Comparing Hazards

Geography: Edexcel A Level, topic 1.2.3

Measuring the Intensity of Tectonic Hazards

The magnitude and intensity of tectonic hazards are measured using a variety of different scales.

The amount of energy released is measured by the Moment Magnitude Scale (MMS), which is a version of the old Richter scale.
The MMS goes from 1 to 10, with a higher number meaning greater magnitude.
The scale is also logarithmic. This means that an increase of 1 unit of magnitude increases the amount of shaking by 10, but the amount of energy released by 30.

The damaging effects of earthquakes are measured by the Mercalli scale.
The Mercalli scale measures the intensity of shaking on a scale of 1 to 12 (usually given as Roman numerals).

Volcano magnitude is measured by the Volcanic Explosivity Index (VEI).
The VEI is based on the volume, duration and column height of eruptions.
The explosivity is related to the type of magma and the type of plate boundary that the volcano is located on.

Effusive eruptions of basaltic lavas with low VEI (0 to 3) normally happen on constructive boundaries.
Explosive eruptions with high VEI (4 to 7) happen on destructive boundaries.

Hazard profiles can be used to compare the characteristics of tectonic hazards.

Hazard profiles summarise the physical processes shared by all hazards so that decision-makers can determine which areas are most at risk.
Hazard profiles show how different types of hazards vary in:
Magnitude.
Speed of onset.
Areal extent (the area that will be affected).
Duration.
Frequency.
Spatial predictability.

Kashmir, in Northern India, was hit by a severe earthquake in 2005, killing at least 79,000 people.

A high magnitude (MMS 7.6) earthquake hit Kashmir in 2005, with a ground-shaking intensity of VII (severe).
The speed of onset was very rapid, so there was a limited chance of evacuating to a safe area.
Damage was centred on Muzaffarabad but spread over a large area of more than 1000 km²
Ground shaking lasted 30–45 seconds (a short duration), but landslides triggered by the earthquake continued for some time, as did aftershocks up to magnitude MMS 6.4.

Earthquakes don't hit Kashmir often. The previous major earthquake in Kashmir was in 1905, so there was no ‘collective memory’ of the risks and impacts of earthquakes in the region.
Spatial predictability was low. Kashmir is in a ‘seismic gap’, i.e. an area of known risk that had not experienced an earthquake for some time.