14.3 What are the Main Hazards Generated from Seismic Activity? Flashcards
(21 cards)
What are the characteristics of a shallow focus earthquake?
- Depth of up to 70km (into crust)
- Occur in cold brittle rocks resulting from factors due to stress or from magma ascension
- Very common
Commonly low energy, sometimes high energy but cause significant impacts
What are the characteristics of a deep-focus earthquake?
- 70-700km into the crust
- Poorly understood. Perhaps mineral charge types or temperature changes or pressure changes
- less frequent
- powerful, high energy earthquakes- usually
What are earthquakes?
Earthquakes are the sudden violent shaking of the ground. This happens because the Earth’s plates are constantly moving. Sometimes, because of friction, plates try to move and become stuck. pressure builds up because the plates are still trying to move. When the pressure is released, it sends out huge amounts of energy causing the Earth’s surface to shake violently.
The point inside the Earth’s crust where the earthquake originates from is known as the focus. The earthquake’s energy is released in seismic waves and they spread out from the focus. The seismic waves are most powerful at the epicentre. The epicentre is the point on the Earth’s surface directly above the focus.
earthquakes are indicators of magma moving through cracks of weakness
- sign of potential volcano activity
Why is the depth of focus key?
- Is key in determining vulnerability and education- shallow focus= increased severity, more energy
continent-continent collision= shallow focus
ocean-continent= deep focus
Why are deep focused earthquakes higher in magnitude, but lower intensity
- Waves have to travel a long distance through the crust to reach the epicentre, so their energy dissipates.
- Wave lengths are longer for deep focused earthquakes so buildings sway less, because they may sit on either the crest or trough- Short S-wave wavelength causes more damage
- By the time seismic waves reach the surface, they are spread across a greater distance, so affect a larger area, but less intensely.
REFER TO TRIANGLE DIAGRAMS
Himalayas are in a subduction zone causing high seismic activity
What is compression stress?
Causes the rocks to push or squeeze against one another
What is tension stress?
forces the rocks to pull apart and can happen in two ways:
- separate plates can move farther away from each other
- ends of plates can move in different directions
What is shear stress?
the force of the stress pushes some of the crust in different directions
What are P waves?
longitudinal
fast moving
travels through liquids and solids
vibrate in the directions they travel
What are S waves?
transverse (vibrate at right angles to direction of travel)
slower moving
travel through solids only
What is earthquake intensity?
perceived strength of an earthquake based on relative effect to people and structures; generally higher near the equator
What is earthquake magnitude?
based on instrumentally derived information and correlates strength with the amount of total energy released at the earthquakes point of origin
What is the richter scale?
The richter scale uses the amplitude of seismic waves to determine earthquake magnitude. The scale is logarithmic so each whole number increase in magnitude represents a ten fold increase in the amplitude of the sesmic wave. This represents a 30 fold increase in the release if energy. It has no upper limit but the largest earthquake recorded had a magnitude of around 9 (Japan, 2011). This scale is not used to express damage. Underestimated the size of the earthquake (magnitude)- moment magnitude scale now more commonly used.
What is the modified mercalli scale?
Measures earthquake intensity and its impact. It relates ground movement to movements that can be felt and seen by anyone in the affected location. It is a qualitative assesment based upon observation and description.
What is the moment magnitude scale?
Measures the enrgey released by an earthquake more accurately than the richter scale. The amount of energy released is related to geological properties such as the rock rigidity, area of the fualt surface and amount of movement on the fault. It is the most accurate as it uses the amount of physical movement caused by an earthquake, which is a direct function of energy. However, it is not used for small earthquakes. Earthquakes with a magnitude of 2.0 or less are micro-earthquakes, 4.5 or greater are large enrough to be recorded globally and felt. Multiple pieces of data= ‘seismic signals’. Possible because more sensitive seismometers and faster computers.
Seismic moment: M0= rock rigidity x area x slip
What is lithology?
the physical characteristics of the rock. Eg. Sedimentary or granite rock, bands of consolidated or unconsolidated rock.
is a local, context specific factor
What is ground shaking?
Direct hazard
release of seismic energy from the focus
The length of the shaking is also important. The 2011 Togoku earthquake created intense shaking for over 5 minutes. Most buildings can withstand vertical movements better than horizontal ones- swaying is dangerous to their stability. Ground movements that cause displacement of rocks along fault lines can rip apart pipelines, sewers and other infrastructure. The displacement of the surface can also disrupt natural drainage, diverting streams and rivers and affecting the movement of groundwater in aquifers.- IMPACTS ON PUBLIC WATER SUPPLIES AND IRRIGATION FOR AGRICULTURE.
What is ground displacement?
direct hazard
Subsidence (depression in the land) or raised elevation (land goes up- crust faults)- caused by the release of energy.
Depends on:
local geology
earthquake magnitude
distance from epicentre
Locations that are close to the epicentre and have surface layers that are unconsolidated with a high water content will experience this.
Most buildings can withstand vertical movements better than horizontal ones
natural drainage, sewage systems, infrastructure etc.
What is liquefaction?
Indirect hazard
When an earthquake strikes an area with fine-grained sands and alluvium with a high water content, the vibrations can cause these materials to behave like liquids. Materials lose their strength and structures tilt and sink as foundations give way.
Loose unconsolidated sediment particles are able to move around more creating spaces which allow liquid to pass through. Heavy material sinks and lighter material rises- for example, a water pipe may rise up to the above ground as it is lighter than the surrounding earth. Usually triggered by large earthquakes and occur close to rivers where there are wet soils underneath. Eg, 2011 Christchurch earthquake created huge problems of liquefaction. Block of flats can also be vulnerable to this.
What are landslides and avalanches?
Indirect hazard
Ground shaking or ground displacement- rocks are destabilised and fall down the hill. Weathering and erosion weakens the rock faces and allow them to fall. Valleys are particularly vulnerable- Nepalese earthquake- killed many people in villages at bottom of valley.
Areas with reservoirs upland can experience bad landslides if these are damaged
Slope failure is caused by both ground shaking and liquefaction. Steep slopes in the Himilayas are well known for being unstable and prone to landslides. Vulnerability is increased by deforestation and heavy monsoon rains
Both a result of ground displacement and shaking
What about tsunamis?
seabed uplift- displaces water above it producing powerful waves at the surface which spread out at high velocity from the epicentre
underwater landslides- caused by earthquakes, when a large volume of rock is shaken and slides downslope= generates a tsunami wave
