Seismic Hazards Flashcards
What is an earthquake?
- a sudden violent shaking of the ground as a result of the movements within the earth’s crust
- As the crust of the earth is constantly moving, there tends to be a slow build up of stress, within the rocks. When the pressure is released, parts of the surface experience for a short period, an intense shaking motion.
What is the focus?
The point at which the pressure release occurs within the crust
What is the epicentre
The point immediately above the four on the earth’s centre
Explain the importance of the earthquake’s focus and epicentre?
the depth of the focus is significant in terms of the effects on the surface and three broad categories of earthquakes are recognised; shallow focus (0 - 70 km deep), intermediate focus (70-300 km deep) and deep focus (300 - 700 km deep)
What are shockwaves?
An area of very high pressure moving through the air, water or earth
What is the Moment Magnitude Scale (MMS)?
it measures the size of the earthquake in terms on energy released
What is the Mercalli Scale?
it measures the intensity of an event and its impact
What are the strengths and weaknesses of the two contrasting approaches for measuring earthquake magnitude?
MMS
Good at showing quantitative measure - can compare data with clear comparisons
It doesn’t tell you about the hazards to humans
Mercalli Scale
Meaningful description of hazard intensity - can compare impacts
It doesn’t link damage to characteristics of place which can lead to a weak understanding of the event
Explain the relationship between plate boundaries and earthquake distribution?
The vast majority of earthquakes occur along plate boundaries, the most powerful being associated with destructive margins. At conservative margins, the boundary is marked by transform faults and sudden differential movements along these faults produce earthquakes
Explain why earthquakes occur away from plate boundaries?
it has been suggested that human activity could cause minor earthquakes, through building large reservoirs which puts pressure on the underlying rocks, or subsidence of deep mine workings
Evaluate the relationship between the nature of seismicity and plate tectonic theory? (9marks) - show plan
constructive
High frequency and low magnitude
Eg. Iceland on the mid Atlantic ridge
The plates move reasonably slowly so large stresses can not build up
destructive
Low frequency and very high magnitude
Subduction and young fold mountains - eg. The 2011 earthquake in Japan had a magnitude if 9.1 undersea megathrust earthquake
conservative
Middle of distribution
Plates move at different speeds
Usually has a high frequency and low magnitude
Rare: plates get stuck, build pressure - therefore high magnitude
Eg. 2014 south napa had a MMS of 6 and a maximum Mercalli intensity of 8
conclusion
Very strong relationship only rare in trapeze the earthquakes are an exception
Explain why the primary effects (hazards) of an earthquake?
The initial or primary hazard (impact), of an earthquake is ground shaking caused by shock waves travelling through the crust from the focus up to the crust. Another primary hazard is ground rupture; the visible breaking and displacement of the earth’s surface.
Explain the secondary effects (hazards) of an earthquake?
soil liquefaction - soils with a high water content lose their mechanical strength and start to behave like a fluid.
Landslides - slope failure as a result of ground shaking
Tsunamis - giant sea waves generated by shallow focus underwater earthquakes + movements in the sea belt
Fires - resulting from broken gas pipes and collapsed electricity transmission systems
soil liquefaction - soils with a high water content lose their mechanical strength and start to behave like a fluid.
Landslides - slope failure as a result of ground shaking
Tsunamis - giant sea waves generated by shallow focus underwater earthquakes + movements in the sea belt
Fires - resulting from broken gas pipes and collapsed electricity transmission systems
the height of the waves and the distance they have travelled
The length of the event (at source)
The extent to which warnings could be given
Coastal physical geography, both offshore and on the coastal area
Coastal land and population density
What is Ground Rupture
it is the visible breaking and displacement of the Earth’s surface, probably along the line of a fault. It poses a major risk for large engineered structures such as dams, bridges and nuclear power stations
What is liquefaction?
Is the process which causes soil to behave more like a liquid than a solid during an earthquake
What kind of hazard intensity and areal extent do earthquakes have?
high hazard intensity
Small area extent
What kind of hazard intensity and areal extent do primary hazards have?
a very high hazard intensity
Very small area extent
What kind of hazard intensity and areal extent do secondary hazards have from an earthquake?
Some hazards such as building collapse and soil liquefaction have a high hazard intensity but a small areal extent
Other, secondary hazards such as and slides have a large areal extent and high hazard intensity?
Tsunamis are the exception with both a larger areal extent and a high hazard intensity
What is a seismic gap?
an area that has not had any real seismic activity for the past 20 years
What is retrofitting?
adding extra components to something previously constructed or manufactured
What is aseismic design?
Designed to protect buildings to some or greater extent from earthquakes
What are the physical factors affecting the impact of an earthquake?
Pressure waves
Fast + longitudinal (causes surface objects to rise and fall)
Causes the least damage
shear waves
Slower + latitudinal (causes surface objects to move side-to-side)
Causes significant damage (especially to buildings
surface waves
Move on surface
Lower velocity but higher amplitude
Greatest damage, especially to buildings
What are the human factors that influence impact?
Earthquakes don’t kill, buildings do
Reinforce buildings / improve building codes
transport, communications and utilities must be made secure, otherwise response to hazard will be hindered/ secondary hazards (eg. Fire) will be worse
land use planning
Eg. Don’t build schools on a geology vulnerable to liquefaction
Using examples, explain the management response (PREDICTION) for earthquake hazards?
The prediction of earthquakes is difficult. Regions at risk can be identified through plate tectonics. Some attempts have been made to predict exactly when and where earthquakes might strike based upon monitoring groundwater levels, release of radon gas and unassailable animal behaviour. Fault lines are monitored for signs of movement.
Using examples, explain the management response (PREVENTION) for earthquake hazards?
Trying to prevent an earthquake is thought to be impossible. However it has not stopped studies into the feasibility of schemes to lubricate active faults with water or oil to stop them being stuck. Some people have gone as far as to suggest nuclear explosions at depth
Using examples, explain the management response (ADAPTION) for earthquake hazards?
Long term change in people’s behaviour depends on the levels of economic development, education and national/regional priorities
Land use planning measures include; identifying areas that are most at risk from an seismic event and regulate the land use, putting key buildings, such as schools and hospitals in low risk areas, including open spaces in plans to allow for safe areas
Using examples, explain the management response (PROTECTION, MITIGATION, PREPARATION) for earthquake hazards?
earthquakes can occur with no warnings whatsoever so actions can be taken in advance such as:
Within homes heavy items should be secured and breakable items should be stored at low levels
If financially possible, people should take out an earthquake insurance policy
Homes should be made structurally sound. This applies to most counties were building codes are relatively lax or non-existent
Households should have an emergency supply kit to last for a few days
Children at a young age should be taught to, ‘stop, drop and cover’
Families should create a communication plan and meeting places
Through mitigation techniques, early warning systems can be put in place providing people with a short period of time to find shelter
Through hazard resistant structures buildings can be designed to be earthquake resistant
Putting large concrete weights on top of buildings that move in the opposite direction of the force to counteract stress
Placing rubber shock absorbers on the foundation
Adding cross-bracing to buildings to hold them together better when they shake
Tsunami protection: includes automated systems to give warnings by using sea-bed pressure sensors which constantly measure the pressure of overlying water. Which give people warnings who live in high risk areas
