Chapter 5 Megathrust Earthquakes and Tsunami PDF

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Sean Sheih...

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Chapter 5: Megathrust Earthquakes and Tsunamis Introduction 

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A large scale thrust fault (due to compressive stress) is called a megathrust fault o One block moves up a slope, relative to the other We consider the whole boundary zone between a subducting and overriding plate, rather than a specific easily defined plate Compressive activity between plates never stops in the lifetime of a subduction zone (ka10) The plates typically do not slip continually and gently past each other Typically, a high degree of friction means they get locked at various contact points for some time o A high amount of stress accumulates – rocks near the thrust zone are greatly deformed These contacts will release and jump ahead In this figure, the font (locked edge) of the right plate slides until a point is reached that the stress breaks the lock, producing a megathrust earthquake The largest earthquakes are all megathrust earthquakes – responsible for 90% of seismic activity

Megathrust Earthquakes 1. Occurs at an interpolate zone where one plate subducts beneath another 2. Occurs upon sudden release of a locked position 3. Has a magnitude greater than 7.0, commonly around 9.0 a. Remember: The Richter scale may be used accurately up to about 7.0 magnitude, but higher magnitudes should use the Moment Magnitude after getting a general idea using the Richter scale b. High number magnitudes produce shaking that lasts much longer than low magnitudes Magnitudes    

How many 2 magnitude earthquakes release the same energy as one 9 magnitude earthquake? The amount of energy released increases about 30 times with every unit increase 30x30x30x30x30x30x30 2 magnitude earthquakes = 1 9 magnitude earthquake If a 9.0 magnitude earthquake occurs every 500 years, it would take 1 million 2 magnitude earthquakes every day to release the same amount of energy

Frequency 

Anywhere between 200 and 800 years for any given segment of a subduction fault prone to periodic locking

Evidence of Megathrust Earthquakes 1. Tsunami Evidence a. A series of large waves that travel outward from the point of a sudden, large, vertical displacement of water 2. Submerged Coastlines a. Can be recognized by drowned trees and other vegetation within the sediment b. We can date the organic matter and get a time of when the tsunami occurred, and thus the earthquake 3. Large Underwater Landslides a. Particularly on the edge of continental shelves, great deposits of loosely compacted sediments accumulate b. These sediments originate from discharge of large rivers c. Any large earthquake can bring down these sediments in enormous landslides Case Study: The Cascadia Subduction Megathrusts 

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A magnitude 9.0 earthquake occurred on January 26, 1700 o Ripped apart the locked section of the Cascadia Subduction Zone west of Vancouver Island The resulting tsunami became North American Indian legend, and led to death and destruction in Japan and China!

Cascadia Plate Tectonics   

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The west coast of NA has every type of plate tectonic, and generates every category of earthquake An older plate, the Farallon plate, has been subducted under the North American plate The net effect produced a strong transform motion along the edge of the North American plate o The major transform boundary to the north is the Queen Charlotte Fault o The south boundary is the San Andreas Fault Part of the old Farallon plate is now called the Juan de Fuca plate, which has been subducting more and more, has been moving closer to the subduction trench The plate is still young, it is warm and buoyant, making it skim the bottom of the continental lithosphere of the North American Plate o Tends to get stuck temporarily – stress builds at sticking points o Release of this energy results in megathrust earthquakes

The 1700 Megathrust Event The Evidence   

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In 1805, Lewis and Clark wrote about strange stumps of large trees that stuck out, and an abundance of chaotically strewn boulders with no apparent local origin These features are a forest of redwood trees suddenly drowned by a sudden land subsidence and the deposits of a tsunami, respectively A movement to record stories from North American natives took place in 1865 o Stories from Pacific coast Indians consistently references a time of great shaking and flooding In 1987, geologists reported evidence of dramatic land subsidence in the past that could only be produced by a large earthquake and subsequent tsunami o Carbon age-dating revealed that the drowned trees were alive throughout 1699, but died afterwards Japan has documented tsunamis since the 1500s, one of which describes an “orphaned” tsunami (no documented associated earthquake) o After modelling the time of tsunami arrival against travel time through the Pacific Ocean, it was determined the earthquake took place January 26, 1700

