5.4- seismic hazards Flashcards
what is an earthquake?
as the crust of the Earth is mobile, there tends to be a slow build up of stress within the rocks; where this pressure is suddenly released, parts of the surface experience an intense shaking motion that lasts for just a few seconds= this is an earthquake
what is the focus of an earthquake?
point at which pressure release occurs within the crust
what is the epicentre of an earthquake?
point immediately above focus on Earth’s surface
explain how the depth of the focus is significant in the effects on the surface
shallow focus= tend to cause greatest damage as opposed to intermediate or deep focus
what are the main causes of seismicity?
- vast majority of earthquakes occur along plate boundaries, the most powerful being associated with destructive plate margins
- at conservative margins, the boundary is marked by a fault, movement along which produces the earthquake
- perhaps most famous of these is the San Andreas fault in California
- it has been suggested that human activity could be the cause of some minor earthquakes, through building large reservoirs which puts pressure on the underlying rocks
- in recent times people have become increasingly worried about the process of fracking (hydraulic fracturing of rock in order to release gas)
E.g. minor earthquakes in Oklahoma
scientists have identified different type of seismic waves:
- primary
- secondary
- surface love
- rayleigh
what are the characteristics of primary waves?
- fastest and reach the surface first
- high frequency and pushing like balls in a line
- ‘push’ through the crust, mantle and core
what are the characteristics of secondary waves?
- half as fast as primary waves and reach the surface next
- like primary waves, they are high-frequency
- ‘shake’ through crust and mantle only
what are the characteristics of surface love waves?
- slowest wave
- cause the most damage
what are the characteristics of rayleigh waves?
radiate from the epicentre in complicated low-frequency rolling motions
the magnitude of earthquakes is measured in a variety of ways:
- Richter scale
- moment magnitude scale (MMS)
- Mercalli scale
summarise the Richter scale
- this logarithmic scale applies simple mathematical formula to interpret the distance moved by the vibrating pen on the seismograph
- the scale starts at 0 and each number is 10x the magnitude of the one before it,so a slight increase in value equates to an enormous effect on the ground
- destructive earthquakes tend to have a value in excess of 6- they rarely exceed 9 (9.5 in Chile in 1960s highest ever recorded)
some geologists have been unhappy with the fundamentals of the Richter scale for some time now and it is more usual to use the
moment magnitude scale which identifies energy release
summarise the Mercalli scale
- measures the intensity of an event and its impact
- 12-point scale where Level 1 (not felt) is approximately to 2 on the Richter scale
- goes up to Level XII (extreme)
seismic records enable earthquake frequency to be observed, but these records only date back to 1800s when
an instrument capable of recording seismic waves was first developed = before that we have to consult historical records as to the date and the effects of the identified event
what are the primary effects of seismic events?
- ground shaking
- ground rupture
the initial effect of an earthquake will be ground shaking, the severity of which will depend upon:
the magnitude of the earthquake and its depth
what is ground rupture?
the visible breaking and displacement of the earth’s surface, probably along the line of the fault- they poses a major risk for large engineered structures such as dams, bridges and nuclear power stations
what are the main secondary events of seismic events?
- soil liquefaction
- landslides/avalanches
- tsunamis
- fires
- effects on people and the built environment
what is soil liquefaction?
when violently shaken soils with a high water content lose their mechanical strength and start to behave like a fluid
name some effects on people and the built environment?
- collapsing buildings
- destruction of service provision such as water, electricity and gas
- disease
- food shortages
- disruption to the local economy, either subsistence or commerical
- some of the effects on the human environment are short term; others occur over a longer period and will depend to a large extent on the ability of the area to recover
what are tsunamis?
giant sea waves generated by shallow-focus underwater earthquakes (the most common cause), volcanic eruptions, underwater debris slides and landslides into the sea
what are the general characteristics of tsunamis?
