The size of the earthquake recording on seismogrammes is related to the size of the event at its source. Scientists, therefore, use various characteristics of the signature of the recording in mathematical equations to calculate the earthquake size or magnitude. As a result, there are different types of magnitude depending on the characteristics used in its determination e.g. body wave, surface wave and duration. The values so obtained may differ, but those using the same type of waves usually agree within a few decimal units.
Magnitude is a value related to the energy generated by an earthquake. It is a fixed number that does not vary regardless of which island you are located. For example, the duration magnitude of the Martinique Earthquake (2007/11/29) which was widely felt throughout the Eastern Caribbean was 7.3.
Intensity scales categorise the severity of an earthquake at a given location by describing the effects on people, structures and geological formations (e.g. whether or not buildings were damaged, whether items fell from shelves etc.) Each degree of intensity is described by a Roman numeral, (I, II, III etc.) the largest being XII, and the effects of the earthquake roughly double in severity for each one-division increase in intensity. The intensity of an earthquake varies depending on where you are in relation to the earthquake’s epicenter. For example, the intensity of the Martinique Earthquake (2007/11/29) was roughly V in Trinidad and VI and higher in Martinique.
A Richter Scale is not a physical scale. Charles F. Richter was the first person to devise a mathematical formula to calculate the sizes of the earthquakes he was studying. The equation he used may be found in any seismology text.
Each year, over 1200 earthquakes are recorded in the Eastern Caribbean. Not all of these events are felt but they serve as a reminder that the region is seismically active.
The answer to this question depends on what is meant by “susceptible”. Every island in the Eastern Caribbean is within 200 km of the largest events to have occurred in the past and as such experienced damage during those earthquakes. In this context all of the islands in the Eastern Caribbean are susceptible to earthquakes. If “susceptible” means feels earthquakes most often, then those areas are near Trinidad and near Martinique.
The largest recorded earthquake occurred on 22nd May, 1960 off the coast of Chile, South America. The earthquake was of magnitude 9.5, resulted in over half billion dollars in damage and generated tsunamis which reached the shores of Hawaii, Phillipines and Japan.
The largest recorded earthquake to have occurred in the Caribbean is believed to have been the El Cibao earthquake in the Dominican Republic in 1946 with aftershocks extending into 1947-48. The earthquake was of magnitude 8.1 and generated a tsunami which caused 75 deaths and rendered 20,000 homeless.
The largest earthquake to have occurred in the Eastern Caribbean (St. Kitts-Nevis to Trinidad & Tobago region) since continuous instrumental monitoring began in the region was the earthquake near Antigua on 8th October, 1974. The earthquake was of magnitude 7.5.
On 8th Feb 1843, the biggest earthquake known to have affected the Eastern Caribbean occurred. Damaging intensities were experienced from St. Maarten to Dominica. In Antigua, the English Harbour sank and in Point-a-Pitre, Guadeloupe, all masonry was destroyed in the earthquake, with an associated fire consuming wooden structures. One third of the population, estimated at 4,000-6,000 persons, perished. The event was felt as far south as Caracas and British Guiana and was even felt 2,000 km away in Washington, Vermont and Charlestown, U.S.A. This earthquake was not instrumentally recorded. The magnitude is estimated to have been in the range 8.0-8.5.
The level of shaking from an earthquake diminishes with distance. Mathematical relationships between the area experiencing a specified level of shaking and events with instrumental magnitudes have been derived. These mathematical equations can then be used to estimate the magnitude of pre-instrumental earthquakes, for which the area with the specified level of shaking is known.
