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WHAT ARE EARTHQUAKES?

An earthquake is sudden motion or trembling of the ground produced by the abrupt displacement of rock masses.

 

HOW DO THEY OCCUR?

The earth's outer shell is divided into seven major and some smaller plates which are constantly in a dynamic state, pushing against, pulling away from, or grinding past one another. Forces build up as the plates attempt to move in relation to each other. When the adhesions along the fault give way, stored energy is released in the form of earth tremors, volcanic activity etc.


Types of plate movements and principal effects:

  • Oceanic plates pulling away from each other leads to hot volcanic material being expelled from cracks to form mid-ocean ridges.

  • Oceanic plates colliding with and forced under continental plates leads to mountain ranges being pushed up, accompanied by earthquakes and volcanic eruptions.

  • Collisions of continental plates force up mountain ranges; release compression energy in quakes.

 

FAULTING

Many earthquakes occur as a result of movement along pre-existing faults or fractures occurring in the earth's crust (e.g. along plate margins). Forces of tension and compression created in the earth's crust largely due to tectonic movements of the plate produce sudden movements along the line of fracture (fault lines) resulting in two blocks of crust slipping with respect to each other. This sudden movement produces seismic waves which travel from the source through the crust to the earth's surface. When they arrive, the vibratory motion felt is called an earthquake.


The Caribbean Plate interacts with the North-American Plate at its eastern boundary, where the Atlantic sea floor attached to the North American and South American Plate is being forced or subducted beneath the Caribbean Plate. The subducted material underwent partial melting which because of its then less buoyant nature was, sometimes forced to the surface in the form of volcanic activity to produce the chain of Caribbean islands.


This process of subduction is still occurring and is largely responsible for the seismicity felt today in these islands. In addition, the sometimes slow, upward, less violent intrusion of molten magma into the crust of these active volcanic areas produces seismic activity which is felt on the surface as earthquakes.

 

LIKELY IMPACT

Primary effect of earthquakes:

  • Total or partial destruction of structures.

  • Blockage or breakage of transport activities.

  • Interruption of Water Supply.

  • Breakage of Sewage Disposal Systems.

  • Loss of Public Utilities, eg. electricity & gas

Casualties may be due to:

 

Direct Hazards

  • Total or partial collapse of structures due to poor design and construction materials.

  • Falling debris and dust from rubble.

  • Transportation casualties due to collapse of bridges etc.

  • Floods from collapsed dams or river banks.

  • Release of hazardous materials.

  • Landslides.

  • Tsunamies, loss of life by inundating low-lying coastal areas.

Indirect Hazards

  • Fires.

  • Release of hazardous material.

  • Electrocution.

  • Exacerbation of chronic diseases

The Eastern Caribbean Islands are particularly vulnerable to impact from submarine eruptions from the "Kick-em-Jenny" volcano located five miles north of Grenada, about 450 feet under water. Also historical records have shown that a tsunami was generated by the earthquake of 1867 which affected St. Thomas and Tortola in the Virgin Islands.


HOW EARTHQUAKES ARE MEASURED


Earthquakes generate different types of waves. By measuring the strength of these waves, seismographs record the characteristics of these waves and from the measurements, scientists can determine their location.


The strength of an earthquake can be measured by magnitude and intensity.


Magnitude is a measure of the physical energy released or the vibrational energy of the shock. It is commonly measured on the Richter Scale which is an open-ended logarithmic scale. The Richter Scale measures the magnitude and the Modified, Mercalli Scale measures the intensity. Below both scales are shown followed by a comparison of the two.

 

Modified Mercalli Scale


Intensity is the effect of an earthquake on the earth's surface. The intensity scale consists of certain key responses such as people awakening, movement of furniture, structural damage and finally, total destruction. The most commonly used measure in the Caribbean and the USA is the Modified Mercalli Intensity Scale (MMI).


The version of the modified Mercalli Intensity Scale that is currently used is a 1956 modification by Richter of the 1931 version by the American seismologists Harry Wood and Frank Neumann. This scale, composed of 12 increasing levels of intensity which range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. It does not have a mathematical basis; instead, it is an arbitrary ranking based on observed effects.


