Earthquakes

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

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.

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.

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.

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.

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.

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:

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.

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:

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.

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.
2	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.
3	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.
5	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.
7	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.
8	XII	Catastrophic. Damage total. Lines of sight and level are distorted. Objects thrown into the air.

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.

Earthquakes & seismic activity