Two Minor Earthquakes Felt In Tobago Within An Hour

At 10:15 PM Monday 8th February 2021, a (reviewed) minor Magnitude 3.8 (Md or Mt) earthquake occurred 2.72 KM ESE of Plymouth, Trinidad and Tobago, 3.95 KM NNW of Scarborough, Trinidad and Tobago, 9.01 KM ENE of Castara, Trinidad and Tobago. This event was located at 60.747°W and 11.219°N, at a depth of 37.0 Kilometers.

This information (above) is preliminary from the U.W.I. Seismic Research Centre, the authority for seismic and volcanological information in the Eastern Caribbean. This information may change when additional data is processed by a seismologist.

Information from the University of the West Indies Seismic Research Centre concerning the M3.8 Earthquake on land Tobago.
Information from the University of the West Indies Seismic Research Centre concerning the M3.8 Earthquake on land Tobago.

Just an hour later, at 11:15 PM, another minor quake was felt in Tobago, though no preliminary or reviewed solution was posted by the University of the West Indies Seismic Research Centre. However, the seismic event was recorded on nearby seismometers in T&T and in Grenada.

Seismic data from a seismic station located in Grenville, Grenada picking up Tobago's dual quakes on Monday night.
Seismic data from a seismic station located in Grenville, Grenada picking up Tobago’s dual quakes on Monday night.

Both events have been reported felt across Tobago with one or two short jolts. Some have reported hearing a rumble before the shaking. You can submit felt reports to the University of the West Indies Seismic Research Centre.

These events are shallow and are on land (confirmed location for the first event) so that’s why these quakes are being felt although they are at or below the 3.8 magnitude threshold (the magnitude at which most earthquakes are felt by humans).

There is no tsunami threat.

There are four conditions necessary for an earthquake to cause a tsunami:

  1. The earthquake must occur beneath the ocean or cause material to slide in the ocean.
  2. The earthquake must be strong, at least magnitude 6.5.
  3. The earthquake must rupture the Earth’s surface and it must occur at shallow depth – less than 70 KM below the surface of the Earth.
  4. The earthquake must cause vertical movement of the seafloor (up to several meters).

None of these conditions occurred.

Note that across the globe, different seismic monitoring agencies use different methods, or several methods, for processing quake 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.

Based on felt reports and other seismic solutions, this quake occurred in seismic zone 6, on-land Trinidad.

Seismic Zone 7: (Southwest) Tobago
Seismic Zone 7: (Southwest) Tobago

Zone 7 is a fairly complex area, where earthquake swarms associated with the Caribbean-South American plate boundary at depth (subducting) have occurred several times in the past (Latchman 2009Weber 2009Burmester et al. 1996). At this location, there are intersections between the transform faults and subduction zones of the Lesser Antilles. Until 1982, this area was considered a seismically quiet area, with low seismic hazard based on the low level, low magnitude output.

Earthquakes *cannot* be predicted – meaning the precise time, date, magnitude, depth, etc. cannot be known ahead of time based on current research and technology.

Generally, across the Eastern Caribbean, a seismically active area, earthquakes of this magnitude, up to M8.0 and greater, are possible and this statement has been repeated by seismologists at the U.W.I. Seismic Research Centre for decades.

Each year, over 2,200 seismic events are recorded in the Eastern Caribbean. On average, the Eastern Caribbean has seen a pattern of major (M7.0-M7.9) quakes every 20 to 30 years. That pattern has stayed true. The last major (M7.0-7.9) quake occurred north of Martinique in 2007. 

Historical patterns indicate great quakes (M8.0+) on the Richter Scale have occurred every century or so in the region. The probability of another event at that level is high since the last >M8.0 earthquake occurred in 1843.

Now is the time to create or go over your earthquake preparedness plan and know what to do during, before and after an earthquake. See here for more details.

Why did I hear the earthquake?

The rumbling you hear before the shaking occurs is the P-Wave, a quake’s first seismic wave creating a sound wave.

The speed of sound in air is 330 meters per second. The speed of seismic waves in the earth can not be described by a single number because it depends heavily on the material through which the waves are passing. 

It is well known that sound is created in air when an oscillating object is in contact with it. The sound really is oscillations in pressure, displacement and speed of the air, in concordance with the object creating them. 

Now, the seismic waves themselves include oscillations of the surface of the earth which is in contact with the air. Therefore, they cause oscillations in the air. However, the frequency of these oscillations is so low that we hear the least part of them as sound. Still, the frequency of the fastest waves, the P-waves, may be more than 30 Hertz and thus be audible as sound. The sound induced by seismic waves in the air have been compared to the boom in underground stations when a train is approaching.

We normally sense P-waves as an earthquake to a lesser degree than for instance the S-waves following them. If an earthquake has not been very strong or we are reasonably far away from its center we will not at all sense the P-waves as an earthquake but only hear the sound induced by them in the air. Still, we may feel the S-waves quite clearly and then other waves which often arrive later, especially at a distance.

Why did I feel two shakes?

An earthquake generates seismic waves that (1) penetrate the Earth as body waves (P & S) or (2) travel as surface waves (Love and Rayleigh). Each wave has a characteristic speed and style of motion. Here we exaggerate the motion by bouncing a building to show what sensitive instruments record as seismic waves arrive at the station. Note that waves travel in all directions from an earthquake. (IRIS)
An earthquake generates seismic waves that (1) penetrate the Earth as body waves (P & S) or (2) travel as surface waves (Love and Rayleigh). Each wave has a characteristic speed and style of motion. Here we exaggerate the motion by bouncing a building to show what sensitive instruments record as seismic waves arrive at the station. Note that waves travel in all directions from an earthquake. (IRIS)

An earthquake releases a number of seismic waves, which cause different types of shaking.

The compressive P-waves – essentially sound passing through rock – are the fastest seismic waves, and arrive first. You may feel an up-and-down jolt.

The sideways-shaking S-waves (following the green lines) arrive some seconds later, traveling a little over half the speed of the P-waves; the delay is a direct indication of the distance to the quake. Both of these are body-waves, that pass directly through the earth’s crust.

Following the S-waves are various kinds of surface-waves – Love waves and Rayleigh waves – that travel only at the earth’s surface. Surface waves are smaller for deep earthquakes, which have less interaction with the surface. For shallow earthquakes – less than roughly 60 km deep – the surface waves are stronger, and may last several minutes; these carry most of the energy of the quake and cause the most severe damage. 

For shallow quakes like this morning’s, people would have felt the first jolt of the P-waves and these the side-to-side shaking of the S-waves.

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