12:51 AM – Strong M6.0 Earthquake Strikes Leeward Islands

At 12:51 AM, Friday 26th March 2021, a strong (preliminary) Magnitude 6.0 (Mt) earthquake struck approximately 101 km NE of Point-à-Pitre, Guadeloupe, 130 km SE of Saint John’s, Antigua and Barbuda and 171 km NE of Roseau, Dominica. This event was located at 60.75°W and 16.70°N, at a depth of 10.0 Kilometers.

There have been no immediate reports of damage or injuries across the islands though strong to very strong shaking was reported in Guadeloupe.

UWI SRC preliminary solution for the M6.0 Northeast of Guadeloupe

This information (above) is preliminary from the U.W.I. Seismic Research Centre, the authority for seismic and volcanological information in the Eastern Caribbean. Quake parameters such as location, depth, and magnitude may change upon review from seismologists at the SRC.

This event was widely reported felt across the Leewards, as far south as St. Lucia and as far west as Puerto Rico. Shaking reportedly lasted up to 15 seconds and was strongly felt in Guadeloupe, the country closest to the epicenter of the quake. You can submit felt reports to the University of the West Indies Seismic Research Centre.

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).

These conditions were not met.

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

Information from the USGS concerning the earthquake in the Leeward Islands.
Information from the USGS concerning the earthquake in the Leeward Islands.

The United States Geological Survey (USGS) recorded this quake in a similar location, slightly south and west of the UWI SRC’s solution, at a magnitude 5.8 event with an estimated depth of 26.1 kilometers.

Information from the EMSC concerning the earthquake in the Leeward Islands.
Information from the EMSC concerning the earthquake in the Leeward Islands.

The European-Mediterranean Seismological Centre (EMSC) recorded this quake in a similar location, slightly south of the UWI SRC’s solution, at a magnitude 5.8 event with an estimated depth of 30.0 kilometers.

Based on the earthquake’s focal mechanism, this event occurred on a thrust fault, which is typical for the area.

Within 20 kilometers of the UWI SRC’s solution, over 200 seismic events have been recorded, the largest being a magnitude 5.6 on December 1st, 1969. Earthquakes in this area generally occur at depths less than 50 kilometers, with most events occurring near the 25 kilometer depth, near the inferface of the subducting slab of the South American plate and the overriding Caribbean Plate.

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.

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 the air is 330 meters per second.  a single number can not describe the speed of seismic waves in the earth 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 earth’s surface, 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.

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