Comparing Earthquakes

Why do more people die in some earthquakes than others?

One of the crucial factors in determing the severity of the effects caused by an earthquake is the magnitude of the quake. The magnitude of an earthquake is measured on the richter scale, which is logarithmic (hence each level of magnitude is 10 times greater than the one before it on the scale). However, magnitude is not the only factor to be taken into consideration; indeed in December 2003 - 2 quakes of a simillar size resulted in very different death tolls - in California 3 people died in a quake measuring 6.5 whilst around 40,000 died in Bam (Iran) in a quake measuring 6.6. So why do some earthquakes result in more fatalities than others? What are the factors which contribute to the severity of the effects of a quake?

1. Location of the Epicentre

The epicentre is the point on the surface directly above the focus (start of the earthquake). It is at this point where the energy from an earthquake is usually at its greatest. The distance from the epicentre therefore has a big impact. The epicentre of the 1989 San Francisco earthquake was around 80km from San Francisco whereas in the 2003 Iranian earthquake, the epicentre was very close to the city of Bam (accounting for the high death toll)

2. Level of development of the Country

Earthquakes which occur in the richer countries of the world often have fewer fatalities simply due to the greater state of preparedness which is facilitated by the greater amount of money available to put into earthquake research, monitoring and preparation. You should be able to compare examples of earthquakes in MEDCs and LEDCs (see posts on the San Francisco 1989 quake and Bam 2003 quake - both of which were around 6.6-6.9 on the richter scale yet with vast differences in the number of fatalities - 67 in the San Francisco quake compared to around 40,000 in the Bam quake).

Some examples of reasons for an often greater death toll in LEDCs:
- buildings are often not earthquake proof and may be built out of flimsy materials not suited to quakes (e.g. mud brick used in Bam); with pressures on population, buildings are also often built quickly and as a result are often sub-standard and not built to meet building codes;
- emergency services in LEDCs are usually not as well funded and therefore not as able to cope, due to fewer training opportunities and less money for essential equipment / supplies;
- lack of money for prediction technology and monitoring of sesimic activity
- many LEDC cities are very densely populated with houses packed close together, resulting in great danger from collapsing buildings and the rapid spread of fire;
- in some LEDCs, difficult political situations can mean response to earthquakes by government officials is not as quick as it should be.

Also have a look at this interesting article from the BBC "Can money stop an earthquake?"

3. Time of the Day / Time of Year

If an earthquake occurs at night, most people are in bed. In areas where buildings collapse easily this can result in a higher death toll, although in areas where fewer buildings are likely to collapse and where deaths are often higher due to collapsing roadways / falling debris, fewer people may die if the quake occurs at night. The time of year can also be important due to seasonal differences in temperature which can exacerbate the effects of a quake. Following the Bam 2003 quake almost 60,000 were left homeless, forced to take shelter in simply blankets and makeshift tents in freezing evening temperatures. Where conditions are much warmer, this can facilitate more rapid decay of bodies and lead to an increase in the spread of disease following a quake, particularly in areas where access to clean water is poor.

4. Population Density

An area of dense population is likely to experience more deaths than a rural area simply due to a greater liklihood of people being affected by the quake and more buildings, road networks and bridges which may collapse. A major difficulty however in earthquakes which occur in rural areas is getting rescue teams and aid to the affected areas.

5. Land that buildings are constructed on

Where buildings are constructed on soft granular sediments or areas of landfill, the effects of an earthquake maybe more severe due to the process of liquefaction. This process, which results in ground failure, occurs when ground shaking causes water to rise, filling pore spaces between granular sediments, increasing pore water pressure and causing the sediment to act as a fluid rather than a solid. This can result in the collapse of overlying buildings, roads etc., such as occured in the Marine District in San Francisco during the 1989 quake due to it being built on landfill from the 1906 quake (see this excellent article on liquefaction in earthquakes for examples)

Likewise, if earthquakes occur in areas with steep surrounding slopes, ground movements can trigger landslides causing great loss of life.

For further information there is an excellent article on "The Geography Site" called "What determines the impact of an earthquake?" - has a good level of detail and lots of examples - well worth reading! Also see this article Factors that Affect Damage in an Earthquake for information on effects and factors that can affect them.

Key Term Check:
Liquefaction - where sediments act like as a liquid rather than a solid due to ground shaking causing water to rise and increase pore water pressure.
Epicentre - the point on the surface directly above the focus
Focus - the point at which an earthquake starts
Magnitude - this refers to the strength of an earthquake and is measured on the Richter Scale