EESA05 - Environmental Hazards - Lec 7 (near-verbatim).docx

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University of Toronto Scarborough
Environmental Science
Mandy Meriano

EESA05 Lecture 7: Tsunami PY Date: Oct 30, 2012 Slide 1: Tsunami  Hurricane Sandy and EQ this week  West coast of Canada more prone to EQs; Queen Charlotte’s Slide 2: BC Earthquake – Oct 27, 2012  7.7 on Richter scale = no injuries  No tsunami  American gvt sent warning earlier than CDN gvt  West coast of Canada = prone to EQs because can find all the recognized faults in West coast – get diff boundaries here – divergent, convergent boundaries in this area and transform faults (strike-slip)  Purple line delineates the plate boundaries o One very well defined boundary with straight lines on the western side = that’s one plate = juan fuca plate o North American plate to the east of it and the much larger Pacific plate to the West  Juan fuca plate is being subducted beneath N. American plate = subduction o How it’s moving – it’s because of the ridges along the straight lines  One ridge above it = called explorer ridge  And the ridge below it = called endeavour ridge  Have these 3 ridges that are acting as a rift where magma is coming out and pushing these plates apart  The force that’s required to push that smaller plate beneath the N.American plate is created by these ridges by these divergent zones  Along these, also have fracture zones where you have transform mvt  Fig A: shows general mvt of Juan Fuca plate by using arrows – northeasterly direction = 45 cm every year moving in that direction o Way above it, top part of diagram = see Queen Charlotte’s fault = strike-slip transform fault = this is Canada’s San Andreas fault – EQ experienced on Oct 27,2012 = mvt along this transform fault  Focus = 10 km below surface  So, along West coast have convergent, divergent and transform boundaries  EQs @ divergent boundaries not as disastrous but along convergent and transform boundary zones – great magnitude, felt a great distance away o Most of the EQs in this case, the focus is quite shallow, 70km depth below surface o Epicenter = Northwest of Vancouver island = circles  EQ happened along subduction zone = Cascadia subduction zone where Juan Fuca plate is being subducted beneath N.American plate o From Vancouver to Washington, Oregon and Northern part of Cali Slide 3: EQ in Turkey – Oct 22, 2011  Caused a lot of havoc 1 EESA05 Lecture 7: Tsunami PY Date: Oct 30, 2012  Epicentre in Turkey  Some larger boundaries have designated smaller plate names based on fault lines  Mvt from Arabian plate being pushed against the Eurasia plate = get mega thrust fault as it’s being pushed up o Convergence of these plates against one another = caused this very large EQ  The movement of the plate Juan Fuca is not perpendicular to the North American plate but is rather moving at an oblique – whenever you have oblique angles you actually get these huge thrust faults – mega thrusts = causes major EQs – Vancover and Turkey are examples  Also get a lot of strike-slip fault around this area b/c of a lot of fractures that happen with these thrust faults o In Canada, whenever you get oblique faults, convergence, and you get subduction zones a lot of strike-slip faults that you get actually happen in continental part; mega thrust fault but the expression in continent is more in the shape of strike-slip fault  Turkey major plates: o Arabian plate pushed in northerly direction below Eurasian plate Slide 4: Learning Objectives  Understand the process of tsunami formation and propagation  Understand effects of tsunamis and the hazards they pose to coastal regions  Know that geographic regions are at risk for tsunamis  Recognize the linkages between tsunamis and other natural hazards  Know what nations, communities and individuals can do to minimize the tsunami hazard Slide 5: Intro  Tsunami is Japanese for ‘harbor wave’  Caused by the vertical displacement of ocean water  Triggered by: o Large EQs that cause uplift or subsidence of sea floor o Underwater landslides o Volcano flank collapse o Submarine volcanic explosion o Asteroids  Can produce mega tsunami Slide 6: The EQ Cycle  Tectonic activity is gradual and relentless o Tectonic activity – things that are pushed together, transform faults slide; move apart  all of these = applying a lot of stress to these rocks – strain builds up until they don’t have the capacity to take any more of the stress – this is when you get rupture in the rock – an elastic = pull it so much that you actually snap it 2 EESA05 Lecture 7: Tsunami PY Date: Oct 30, 2012 o Rocks have elastic quality, can’t take a lot of stress – will eventually deform – then break/rupture of the rock –T1 to T4 = 4 diff stages of EQ o 1000s of years, applying stress to these stress o Under the strain = being deformed = still within elastic limitations o Elastic strain accumulates and rock bends o Then rupture of the rocks happens when the fracture is in rock – have displacement of the rocks on either side of the fault – strain is now replaced by horizontal displacement o Type of fault in this diagram – strike slip = right lateral  Over millions of years, stresses are gradually applied to rocks @ continental margins  The rocks deform continuously until there is too much strain and then they break  After the EQ occurs, the stress levels in the immediate area are reduced and it will likely require considerable time before another EQ occurs Slide 7: The EQ cycle  Animation: o 4 stages of EQ cycle o Fault line = survey line for us to know what is moving and which direction moving o Elastic rebound  Applying stress, rocks are being strained – bending a bit  See how the 2 sides are moving but the rock hasn’t ruptured yet  Rocks are being strained  B4 EQ happens can get foreshocks – this is when you can get foreshocks  Foreshocks are happening as the rocks reach its limitation of strain but before it breaks – when it starts to shake  Don’t always