Chapter 5

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Western University
Earth Sciences
Earth Sciences 2240F/G
Kristy F Tiampo

EARTH SCIENCE Chapter 5: Megathrust Earthquakes and Tsunami 1.0 Introduction  Similar to a thrust fault, a megathrust fault also deals with one rock block thrusting up and over the other by compressive stress, but on a much larger scale. o For a megathrust fault we consider the whole boundary zone between a subducting and overriding plate rather than a specific, easily defined plate  During the lifetime of a subduction zone (where two plates meet), compressive activity is always happening. The plates do not continually slide against each other, but build up friction at contact points over the course of time. During this time massive amounts of stress accumulate, causing the rocks in the vicinity to deform while locking against one another.  When the point is reached that the accumulated stress of the deformed plate is sufficient to cause the lock to break, the sudden release of energy produces a megathrust earthquake.  All of the world‟s largest earthquakes are megathrust earthquakes accounting for the release of 90% of the earth‟s seismic energy.  Megathrust earthquakes: 1. Occur at an interplate zone where one plate subducts beneath another 2. Occur upon the sudden release of a previously locked section 3. Has a magnitude greater than 7.0 and commonly in the range of 9.0  Magnitudes: Richter scale can measure magnitudes up to about 7 o For higher magnitudes we start with the Richter Scale but get the final number using the Moment Magnitude Scale (Chap 3) o Much higher magnitudes cause great energy release that produces shaking that lasts much longer than lower magnitudes o Amount of energy released increases about 10x with every unit increase on the magnitude scale (40x over 7.0) o There is a 9.0 magnitude earthquake about once every 500 years  Evidence of past megathrust earthquakes include: o Tsunami evidence: generated by the sudden upward motion of a plate edge by a megathrust action o Submerged coastlines: recognizable by drowned trees and other vegetation within the sedimentary layer o Large underwater landslides: massive earthquakes will send great deposits of sediments into underwater landslides 2.0 Case Study 1: Cascadia Subduction Megathrusts  January 26 1700 just west of Vancouver Island, a megathrust earthquake of approximately a 9.0 magnitude ripped apart the locked section of the Cascadia Subduction Zone. o The resulting tsunami became North American Indian legend as it caused death and destruction as far away as China and Japan 2.1 Cascadia Plate Tectonics  West coast of North America has every type of plate tectonics, thus every category of earthquake can happen in the area  SEE CHAPTER 5, PAGES 2-3 FOR EXPLANATION AND DIAGRAMS The 1700 megathrust event History  In 1805 explorers Lewis and Clark wrote about their findings that today we recognize as evidence of a tsunami: stumps of trees sticking out of rivers, boulders and rocks that did not originate locally, and a redwood forest drowned by a sudden land subsidence  1865, an attempt was made to record native‟s stories so they would not be lost. Many stories from the Pacific coast Indians included reference to a time of great shaking and flooding, figured to be a massive earthquake/tsunami event around the year 1700  At the time, the stories were dismissed because although they knew it was a subduction zone, they believed it was completely stable and not moving EARTH SCIENCE  1987 geologists reported findings of dramatic land subsidence in the past from the same evidence found by Lewis and Clark, which they concluded could only be produced by a large earthquake/tsunami. By carbon age dating the tree rings, they discovered the trees had been alive throughout 1699 and died a few months later, concluding the earthquake occurred in January 1700  With the help of the Japanese who had been recording tsunamis since the 1500s, records found an orphaned tsunami (no earthquake), determined the time it would have taken to travel across the Pacific Ocean, and concluded the earthquake would have occurred Tuesday January 26 1700 The event  Broke the length of the Cascadia subduction zone (1100km) to a width of 50-150km and an offset averaging 20 meters. = Magnitude 9.0 or a bit more  Upheaval of the very large block of ocean floor crust caused the tsunami, and within minutes the waves washed over nearby coasts and also travelled across the Pacific Ocean.  Wave heights reached 10 meters on the coast of Washington State and about 5 meters on the coast of Japan across the Ocean. Pre-1700 events  1 megathrust 9800 years ago o 800 year gap  3 megathrusts 8300-9000 years ago o 700 year gap  5 megathrusts 5700-7600 years ago o 1000 year gap  2 megathrusts 3900-4700 years ago o 500 year gap  3 megathrusts 2500-3400 years ago o 700-900 year gap  4 megathrusts in past 1600 years o last two: around 1500 AD, and 1700 AD Cascadia subduction zone today and tomorrow  Overall current subduction rate horizontally is 40mm/y but that doesn‟t produce any movement on the locked portion, which extends to about 60km down slope below the lip of the slab.  Below the locked zone is the „transition‟ zone where some motion occurs  Below that there is complete response to compression stress continuously without significant earthquakes.  As an example, in August 1999 the equivalent to a 6.7 magnitude earthquake occurred under Vancouver Island and Washington State. The movement took place over one month and in many small „silent‟ steps. All the motion took place in the transition zone, nothing moved in the locked portion.  Since 1999, very sensitive seismographs have been put in place and has seen the period of silent creeps take place about once a year (creeping along instead of one big release jump) o However, since all the motion is occurring under the locked zone, more and more stress is being put on the locked zone. o Its possible that at some point in the near future during a period of silent creeping, the lock on the upper zone will be released and cause the anticipated megathrust earthquake to occur o When it will occur is completely random and there is no way of predicting the exact time the stress release may happen Case Study 2: Sunda Megathrust Fault Zone  December 26, 2004 – second largest seismic event in 40+ years  Magnitude of 9.2 ripped open a section of the Sunda Subduction Fault Zone, inducing the most devastating tsunami in history  It roared around the Indian Ocean taking the lives of 186,983 and 42,883 missing (total of 230,000) Plate Tectonics o Rupture at the boundary between the Indo-Australian plate (moving northward at 40-50mm/y) and the SE portion of the Eurasian plate, which is segmented into the Burma and Sunda subplates o As a result of the slanting motion between the Indo-Australian plate and Burma and Sunda subplates, a plate sliver (Andaman micro plate) has sheared off parallel to the subduction zone. EARTH SCIENCE o Past earthquakes in the region: 1797 magnitude 8.4, 1844 magnitude 9, and several others o In March 2005 following the 2004 event there was a magnitude 8.6 in the same region as the 1861 and 1907 quakes, however the 2004 event was by far the largest 2004 event Earthquake o The rupture was at a depth of 30km north of the section broken in the previously mentioned events o Aftershocks in the weeks after amounted to the greatest „swarm‟ of earthquakes every recorded, and showed the rupture had slipped an average of 5m over the length of 1300km (longest known rupture) and a max 240km of fault width. o When the magnitude calculations were done it was discovered that the total energy released by the main event was equal to the cumulative total of all global earthquakes over the previous 10 years. o The release of energy was so large that it altered the Earth‟s rotation, shortening each day by 2.68 microseconds, also decreasing the earth‟s oblate sh
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