The Event 

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The rupture broke the entire length of the Cascadia subduction zone (1100km, width of 50-150km, offset of about 20m) o These numbers equate to a magnitude of 9.0 or higher When coastal lands sink below sea level, plants and trees are killed (cannot live in salt water) Eventually the area fills with salty mud and becomes a marsh – trees here were carbon-dated o Buried plants have been uncovered by trenching and carbon-age dated The upheaval of a very large block of ocean floor crust drove the overlying water column, causing the tsunami o The tsunami inundated the coast and also traveled across the Pacific Ocean Calculated from mapped deposits, the wave heights probably reached 5-10m on the coast of Japan In the past 9800 years, there have been 18 megathrust earthquakes – quiet periods lasted 200 to 1000 years o The last two occurred at ~1500 and ~1700AD There is currently no pattern in frequency of the earthquakes

The Cascadia Subduction Zone Today and Tomorrow

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The overall subduction rate is currently about 40mm/y (horizontally) o This does not produce any movement on the locked portion, which extends about 60km down below the “lip” of the slab Below the locked zone is the transition zone, where some motion occurs o Below here is complete response to compression stress (almost continuously) without significant earthquakes August 1999 saw the equivalent of a magnitude 6.7 earthquake that occurred deep under Vancouver Island and northwest Washington State o Movement took place over a month in many “silent” steps in the transition zone Since 1999, very sensitive seismographs have been implemented – we can see that the “silent creep” occurs about once every year Since motion occurs below the locked zone, more stress is placed on the locked zone In the future years, the silent creep will eventually release the lock on the upper zone and a megathrust earthquake will occur! o We don’t know when! Case Study 2: The Sunda Megathrust Fault Zone

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On December 26, 2004, a 9.2 moment magnitude megathrust earthquake ripped open a section of the Sunda Subduction Fault Zone o The 1960 Chile earthquake was a bit greater! The ensuing tsunami claimed 186, 983 lives and made 42, 883 people missing, and presumed dead (total of 229, 866)

Plate Tectonic Setting 

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The Indo-Australian plate (divided into the Burma and Sunda subplates) moves north towards the southeastern portion of the Eurasian plate about 4050mm/y An earthquake ruptured at the boundary in 2004 Caused by oblique motion between the plates, which made a plate sliver shear off parallel to the subduction zone

The 2004 Earthquake    

Occurred at a depth of about 30km on a fault plane that dipped about 8° eastward Aftershocks continued for many weeks The rupture slipped 5m over 1300km, with a maximum 240km fault width The release of energy was equivalent to all global earthquakes within the past 10 years o This amount of energy altered Earth’s rotation (by a tiny bit) – 2.68μs shorter! o The effect is short-lived since the “wobble” of Earth increases Earth’s day a tiny amount every year

The Tsunami Event

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A section of ocean floor 600 – 800km N of the quake epicenter uplifted and displaced water It took about 15min to many hours to reach various coastlines o Indonesian coastlines were hit quickly o South Africa was hit 16 hours later Virtually all victims were taken by surprise – there was no tsunami warning system in the entire Indian Ocean region o The cost could be high, and countries that would be affected could not afford it o The Pacific Ocean has a sophisticated and advanced warning system because subduction zones bordering the Pacific commonly generate tsunami The tremor itself could indicate potential tsunami A draw-down event is a warming of an imminent tsunami A fax sent to the Thailand Meteorological Service warned of the tsunami, but was ignored o Prior earthquakes that generated a tsunami warning (although none developed) decreased tourism considerably

Death and Destruction 

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Ranked among the top ten worst earthquakes recorded in history, and the single worst tsunami in history o 229,866 deaths and 1,126,900 displaced persons The tsunami deposited many tonnes of debris across coastlines and inland – can lead to contamination of groundwater and spread of disease o Sewage, oil spills, industrial runoff, salt and heavy metals

The Human Role   

In further areas, the death toll was less because the effects of the tsunami were mitigated (lower speed and amplitude of the wave) Coastal modification (destruction of coral reefs, often with dynamite) often remove things that would dampened a tsunami wave (mangrove swamps, sand dunes) Ruins of a 1,200-year-old city was uncovered in South India, where a half-buried granite lion was discovered Tsunamis