- have a very long wavelength (sometimes over 100km)
- low wave height (under 1m) in the open ocean
- travel very quickly at speeds over 700km per hour
- but, when reaching shallow water bordering land, increase rapidly in height
- they usually consist of a series of waves, with the first not always being the biggest
quite often, the first warning of tsunamis given to coastal populations is
the wave trough in front of the tsunami which results in a reduction in sea level known as a drawdown; behind this comes the tsunami itself which can reach heights in excess of 25m
when a tsunami reaches land, its effects will depend upon:
- the height of the waves and the distance they have travelled
- the length of the event (at source)
- the extent to which warnings can be given
- coastal physical geography, both offshore and on the coastal area
- coastal land use and population density
around 90% of all tsunamis are generated within
the pacific basin, associated with the tectonic activity taking place around its edges- most are generated at convergent plate boundaries where subduction is taking place
the prediction of earthquakes is very difficult. Regions at risk can be identified through plate tectonics, but attempts to predict a few hours before the event are questionable- such attempts are based upon:
- monitoring groundwater levels
- release of radon gas
- unusual animal behaviour
- faultlines such as the San Andreas can be monitored and the local magnetic fields measured
even in areas where earthquakes are expected, issuing warnings based on such flimsy evidence could never be risked as panic and chaos result which is where
prevention, protection and prepardeness come in (managing seismic hazards- long term responses)
trying to prevent an earthquake is thought by almost all to be impossible. This however has not stopped studies into the feasibility of schemes to
keep plates sliding past each other rather than ‘sticking’ and releasing, which is the main cause of earthquakes. Suggestions so far as to lubricating this movement have focused on water and oil; some people have even gone as far as to suggest nuclear explosions at depth
regarding protection, since earthquakes strike suddenly,violently and without warning, it is essential that everyone from civil authorities to individuals are prepared. In the USA, the Federal Emergency Management Agency’s (FEMA) programme has the following objectives:
- to promote understanding of earthquakes and their effects
- to improve earthquake-resistant design
protection therefore means preparing for the event by modifying the human and built environments in order to decrease vulnerability. It also includes attempts to modify the loss by insurance and aid- what are some of these ways:
- hazard-resistant structures
- education
- fire prevention
- land-use planning
- insurance
- aid
- tsunami protection
discuss hazard-resistant structures
buildings can be designed to be earthquake resistant- there are 3 main ways in which this can be achieved:
1- by putting a large concrete weight on top of the building which will move, with the aid of a computer programme, in the opposite direction to the force of the earthquake in order to counteract stress
2- putting large rubber shock absorbers in the foundations which will allow some movement of the building
3- by adding cross-bracing to the structure to hold it together better when it shakes
- older buildings and structures, such as elevated motorways can be retrofitted with such devices to make them more earthquake proof
discuss education
- for many areas, this is the main way that loss of life can be minimised
- instructions are issued by the authorities in how to prepare for such events by securing homes and assembling ‘earthquake kits’
- children have earthquake drills at school as do people in offices and factories
- American red cross issued a list of supplies that people should keep in hand incase of an earthquake from water and foodsuffs to torch batteries and pliers
discuss fire prevention
‘smart meters’ have been developed that can cut off gas if an earthquake of sufficient magnitude occurs- in Tokyo, the gas company has a network that transmits seismic information to a computer which then informs employees where to switch off major pipelines, thus reducing number of fires
discuss land-use planning
certain types of buildings should be put in areas of identified low risk such as schools and hospitals- it is also important to have sufficient open space, as this forms a safe area away from fires and aftershock damage to buildings
discuss insurance
in richer areas, people are strongly advised to take out insurance to cover their losses- the only problem being that for individuals, this is very expensive
discuss aid
most aid to poorer countries (medical services, tents, water purification equipment etc) has generally been to help in the few days after the events but aid over the longer term is much more problematic; it is something that is needed for the reconstruction of the built environment and the redevelopment of the economy
discuss tsunami protection
- certain automated systems can be installed to give warnings, the best of which uses bottom pressure sensors, attached to buoys, which constantly measure the pressure of the overlying water column
- The Pacific Warning System is based on Hawaii- it monitors earthquake activity and issues warnings to countries around the Pacific edge if tsunamis are likely
- some countries have built prevention walls over 10m but have not proved very effective as large tsunamis are likely to overwhelm them
