Prediction of Seismic Hazards (AQA A Level Geography)

Measuring Earthquakes

• Magnitude is the most common way to measure an earthquake

• It measures the energy released at the source (focus)

  • The magnitude can be measured on different scales but the one which is reported on the news when an earthquake occurs is the Moment Magnitude Scale (MMS)

  • It is measured using seismographs

• The MMS goes from 1 which is not felt by humans to over 10

• The MMS is a logarithmic scale which means that a 6 on the scale is a ten times increase in amplitude from a 5 

  • The energy release is 32 times greater

Earthquake intensity

• The intensity of an earthquake is its impact on people, as well as on the built and natural environments

• The Modified Mercalli Intensity Scale is used to measure the intensity

• The scale goes from I to XII

Frequency & Regularity

• Low magnitude earthquakes occur much more frequently than high magnitude earthquakes

  • There are around 20,000 earthquakes around the world every year – that equates to around 55 per day

  • Most of these are low magnitude and may not even be felt by humans

• According to records from 1900, we expect around 16 major earthquakes (magnitude 7.0 or greater) every year

  • Some years there are more major earthquakes than the long-term average e.g. in 2010, there were 23 major earthquakes

  • Other years there are less e.g. in 1989, there were only 6

• Seismic gap theory suggests that if there has been little or no earthquake activity at a plate margin for a long period of time, it is more likely to experience a large earthquake in future

  • This is because stress will have built up for a longer period of time, so more energy will be released

  • The theory has been used to identify area at higher risk of large earthquakes

  • However, it is not precise and not all seismic gaps result in large earthquakes

Predicting Earthquakes

• It is not possible to predict earthquakes

  • An understanding of tectonic activity can help scientists identify areas most at risk

  • 95% of earthquakes occur on or near plate boundaries

• Scientists are constantly researching to improve forecasting - research has focussed on monitoring:

  • Pre-cursor earthquakes or foreshocks - these are measured using seismometers – they may indicate that a larger earthquake will follow

  • Deformation - this is measured using strainmeters and tiltmeters - it might indicate that pressure is building up

  • Groundwater levels - these are measured using sensors – water levels in wells can rise or fall suddenly indicating that an earthquake may be imminent

  • Radon emissions - these are measured using radon detectors - concentrations of radon gas in the atmosphere can increase prior to an earthquake

  • Animal behaviour - unusual behaviour can indicate that an earthquake might be about to happen

• No method of prediction has yet proved to be reliable

• The United States Geological Survey have made forecasts about the chances of an earthquake in the Los Angeles area:

  • Within the next 30 years there is a 60% probability of a 6.7 magnitude earthquake