Earthquakes

Earthquakes are undoubtedly one of our most common natural phenomena in Trinidad and Tobago. In fact, our area, in the southeastern Caribbean, is one of the most seismically active areas in the entire Caribbean!

Content:
Causes of An Earthquake
Seismic Waves
Primary Waves
Shear (Secondary) Waves
Love Waves
Rayleigh Waves
Earthquake Frequency

Earthquake Sequences
Seismic Swarms
Earthquake Monitoring & Reporting
Measuring an Earthquake
Which is the “correct” magnitude and location?
Earthquake Prediction & Forecasting
Hazards
Major Earthquakes in T&T
Safety

What Causes An Earthquake?

On the most basic level, an earthquake occurs when there is a release of stress within a layer of rock – usually at a zone of existing weakness within the rock layer. This “zone of weakness” is typically a fault, but quakes do not always occur on a fault.

This definition applies to naturally occurring quakes, but there are also several other ways the ground can shake!

  • Tectonic Earthquakes: These are the most common, occurring along existing tectonic plates where stresses are built up along and between rock layers. Quakes here occur along existing or new normal, strike-slip or reverse/thrust faults.
  • Induced Earthquakes: These quakes are common in areas where hydraulic fracturing occurs but also includes events induced by tunneling, quarry blasts, filling reservoirs, geothermal projects, and even nuclear detonations. Earthquakes that are considered induced are seismic events caused by human activity.
  • Volcanic Earthquakes: These quakes are triggered by changes in pressure within the earth, generally associated with the movement of magma or other fluids such as water. According to the University of the West Indies Seismic Research Centre, there are 19 “live” volcanoes (which are likely to erupt again) in the Eastern Caribbean, all of which can and do produce volcanic earthquakes – most of which are not felt by inhabitants of the respective islands. There are also several subtypes of volcanic earthquakes.

Types of Waves

Types of seismic waves generated in an earthquake. Animation: IRIS
Types of seismic waves generated in an earthquake. Animation: IRIS

When any type of earthquake occurs, waves of energy, known as seismic waves, are generated. There are two types – body waves which move within the Earth and surface waves that move along the Earth’s surface.

Seismic waves help seismic analysts and seismologists to determine the earthquake’s location, magnitude and depth. It can also help in understanding the Earth’s inner structure and understanding the different types for shaking to help build safer infrastructure.

Primary (P-Waves)

P-Wave Animated Diagram showing the push-pull motion of the wave after an earthquake. Animation: Michigan Technological University
P-Wave Animated Diagram showing the push-pull motion of the wave after an earthquake. Animation: Michigan Technological University
  • A P-Wave is a body wave, meaning it moves through the Earth.
  • This is the fastest (by velocity) of all the waves. It reaches the seismometer first and is the first wave recorded.
  • The shaking feels like a push or a pull vertically.
  • This wave moves through solids and liquids through the earth.
  • It is also sometimes heard as a rumble before the shaking occurs.
  • It is the least damaging wave to life and property.

Shear (Secondary) (S-Waves)

S-Wave Animated Diagram showing the motion of the wave after an earthquake. Animation: Michigan Technological University
S-Wave Animated Diagram showing the motion of the wave after an earthquake. Animation: Michigan Technological University
  • An S-Wave is also a body wave, meaning it moves through the Earth.
  • This is the second-fastest (by velocity) of all the waves.
  • The shaking can feel sharp, with either side to side or forward/backward movement.
  • This wave only moves through solids through the earth.
  • This is often the first shaking persons feel, like that small jolt we feel in our minor to light quakes.
  • This can cause some damage.

Love (L-Waves)

L-Wave Animated Diagram showing the motion of the wave after an earthquake. Animation: Michigan Technological University
L-Wave Animated Diagram showing the motion of the wave after an earthquake. Animation: Michigan Technological University
  • An L-Wave is a surface wave, meaning it moves along the surface of the Earth.
  • This is the fastest (by velocity) of the two common surface waves.
  • The shaking can feel like side to side, forward to backward or a combination of both.
  • This wave only moves through solids along the earth.
  • It is the second most damaging wave to life and property.

