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What is an earthquake?

An earthquake is a natural phenomenon like rain. Earthquakes have occurred for billions of years. Descriptions as old as recorded history show the significant effects they have had on people’s lives.

Records of earthquakes in Jamaica date back to the 16th century where Europeans were the first to keep records of such events.

It has been pointed out that Jamaica may experience up to two hundred (200) felt earthquakes in any one year. From 1667 to 1888 at least 55 earthquakes of intensity IV on the Modified Mercalli Scale have occurred in Jamaica and since 1776 we have had nine earthquakes of intensity VII, enough to cause damage to stone buildings.

Earthquake activity in Jamaica is a direct result of various types of movements taking place within boundary zones of the Caribbean Plate.

Jamaica is situated near the northern edge of the Caribbean Plate, which is thought to be moving in an east-north easterly direction with respect to the North American Plate.

In simple terms, the constant motion of the earth’s surface causes an earthquake. The earth’s rock layer is broken into large pieces. These pieces are in slow but constant motion. They may slide by each other smoothly and almost imperceptibly.

From time to time, the pieces may lock together and energy that accumulates between the pieces may be suddenly released. The energy that is released travels through the Earth in the form of waves. People on the surface of the earth then experience an earthquake.

Earthquakes are the sudden, rapid release of energy stored in rocks.

What is the Earthquake Epicenter?

The epicenter of an earthquake is the place on the Earth’s surface directly above the focus or hypocenter, the place inside the earth where the quake originates. Earthquake foci are usually somewhere between the surface and 100 km in depth. In some areas, however, the foci may be as deep as 700 km.

Maps of earthquake epicenters show that most earthquakes have occurred in certain well-defined regions of the Earth. Because these regions tend to be relatively long and narrow, they are sometimes referred to as earthquake belts.

One large belt of epicenters run through the Mediterranean Sea, Asia Minor, and the Himalayan Mountains and into the eastern Indian Ocean. A second large belt runs northwards through the western Pacific Ocean, the Japanese Islands, the Aleutian Islands and the west coasts of North and South America. The longest belt of earthquake epicenters runs through the central regions of most ocean basins.

What are the Layers of the Earth?

To really understand how earthquakes happen we must understand the make-up of the Earth.

The simplest way of describing the Earth’s layers is to compare the globe to a hard-boiled egg. It has a crust (something like the shell), a middle layer or mantle (something like the egg white) and a core that is something like the yolk.

The crust and the upper portion of the mantle are often referred together as the lithosphere or rock sphere.

The Earth’s crust varies in thickness from about 65km on the continents to only about 10km on the ocean floors. The lithosphere is the outer solid portion of the Earth that includes the crust and the uppermost part of the mantle. The lithosphere has an average depth of 100 km.

Directly below the lithosphere is the asthenosphere, a region of the mantle with a plasmic, semisolid consistency which reaches to about 2900 km below the surface. The solid, metallic inner core goes the rest of the way to the center of the earth. Both are composed primarily of iron and nickel.

The Earth’s Plates

Scientists think that the lithosphere broke into pieces, called tectonic plates, some 3.8 billion years ago. Most earthquakes are caused by the large-scale movement of these lithospheric plates and occur at boundaries between the plates.

Experts recognize seven to twelve major plates and a number of smaller ones. The plates take their names from continents (the North American Plate) from oceans (the Pacific Plate) and from geographic areas (the Caribbean Plate).

The plates are in very slow but constant motion so that, seen from above the Earth’s surface, it might look like a slow-moving spherical jigsaw puzzle. The plates move at a rate of 2-15 cm, or several inches, in a year.

Three Kinds of Plate Movements

The movement of the plates is generally one of three kinds: spreading, colliding or sliding.

  1. Spreading: when plates are spreading or separating from each other, we call the movement divergent.
  2. Colliding: when plates are colliding, or pushing each other, we call the movement convergent.
  3. Sliding: when plates are sliding past each other we call this movement lateral plate movement.

Earthquakes can accompany each of the three types of movement.

What is a Fault?