The following is an abbreviated description of the 12 levels of intensity and likely impact:

  1. Instrumental. Not felt except by a very few under especially favourable conditions detected mostly by Seismography.

  2. Feeble. Felt only by a few persons at rest, especially on upper floors of buildings.

  3. Slight. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck.

  4. Moderate. Felt indoors by many, outdoors by few during the day. At night, some awakening. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like a heavy truck striking building. Standing motor cars rock noticeably.

  5. Rather Strong. Felt by nearly everyone; many awakened. Some dishes, windows broken. Un-stable objects overturned. Pendulum clocks may stop.

  6. Strong. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.

  7. Very Strong. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in ordinary structures; considerable damage in poorly built or badly designed structures.

  8. Destructive. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of factory stacks, columns, monuments, walls. Heavy furniture overturned.

  9. Ruinous. 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.

  10. Disastrous. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bend greatly.

  11. Very Disastrous. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bend greatly.

  12. Catastrophic. Damage total. Lines of sight and level are distorted. Objects thrown into the air.

Above Intensity Level 8, impact is high on the Economy and Health with many deaths and injuries. Food remains available for the first few weeks.

 

 

Richter Scale

 

Seismologists use a Magnitude scale to express the seismic energy released by each earthquake. Here are the typical effects of earthquakes in various magnitude ranges:

Earthquake Severity

Richter magnitudes Earthquake effects
Less than 3.5 Generally not felt, but recorded
3.5-5.4 Often felt, but rarely causes damage
Under 6.0 At most slight damage to well-designed buildings. Can cause major damage to poorly constructed buildings over small regions.
6.1-6.9 Can be destructive in areas up to about 100 kilomete
across where people live.
7.0-7.9 Major earthquake. Can cause serious damage over larger areas.
8 or greater Great earthquake. Can cause serious damage in areas several hundred kilometers across.

Although each earthquake has a unique Magnitude, its effects will vary greatly according to distance, ground conditions, construction standards, and other factors. Seismologists use a different Modified Mercalli Scale to express the variable effects of an earthquake.

Each earthquake has a unique amount of energy, but magnitude values given by different seismological observatories for an event may vary. Depending on the size, nature, and location of an earthquake, seismologists use several different methods to estimate magnitude. The uncertainty in an estimate of the magnitude is about plus or minus 0.3 units, and seismologists often revise magnitude estimates as they obtain and analyze additional data.

Comparing the Richter and Modified Mercalli scales

Remember - Richter measures energy of the seismic wave or shock while Mercalli measures the intensity or effect on the surface of the earth.

 

Richter Modified Mercalli

Effect

2 I

Instrumental. Not felt except by a very few under especially favourable conditions detected mostly by Seismography.

II

Feeble. Felt only by a few persons at rest, especially on upper floors of buildings.

3 III

Slight. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck.

IV

Moderate. Felt indoors by many, outdoors by few during the day. At night, some awakening. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like a heavy truck striking building. Standing motor cars rock noticeably.

4 V

Rather Strong. Felt by nearly everyone; many awakened. Some dishes, windows broken. Un-stable objects overturned. Pendulum clocks may stop.

5 VI

Strong. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.

VII

Very Strong. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in ordinary structures; considerable damage in poorly built or badly designed structures.

6 VIII

Destructive. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of factory stacks, columns, monuments, walls. Heavy furniture overturned.

7 IX

Ruinous. 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

Disastrous. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bend greatly.

8 XI

Very Disastrous. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bend greatly.

XII Catastrophic. Damage total. Lines of sight and level are distorted. Objects thrown into the air.

 

 

EMERGENCY ACTION

 

Before an Earthquake

  • Potential earthquake hazards in the home and work place should be removed or corrected.
  • Top-heavy objects and furniture should be fastened to the wall and the largest and heaviest objects placed on lower shelves.
  • Water heaters and other appliances should be firmly bolted down.
  • Pay special attention to emergency equipment, such as radio transmitters and medical equipment.
  • Set aside emergency supplies and equipment.
  • Learn First Aid.
  • Teach family members how to turn off electricity, gas and water by learning location of safety valves and main switches.

