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Modified Mercalli Intensity Scale Essay

1203 words - 5 pages

1. Ground shaking intensity is defined as the severity of ground motion, i.e. displacement, during an earthquake and is assessed using the Modified Mercalli Intensity Scale (MMI). This scale is a good indicator on the effects and earthquake has on the environment and its population. It is based on three features:

 People’s perception
 Building performance
 Changes in the natural environment

The MMI is often associated to Peak Ground Acceleration (PGA), which is used to quantify the severity of ground shaking as shown in Table 1.

The intensity of an earthquake is a qualitative measure of the shaking at a location and it usually depends on:

 The Magnitude of an earthquake
 Distance from the fault
 Local Geology
These will all affect the way in which seismic waves will propagate through the ground and therefore will be responsible for all the subsequent event which will occur as a result of the earthquake.
Subsidence is a result of earthquake shaking. As the ground shakes, unconsolidated sediments lose their bearing strengths and the ground “shifts” downwards relative to sea level and this resulted in liquefaction and landslides and flooding. Subsidence can also occur as a result of downward displacement on one side of a fault which may sometimes affect large areas of land.
The 4th of September 2010 Darfield event generated a magnitude 7.1 earthquake with a MMI of 9. This earthquake resulted in extensive liquefaction and differential subsidence which was located close to major streams and rivers around Christchurch.
The 22nd of February 2011 Christchurch City event created a magnitude 6.3 earthquake with a MMI of 9. This earthquake occurred within 10 kilometres of the city at a shallow depth of 5 kilometres. This caused further damage to the city of Christchurch and resulted in further subsidence and liquefaction around the city of Christchurch.
Further information for the Canterbury Earthquake Sequence can be observed in Table 1 located in the appendix.
To conclude, different types of hazards will depend on the strength of seismic waves and the intensity of ground motion. Factors such as topography, building types, local geology and groundwater will play an essential role in defining what types of hazards may occur within a given area.

2. The four ways in which earthquake shaking has increased flood hazards is through

 Subsidence
 Liquefaction
 Soil disruption
 Sea level rise + tides
The September earthquake occurred as a result of hidden faults, located under the Canterbury Plains Pleistocene glacial outwash, which ruptured the Greendale fault creating a fault trace and the 7.1 magnitude earthquake (Figure 1). The aftershocks following this event, including the February 2011 6.2 earthquake, caused extensive damage to Christchurch City and its environment. Damage to properties was caused by the action of liquefaction, subsidence and lateral spreading which was caused by ground shaking intensity (both vertical and...

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