There are approximately 100,000 tectonic earthquakes (plate movements) recorded each year on the planet, but very few are felt by humans.

The study of earthquakes and their mechanisms is called seismology, that is, the study of the propagation of seismic waves within the Earth. Modern seismology uses the physical concepts of Newtonian mechanics applied to the knowledge of the Earth.

Before talking about the earthquake, two associated principles must be explained :

• The rupture of rocks along the fault line, depending on the magnitude of the earthquake, can permanently alter the landscape surrounding the fault.
• The emission of seismic waves that propagate through the Earth in all directions, reaching the surface where they can cause serious damage (material and human).

Seismic waves and their mechanisms

Global seismology studies the internal structure of the Earth using recordings of waves produced by earthquakes at very large distances. Indeed, when the magnitude of the earthquake is sufficient (greater than 5), the waves it emits can be measured over the entire surface of the Earth. The paths and shapes of these waves depend on the environments crossed. Their study therefore makes it possible to define the structure but also the physicochemical properties of the different Earth envelopes.

Seismology has also made it possible to highlight “large” discontinuities (interfaces) in the deep structures of the globe, subsequently delimiting a very precise internal structure. You have understood that thanks to these waves (P, S and surface waves), we have been able to guess the structure of the Earth. How? Through the medium in which they propagate, because these waves propagate differently depending on the medium.

P waves propagate in solids, liquids and gases, while S waves propagate only in solids. We also know that the propagation speed of seismic waves is proportional to the density of the material in which they propagate. Thanks to this, we can identify the layers of the Earth up to the core as well as their respective state. Like light rays (studied in optics), the paths of seismic waves in the Earth depend on the physical properties of the media crossed (density, speed, reflection, transmission, refraction coefficients, etc.). Thus, S waves do not propagate in the outer core of the Earth: it is therefore liquid!

By combining the measurements of several earthquakes at different locations on Earth, we can deduce the Earth’s internal structure. This is called seismic tomography.

The seismometer will be the device that will be used to measure these waves, very precisely the movement of the ground in a wide frequency range. Here is the type of physical measurement carried out when we are going to classify them :

• Sensors recording ground displacement -> mechanical seismometers
• Sensors recording ground velocity -> velocimeters
• Sensors recording ground acceleration -> accelerometers

There are seismic networks at different scales: local, regional, global, depending on the type of application or study envisaged.

Location

In order to locate an earthquake, 2 parameters are taken into account:

• Its time of origin, that is, at a moment (t) when the seismic shock occurs.
• Its hypocenter (the focus), the location (coordinate = x, y, z or latitude, longitude, depth) where the earthquake occurs. When we speak of the epicenter, we are referring to the projection of the hypocenter onto the Earth’s surface.

“Earthquake localization relies heavily on analyzing the arrival times of bulk seismic waves (P and S) and studying waveforms. It can be done independently of knowledge of the size and duration of the event.”

Earthquake ranking

(Currently being published)

The Lesser Antilles arc is a place where many earthquakes occur.

It is worth remembering that the Earth’s crust is a layer made up of enormous plates of rock (tectonic plates) that are irregularly divided. Thus, the area where two (or more) plates meet is called a plate boundary. This whole process will be explained by what is called plate tectonic theory. The division of the lithosphere into rigid plates that move relative to each other is the surface expression of convection movements that take place deep within the Earth’s mantle.

Plates are constantly moving, and this movement is neither regular nor continuous. Much of the world’s seismic and volcanic activity occurs along plate boundaries. Their movements release energy and cause the earthquakes we feel (provided they are shallow and close to human habitation).

At these boundaries, the plates interact with each other in different ways, there are movements of :

• Divergence (plates move apart)
• Convergence (plates move together)
• Sliding (plates slide against each other)

The convergent movement, which creates subduction zones, is the main movement occurring in the eastern Caribbean. As the North American plate slides beneath the lighter Caribbean plate, the lithosphere thickens at the plate edge, allowing the formation of the Caribbean islands and the Central American mountain ranges.

Although we most often talk about tectonic earthquakes, there are also earthquakes of volcanic origin. They are generated by the movement of magma within the lithosphere (rock). The magma, being less dense than the surrounding rock, rises to the surface, breaks the rock during its movement and generates earthquakes.

Since earthquakes are not predictable events, they must be prevented. Our area is considered high-risk, so construction must comply with current earthquake regulations and learn how to behave before, during, and after an earthquake.

To do this, we have listed some educational or informative links/videos on the subject :

• To know how to react during an earthquake https://www.sikana.tv/fr/health/natural-disaster/major-disasters-how-to-protect-yourself-during-an-earthquake
• If you are interested in seismic activity, you can follow Haiti’s seismic activity in real time https://ayiti.unice.fr/ayiti-seismes/

We often think of Haiti when we talk about earthquakes because it has already been the victim of many very destructive events. The cause is very simply explained by its geographical location and the interaction of the plates under its soil. The island of Hispaniola is located at the border of the Caribbean and North American plates. A microplate named Gonâve was then formed between these two major plates. The latter then cut Haiti off from its capital, its movements then causing the damage that we all know.

This border is also fragmented by various faults such as: The Southern Peninsula Fault (Enriquillo/Plantain Garden), the Northern Fault, the North Hispaniola Fault and the Muertos-Neiba-Matheux Fault which are potentially seismogenic.