Seismic hazards

Earthquake

As the crust of the earth is constantly moving there tends to be slow build up of stress withing the rocks.When this pressure is released parts of the surface experience for a short amount of period an intense shaking motion

Focus

the point at which this pressure release occurs within the crust

epicentre

the point immediately above the focus on earths surface

The depth of the focus is significant in terms of the effects on the surface.There are three broad categories of earthquakes these are..

shallow focus (0-70km deep)- causes the greatest damage and accounts for 75% of all the earthquake energy released

intermediate focus (70-300km deep)

deeo focus(300-700km deep)

the relationship between plate boundaries and earthquake distribution

vast majority of earthquakes occur along plate boundaries

most powerful being associated with destructive margins

at conservative margins the boundary is marked by transform faults and sudden differential movement along these faults produces the earthquake.

why can earthquakes occur away from plate boundaries

it has been suggested that human activity could be the cause of some minor earthquakes through building large reservoiurs which puts pressure on the underlying rocks or subsidence of deep mine workings

moment magnitude scale

measures the size of an earthquake in terms of energy released (Mw)

it is a logarithmic scale which means that an increase of one step on the scale corresponds to about 32 times the amount of energy released

calculated using a formula that includes the rigidity of the rock affected ,the distance moved and the size of the area

but doesnt tell us about what matters to humans

mercalli scale

meaningful description of hazard intensity which allows comparison of impacts but doesnt link damage to characteristics of the place which can lead to weak understanding of the event

impacts of seismic events

very few people are killed by the earthquake if they are standing in the middle of an open area.

people are killed by what the earthquakes do to buildings

primary effects

initial impact is ground shaking caused by shock waves travelling through the crust from the focus up to the surface and then radiating outwards

another primary hazard is ground rupture which is the visible breaking and displacement of the earths surface,probably along the line of a fault line. it poses a major risk for large engineered structures such as dams

secondary effects

soil liquefaction

land slides

tsunamis

fires

soil liquefaction

when violently shaken,soil with high water content lose their mechanical strength and start to behave like fluid it can sometimes lead to mudflows and submerge many buildings

land slides/avalanches

slope failure as a result of ground shaking

tsunamis

giant sea waves generated by shallow focus underwater earthquakes involving movements of the sea bed or landslides in to the sea

tsunamis have very long wavelength and low wave height

in open ocean they travel at speeds of over 700km per hour

when they reach shallow water bordering land they slow down and increase in height

Fires

resulting from broken broken gas pipes and collapsed electricity transmission systems

more on tsunamis

quite often first warning given to coastal populations is the wave trough in front of the tsunami which results in a reduction in sea level known as drawdown

behind this the tsunami comes itself

when a tsunami reaches the land the affect will depend upon the height of the waves and the distance they have travelled

the length of the event

the extent to which warnings could be given

coastal physical gegoraphy

coastal land use and population density

relationship between tsunamis and plate boundaries

around 90% of al tsunamis are generated within the pacific basin associated with tectonic activity taking place around the edges

most are generated at convergent plate boundaries where subduction is taking place

preparedness

manta of ‘ drop, cover and hold on’ should be instilled in children froma very young age and practised regularly

within the home any heavy items should be secure and breakable items stored at a low level

create a communication plan

emergency supply kits

homes should be structurally sound

earthquake insurance policy

prediction

it is difficult but regions at risk can be identified through plate tectonics.Some attempts have been made e.g monitoring groundwater levels and fault lines etc but the ability to predict an earthquake a few hours before the event is unrealistic

mitigation

early warning systems- Early warning systems give people a short time to make themselves safe

hazard resistant structures-buildings can be designed to be earthquake resistant by

putting a large concrete weight on top of the building that will move in the opposite direction to the force of the earthquake in order to counteract the stress

large rubber shock obsorbers in the foundations that will allow some movement of the building without structural failure

Tsunami protection- tsunamis cannot be predicted entirely of the magnitude and location of an earthquake is known

certain automated systems can be installed to give warnings

prevention

lubricating active faults with water and oil to stop them becoming stuck which has not been proven to work

adaptation

log term change depends on economic development,education and national and regional priorities HIC are generally more able to adapt their environment than those in LIC

land use planning measures -

• identifying areas with most risk from seismic event and regulating land use planning for those areas or limiting the way in which the land can be used

• putting key buildings such as school in low risk areas

• including open spaces in plans to allow safe areas away from fires and aftershock damage to buildings

emergency services should adapt their organisations and planning to deal with seismic events

what does most earthquake hazards have in temrs of intensity and areal extent?

high hazard intensity and few have a large areal extent

what does primary hazards tend to have in terms of areal extent ?

low areal extent

what does some secondary hazards have in terms of intensity and areal extent such as liquefaction and building collapse?

high hazard intensity but low areal extent

what does other secondary hazards such as landslides have in terms of intensity and areal extent ?

large areal extent remain very deadly low hazard intensity

what is one exception?

tsunamis have large hazard intensity and large areal extent

what is the relationship between earthquake magnitude and social cost?

there is some relationship

however this relationship is weak becasue many earthquakes don’t fit this pattern

what are physical factors influencing the impact of seismic hazards

p waves(pressure waves)

• fast

• longitudinal (causes surface objects to rise and fall)

• causes least damage

s waves (shear waves)

• slower

• latitudinal(causes surface objects to move side to side)

• causes significant damage especially to buildings

surface waves(rayleigh waves)

• move on surface

• lower velocity but higher amplitude

• greatest damage

what are human factors influencing impact of seismic hazards?

• land use planning

• reinforce buildings

• transport,communications and utilities must be made secure

Haiti

characteristics of the hazard

• 7MMS

• destructive boundary

• deeper focus 13km

characteristics of the place

• unconsolidated geology and steep terrain

• corrupt government,building codes not enforced

• dense and informal settlement

• poorest country in western hemisphere

• high poverty

• fatalistic attitude to hazards

disaster

• 220000 dead

• 188000 buildings destroyed

• 11.5 b damages

Napa

characteristics of the hazard

6MMS

conservative boundary

shallow focus 11km

characteristics of the place

• unconsolidated geology and steep terrain

• stable government ,building codes enforced including land use planning

• well planned residential zones

• excellent public services

• hazard perception is adaption

disaster

• 1 dead

• 4 buildings destroyed

• 362m damages

• fire

how is the park model and the hazard management cycle helpful in understanding the impact of seismic events

• can evaluate attempts in terms of their success

• learn from past experiences to prepare for future events

• guide for aid

• promote sustainable lifestyles in the context of hazards

why cant models on their own keep people safe?

learning needs to be implemented

preparation can be overwhelmed by scale of the hazard and human factors