Tsunami is derived from a the Japanese words “tsu” (harbor) and “nami” (wave); both singular and plural use of the word have the same spelling. Normal waves have intervals of 10s and wave lengths of 150m. Tsunamis can have wavelengths more than 200km and periods over an hour. Long wavelengths leads to increased speed of the wave, related to the water depth. In the deep Pacific Ocean, a tsunami can reach speeds up to 700km/h and travel over large distances with little loss in energy. 1. Tsunamis are unrelated to tides 2. Are not “seismic sea waves”, which implies they are related to earthquakes alone a. Some tsunami may be caused by landslides, volcanic eruptions, or meteoroid/asteroid impact

Measurement of Tsunami Magnitude 

The Imamura-Iida scale measures magnitude as a function of maximum wave height measured in the open ocean o May be difficult to apply because of the topography of the shore line hit may attenuate the wave height as it moves in

Tsunami Generation    

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A tsunami can be generated by any disturbance that displaced a large water mass from its equilibrium position Displacement of water on a large scale can lead to large waves Since many earthquakes take place in subduction zones in the ocean, these can lead to tsunamis Earthquakes deforms the crust rapidly from a sudden release of energy o Rather, the rust is slowly deformed as strain is build up, and rapidly deformed when the energy is released Movement of the crust will vertically displace the water, which will act under the influence of gravity to regain its equilibrium position, forming waves Submarine landslides caused by large earthquakes, or the collapse of oceanic volcanoes during eruption can disrupt the water column Violent underwater volcanic eruptions can also uplift the water column Landslides and meteorite impacts can also displace water – tsunami generated this way are generally smaller

What Happens as a Tsunami Approaches Land? 

As the tsunami approaches land, the water becomes shallower and the tsunami slows down – however the energy remains the same o The water rises to compensate for the decreased speed – the shoaling effect

What Happens as a Tsunami Encounters Land?  

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The level of water at the coast recedes noticeably before a tsunami strikes – known as the drawdown phenomenon Other times a rise in water level occurs, and due to the long wavelength, it can take a long time for a tsunami to crest and recede o Water may rise and peak, and remain there for several minutes o It can take up to an hour for successive crests of a tsunami to reach the shore Following the first wave, there is a hissing sound of water as it is drawn out o Usually the second or third waves are the greatest Part of the wave energy is reflected offshore, and energy of the forward moving wave begins to dissipate as it encounters friction and turbulence (from moving large objects) o Able to uproot trees, destroy buildings, and strip away sand from beaches Water can flood hundreds of meters inland Run-Up Height: Maximum height of the tsunami o Can reach 10, 20 or even 30 meters high!  The highest tsunami experienced was at Lituya Bay, which had a height of 150m and speeds of 150-210km/h

Pacific Tsunami Warning Center (PTWC) The PTWC originally started as the “Seismic Sea Wave Warming System”, but changed into the PTWC by 1949. Its headquarters are at Ewa Beach, Hawaii, and is headed by the National Oceanic Atmospheric Administration. Its objectives are; 1. To detect major earthquakes in the Pacific region 2. To determine whether they have generated tsunami 3. To provide warnings to the population of the Pacific to minimize hazard Seismograph detects a magnitude 6.5 or greater earthquake  Signal is sent to PTWC HQ  Location and possibility of tsunami generation is determined  Automatic tsunami watch is issued to Civil Defense agencies  Data from tidal gauges in the region determined if a tsunami developed  A tsunami warning is issued for areas that may be impacted  Boats head out to sea 1946 Hilo Tsunami 

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Two large subduction ruptures in the Aleutian Trench (one of which was magnitude 8.1) produced a large tsunami o Reached 30.5m high Hit the coast of Scotch Cap on Unimak Island 48min after the earthquake The US Coast Guard lighthouse was destroyed and its 5 inhabitants were dead The tsunami lost most of its energy as it crossed the Aleutian Islands Five hours after the earthquake, the tsunami waves hit the Hawaiian Islands o Killed 159 individuals...