Small geological fault lines occur is various areas of the Cape Town, and in the mountains to the east. Cape Town lies in a low earthquake activity region, but earthquakes of varying magnitude have occurred in the past and are still likely to occur at some stage in the future.
An earthquake may occur with one or more of the following indicators:
- Strong ground motion
- Fault rupturing
- Soil liquefaction (When saturated soil loses strength and stiffness causing it to behave like a liquid.)
Earthquakes that have shaking intensities greater than 7 (VII) on the Modified Mercalli scale are likely to cause widespread damage. The extent of this damage will depend on the conditions of a particular site, e.g. liquefaction potential, ground shaking amplification, instability and fault rupture, building and infrastructure condition, response and recovery plans, and the community awareness of what to do.
By drafting your own Family Emergency Plan and identifying all your risks, including those hazards affecting your own environment, you can make a positive contribution to preparedness.
Potential effects of an earthquake
Ground shaking that is severe enough to damage buildings built to earthquake code standards (~0.26 g) is expected to occur in Cape Town, on average, once every two thousand years. The expected impacts of an earthquake of this magnitude include:
- 30-90% of liquefiable soils liquefy, causing very high probability of damage to structures founded in or on these soils. There is also significant risk of slope failure.
- Loss of key engineering lifelines such as communications, energy supplies, fuel pipelines, water supply, wastewater, sewerage and stormwater.
- Damage to road and rail bridges.
- Possible high economic losses.
- Possible high social disruption - real or perceived need for relocation of communities.
- High risk of loss of life and risk to human health.
(Reference)
The structural collapse hazard is also linked to the earthquake hazard. Any built structure may be damaged by external forces like earthquakes, flooding, winds, storms or other natural phenomena, but collapse may also be the result of inadequate planning, inadequate foundations, lower quality building materials and methods, too little supervision and testing, the lack of proper maintenance and incidents such as fires, or even due to malicious acts such as bombings.
The Modified Mercalli Intensity Scale (MM)
The Modified Mercalli Scale measures the intensity of an earthquake. The intensity of an earthquake is its destructiveness due to the amount of ground movement, at a particular place. The table below gives Modified Mercalli Scale intensities that are typically observed at locations near the epicenter of an earthquake.
|
I. Instrumental |
Generally not felt by people unless in favourable conditions. |
|
II. Weak |
Felt only by a few people at best, especially on the upper floors of buildings. Delicately suspended objects may swing. |
|
III. Slight |
Felt quite noticeably by people indoors, especially on the upper floors of buildings. Many do not recognize it as an earthquake. Stationary motor cars may rock slightly. Vibration similar to the passing of a truck. Duration estimated. |
|
IV. Moderate |
Felt indoors by many people, outdoors by few people during the day. At night, some awaken. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Stationary motor cars rock noticeably. Dishes and windows rattle alarmingly. |
|
V. Rather Strong |
Felt outside by most, may not be felt by some outside in non-favourable conditions. Dishes and windows may break and large bells will ring. Vibrations like large train passing close to house. |
|
VI. Strong |
Felt by all; many frightened and run outdoors, walk unsteadily. Windows, dishes, glassware broken; books fall off shelves; some heavy furniture moved or overturned; a few instances of fallen plaster. Damage slight. |
|
VII. Very Strong |
Difficult to stand; furniture broken; damage negligible in building of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. Noticed by people driving motor cars. |
|
VIII. Destructive |
Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture moved. |
|
IX. Violent |
General panic; damage considerable in specially designed structures, well designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations. |
|
X. Intense |
Some well built wooden structures destroyed; most masonry and frame structures destroyed with foundation. Rails bent. |
|
XI. Extreme |
Few, if any masonry structures remain standing. Bridges destroyed. Rails bent greatly. |
|
XII. Cataclysmic |
Total destruction - Everything is destroyed. Lines of sight and level distorted. Objects thrown into the air. The ground moves in waves or ripples. Large amounts of rock move position. Landscape altered, or levelled by several meters. In some cases, even the routes of rivers are changed. |
(References)
The Richter Scale
The Richter Scale, also known as the Richter Magnitude Scale, is an alternative measurement of the magnitude of earthquakes. This scale was developed by Charles F. Richter of the California Institute of Technology in 1935. It is a logarithmic scale that ranges from 0 to over 10. Each unit of increase on the Richter Scale corresponds to an increase by a factor of 10. Magnitude on the Richter Scale is expressed in the form of whole numbers and decimal fractions.
The magnitude of an earthquake is measured using the instrument known as a seismograph. The magnitude is determined by the Richter Scale from the logarithm of the amplitude of the waves, that are recorded by the seismograph. For example, a moderate earthquake shows a magnitude of 5.4 on the scale, whereas a strong one shows the magnitude of just 6.2. This is because each whole number on the Richter Scale corresponds to a tenfold increase in the measured amplitude. Based on the values expressed by the Richter Scale, earthquakes with a magnitude of about 2.0 or less are generally referred to as micro earthquakes and are not commonly felt by people. Moderate earthquakes are the ones with a magnitude of about 4.5. Several such shocks are experienced in a year. Earthquakes with magnitude of 8.0 or higher are termed as great earthquakes and generally occur once in a year, in some part of the world.
Richter Magnitude
(at the epicentre) |
Description |
Effects |
Frequency of Occurrence Worldwide |
|
0 - 2.0 |
Micro |
Not felt |
About 8 000 per day |
|
2.0 - 2.9 |
Minor |
Not felt but recorded on seismographs |
About 1 000 per day |
|
3.0 - 3.9 |
Minor |
Often felt, but causes no damage |
49 000 per year |
|
4.0 - 4.9 |
Light |
Often felt with shaking and rattling noises, but causes no significant damage |
6 200 per year |
|
5.0 - 5.9 |
Moderate |
Can cause major damage, especially to poorly constructed buildings |
800 per year |
|
6.0 - 6.9 |
Strong |
Can be destructive in areas up to about 160 km from the epicentre |
120 per year |
|
7.0 - 7.9 |
Major |
Can cause serious damage |
18 per year |
|
8.0 - 8.9 |
Great |
Causes serious damage over large areas |
1 per year |
|
9.0 - 9.9 |
Great |
Can be very destructive in areas several kilometres across |
1 per year |
|
10.0 and more |
Epic |
Has never been recorded |
Extremely Rare |
(Reference)
Comparison between the Richter and Mercalli scales
The effects of any one earthquake can vary greatly from place to place, so there may be many Mercalli intensity values measured for the same earthquake. These values can be best displayed using a contoured map. Each earthquake, on the other hand, has only one magnitude. The table below is a rough guide to the degrees of the Modified Mercalli Intensity Scale in comparison to the Richter Earthquake Magnitude Scale.
|
Richter Magnitude |
Typical Maximum
Modified Mercalli Intensity |
|
1.0 - 3.0 |
I |
|
3.0 - 3.9 |
II - III |
|
4.0 - 4.9 |
IV - V |
|
5.0 - 5.9 |
VI - VII |
|
6.0 - 6.9 |
VII - IX |
|
7.0+ |
VIII or higher |