As a result of plate motions, the built-up stress and strain within the rocks of the lithosphere may cause great warps or folds in rock layers. Where rock is strained beyond its limit, it will fracture and the rock mass on either side will move abruptly.

A fault is a fracture within the Earth’s crust along which a significant movement has occurred. 

Faults are often classified according to the direction of movement and whether that movement is predominantly horizontal or vertical. Displacement or movement of rock along a fault can occur as a result of vertical or horizontal fault movement:

Folding Rock Layers

Folding is another way that rock layers respond to stress. They may crumple sideways, without fracturing, like wrinkles in a rug. Small folds can be seen in specimens of sedimentary rock; larger examples of folded rock layers can be seen in mountainsides and road cuts.

How are Earthquakes Measured?

The Mercalli Scale, the older of the two, measures the intensity of the earthquake, i.e. the impact of a quake on people and their property.

This speaks to the observed effects of an earthquake over a limited geographical area. Intensity scales assign whole numbers usually from 1 to 12 to describe these observed levels of shaking. An intensity of 1 means the earthquake was not felt, while 12 means absolute and total destruction. 

In Jamaica, we formerly used the Modified Mercalli Scale (1956 version). Now we use the European Macroseismic Scale (1992), which has been developed and tested over a period of years by a working group of the European Seismological Commission.

The EMS makes the imprecise and subjective nature of assigning intensities more robust and straightforward with regard to earthquake effects on humans, objects and buildings.

The Richter Scale measures the magnitude or amount of energy released by the earthquake. This is a measure of size for earthquakes based on recordings of ground motions by instruments. 

Charles Richter developed the first magnitude scale in 1935. He used the logarithmic scale (which scales numbers by a factor of 10) to accommodate the wide range of ground motions, which earthquakes can cause, and this has persisted to the present.

Scales today are based on various aspects of the seismograms as told by the following names: body-wave magnitude, duration magnitude, and moment magnitude. Jamaica uses the latter two scales for magnitude. The moment magnitude is the truest indication of the size of an earthquake because it is based on the amount of movement on the fault.

What are Some of the Physical Results of Earthquakes?

As the plates of the Earth’s surface move, warping slowly, up, down and sideways in relation to each other, we may feel these movements as earthquakes. The waves of energy they release not only shake the Earth but also alter the nature of many soils, giving them an unstable liquid-like consistency. Then structures sink or tip, and hillsides topple.

Sections of ground may be elevated or may subside during an earthquake. Sometimes one side of a fault will rise or sink, creating a scarp (an earthquake caused cliff). Scarps may be lifted again and again in successive earthquakes, with the uplifts in any one earthquake ranging from a few centimeters to several metres or more.

Unstable hillsides may slump or slide during or after the shaking. Rocks may break loose and slide downhill, sometimes creating rock avalanches. The potential for landsliding is highest in soft sediments on steep slopes; where seasonal rainfall is high, vegetation is shallow, rotted or sparse; the rate of erosion is high; and where ground shaking is intense.

The underground water system may also be disturbed by an earthquake, causing fluctuations in water pressure and stream volume, and the appearance or disappearance of springs. Well, water levels or temperature can change, and the water can become cloudy and muddy. Underground oil and gas deposits could be similarly disturbed.

When an enclosed body of water such as a bay, a dam, a swimming pool, or even a pan of water is rocked, the water may begin to slosh back and forth rhythmically. During this phenomenon, known as a “seiche” (pronounced sash), the water surges from one side to the other often gaining in intensity and may overflow its basin before gradually slowing down and stopping.

Loose soils with a high water table (water in the soil close to the surface) may experience the phenomenon of liquefaction. As the earthquake’s vibrations pass through the loose soil, it becomes like quicksand. Heavy objects such as buildings and other structures situated over such areas may sink or tilt into the liquefied soil. Hillsides or earth-filled dams situated over such an area could also collapse. The effect is temporary, but the results can be very damaging.

Tsunamis (pronounced is a Japanese word that means “wave in the harbor”.

Tsunamis are caused by earthquakes undersea or near the coastline strong enough to rock the seafloor and disturb the mass of water over it. These movements generate waves that travel at speeds up to 800 km (500 miles) per hour.

