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ISNS 4359 Earthquakes and Volcanoes (aka shake and bake) Lecture 7: Tsunami Fall 2005 Tsunami • Created most often by earthquakes – Vertical shift of ocean floor that offsets water mass, transmitted throughout ocean in tsunami – Usually vertical fault motions at subduction zones, mostly in Pacific Ocean • 70,000 people killed by tsunami in 20th century • Seiche (tsunami in enclosed area) – Largest ever known was caused by rockfall in Lituya Bay in Alaska, 1958 – up to 520 m high • Largest tsunami possible probably from asteroid or comet impact into ocean – never witnessed by humans Tsunami vs. Wind-Caused Waves • Wind waves – Velocities depend on period • 17 mph for 5-second wave • 70 mph for 20-second wave • Tsunami – Velocity: v = (g x D) ½ • • • • g – acceleration due to gravity D – depth of ocean water For average D = 5,500 m, v = 232 m/sec Will slow down as nearing shoreline but still freeway speeds – Height: ~1 m in open ocean, 6 to 15 m in shallow water, higher in narrow topography Tsunami vs. Wind-Caused Waves • Tsunami at the shoreline – Not a gigantic version of breaking wave – Very rapidly rising tide, rushing inland – Height of wave is typically less important than volume of water flooding area, before retreating to form next wave – Tsunami waves are usually separated by 10-60 minutes Tsunami Warnings • Warnings – Feel the earthquake – See sea level draw down significantly – Hear wave coming • What to do – Seek high ground immediately – Go upstairs in well-built building • Warning system – First sensors activated in 2003 – Tsunami warning center in Honolulu for Pacific Ocean Fig. 4.40 Tsunami hazard zones Tsunami travel times in hours Tsunami Case Histories • Alaska, 1 April 1946 • Two large subduction zone earthquakes in Aleutian Islands created 30 m high tsunami • Destroyed local lighthouse • Raced across Pacific Ocean to Hilo, Hawaii – killed 159 people Fig. 4.36 Hilo, HA 1946 Alaska quake Fig. 4.39 Hilo 1946 Alaska earthquake Tsunami Case Histories • Chile, 22 May 1960 • Magnitude 9.5 subduction event was most powerful earthquake ever recorded, created large tsunami • Three waves, each successively larger, hit Chilean coast, killing 1,000 Chileans • Adequate warning was given in Hawaii but 61 people killed • Tsunami continued to Japan, killing 185 people • Could continue to be measured in Pacific Ocean for a week Hilo—note distance to shore From 1960 Chile earthquake Tsunami Case Histories • Alaska, 27 March 1964 • Magnitude 9.2 subduction earthquake killed 122 people in sparsely populated Alaskan coast • Tsunami hit Vancouver Island, then California • Series of waves, with fifth one largest • Which wave in series will be largest is not predictable Tsunami Case Histories • Nicaragua, 1 September 1992 • Very slow but large (magnitude 7.6) earthquake shook ground very little so was barely noticed • 100 km segment of plate moved 1 m over two minutes • Generated 10 m high tsunami to hit 300 km of coastline without warning Tsunami Case Histories • Papua New Guinea, 17 July 1998 • Magnitude 7.1 earthquake 20 km offshore, triggered underwater landslide that caused tsunami • Hit coastline of Papua New Guinea about 5 minutes later, washing four villages on barrier beaches into lagoons • Caused re-thinking of tsunami threat, as caused not just by large earthquakes but also by landslides triggered by moderate earthquakes Tsunami Hazards from Landslides • Southern California: – Protected from distant tsunami by offshore islands – Offshore islands were formed by active faults, that could cause landslides to generate local tsunami • Atlantic coastline: – Off east coast of North America, images show scars of big submarine landslides • 1929 magnitude 7.2 Grand Banks earthquake triggered landslide that caused three tsunami waves to hit Newfoundland • Lake shores are at risk from seiche waves also – Active faults run under Lake Tahoe Fig. 4.42 Tsunami swept sandbar, New Guinea,1998 Sumatra Tsunami, December 26, 2004 Sumatra earthquake, 2005 At 00:58:53 UTC on 26 December 2004, Great earthquake off northeast coast of Sumatra, Indonesia. Location map of Indonesia from the National Earthquake Information Center (http://neic.usgs.gov/neis/bulletin/neic_slav_l.html) Earthquake caused by subduction of Indian plate beneath the Burma microplate Relative motion of Indian plate to Burma microplate = 0.06 m per year (= 6 cm or 2.4 inches per year) Other tectonic elements in region: Yangtze Plate Eurasia Plate Sunda Trench Sunda Plate Australian Plate Ninety East Ridge Tectonic Setting of Earthquake of 26 December 2004 from United States Geological Survey (http://earthquake.usgs.gov/eqinthenews/2004/usslav/) Mw = 9.0 Hypocenter = 30 km depth Focal mechanism = thrust Largest quake since 1964 Good Friday event, Alaska Large magnitude quakes at subduction zones are often tsunamigenic Local tsunamis propagate toward nearest shoreline Teletsunamis spread out across ocean basins Map showing epicenter (star) of earthquake from the National Earthquake Information Center (http://neic.usgs.gov/neis/bulletin/neic_slav_l.html) Animation of Before and After Images of Banda Aceh, Indonesia Seismicity 1990-2004 Star is 26 Dec 2004 event Well-defined Benioff Zone associated with subduction of Indian plate beneath Burmas