Rayleigh Waves

Rayleigh Wave Animated Diagram showing the motion of the wave after an earthquake. Animation: Michigan Technological University
Rayleigh Wave Animated Diagram showing the motion of the wave after an earthquake. Animation: Michigan Technological University
  • A Rayleigh Wave is a surface wave, meaning it moves along the surface of the Earth.
  • This is the slowest (by velocity) of all the waves.
  • The shaking feels like a rolling or tumbling beach wave and can be in any direction.
  • In combination with the Love Wave, Rayleigh waves can be the most destructive to life and property.

Frequency of Earthquakes

Seismic Zones across Trinidad and Tobago

Trinidad and Tobago, and the surrounding region is a very seismically active area. Across the Eastern Caribbean, over 2,200 earthquakes are recorded annually. Since 1990, the University of the West Indies Seismic Research Centre records an annual average of 280 earthquakes in the Trinidad and Tobago region (area bounded by 9.5°- 11.5°N & 59.5°W – 63.5°W). Of these 280 quakes, 50 of these seismic events are on average, above magnitude 3.5.

However, different zones across Trinidad and Tobago have different overall seismicity (frequency of earthquakes). To make this easier, seismic zones are created which separate areas or regions of seismicity where there is a common cause of seismic events.

The Earthquake Sequence

Generally, in a typical earthquake sequence, there are sometimes foreshocks, then the mainshock (the largest magnitude seismic event) and then an aftershock sequence (all seismic events proceeding the largest registered magnitude earthquake).

These are rolling definitions, meaning that during the “aftershock sequence,” if a larger magnitude quake occurs in the same area at a similar depth, that would be considered the new mainshock, all prior quakes are now foreshocks and subsequent smaller magnitude quakes are aftershocks.

In addition, there are no set definitions or time periods of what constitutes as a mainshock or foreshock. In fact, based on present definitions, we can state that some of our current large magnitude events (which occur in the same location and depth) of historical large magnitude seismic events can be considered part of that aftershock sequence.

Seismic Swarms

Earthquake swarm of December 2004 across Eastern Trinidad
Earthquake swarm of December 2004 across Eastern Trinidad

Earthquake swarms are a series of earthquakes within a local area, occurring within a relatively short period of time. These swarms are usually associated with volcanic earthquakes or fracking and wastewater injection sites.

However, In the Trinidad and Tobago region, neither of the above causes of swarms occur. Within the seismic zones, there is little to no subsurface wastewater disposal, nor are there magmatic volcanoes triggering earthquake swarms. The prevailing theory for regions such as ours is that these quakes are related to stress release on smaller faults, within a fault system.

Monitoring & Reporting

The University of the West Indies Seismic Research Centre (SRC) remains the authoritative source for information on earthquake and volcanic activity in the English-speaking Eastern Caribbean.

With over 50 seismic stations in the region, the SRC has the densest monitoring network across the Eastern Caribbean, supplemented by stations from other monitoring agencies.

Map of Seismic Stations as of 2010 across the Eastern Caribbean. Image; UWI SRC
Map of Seismic Stations as of 2010 across the Eastern Caribbean. Image; UWI SRC

Automated posts from the U.W.I. Seismic Research Centre are posted onto their social media platforms within minutes of the event. These events are always then reviewed by a seismologist or seismic analyst at the SRC within the next few days. In most instances, the reviewed solution is then posted to their website.

However, the SRC only publishes seismic events that are greater than magnitude (md) 3.8 on their social media platforms and website or events that have been reported felt to them.

We also have the neighboring seismic monitoring agency, the Venezuelan Foundation for Seismological Research (FUNVISIS), which publishes earthquake solutions for seismic events that are magnitude (Mw) 2.5 or greater in their area of responsibility. This means we often also get information on seismic events that occur near T&T, including the Gulf of Paria and north of the Paria Peninsula and Trinidad.

Internationally, other seismic monitoring agencies such as the United States Geological Survey and the European-Mediterranean Seismological Centre also provide earthquake solutions for T&T and the Eastern Caribbean. Typically, these agencies report on seismic events larger than magnitude 4.0 or events that are reported felt to the respective agencies which may have been below that magnitude threshold.

