Seismic Hazards Flashcards
(32 cards)
What are earthquakes?
The movement of tectonic plates causes pressure to build up in the Earth’s crust. When the pressure is released, a series of tremors known as earthquakes occur.
The focus is the point underground where the earthquake originates from. The epicentre is the point above ground that is directly above the focus.
What is the spatial distribution of earthquakes?
-Found along all plate boundaries.
-The ring of fire accounts for 90% of all the worlds earthquakes
-While about 90% of all earthquakes occur at plate boundaries, approximately 10% are intraplate.
-The UK experiences minor earthquake activity, it is estimated that the UK experiences between 20 and 30 earthquakes each year
Seismic activity in relation to plate boundaries
-At constructive plate boundaries, shallow focus earthquakes occur as a result of tensional forces in the crust. Often these earthquakes occur at mid- ocean ridges and therefore pose little hazard to people
-Shallow focus earthquakes also occur at a collisional plate boundaries where continental crust is pushing against continental crust to form fold mountain belts, and at conservative margins where two sections of the earths crust move laterally
-Earthquakes that occur at destructive plate boundaries tend to have deeper foci. They occur in a narrow zone known as the Benioff zone. Here, there are compressional forces as the oceanic crust subducts underneath the continental crust.
What is the Benioff Zone?
The zone where earthquakes tend to occur as the oceanic crust is being subducted under the continental crust at a destructive plate boundary.
Primary hazards of earthquakes/ Shockwaves
- Shockwaves:- When two plates move side by side, friction builds up and pressure increases; this pressure is stored as potential energy, it cannot move so it just builds up. When
the pressure becomes too much, the plates eventually move. All of the
energy that has been built up must go somewhere, so it is transferred into kinetic energy,
which is released and vibrates throughout the ground. The further away from the focus, the
weaker the shockwaves, as the energy is transferred into the surroundings.
What are body waves?
Body Waves: they travel through the earth. They can be further divided into Primary (P) waves and secondary (S) waves. Primary waves travel through both solids and liquids. These are the faster of the two and reach the surface first. Secondary waves also travel through the Earth however, they cannot travel through liquids.
What are surface waves?
Travel along the Earths surface. Surface waves can also be further divided into Love and Rayleigh waves. Love waves shake the ground at right angles to the direction of movement and cause the most damage. Rayleigh waves radiate from epicentre in complicated. low frequency rolling motions.
How does geology of an area effect how surface waves travel?
The underlying geology of the area can affect the way in which surface waves travel. For example, unconsolidated sediment can cause the seismic waves to be amplified. During an earthquake event, it is the surface waves that are the greatest hazard as they tend to cause the most damage to buildings, which may collapse and injure or kill their occupants.
Secondary hazards- Liquefaction
Liquefaction occurs in unconsolidated sediments that are saturated with water. The sediments act as a liquid when the ground begins to shake, which can cause the subsidence of building foundations or the destruction of utility pipes.
Secondary hazards- Landslides
Landslides and avalanches can occur on even fairly gentle slopes as a result of the ground shaking. Falling rocks can destroy buildings and injure people and can also block roads, hindering the efforts of the emergency services.
Secondary hazards- Tsunamis
Tsunamis occur when oceanic crust is jolted in an earthquake, making all the water above it be displaced. The water travels fast but at a low height. As it gets closer to the coast, sea level decreases, so there is friction between the sea bed and the waves. This causes the waves to slow down and gain height. This creates a wall of water which can reach 10 metres high and cause widespread coastal flooding and destruction
How do we measure magnitude of seismic events?
Richter scale
Seismicity is measured using the logarithmic Richter Scale, a measure of the strength of seismic waves. First used in 1934, it was a formula based upon the amplitude of the largest recorded seismic wave and the distance between the earthquake focus and the seismometer.
However, while the Richter scale is useful for measuring small-scale earthquakes , it is less accurate when measuring large events.
Doesn’t highlight the impact of the Earthquake.
Magnitude/ modified Mercalli Intensity Scale
This uses observations of people to determine the amount of damage caused by an event. The 12- point scale describes the impacts of the event and ranges from 1 where the earthquake is not felt to 12, where damage is total and objects are thrown into the air.
Mercalli scale is subjective, meaning sometimes it is disputed as it is dependent on human development being present rather than the strength of the seismic waves.
Magnitude/ The Moment Magnitude Scale
This is used globally today. It measures the total moment release of the earthquake, where moment is a product of the distance the fault has moved and the force required to move it. For most earthquakes, the MMS is roughly equivalent to the Richter scale, although for events greater than magnitude 8, it is significantly more accurate.
What does the magnitude of an earthquake depend on?
The magnitude of the earthquake is also dependent on the depth of focus. Conservative boundaries have the shallowest boundaries, meaning they are closer to the epicentre and the seismic waves are stronger. Destructive boundaries usually have deeper focuses, meaning the seismic waves are spread over a larger area before they reach the epicentre. This is dependent on the
earthquake.
Frequency of Seismic events
Earthquakes are frequent around the world and occur every day at boundaries. Hundreds of smaller magnitude earthquakes that cannot be felt by humans occur every day, whereas the larger earthquakes are less frequent.
Regularity of seismic events
Earthquakes follow no pattern and are random
events. So there is irregularity between events.
Predictability of seismic events
Earthquakes are almost impossible to predict. Microquakes may give some indication but the magnitude cannot be predicted as how strong they are is random.
Primary impacts SOCIAL
-Buildings collapse/ There is a saying. “earthquakes don’t kill people, buildings do”. Around 3/4 of all deaths during earthquakes are caused by collapsing buildings. In many cases, rapid urbanisation has increased the earthquake risk. Low cost, poorly built housing is a particular hazard and so people living in slums are disproportionately vulnerable.
-Earthquakes also destroy infrastructure, including roads, which then may hinder the rescue effort. Water pipes may also burst which may contaminate or disrupt the water supply, which can cause disease to spread. The problem can be exacerbated by the increase in refugee camps, following the collapse of buildings and the need for people to be rehoused.
Primary impacts- ECONOMIC
-Businesses destroyed
-loss of jobs resulting from damage to communication and infrastructure
-Damage to water, electricity, gas, and communication systems disrupts services and adds to repair costs.
-Businesses may shut down during and after the event, especially in heavily affected areas.
-Immediate expenses for search and rescue, medical aid, and temporary shelters.
Primary impacts - ENVIRONMENTAL
-Earthquake can cause fault lines which destroy the environment
- Liquefaction
-Landslides and Avalanches: Triggered by shaking, these can devastate forests, farmlands, and wildlife habitats.
Primary impacts- POLITICAL
-Government buildings destroyed
-Poor handling of rescue, aid distribution, or rebuilding efforts can erode political legitimacy and trust in institutions.
Secondary impacts -SOCIAL
-gas pipes rupture, starting fires which can kill
- Water supplies are contaminated as pipes burst, spreading disease and causing floods
- Tsunamis which lead to damaging flooding
Secondary impacts- ECONOMIC
-Economic decline as businesses are destroyed (tax breaks etc)
-High cost of insurance payout and rebuilding
-Sources of income lost
-Lower GDP Growth: Especially in developing countries, a significant earthquake can reduce GDP growth due to lasting damage and slow recovery.
-Decline in Investment: Perceived risk of future earthquakes can deter both domestic and foreign investment
