What is an Earthquake?
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.
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What is the Earthquake Epicentre?
The epicentre of an earthquake is the place on the Earth’s surface directly above the focus or (hypocentre), 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 epicentres run through the Mediterranean Sea, Asia Minor, and the Himalayan Mountains and into the eastern Indian Ocean. A second large belt runs northwards though the western Pacific Ocean, the Japanese Islands, the Aleutian islands and the west coasts of North and South America. The longest belt of earthquake epicentres runs through the central regions of most ocean basins.
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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 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.
- Crust and Lithosphere:
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.
- Lower Mantle and Core:
Directly below the lithosphere is the asthenosphere, a region of the mantle with a plastic, 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 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.
Earthquakes can accompany each of the three types of movement.
- Spreading: when plates are spreading or separating from each other, we call the movement divergent.
- Colliding: when plates are colliding, or pushing each other, we call the movement convergent.
- Sliding: when plates are sliding past each other we call this movement lateral plate movement.
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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:
- Vertical fault movement changes the elevation or height of a rock mass on one side of the fault in relation the rock mass on the opposite side. Rock masses on one side of the fault can also shift horizontally in relation to the opposite side. Vertical fault movement may result in cliffs along the fault line.
- Horizontal or sideways fault movement may cause roads and riverbanks to change their position.
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.
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How are Earthquakes Measured?
Two scales are commonly used to measure earthquakes: the Mercalli Scale and the Richter Scale.
A Measure of Intensity - The Mercalli Scale
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 Macroseimic 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.
A Measure of Magnitude - The Richter Scale
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. Moment magnitude is the truest indication of the size of an earthquake because it is based on the amount of movement on the fault.
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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.
- Land Shifts and Scarps:
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.
- Water Changes:
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.
- Soil Liquefaction:
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 soo.nah.me) is a Japanese word that means "wave in the harbour".
Tsunamis are caused by earthquakes undersea or near the coastline strong enough to rock the sea floor 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 damage caused by hurricanes and other kinds of storm waves.
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