For larger events, the GFZ German Research Centre for Geosciences may also produce a solution for the seismic event.

Regionally, the French Central Seismological Office monitors seismic events in the French Antilles, supported by the Guadeloupe and Martinique Volcano Observatories. The UWI SRC also manages the Montserrat Volcano Observatory, with a dense network of equipment on that island. Further North, for the Caribbean Netherlands, the Royal Netherlands Meteorological Institute (KNMI) supports seismic monitoring for that area. Lastly, the Puerto Rico Seismic Network monitors seismic activity for Puerto Rico and the Virgin Islands.

Measuring an Earthquake

Earthquakes can be measured in two ways – with a number (magnitude) or by a feeling (intensity of shaking). Earthquake magnitude is a quantitative, mathematical calculation to measure the energy released at the source of an earthquake. On the other hand, earthquake intensity is the qualitative measurement of the strength of shaking produced by an earthquake.

There are both a number of different magnitude scales and intensity scales. For Trinidad and Tobago, authoritative information comes from the University of the West Indies Seismic Research Centre which reports using the duration magnitude (md) method and intensity using the Modified Mercalli Intensity (MMI).

Which is the “correct” magnitude and location?

Across the globe, different seismic monitoring agencies use different methods, or several methods, for processing earthquake parameters. Each method has its limitations and will likely produce different results within the ranges of the uncertainty of that data. This is generally accepted within the scientific community.

Additionally, to meet the public’s need for information, seismological agencies have now begun publishing and updating information as an earthquake occurs. The data is usually automatically processed with limited data by computers and published either directly online for the public (USGS, EMSC, etc.), or sent to stakeholders, then to the public (U.W.I. SRC).

Because limited amounts of data are used for these preliminary outputs, it is common to see minor changes in latitude, longitude, depths, and magnitude.

Furthermore, because of the difference in magnitude types, you will likely see even more variation amongst seismic reports from different organizations (UWI SRC, FUNVISIS, USGS, BSCF (France), etc.).

However, nearly all international seismic monitoring agencies do not receive seismic data from FUNVISIS or UWI SRC. This means that in most cases, with reporting stations mainly north of T&T, the epicenter of quakes nearly always have a northward bias when it comes to latitude and longitude of a quake. It is also important to note that there is no exact location of a quake, as these seismic events occur due to a slip across a fault.

No matter how dense the seismic network is, there is always uncertainty which by the density of stations is reduced but never eliminated. When a solution is produced, the longitude and latitude are generated. All processing algorithms also provide the small and big axis of the eclipse with that location in the center, hence the location of an earthquake is not one point on the earth, but an area defined by those axes.

Prediction and Forecasting

Earthquakes cannot be predicted based on current research and technology.

Earthquake prediction is a branch of seismology focused on predicting an exact time, location and magnitude of an earthquake within a specific location. Earthquake forecasting is the probability of the general seismic hazard of an area, including the frequency and magnitude of earthquakes in an area over years or decades.

Both forecasting and prediction differ from earthquake warning systems, which detect initial seismic waves (P-waves) and provide a real-time warning to regions that might be affected.

Hazards

Earthquakes bring with it a multitude of hazards. Primary hazards result from direct interaction of seismic wave energy with the ground. This causes ground shaking and possible surface ruptures.

Secondary hazards are usually more devastating. These include landslides, liquefaction, tsunamis, seiches, flooding, fires, power outages, and even mud volcanism in Trinidad and Tobago.

Major Seismic Events in Trinidad and Tobago

Largest Caribbean Earthquakes from 2150 B.C. to 2018
Largest Caribbean Earthquakes from 2150 B.C. to 2018

Moderate (magnitude 5.0 to 5.9), strong (magnitude 6.0 to 6.9) and major (magnitude 7.0 to 7.9) are no stranger to Trinidad and Tobago and the Eastern Caribbean.

Safety

Since Trinidad and Tobago experiences an annual average of 280 seismic events, it is not only imperative that we practice “Drop. Cover. Hold.” during an earthquake, but how best to prepare. This is of particular importance since Trinidad and Tobago is long overdue for our “Big One.”

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