In deep water, on the open ocean, Tsunamis cause no damage and are hardly noticed. When they meet shallow water, however, they can batter coastlines with waves as high as 60 meters (200 ft.).

Tsunami damage is very similar to the damage caused by hurricanes and other kinds of storm waves.

What to do Before an Earthquake - Conduct a Hazard Hunt

Especially in small earthquakes, which make up the vast majority of all earthquakes, most injuries and fatalities occur because the ground shaking dislodges loose objects in and on buildings.

Conduct an earthquake hazard hunt of your environment and eliminate objects that have the potential to cause injuries. Foresight and common sense are all that is needed as you go from room to room and imagine what would happen in an earthquake.

Some common earthquake hazards are:

These may topple over during an earthquake unless they are securely anchored to the wall. Either bolt directly through the back of the furniture into the wall or use steel angle brackets. Fallen furnishings could block your escape route, in addition to causing injury and damage. Freestanding bookshelves, especially in an office setting, should be bolted to the floor and to ceiling posts and put guard rails or ‘fences’ on open shelves so that items cannot slide off.

If you display fragile items on open shelves or tables use industrial Velcro to attach items to stands.

These tend to swing widely during earthquakes. Hang planters on hooks that can be curved over to form a circle so that the wire or cord cannot jump freely. Use lightweight, plastic containers or baskets instead of heavy, ceramic ones that may cause serious injuries if they strike someone in the head.

Ensure that mirrors are securely bolted to walls.

Ensure that shelves are securely bolted to walls. Adjustable shelves, the board of which rest on wall brackets, can be stabilized with clips or wire to connect the board to the bracket. Remember to use guardrails on shelves and do not place chairs, desks, beds, etc. beneath shelves where items can fall on people.

Make sure they are securely fastened and bolted into their spaces so they do not fall out.

Some other common earthquake hazards are:

Make sure they are securely fastened to the ceiling. Put a mesh or plastic guard around fluorescent bulbs to catch any splinters.

Attach TVs and computers to their stands with industrial Velcro or bolt the items to the stand. Wheels on carts must be able to be locked to ensure that the cart will not roll around wildly.

locate bed near an interior wall and away from windows and hanging light fixtures or any item that may fall on you while in bed. If the bed must be next to a glass window, install shatter-resistant plastic film (like the material used to tint windshields) over the glass to hold shattered glass in place and prevent it from flying around the room. Another alternative is tempered glass, which breaks into tiny rounded pieces, but this is more expensive than the film. Be sure that the bed is not on rollers, and if it is on the bare floor use plastic non-skid coasters to reduce sliding.

Hang these from fixtures that can adequately bear their weight. Items such as hanging pictures and items on shelves will weigh twice as much when they fall. For example, if it weighs 2 lbs. on the wall, it weighs 4 lbs. when it hits, so do not place over beds, desks or chairs.

Install proper latches on cupboard doors that will not open if the object tilts over or is shaken. Heavy objects inside your cupboards can lean or fall against the inside of the doors, so the latches must be strong enough to withstand this pushing. Be careful not to stand directly in front of cupboards as items lying against the doors can come crashing out on you.

The primary hazard in the bathroom during an earthquake is broken glass. Mirrors, toiletries, and medicines can fall and break. Most personal care products are now being packaged, but liquid medicines, perfumes, and colognes are sometimes supplied in glass containers. Select products in unbreakable containers where possible and make sure the doors of your medicine cabinet can be secured with a latch.

These are very vulnerable to earthquake damage. They are likely to “walk” or even topple over disconnecting the utility lines, causing gas or water leakage, or electrical shorts, fires or explosions. To prevent the water heater from moving or toppling over, wrap it with two metal straps or chains, near the top and bottom and bolt the ends to the wall.

Use flexible gas lines that will not break during an earthquake and release gas. Anchor the gas cylinder to the wall with chains and if you are cooking, turn off the stove before taking cover.

Ensure that houses/buildings are properly attached to their foundations.



What to do During an Earthquake

Earthquakes can happen at any time, anywhere. There is a chance, when an earthquake hits, that you may have a few seconds between the realization that this is an earthquake and the time when the shaking stops.

