1,100 years ago it was the scene of a major earthquake of about magnitude 7 – an event that entered Native American oral legend. Extensive research has since shown the Seattle Fault to be part of a regional system of faults.
The Cascadia subduction zone is a convergent plate boundary that stretches from northern Vancouver Island in Canada to Northern California in the United States. It is a very long, sloping subduction zone where the Explorer, Juan de Fuca, and Gorda plates move to the east and slide below the much larger mostly continental North American Plate. The zone varies in width and lies offshore beginning near Cape Mendocino, Northern California, passing through Oregon and Washington, and terminating at about Vancouver Island in British Columbia.
Megathrust earthquakes occur at subduction zones at destructive convergent plate boundaries, where one tectonic plate is forced underneath another, caused by slip along the thrust fault that forms the contact between them. These interplate earthquakes are the planet's most powerful, with moment magnitudes (Mw) that can exceed 9.0. Since 1900, all earthquakes of magnitude 9.0 or greater have been megathrust earthquakes.
A slow earthquake is a discontinuous, earthquake-like event that releases energy over a period of hours to months, rather than the seconds to minutes characteristic of a typical earthquake. First detected using long term strain measurements, most slow earthquakes now appear to be accompanied by fluid flow and related tremor, which can be detected and approximately located using seismometer data filtered appropriately. That is, they are quiet compared to a regular earthquake, but not "silent" as described in the past.
The Wrangellia Terrane is a terrane extending from the south-central part of Alaska through southwestern Yukon and along the Coast of British Columbia in Canada. Some geologists contend that Wrangellia extends southward to Oregon, although this is not generally accepted.
Episodic tremor and slip (ETS) is a seismological phenomenon observed in some subduction zones that is characterized by non-earthquake seismic rumbling, or tremor, and slow slip along the plate interface. Slow slip events are distinguished from earthquakes by their propagation speed and focus. In slow slip events, there is an apparent reversal of crustal motion, although the fault motion remains consistent with the direction of subduction. ETS events themselves are imperceptible to human beings and do not cause damage.
A supershear earthquake is an earthquake in which the propagation of the rupture along the fault surface occurs at speeds in excess of the seismic shear wave (S-wave) velocity. This causes an effect analogous to a sonic boom.
The 2001 Kunlun earthquake also known as the 2001 Kokoxili earthquake, occurred on 14 November 2001 at 09:26 UTC, with an epicenter near Kokoxili, close to the border between Qinghai and Xinjiang in a remote mountainous region. With a magnitude of 7.8 Mw it was the most powerful earthquake in China for 5 decades. No casualties were reported, presumably due to the very low population density and the lack of high-rise buildings. This earthquake was associated with the longest surface rupture ever recorded on land, ~450 km.
The 2002 Denali earthquake occurred at 22:12:41 UTC November 3 with an epicenter 66 km ESE of Denali National Park, Alaska, United States. This 7.9 Mw earthquake was the largest recorded in the United States in 37 years. The shock was the strongest ever recorded in the interior of Alaska. Due to the remote location, there were no fatalities and only a few injuries.
The Denali Fault is a major intracontinental dextral strike-slip fault in western North America, extending from northwestern British Columbia, Canada to the central region of the U.S. state of Alaska.
The 2009 Karonga earthquakes occurred near Karonga, Malawi in December 2009 near the northern tip of Lake Malawi in southeast Africa.
The 2002 Sumatra earthquake occurred at 01:26 UTC on 2 November. It had a magnitude of 7.3 on the moment magnitude scale with an epicenter just north of Simeulue island and caused three deaths. This earthquake is regarded as a foreshock of the 2004 Indian Ocean earthquake, which had an epicenter about 60 km to the northwest.
An earthquake rupture is the extent of slip that occurs during an earthquake in the Earth's crust. Earthquakes occur for many reasons that include: landslides, movement of magma in a volcano, the formation of a new fault, or, most commonly of all, a slip on an existing fault.
The Leech River Fault extends across the southern tip of Vancouver Island in British Columbia, Canada, creating the distinctively straight, narrow, and steep-sided valley, occupied by Loss Creek and two reservoirs, that runs from Sombrio Point due east to the Leech River, and then turns southeast to run past Victoria. It is a thrust fault that marks the northernmost exposure of the Crescent Terrane, where basalt of the Metchosin Igneous Complex is dragged under Vancouver Island by the subducting Juan de Fuca Plate. About ten kilometers north the nearly parallel San Juan Fault marks the southern limit of rock of the Wrangellia terrane, which underlies most of Vancouver Island. Between these two northeast-dipping thrust faults are the Leech River Complex and the Pandora Peak Unit. These, along with the Pacific Rim Complex further up the coast, are remnants of the Pacific Rim Terrane which was crushed between Wrangellia and Siletzia. The contact between the bottom of Wrangellia and the top of the subducted PRT continues northwest along the coast as the West Coast Fault, and southeast towards Victoria as the Survey Mountain Fault. The Leach River Fault (LRF) extends off-shore towards Cape Flattery, where the Crescent—Pacific Rim contact continues northwest as the Tofino Fault (TF).
Surface rupture is the visible offset of the ground surface when an earthquake rupture along a fault affects the Earth's surface. Surface rupture is opposed by buried rupture, where there is no displacement at ground level. This is a major risk to any structure that is built across a fault zone that may be active, in addition to any risk from ground shaking. Surface rupture entails vertical or horizontal movement, on either side of a ruptured fault. Surface rupture can affect large areas of land.
The 1958 Huslia earthquake on April 7 struck in an unusual part of state, near the small city of Huslia, about 415 km from Fairbanks. The mb 7.3 earthquake is one of two magnitude 7.0 or greater earthquakes recorded north of 65° latitude, the other being the 1933 Baffin Bay earthquake, and is one of the strongest earthquakes within the interior of Alaska. The earthquake was a result of compression of the crust due to the subduction of the Pacific Plate under the North American Plate.
The 2013 Craig, Alaska earthquake struck on January 5th, at 12:58 am (UTC–7) near the city of Craig and Hydaburg, on Prince of Wales Island. The Mw 7.5 earthquake came nearly three months after an Mw 7.8 quake struck Haida Gwaii on October 28, in 2012. The quake prompted a regional tsunami warning to British Columbia and Alaska, but it was later cancelled. Due to the remote location of the quake, there were no reports of casualties or damages.
Donna Eberhart-Phillips is geologist known for her research on subduction zones, especially in Alaska and New Zealand.
References
- ↑ Eberhart-Phillips, Donna; Haeussler, Peter J.; Freymueller, Jeffrey T.; Frankel, Arthur D.; Rubin, Charles M.; Craw, Patricia; Ratchkovski, Natalia A.; Anderson, Greg; Carver, Gary A; et al. (May 2003). "The 2002 Denali Fault Earthquake, Alaska: A Large Magnitude, Slip-Partitioned Event". Science . Elsevier. 300 (5622, number 5622): 1113–1118. Bibcode:2003Sci...300.1113E. doi:10.1126/science.1082703. PMID 12750512.
- ↑ Trop, J.M.; Benowitz, J.A.; Koepp, D.Q.; Sunderlin, D.; Brueseke, M.E.; Layer, P.W.; Fitzgerals, P.G. (2019). "Stitch in the ditch: Nutzotin Mountains (Alaska) fluvial strata and a dike record ca. 117–114 Ma accretion of Wrangellia with western North America and initiation of the Totschunda fault". Geosphere. 16. doi:10.1130/GES02127.1.
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