Lecture 3 Flashcards
What is a tsunami?
- Japanese word for “harbour wave”
- produced by sudden displacement of water
- events capable of triggering tsunamis include: earthquakes that cause uplift of the seafloor, landslides, volcano flank collapse, submarine volcanic eruptions, meteorites
What are 4 historic tsunamis?
- 1755 Lisbon earthquake (M9)- 20 000 human casualties
- 1883 Krakatoa volcanic eruption, Indonesia- 36000 human casualties
- 2004 Sumatra earthquake (M9.1), Indonesia- 230 000 human casualties
- 2011 Tohoku earthquake (M9), Japan- 15000 human casualties
How can earthquakes cause tsunamis?
- displacement of the seafloor
- by triggering a landslide that enters water
- generally earthquake must be at least M7.5 to trigger a tsunami (subduction zones cause these earthquakes)
What are the stages of earthquake-triggered tsunami development?
Stage 1: displacement of the seafloor sets waves in motion that transmit energy outward and upward. When the waves reach the surface of the water, they spread outward.
Stage 2: In the deep ocean, the waves move rapidly (over 500km/h). The spacing of the wave crests is very large (more than 100km). The height of the waves is often small. Passengers on ships in the ocean rarely notice tsunamis passing beneath them.
Stage 3: As the tsunami approaches land, the water depth decreases. This results in the water “piling up” and causes: decrease in speed, decrease is spacing of the waves, increase in wave amplitude.
Stage 4: As the tsunami impacts land, waves can reach heights of dozens of metres. The wave speed at this time can be up to 50km/h making them impossible to outrun. During some tsunamis, the water first recedes from the shore and exposes the seafloor.
What occurs during a tsunami event? What is run up?
- tsunami event consists of a series of large waves reaching shore that can last for several hours
- run up: maximum vertical distance that the largest wave of a tsunami reaches as it travels inland
What is a distant vs local tsunami?
- distant tsunami: a tsunami that travels thousands of kms across the open ocean, on remote shorelines across the ocean reduced energy lessens the impact (also called tele tsunamis)
- local tsunami: a tsunami that affects shorelines a few km to about 100km from its source (earthquake), because of this short distance local tsunamis provide little warning
What regions are at risk for tsunamis?
- coasts located near subduction zones or across ocean basins from subduction zones are most at risk
- areas at greatest risk are the Pacific Ocean and the Mediterranean Sea
- Indonesia has a risk but the rest of the Indian ocean does not have much risk
What are the primary effects of a tsunami?
- flooding and erosion destroy beaches, coastal vegetation, and infrastructure
- after the tsunami retreats to the ocean, scattered debris is left behind
- most tsunami deaths are from drowning
- injuries result from physical impacts with debris
What are secondary effects of tsunamis?
- these are effects that generally occur after the event is over
- fires may develop due to ruptured gas lines or from ignition of flammable chemicals
- water supplies may become contaminated and water-borne diseases (cholera) may spread
How did the Indian Ocean tsunami of 2004 occur?
- M9.1 earthquake off the west coast of Sumatra
- occurred in a subduction zone between the Burma and Indian-Australian plates
- relatively small plates but the size of the plate doesn’t matter, only the interaction between the plates matters
- these plates had been locked for about 150 years thus allowing strain to build
- the rupture caused some land areas along the coastline to subside below sea level
How were other countries bordering the Indian ocean affected by the tsunami in 2004?
- countries bordering the Indian Ocean did not have a tsunami warning system like those bordering the Pacific Ocean
- people were caught by surprise and over 230 000 died
- many were unfamiliar with tsunamis and some where intrigued by the approaching waves
- most people in the area were ignorant of an early warning sign (receding sea)
- went the whole 1900’s with no large tsunami so people weren’t as aware and were more intrigued
What lessons were learned from the 2004 tsunami in the Indian Ocean?
- effective tsunami warning systems are needed around all oceans where tsunamis can occur
- in 2006, a new warning system became operational in the Indian Ocean
- emergency officials must have an organized plan for evacuating residents during a warning
- earthquake and tsunami education is necessary for people who live along or visit coast lines
How does the Pacific Ocean warning system detect tsunamis?
- uses a network of seismographs to estimate earthquake magnitude
- sensors electronically connected to buoys verify that a tsunami was produced
- they rest on the seafloor and measure changes in water pressure passing over them
- these sensors are called tsunameters
What are some structural controls that can be put in place to protect from tsunami damage?
