Tectonic Processes & Hazards - EQ1 - 1.3C - Tsunamis Flashcards
What is a tsunami
- a series of larger-than-normal waves (know as a wave train) caused by seabed displacement
Tsunamis are usually linked to……, …% occur along ……………………- particularly located within the ………….
- tectonic events
- 90%
- subduction zones
- pacific Basin’s ‘Ring of fire’
What is the wavelength of tsunamis
long (much longer than normal sea waves) typically 150- 1000 km
What is the waveheight of tsunamis
- Wave height (amplitude) is low (0.5- 5m)
What is the speed of tsunamis
- Velocities are fast, up to 600 kph in deep water
The first wave in a Tsunami is not………………………….., so often there is an……………….
The first wave in a Tsunami is not usually the most destructive, so often there is an escalation effect in terms of damage and loss of life
The amount of time between ………………waves (………………………) is only a …….. apart posing a …………… risk, as….
the amount of time between successive waves (wave periods) is only a few minutes apart posing a greater risk, as people have lost their lives after returning home between waves thinking it has stopped coming
Why are not all tsunamis a hazard
- Out to sea Tsunamis do not represent a hazard since they are generally low in height (often 300 mm) and generally go unnoticed
- It is only as they approach a coastline that they grow in height as the water becomes shallower
Give an example of the largest wave not being the first in tsunamis
- For example, in the 1960 Chile tsunami at Hawaii, it was the third wave that did the damage
How does the wavelength of tsunamis vary
Long wavelength in open ocean but short in shallow water
How does the speed of tsunamis develop
- Very fast speed in deep water but much slower (35 km/h) in shallow water
How does the waveheight of tsunamis vary
- Low height in open ocean (about 3 m), but significant height in shallow water (up to 20 times higher than in deep water)
Causes of tsunamis
Tsunami waves are caused by….
- the displacement of large quantities (columns) of water
- The displaced water become tsunami waves as the waves reach shallower water in coastal areas (as the topography of the seabed changes) the waves become higher
- In shallower water, friction between the tsunami wave and the seabed increases and the tsunami waves slow down,
- decreasing wavelength but increasing waveheight
Causes of tsunamis
How can earthquakes cause tsunamis
- An undersea earthquake at a subduction zone may cause a rapid movement of the seabed
- If this movement is up or down then a column of seawater above the epicentre is displaced, creating a powerful wave motion
- Undersea landslides displace water when material falls from a continental shelf on the seabed
Causes of tsunamis
What can volcanic erruptions cause tsunamis
- Volcanic eruptions displace water when materials ejected from the volcano falls into the sea
Causes of tsunamis
how do landslides cause tsnuamis
Landslides displace water when large quantities of water are displaced by land falling into the sea
What determines the impact of Tsunamis
- The duration of the event
- The wave amplitude, water column displacement and the distance travelled.
- The physical geography of the coast, especially water depth and gradient at the shoreline.
- The degree of coastal ecosystem buffer, for example protection by mangroves and coral reefs.
- The timing of the event – night versus day – and the quality of early warning systems.
- The degree of coastal development and its proximity from the coast, especially in tourist areas.
What are tsunamis measured on
The Tsunami Intensity Scale
How are tsunamis measured
- Tsunamis are measured by their run-up height, which is the maximum vertical distance between the normal sea level and the highest point the water reaches on land.
- They are also measured by wave height, which refers to the vertical distance from the wave’s trough to its crest in the open ocean.
- Today, satellite altimeters and deep-ocean sensors (such as DART buoys) are used to detect and measure tsunami wave height and speed in real time.
What is the diffculty of predicting tsunamis to send warnings
- Since most tsunami are caused by underwater earthquakes, the lack of a way to predict earthquakes also means that there is no way to predict tsunami before they occur
- although it is possible to give people some early warning before an activated tsunami actually reaches the coast and threatens lives
Tsunami early warning systems are now in place in both the ……..
Pacific and Indian Oceans
What are tsunami early warning systems
- These systems use seismic sensors to detect undersea earthquakes
- Yet, because not all undersea earthquakes cause tsunami, scientists use additional scientific equipment to gather more information
- This includes a system called DART (Deep-ocean Assessment and Reporting of Tsunami)
Describe the DART system
- The DART system uses seabed sensors and surface buoys to monitor changes in sea level and pressure
- When tsunami waves are detected, the system sends the information via a satellite to tsunami warning stations
- These stations review the transmitted information and use computer modelling to estimate the size and direction of the tsunami, before informing the areas at risk
- Depending on where the tsunami originates, some people might receive a warning hours in advance of any threat - giving them time to move to higher ground further inland
Describe how early warning tsunamis sensors could have be useful at the 2004 Boxing Day Tsunami
- In 2004, the Indian Ocean didn’t have an early warning system The Boxing Day tsunami took 2 hours to reach Sri Lanka, where over 31 000 people died
- Had an early warning system been in place, many people might have had time to evacuate
- Therefore, because of the terrible impacts of the 2004 tsunami , an Indian Ocean early warning system was developed and began operating in 2006
