Seismic Zones of Trinidad and Tobago
Seismic zones are areas or regions of seismicity where there is a common cause of seismic events.
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.
Within the Trinidad and Tobago region, we can see periods of elevated seismicity in the form of earthquake swarms. These swarms can occur in any of the seismic zones, but over the last year, we’ve seen swarms occur in the Gulf of Paria and near Carúpano, Venezuela.
Because of the complex geophysics and tectonics associated with Trinidad, earthquakes within these zones are mainly attributed to two processes: subduction and strike-slip faults associated with the transverse boundary of the South American and Caribbean plate.
To the east of Trinidad and Tobago, the South American plate begins to be subducted under the Caribbean plate. This subduction continues under Trinidad and Tobago, with a gentle gradient. Across Tobago, according to estimates from the United States Geological Service’s Slab 2.0 Model, the South American plate begins at 42-46 kilometers below the island. Across Trinidad, the South American plate is 46-56 kilometers below the island.
However, at the eastern-most tip of Venezuela’s Paria Peninsula, the depth of the South American plate starts at 60 kilometers, and rapidly dives into the earth’s crust as you move westward, with the slab of the plate starting at 140 kilometers at Carúpano, Venezuela.
Closer to the surface, generally above 40 KM depth, the South American plate and the Caribbean plate are sliding past one another, with the Caribbean plate moving in an easterly direction at a rate of about 20 millimeters per year.
Rather than moving continuously, the plates tend to become locked along planar fault segments, accumulate strain then rupture in zones.
Zone 1: The Paria Peninsula
Seismic zone one is a complex, and without a doubt, the most seismically active area near Trinidad. Within zone one, the South American plate is beginning to rapidly descend into the Earth’s mantle as the detached oceanic lithosphere exists at depths between 50 to 300 kilometers. (Russo et al. 1993). This zone is one of the most active seismogenic sources in the Eastern Caribbean and has the potential to generate earthquakes up to Magnitude 8.0.
At shallower levels, the North Coast Fault Zone, as well as the El Pilar Fault, part of the Boconó-San Sebastian-El Pilar Fault system, run across Zone 1. These fault systems compensate for the stress built up as the Caribbean plate slides past the South American plate. Hence, most of these earthquakes from these fault systems are shallow to moderate between 0 to 70 kilometers.
Zone 2: The El Pilar Fault (Irapa- Carúpano Area)
This zone comprises part of the boundary between the Caribbean and the South American plate. The events that have their origin in the fault are shallow – less than 50 kilometers depth, and they are usually characterized mainly by right lateral strike slip along the northern coast of South America.
The El Pilar Fault, in the vicinity of Carúpano, has a number of smaller, conjugate or perpendicular faults which frequently produce earthquake swarms. This location is hypothesized to have a higher frequency of earthquakes compared to other locations along the El Pilar Fault Zone due to a kink in the fault system, with higher stress levels building in this zone than elsewhere.
Zone 3: Central Gulf of Paria & East Central Coastal Venezuela
The Gulf of Paria is yet another very faulted, seismically complex area surrounding Trinidad. At this zone, the extension of the Los Bajos fault from southwestern Trinidad, and the Warm Springs Fault from Central Trinidad meets the El Pilar fault. This complex network of faults also includes small, conjugate or perpendicular faults.
High levels of seismic activity occur in this zone, with both shallow and moderate depth earthquakes, generally remaining less than 50 kilometers depth. The UWI SRC has stated during a Q&A of the earthquake swarm between January and February 2018, this location is capable of generating a magnitude 6.5 or greater earthquake.
Zone 4: North of Trinidad
Earthquakes in Zone 4 can be associated with the subducting slab of the South American Plate, resulting in a deeper event of depths between 40-55 KM, but it can be deeper. Quakes in this area can also be associated with the North Coast Fault System, which runs just offshore the Northern Coast of Trinidad, where events are usually less than 40 KM depth. This area typically produces light (M4.0-4.9) or moderate (M5.0-5.9) magnitude earthquakes. Earthquakes in this zone tend to produce less aftershocks than earthquakes elsewhere across Trinidad and Tobago.
Zone 5: South of Trinidad
This zone is fairly seismically quiet with regards to earthquakes larger than magnitude 3.8. Russo et al. (1993) defined this zone as a passive margin edge in the Foreland basin in North of South America continent, covering events with strike slip and mixed thrust and thrust, around the Orinoco-Delta region in Venezuela with an average depth of 50 kilometers and a maximum magnitude of 6.5. Generally, we see episodic moderate (M5.0-M5.9) earthquakes.
Zone 6: On Land Trinidad
Trinidad is a highly faulted area, with several fault systems running across the island – all due to compensation of the lateral movement of the Caribbean and South American plates. There are several major fault systems that run across, on land, Trinidad including the El Pilar Fault system, the Central Range Fault, Northern Range Fault, Darien Ridge, Los Bajos Fault and the Arima Fault. Earthquakes on-land across Trinidad are typically less than 50 kilometers.
There was some variance in this typical depth due to an earthquake swarm in the Toco area in 2001. There have also been some moderate magnitude earthquakes occurring at a deeper depth.
On December 2nd 2004, events with a magnitude 5.5 and 5.1 occurred in the central north-east of Trinidad. Fault plane solutions suggest a normal motion with a component of right lateral strike-slip. The location of these earthquakes and the corresponding focal mechanisms coincides with the Northern Range faults dipping southward mapped by Algar & Pindell (1993) beneath the Caroni Swamp Area.
Zone 7: South & West of 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 2009, Weber 2009, Burmester 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.
On 20th September 1982, a magnitude 5.2 earthquake occurred, beginning an earthquake sequence for many months thereafter. In April 1997, a strong earthquake, measuring Mt 6.1 occurred, being the largest magnitude earthquake recorded in this zone during the instrumental era. This quake caused at least TT$18M worth of damage in Tobago alone.
Shallow to moderate earthquakes (0 to generally 60 KM depth) occurred in the past, with right lateral strike slip and normal faulting occurring (Morgan et al. 1988; Latchman 2009). Earthquakes of magnitudes 6.0-6.5 have occurred in this region in the past.
Zone 8: East of Trinidad, South of Tobago.
This zone, similar to Zone 7, 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 have 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).
Why have seismic zones?
Ultimately, earthquakes can occur at anytime, and at any location across Trinidad and Tobago. Seismic zones help emergency planners, seismologists and other relevant personnel to determine earthquake potential of different locations. In addition, it can help determine the most likely areas an earthquake may occur.
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, in the Eastern Caribbean, a seismically active area, earthquakes of this magnitude, up to M8.0 and greater, are possible in area, 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 earthquakes (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.
Related: Earthquake Safety
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.