- regulations on buildings and structures
- some cities in Hawaii require flood proofing measures such as basement window sealing and bolting homes to their foundation
- concrete levees are other preventative measures but can be very expensive
- offshore barriers are only feasible outside cities with very large populations (deflect waves away from a city)
What are inundation maps?
- maps showing the geographic area that can be potentially impacted by tsunamis
- can help plan for future events
- historical records, geologic data, and aerial photography aid in making the maps
- many NA cities on the Pacific Coast have such maps and development restrictions may exist there in areas at high risk of tsunamis
How can land use impact tsunami damage?
- vegetation plays a role in determining tsunami damage
- in areas impacted by smaller waves, trees and dense vegetation protect areas farther inland
- problem is that a lot of this vegetation gets cut down to sell
What should education on tsunamis include?
- information on where to go and what to take
- evacuation routes should be clearly marked to allow for a mass exodus of people
- people must be aware that tsunamis are a series of waves rather than just one
- coastal residents should be aware of the differences between a tsunami watch and a tsunami warning
How should people adjust for tsunami hazard?
- those located in coastal areas during an earthquake should leave for higher ground
- if sea level noticeably recedes, leave the coast
- people should not stay to watch a tsunami; based on typical tsunami speeds, if you can see it, you are already in danger
What are coastal hazards?
- the landscapes where continents meet oceans are dynamic and capable of rapid change
- major hazards include rip currents, erosion, sea level rise, storm surges, and tsunamis
What are coastline margins?
- some coastline margins are influenced by plate tectonics
- passive margins are distant from plate boundaries
- active margins are relatively close to plate boundaries
- West coast of NA- Cascadia is an active margin
- most of east coast is a passive margin (plate boundary is in middle of atlantic ocean)
What is wave height, wavelength, and wave period?
Wave height: the difference in height between a crest and a trough
Wavelength: the distance between successive wave crests
Wave period: the time for successive wave crests to pass a reference point
What is the motion of waves in the open ocean?
- motion is circular
- circles decrease in diameter with depth because energy decreases with depth
How do waves vary along a coastline?
- irregularities in topography cause variations in wave height as they approach the shore
- irregular coastlines have headlands that the waves reach first; resultant waves are typically large and high energy
What is a sea cliff (bluff)?
- bluff is freshwater
- sea cliff in ocean
- erosional landform that marks the landward boundary of a beach
What is a berm?
-an onshore portion of the beach that is generally flat and formed by deposition of sediment
What is a beach face?
- an onshore portion of the beach that slopes seaward
- it lies within the swash zone where waves repeatedly rush up and then back
What is a surf zone?
-an area where waves move toward the shore after they break
What is a breaker zone?
-an area where incoming waves peak and break
What is a longshore bar?
- a low ridge on the seafloor in the breaker zone
What is a longshore trough?
-a depression on the seafloor formed by wave action landward of the longshore bar
What is littoral transport?
- sand movement parallel to the shore
- consists of:
Beach drift: sand moving in a zig zag pattern in the swash zone
Longshore drift: transport of sediment by currents that flow parallel to the shoreline
-both types occur where waves strike the coast at an angle other than 90 degrees
What landforms are produced by littoral transport?
- spits and ridges that extend parallel to the shore from a point of land on a coast
- barrier islands are long, narrow islands separated from the mainland by a bay or lagoon
What are rip currents?
- horizontal currents that move away from a shoreline
- sometimes called undertow
- they develop when waves pile up water between the longshore bar and the swash zone; a breakage develops in the longshore bar and then water rushes back through the break
- in NA, 100 people drown in rip currents each year
What are seawalls?
- seawalls are structures built parallel to the shoreline
- vertical design reflects waves and redirects energy to the shore
- they tend to enhance beach erosion seaward of their base
What are groins?
- groins are structures built perpendicular to the shoreline usually in groups
- they trap sand carried by longshore drift
- the accumulation of sand contributes to a wider beach but erosion still occurs in the downdrift area
What are breakwaters? Jetties?
- built parallel to shores and designed to protect boats in a harbour or marina
- jetties extend perpendicular to shores at the mouth of a river commonly built in pairs
- the objective of a jetty is to prevent sediment from accumulating at a river mouth and to shelter the river channel from large waves
What is the e-line and e-zone?
- after average erosion rates have been determined, maps can be produced to forecast future erosion
- e line: the expected position of the shoreline after a specified number of years
- e zone: the area between the present shoreline and the respective e line