get foreshocks but in many cases, right b4 big EQs, get little tremors o Stage 1:  Longer period of inactivity o Stage 2:  When the elastic strain actually produces some small EQs  Tremors can happen for very long time as the rock is being stressed o Stage 3:  Foreshocks happening right before major EQ; happen days b4 major EQ event  Doesn’t necessarily have to happen o Stage 4:  Break rocks, get major quake – rock has released all the NRG lock up inside of it and it breaks o After EQ get aftershocks (this is what they’re feeling in Vancouver now)  Can happen for very long time after major event (can be from days to even a year after the major quake)  Big EQ in NZ, a year and a half after big EQ, still had aftershocks 3 EESA05 Lecture 7: Tsunami PY Date: Oct 30, 2012  Central America = things are shaking all the time but no one does anything because used to it – tectonic activity that causes tremors, these aren’t necessarily foreshocks; these are periods where the rock is taking up all the stress Slide 8: Question  Brittleness is the tendency of a solid material to fracture when a deforming stress exceeds the material’s elastic limit. o Option D = ductile = ability to take new form so it doesn’t make sense in this case Slide 9: Tsunami  Tsunamis are large waves generated by any significant and rapid disturbance of the ocean. The most common cause is an EQ on the ocean floor (displacement of the ocean floor – dropping or lifting – all the water on top of it will be displaced), although they can be caused by underwater volcanoes, landslides or meteorite impacts o Tidal wave = inappropriate way to describe tsunami b/c it has nothing to do with tides  Focus = on ocean floor – result of the EQ = movement of one plate being forced further up – causes water above it to lift up – tsunami is generated  500 – 800km / hr wave  Tsunami are very diff from wind-generated waves = water moc in wind-generated wave moves in circle whereas in tsunami it moves straight on (therefore, doesn’t lose a lot of NRG b/c doesn’t spin)  so, water displace and it radiates out and starts to move  when it gets to shore – a lot shallower area – it’s almost like someone’s trying to stop the wave and it slows down from 500 km/hr to 50 km/hr o meanwhile, the water still moves @ 500km/hr, the water behind it stacks up – huge wall of water Slide 10:  In deep water, tsunamis are fast-moving waves of low amplitude and very long wavelength and are nearly undetectable. As the water becomes shallower, the front of the wave begins to slow down and the trailing portion of the wave catches up and the wave stacks up – large wall of water when it gets close to shore o Part of wave:  Amplitude = height of wave  Wavelength = distance between 2 wave crests = lambda  Crest  Trough Slide 11:  The destructive power of the waves can’t be appreciated at sea as the amplitude is so low. Once the waves reach shore, tremendous damage can result, often with little warning. 4 EESA05 Lecture 7: Tsunami PY Date: Oct 30, 2012 o Tsunamis have very long wavelength and very small amplitude and doesn’t travel in circular motion – goes straight on  Because of these characteristics, if you have a boat in the ocean (away from the shore), you wouldn’t even know that there’s a tsunami travelling right below you Slide 12: Animation – Indonesia EQ  Motion of ocean floor = up – EQ  All along this fault, the water and everything above it has been displaced  Displacement of entire water column creates tsunami which travels at high speed  Travelling away from epicentre  Boat sitting off shore – because tsunami wavelength so large and amplitude so low, they don’t feel the tsunami  Not until it reaches shallow areas, near shore, that the slowing down of the wave causes build- up and b/c it builds up so high it can surge for very large distance from shore o Not only does it reach large heights once it reaches shore, it can go straight on for long distances and take out whatever is in its way  Indonesian tsunami – before and after o Can see large distance away from shore has taken everything in its way out Slide 13:  Wind waves move in circle whereas tsunami waves move straight on o Wind waves come and go without flooding higher areas o Tsunamis run quickly over the land as a wall of water – don’t lose NRG – keep NRG as they travel which is why they’re so dangerous  Tsunami isn’t a simple wave o If you’re in a place where tsunami hit, don’t believe that it’s safe to go back to lower grounds o Tsunamis come in wave trains – never one wave, should always anticipate more Slide 14: Which of the following statements is true?  All tsunamis are caused by displacement of water Slide 15: Tsunami  The destructive power of the waves can’t be appreciated at sea as the amplitude is so low. Once the waves reach shore, tremendous damage can result, often with little warning Slide 16: Tsunami @ Japan  Magnitude 9.0 – near the east coast of Honshu, Japan on March 11, 2011  Pacific plate on right side moving below N.American plate o Plate tectonics 5 EESA05 Lecture 7: Tsunami PY Date: Oct 30, 2012 o Result of major convergence is a big thrust fault and this thrust fault is what caused the large tsunami o Fault itself happened along 300km long fault line = strike was 300km long; and it was 150 km wide and plate moved by few meters up – large amount of water displaced o It caused a lot of damage; killed 15703 ppl; 4500 ppl missing; more than 13000 were displaced; caused $309 B US in damage o Huge hazard = very significant event Slide 17: Tsunami @ Sumatra (2004)  The amount of ground motion has been measured by satellite  Note that motion of seafloor west of Sumatra was toward the northeast, whereas motion to the east of the fault was directed to the south west  Tsunami caused by EQ in Dec 2004; happened along seismic boundaries of India plate being subducted below Andeman plate (smaller plate); subduction in oblique fashion and it was this
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