Where Did Monday’s Earthquake Actually Strike?

At 6:32 PM Monday 4th May 2020, a light (preliminary) Magnitude 4.9 (Md or Mt) earthquake struck 47 km SE of Arima, Trinidad and Tobago, 65 km ENE of San Fernando, Trinidad and Tobago, and 72 km SE of Port of Spain, Trinidad and Tobago.

UWI SRC's preliminary solution for the light earthquake east of Trinidad
UWI SRC’s preliminary solution for the light earthquake east of Trinidad

This event occurred at a (preliminary) depth of 22 Kilometers. This information 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 a seismologist at the SRC.

This event has been reported felt across parts of Trinidad as a small jolt, lasting no longer than 5 seconds. You can submit felt reports to the University of the West Indies Seismic Research Centre.

The Mean Spectral Acceleration in g for Point Galeota for the main event and the first aftershock, 4 minutes later Based on the acceleration recorded, strong shaking was possible near the epicenter of the quake. Image & information: Dr. Ilias Papadopoulos  (University of the West Indies Seismic Research Centre)
The Mean Spectral Acceleration in g for Point Galeota for the main event and the first aftershock, 4 minutes later Based on the acceleration recorded, strong shaking was possible near the epicenter of the quake. Image & information: Dr. Ilias Papadopoulos (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.

With this earthquake, a larger than normal amount of earthquake solutions began to be published online concerning this quake – casting some doubt on the University of the West Indies SRC’s preliminary solution. One of the theories for the increased number of solutions would be a lack of seismic noise due to the COVID-19 pandemic, allowing for distant seismometers to pick up fainter seismic waves.

The different locations of Monday's quake based on different reporting seismic organizations and agencies.
The different locations of Monday’s quake based on different reporting seismic organizations and agencies.

While some of these solutions were reviewed by a seismic analyst, it is uncertain exactly how much data each organization was privy to in both automatic and manually reviewed solutions. As seen above, the location of the epicenter of the quake ranged from off the south coast of Trinidad, offshore eastern Trinidad, between Trinidad and Tobago and even north of Trinidad itself. The magnitudes, which some were calculated differently, ranged from magnitude 5.3 to magnitude 4.7. Even the depths varied wildly, from 10 kilometers to as deep as 110 kilometers.

Though the University of the West Indies Seismic Research Centre’s solution remains preliminary, it’s the authoritative information for our region. The Centre not only as access to other seismic networks in the area, but their own dense network of seismometers and accelerometers to determine more precise and accurate quake parameters for seismic events in our region.

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.

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 the location and depth of this earthquake, using the University of the West Indies Seismic Research Centre’s preliminary solution, it is tectonic in origin.

Seismic activity east of Trinidad was considered to be a low seismic hazard based on the low level, low magnitude output., until 10th March 1988, where an earthquake, measuring Mt 6.3, east of Trinidad effectively activated that zone. Since that time, earthquakes are episodic, with moderate (M5.0-M5.9) magnitude earthquakes.

Earthquakes within this zone are generally shallow to moderate in-depth, generally remaining less than 70 kilometers. Research has suggested that earthquakes located within this zone are consistent with the detachment and bending-flexure of the South American plate (slab) subducting and moving toward the collision zone (Russo & Speed, 1992).

Earthquakes at this location are typically due to normal faulting or strike-slip faults. These quakes occur at an average depth of 45 kilometers.

Closer view of Monday's earthquake east of Trinidad based on the UWI SRC's preliminary location.
Closer view of Monday’s earthquake east of Trinidad based on the UWI SRC’s preliminary location.

For this particular quake, and its two recorded aftershocks, it occurred just south of the Dunmore Hill Thrust Fault and west of a normal fault just offshore eastern Trinidad.

Seismograph between 2:00 PM and 9:00 PM Monday 4th May 2020 from Grenada showing the main shock of the earthquake east of Trinidad, with two subsequent aftershocks.
Seismograph between 2:00 PM and 9:00 PM Monday 4th May 2020 from Grenada showing the main shock of the earthquake east of Trinidad, with two subsequent aftershocks.

Within 10 kilometers of the epicenter of this earthquake, since 1960, there have been 33 recorded events with varying depths, generally between 20 kilometers to 65 kilometers, with two events deeper than 100 kilometers.

Based on records going back to the 1960s, this preliminary magnitude 4.9 (Md) earthquake would be the largest in the area, within 10 kilometers of the epicenter – provided the coordinates of the quake does not drastically change upon review.

Prior to this event, the largest quake that occurred in this area was recorded as magnitude 3.6 (Md) quakes, occurring on May 9th, 194, and October 20th, 1996. All other quakes have registered below magnitude 3.6.

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.

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