Shonel Dwyer | Earthquakes … the threat is in the fault

It behoves us to understand the world around us or as we experience it, whether it is the latest music that we dive into or the earthquake that moved most of us on the evening of September 21.

Let us explore the less acknowledged seismic ‘drift’, a phenomenon intricately woven into the grand orchestration of plate tectonics, sometimes leading to widespread tragedy and devastation.

REVISITING RUDIMENTS

Chemically, the composition of the Earth is layered as crust, mantle, and core, one of our first recollections from the back of the class in geography.

If you sat a bit closer to the teacher, you would have then realised that the Earth is also layered in terms of physical or mechanical behaviour. In those terms, the outer layer of the Earth is a rigid lithosphere (outer layer) and underneath it, a weak, soft asthenosphere (inner layer).

This chemical/mechanical intersection of the Earth’s layers allows geologists to understand the origins of and relationships between

(1) the world’s volcanic arcs (chain of volcanoes) and deep earthquake zones;

(2) exotic terranes and thrust fault zones; and

(3) transform faults and shallow earthquake zones such as in the Northern Caribbean.

With this understanding, we shall discover that Earth’s crust is constantly under pressure, resisting forces of movement at times, but always in motion.

In the realm of plate tectonic theory, conceived in the 1970s, the Earth’s lithosphere is imagined as a complex jigsaw puzzle. Following the breakup of Pangea (the supercontinent that existed millions of years ago), the Earth’s lithosphere fragmented into several distinct plates. According to this theory, these rigid plates float on the semi-fluid asthenosphere, and their movements drive various geological phenomena that certainly get our attention.

This puzzle of enormous plates intricately interlocking are far from seamless. They exist at highly dynamic and also transformative boundaries. Depending on tectonic forces (push or pull), these interactions can significantly impact how plates move or transform (slide) concerning one another. ‘Transform plate boundaries’ exemplifying one of these interactions, are associated with shallow focus earthquakes in association with faults and other deformations such as folds.

Earthquakes occur due to the sudden breaking of rocks beneath the Earth’s surface. These rocks are under immense tectonic pressure. This intense pressure causes the rocks to break abruptly, leading to rapid movement along a fault line (a fracture or weakness) in the Earth’s crust, where rocks have moved. This rapid movement results in a sudden release of energy, generating seismic waves. These waves travel through the Earth, causing the ground to shake or tremble.

EARTHQUAKES IN GREATER ANTILLES

Professor Simon Mitchell from The University of the West Indies, Mona, says that the current Caribbean Plate is in a relatively “stable position” while the Americas are, in fact, sliding westward in respect of the plate’s perceived stability. So what then causes earthquakes to occur in Jamaica and the rest of the Greater Antilles?

Certainly, based on information and recorded history, the vulnerability to earthquakes in the region is significant due to the presence of two massive transform plate boundaries. The first, the Septentrional-Oriente Fault Zone, separates the Gonave microplate from the North American Plate, while the second, the Enriquillo–Plantain Garden Fault Zone (EPGFZ), separates the Gonave microplate from the Caribbean Plate. These fault zones also facilitate a slight eastward extension of the Greater Antillean group of islands in the order of two centimetres per year, says Mitchell.

Moreover, smaller yet significant fault systems have been propagated within each Greater Antillean island, sharing a similar fault trend/orientation as the major transform faults and creating multiple areas of weakness in the Earth’s crust. These areas of weakness serve as potential epicentres for earthquakes, making the Northern Caribbean region susceptible to seismic activity. Slight movement of the Caribbean Plate, combined with the presence of interconnected fault systems, amplifies the vulnerability of the area. During an earthquake, the two sides of a fault slide at a velocity of the order of a metre per second – about a billion times faster than the steady-state background motion of the Caribbean Plate.

CAN WE PREDICT EARTHQUAKES?

Seismologists help with long-term planning and monitoring. They analyse past earthquakes to assess the risk of future ones and provide information needed for safe construction and land management policies.

The public often expects seismologists to predict earthquakes on a short-term scale (hours to days). However, this remains a challenge due to the complexity of the processes causing earthquakes. Recent progress in seismology has allowed scientists to detect subtle changes in the Earth’s surface, including slow earthquakes. These are events that bridge the gap between regular tectonic motion and rapid seismic ruptures. Without a doubt, Jamaica experienced its most significant ‘drift’ this past summer, although not seismic in nature, but rather, through the airwaves.

Shonel Dwyer is senior technology officer at the National Commission on Science and Technology (NCST). She is a geologist who has worked as a specialist in natural/water resources management and the local oil and gas sector. Send feedback to columns@gleanerjm.com.