Mon | Jan 18, 2021

Simon Mitchell | Why does Jamaica have so many earthquakes?

Published:Sunday | January 10, 2021 | 12:09 AM
Figure 2: Felt earthquakes in Jamaica in 2020. Note the large earthquake on the Oriente Fault Zone (magnitude 7.7, top left in the figure) that occurred on the January 28, 2020 (and an aftershock the next day).
Figure 2: Felt earthquakes in Jamaica in 2020. Note the large earthquake on the Oriente Fault Zone (magnitude 7.7, top left in the figure) that occurred on the January 28, 2020 (and an aftershock the next day).
Figure 3: Regional distribution of earthquakes recorded by the Earthquake Unit (University of the West Indies) excluding Jamaica. The Oriente Fault Zone and Cayman Spreading Centre are clearly indicated by earthquake activity, whereas the Walton and Enriqu
Figure 3: Regional distribution of earthquakes recorded by the Earthquake Unit (University of the West Indies) excluding Jamaica. The Oriente Fault Zone and Cayman Spreading Centre are clearly indicated by earthquake activity, whereas the Walton and Enriquillo fault zones show few earthquakes.
Figure 4: Local distribution of earthquakes in and around Jamaica recorded by the Earthquake Unit (University of the West Indies). Note the usual concentration of earthquakes in the east of the island (associated with the South-West Blue Mountains), and ot
Figure 4: Local distribution of earthquakes in and around Jamaica recorded by the Earthquake Unit (University of the West Indies). Note the usual concentration of earthquakes in the east of the island (associated with the South-West Blue Mountains), and other earthquakes extending through the southern parishes of Jamaica.
Figure 1: Faults surrounding the Gonâve Microplate and their relationship with Jamaica.
Figure 1: Faults surrounding the Gonâve Microplate and their relationship with Jamaica.
Holy Trinity Cathedral, Duke Street, after the January 13, 1907 great earthquake struck Kingston.
Holy Trinity Cathedral, Duke Street, after the January 13, 1907 great earthquake struck Kingston.
Prof Simon Mitchell
Prof Simon Mitchell
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Every year, many people in Jamaica feel earthquakes. Some years, there are many, some years there are few. But why do we get earthquakes in Jamaica? This is related to plate tectonics. Plates are relatively ridged parts of the Earth’s crust, and within the plates there are relatively few earthquakes. But the plates move relative to each other, and there are three possibilities: the plates move towards one another (convergent – which form, volcanoes as in the Lesser Antilles); the plates move away from one another (divergent – which forms oceanic ridges); or the plates slide sideways against each other (transform or strike-slip faults).

A large part of the Caribbean and Central America is situated on a relatively small plate that we call the Caribbean Plate. Both the North American Plate and the South American Plate are moving to the west with regard to the Caribbean Plate, and this means that we have strike-slip movements along the southern boundary (e.g., Trinidad and north of Venezuela) and the northern boundary (Jamaica, Hispaniola, and Puerto Rico) of the plate. The two American plates are moving at about two centimetres a year with regard to the Caribbean Plate, and over many years, this builds up stress in the plate margin zones.

But plate boundaries are never simple and are often made up of several faults and/or fault zones. The northern boundary of the Caribbean Plate is made up of several fault zones. In the west, the plate boundary is formed by the Swan Island Fault Zone, which splits at the middle of the Cayman Trough into two sets of faults that extend to the west. The Cayman Spreading Centre in the middle of the Cayman Trough represents a short segment of divergent plate boundary, and it is here that volcanoes at water depths of five kilometres (the deepest part of the Caribbean) produce new oceanic crust (the type of crust (basaltic) that underlies the oceans). The fault that extends west along the northern margin of the Cayman Trough includes segments called the Oriente Fault Zone and the Septentrional Fault Zone, which passes south of Cuba and through northern Hispaniola. Movement along these faults is at about 11-12mm a year.

The fault that extends west along the southern margin of the Cayman Trough has segments called the Walton Fault Zone, the Plantain Garden Fault Zone, and the Enriquillo Fault Zone. It is moving at about 8-9 millimetres a year. These two fault zones and the Cayman Spreading Centre define the Gonâve Microplate (Figure 1). The Enriquillo Fault Zone passes through the south-west Peninsula of Haiti and was responsible for the Port-au-Prince earthquake in Haiti in 2010 (an earthquake that was felt in Jamaica). The Walton Fault Zone and Plantain Garden Fault Zone cannot be traced through Jamaica, where they break up in a series of faults across the island (thus the boundary between the Gonâve Microplate and the Caribbean Plate is difficult to define in this area). Because there is no single fault, a stress field is set up across the island of Jamaica, with the north-eastern part moving at a greater rate towards the west than the south-western part.

STRESS RELEASED

As the stress builds up, it is released by earthquakes, and this explains why earthquakes can occur across many parts of the island. When an earthquake happens, the stress is released on that fault and transferred to other faults, which, in turn, will produce further earthquakes. In this way, the stress is progressively released across Jamaica. If enough stress builds up on a long fault (say 100 kilometres) and much of that fault ruptures, it produces a large earthquake. This is what caused the major earthquakes in Jamaica: the Great Port Royal Earthquake of 1692 and the Kingston Earthquake of 1907.

The year 2020 will always be known for the COVID-19 epidemic, but it was also a year in which Jamaica had 18 felt earthquakes (Figure 2), a relatively large number. This is partly due to two earthquakes on the Oriente Fault Zone being felt in western Jamaica in January.

The regional earthquakes recorded by the Earthquake Unit (Figure 3) clearly show earthquakes along the Cayman Spreading Centre and the Oriente Fault Zone. Note the earthquakes located in a line orientated East North East-West North West extending from just south of Cuba to just south of The Cayman Islands. This trend turns down towards the south, west of the Cayman Islands, indicating the divergent plate boundary (spreading centre) at the centre of the Cayman Trough. In contrast, there is relatively little activity along the Walton and Enriquillo fault zones.

TYPICAL PATTERN

The earthquakes that were recorded across Jamaica (Figure 4) show a relatively typical pattern. There is a cluster of earthquakes centred on SW Portland at the eastern end of the island. These earthquakes are associated with a series of North West-South East faults that cause a NE-SW shortening across Jamaica, which is responsible for the continued uplift of the Blue Mountains, which recent research suggests are rising at about 2mm a year. Earthquakes elsewhere in Jamaica are linked to movements on faults across the southern parishes, where active faults produce steep hills, such as at Spur Tree in western Manchester and the Santa Cruz Mountains in St Elizabeth. No earthquakes are associated with the Walton and Plantain Garden faults zones (see Figure 1), and as regionally, there appears to have been little movement on the faults bordering the southern margin of the Gonâve Microplate in 2020.

Jamaica has earthquakes because of plate tectonics and its position in relationship to the northern margin of the Caribbean Plate and the Gonâve Microplate. While none of us wants a major earthquake, just as in the past, it will happen in the future. We are still not in a position to predict earthquakes, and, therefore,it is important that we build using the right building codes and make sure that we place important infrastructure in areas where earthquakes will produce less damage. It is also worth remembering that without plate tectonics, there would be no Jamaica. Jamaica only rose out of the sea 10-12 million years ago because of the power of plate tectonics.

- Simon Mitchell is professor of sedimentary geology, Department of Geography and Geology, and director of research, The Earthquake Unit, at The University of the West Indies, Mona. Send feedback to simon.mitchell@uwimona.edu.jm. January is observed as Earthquake Awareness Month. This article is one in a series of articles highlighting earthquakes and Jamaica.