Multiple Aftershocks Recorded East of Leewards Following M6.0 Earthquake

At 12:51 AM Friday 26th March 2021, a strong magnitude 6.0 (Mt) earthquake struck northeast of Guadeloupe, jolting islands as far west as Puerto Rico to as far south as St. Lucia. The quake was located at 60.75°W and 16.70°N, 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. The event was located at a preliminary depth of 10.0 Kilometers. No damage nor injuries were reported across the Leewards but shaking up to 15 seconds occurred.

Aftershock sequence of the M6.0 earthquake that struck at 12:51 AM Friday 26th March 2021. Data:BCSF-RéNaSS
Aftershock sequence of the M6.0 earthquake that struck at 12:51 AM Friday 26th March 2021. Data: BCSF-RéNaSS

Over the last 72 hours, the French Central Seismological Office (BCSF-RéNaSS) has detected at least 13 aftershocks in the area over magnitude 2.0. Based on BCSF-RéNaSS data, 9 quakes were between magnitude 2.0 and 3.0, 4 quakes were between magnitude 3.0 and 4.0, and one event, occurring at 9:19 AM Friday 26th March 2021 registered a magnitude 5.0 (MLv).

M4.9 aftershock in the Leewards based on UWI SRC data.
M4.9 aftershock in the Leewards based on UWI SRC data.

According to the University of the West Indies Seismic Research Centre (UWI-SRC), the authority for seismic and volcanological information in the Eastern Caribbean, this aftershock registered at a light magnitude 4.9 (Mt). The quake was preliminarily located at 60.77°W and 16.62°N, approximately 94 km NE of Point-à-Pitre, Guadeloupe, 131 km SE of Saint John’s, Antigua and Barbuda, 162 km NE of Roseau, Dominica. The event was located at a preliminary depth of 10.0 Kilometers.

In the Lesser Antilles, Friday’s M6.0 earthquake was the largest seismic event since T&T’s M6.9 earthquake on August 21st, 2018 and the subsequent M6.0 aftershock on August 22nd, 2018.

Aftershocks are normal after a large earthquake and can continue for days, weeks, months and even years after the main shock.

There was 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.

This is why you’d see other seismic agencies reporting differing magnitudes and other earthquake parameters for both the mainshock (M6.0) and aftershock (M5.0-M4.9).

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.

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