microplate Thus, main event occurred in seismically active area Many earthquakes annually Many have Mw > 6.5 (damaging magnitudes) Map showing historical seismicity (1990 - 2004) from the National Earthquake Information Center (http://neic.usgs.gov/neis/bulletin/neic_slav_l.html) Est. rupture length = 1200 km Est. rupture width = 100 km Est. thrust fault offset = 15m Est. seafloor uplift = m? Uplift of seafloor is responsible for excitation of tsunamis Map showing historical seismicity (1900 - 2004 ≥ M6) from the National Earthquake Information Center (http://neic.usgs.gov/neis/bulletin/neic_slav_l.html) http://earthquake.usgs.gov/eqinthe news/2004/usslav/rupture_areanw.html Areas potentially affected by tsunami inundation and run-up in red Map showing areas potentially inundated by tsunami on 26 December 2004 from UNOSAT. (http://cern.ch/unosat/freeproducts/Tsunami/JRC/Asia_Tsunami_07January_landcover.pdf) Potential population affected by tsunami in red zone Potential population affected by tsunami on 26 December 2004 from UNOSAT (http://cern.ch/unosat/freeproducts/Tsunami/JRC/Asia_Tsunami_04January.pdf) Areas potentially affected by tsunami inundation and run-up in yellow (i.e. shore zones < 20 m above sea-level) Areas Potentially Affected by Tsunami of 26 December 2004 from UNOSAT (www.unosat.org) Predicted Arrival Time of First Tsunami Wave (in hours after earthquake) QuickTime™ and a Cinepak decompressor are needed to see this picture. Numerical simulation (computer model) of tsunami Dr. A. Piatanesi of the National Institute of Geophysics and Volcanology (INGV) in Rome, Italy. Numerical simulation (computer model) of tsunami http://www.noaanews.noaa.gov/stories2005/s2365.htm Areas potentially affected by tsunami inundation and run-up in yellow (i.e. shore zones < 20 m above sea-level) http://www.nasa.go v/vision/earth/looki ngatearth/tsunamiimages.html Sri Lanka http://www.nasa.go v/vision/earth/looki ngatearth/tsunamiimages.html QuickTime™ and a Video decompressor are needed to see this picture. http://earthquake.usgs.gov/eqinthenews/2004/usslav/andaman_fig1.gif http://earthquake.usgs.gov/eqinthenews/2004/usslav/andaman_fig2.gif http://earthquake.usgs.gov/eqinthenews/2005/usweax/ Animation of Before and After Images of Banda Aceh, Indonesia Animation produced at the Earth Visualization Laboratory, University of Arkansas, USA By Dr. Stephen K. Boss Countries Reporting Direct Casualties from Earthquake and Tsunami Indonesia Thailand Andaman Islands (India) Nicobar Islands (India) Myanmar Bangladesh India Sri Lanka The Maldives The Seychelles Kenya Somalia Tanzania In addition, many western nations are reporting fatalities and missing among tourists who were visiting the affected countries Fundamental Features of Water Waves Wave Crest: is the highest portion of the wave. Wave Trough: is the lowest portion of the wave. Wavelength: is the linear distance separating wave crests (or separating wave troughs). Wave Amplitude: is the displacement of a crest or trough about the mean position or water level. Wave Height: is the total vertical distance from crest to trough (equal to twice the amplitude). Wave Period: is the time required for successive wave crests (or troughs) to pass a fixed point. from An Introduction to the World's Oceans, 8th edition by K.A. Sverdrup, A.C. Duxbury, and A.B. Duxbury (2004) deep-water waves: water depth is greater than one-half the waves' length. shallow-water waves: depth becomes shallower than 1/20th of the wavelength Tsunamis have wavelengths ranging from 100 – 200 km! Tsunamis are, therefore, shallow-water waves (even in the open ocean)! Though tsunamis have very long wavelengths, their amplitude in the open ocean is often relatively small - commonly only a meter or two - and this amplitude is distributed over the very long wavelength so that tsunamis are quite imperceptible on the surface of the ocean. The speed of tsunamis is called celerity and is equal to: 3.31 x D0.5, where D = water depth (in meters) In the deep ocean, tsunamis travel at speeds up to 750 km per hour (465 mi/hr)! Wave Refraction occurs when waves enter shallow water over an irregular bottom Wave traveling in shallowest water will also move with the slowest celerity Waves in deeper water continue moving with relatively greater celerity Wave crests will curve or refract as the wave moves forward Waves spread or disperse upon passing through the gaps in barriers. This process is called "wave diffraction“ Barriers with many gaps scatter wave energy, diminish wave height and power Tsunami Triggered by Explosive Volcanic Eruption Krakatau, Indonesia, 1883 On 26-27 August 1883, Krakatau volcano erupted Among most violent volcanic eruptions in last 200 years Tsunamis triggered by volcanic explosion and caldera collapse Destroyed 165 coastal Indonesian villages on Java and Sumatra 36,000 Fatalities Prior to 26 December 2004, most destructive tsunamis on record Tsunami Triggered by Impacts of Large Extraterrestrial Objects Chicxulub, Mexico Asteroid impact site and tsunami trigger terminating the Cretaceous Period Asteroid believed to have had 10 km diameter (6.2 miles) Impact in shallow tropical seas Tsunami deposits widespread across Caribbean basin & Gulf of Mexico Impact event also associated with mass extinction of terrestrial and marine biota