This is when your advanced planning becomes important. If you know what to expect and what to do, you can make the right decisions that may mean the difference between injury, life or death. 

Take cover where you are. If you are outside during an earthquake take cover there, do not rush indoors or vice versa.

Protect yourself from things that may fall on top of you, whether it is broken glass or a whole building. Once you take over in your safe place, stay there until the shaking stops, earthquakes seldom last longer than a minute although it seems longer.
Duck, Cover, and Hold

Practice the Duck, Cover and Hold procedure until it becomes second nature.

  • Duck: get under a sturdy piece of furniture, making yourself into a little ball (do not duck under beds or other objects that could collapse)>

  • Cover: keep your head and eyes protected from falling or flying objects. Cover your head with one hand.
  • Hold: with your other hand, hold onto the piece of furniture. If it moves, move with it. Stay under shelter until you are sure the shaking has stopped.

If you cannot shelter under furniture or a doorway, move against an interior wall if you are indoors, duck, put your arms over your head and across the back of your neck for protection. If there is a book, pillow, tray or other protection at hand, hold it over your head and neck.

It is better to break your arms than to have something fall on your head or neck, which will probably result in unconsciousness, paralysis, brain damage or death.
Door Way for Protection

If you are not near any sturdy furniture, take cover in a sturdy doorway. The extra construction around a doorframe makes

it one of the strongest parts of a building. Also, there is rarely anything over a doorway to fall on you.

Avoid doorways that have transoms or air conditioners above them.

Also, beware of the door that can swing back and forth during an earthquake. Brace yourself and try to hold off the door with your shoulder or hip and hold on tight, feet spread wide apart for balance, leaning across to hold onto the opposite side.

What to do After an Earthquake If...

  • Put on heavy-soled shoes: If you are bare footed, put on shoes before you walk anywhere after an earthquake.
  • Check for injuries: Check yourself and other family members for injuries and seek medical attention for serious conditions.
  • Do not use the telephones unless there is a serious injury: Rescue workers will need all available lines. If the receiver is off the hook, replace it. Do not jiggle the hook if you do not get a dial tone as this could further jam the wires.
  • Check for fires: If possible one person or group should check for injuries while another immediately checks for fires. Do not light matches or candles as naked flames can ignite leaking gas and spilled flammable products. If you smell gas or have reason to suspect that lines might be broken, immediately disconnect the cylinder.
  • Check stairs before using them: Stairs may have weakened after an earthquake; when evacuating check these carefully before placing your full weight on them.
  • Check the building: Carefully inspect the interior and exterior of the building. Look for cracks in the walls, shifted posts or pillars, and cracks in porches and sidewalks. If you see anything other than minor cracks, evacuate the building immediately and do not re-enter the building until a professional has checked it for safety.
  • Assist your neighbors: Once your home is secure, check with your neighbors to see if they need assistance.
  • Listen to the radio: Listen to your radio for evacuation orders and other information.
  • Stay where you are: If you are in your car, at a movie or store, or some place where you do not feel safe, you will probably try to go home. Stay where you are for a while and wait for aftershocks and information on the radio. Remember that aftershocks, particularly those following a big earthquake, can cause a lot of damage. Overpasses, bridges and some buildings might survive the main shock, but fall during an aftershock.
  • Do not drive unless: You are away from tall buildings and bridges, and then your driving should only be to safety or to render assistance. The roads should be kept open for emergency vehicles. When you arrive home, do not rush in. Look at the building from a distance for damage; if it looks okay make a closer inspection. If it still looks undamaged, open the door and smell for gas. If you don’t smell gas, enter and check for other fire hazards.
  • Recall your location: If you find yourself alone in the dark after an earthquake, take a few minutes to recall the location of exits and the layout of the building.
  • Plan your escape route: Plan your escape route and then move slowly, using your hands to guide you. If you heard the sound of breaking glass during the earthquake, wrap your hands in a jacket or other material to protect them. Carefully exit the building, take cover if there are any aftershocks and be alert for the smell of leaking gas.