During an Earthquake

  • Protect head and face.
  • Remain calm. Do not rush for exits.
  • If inside, stay there, if outside, stay there.
  • If inside a building, stand in a strong doorway, or get under a desk, table or bed.
  • Move away from windows, glass doors, heavy mirrors, pictures, bookcases, hanging plants and heavy objects.
  • Watch for falling plaster, bricks, light fixtures and other objects.
  • Avoid using elevators as power may fail.
  • If you're in an automobile, don't stop on a bridge, under or near to electrical poles or close to buildings from which debris may fall.
  • Try to remain calm and reassure others.
  • If you are in a store or shop, move away from display shelves containing bottles, cans, or other objects that may fall.

After an Earthquake


DO's

  • Check for fires.
  • Check Utilities - shut off if necessary.
  • Check your house for serious damage - evacuate if there's threat of collapse.
  • Check for injuries - administer First Aid.
  • Be prepared for additional earthquake shocks.
  • Collect water.
  • Clear up hazardous materials.
  • Assist others.
  • Turn on a transistor for emergency bulletins.
  • Stay away from landslide prone areas.
  • Stay away from buildings that might have been weakened by the earthquake.
  • Remain calm.

DON'T's

  • Do not light a match or turn on a light switch. Use a flashlight!
  • Never touch fallen power lines.
  • Do not use telephone except in extreme emergency.
  • Do not go to the beach to watch for giant sea waves.
  • Do not go sightseeing! Keep the streets clear for the passage of emergency. and rescue vehicles.
  • Do not attempt to move seriously injured persons unless they are in danger of further injury.

 

MITIGATION MEASURES

 

There are many ways to reduce earthquake damage. Possible actions include:

  • Developing construction techniques that are seismic resistant.
  • Conducting a program to introduce improved construction techniques to the building industry and the general public.
  • Determining which sites are safe for construction through analysis of the soil type and geological structure.
  • Instituting incentives to remove unsafe buildings and buildings on unsafe sites or, more probably, to upgrade their level of safety.
  • Instituting incentives to encourage future development on safer sites and safer methods of construction through:
    • Land use controls (zoning).
    • Building Codes and standards and means of enforcing them.
    • Favourable taxation, loans, or subsidies to qualify buildings, methods and sites.
    • Land development incentives.
  • Reducing possible damage from secondary effects by:
    • Identifying potential landslide sites and restricting construction in those areas.
    • Installing devices that will keep breakages in electrical lines and gas mains from producing fires.
    • Verifying the capability of dams to resist earthquake forces, and upgrading as necessary.

 

EARTHQUAKES IN THE CARIBBEAN

The Caribbean is considered to be a seismically active area. The following are some examples of damage and destruction caused by earthquakes in the past.


On 7th June, 1692, 90% of the town of Port Royal in Jamaica was destroyed by an earthquake. Over 2,000 people were killed, including several killed by a fever epidemic that followed the event. Jamaica has a long history of earthquakes. The island was again affected by an earthquake in January 1902 and, in January, 1993, an earthquake measuring 5.3 on the Richter scale caused minor damage.


On 8th Feb 1843, the biggest earthquake known to have affected the Eastern Caribbean occurred. It was felt from St Kitts to Dominica. In Antigua, the English Harbour sank and in Point-a-Pitre, Guadeloupe, all masonry was destroyed. The number of deaths included more than 5,000 in Guadeloupe, 30 in Antigua, 6 in Montserrat and 1 in Dominica.


Between April and July 1997, a series of earthquakes affected the southern Caribbean, particularly Trinidad and Tobago. Two of these events, on April 2 and April 22, caused damage to homes, public and commercial buildings and other property in Tobago. The cost of damage was estimated at approximately US $3 million and about 200 people were affected.


Additional Sources of Information


Earthquake FAQ (from US Geological Survey)

 

Vacancies

CDEMA Annual Reports