Editing Tsunami (section)

===Seismicity===
Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that are associated with the Earth's crustal deformation; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position.<ref>{{cite web|title=How do earthquakes generate tsunamis?|url=http://www.geophys.washington.edu/tsunami/general/physics/earthquake.html|publisher=University of Washington|archive-url=https://web.archive.org/web/20070203154326/http://www.geophys.washington.edu/tsunami/general/physics/earthquake.html|archive-date=2007-02-03}}</ref> More specifically, a tsunami can be generated when [[thrust fault]]s associated with [[convergent boundary|convergent]] or destructive [[plate boundaries]] move abruptly, resulting in water displacement, owing to the vertical component of movement involved. Movement on [[Fault (geology)#Dip-slip faults|normal (extensional) faults]] can also cause displacement of the seabed, but only the largest of such events (typically related to flexure in the [[outer trench swell]]) cause enough displacement to give rise to a significant tsunami, such as the [[1977 Sumba earthquake|1977 Sumba]] and [[1933 Sanriku earthquake|1933 Sanriku]] events.<ref>{{cite journal|title=Source Process of the Great 1977 Sumba Earthquake|url=http://www.es.ucsc.edu/~thorne/TL.pdfs/LL_Sumba_JGR1988.pdf|first1=C. S.|last1=Lynnes|first2=T.|last2=Lay|authorlink2=Thorne Lay|year=1988|journal=Geophysical Research Letters|publisher=[[American Geophysical Union]]|volume=93|issue=B11|pages=13, 407–413, 420|doi=10.1029/JB093iB11p13407|bibcode=1988JGR....9313407L}}</ref><ref name="Kanamori_1971">{{cite journal|title=Seismological evidence for a lithospheric normal faulting – the Sanriku earthquake of 1933|last=Kanamori|first=Hiroo|author-link=Hiroo Kanamori|journal=[[Physics of the Earth and Planetary Interiors]]|year=1971|volume=4|issue=4|pages=298–300|doi=10.1016/0031-9201(71)90013-6|bibcode=1971PEPI....4..289K}}</ref>

<gallery mode="packed">
File:Eq-gen1.svg|Drawing of [[tectonic plate boundary]] before [[earthquake]]
File:Eq-gen2.svg|Over-riding plate bulges under strain, causing [[tectonic uplift]].
File:Eq-gen3.svg|Plate slips, causing [[subsidence]] and releasing energy into water.
File:Eq-gen4.svg|The energy released produces tsunami waves.
</gallery>

Tsunamis have a small wave height offshore, and a very long [[wavelength]] (often hundreds of kilometres long, whereas normal ocean waves have a wavelength of only 30 or 40 metres),<ref>[http://www.australiangeographic.com.au/journal/facts-and-figures-how-tsunamis-form.htm/ Facts and figures: how tsunamis form] {{Webarchive|url=https://web.archive.org/web/20131105033906/http://www.australiangeographic.com.au/journal/facts-and-figures-how-tsunamis-form.htm/ |date=2013-11-05 }}, Australian Geographic, March 18, 2011.</ref> which is why they generally pass unnoticed at sea, forming only a slight swell usually about {{convert|300|mm|in}} above the normal sea surface. They grow in height when they reach shallower water, in a [[wave shoaling]] process described below. A tsunami can occur in any tidal state and even at low tide can still inundate coastal areas.

On April 1, 1946, the 8.6 {{M|w}} [[1946 Aleutian Islands earthquake|Aleutian Islands earthquake]] occurred with a maximum [[Mercalli intensity scale|Mercalli intensity]] of VI (''Strong''). It generated a tsunami which inundated [[Hilo, Hawaii|Hilo]] on the island of Hawaii with a {{convert|14|m|ft|adj=mid|high}} surge. Between 165 and 173 were killed. The area where the earthquake occurred is where the [[Pacific Ocean]] floor is [[subducting]] (or being pushed downwards) under Alaska.

Examples of tsunamis originating at locations away from convergent boundaries include [[Storegga]] about 8,000 years ago, [[Grand Banks]] in 1929, and [[Papua New Guinea]] in 1998 (Tappin, 2001). The Grand Banks and Papua New Guinea tsunamis came from earthquakes which destabilised sediments, causing them to flow into the ocean and generate a tsunami. They dissipated before travelling transoceanic distances.

The cause of the Storegga sediment failure is unknown. Possibilities include an overloading of the sediments, an earthquake or a release of gas hydrates (methane etc.).

The [[1960 Valdivia earthquake]] ([[Moment magnitude scale|''M''<sub>w</sub>]] 9.5), [[1964 Alaska earthquake]] (''M''<sub>w</sub> 9.2), [[2004 Indian Ocean earthquake]] (''M''<sub>w</sub> 9.2), and [[2011 Tōhoku earthquake and tsunami|2011 Tōhoku earthquake]] (''M''<sub>w</sub>9.0) are recent examples of powerful [[megathrust earthquake]]s that generated tsunamis (known as [[teletsunamis]]) that can cross entire oceans. Smaller (''M''<sub>w</sub> 4.2) earthquakes in Japan can trigger tsunamis (called local and regional tsunamis) that can devastate stretches of coastline, but can do so in only a few minutes at a time.